Noradrenergic depletion causes sex specific alterations in the endocannabinoid system in the Murine prefrontal cortex

Brain endocannabinoids (eCB), acting primarily via the cannabinoid type 1 receptor (CB1r), are involved in the regulation of many physiological processes, including behavioral responses to stress. A significant neural target of eCB action is the stress-responsive norepinephrine (NE) system, whose dysregulation is implicated in myriad psychiatric and neurodegenerative disorders. Using Western blot analysis, the protein expression levels of a key enzyme in the biosynthesis of the eCB 2-arachidonoylglycerol (2-AG), diacylglycerol lipase-α (DGL-α), and two eCB degrading enzymes monoacylglycerol lipase (MGL) and fatty acid amide hydrolase (FAAH) were examined in a mouse model that lacks the NE-synthesizing enzyme, dopamine β-hydroxylase (DβH-knockout, KO) and in rats treated with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4). In the prefrontal cortex (PFC), DGL-α protein expression was significantly increased in male and female DβH-KO mice (P < 0.05) compared to wild-type (WT) mice. DβH-KO male mice showed significant decreases in FAAH protein expression compared to WT male mice. Consistent with the DβH-KO results, DGL-α protein expression was significantly increased in male DSP-4-treated rats (P < 0.05) when compared to saline-treated controls. MGL and FAAH protein expression levels were significantly increased in male DSP-4 treated rats compared to male saline controls. Finally, we investigated the anatomical distribution of MGL and FAAH in the NE containing axon terminals of the PFC using immunoelectron microscopy. MGL was predominantly within presynaptic terminals while FAAH was localized to postsynaptic sites. These results suggest that the eCB system may be more responsive in males than females under conditions of NE perturbation, thus having potential implications for sex-specific treatment strategies of stress-related psychiatric disorders.

Cortical adrenoceptor expression, function and adaptation under conditions of cannabinoid receptor deletion

A neurochemical target at which cannabinoids interact to have global effects on behavior is brain noradrenergic circuitry. Acute and repeated administration of a cannabinoid receptor synthetic agonist is capable of increasing multiple indices of noradrenergic activity. This includes cannabinoid-induced 1) increases in norepinephrine (NE) release in the medial prefrontal cortex (mPFC); 2) desensitization of cortical α2-adrenoceptor-mediated effects; 3) activation of c-Fos in brainstem locus coeruleus (LC) noradrenergic neurons; and 4) increases in anxiety-like behaviors. In the present study, we sought to examine adaptations in adrenoceptor expression and function under conditions of cannabinoid receptor type 1 (CB1r) deletion using knockout (KO) mice and compare these to wild type (WT) controls. Electrophysiological analysis of α2-adrenoceptor-mediated responses in mPFC slices in WT mice showed a clonidine-induced α2-adrenoceptor-mediated increase in mPFC cell excitability coupled with an increase in input resistance. In contrast, CB1r KO mice showed an α2-adrenoceptor-mediated decrease in mPFC cell excitability. We then examined protein expression levels of α2- and β1-adrenoceptor subtypes in the mPFC as well as TH expression in the locus coeruleus (LC) of mice deficient in CB1r. Both α2- and β1-adrenoceptors exhibited a significant decrease in expression levels in CB1r KO mice when compared to WT in the mPFC, while a significant increase in TH was observed in the LC. To better define whether the same cortical neurons express α2A-adrenoceptor and CB1r in mPFC, we utilized high-resolution immunoelectron microscopy. We localized α2A-adrenoceptors in a knock-in mouse that expressed a hemoagglutinin (HA) tag downstream of the α2A-adrenoceptor promoter. Although the α2A-adrenoceptor was often identified pre-synaptically, we observed co-localization of CB1r with α2-adrenoceptors post-synaptically in the same mPFC neurons. Finally, using receptor binding, we confirmed prior results showing that α2A-adrenoceptor is unchanged in mPFC following acute or chronic exposure to the synthetic cannabinoid receptor agonist, WIN 55,212-2, but is increased, following chronic treatment followed by a period of abstinence. Taken together, these data provide convergent lines of evidence indicating cannabinoid regulation of the cortical adrenergic system.

Localization of the delta opioid receptor and corticotropin-releasing factor in the amygdalar complex: role in anxiety

It is well established that central nervous system norepinephrine (NE) and corticotropin-releasing factor (CRF) systems are important mediators of behavioral responses to stressors. More recent studies have defined a role for delta opioid receptors (DOPR) in maintaining emotional valence including anxiety. The amygdala plays an important role in processing emotional stimuli, and has been implicated in the development of anxiety disorders. Activation of DOPR or inhibition of CRF in the amygdala reduces baseline and stress-induced anxiety-like responses. It is not known whether CRF- and DOPR-containing amygdalar neurons interact or whether they are regulated by NE afferents. Therefore, this study sought to better define interactions between the CRF, DOPR and NE systems in the basolateral (BLA) and central nucleus of the amygdala (CeA) of the male rat using anatomical and functional approaches. Irrespective of the amygdalar subregion, dual immunofluorescence microscopy showed that DOPR was present in CRF-containing neurons. Immunoelectron microscopy confirmed that DOPR was localized to both dendritic processes and axon terminals in the BLA and CeA. Semi-quantitative dual immunoelectron microscopy analysis of gold-silver labeling for DOPR and immunoperoxidase labeling for CRF revealed that 55 % of the CRF neurons analyzed contained DOPR in the BLA while 67 % of the CRF neurons analyzed contained DOPR in the CeA. Furthermore, approximately 41 % of DOPR-labeled axon terminals targeted BLA neurons that expressed CRF while 29 % of DOPR-labeled axon terminals targeted CeA neurons that expressed CRF. Triple label immunofluorescence microscopy revealed that DOPR and CRF were co-localized in common cellular profiles that were in close proximity to NE-containing fibers in both subregions. These anatomical results indicate significant interactions between DOPR and CRF in this critical limbic region and reveal that NE is poised to regulate these peptidergic systems in the amygdala. Functional studies were performed to determine if activation of DOPR could inhibit the anxiety produced by elevation of NE in the amygdala using the pharmacological stressor yohimbine. Administration of the DOPR agonist, SNC80, significantly attenuated elevated anxiogenic behaviors produced by yohimbine as measured in the rat on the elevated zero maze. Taken together, results from this study demonstrate the convergence of three important systems, NE, CRF, and DOPR, in the amygdala and provide insight into their functional role in modulating stress and anxiety responses.

Alcohol-induced dysregulation of stress-related circuitry: The search for novel targets and implications for interventions across the sexes

While the ability to process fermented fruits and alcohols was once an adaptive trait that improved nutrition and quality of life, the availability and prevalence of high potency alcoholic drinks has contributed to alcohol abuse disorders in a vulnerable portion of the population. Although the neural reward systems take part in the initial response to alcohol, negative reinforcement and stress, which are normally adaptive responses, can intersect to promote continued alcohol use at all stages of the addiction cycle. Eventually a point is reached where these once adaptive responses become dysregulated resulting in uncontrolled intake that constitutes a clinically important condition termed alcohol use disorder (AUD). Current research is targeted at both the behavioral and molecular adaptations in AUDs in an effort to better develop novel approaches to intervention. In this review, historical context is provided demonstrating the societal burden of alcohol use and abuse disorders. The importance of gender in the mechanism of action of alcohol is discussed. Finally, the impact of alcohol on stress-related circuitry, uncovered by preclinical research, is outlined to provide insight into potential novel pharmacological approaches to the treatment of AUD.

Ultrastructural evidence for synaptic contacts between cortical noradrenergic afferents and endocannabinoid-synthesizing post-synaptic neurons

Endocannabinoids (eCBs) are involved in a myriad of physiological processes that are mediated through the activation of cannabinoid receptors, which are ubiquitously distributed within the nervous system. One neurochemical target at which cannabinoids interact to have global effects on behavior is brain noradrenergic circuitry. We, and others, have previously shown that CB type 1 receptors (CB1r) are positioned to pre-synaptically modulate norepinephrine (NE) release in the rat frontal cortex (FC). Diacylglycerol lipase (DGL) is a key enzyme in the biosynthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG). While DGL-α is expressed in the FC in the rat brain, it is not known whether noradrenergic afferents target neurons expressing synthesizing enzymes for the endocannabinoid, 2-AG. In the present study, we employed high-resolution neuroanatomical approaches to better define cellular sites for interactions between noradrenergic afferents and FC neurons expressing DGL-α. Immunofluorescence microscopy showed close appositions between processes containing the norepinephrine transporter (NET) or dopamine-β-hydroxylase (DβH) and cortical neurons expressing DGL-α-immunoreactivity. Ultrastructural analysis using immunogold-silver labeling for DGL-α and immunoperoxidase labeling for NET or DβH confirmed that NET-labeled axon terminals were directly apposed to FC somata and dendritic processes that exhibited DGL-α-immunoreactivity. Finally, tissue sections were processed for immunohistochemical detection of DGL-α, CB1r and DβH. Triple label immunofluorescence revealed that CB1r and DβH were co-localized in common cellular profiles and these were in close association with DGL-α. Taken together, these data provide anatomical evidence for direct synaptic associations between noradrenergic afferents and cortical neurons exhibiting endocannabinoid synthesizing machinery.

Chronic alcohol exposure differentially affects activation of female locus coeruleus neurons and the subcellular distribution of corticotropin releasing factor receptors

Understanding the neurobiological bases for sex differences in alcohol dependence is needed to help guide the development of individualized therapies for alcohol abuse disorders. In the present study, alcohol-induced adaptations in (1) anxiety-like behavior, (2) patterns of c-Fos activation and (3) subcellular distribution of corticotropin releasing factor receptor in locus coeruleus (LC) neurons was investigated in male and female Sprague-Dawley rats that were chronically exposed to ethanol using a liquid diet. Results confirm and extend reports by others showing that chronic ethanol exposure produces an anxiogenic-like response in both male and female subjects. Ethanol-induced sex differences were observed with increased c-Fos expression in LC neurons of female ethanol-treated subjects compared to controls or male subjects. Results also reveal sex differences in the subcellular distribution of the CRFr in LC-noradrenergic neurons with female subjects exposed to ethanol exhibiting a higher frequency of plasmalemmal CRFrs. These adaptations have implications for LC neuronal activity and its neural targets across the sexes. Considering the important role of the LC in ethanol-induced activation of the hypothalamo-pituitary-adrenal (HPA) axis, the present results indicate important sex differences in feed-forward regulation of the HPA axis that may render alcohol dependent females more vulnerable to subsequent stress exposure.

Amygdalar neuronal plasticity and the interactions of alcohol, sex, and stress

Alcohol abuse and alcoholism are major medical problems affecting both men and women. Previous animal studies reported a difference in c-Fos neuronal activation after chronic alcohol exposure; however, females remain an understudied population. To model chronic alcohol exposure match-pair fed adult male and female rats were administered 14 days of a liquid ethanol containing diet. Analysis focused on the central nucleus of the amygdala (CeA), a region integral to stress sensitivity and substance abuse. Immunocytochemical approaches identified cells containing ΔFosB, a marker of sustained neuronal activation, and activity patterns within the CeA were mapped by subdivision and rostral-caudal extent. Significant interactions were present between all groups, with gender differences noted among control groups, and ethanol exposed animals having the greatest number of ΔFosB immunoreactive cells indicating baseline dysregulation. Compared with c-Fos, a marker of recent neuronal activation, male ethanol treated animals had similar activity to controls, indicating a neuronal habituation not seen in females. Next, a cohort of animals were exposed to the forced swim test (FST), and c-Fos was examined in addition to FST behavior. Neuronal activity was increased in ethanol exposed animals compared to controls, and control females compared to males, indicating a potentiated stress response. Further, a population of activated neurons were shown to contain either corticotropin releasing factor or enkephalin. The present data suggest that dysregulation in the CeA neuronal activity may underlie some of the negative sequelae of alcohol abuse, and may, in part, underlie the distinctive response seen between genders to alcohol use.

Using high resolution imaging to determine trafficking of corticotropin-releasing factor receptors in noradrenergic neurons of the rat locus coeruleus

Trafficking of G protein-coupled receptors (GPCRs) is a critical determinant of cellular sensitivity of neurons. To understand how endogenous or exogenous ligands impact cell surface expression of GPCRs, it is essential to employ approaches that achieve superior anatomical resolution at the synaptic level. In situations in which light and fluorescence microscopy techniques may provide only limited resolution, electron microscopy provides enhanced subcellular precision. Dual labeling immunohistochemistry employing visually distinct immunoperoxidase and immunogold markers has been an effective approach for elucidating complex receptor profiles at the synapse and to definitively establish the localization of individual receptors and neuromodulators to common cellular profiles. The immuno-electron microscopy approach offers the potential for determining membrane versus intracellular protein localization, as well as the association with various identifiable cellular organelles. Corticotropin-releasing factor (CRF) is an important regulator of endocrine, autonomic, immunological, behavioral and cognitive limbs of the stress response. Dysfunction of this neuropeptide system has been associated with several psychiatric disorders. This review summarizes findings from neuroanatomical studies, with superior spatial resolution, that indicate that the distribution of CRF receptors is a highly dynamic process that, in addition to being sexually dimorphic, involves complex regulation of receptor trafficking within extrasynaptic sites that have significant consequences for adaptations to stress, particularly within the locus coeruleus (LC), the major brain norepinephrine-containing nucleus.

Direct targeting of peptidergic amygdalar neurons by noradrenergic afferents: linking stress-integrative circuitry

Amygdalar norepinephrine (NE) plays a key role in regulating neural responses to emotionally arousing stimuli and is involved in memory consolidation of emotionally charged events. Corticotropin-releasing factor (CRF) and dynorphin (DYN), two neuropeptides that mediate the physiological and behavioral responses to stress, are abundant in the central nucleus of the amygdala (CeA), and directly innervate brainstem noradrenergic locus coeruleus (LC) neurons. Whether the CRF- and DYN-containing amygdalar neurons receive direct noradrenergic innervation has not yet been elucidated. The present study sought to define cellular substrates underlying noradrenergic modulation of CRF- and DYN-containing neurons in the CeA using immunohistochemistry and electron microscopy. Ultrastructural analysis revealed that NE-labeled axon terminals form synapses with CRF- and DYN-containing neurons in the CeA. Semi-quantitative analysis showed that approximately 31 % of NET-labeled axon terminals targeted CeA neurons that co-expressed DYN and CRF. As a major source of CRF innervation to the LC, it is also not known whether CRF-containing CeA neurons are directly targeted by noradrenergic afferents. To test this, retrograde tract tracing using FluoroGold from the LC was combined with immunocytochemical detection of CRF and NET in the CeA. Our results revealed a population of LC-projecting CRF-containing CeA neurons that are directly innervated by NE afferents. Analysis showed that approximately 34 % of NET-labeled axon terminals targeted LC-projecting CeA neurons that contain CRF. Taken together, these results indicate significant interactions between NE, CRF and DYN in this critical limbic region and reveal direct synaptic interactions of NE with amygdalar CRF that influence the LC-NE arousal system.

Coordinate regulation of noradrenergic and serotonergic brain regions by amygdalar neurons

Based on the importance of the locus coeruleus-norepinephrine (LC-NE) system and the dorsal raphe nucleus-serotonergic (DRN-5-HT) system in stress-related pathologies, additional understanding of brain regions coordinating their activity is of particular interest. One such candidate is the amygdalar complex, and specifically, the central nucleus (CeA), which has been implicated in emotional arousal and is known to send monosynaptic afferent projections to both these regions. Our present data using dual retrograde tract tracing is the first to demonstrate a population of amygdalar neurons that project in a collateralized manner to the LC and DRN, indicating that amygdalar neurons are positioned to coordinately regulate the LC and DRN, and links these brain regions by virtue of a common set of afferents. Further, we have also characterized the phenotype of a population of these collaterally projecting neurons from the amygdala as containing corticotropin releasing factor or dynorphin, two peptides heavily implicated in the stress response. Understanding the co-regulatory influences of this limbic region on 5HT and NE regions may help fill a gap in our knowledge regarding neural circuits impacting these systems and their adaptations in stress.

Ultrastructural analysis of rat ventrolateral periaqueductal gray projections to the A5 cell group

Stimulation of neurons in the ventrolateral periaqueductal gray (PAG) produces antinociception as well as cardiovascular depressor responses that are mediated in part by pontine noradrenergic neurons. A previous report using light microscopy has described a pathway from neurons in the ventrolateral PAG to noradrenergic neurons in the A5 cell group that may mediate these effects. The present study used anterograde tracing and electron microscopic analysis to provide more definitive evidence that neurons in the ventrolateral PAG form synapses with noradrenergic and non-catecholaminergic A5 neurons in Sasco Sprague-Dawley rats. Deposits of anterograde tracer, biotinylated dextran amine, into the rat ventrolateral PAG labeled a significant number of axons in the region of the rostral subdivision of the A5 cell group, and a relatively lower number in the caudal A5 cell group. Electron microscopic analysis of anterogradely-labeled terminals in both rostral (n=127) and caudal (n=70) regions of the A5 cell group indicated that approximately 10% of these form synapses with noradrenergic dendrites. In rostral sections, about 31% of these were symmetric synapses, 19% were asymmetric synapses, and 50% were membrane appositions without clear synaptic specializations. In caudal sections, about 22% were symmetric synapses, and the remaining 78% were appositions. In both rostral and caudal subdivisions of the A5, nearly 40% of the anterogradely-labeled terminals formed synapses with non-catecholaminergic dendrites, and about 45% formed axoaxonic synapses. These results provide direct evidence for a monosynaptic pathway from neurons in the ventrolateral PAG to noradrenergic and non-catecholaminergic neurons in the A5 cell group. Further studies should evaluate if this established monosynaptic pathway may contribute to the cardiovascular depressor effects or the analgesia produced by the activation of neurons in the ventrolateral PAG.

Stress-induced sensitization of cortical adrenergic receptors following a history of cannabinoid exposure

The cannabinoid receptor agonist, WIN 55,212-2, increases extracellular norepinephrine levels in the rat frontal cortex under basal conditions, likely via desensitization of inhibitory α2-adrenergic receptors located on norepinephrine terminals. Here, the effect of WIN 55,212-2 on stress-induced norepinephrine release was assessed in the medial prefrontal cortex (mPFC), in adult male Sprague-Dawley rats using in vivo microdialysis. Systemic administration of WIN 55,212-2 30 min prior to stressor exposure prevented stress-induced cortical norepinephrine release induced by a single exposure to swim when compared to vehicle. To further probe cortical cannabinoid-adrenergic interactions, postsynaptic α2-adrenergic receptor (AR)-mediated responses were assessed in mPFC pyramidal neurons using electrophysiological analysis in an in vitro cortical slice preparation. We confirm prior studies showing that clonidine increases cortical pyramidal cell excitability and that this was unaffected by exposure to acute stress. WIN 55,212-2, via bath application, blocked postsynaptic α2-AR mediated responses in cortical neurons irrespective of exposure to stress. Interestingly, stress exposure prevented the desensitization of α2-AR mediated responses produced by a history of cannabinoid exposure. Together, these data indicate the stress-dependent nature of cannabinoid interactions via both pre- and postsynaptic ARs. In summary, microdialysis data indicate that cannabinoids restrain stress-induced cortical NE efflux. Electrophysiology data indicate that cannabinoids also restrain cortical cell excitability under basal conditions; however, stress interferes with these CB1-α2 AR interactions, potentially contributing to over-activation of pyramidal neurons in mPFC. Overall, cannabinoids are protective of the NE system and cortical excitability but stress can derail this protective effect, potentially contributing to stress-related psychopathology. These data add to the growing evidence of complex, stress-dependent modulation of monoaminergic systems by cannabinoids and support the potential use of cannabinoids in the treatment of stress-induced noradrenergic dysfunction.

Opiate agonist-induced re-distribution of Wntless, a mu-opioid receptor interacting protein, in rat striatal neurons

Wntless (WLS), a mu-opioid receptor (MOR) interacting protein, mediates Wnt protein secretion that is critical for neuronal development. We investigated whether MOR agonists induce re-distribution of WLS within rat striatal neurons. Adult male rats received either saline, morphine or [d-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO) directly into the lateral ventricles. Following thirty minutes, brains were extracted and tissue sections were processed for immunogold silver detection of WLS. In saline-treated rats, WLS was distributed along the plasma membrane and within the cytoplasmic compartment of striatal dendrites as previously described. The ratio of cytoplasmic to total dendritic WLS labeling was 0.70±0.03 in saline-treated striatal tissue. Morphine treatment decreased this ratio to 0.48±0.03 indicating a shift of WLS from the intracellular compartment to the plasma membrane. However, following DAMGO treatment, the ratio was 0.85±0.05 indicating a greater distribution of WLS intracellularly. The difference in the re-distribution of the WLS following different agonist exposure may be related to DAMGO's well known ability to induce internalization of MOR in contrast to morphine, which is less effective in producing receptor internalization. Furthermore, these data are consistent with our hypothesis that MOR agonists promote dimerization of WLS and MOR, thereby preventing WLS from mediating Wnt secretion. In summary, our findings indicate differential agonist-induced trafficking of WLS in striatal neurons following distinct agonist exposure. Adaptations in WLS trafficking may represent a novel pharmacological target in the treatment of opiate addiction and/or pain.

Amygdalar peptidergic circuits regulating noradrenergic locus coeruleus neurons: linking limbic and arousal centers

The endogenous opioid peptides, met- or leu-enkephalin, and corticotropin-releasing factor (CRF) regulate noradrenergic neurons in the locus coeruleus (LC) in a convergent manner via projections from distinct brain areas. In contrast, the opioid peptide dynorphin (DYN) has been shown to serve as a co-transmitter with CRF in afferents to the LC. To further define anatomical substrates targeting noradrenergic neurons by DYN afferents originating from limbic sources, anterograde tract-tracing of biotinylated dextran amine (BDA) from the central amygdaloid complex was combined with immunocytochemical detection of DYN and tyrosine hydroxylase (TH) in the same section of tissue. Triple labeling immunocytochemistry was combined with electron microscopy in the LC where BDA was identified using an immunoperoxidase marker, and DYN and TH were distinguished by the use of sequential immunogold labeling and silver enhancement to produce different sized gold particles. Results show direct evidence of a monosynaptic pathway linking amygdalar DYN afferents with LC neurons. To determine whether DYN-containing amygdalar LC-projecting neurons colocalize CRF, retrograde tract-tracing using fluorescent latex microspheres injected into the LC was combined with immunocytochemical detection of DYN and CRF in single sections in the central amygdala. Retrogradely labeled neurons from the LC were distributed throughout the rostro-caudal extent of the central nucleus of the amygdala (CeA) as previously described. Cell counts showed that approximately 42% of LC-projecting neurons in the CeA contained both DYN and CRF. Taken with our previous studies showing monosynaptic projections from amygdalar CRF neurons to noradrenergic LC cells, the present study extends this by showing that DYN and CRF are co-transmitters in monosynaptic projections to the LC and are poised to coordinately impact LC neuronal activity.

Intracisternal rSV40 administration provides effective pan-CNS transgene expression

Potential genetic treatments for many generalized central nervous system (CNS) diseases require transgene expression throughout the CNS. Using oxidant stress and apoptosis caused by HIV-1 envelope gp120 as a model, we studied pan-CNS neuroprotective gene delivery into the cisterna magna (CM). Recombinant SV40 vectors carrying Cu/Zn superoxide dismutase or glutathione peroxidase were injected into rat CMs following intraperitoneal administration of mannitol. Sustained transgene expression was seen in neurons throughout the CNS. On challenge, 8 weeks later with gp120 injected into the caudate putamen, significant neuroprotection was documented. Thus, intracisternal administration of antioxidant-carrying rSV40 vectors may be useful in treating widespread CNS diseases such as HIV-1-associated neurocognitive disorders characterized by oxidative stress.

Agonist-induced internalization of κ-opioid receptors in noradrenergic neurons of the rat locus coeruleus

Kappa-opioid receptors (κOR) are positioned to modulate pre- and post-synaptic responses of norepinephrine-containing neurons in the rat locus coeruleus (LC). The ability of an acute systemic injection of a long acting κOR agonist, U50,488, to induce trafficking of κOR was assessed in the LC using immunogold-silver detection in male Sprague-Dawley rats. U50,488 administration shifted immunogold-silver labeling indicative of κOR from primarily plasmalemmal sites to intracellular sites when compared to vehicle-treated subjects. This translocation from the plasma membrane to the cytoplasmic compartment was prevented by pre-treatment with the κOR antagonist, norbinaltorphimine (norBNI). To determine whether agonist stimulation could induce adaptations in the expression of the noradrenergic synthesizing enzyme, dopamine beta hydroxylase (DβH), and κOR expression, Western blot analysis was used to compare expression levels of DβH and κOR following U50,488 administration. Expression levels for DβH and κOR were significantly increased following U50,488 administration when compared to controls. These data indicate that a systemic injection of a κOR agonist stimulates internalization of κORs in noradrenergic neurons and can impact κOR and DβH expression levels in this stress-sensitive brain region.

The locus coeruleus: A key nucleus where stress and opioids intersect to mediate vulnerability to opiate abuse

The interaction between the stress axis and endogenous opioid systems has gained substantial clinical attention as it is increasingly recognized that stress predisposes to opiate abuse. For example, stress has been implicated as a risk factor in vulnerability to the initiation and maintenance of opiate abuse and is thought to play an important role in relapse in subjects with a history of abuse. Numerous reports indicating that stress alters individual sensitivity to opiates suggest that prior stress can influence the pharmacodynamics of opiates that are used in clinical settings. Conversely, the effects of opiates on different components of the stress axis can impact on individual responsivity to stressors and potentially predispose individuals to stress-related psychiatric disorders. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC), the major brain norepinephrine (NE)-containing nucleus. This review summarizes our current knowledge regarding the anatomical and neurochemical afferent regulation of the LC. It then presents physiological studies demonstrating opposing interactions between opioids and stress-related neuropeptides in the LC and summarizes results showing that chronic morphine exposure sensitizes the LC-NE system to corticotropin releasing factor and stress. Finally, new evidence for novel presynaptic actions of kappa-opioids on LC afferents is provided that adds another dimension to our model of how this central NE system is co-regulated by opioids and stress-related peptides.

Cannabinoid modulation of cortical adrenergic receptors and transporters

We previously reported that administration of the synthetic cannabinoid agonist WIN 55,212-2 causes an increase in norepinephrine (NE) efflux in the frontal cortex (FC). The present study examined the expression levels of alpha2- and beta1-adrenergic receptors (ARs) as well as the norepinephrine transporter (NET) in the FC of rats following exposure to WIN 55,212-2. Rats received systemic injection of WIN 55,212-2 (3 mg/kg) acutely or for 7 days. Another group of rats received repeated WIN 55,212-2 treatment followed by a period of abstinence. Control rats received vehicle injections. Rats were euthanized 30 min after the last WIN 55,212-2 injection, the FC was microdissected, and protein extracts were probed for alpha2-AR, beta1-AR, and NET. Results showed that beta1-AR expression was significantly decreased following repeated WIN 55,212-2 treatment but significantly increased following a period of abstinence. alpha2-AR expression showed no significant change in all groups examined. NET expression was significantly decreased following acute WIN 55,212-2 treatment, with no changes following chronic administration or a period of abstinence. Alterations in NET may arise from modulation of cannabinoid receptors (CB1) that are localized to noradrenergic axon terminals as we demonstrate colocalization of CB1 receptor and NET in the same cortical axonal processes. The present findings support significant alterations in adrenergic receptor and NET expression in the FC after WIN 55,212 exposure that may underlie the reported changes in attention, cognition, and anxiety commonly observed after cannabinoid exposure.

Dynorphin and stress-related peptides in rat locus coeruleus: contribution of amygdalar efferents

The interaction between the stress axis and endogenous opioid systems has gained substantial attention, because it is increasingly recognized that stress alters individual sensitivity to opiates. One site at which opiates and stress substrates may interact to have global effects on behavior is within the locus coeruleus (LC). We have previously described interactions of several opioid peptides [e.g., proopiomelanocortin, enkephalin (ENK)] with the stress-related peptide corticotropin-releasing factor (CRF) in the LC. To examine further the interactions among dynorphin (DYN), ENK, and CRF in the LC, sections were processed for detection of DYN and CRF or DYN and ENK in rat brain. DYN- and CRF-containing axon terminals overlapped noradrenergic dendrites in this region. Dual immunoelectron microscopy showed coexistence of DYN and CRF; 35% of axon terminals containing DYN were also immunoreactive for CRF. In contrast, few axon terminals contained both DYN and ENK. A potential DYN/CRF afferent is the central nucleus of the amygdala (CeA). Dual in situ hybridization showed that, in CeA neurons, 31% of DYN mRNA-positive cells colocalized with CRF mRNA, whereas 53% of CRF mRNA-containing cells colocalized with DYN mRNA. Finally, to determine whether limbic DYN afferents target the LC, the CeA was electrolytically lesioned. Light-level densitometry of DYN labeling in the LC showed a significant decrease in immunoreactivity on the side of the lesion. Taken together, these data indicate that DYN- and CRF-labeled axon terminals, most likely arising from amygdalar sources, are positioned dually to affect LC function, whereas DYN and ENK function in parallel.

Local administration of a cannabinoid agonist alters norepinephrine efflux in the rat frontal cortex

Delta(9)-tetrahydrocannabinol, the main psychoactive ingredient in marijuana, activates specific cannabinoid (CB) receptors to exert complex actions on modulatory neurotransmitters involved in attention and cognition. Previous research has demonstrated that systemic administration of the synthetic cannabinoid agonist, WIN 55,212-2, increases norepinephrine efflux in the frontal cortex. The distribution of CB1 receptors on noradrenergic fibers in the frontal cortex suggests this may be one potential site for the regulation of norepinephrine release. In the present study, we first examined the ability of a CB1 antagonist, applied locally in the frontal cortex of adult male Sprague-Dawley rats, to block the actions of systemic WIN 55,212-2. Pretreatment with SR 141716A (300 microM) significantly attenuated the excitatory effects of WIN 55,212-2 (15 mg/kg, i.p.). Next, the impact of direct perfusion of WIN 55,212-2 into the frontal cortex on extracellular norepinephrine efflux was measured. Direct application of WIN 55,212-2 (100 microM) into the frontal cortex elicited a significant increase in extracellular norepinephrine efflux suggesting that activation of cortical cannabinoid receptors contributes to alterations in norepinephrine levels in this brain region. Finally, local administration of SR 141716A followed by local administration of WIN 55,212-2 revealed a paradoxical inhibition of norepinephrine efflux.

Protecting neurons from HIV-1 gp120-induced oxidant stress using both localized intracerebral and generalized intraventricular administration of antioxidant enzymes delivered by SV40-derived vectors

Human immunodeficiency virus-1 (HIV-1) is the most frequent cause of dementia in adults under 40. We sought to use gene delivery to protect from HIV-1-related neuron loss. Because HIV-1 envelope (Env) gp120 elicits oxidant stress and apoptosis in cultured neurons, we established reproducible parameters of Env-mediated neurotoxicity in vivo, then tested neuroprotection using gene delivery of antioxidant enzymes. We injected 100-500 ng mul(-1)gp120 stereotaxically into rat caudate-putamens (CP) and assayed brains for apoptosis by terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) 6-h to 14-day post-injection. Peak apoptosis occurred 1 day after injection of 250 and 500 ng microl(-1)gp120. TUNEL-positive cells mostly expressed neuronal markers (NeuroTrace), although some expressed CD68 and so were most likely microglial cells. Finally, we compared neuroprotection from gp120-induced apoptosis provided by localized and generalized intra-central nervous system (CNS) gene delivery. Recombinant SV40 vectors carrying Cu/Zn superoxide dismutase (SOD1) or glutathione peroxidase (GPx1) were injected into the CP, where gp120 was administered 4-24 weeks later. Alternatively, we inoculated the vector into the lateral ventricle (LV), with or without prior intraperitoneal (i.p.) administration of mannitol. Intracerebral injection of SV(SOD1) or SV(GPx1) significantly protected neurons from gp120-induced apoptosis throughout the 24-week study. Intraventricular vector administration protected from gp120 neurotoxicity comparably, particularly if preceded by mannitol i.p. Thus, HIV-1 gp120 is neurotoxic in vivo, and intracerebral or intra-ventricular administration of rSV40 vectors carrying antioxidant enzymes is neuroprotective. These findings suggest the potential utility of both localized and widespread gene delivery in treating neuroAIDS and other CNS diseases characterized by excessive oxidative stress.

Betaxolol, a selective beta(1)-adrenergic receptor antagonist, diminishes anxiety-like behavior during early withdrawal from chronic cocaine administration in rats

BACKGROUND

Anxiety has been indicated as one of the main symptoms of the cocaine withdrawal syndrome in human addicts and severe anxiety during withdrawal may potentially contribute to relapse. As alterations in noradrenergic transmission in limbic areas underlie withdrawal symptomatology for many drugs of abuse, the present study sought to determine the effect of cocaine withdrawal on beta-adrenergic receptor (beta(1) and beta(2)) expression in the amygdala.

METHODS

Male Sprague Dawley rats were administered intraperitoneal (i.p.) injections of cocaine (20 mg/kg) once daily for 14 days. Two days following the last cocaine injection, amygdala brain regions were micro-dissected and processed for Western blot analysis. Results showed that beta(1)-adrenergic receptor, but not beta(2)-adrenergic receptor expression was significantly increased in amygdala extracts of cocaine-withdrawn animals as compared to controls. This finding motivated further studies aimed at determining whether treatment with betaxolol, a highly selective beta(1)-adrenergic receptor antagonist, could ameliorate cocaine withdrawal-induced anxiety. In these studies, betaxolol (5 mg/kg via i.p. injection) was administered at 24 and then 44 h following the final chronic cocaine administration. Anxiety-like behavior was evaluated using the elevated plus maze test approximately 2 h following the last betaxolol injection. Following behavioral testing, betaxolol effects on beta(1)-adrenergic receptor protein expression were examined by Western blotting in amygdala extracts from rats undergoing cocaine withdrawal.

RESULTS

Animals treated with betaxolol during cocaine withdrawal exhibited a significant attenuation of anxiety-like behavior characterized by increased time spent in the open arms and increased entries into the open arms compared to animals treated with only saline during cocaine withdrawal. In contrast, betaxolol did not produce anxiolytic-like effects in control animals treated chronically with saline. Furthermore, treatment with betaxolol during early cocaine withdrawal significantly decreased beta(1)-adrenergic receptor protein expression in the amygdala to levels comparable to those of control animals.

CONCLUSIONS

The present findings suggest that the anxiolytic-like effect of betaxolol on cocaine-induced anxiety may be related to its effect on amygdalar beta(1)-adrenergic receptors that are up-regulated during early phases of drug withdrawal. These data support the efficacy of betaxolol as a potential effective pharmacotherapy in treating cocaine withdrawal-induced anxiety during early phases of abstinence.

Repeated cannabinoid administration increases indices of noradrenergic activity in rats

The present study examined the impact of repeated administration of a synthetic cannabinoid agonist, WIN 55,212-2 on the coeruleo-cortical pathway, a circuit implicated in anxiety. Male Sprague-Dawley rats received repeated systemic injections of WIN 55,212-2 (3.0 mg/kg). A separate group of rats received repeated WIN 55,212-2 injections followed by a period of abstinence. Control animals received vehicle injections. Ninety minutes following the last injection on day 8, anxiety-related behavior was assessed using the elevated plus maze. The abstinent group was tested after another 8 days. Following behavioral testing, brain tissue was extracted from the locus coeruleus (LC) and probed for tyrosine hydroxylase (TH) expression. In a separate group of animals, in vivo microdialysis was used to monitor extracellular norepinephrine efflux in the frontal cortex following repeated WIN 55,212-2 administration and following a period of abstinence. Repeated administration of WIN 55,212-2 evoked an anxiogenic-like response that was accompanied by an increase in TH protein expression in the LC. A similar neurochemical profile was observed using in vivo microdialysis where an augmented increase in cortical norepinephrine efflux was identified in response to a systemic injection of WIN 55,212-2 on day 8. Anxiety-like behavior, catecholamine synthesizing enzyme levels and NE efflux returned to control values after 8 days of abstinence. The present findings indicate that repeated administration of a synthetic cannabinoid receptor agonist induces transient anxiety-like behaviors that correlate with increases in catecholamine synthesizing enzyme expression in the LC and augmented norepinephrine efflux in response to a challenge injection of WIN 55,212-2.

Antioxidant enzyme gene delivery to protect from HIV-1 gp120-induced neuronal apoptosis

Human immunodeficiency virus-1 (HIV-1) infection in the central nervous system (CNS) may lead to neuronal loss and progressively deteriorating CNS function: HIV-1 gene products, especially gp120, induce free radical-mediated apoptosis. Reactive oxygen species (ROS), are among the potential mediators of these effects. Neurons readily form ROS after gp120 exposure, and so might be protected from ROS-mediated injury by antioxidant enzymes such as Cu/Zn-superoxide dismutase (SOD1) and/or glutathione peroxidase (GPx1). Both enzymes detoxify oxygen free radicals. As they are highly efficient gene delivery vehicles for neurons, recombinant SV40-derived vectors were used for these studies. Cultured mature neurons derived from NT2 cells and primary fetal neurons were transduced with rSV40 vectors carrying human SOD1 and/or GPx1 cDNAs, then exposed to gp120. Apoptosis was measured by terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) assay. Transduction efficiency of both neuron populations was >95%, as assayed by immunostaining. Transgene expression was also ascertained by Western blotting and direct assays of enzyme activity. Gp120 induced apoptosis in a high percentage of unprotected NT2-N. Transduction with SV(SOD1) and SV(GPx1) before gp120 challenge reduced neuronal apoptosis by >90%. Even greater protection was seen in cells treated with both vectors in sequence. Given singly or in combination, they protect neuronal cells from HIV-1-gp120 induced apoptosis. We tested whether rSV40 s can deliver antioxidant enzymes to the CNS in vivo: intracerebral injection of SV(SOD1) or SV(GPx1) into the caudate putamen of rat brain yielded excellent transgene expression in neurons. In vivo transduction using SV(SOD1) also protected neurons from subsequent gp120-induced apoptosis after injection of both into the caudate putamen of rat brain. Thus, SOD1 and GPx1 can be delivered by SV40 vectors in vitro or in vivo. This approach may merit consideration for therapies in HIV-1-induced encephalopathy.

Distribution and ultrastructural localization of GEC1 in the rat CNS

We have previously demonstrated that GEC1 interacts with the kappa opioid receptor and GEC1 expression enhances cell surface expression of the receptor in Chinese hamster ovary cells. In this study, we generated an antiserum (PA629) directed against GEC1 in rabbits, characterized its specificity, and investigated distribution of GEC1 in tissues and in brain regions and spinal cord and its subcellular localization in hypothalamic neurons in the rat. Immunofluorescence staining demonstrated that PA629 recognized HA-GEC1 transfected into Chinese hamster ovary cells, but not HA-GABARAP or HA-GATE-16, although the three share high homology. Pre-incubation of PA629 with GST-GEC1, but not GST, abolished the staining. In immunoblotting, affinity-purified PA629 (PA629p) recognized GEC1, GABARAP and GATE-16. GEC1 migrated slower than GABARAP and GATE-16, with a M(r) of 16 kDa for GEC1 and M(r) of 14 kDa for GABARAP and GATE-16. Immunoblotting results showed that GEC1 level was higher in liver and brain than in lung and heart, and very low in kidney and skeletal muscle. GEC1 was present in all rat brain regions examined and spinal cord. Immunohistochemistry demonstrated that GEC1 immunoreactivity was distributed ubiquitously in the rat CNS with highly intense immunoreactivity in various brain nuclei and motor neurons of the spinal cord. Ultrastructural examination of neurons in the paraventricular nucleus of the hypothalamus showed that GEC1 was associated with endoplasmic reticulum and Golgi apparatus and distributed along plasma membranes and in cytosol. Coupled with our previous observation that GEC1 interacts with N-ethylmaleimide-sensitive factor, these findings strongly suggest that GEC1 functions in intracellular trafficking in the biosynthesis pathway and perhaps also the endocytic pathway. The widespread distribution of GEC1 suggests that GEC1 may be associated with many proteins, in addition to the kappa opioid receptor.

Elk-1 associates with the mitochondrial permeability transition pore complex in neurons

The nuclear transcription factor E-26-like protein 1 (Elk-1) is thought to impact neuronal differentiation [Sharrocks, A. D. (2001) Nat. Rev. Mol. Cell Biol. 2, 827-837], cell proliferation [Sharrocks, A. D. (2002) Biochem. Soc. Trans. 30, 1-9], tumorigenesis [Chai, Y. L., Chipitsyna, G., Cui, J., Liao, B., Liu, S., Aysola, K., Yezdani, M., Reddy, E. S. P. & Rao, V. N. (2001) Oncogene 20, 1357-1367], and apoptosis [Shao, N., Chai, Y., Cui, J., Wang, N., Aysola, K., Reddy, E. S. P. & Rao, V. N. (1998) Oncogene 17, 527-532]. In addition to its nuclear localization, Elk-1 is found throughout the cytoplasm, including localization in neuronal dendrites [Sgambato, V., Vanhoutte, P., Pages, C., Rogard, M., Hipskind, R., Besson, M. J. & Caboche, J. (1998) J. Neurosci. 18, 214-226], raising the possibility that Elk-1 may have alternative extranuclear functions in neurons. Using coimmunoprecipitation and reciprocal coimmunoprecipitation from adult rat brain, we found an association between Elk-1 protein and the mitochondrial permeability transition pore complex (PTP), a structure involved in both apoptotic and necrotic cell death. Electron microscopy in adult rat brain sections confirmed this association with mitochondria. Elk-1 was also identified from purified mitochondrial fractions by using Western blotting, and Elk-1 increased its association with mitochondria following proapoptotic stimuli. Consistent with a role for Elk-1 in neuron viability, overexpression of Elk-1 in primary neurons decreased cell viability, whereas Elk-1 siRNA-mediated knockdown increased cell viability. This decrease in viability induced by Elk-1 overexpression was blocked with application of a PTP inhibitor. These results show an association of the nuclear transcription factor Elk-1 with the mitochondrial PTP and suggest an additional extranuclear function for Elk-1 in neurons.

Dopamine-D1 and delta-opioid receptors co-exist in rat striatal neurons

Cocaine's enhancement of dopaminergic neurotransmission in the mesolimbic pathway plays a critical role in the initial reinforcing properties of this drug. However, other neurotransmitter systems are also integral to the addiction process. A large body of data indicates that opioids and dopamine together mediate emotional and reinforced behaviors. In support of this, cocaine-mediated increases in activation of dopamine D1 receptors (D1R) results in a desensitization of delta-opioid receptor (DOR) signaling through adenylyl cyclase (AC) in striatal neurons. To further define cellular mechanisms underlying this effect, the subcellular distribution of DOR and D1R was examined in the rat dorsolateral striatum. Dual immunoperoxidase/gold-silver detection combined with electron microscopy was used to identify DOR and D1R immunoreactivities in the same section of tissue. Semi-quantitative analysis revealed that a subset of dendritic cellular profiles exhibited both DOR and D1R immunoreactivities. Of 198 randomly sampled D1R immunoreactive profiles, 43% contained DOR. Similarly of 165 DOR-labeled cellular profiles, 52% contained D1R. The present data provide ultrastructural evidence for co-existence between DOR and D1R in striatal neurons, suggesting a possible mechanism whereby D1R modulation may alter DOR function.

Elevated μ-opioid receptor expression in the nucleus of the solitary tract accompanies attenuated withdrawal signs after chronic low dose naltrexone in opiate-dependent rats

We previously described a decrease in withdrawal behaviors in opiate-dependent rats that were chronically treated with very low doses of naltrexone in their drinking water. Attenuated expression of withdrawal behaviors correlated with decreased c-Fos expression and intracellular signal transduction elements [protein kinase A regulatory subunit II (PKA) and phosphorylated cAMP response element binding protein (pCREB)] in brainstem noradrenergic nuclei. In this study, to determine whether similar cellular changes occurred in forebrain nuclei associated with drug reward, expressions of PKA and pCREB were analyzed in the ventral tegmental area, frontal cortex, striatum, and amygdala of opiate-treated rats that received low doses of naltrexone in their drinking water. No significant difference in PKA or pCREB was detected in these regions following drug treatment. To examine further the cellular mechanisms in noradrenergic nuclei that could underlie attenuated withdrawal behaviors following low dose naltrexone administration, the nucleus of the solitary tract (NTS) and locus coeruleus (LC) were examined for opioid receptor (OR) protein expression. Results showed a significant increase in muOR expression in the NTS of morphine-dependent rats that received low doses of naltrexone in their drinking water, and increases in muOR expression were also found to be dose dependent. Protein expression of muOR in the LC and deltaOR in either brain region remained unchanged. In conclusion, our previously reported decreases in c-Fos and PKA expression in the NTS following pretreatment with low doses of naltrexone may be partially explained by a greater inhibition of NTS neurons resulting from increased muOR expression in this region.

Systemic administration of WIN 55,212-2 increases norepinephrine release in the rat frontal cortex

Cannabinoid agonists modulate a variety of behavioral functions by activating cannabinoid receptors that are widely distributed throughout the central nervous system. In the present study, norepinephrine efflux was assessed in the frontal cortex of rats that received a systemic administration of the cannabinoid agonist, WIN 55,212-2. The synthetic cannabinoid agonist dose-dependently increased the release of norepinephrine in this brain region. Pretreatment with the cannabinoid receptor antagonist, SR 141716A, blocked the increase in norepinephrine release. To identify sites of cellular activation, immunocytochemical detection of c-Fos was combined with detection of the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), in the brainstem nucleus locus coeruleus (LC), a region that is the sole source of norepinephrine to the frontal cortex. Systemic administration of WIN 55,212-2 significantly increased the number of c-Fos immunoreactive cells within TH-containing neurons in the LC compared to vehicle-treated rats. Pretreatment with SR 141716A inhibited the WIN 55,212-2 induced c-Fos expression, while the antagonist alone did not affect c-Fos expression. Taken together, these data indicate that systemically administered cannabinoid agonists stimulate norepinephrine release in the frontal cortex by activating noradrenergic neurons in the coeruleo-frontal cortex pathway. These effects may partially underlie changes in attention, arousal and anxiety observed following exposure to cannabis-based drugs.

Ultrastructural evidence for co-localization of dopamine D2 and micro-opioid receptors in the rat dorsolateral striatum

Previous studies have shown significant changes in dopamine and opioid receptors in the basal ganglia following administration of cocaine. Cocaine administration results in a significant increase in the number of opioid receptors in dopamine-enriched brain regions. The aim of this study was to determine if dopamine D2 receptors (D2r) and micro-opioid receptors (microOr) are localized to the same neurons in the dorsolateral striatum. Immunoperoxidase and immunogold-silver labeling combined with electron microscopy was used to examine the ultrastructural localization of both receptors in the dorsolateral striatum. Approximately half of the microOr-labeled somatodendritic processes showed immunolabeling for the D2r. Similarly, about half of the D2r-labeled dendrites and cell bodies showed immunolabeling for the microOr. In conclusion, our results indicate that individual neurons in the rat dorsolateral striatum may be directly modulated by both dopaminergic and opioid ligands. These data also suggest that the molecular mechanism responsible for the up-regulation of microOrs in the caudate and putamen following cocaine exposure may depend, in part, on the co-existence of D2rs and micro-Ors in these cells.

Expression of connexins during development and following manipulation of afferent input in the rat locus coeruleus

Synchronous activity of locus coeruleus (LC) neurons during early postnatal development is regulated, in part, by electrotonic coupling. Connexin (Cx) proteins that make up gap junction channels are localized to both neurons and glia in the LC during this period. In adult rats, however, synchrony exists only under certain experimental conditions. The expression of Cx proteins was examined using western blot analysis at several developmental time points. Immunoblot analysis revealed little to no expression of Cx26 while Cx32, Cx43 and Cx36 were present at all time points examined. A progressive increase in Cx43 was identified from the first postnatal week through adulthood. Immunocytochemical detection of Cx36 and Cx43 in adult LC showed that Cx36 was associated with neuronal processes while Cx43 was localized to glia. In adult LC, in vitro intracellular recordings combined with neurobiotin injections confirmed the presence of gap junctional communication albeit to a lesser extent than in early postnatal periods. The degree to which synaptic inputs to LC neurons impact on Cx protein expression was also evaluated. Samples of the LC from rats that received an electrolytic lesion of the amygdala were processed for western blot analysis of Cx36 and Cx43. The predominantly neuronal Cx36 exhibited an increase in expression while the glial Cx43 was unchanged. The present results indicate that, despite subtype-specific changes during development, several Cx proteins are expressed in the adult LC. In addition, manipulating afferent input to the LC, in adult rats, results in increases in neuronal Cx protein levels but not in glial Cx levels suggesting that altering synaptic inputs to the LC may alter synchronous activity in noradrenergic neurons.

Cellular interactions between axon terminals containing endogenous opioid peptides or corticotropin-releasing factor in the rat locus coeruleus and surrounding dorsal pontine tegmentum

Recent evidence suggests that certain stressors release both endogenous opioids and corticotropin-releasing factor (CRF) to modulate activity of the locus coeruleus (LC)-norepinephrine (NE) system. In ultrastructural studies, axon terminals containing methionine(5)-enkephalin (ENK) or CRF have been shown to target LC dendrites. These findings suggested the hypothesis that both neuropeptides may coexist in common axon terminals that are positioned to have an impact on the LC. This possibility was examined by using immunofluorescence and immunoelectron microscopic analysis of the rat LC and neighboring dorsal pontine tegmentum. Ultrastructural analysis indicated that CRF- and ENK-containing axon terminals were abundant in similar portions of the neuropil and that approximately 16% of the axon terminals containing ENK were also immunoreactive for CRF. Dually labeled terminals were more frequently encountered in the "core" of the LC vs. its extranuclear dendritic zone, which included the medial parabrachial nucleus (mPB). Triple labeling for ENK, CRF, and tyrosine hydroxylase (TH) showed convergence of opioid and CRF axon terminals with noradrenergic dendrites as well as evidence for inputs to TH-labeled dendrites from dually labeled opioid/CRF axon terminals. One potential source of ENK and CRF in the dorsal pons is the central nucleus of the amygdala (CNA). To determine the relative contribution of ENK and CRF terminals from the CNA, the CNA was electrolytically lesioned. Light-level densitometry revealed robust decreases in CRF immunoreactivity in the LC and mPB on the side ipsilateral to the lesion but little or no change in ENK immunoreactivity, confirming previous studies of the mPB. Degenerating terminals from the CNA in lesioned rats were found to be in direct contact with TH-labeled dendrites. Together, these data indicate that ENK and CRF may be colocalized to a subset of individual axon terminals in the LC "core." The finding that the CNA provides, to dendrites in the area examined, a robust CRF innervation, but little or no opioid innervation, suggests that ENK and CRF axon terminals impacting LC neurons originate from distinct sources and that terminals that colocalize ENK and CRF are not from the CNA.

Opioid circuits originating from the nucleus paragigantocellularis and their potential role in opiate withdrawal

Neurons in the rat nucleus paragigantocellularis (PGi), located in the ventrolateral medulla, send collateral projections to the locus coeruleus (LC) and to the nucleus of the solitary tract (NTS). The present study examined whether neurons in the PGi that project to both the LC and NTS contain leucine(5)-enkephalin (ENK), and also whether opioid-containing neurons in the PGi are activated following withdrawal from opiates. Retrograde transport of Fluoro-Gold (FG) from the LC and transport of a protein-gold tracer from the NTS was combined with detection of an antibody directed against ENK in the PGi. Using fluorescence and brightfield microscopy, it was established that more than half of the neurons containing both FG and the protein-gold tracer, also exhibited immunolabeling for ENK. The most frequent location of triply labeled neurons was the retrofacial portion of the PGi. In a separate series, rats were chronically implanted with morphine or placebo pellets and, on the fifth day, were subjected to an intraperitoneal injection of naltrexone. Two hours following initiation of withdrawal, rat brains were obtained and processed for detection of c-fos and in situ hybridization labeling of preproenkephalin (PPE) mRNA. Naltrexone injections into morphine-dependent rats caused a dramatic increase in c-fos as compared to control rats. Approximately 66% of the c-fos-labeled neurons exhibited labeling for PPE mRNA. These were also enriched in the retrofacial portion of the PGi. Taken together, the present data indicate that withdrawal from opiates engages opioid neurons in the PGi, some of which may coordinate activity of neurons in both the NTS and the LC.

Internalization of mu-opioid receptors produced by etorphine in the rat locus coeruleus

Chronic administration of mu-opioid receptor agonists is known to produce adaptive changes within noradrenergic neurons of the locus coeruleus. Although mu-opioid receptors are densely expressed by locus coeruleus neurons, the effects of acute and chronic administration of agonists on the subcellular distribution of mu-opioid receptors remain poorly understood. Therefore, we examined the ultrastructural distribution of mu-opioid receptor immunoreactivity in the locus coeruleus of rats subjected to either acute morphine, or etorphine, or chronic morphine treatment. In the locus coeruleus of control rats receiving acute saline injections or placebo pellet implants, immunogold-silver labeling for mu-opioid receptors was localized to parasynaptic and extrasynaptic portions of the plasma membranes of perikarya and dendrites. Only 8% of the gold-silver particles analyzed were distributed within the cytoplasm of dendrites and perikarya in vehicle-treated rats. Immunolabeling for mu-opioid receptors was distributed along portions of the plasma membrane that were often apposed by astroglial sheaths. After acute injections of etorphine, there was a dramatic internalization of mu-opioid receptors to intracellular compartments. Quantitative analysis of gold-silver particles indicative of mu-opioid receptors showed that a substantial number of gold particles shifted from the plasma membrane to early endosomes in dendrites from etorphine-treated rats. In dendrites sampled from etorphine-treated rats, 85% of the gold-silver grains indicative of mu-opioid receptor labeling were located in intracellular compartments as compared to 15% that were distributed along the plasma membrane. In animals that received either acute morphine injections or chronic morphine via pellet implantation, no change in the subcellular distribution of immunogold particles indicative of mu-opioid receptors was detected when compared to matched control animals. These results provide the first ultrastructural evidence that mu-opioid receptors are internalized by agonists such as etorphine, but not the partial agonist morphine, in the locus coeruleus.

Corticotropin-releasing factor is preferentially colocalized with excitatory rather than inhibitory amino acids in axon terminals in the peri-locus coeruleus region

Corticotropin-releasing factor(CRF)-immunoreactive terminals form synaptic specializations with locus coeruleus (LC) dendrites in rat brain. Within these terminals, CRF-immunoreactive dense core vesicles are colocalized with non-labeled dense core vesicles and clear vesicles, implicating other neuromodulators in the actions of CRF on LC neurons. Excitatory (glutamate) and inhibitory (GABA) amino acid afferents to the LC, have been identified which regulate noradrenergic responses to sensory stimuli. This study was designed to determine whether these amino acid neurotransmitters are colocalized with CRF in terminals within the LC/peri-LC region in the rat. Sections through the LC region that were dually labeled using immunohistochemical techniques to visualize either CRF and glutamate or CRF and GABA were examined using electron microscopy. Numerous terminals that contained immunolabeling for both CRF and glutamate (e.g. 30% of 106 CRF-immunoreactive terminals and 13% of 232 glutamate-immunolabeled terminals) were observed in the peri-LC. Additionally, single labeled CRF and glutamate terminals were often apposed to one another or found to converge on common dendritic targets. In contrast, relatively few terminals exhibited immunolabeling for both GABA and CRF (5% of 317 CRF-immunoreactive terminals). However, evidence for a postsynaptic effect of CRF on GABA-containing profiles included synapses between CRF axon terminals and GABA-labeled dendrites (10% of 317 CRF-labeled terminals), as well as appositions between CRF- and GABA-labeled terminals. These results indicate that CRF is preferentially colocalized with glutamate in the rostrolateral LC region and may impact on glutamate neurotransmission in the LC via presynaptic or postsynaptic actions. They argue against colocalization of CRF with GABA, although CRF may modulate GABA release via postsynaptic effects in the peri-LC region.

Ultrastructural analysis of ventrolateral periaqueductal gray projections to the A7 catecholamine cell group

Stimulation of neurons in the ventrolateral periaqueductal gray produces antinociception that is mediated in part by pontine noradrenergic neurons. Previous light microscopic analysis provided suggestive evidence for a direct projection from neurons in the ventrolateral periaqueductal gray to noradrenergic neurons in the A7 cell group that innervate the spinal cord dorsal horn. Therefore, the present ultrastructural study used anterograde tracing combined with tyrosine hydroxylase immunoreactivity to provide definitive evidence that neurons in the ventrolateral periaqueductal gray form synapses with the somata and dendrites of noradrenergic neurons of the A7 cell group. Injections of the anterograde tracers biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin into the ventrolateral periaqueductal gray of Sasco Sprague-Dawley rats yielded a dense innervation in the region of the lateral pons containing the A7 cell group. Electron microscopic analysis of anterogradely labeled terminals (n=401) in the region of the A7 cell group indicated that approximately 10% of these formed plasmalemmal appositions to tyrosine hydroxylase-immunoreactive dendrites with no intervening astrocytic processes. About 23% of these were asymmetric synapses, 10% were symmetric synapses, and 67% did not exhibit clearly differentiated synaptic specializations. The majority of anterogradely labeled terminals (60%) formed plasmalemmal appositions with dendrites and somata that lacked detectable tyrosine hydroxylase immunoreactivity. About 35% of these were symmetric synapses, 9% were asymmetric synapses and 56% did not form synaptic specializations. Approximately 30% of all anterogradely labeled terminals displayed features characteristic of axo-axonic synapses.The present results provide direct ultrastructural evidence to support the hypothesis that the analgesia produced by stimulation of neurons in the ventrolateral periaqueductal gray is mediated, in part, by activation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group.

Evidence for regional heterogeneity in corticotropin-releasing factor interactions in the dorsal raphe nucleus

The dorsal raphe nucleus (DR) is innervated by fibers containing the stress-related neurohormone corticotropin-releasing factor (CRF), which alters DR neuronal activity and serotonin release in rats. This study examined the relative distribution of CRF-immunoreactive fibers in the rat DR by using light level densitometry. Additionally, CRF-immunoreactive processes within specific subregions of the DR were examined at the ultrastructural level by using electron microscopy. CRF-immunoreactive fibers were organized within the DR along a caudal-rostral gradient, such that proceeding rostrally, innervation shifted from dorsolateral to ventromedial. Numerous CRF-immunoreactive axon terminals containing dense-core vesicles were found in both the caudal dorsolateral region and the rostral ventromedial/interfascicular region. These formed synaptic specializations with unlabeled dendrites and frequently contacted nonlabeled axon terminals. Semiquantitative analysis revealed certain differences between the two regions with respect to the types of associations made by CRF-immunoreactive terminals. Associations with dendrites were more frequent in the dorsolateral vs. ventromedial region (65% of 171 terminals vs. 39% of 233 terminals, respectively), whereas associations with axon terminals were more frequent in the ventromedial/interfascicular vs. the dorsolateral region (72% of 233 terminals vs. 57% of 171 terminals, respectively). Additionally, synaptic specializations between CRF-immunoreactive terminals and dendrites were more frequently asymmetric in the dorsolateral region (60%) and symmetric (49%) in the ventromedial/interfascicular region. Regional differences in CRF terminal interactions in the DR could account for the reported heterogeneous effects of CRF on DR neuronal activity and forebrain serotonin release. Importantly, the present results provide anatomical substrates for regulation of the DR by endogenous CRF.

Topographic architecture of stress-related pathways targeting the noradrenergic locus coeruleus

Peripheral sympathetic nerves and brainstem noradrenergic neurons of the locus coeruleus (LC) respond in parallel to a variety of stress-related stimuli which results in norepinephrine release both peripherally and centrally. Elucidation of central pathways subserving modulation of LC neurons point to extranuclear noradrenergic dendrites of LC somata that extend into peri-coerulear areas as a major target of afferents that participate in behavioral and physiological responses to stress. Anterograde tract tracing combined with immunoelectron microscopic detection of the catecholamine synthesizing enzyme tyrosine hydroxylase (TH) has demonstrated that the nucleus of the solitary tract (NTS) and the ventrolateral aspect of the periaqueductal gray (PAG), regions that participate in coordinating autonomic and motor behavior in response to stress, preferentially target the rostral ventromedial aspect of the peri-LC. In contrast, limbic forebrain afferents including the central nucleus of the amygdala (CNA) and the bed nucleus of the stria terminalis (BNST), regions that coordinate emotional responses to external stressors, provide direct synaptic input to noradrenergic dendrites that extend into rostral dorsolateral peri-coerulear areas. Neurochemical identification of transmitter systems impinging on LC indicate that the CNA provides corticotropin-releasing factor (CRF), a peptide essential for integrated physiological responses to stress, to the dorsolateral LC. Endogenous opioid peptides that originate from medullary sources, however, target primarily the "core" of the LC. Our physiological data suggest that stress engages CRF and opioid afferents to the LC, which have opposing influences on this noradrenergic system. The balance between opioid and CRF influences acting in the LC may, in part, maintain the balance of active and passive coping behaviors in response to stress. Understanding the afferent and neurochemical organization of the LC may help elucidate adaptations in neural circuits associated with stress which impact on central noradrenergic function.

Neuroadaptive responses in brainstem noradrenergic nuclei following chronic morphine exposure

Opiate dependence and withdrawal involve neuroadaptive responses in the central nervous system. A host of studies have previously implicated the A6 noradrenergic neurons of the pontine nucleus locus coeruleus (LC) as an important mediator of somatic signs observed upon withdrawal from opiates. Recent studies, however, are showing that noradrenergic neurons of the LC may not be solely involved in mediating somatic signs of withdrawal. The A2 noradrenergic neurons of the nucleus of the solitary tract (nucleus tractus solitarius [NTS]) in the caudal brainstem may be another possible site. Neurons in the nucleus paragigantocellularis lateralis (PGi), located in the rostral ventral medulla, which are known to send collateral projections to both the LC and the NTS, may co-modulate both noradrenergic nuclei in a parallel fashion, which may represent an anatomical substrate underlying the behavioral expression of opiate withdrawal. The PGi provides glutamatergic and opioid innervation to LC neurons. Hyperactivity of LC during opiate withdrawal arises, in part, from increased glutamate transmission in this pathway. The authors have recently shown that the excitatory transmitter, glutamate, co-exists with the endogenous opioid peptide, enkephalin, in a subset of axon terminals in the LC. Decreases in endogenous opioids in afferents to LC and NTS, following chronic opiate administration, may be equally important in modulating noradrenergic neurons following chronic opiate exposure, by removing a neurochemical system that would inhibit noradrenergic neurons. A persistent decrease in opioid peptide release from afferents during withdrawal would result in glutamate acting on postsynaptic targets, in an unopposed fashion. A parallel effect in opioid projections from PGi to the NTS would potentially support similar actions in this noradrenergic nucleus. The authors' recent data show that opioid-containing neurons in the PGi project to the NTS, and that enkephalin levels are decreased in opioid afferents to the NTS. This review summarizes data that the authors have collected regarding opioid expression changes in brainstem circuits (PGi-LC and PGi-NTS), following chronic morphine treatment, which may represent a model for understanding of adaptations in endogenous opioid circuits during drug dependence and withdrawal.

Anatomical evidence for presynaptic modulation by the delta opioid receptor in the ventrolateral periaqueductal gray of the rat

The delta opioid receptor (DOR) modulates nociception and blood pressure in the periaqueductal gray (PAG). To examine the cellular basis for DOR effects, the ultrastructural distribution of DOR immunoreactivity was examined in the caudal ventrolateral PAG. DOR immunoreactivity was located predominantly in axon terminals that formed asymmetric (excitatory-type) synaptic contacts. However, rather then localized to the plasma membrane of synaptic boutons, immunolabeling for the DOR was intracellular, often associated with large dense-core vesicles. This finding suggests that dense-core vesicles may play a role in targeting the DOR, as vesicle fusion would shift the distribution of the DOR to the plasma membrane. To investigate the neural circuits in which DOR may function, dual-immunolabeling was used to determine the relationship of the DOR to an endogenous ligand, enkephalin, and to a potential target, GABAergic neurons. Approximately a third (38 of 127) of DOR containing axons had enkephalin immunoreactivity, indicating DOR may act in part as a presynaptic autoreceptor. Although single axon terminals containing immunoreactivity for both DOR and GABA were not detected, some DOR-immunolabeled axon terminals (26 of 86) contacted soma or dendrites containing GABA. These data suggest that the DOR may act in part as an autoreceptor to regulate synaptic input to GABAergic as well as non-GABAergic PAG neurons. Furthermore, the exposure of the DOR to the extracellular space may be contingent upon dense-core vesicle fusion with the plasma membrane.

Decreases in endogenous opioid peptides in the rat medullo-coerulear pathway after chronic morphine treatment

Several biochemical changes have been described in noradrenergic neurons of the locus coeruleus (LC) after chronic morphine treatment. Changes in neurochemical expression in opioid afferent projections to the LC may be equally important in modulating noradrenergic neurons during chronic opiate exposure. To test the hypothesis that opioid peptides in LC afferents are altered after chronic opiate administration, we exposed adult male rats to either morphine or placebo pellets for 5 d. Tissue sections through the LC were processed for peroxidase or gold-silver labeling of methionine(5)-enkephalin (met-ENK) and analyzed using light or electron microscopy, respectively. Light level densitometry and ultrastructural analysis showed that there was a significant decrease in immunolabeling for ENK in LC-afferent terminals of morphine-treated rats. Western immunoblot analysis confirmed that protein levels for both leucine(5)- and methionine(5)-ENK were significantly decreased in tissue samples containing the LC after chronic morphine treatment. To test whether decreases in ENK protein expression were mirrored by decreases in gene expression, Northern blot analysis of preproenkephalin (PPE) mRNA was conducted in tissue samples obtained through the medulla, a brainstem area that contains the major opioid afferents to the LC. PPE mRNA was reduced in samples obtained from morphine-treated rats. Finally, in situ hybridization experiments confirmed significant decreases in PPE mRNA expression in the nucleus paragigantocellularis, a region known to provide a robust opioid input to the LC. These data suggest that there is a decrease in the synthesis of the opioid peptide mRNA and protein in the medullo-coerulear pathway after chronic exposure to morphine. Such alterations in opioid peptide levels during opiate dependence may contribute to the observed hyperactivity of LC neurons during opiate withdrawal.

Periaqueductal gray neurons monosynaptically innervate extranuclear noradrenergic dendrites in the rat pericoerulear region

Previous reports using light microscopy have provided anatomical evidence that neurons in the ventrolateral periaqueductal gray (PAG) innervate the medial pericoerulear dendrites of noradrenergic neurons in the nucleus locus coeruleus (LC). The present study used anterograde tracing and electron microscopic analysis to provide more definitive evidence that neurons in the ventrolateral PAG form synapses with the somata or dendrites of noradrenergic LC neurons. Deposits of either biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin into the rat ventrolateral PAG labeled a moderate to high number of axons in the region of the medial pericoerulear region and Barrington's nucleus, but a relatively low number were labeled in the nuclear core of the LC. Ultrastructural analysis of anterogradely labeled terminals at the levels of the rostral (n = 233) and caudal (n = 272) subdivisions of the LC indicated that approximately 20% of these form synapses with tyrosine hydroxylase-immunoreactive dendrites; most of these were located in the medial pericoerulear region. In rostral sections, about 12% of these were symmetric synapses, 9% were asymmetric synapses, and 79% were membrane appositions without clear synaptic specializations. In caudal sections, about 30% were symmetric synapses, 11% were asymmetric synapses, and 59% were appositions. In both rostral and caudal sections, 60% of the anterogradely labeled terminals formed synapses with noncatecholamine dendrites, and 20% formed axoaxonic synapses. These results provide direct evidence for monosynaptic projections from neurons in the ventrolateral PAG to the extranuclear dendrites of noradrenergic LC neurons. This monosynaptic pathway may mediate in part the analgesia, reduced responsiveness to external stimuli, and decreased excitability of somatic motoneurons produced by stimulation of neurons in the ventrolateral PAG.

Functional coupling between neurons and glia

Neuronal-glial interactions play an important role in information processing in the CNS. Previous studies have indicated that electrotonic coupling between locus ceruleus (LC) neurons is involved in synchronizing the spontaneous activity. The results of the present study extend the functional electrotonic coupling to interactions between neurons and glia. Spontaneous oscillations in the membrane potential were observed in a subset of glia. These oscillations were synchronous with the firing of neurons, insensitive to transmitter receptor antagonists and disrupted by carbenoxolone, a gap junction blocker. Hyperpolarization of neurons with [Met] (5)enkephalin blocked the oscillations in glia. Selective depolarization of glia with a glutamate transporter substrate (l-alpha-aminoadipic acid) increased the neuronal firing rate, suggesting that changes in the membrane potential of glia can modulate neuronal excitability through heterocellular coupling. Dye-coupling experiments further confirmed that small molecules could be transferred through gap junctions between these distinct cell types. No dye transfer was observed between neurons and oligodendrocytes or between astrocytes and oligodendrocytes, suggesting that the junctional communication was specific for astrocytes and neurons. Finally, immunoelectron microscopy studies established that connexins, the proteins that form gap junctions, were present on portions of the plasmalemma, bridging the cytoplasm of neurons and glia in LC. This heterocellular coupling extends the mechanisms by which glia participate in the network properties of the LC in which the degree of coupling is thought to influence cognitive performance.

Multiple substrates for serotonergic modulation of rat locus coeruleus neurons and relationships with kainate receptors

The ultrastructural substrates of serotonin (5-hydroxytryptamine [5-HT]) immunoreactive terminals with respect to noradrenergic neurons of the locus coeruleus (LC) have only been suggested from immunocytochemical analysis in adjacent tissue sections using antisera directed against tryptophan and tyrosine hydroxylase (TH) enzymes. Here, we conducted dual immunoelectron microscopy in the same section of tissue using antisera directed against 5-HT and TH to determine cellular substrates for proposed interactions between these two transmitter systems. Axon terminals containing peroxidase labeling for 5-HT possessed small clear as well as large dense core vesicles. Of 176 5-HT-labeled axons and terminals, 19% contacted TH-labeled dendrites. When a synaptic specialization was detectable, it was more often of the asymmetric type. Electrophysiological studies have also shown that 5-HT selectively attenuates excitatory amino acid-induced activation of neurons in the LC. Thus, to further examine the cellular relationship between 5-HT-labeled axon terminals and excitatory amino acid receptors, we conducted immunogold-silver labeling of an antibody which recognized the three identified members of the kainate receptor (KAr) subunit class (GluR 5,6,7) and peroxidase localization of 5-HT. Similar proportions of 5-HT-labeled terminals (9%) were either apposed to KAr-labeled dendrites or exhibited KAr immunoreactivity. Twenty-four percent of the 5-HT axon terminals examined were apposed by glial processes that contained KAr. These data indicate that 5-HT axon terminals are in direct contact with LC neurons and also suggest pre- and postsynaptic sites for modulation of 5-HT terminals by excitatory amino acid ligands as well as indirect sites via glial processes.

Evidence for coexistence of enkephalin and glutamate in axon terminals and cellular sites for functional interactions of their receptors in the rat locus coeruleus

The authors previously showed that a subset of axon terminals in the locus coeruleus (LC) contains methionine5-enkephalin (ENK) and gamma-aminobutyric acid (GABA) immunoreactivities. However, numerous ENK-labeled terminals lacked GABA and exhibited synaptic specializations that were characteristic of excitatory-type transmitters. To determine whether ENK coexists with glutamate in the LC, preembedding immunoperoxidase detection of ENK or immunogold-silver was combined with postembedding identification of glutamate using a gold marker. Indeed, 28% of the ENK-labeled axon terminals examined (n = 250 axon terminals) also contained glutamate. To define further sites for functional interactions between opiate ligands and excitatory amino acid receptors, the ultrastructural localization of the mu-opioid receptor (MOR) was examined with respect to either the kainate receptor (KAR) or the R1 subunit of the N-methyl-D-aspartate (NR1)-type glutamate receptor in the LC. Gold-silver labeling for MOR and peroxidase labeling for either KAR or NR1 indicated that the MOR often was localized to the plasma membrane of dendrites that also exhibited immunolabeling for either glutamate receptor subtype. In contrast to the KAR, which was identified primarily in somata and dendrites, NR1 immunoreactivity also was found frequently in axon terminals as well as in glial processes. Glial processes containing NR1 occasionally exhibited immunolabeling for MOR and sometimes were directly apposed to MOR-containing dendrites in the LC. Furthermore, NR1-labeled receptors in axon terminals sometimes were presynaptic to MOR-labeled dendrites. The authors concluded that ENK and glutamate may be cotransmitters in LC afferents. Moreover, ligands at the KAR may modulate directly MOR-containing neurons in the LC, whereas actions at NR1 receptors may affect opioid-sensitive neurons through multiple cellular mechanisms, i.e., through presynaptic, postsynaptic, or glial actions.

A.E. Bennett Research Award. Anatomic basis for differential regulation of the rostrolateral peri-locus coeruleus region by limbic afferents

BACKGROUND

Neurochemical and electrophysiological studies indicate that the locus coeruleus (LC)-norepinephrine system is activated by physiological and external stressors. This activation is mediated in part by corticotropin-releasing factor (CRF), the hypothalamic neurohormone that initiates the endocrine response to stress. We have previously shown that the central nucleus of the amygdala (CNA) provides CRF afferents to noradrenergic processes in the peri-LC area that may serve to integrate emotional and cognitive responses to stress. The bed nucleus of the stria terminalis (BNST) shares many anatomical and neurochemical characteristics with the CNA, including a high density of CRF-immunoreactive cells and fibers; however, recent studies have suggested that the CNA and the BNST may differentially regulate responses to conditioned and unconditioned fear, respectively, suggesting divergent neuroanatomical circuits underlying these processes.

METHODS

In the present study, neuroanatomical substrates subserving regulation of the LC by the BNST were examined. Anterograde tract-tracing was combined with immunoelectron microscopy to test the hypotheses that BNST efferents target noradrenergic neurons of the LC and that these efferents exhibit immunolabeling for CRF.

RESULTS

Ultrastructural analysis of sections that were dually labeled for the anterograde tracer biotinylated dextran amine (BDA) injected into the BNST and tyrosine hydroxylase (TH)-immunoreactivity demonstrated that BDA-labeled axon terminals formed synaptic specializations (primarily inhibitory) with TH-labeled dendrites and dendrites that lacked TH immunoreactivity. In contrast to CNA efferents that exhibited substantial immunolabeling for CRF, far fewer BDA-labeled terminals from the BNST in the rostrolateral peri-LC contained CRF.

CONCLUSIONS

The present results indicate that the BNST may provide distinct neurochemical regulation of the peri-LC as compared to other limbic afferents such as the CNA. These data are interesting in light of behavioral studies showing that the CNA and BNST may be differentially involved in fear versus anxiety, respectively.

Localization of mu-opioid receptors to locus coeruleus-projecting neurons in the rostral medulla: morphological substrates and synaptic organization

The increase in discharge activity of locus coeruleus (LC) neurons following precipitated opiate withdrawal has been reported to be caused, in part, by excitatory amino acid release most likely originating from the nucleus paragigantocellularis lateralis (PGCl) in the rostral ventral medulla. Activation of glutamate-containing neurons in the PGCl may depend on changes in the occupancy of opioid receptive sites located on LC-projecting neurons which subsequently effect excitatory amino acid release in the LC during opiate withdrawal. To determine whether the mu-opioid receptor (MOR) is localized to plasmalemmal sites of LC-projecting neurons in the PGCl, we combined retrograde transport of the protein-gold tracer, wheat germ agglutinin-conjugated to inactive horseradish peroxidase (WGA-AU-apoHRP), from the LC with immunocytochemical detection of MOR in the same section of tissue throughout the rostral medulla. Light microscopic analysis indicated that neurons containing either the retrograde tracer or immunoperoxidase labeling for the MOR were numerous throughout the ventral medulla and that individual PGCl neurons contained both WGA-Au-apoHRP as well as MOR. By electron microscopy, WGA-Au-apoHRP was commonly identified in lysosomes within somata and large proximal dendrites. The somata contained either spherical or invaginated nuclei and were often surrounded by numerous myelinated axons. Gold deposits could also be identified in the cytoplasm of smaller dendritic processes in the PGCl, although these were not necessarily associated with lysosomes. The smaller dendritic processes were often the target of afferent input by axon terminals containing heterogeneous types of synaptic vesicles. Of 150 cellular profiles exhibiting WGA-Au-apoHRP retrograde labeling, 31% contained immunoperoxidase labeling for MOR. These results indicate that the MOR is distributed along plasmalemmal sites of morphologically diverse neurons in the PGCl which project to the LC.

Efferent projections of the nucleus of the solitary tract to peri-locus coeruleus dendrites in rat brain: evidence for a monosynaptic pathway

Locus coeruleus (LC) neurons respond to autonomic influences, are activated by physiological stressors, and discharge in parallel with peripheral sympathetic nerves. The circuitry underlying modulation of LC activity by physiological manipulations (i.e., hemodynamic stress, hypovolumia) remains unclear. Specifically, monosynaptic projections from primary baroreceptor centers to the LC have been suggested by electrophysiological studies but have not been unequivocally established. Light microscopic anterograde tract-tracing studies have previously shown that neurons originating in the nucleus of the solitary tract (NTS) project to a region of the rostrodorsal pontine tegmentum, which contains noradrenergic dendrites of the LC; however, it is not known whether these NTS efferents specifically target LC dendrites. Therefore, we combined peroxidase labeling of biotinylated dextran amine (BDA) or Phaseolus vulgaris-leucoagglutinin (PHA-L) from the NTS with gold-silver labeling for tyrosine hydroxylase (TH) in the rostrolateral peri-LC region. Injections placed into neighboring nuclei (nucleus gracilis, hypoglossal nucleus) served as controls. Only injections centered in the NTS produced anterograde labeling in peri-LC regions containing TH processes. By electron microscopy, BDA- or PHA-L-labeled axon terminals originating from the NTS contained small, clear, and some large dense-core vesicles and formed heterogeneous synaptic contacts characteristic of both excitatory- and inhibitory-type transmitters. Approximately 19% of the BDA and PHA-L axon terminals examined originating from the commissural portion of the NTS formed synaptic specializations with dendrites exhibiting TH immunoreactivity in the peri-LC. These results demonstrate that neurons projecting from the cardiovascular-related portion of the NTS target noradrenergic dendrites, indicating that barosensitive NTS neurons may directly modulate the activity of LC neurons and may serve to integrate autonomic responses in brain by influencing the widespread noradrenergic projections of the LC. In addition, these findings demonstrate that extranuclear dendrites are an important termination site for afferents to the LC.

Amygdaloid corticotropin-releasing factor targets locus coeruleus dendrites: substrate for the co-ordination of emotional and cognitive limbs of the stress response

Corticotropin-releasing factor (CRF), the neurohormone that initiates the endocrine limb of the stress response via its actions on the anterior pituitary, also acts as a neurotransmitter in the noradrenergic locus coeruleus (LC) to activate this system during stress. Because the central nucleus of the amygdala contains numerous CRF-immunoreactive neurones, the present study examined whether CRF projections from the central nucleus of the amygdala target LC dendrites, thereby providing a mechanism for limbic-CRF modulation of brain noradrenergic activity. Retrograde tracers injected into the rostrolateral pericoerulear region, where CRF-immunoreactive fibres are dense, labelled numerous CRF-immunoreactive neurones in the central nucleus of the amygdala. Consistent with this, ultrastructural analysis of the rostrolateral pericoerulear region in sections that were dually labelled for an anterograde tracer (biotinylated dextran amine, BDA) injected into the central nucleus of the amygdala and CRF immunoreactivity revealed that a substantial percentage (35%) of amygdaloid axon terminals were CRF-immunoreactive. These terminals formed synaptic specializations with unlabelled dendrites that were more often of the asymmetric (excitatory) type. Additionally, ultrastructural analysis of sections that were dually labelled to visualize CRF-and tyrosine hydroxlase-immunoreactivity demonstrated synaptic specializations between CRF-immunoreactive terminals and LC dendrites in the rostrolateral peri-LC, which were also frequently asymmetric. Taken together with previous ultrastructural findings that LC dendrites in the rostrolateral pericoerulear region are targeted by anterogradely labelled terminals from the central nucleus of the amygdala, the present results implicate this nucleus as a source of CRF that can impact on LC activity via effects on dendrites in the rostrolateral pericoerulear region. This cellular substrate for amygdaloid-CRF modulation of brain noradrenergic activity may serve as a mechanism for the integration of emotional and cognitive responses to stress.