Mu-opioid-mediated inhibition of glutamate synaptic transmission in rat central amygdala neurons

Calcium imaging of central amygdala activity after escalation of fentanyl self-administration

The central amygdala (CeA) is involved in opioid relapse-associated behaviors. This study determined if escalation of fentanyl intake as modeled by long-access (LgA) self-administration (SA) alters ex vivo neuronal activity in CeA in response to fentanyl during acute withdrawal and protracted abstinence. Adult male and female Sprague-Dawley rats were trained to self-administer fentanyl or saline across 7 daily 1-h short access (ShA) sessions, followed by 21 6-h long access (LgA) sessions. Following acute (17 h) or protracted (30 days) withdrawal, withdrawal signs were assessed and rats were euthanized for CeA calcium imaging in brain slices. Fentanyl rats demonstrated reduced basal frequency of activity after 30 days withdrawal, but not after 17 h withdrawal. Regardless of SA group, acute fentanyl application in slices reduced activity (frequency, duration, active cell number) of CeA neurons. In acute withdrawal, the magnitude to which acute fentanyl suppressed CeA neuronal activity was smaller in fentanyl SA rats, relative to saline SA controls. However, the magnitude of acute fentanyl effect on suppression of CeA activity was greater in fentanyl SA rats (vs. saline SA controls) after protracted abstinence.

CCK-expressing neurons in the NTS are directly activated by CCK-sensitive C-type vagal afferents

Vagal sensory afferents carrying information from the gastrointestinal tract (GI) terminate in the nucleus of the solitary tract (NTS). Different subpopulations of NTS neurons then relay this information throughout the brain. Cholecystokinin (CCK) is a satiety peptide that activates vagal afferents in the GI. However, CCK is also expressed by neurons in the NTS, and activation of these neurons decreases food intake. What is less clear is how these NTS CCK neurons are activated by vagal afferents and what type of information they integrate about meal size and content. To address this, we identified NTS-CCK neurons by crossing CCK-IRES-Cre mice with floxed-Rosa-tdtomato mice and made a horizontal brain slice containing vagal afferents in the solitary tract (ST). Voltage clamp recordings of NTS-CCK neurons show that activation of the ST evokes excitatory postsynaptic currents (EPSCs) mediated by both α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptors. Analysis of these EPSCs revealed that 80% of NTS-CCK neurons receive direct, monosynaptic inputs, with many also receiving indirect, or polysynaptic, inputs. NTS-CCK neurons are sensitive to the transient receptor potential vanilloid type 1 agonist capsaicin, suggesting that they are downstream of C-fibers. In addition, both CCK and a 5 hydroxytryptamine 3 receptor (5-HT3R) agonist increased spontaneous EPSC (sEPSC) frequency in NTS-CCK neurons, with 69% of NTS-CCK neurons sensitive to CCK and 42% to the 5-HT3 receptor agonist, as well as 45% sensitive to both and 10% to neither. Taken together with previous studies, this suggests that NTS-CCK neurons are driven primarily by vagal afferents that are sensitive to CCK and are only weakly driven by those sensitive to serotonin.NEW & NOTEWORTHY Nucleus of the solitary tract (NTS) cholecystokinin (CCK) expressing neurons are directly activated by glutamate released from vagal afferents. They are downstream of primarily C-type CCK-sensitive afferents, with a small proportion also downstream of serotonin-sensitive afferents. These findings suggest that NTS-CCK neurons integrate signals from the gut about ingestion of fats and proteins as well as stretch of the stomach, which they then relay to other brain regions important for the control of food intake.

Presynaptic inhibition of excitatory synaptic transmission from the calcitonin gene-related peptide-containing parabrachial neurons to the central amygdala in mice - unexpected influence of systemic inflammation thereon

The monosynaptic connection from the lateral parabrachial nucleus (LPB) to the central amygdala (CeA) serves as a fundamental pathway for transmitting nociceptive signals to the brain. The LPB receives nociceptive information from the dorsal horn and spinal trigeminal nucleus and sends it to the "nociceptive" CeA, which modulates pain-associated emotions and nociceptive sensitivity. To elucidate the role of densely expressed mu-opioid receptors (MORs) within this pathway, we investigated the effects of exogenously applied opioids on LPB-CeA synaptic transmission, employing optogenetics in mice expressing channelrhodopsin-2 in LPB neurons with calcitonin gene-related peptide (CGRP). A MOR agonist ([D-Ala2,N-Me-Phe4,Glycinol5]-enkephalin, DAMGO) significantly reduced the amplitude of light-evoked excitatory postsynaptic currents (leEPSCs), in a manner negatively correlated with an increase in the paired-pulse ratio. An antagonist of MORs significantly attenuated these effects. Notably, this antagonist significantly increased leEPSC amplitude when applied alone, an effect further amplified in mice subjected to lipopolysaccharide injection 2 h before brain isolation, yet not observed at the 24-h mark. We conclude that opioids could shut off the ascending nociceptive signal at the LPB-CeA synapse through presynaptic mechanisms. Moreover, this gating process might be modulated by endogenous opioids, and the innate immune system influences this modulation.

Human OPRM1 and murine Oprm1 promoter driven viral constructs for genetic access to μ-opioidergic cell types

With concurrent global epidemics of chronic pain and opioid use disorders, there is a critical need to identify, target and manipulate specific cell populations expressing the mu-opioid receptor (MOR). However, available tools and transgenic models for gaining long-term genetic access to MOR+ neural cell types and circuits involved in modulating pain, analgesia and addiction across species are limited. To address this, we developed a catalog of MOR promoter (MORp) based constructs packaged into adeno-associated viral vectors that drive transgene expression in MOR+ cells. MORp constructs designed from promoter regions upstream of the mouse Oprm1 gene (mMORp) were validated for transduction efficiency and selectivity in endogenous MOR+ neurons in the brain, spinal cord, and periphery of mice, with additional studies revealing robust expression in rats, shrews, and human induced pluripotent stem cell (iPSC)-derived nociceptors. The use of mMORp for in vivo fiber photometry, behavioral chemogenetics, and intersectional genetic strategies is also demonstrated. Lastly, a human designed MORp (hMORp) efficiently transduced macaque cortical OPRM1+ cells. Together, our MORp toolkit provides researchers cell type specific genetic access to target and functionally manipulate mu-opioidergic neurons across a range of vertebrate species and translational models for pain, addiction, and neuropsychiatric disorders.

Pain-related cortico-limbic plasticity and opioid signaling

Neuroplasticity in cortico-limbic circuits has been implicated in pain persistence and pain modulation in clinical and preclinical studies. The amygdala has emerged as a key player in the emotional-affective dimension of pain and pain modulation. Reciprocal interactions with medial prefrontal cortical regions undergo changes in pain conditions. Other limbic and paralimbic regions have been implicated in pain modulation as well. The cortico-limbic system is rich in opioids and opioid receptors. Preclinical evidence for their pain modulatory effects in different regions of this highly interactive system, potentially opposing functions of different opioid receptors, and knowledge gaps will be described here. There is little information about cell type- and circuit-specific functions of opioid receptor subtypes related to pain processing and pain-related plasticity in the cortico-limbic system. The important role of anterior cingulate cortex (ACC) and amygdala in MOR-dependent analgesia is most well-established, and MOR actions in the mesolimbic system appear to be similar but remain to be determined in mPFC regions other than ACC. Evidence also suggests that KOR signaling generally serves opposing functions whereas DOR signaling in the ACC has similar, if not synergistic effects, to MOR. A unifying picture of pain-related neuronal mechanisms of opioid signaling in different elements of the cortico-limbic circuitry has yet to emerge. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Transcriptomics reveals amygdala neuron regulation by fasting and ghrelin thereby promoting feeding

The central amygdala (CeA) consists of numerous genetically defined inhibitory neurons that control defensive and appetitive behaviors including feeding. Transcriptomic signatures of cell types and their links to function remain poorly understood. Using single-nucleus RNA sequencing, we describe nine CeA cell clusters, of which four are mostly associated with appetitive and two with aversive behaviors. To analyze the activation mechanism of appetitive CeA neurons, we characterized serotonin receptor 2a (Htr2a)-expressing neurons (CeA^Htr2a) that comprise three appetitive clusters and were previously shown to promote feeding. In vivo calcium imaging revealed that CeA^Htr2a neurons are activated by fasting, the hormone ghrelin, and the presence of food. Moreover, these neurons are required for the orexigenic effects of ghrelin. Appetitive CeA neurons responsive to fasting and ghrelin project to the parabrachial nucleus (PBN) causing inhibition of target PBN neurons. These results illustrate how the transcriptomic diversification of CeA neurons relates to fasting and hormone-regulated feeding behavior.

Modulation of GABAA receptors and of GABAergic synapses by the natural alkaloid gelsemine

The Gelsemium elegans plant preparations have shown beneficial activity against common diseases, including chronic pain and anxiety. Nevertheless, their clinical uses are limited by their toxicity. Gelsemine, one of the most abundant alkaloids in the Gelsemium plants, have replicated these therapeutic and toxic actions in experimental behavioral models. However, the molecular targets underlying these biological effects remain unclear. The behavioral activity profile of gelsemine suggests the involvement of GABA_A receptors (GABA_ARs), which are the main biological targets of benzodiazepines (BDZs), a group of drugs with anxiolytic, hypnotic, and analgesic properties. Here, we aim to define the modulation of GABA_ARs by gelsemine, with a special focus on the subtypes involved in the BDZ actions. The gelsemine actions were determined by electrophysiological recordings of recombinant GABA_ARs expressed in HEK293 cells, and of native receptors in cortical neurons. Gelsemine inhibited the agonist-evoked currents of recombinant and native receptors. The functional inhibition was not associated with the BDZ binding site. We determined in addition that gelsemine diminished the frequency of GABAergic synaptic events, likely through a presynaptic modulation. Our findings establish gelsemine as a negative modulator of GABA_ARs and of GABAergic synaptic function. These pharmacological features discard direct anxiolytic or analgesic actions of gelsemine through GABA_ARs but support a role of GABA_ARs on the alkaloid induced toxicity. On the other hand, the presynaptic effects of the alkaloid provide an additional mechanism to explain their beneficial effects. Collectively, our results contribute novel information to improve understanding of gelsemine actions in the mammalian nervous system.

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Current research in pathophysiology of opioid-induced respiratory depression, neonatal opioid withdrawal syndrome, and neonatal antidepressant exposure syndrome

Respiratory depression (RD) is the primary cause of death due to opioids. Opioids bind to mu (µ)-opioid receptors (MORs) encoded by the MOR gene Oprm1, widely expressed in the central and peripheral nervous systems including centers that modulate breathing. Respiratory centers are located throughout the brainstem. Experiments with Oprm1-deleted knockout (KO) mice undertaken to determine which sites are necessary for the induction of opioid-induced respiratory depression (OIRD) showed that the pre-Bötzinger complex (preBötC) and the pontine Kölliker-Fuse nucleus (KF) contribute equally to OIRD but RD was not totally eliminated. Morphine showed a differential influence on preBötC and KF neurons - low doses attenuated RD following deletion of MORs from preBötC neurons and an increase in apneas after high doses whereas deletion of MORs from KF neurons but not the preBötC attenuated RD at both high and low doses. In other KO mice studies, morphine administration after deletion of Oprm1 from both the preBötC and the KF/PBN neurons, led to the conclusion that both respiratory centres contribute to OIRD but the preBötC predominates. MOR-mediated post-synaptic activation of GIRK potassium channels has been implicated as a cause of OIRD. A complementary mechanism in the preBötC involving KCNQ potassium channels independent of MOR signaling has been described. Recent experiments in rats showing that morphine depresses normal, but not gasping breathing, cast doubt on the belief that eupnea, sighs, and gasps, are under the control of preBötC neurons. Methadone, administered to alleviate symptoms of neonatal opioid withdrawal syndrome (NOWES), desensitized rats to OIRD. Protection lost between postnatal days 1 and 2 coincides with the preBötC becoming the dominant generator of respiratory rhythm. Neonatal antidepressant exposure syndrome (NADES) and serotonin toxicity (ST) show similarities including RD. Enzyme CYP2D6 involved in opioid detoxification is polymorphic. Individuals of different CYP2D6 genotype may show increased, decreased, or no enzyme activity, contributing to the variability of patient responses to different opioids and OIRD.

Neurodegeneration Within the Amygdala Is Differentially Induced by Opioid and HIV-1 Tat Exposure

Opioid use disorder (OUD) is a critical problem that contributes to the spread of HIV and may intrinsically worsen neuroHIV. Despite the advent of combined antiretroviral therapies (cART), about half of persons infected with HIV (PWH) experience cognitive and emotional deficits that can be exacerbated by opioid abuse. HIV-1 Tat is expressed in the central nervous system (CNS) of PWH on cART and is thought to contribute to neuroHIV. The amygdala regulates emotion and memories associated with fear and stress and is important in addiction behavior. Notwithstanding its importance in emotional saliency, the effects of HIV and opioids in the amygdala are underexplored. To assess Tat- and morphine-induced neuropathology within the amygdala, male Tat transgenic mice were exposed to Tat for 8 weeks and administered saline and/or escalating doses of morphine twice daily (s.c.) during the last 2 weeks of Tat exposure. Eight weeks of Tat exposure decreased the acoustic startle response and the dendritic spine density in the basolateral amygdala, but not the central nucleus of the amygdala. In contrast, repeated exposure to morphine alone, but not Tat, increased the acoustic startle response and whole amygdalar levels of amyloid-β (Aβ) monomers and oligomers and tau phosphorylation at Ser396, but not neurofilament light chain levels. Co-exposure to Tat and morphine decreased habituation and prepulse inhibition to the acoustic startle response and potentiated the morphine-induced increase in Aβ monomers. Together, our findings indicate that sustained Tat and morphine exposure differentially promote synaptodendritic degeneration within the amygdala and alter sensorimotor processing.

Engaging endogenous opioid circuits in pain affective processes

The pervasive use of opioid compounds for pain relief is rooted in their utility as one of the most effective therapeutic strategies for providing analgesia. While the detrimental side effects of these compounds have significantly contributed to the current opioid epidemic, opioids still provide millions of patients with reprieve from the relentless and agonizing experience of pain. The human experience of pain has long recognized the perceived unpleasantness entangled with a unique sensation that is immediate and identifiable from the first-person subjective vantage point as "painful." From this phenomenological perspective, how is it that opioids interfere with pain perception? Evidence from human lesion, neuroimaging, and preclinical functional neuroanatomy approaches is sculpting the view that opioids predominately alleviate the affective or inferential appraisal of nociceptive neural information. Thus, opioids weaken pain-associated unpleasantness rather than modulate perceived sensory qualities. Here, we discuss the historical theories of pain to demonstrate how modern neuroscience is revisiting these ideas to deconstruct the brain mechanisms driving the emergence of aversive pain perceptions. We further detail how targeting opioidergic signaling within affective or emotional brain circuits remains a strong avenue for developing targeted pharmacological and gene-therapy analgesic treatments that might reduce the dependence on current clinical opioid options.

Central regulation of body fluid homeostasis

Extracellular fluids, including blood, lymphatic fluid, and cerebrospinal fluid, are collectively called body fluids. The Na⁺ concentration ([Na⁺]) in body fluids is maintained at 135-145 mM and is broadly conserved among terrestrial animals. Homeostatic osmoregulation by Na⁺ is vital for life because severe hyper- or hypotonicity elicits irreversible organ damage and lethal neurological trauma. To achieve "body fluid homeostasis" or "Na homeostasis", the brain continuously monitors [Na⁺] in body fluids and controls water/salt intake and water/salt excretion by the kidneys. These physiological functions are primarily regulated based on information on [Na⁺] and relevant circulating hormones, such as angiotensin II, aldosterone, and vasopressin. In this review, we discuss sensing mechanisms for [Na⁺] and hormones in the brain that control water/salt intake behaviors, together with the responsible sensors (receptors) and relevant neural pathways. We also describe mechanisms in the brain by which [Na⁺] increases in body fluids activate the sympathetic neural activity leading to hypertension.

Inhibition of glutamatergic transmission and neuronal excitability by oxycodone in the rat hippocampal CA3 neurons

Oxycodone, a semisynthetic opioid analgesic with actions similar to morphine, is extensively prescribed for treatment of moderate to severe acute pain. Given that glutamate plays a crucial role in mediating pain transmission, the purpose of this study was to investigate the effect of oxycodone on glutamatergic synaptic transmission in rat hippocampal CA3 area, which is associated with the modulation of nociceptive perception. Whole-cell patch-clamp recordings revealed that oxycodone effectively reduced presynaptic glutamate release, as detected by decreased frequencies of spontaneous excitatory postsynaptic currents (sEPSCs) and miniature EPSCs (mEPSCs), without eliciting significant changes in the amplitudes of sEPSCs and mEPSCs and glutamate-evoked inward currents. The inhibitory effect of oxycodone on the frequency of sEPSCs was blocked by the nonselective opioid receptor antagonist naloxone. In addition, oxycodone suppressed burst firing induced by 4-aminopyridine and tonic repetitive firing evoked by the applied depolarizing current. These results suggest that oxycodone inhibits spontaneous presynaptic glutamate release possibly by activating opioid receptors and consequently suppressing the neuronal excitability of hippocampal CA3 neurons.

Mu opioid receptors on vGluT2-expressing glutamatergic neurons modulate opioid reward

The role of Mu opioid receptor (MOR)‐mediated regulation of GABA transmission in opioid reward is well established. Much less is known about MOR‐mediated regulation of glutamate transmission in the brain and how this relates to drug reward. We previously found that MORs inhibit glutamate transmission at synapses that express the Type 2 vesicular glutamate transporter (vGluT2). We created a transgenic mouse that lacks MORs in vGluT2‐expressing neurons (MORflox‐vGluT2cre) to demonstrate that MORs on the vGluT2 neurons themselves mediate this synaptic inhibition. We then explored the role of MORs in vGluT2‐expressing neurons in opioid‐related behaviors. In tests of conditioned place preference, MORflox‐vGluT2cre mice did not acquire place preference for a low dose of the opioid, oxycodone, but displayed conditioned place aversion at a higher dose, whereas control mice displayed preference for both doses. In an oral consumption assessment, these mice consumed less oxycodone and had reduced preference for oxycodone compared with controls. MORflox‐vGluT2cre mice also failed to show oxycodone‐induced locomotor stimulation. These mice displayed baseline withdrawal‐like responses following the development of oxycodone dependence that were not seen in littermate controls. In addition, withdrawal‐like responses in these mice did not increase following treatment with the opioid antagonist, naloxone. However, other MOR‐mediated behaviors were unaffected, including oxycodone‐induced analgesia. These data reveal that MOR‐mediated regulation of glutamate transmission is a critical component of opioid reward.

Selective modulation of tonic aversive qualities of neuropathic pain by morphine in the central nucleus of the amygdala requires endogenous opioid signaling in the anterior cingulate cortex

The amygdala is a key subcortical region believed to contribute to emotional components of pain. As opioid receptors are found in both the central (CeA) and basolateral (BLA) nuclei of the amygdala, we investigated the effects of morphine microinjection on evoked pain responses, pain-motivated behaviors, dopamine release in the nucleus accumbens (NAc), and descending modulation in rats with left-side spinal nerve ligation (SNL). Morphine administered into the right or left CeA had no effect on nerve injury-induced tactile allodynia or mechanical hyperalgesia. Right, but not left, CeA morphine produced conditioned place preference (CPP) and increased extracellular dopamine in the NAc selectively in SNL rats, suggesting relief of aversive qualities of ongoing pain. In SNL rats, CPP and NAc dopamine release following right CeA morphine was abolished by blocking mu opioid receptor signaling in the rostral anterior cingulate cortex (rACC). Right CeA morphine also significantly restored SNL-induced loss of the diffuse noxious inhibitory controls, a spino-bulbo-spinal pain modulatory mechanism, termed conditioned pain modulation in humans. Microinjection of morphine into the BLA had no effects on evoked behaviors and did not produce CPP in nerve-injured rats. These findings demonstrate that the amygdalar action of morphine is specific to the right CeA contralateral to the side of injury and results in enhancement of net descending inhibition. In addition, engagement of mu opioid receptors in the right CeA modulates affective qualities of ongoing pain through endogenous opioid neurotransmission within the rACC, revealing opioid-dependent functional connections from the CeA to the rACC.

Amygdala, neuropeptides, and chronic pain-related affective behaviors

Neuropeptides play important modulatory roles throughout the nervous system, functioning as direct effectors or as interacting partners with other neuropeptide and neurotransmitter systems. Limbic brain areas involved in learning, memory and emotions are particularly rich in neuropeptides. This review will focus on the amygdala, a limbic region that plays a key role in emotional-affective behaviors and pain modulation. The amygdala is comprised of different nuclei; the basolateral (BLA) and central (CeA) nuclei and in between, the intercalated cells (ITC), have been linked to pain-related functions. A wide range of neuropeptides are found in the amygdala, particularly in the CeA, but this review will discuss those neuropeptides that have been explored for their role in pain modulation. Calcitonin gene-related peptide (CGRP) is a key peptide in the afferent nociceptive pathway from the parabrachial area and mediates excitatory drive of CeA neurons. CeA neurons containing corticotropin releasing factor (CRF) and/or somatostatin (SOM) are a source of long-range projections and serve major output functions, but CRF also acts locally to excite neurons in the CeA and BLA. Neuropeptide S (NPS) is associated with inhibitory ITC neurons that gate amygdala output. Oxytocin and vasopressin exert opposite (inhibitory and excitatory, respectively) effects on amygdala output. The opioid system of mu, delta and kappa receptors (MOR, DOR, KOR) and their peptide ligands (β-endorphin, enkephalin, dynorphin) have complex and partially opposing effects on amygdala function. Neuropeptides therefore serve as valuable targets to regulate amygdala function in pain conditions. This article is part of the special issue on Neuropeptides.

Silver nanoparticles (Ag-NPs) in the central amygdala protect the rat conditioned by morphine from withdrawal attack due to naloxone via high-level nitric oxide

Repeated injection of morphine during conditioned place preference (CPP) leads to spatial craving due to high-level nitric oxide (NO) in the central nucleus of amygdala (CeA). Silver nanoparticles (Ag-NPs) can produce oxygen-free radicals that lead to NO formation. We aimed to show the Ag-NPs protective effect on naloxone (NLX)-induced morphine withdrawal in the conditioned rats. Wistar rats (300-350 g) were implanted with cannulae in the CeA. After recovery, they were randomly divided into experimental and saline groups. CPP was conducted by three-phase unbiased program. Morphine (0.5-7.5 mg/kg) was injected subcutaneously (s.c.) once/per day during the conditioning phase. Naloxone (NLX) (0.05-0.4 μg/rat) was given, intra-CeA, 10 min before the CPP test. Ag-NPs (0.0001-0.01 μg/rat) were administered alone or prior to the NLX effective dose (0.4 μg/rat), intra-CeA. Conditioning score and withdrawal signs (wet dog shaking and scratching) were obtained and compared with saline group data. All rats' brains were collected in formalin 10% and after 48-72 h stained with NADPH-diaphorase, the NO marker. All data were analyzed by one-way or two-way ANOVA. Morphine (2.5-7.5 mg/kg, s.c.) induced a significant CPP vs. saline (1 mL/kg, s.c.). The single Ag-NPs had no significant effect, whereas the NLX caused meaningful WDS and scratching. However, the NLX pre-treatment in combination with Ag-NPs eliminated these signs. Furthermore, the NO level increased in the CeA. The Ag-NPs may protect the morphine-conditioned rats against the NLX-induced withdrawal symptoms due to high-level NO in the CeA.

Presynaptic Mechanisms and KCNQ Potassium Channels Modulate Opioid Depression of Respiratory Drive

Opioid-induced respiratory depression (OIRD) is the major cause of death associated with opioid analgesics and drugs of abuse, but the underlying cellular and molecular mechanisms remain poorly understood. We investigated opioid action in vivo in unanesthetized mice and in in vitro medullary slices containing the preBötzinger Complex (preBötC), a locus critical for breathing and inspiratory rhythm generation. Although hypothesized as a primary mechanism, we found that mu-opioid receptor (MOR1)-mediated GIRK activation contributed only modestly to OIRD. Instead, mEPSC recordings from genetically identified Dbx1-derived interneurons, essential for rhythmogenesis, revealed a prevalent presynaptic mode of action for OIRD. Consistent with MOR1-mediated suppression of presynaptic release as a major component of OIRD, Cacna1a KO slices lacking P/Q-type Ca2+ channels enhanced OIRD. Furthermore, OIRD was mimicked and reversed by KCNQ potassium channel activators and blockers, respectively. In vivo whole-body plethysmography combined with systemic delivery of GIRK- and KCNQ-specific potassium channel drugs largely recapitulated these in vitro results, and revealed state-dependent modulation of OIRD. We propose that respiratory failure from OIRD results from a general reduction of synaptic efficacy, leading to a state-dependent collapse of rhythmic network activity.

Mu opioid receptor regulation of glutamate efflux in the central amygdala in response to predator odor

The amygdala plays an important role in the responses to predator threat. Glutamatergic processes in amygdala regulate the behavioral responses to predator stress, and we have found that exposure to ferret odor activates glutamatergic neurons of the basolateral amygdala [BLA] which are known to project to the central amygdala [CeA]. Therefore, we tested if predator stress would increase glutamate release in the rat CeA using in vivo microdialysis, while monitoring behavioral responses during a 1 h exposure to ferret odor. Since injections of mu opioid receptor [MOR] agonists and antagonists into the CeA modulate behavioral responses to predator odor, we locally infused the MOR agonist DAMGO or the MOR antagonist CTAP into the CeA during predator stress to examine effects on glutamate efflux and behavior. We found that ferret odor exposure increased glutamate, but not GABA, efflux in the CeA, and this effect was attenuated by tetrodotoxin. Interestingly, increases in glutamate efflux elicited by ferret odor exposure were blocked by infusion of CTAP, but CTAP did not alter the behavioral responses during predator stress. DAMGO alone enhanced glutamate efflux, but did not modulate glutamate efflux during predator stress. These studies demonstrate that ferret odor exposure, like other stressors, enhances glutamate efflux in the CeA. Further, they suggest that activation of MOR in the CeA may help shape the defensive response to predator odor and other threats.

An amygdalar neural ensemble that encodes the unpleasantness of pain

Pain is an unpleasant experience. How the brain’s affective neural circuits attribute this aversive quality to nociceptive information remains unknown. By means of time-lapse in vivo calcium imaging and neural activity manipulation in freely behaving mice encountering noxious stimuli, we identified a distinct neural ensemble in the basolateral amygdala that encodes the negative affective valence of pain. Silencing this nociceptive ensemble alleviated pain affective-motivational behaviors without altering the detection of noxious stimuli, withdrawal reflexes, anxiety, or reward. Following peripheral nerve injury, innocuous stimuli activated this nociceptive ensemble to drive dysfunctional perceptual changes associated with neuropathic pain, including pain aversion to light touch (allodynia). These results identify the amygdalar representations of noxious stimuli that are functionally required for the negative affective qualities of acute and chronic pain perception.

Interactive effects of morphine and nicotine on memory function depend on the central amygdala cannabinoid CB1 receptor function in rats

The present study investigated the possible involvement of the central amygdala (CeA) cannabinoid receptors type-1 (CB1Rs) in the interactive effects of morphine and nicotine on memory formation in a passive avoidance learning task. Our results showed that systemic administration of morphine (3 and 6mg/kg, s.c.) immediately after training phase impaired memory consolidation and induced amnesia. Administration of nicotine (0.3 and 0.6mg/kg, s.c.) before testing phase significantly restored morphine-induced amnesia, suggesting a cross state-dependent learning between morphine and nicotine. The results showed that while the administration of the lower dose of nicotine (0.1mg/kg, s.c.) per se did not induce a significant effect on morphine-induced amnesia, intra-CeA injection of arachidonylcyclopropylamide (ACPA), a cannabinoid CB1 receptor agonist (3 and 4ng/rat), significantly potentiated the nicotine response. Furthermore, the blockade of the CeA cannabinoid CB1 receptors by the injection of AM251 (0.75 and 1ng/rat) reversed the potentiative effect of nicotine (0.6mg/kg, s.c.) on morphine-induced amnesia. It should be considered that bilateral injection of the same doses of ACPA or AM251 (0.5-1ng/rat) into the CeA by itself had no effect on morphine response in a passive avoidance learning task. Confirmed by the cubic interpolation planes, the dose-response data revealed a cross-state-dependent learning between morphine and nicotine which may be mediated by the CeA endocannabinoid system via CB1 receptors.

Tramadol ameliorates behavioural, biochemical, mitochondrial and histological alterations in ICV-STZ-induced sporadic dementia of Alzheimer's type in rats

Alzheimer disease represents a major public health issue with limited therapeutic interventions. We explored the possibility of therapeutic approach by repurposing of tramadol in a sporadic animal model of Alzheimer's type. Streptozocin (STZ 3 mg/kg; bilaterally) was injected to male SD rats through intracerebroventricular (ICV) route. Drug treatment was started just after streptozocin administration and continued for 3 weeks. The rats were killed on the 21st day following the last behavioral test, and cytoplasmic fractions of the hippocampus and pre-frontal cortex were prepared for the quantification of acetylcholinesterase, oxidative stress parameter, mitochondrial enzymes activity and histological examination. Tramadol (5, 10 and 20 mg/kg, i.p.) was used as a treatment drug, and memantine (10 mg/kg, i.p.) was used as a standard. Tramadol significantly attenuated behavioral, biochemical, mitochondrial and histological alterations at low (5 mg/kg) and intermediate (10 mg/kg) dose, suggesting its neuroprotective potential in ICV-STZ-treated rats. Further, the neuroprotective effect of tramadol (10 mg/kg) was comparable to memantine (10 mg/kg). In conclusion, our results indicate the effectiveness of tramadol in preventing ICV-STZ-induced cognitive impairment as well as mito-oxidative stress. Further, these findings reveal the possibility of MOR agonist as a therapeutic approach for sporadic Alzheimer disease.

Enkephalins: Endogenous Analgesics with an Emerging Role in Stress Resilience

Psychological stress is a state of mental or emotional strain or tension that results from adverse or demanding circumstances. Chronic stress is well known to induce anxiety disorders and major depression; it is also considered a risk factor for Alzheimer's disease. Stress resilience is a positive outcome that is associated with preserved cognition and healthy aging. Resilience presents psychological and biological characteristics intrinsic to an individual conferring protection against the development of psychopathologies in the face of adversity. How can we promote or improve resilience to chronic stress? Numerous studies have proposed mechanisms that could trigger this desirable process. The roles of enkephalin transmission in the control of pain, physiological functions, like respiration, and affective disorders have been studied for more than 30 years. However, their role in the resilience to chronic stress has received much less attention. This review presents the evidence for an emerging involvement of enkephalin signaling through its two associated opioid receptors, μ opioid peptide receptor and δ opioid peptide receptor, in the natural adaptation to stressful lifestyles.

Desensitization-resistant and -sensitive GPCR-mediated inhibition of GABA release occurs by Ca2+-dependent and -independent mechanisms at a hypothalamic synapse

Whereas the activation of Gαi/o-coupled receptors commonly results in postsynaptic responses that show acute desensitization, the presynaptic inhibition of transmitter release caused by many Gαi/o-coupled receptors is maintained during agonist exposure. However, an exception has been noted where GABAB receptor (GABABR)-mediated inhibition of inhibitory postsynaptic currents (IPSCs) recorded in mouse proopiomelanocortin (POMC) neurons exhibit acute desensitization in ∼25% of experiments. To determine whether differential effector coupling confers sensitivity to desensitization, voltage-clamp recordings were made from POMC neurons to compare the mechanism by which μ-opioid receptors (MORs) and GABABRs inhibit transmitter release. Neither MOR- nor GABABR-mediated inhibition of release relied on the activation of presynaptic K(+) channels. Both receptors maintained the ability to inhibit release in the absence of external Ca(2+) or in the presence of ionomycin-induced Ca(2+) influx, indicating that inhibition of release can occur through a Ca(2+)-independent mechanism. Replacing Ca(2+) with Sr(2+) to disrupt G-protein-mediated inhibition of release occurring directly at the release machinery did not alter MOR- or GABAB -mediated inhibition of IPSCs, suggesting that reductions in evoked release can occur through the inhibition of Ca(2+) channels. Additionally, both receptors inhibited evoked IPSCs in the presence of selective blockers of N- or P/Q-type Ca(2+) channels. Altogether, the results show that MORs and GABABRs can inhibit transmitter release through the inhibition of calcium influx and by direct actions at the release machinery. Furthermore, since both the desensitizing and nondesensitizing presynaptic receptors are similarly coupled, differential effector coupling is unlikely responsible for differential desensitization of the inhibition of release.

Mu opioid receptor localization in the basolateral amygdala: An ultrastructural analysis

Receptor binding studies have shown that the density of mu opioid receptors (MORs) in the basolateral amygdala is among the highest in the brain. Activation of these receptors in the basolateral amygdala is critical for stress-induced analgesia, memory consolidation of aversive events, and stress adaptation. Despite the importance of MORs in these stress-related functions, little is known about the neural circuits that are modulated by amygdalar MORs. In the present investigation light and electron microscopy combined with immunohistochemistry was used to study the expression of MORs in the anterior basolateral nucleus (BLa). At the light microscopic level, light to moderate MOR-immunoreactivity (MOR-ir) was observed in a small number of cell bodies of nonpyramidal interneurons and in a small number of processes and puncta in the neuropil. At the electron microscopic level most MOR-ir was observed in dendritic shafts, dendritic spines, and axon terminals. MOR-ir was also observed in the Golgi apparatus of the cell bodies of pyramidal neurons (PNs) and interneurons. Some of the MOR-positive (MOR+) dendrites were spiny, suggesting that they belonged to PNs, while others received multiple asymmetrical synapses typical of interneurons. The great majority of MOR+ axon terminals (80%) that formed synapses made asymmetrical (excitatory) synapses; their main targets were spines, including some that were MOR+. The main targets of symmetrical (inhibitory and/or neuromodulatory) synapses were dendritic shafts, many of which were MOR+, but some of these terminals formed synapses with somata or spines. All of our observations were consistent with the few electrophysiological studies which have been performed on MOR activation in the basolateral amygdala. Collectively, these findings suggest that MORs may be important for filtering out weak excitatory inputs to PNs, allowing only strong inputs or synchronous inputs to influence pyramidal neuronal firing.

DAMGO depresses inhibitory synaptic transmission via different downstream pathways of μ opioid receptors in ventral tegmental area and periaqueductal gray

Opioid-induced rewarding and motorstimulant effects are mediated by an increased activity of the ventral tegmental area (VTA) dopamine (DA) neurons. The excitatory mechanism of opioids on VTA-DA neurons has been proposed to be due to the depression of GABAergic synaptic transmission in VTA-DA neurons. However, how opioids depress GABAergic synaptic transmission in VTA-DA neurons remain to be studied. In the present study, we explored the mechanism of the inhibitory effect of [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) on GABAergic synaptic transmission in VTA-DA neurons using multiple approaches and techniques. Our results showed that (1) DAMGO inhibits GABAergic inputs in VTA-DA neurons at presynaptic sites; (2) effect of DAMGO on GABAergic inputs in VTA-DA neurons is inhibited by potassium channel blocker 4-aminopyridine (4-AP) and Gi protein inhibitor N-ethylmaleimide (NEM); (3) phospholipase A2 (PLA2) does not mediate the effect of DAMGO on GABAergic inputs in VTA-DA neurons, but mediates it in the periaqueductal gray (PAG); (4) multiple downstream signaling molecules of μ receptors do not mediate the effect of DAMGO on GABAergic inputs in VTA-DA neurons. These results suggest that DAMGO depresses inhibitory synaptic transmission via μ receptor-Gi protein-Kv channel pathway in VTA-DA neurons, but via μ receptor-PLA2 pathway in PAG neurons.

Acute morphine alters GABAergic transmission in the central amygdala during naloxone-precipitated morphine withdrawal: role of cyclic AMP

The central amygdala (CeA) plays an important role in opioid addiction. Therefore, we examined the effects of naloxone-precipitated morphine withdrawal (WD) on GABAergic transmission in rat CeA neurons using whole-cell recordings with naloxone in the bath. The basal frequency of miniature inhibitory postsynaptic currents (mIPSCs) increased in CeA neurons from WD compared to placebo rats. Acute morphine (10 μ M) had mixed effects (≥20% change from baseline) on mIPSCs in placebo and WD rats. In most CeA neurons (64%) from placebo rats, morphine significantly decreased mIPSC frequency and amplitude. In 32% of placebo neurons, morphine significantly increased mIPSC amplitudes but had no effect on mIPSC frequency. In WD rats, acute morphine significantly increased mIPSC frequency but had no effect on mIPSC amplitude in 41% of CeA neurons. In 45% of cells, acute morphine significantly decreased mIPSC frequency and amplitude. Pre-treatment with the cyclic AMP inhibitor (R)-adenosine, cyclic 3',5'-(hydrogenphosphorothioate) triethylammonium (RP), prevented acute morphine-induced potentiation of mIPSCs. Pre-treatment of slices with the Gi/o G-protein subunit inhibitor pertussis toxin (PTX) did not prevent the acute morphine-induced enhancement or inhibition of mIPSCs. PTX and RP decreased basal mIPSC frequencies and amplitudes only in WD rats. The results suggest that inhibition of GABAergic transmission in the CeA by acute morphine is mediated by PTX-insensitive mechanisms, although PTX-sensitive mechanisms cannot be ruled out for non-morphine responsive cells; by contrast, potentiation of GABAergic transmission is mediated by activated cAMP signaling that also mediates the increased basal GABAergic transmission in WD rats. Our data indicate that during the acute phase of WD, the CeA opioid and GABAergic systems undergo neuroadaptative changes conditioned by a previous chronic morphine exposure and dependence.

Enkephalin knockout male mice are resistant to chronic mild stress

Enhanced stress reactivity or sensitivity to chronic stress increases the susceptibility to mood pathologies such as major depression. The opioid peptide enkephalin is an important modulator of the stress response. Previous studies using preproenkephalin knockout (PENK KO) mice showed that these animals exhibit abnormal stress reactivity and show increased anxiety behavior in acute stress situations. However, the consequence of enkephalin deficiency in the reactivity to chronic stress conditions is not known. In this study, we therefore submitted wild-type (WT) and PENK KO male mice to chronic stress conditions, using the chronic mild stress (CMS) protocol. Subsequently, we studied the CMS effects on the behavioral and hormonal level and also performed gene expression analyses. In WT animals, CMS increased the expression of the enkephalin gene in the paraventricular nucleus (PVN) of the hypothalamus and elevated the corticosterone levels. In addition, WT mice exhibited enhanced anxiety in the zero-maze test and depression-related behaviors in the sucrose preference and forced swim tests. Surprisingly, in PENK KO mice, we did not detect anxiety and depression-related behavioral changes after the CMS procedure, and even measured a decreased hormonal stress response. These results indicate that PENK KO mice are resistant to the CMS effects, suggesting that enkephalin enhances the reactivity to chronic stress.

Enkephalin knockdown in the basolateral amygdala reproduces vulnerable anxiety-like responses to chronic unpredictable stress

The endogenous enkephalins (ENKs) are potential candidates participating in the naturally occurring variations in coping styles and determining the individual capacities for adaptation during chronic stress exposure. Here we demonstrate that there is a large variance in individual behavioral, as well as in physiological outcomes, in a population of Sprague-Dawley rats subjected to 3 weeks of chronic unpredictable stress (CUS). Separation of resilient and vulnerable subpopulations reveals specific long-term neuroadaptation in the ENKergic brain circuits. ENK mRNA expression was greatly reduced in the posterior basolateral nucleus of amygdala (BLAp) in vulnerable individuals. In contrast, ENK mRNA levels were similar in resilient and control (unstressed) individuals. Another group of rats were used for lentiviral-mediated knockdown of ENK to assess whether a decrease of ENK expression in the BLAp reproduces the behavioral disturbances found in vulnerable individuals. ENK knockdown specifically located in the BLAp was sufficient to increase anxiety in the behavioral tests, such as social interaction and elevated plus maze when compared with control individuals. These results show that specific neuroadaptation mediated by the ENKergic neurotransmission in the BLAp is a key regulator of resilience, whereas a decrease of the ENK in the BLAp is a maladaptation mechanism, which mediates the behavioral dichotomy observed between vulnerable and resilient following 3 weeks of CUS.

Opioid receptor mu 1 gene, fat intake and obesity in adolescence

Dietary preference for fat may increase risk for obesity. It is a complex behavior regulated in part by the amygdala, a brain structure involved in reward processing and food behavior, and modulated by genetic factors. Here, we conducted a genome-wide association study (GWAS) to search for gene loci associated with dietary intake of fat, and we tested whether these loci are also associated with adiposity and amygdala volume. We studied 598 adolescents (12-18 years) recruited from the French-Canadian founder population and genotyped them with 530 011 single-nucleotide polymorphisms. Fat intake was assessed with a 24-hour food recall. Adiposity was examined with anthropometry and bioimpedance. Amygdala volume was measured by magnetic resonance imaging. GWAS identified a locus of fat intake in the μ-opioid receptor gene (OPRM1, rs2281617, P=5.2 × 10(-6)), which encodes a receptor expressed in the brain-reward system and shown previously to modulate fat preference in animals. The minor OPRM1 allele appeared to have a 'protective' effect: it was associated with lower fat intake (by 4%) and lower body-fat mass (by ∼2 kg, P=0.02). Consistent with the possible amygdala-mediated inhibition of fat preference, this allele was additionally associated with higher amygdala volume (by 69 mm(3), P=0.02) and, in the carriers of this allele, amygdala volume correlated inversely with fat intake (P=0.02). Finally, OPRM1 was associated with fat intake in an independent sample of 490 young adults. In summary, OPRM1 may modulate dietary intake of fat and hence risk for obesity, and this effect may be modulated by subtle variations in the amygdala volume.

Antitussive activity of Withania somnifera and opioid receptors

Arabinogalactan is a polysaccharide isolated from the roots of the medicinal plant Withania somnifera L. It contains 65% arabinose and 18% galactose. The aim of the present study was to evaluate the antitussive activity of arabinogalactan in conscious, healthy adult guinea pigs and the role of the opioid pathway in the antitussive action. A polysaccharide extract was given orally in a dose of 50 mg/kg. Cough was induced by an aerosol of citric acid in a concentration 0.3 mol/L, generated by a jet nebulizer into a plethysmographic chamber. The intensity of cough response was defined as the number of cough efforts counted during a 3-min exposure to the aerosol. The major finding was that arabinogalactan clearly suppressed the cough reflex; the suppression was comparable with that of codeine that was taken as a reference drug. The involvement of the opioid system was tested with the use of a blood-brain barrier penetrable, naloxone hydrochloride, and non-penetrable, naloxone methiodide, to distinguish between the central and peripheral mu-opioid receptor pathways. Both opioid antagonists acted to reverse the arabinogalactan-induced cough suppression; the reversion was total over time with the latter antagonist. We failed to confirm the presence of a bronchodilating effect of the polysaccharide, which could be involved in its antitussive action. We conclude that the polysaccharide arabinogalactan from Withania somnifera has a distinct antitussive activity consisting of cough suppression and that this action involves the mu-opioid receptor pathways.

BDNF parabrachio-amygdaloid pathway in morphine-induced analgesia

In addition to its neurotrophic role, brain-derived neurotrophic factor (BDNF) is involved in a wide array of functions, including anxiety and pain. The central amygdaloid nucleus (CeA) contains a high concentration of BDNF in terminals, originating from the pontine parabrachial nucleus. Since the spino-parabrachio-amygdaloid neural pathway is known to convey nociceptive information, we hypothesized a possible involvement of BDNF in supraspinal pain-related processes. To test this hypothesis, we generated localized deletion of BDNF in the parabrachial nucleus using local bilateral injections of adeno-associated viruses in adult floxed-BDNF mice. Basal thresholds of thermal and mechanical nociceptive responses were not altered by BDNF loss and no behavioural deficit was noticed in anxiety and motor tests. However, BDNF-deleted animals displayed a major decrease in the analgesic effect of morphine. In addition, intra-CeA injections of the BDNF scavenger TrkB-Fc in control mice also decreased morphine-induced analgesia. Finally, the number of c-Fos immunoreactive nuclei after acute morphine injection was decreased by 45% in the extended amygdala of BDNF-deleted animals. The absence of BDNF in the parabrachial nucleus thus altered the parabrachio-amygdaloid pathway. Overall, our study provides evidence that BDNF produced in the parabrachial nucleus modulates the functions of the parabrachio-amygdaloid pathway in opiate analgesia.

The amygdala between sensation and affect: a role in pain

The amygdala is a structure of the temporal lobe thought to be involved in assigning emotional significance to environmental information and triggering adapted physiological, behavioral and affective responses. A large body of literature in animals and human implicates the amygdala in fear. Pain having a strong affective and emotional dimension, the amygdala, especially its central nucleus (CeA), has also emerged in the last twenty years as key element of the pain matrix. The CeA receives multiple nociceptive information from the brainstem, as well as highly processed polymodal information from the thalamus and the cerebral cortex. It also possesses the connections that allow influencing most of the descending pain control systems as well as higher centers involved in emotional, affective and cognitive functions. Preclinical studies indicate that the integration of nociceptive inputs in the CeA only marginally contributes to sensory-discriminative components of pain, but rather contributes to associated behavior and affective responses. The CeA doesn’t have a major influence on responses to acute nociception in basal condition, but it induces hypoalgesia during aversive situation, such as stress or fear. On the contrary, during persistent pain states (inflammatory, visceral, neuropathic), a long-lasting functional plasticity of CeA activity contributes to an enhancement of the pain experience, including hyperalgesia, aversive behavioral reactions and affective anxiety-like states.

Naltrexone in the treatment of alcohol dependence

Naltrexone is an opioid receptor antagonist that has been shown to be effective for maintaining abstinence in alcohol-dependent persons. It is particularly effective in a subset of persons who suffer from high craving, as it reduces craving for alcohol. Family history has been shown to be a predictor of treatment response and, indeed, allelic variation in the mu opioid receptor gene predicts treatment response to naltrexone. The therapeutic effects of naltrexone are mediated by blockade of central mu opioid receptors. The site of action is under investigation but evidence supports a role of mu opioid receptors in the central nucleus of the amygdala, nucleus accumbens, and ventral tegmental area in the therapeutic actions of naltrexone for alcohol dependence. This article reviews the role of the endogenous opioid system in addictive diseases, especially alcoholism and discusses the pharmacological basis for the use of naltrexone in the treatment of alcohol dependence.

Long-term changes in reward-seeking following morphine withdrawal are associated with altered N-methyl-D-aspartate receptor 1 splice variants in the amygdala

The NR1 subunit of the NMDA receptor can be alternatively spliced by the insertion or removal of the N1, C1, C2, or C2' regions. Morphine dependence and withdrawal were previously demonstrated to lower N1 and C2' in the accumbens and lower N1, C1, and C2' in the amygdala (AMY). Withdrawal has also been demonstrated to increase motivational and anxiety/stress behaviors in rats. We tested the hypothesis that NR1 splicing would be associated with these behaviors during an extended withdrawal period of 2 months. Motivation was measured using an operant orofacial assay at non-aversive temperatures (37°C) while anxiety and stress were measured by examining this behavior at aversive temperatures (46°C). Lower C1 and C2 expression levels were observed in the AMY in a subset of the population of withdrawn rats even after 2 months of morphine withdrawal. These subsets were associated with a hypersensitivity to adverse conditions which may reflect long-term alterations in the withdrawn population.

Opioids inhibit visceral afferent activation of catecholamine neurons in the solitary tract nucleus

Brainstem A2/C2 catecholamine (CA) neurons within the solitary tract nucleus (NTS) influence many homeostatic functions, including food intake, stress, respiratory and cardiovascular reflexes. They also play a role in both opioid reward and withdrawal. Injections of opioids into the NTS modulate many autonomic functions influenced by catecholamine neurons including food intake and cardiac function. We recently showed that NTS-CA neurons are directly activated by incoming visceral afferent inputs. Here we determined whether opioid agonists modulate afferent activation of NTS-CA neurons using transgenic mice with EGFP expressed under the control of the tyrosine hydroxylase promoter (TH-EGFP) to identify catecholamine neurons. The opioid agonist Met-enkephalin (Met-Enk) significantly attenuated solitary tract-evoked excitatory postsynaptic currents (ST-EPSCs) in NTS TH-EGFP neurons by 80%, an effect reversed by wash or the mu opioid receptor-specific antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP). Met-Enk had a significantly greater effect to inhibit afferent inputs onto TH-EGFP-positive neurons than EGFP-negative neurons, which were only inhibited by 50%. The mu agonist, DAMGO, also inhibited the ST-EPSC in TH-EGFP neurons in a dose-dependent manner. In contrast, neither the delta agonist DPDPE, nor the kappa agonist, U69,593, consistently inhibited the ST-EPSC amplitude. Met-Enk and DAMGO increased the paired pulse ratio, decreased the frequency, but not amplitude, of mini-EPSCs and had no effect on holding current, input resistance or current-voltage relationships in TH-EGFP neurons, suggesting a presynaptic mechanism of action on afferent terminals. Met-Enk significantly reduced both the basal firing rate of NTS TH-EGFP neurons and the ability of afferent stimulation to evoke an action potential. These results suggest that opioids inhibit NTS-CA neurons by reducing an excitatory afferent drive onto these neurons through presynaptic inhibition of glutamate release and elucidate one potential mechanism by which opioids could control autonomic functions and modulate reward and opioid withdrawal symptoms at the level of the NTS.

What and when to "want"? Amygdala-based focusing of incentive salience upon sugar and sex

RATIONALE

Amygdala-related circuitry helps translate learned Pavlovian associations into appetitive and aversive motivation, especially upon subsequent encounters with cues.

OBJECTIVES

We asked whether μ-opioid stimulation via microinjections of the specific agonist D-Ala(2), N-MePhe(4), Gly-ol)-enkephalin (DAMGO) in central nucleus of amygdala (CeA), or the adjacent basolateral amygdala (BLA) would magnify sucrose or sex "wanting", guided by available cues.

MATERIALS AND METHODS

CeA or BLA DAMGO enhancement of cue-triggered "wanting" was assessed using Pavlovian to instrumental transfer (PIT). Unconditioned food "wanting" was measured via intake, and male sexual "wanting" for an estrous female was measured in a sexual approach test. Sucrose hedonic taste "liking" was measured in a taste reactivity test.

RESULTS

CeA (but not BLA) DAMGO increased the intensity of phasic peaks in instrumental sucrose seeking stimulated by Pavlovian cues over precue levels in PIT, while suppressing seeking at other moments. CeA DAMGO also enhanced food intake, as well as sexual approach and investigation of an estrous female by males. DAMGO "wanting" enhancements were localized to CeA, as indicated by "Fos plume"-based anatomical maps for DAMGO causation of behavioral effects. Despite increasing "wanting", CeA DAMGO decreased the hedonic impact or "liking" for sucrose in a taste reactivity paradigm.

CONCLUSIONS

CeA μ-opioid stimulation specifically enhances incentive salience, which is dynamically guided to food or sex by available cues.

Signaling cascades for δ-opioid receptor-mediated inhibition of GABA synaptic transmission and behavioral antinociception

Membrane trafficking of the δ-opioid receptor (DOR) from intracellular compartments to plasma membrane in central neurons, induced by various pathological conditions such as long-term opioid exposure, represents unique receptor plasticity involved in the mechanisms of long-term opioid effects in opioid addiction and opioid treatment of chronic pain. However, the signaling pathways coupled to the newly emerged functional DOR in central neurons are largely unknown at present. In this study, we investigated the signaling cascades of long-term morphine-induced DOR for its cellular and behavioral effects in neurons of the rat brainstem nucleus raphe magnus (NRM), a key supraspinal site for opioid analgesia. We found that, among the three phospholipase A(2) (PLA(2))-regulated arachidonic acid (AA) metabolic pathways of lipoxygenase, cyclooxygenase, and epoxygenase, 12-lipoxygenase of the lipoxygenase pathway primarily mediated DOR inhibition of GABA synaptic transmission, because inhibitors of 12-lipoxygenase as well as lipoxygenases and PLA(2) largely blocked the DOR- or AA-induced GABA inhibition in NRM neurons in brainstem slices in vitro. Blockade of the epoxygenase pathway was ineffective, whereas blocking either 5-lipoxygenase of the lipoxygenase pathway or the cyclooxygenase pathway enhanced the DOR-mediated GABA inhibition. Behaviorally in rats in vivo, NRM infusion of 12-lipoxygenase inhibitors significantly reduced DOR-induced antinociceptive effect whereas inhibitors of 5-lipoxygenase and cyclooxygenase augmented the DOR antinociception. These findings suggest the PLA(2)-AA-12-lipoxygenase pathway as a primary signaling cascade for DOR-mediated analgesia through inhibition of GABA neurotransmission and indicate potential therapeutic benefits of combining 5-lipoxygenase and cyclooxygenase inhibitors for maximal pain inhibition.

The NMDA-NR1 receptor subunit and the mu-opioid receptor are expressed in somatodendritic compartments of central nucleus of the amygdala neurons projecting to the bed nucleus of the stria terminalis

The pathway between the central nucleus of the amygdala (CeA) and the bed nucleus of the stria terminalis (BNST) is emerging as a critical mediator of stress-related affective processes. Evidence also indicates that exposure to drugs of abuse, like opioids, is associated with NMDA-type glutamate receptor-dependent plasticity in the CeA and BNST. However, there is little evidence that NMDA receptors are expressed in CeA neurons projecting to the BNST, or are required for opioid-induced BNST neural activation. Immunoelectron microscopy, tract tracing, and conditional gene deletion technology were used to investigate the synaptic organization of the NMDA receptor and the mu-opioid receptor (μOR) in the CeA-BNST pathway. By dual labeling electron microscopy, numerous CeA-BNST projection neurons expressed the NMDA-NR1 receptor subunit (NR1) or μOR. By triple labeling, it was also found that NR1 and μOR were co-expressed in some CeA-BNST projection neurons. Despite being colocalized in somato-dendritic compartments of CeA neurons, NR1 and μOR were rarely expressed in their axonal terminations in the BNST. Deleting the NR1 gene in CeA neurons resulted in a reduction of morphine-induced Fos protein labeling in the ventral BNST. In summary, NR1 and μOR are coexpressed in somatodendritic sites of CeA neurons, including those projecting to the BNST. In addition, expression of the NR1 gene in CeA neurons is required for morphine-induced BNST neural activation. Thus, postsynaptic NMDA receptors and μORs are positioned for the co-modulation of CeA projection neurons to the BNST, which may provide a synaptic substrate for stress-induced emotional processes critically involved in opioid addictive behaviors.