Distinct distributions of mu, delta and kappa opioid receptor mRNA in rat brain

Divergent opioid-mediated suppression of inhibition between hippocampus and neocortex across species and development

Opioid receptors within the CNS regulate pain sensation and mood and are key targets for drugs of abuse. Within the adult rodent hippocampus (HPC), μ-opioid receptor agonists suppress inhibitory parvalbumin-expressing interneurons (PV-INs), thus disinhibiting the circuit. However, it is uncertain if this disinhibitory motif is conserved in other cortical regions, species, or across development. We observed that PV-IN mediated inhibition is robustly suppressed by opioids in HPC but not neocortex in mice and nonhuman primates, with spontaneous inhibitory tone in resected human tissue also following a consistent dichotomy. This hippocampal disinhibitory motif was established in early development when immature PV-INs and opioids already influence primordial network rhythmogenesis. Acute opioid-mediated modulation was partially occluded with morphine pretreatment, with implications for the effects of opioids on hippocampal network activity during circuit maturation as well as learning and memory. Together, these findings demonstrate that PV-INs exhibit a divergence in opioid sensitivity across brain regions that is remarkably conserved across evolution and highlights the underappreciated role of opioids acting through immature PV-INs in shaping hippocampal development.

Kappa Opioid Receptors in Mesolimbic Terminals Mediate Escalation of Cocaine Consumption

Increases in drug consumption over time, also known as escalation, is a key behavioral component of substance use disorder (SUD) that is related to potential harm to users, such as overdose. Studying escalation also allows researchers to investigate the transition from casual drug use to more SUD-like drug use. Understanding the neurobiological systems that drive this transition will inform therapeutic treatments in the aim to prevent increases in drug use and the development of SUD. The kappa opioid receptor (KOR) system is typically known for its role in negative affect, which is commonly found in SUD as well. Furthermore, the KOR system has also been implicated in drug use and importantly, modulating the negative effects of drug use. However, the specific neuronal subpopulation expressing KOR involved has not been identified. Here, we first demonstrated that pharmacologically inhibiting KOR in the nucleus accumbens core (NAcC), as a whole, blocks cocaine escalation under long-access self-administration conditions. We then demonstrated that KOR expressed on ventral tegmental area (VTA) neurons but not NAcC neurons is sufficient for blocking cocaine escalation by utilizing a novel virally-mediated CRISPR-SaCas9 knock-out of the oprk1 gene. Together, this suggests that activation of KOR on VTA terminals in the NAcC drives the transition to the SUD-like phenotype of escalation of cocaine consumption.

Kappa opioid activation changes protein profiles in different regions of the brain relevant to depression

Depression is a widespread disorder with a significant burden on individuals and society. There are various available treatments for patients with depression. However, not all patients respond adequately to their treatment. Recently, the opioid system has regained interest in depression studies. Research in animals and humans suggest that blocking the kappa opioid receptor (KOR) may potentially alleviate the symptoms of depression. The mechanism behind this effect is not fully understood. Stress and alterations in hypothalamic-pituitary-adrenal axis (HPA-axis) activity are thought to play a crucial role in depression. This study aimed to characterize stress hormones and stress-related protein expression following activation of KOR using a selective agonist. The longitudinal effect was investigated 24 h after KOR activation using the selective agonist U50,488 in Sprague Dawley rats. Stress-related hormones and protein expression patterns were explored using multiplex bead-based assays and western blotting. We found that KOR activation caused an increase in both adrenocorticotropic hormone (ACTH) and corticosterone (CORT) in serum. Regarding protein assays in different brain regions, phosphorylated glucocorticoid receptors also increased significantly in thalamus (THL), hypothalamus (HTH), and striatum (STR). C-Fos increased time-dependently in THL following KOR activation, extracellular signal-regulated kinases 1/2 (ERK1/2) increased significantly in STR and amygdala (AMG), while phosphorylated ERK1/2 decreased during the first 2 h and then increased again in AMG and prefrontal cortex (PFC). This study shows that KOR activation alters the HPA axis and ERK signaling which may cause to develop mood disorders.

Opioid Receptors Modulate Firing and Synaptic Transmission in the Paraventricular Nucleus of the Thalamus

The paraventricular nucleus of the thalamus (PVT) is involved in drug addiction-related behaviors, and morphine is a widely used opioid for the relief of severe pain. Morphine acts via opioid receptors, but the function of opioid receptors in the PVT has not been fully elucidated. Here, we used in vitro electrophysiology to study neuronal activity and synaptic transmission in the PVT of male and female mice. Activation of opioid receptors suppresses the firing and inhibitory synaptic transmission of PVT neurons in brain slices. On the other hand, the involvement of opioid modulation is reduced after chronic morphine exposure, probably because of desensitization and internalization of opioid receptors in the PVT. Overall, the opioid system is essential for the modulation of PVT activities.SIGNIFICANCE STATEMENT Opioid receptors modulate the activities and synaptic transmission in the PVT by suppressing the firing rate and inhibitory synaptic inputs. These modulations were largely diminished after chronic morphine exposure.

Striatal μ-opioid receptor activation triggers direct-pathway GABAergic plasticity and induces negative affect

Withdrawal from chronic opioid use often causes hypodopaminergic states and negative affect, which may drive relapse. Direct-pathway medium spiny neurons (dMSNs) in the striatal patch compartment contain μ-opioid receptors (MORs). It remains unclear how chronic opioid exposure and withdrawal impact these MOR-expressing dMSNs and their outputs. Here, we report that MOR activation acutely suppressed GABAergic striatopallidal transmission in habenula-projecting globus pallidus neurons. Notably, withdrawal from repeated morphine or fentanyl administration potentiated this GABAergic transmission. Furthermore, intravenous fentanyl self-administration enhanced GABAergic striatonigral transmission and reduced midbrain dopaminergic activity. Fentanyl-activated striatal neurons mediated contextual memory retrieval required for conditioned place preference tests. Importantly, chemogenetic inhibition of striatal MOR+ neurons rescued fentanyl withdrawal-induced physical symptoms and anxiety-like behaviors. These data suggest that chronic opioid use triggers GABAergic striatopallidal and striatonigral plasticity to induce a hypodopaminergic state, which may promote negative emotions and relapse.

Opioidergic pathways and kisspeptin in the regulation of female reproduction in mammals

Endogenous opioid peptides have attracted attention as critical neuropeptides in the central mechanism regulating female reproduction ever since the discovery that arcuate dynorphin neurons that coexpress kisspeptin and neurokinin B (NKB), which are also known as kisspeptin/neurokinin B/dynorphin (KNDy) neurons, play a role as a master regulator of pulsatile gonadotropin-releasing hormone (GnRH) release in mammals. In this study, we first focus on the role of dynorphin released by KNDy neurons in the GnRH pulse generation. Second, we provide a historical overview of studies on endogenous opioid peptides. Third, we discuss how endogenous opioid peptides modulate tonic GnRH/gonadotropin release in female mammals as a mediator of inhibitory internal and external cues, such as ovarian steroids, nutritional status, or stress, on reproduction. Then, we discuss the role of endogenous opioid peptides in GnRH surge generation in female mammals.

The expression of delta opioid receptor mRNA in adult male zebra finches (Taenopygia guttata)

The endogenous opioid system is evolutionarily conserved across reptiles, birds and mammals and is known to modulate varied brain functions such as learning, memory, cognition and reward. To date, most of the behavioral and anatomical studies in songbirds have mainly focused on μ-opioid receptors (ORs). Expression patterns of δ-ORs in zebra finches, a well-studied species of songbird have not yet been reported, possibly due to the high sequence similarity amongst different opioid receptors. In the present study, a specific riboprobe against the δ-OR mRNA was used to perform fluorescence in situ hybridization (FISH) on sections from the male zebra finch brain. We found that δ-OR mRNA was expressed in different parts of the pallium, basal ganglia, cerebellum and the hippocampus. Amongst the song control and auditory nuclei, HVC (abbreviation used as a formal name) and NIf (nucleus interfacialis nidopallii) strongly express δ-OR mRNA and stand out from the surrounding nidopallium. Whereas the expression of δ-OR mRNA is moderate in LMAN (lateral magnocellular nucleus of the anterior nidopallium), it is low in the MSt (medial striatum), Area X, DLM (dorsolateral nucleus of the medial thalamus), RA (robust nucleus of the arcopallium) of the song control circuit and Field L, Ov (nucleus ovoidalis) and MLd (nucleus mesencephalicus lateralis, pars dorsalis) of the auditory pathway. Our results suggest that δ-ORs may be involved in modulating singing, song learning as well as spatial learning in zebra finches.

Reported Benefits of Low-Dose Naltrexone Appear to Be Independent of the Endogenous Opioid System Involving Proopiomelanocortin Neurons and β-Endorphin

Naltrexone is an opioid receptor antagonist approved for the treatment of alcohol and opioid use disorders at doses of 50–150 mg/d. Naltrexone has also been prescribed at much lower doses (3–6 mg/d) for the off-label treatment of inflammation and pain. Currently, a compelling mechanistic explanation for the reported efficacy of low-dose naltrexone (LDN) is lacking and none of the proposed mechanisms can explain patient reports of improved mood and sense of well-being. Here, we examined the possibility that LDN might alter the activity of the endogenous opioid system involving proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH) in male and female mice. Known actions of POMC neurons could account for changes in pain perception and mood. However, using electrophysiologic, imaging and peptide measurement approaches, we found no evidence for such a mechanism. LDN did not change the sensitivity of opioid receptors regulating POMC neurons, the production of the β-endorphin precursor Pomc mRNA, nor the release of β-endorphin into plasma. Spontaneous postsynaptic currents (sPSCs) onto POMC neurons were slightly decreased after LDN treatment and GCaMP fluorescent signal, a proxy for intracellular calcium levels, was slightly increased. However, LDN treatment did not appear to change POMC neuron firing rate, resting membrane potential, nor action potential threshold. Therefore, LDN appears to have only slight effects on POMC neurons that do not translate to changes in intrinsic excitability or baseline electrical activity and mechanisms beyond POMC neurons and altered opioid receptor sensitivity should continue to be explored.

Acute opioid use may cause choroidal thinning and retinal nerve fiber layer increase

The number of optical coherence tomography (OCT) examinations in substance use disorders is gradually increasing. However, OCT findings in opioid use disorder (OUD) have not yet been investigated. In this study, we compared the retinal nerve fiber layer (RNFL), the ganglion cell layer (GCL), the inner plexiform layer (IPL), and choroid thickness (CT) of OUD and control groups. We included 43 male patients and 43 healthy male controls of similar age (p = 0.296) in the study, prospectively. On the day of OCT application, urine toxic screening test results of all OUD patients were positive for opioid use. There was a significant difference between OUD and control groups in terms of CT (p < 0.05), nasal superior (NS), and nasal (N) sectors of the RNFL (p < 0.05) values of both eyes. According to the binary logistic regression analysis, the sensitivity of mean NS (p = 0.001) and mean CT (p = 0.007) related to the diagnosis of OUD was 72.1 percent, and the specificity was 65.1 percent. Receiver operating characteristic (ROC) analysis revealed that the sensitivity and specificity of mean CT for the diagnosis of OUD were 18.6% and 97.7%, respectively. This is the first study to investigate the OCT findings in OUD. Our findings are important in terms of showing thinning in the choroidal layer and an increase in the volume of the NS and N sectors of RNFL while detecting opioids in the body/urine. Further studies are needed to clarify whether these differences are due to the acute and/or chronic effects of opioids.

κ-Opioid receptors in the medial amygdaloid nucleus modulate autonomic and neuroendocrine responses to acute stress

The medial amygdaloid nucleus (MeA) is a key neural structure in triggering physiologic and behavioral control during aversive situations. However, MeA role during stress exposure has not yet been fully elucidated. Thus, in the present study, we investigated the involvement of the MeA opioid neurotransmission in the modulation of autonomic, neuroendocrine and behavioral responses evoked by acute restraint stress (RS). The bilateral microinjection of naloxone (non-selective opioid antagonist) into the MeA potentiated RS-evoked autonomic responses and increased plasma corticosterone levels, in a dose-dependent manner. However, no effects were observed in RS-evoked increases on plasma oxytocin levels and anxiogenic-like behavior. Similar to naloxone, MeA pretreatment with the selective κ-opioid antagonist (nor-BNI) also enhanced heart rate and corticosterone increases induced by RS, whereas treatment with selective µ- or δ-opioid antagonists did not affect the physiologic and behavioral responses caused by RS. The present results showed MeA κ-opioid receptors modulate heart rate and corticosterone increases evoked by acute RS, reinforcing the idea of an inhibitory role exerted by MeA during aversive situations .

Sex and chronic stress alter delta opioid receptor distribution within rat hippocampal CA1 pyramidal cells following behavioral challenges

Following oxycodone (Oxy) conditioned place preference (CPP), delta opioid receptors (DORs) differentially redistribute in hippocampal CA3 pyramidal cells in female and male rats in a manner that would promote plasticity and opioid-associative learning processes. However, following chronic immobilization stress (CIS), males do not acquire Oxy-CPP and the trafficking of DORs in CA3 pyramidal neurons is attenuated. Here, we examined the subcellular distribution of DORs in CA1 pyramidal cells using electron microscopy in these same cohorts.

CPP

Saline (Sal)-females compared to Sal-males have more cytoplasmic and total DORs in dendrites and more DOR-labeled spines. Following Oxy-CPP, DORs redistribute from near-plasmalemma pools in dendrites to spines in males.

CIS

Control females compared to control males have more near-plasmalemmal dendritic DORs. Following CIS, dendritic DORs are elevated in the cytoplasm in females and near-plasmalemma in males.

CIS plus CPP

CIS Sal-females compared to CIS Sal-males have more DORs on the plasmalemma of dendrites and in spines. After Oxy, the distribution of DORs does not change in either females or males.

Conclusion

Following Oxy-CPP, DORs within CA1 pyramidal cells remain positioned in naïve female rats to enhance sensitivity to DOR agonists and traffic to dendritic spines in naïve males where they can promote plasticity processes. Following CIS plus behavioral enrichment, DORs are redistributed within CA1 pyramidal cells in females in a manner that could enhance sensitivity to DOR agonists. Conversely, CIS plus behavioral enrichment does not alter DORs in CA1 pyramidal cells in males, which may contribute to their diminished capacity to acquire Oxy-CPP.

Neurochemistry of the Kölliker-Fuse nucleus from a respiratory perspective

The Kölliker-Fuse nucleus (KF) is a functionally distinct component of the parabrachial complex, located in the dorsolateral pons of mammals. The KF has a major role in respiration and upper airway control. A comprehensive understanding of the KF and its contributions to respiratory function and dysfunction requires an appreciation for its neurochemical characteristics. The goal of this review is to summarize the diverse neurochemical composition of the KF, focusing on the neurotransmitters, neuromodulators, and neuropeptides present. We also include a description of the receptors expressed on KF neurons and transporters involved in each system, as well as their putative roles in respiratory physiology. Finally, we provide a short section reviewing the literature regarding neurochemical changes in the KF in the context of respiratory dysfunction observed in SIDS and Rett syndrome. By over-viewing the current literature on the neurochemical composition of the KF, this review will serve to aid a wide range of topics in the future research into the neural control of respiration in health and disease.

Different neuronal populations mediate inflammatory pain analgesia by exogenous and endogenous opioids

Mu-opioid receptors (MORs) are crucial for analgesia by both exogenous and endogenous opioids. However, the distinct mechanisms underlying these two types of opioid analgesia remain largely unknown. Here, we demonstrate that analgesic effects of exogenous and endogenous opioids on inflammatory pain are mediated by MORs expressed in distinct subpopulations of neurons in mice. We found that the exogenous opioid-induced analgesia of inflammatory pain is mediated by MORs in Vglut2⁺ glutamatergic but not GABAergic neurons. In contrast, analgesia by endogenous opioids is mediated by MORs in GABAergic rather than Vglut2⁺ glutamatergic neurons. Furthermore, MORs expressed at the spinal level is mainly involved in the analgesic effect of morphine in acute pain, but not in endogenous opioid analgesia during chronic inflammatory pain. Thus, our study revealed distinct mechanisms underlying analgesia by exogenous and endogenous opioids, and laid the foundation for further dissecting the circuit mechanism underlying opioid analgesia.

Opioid receptors mRNAs expression and opioids agonist-dependent G-protein activation in the rat brain following neuropathy

Potent opioid-based therapies are often unsuccessful in promoting satisfactory analgesia in neuropathic pain. Moreover, the side effects associated with opioid therapy are still manifested in neuropathy-like diseases, including tolerance, abuse, addiction and hyperalgesia, although the mechanisms underlying these effects remain unclear. Studies in the spinal cord and periphery indicate that neuropathy alters the expression of mu-[MOP], delta-[DOP] or kappa-[KOP] opioid receptors, interfering with their activity. However, there is no consensus as to the supraspinal opioidergic modulation provoked by neuropathy, the structures where the sensory and affective-related pain components are processed. In this study we explored the effect of chronic constriction of the sciatic nerve (CCI) over 7 and 30 days (CCI-7d and CCI-30d, respectively) on MOP, DOP and KOP mRNAs expression, using in situ hybridization, and the efficacy of G-protein stimulation by DAMGO, DPDPE and U-69593 (MOP, DOP and KOP specific agonists, respectively), using [35S]GTPγS binding, within opioid-sensitive brain structures. After CCI-7d, CCI-30d or both, opioid receptor mRNAs expression was altered throughout the brain: MOP - in the paracentral/centrolateral thalamic nuclei, ventral posteromedial thalamic nuclei, superior olivary complex, parabrachial nucleus [PB] and posterodorsal tegmental nucleus; DOP - in the somatosensory cortex [SSC], ventral tegmental area, caudate putamen [CPu], nucleus accumbens [NAcc], raphe magnus [RMg] and PB; and KOP - in the locus coeruleus. Agonist-stimulated [35S]GTPγS binding was altered following CCI: MOP - CPu and RMg; DOP - prefrontal cortex [PFC], SSC, RMg and NAcc; and KOP - PFC and SSC. Thus, this study shows that several opioidergic circuits in the brain are recruited and modified following neuropathy.

μ-Opioid receptors in primary sensory neurons are involved in supraspinal opioid analgesia

Both inhibiting ascending nociceptive transmission and activating descending inhibition are involved in the opioid analgesic effect. The spinal dorsal horn is a critical site for modulating nociceptive transmission by descending pathways elicited by opioids in the brain. μ-Opioid receptors (MORs, encoded by Oprm1) are highly expressed in primary sensory neurons and their central terminals in the spinal cord. In the present study, we tested the hypothesis that MORs expressed in primary sensory neurons contribute to the descending inhibition and supraspinal analgesic effect induced by centrally administered opioids. We generated Oprm1 conditional knockout (Oprm1-cKO) mice by crossing Advillin^Cre/+ mice with Oprm1^flox/flox mice. Immunocytochemical labeling in Oprm1-cKO mice showed that MORs are completely ablated from primary sensory neurons and are profoundly reduced in the superficial spinal dorsal horn. Intracerebroventricular injection of morphine or fentanyl produced a potent analgesic effect in wild-type mice, but such an effect was significantly attenuated in Oprm1-cKO mice. Furthermore, the analgesic effect produced by morphine or fentanyl microinjected into the periaqueductal gray was significantly greater in wild-type mice than in Oprm1-cKO mice. Blocking MORs at the spinal cord level diminished the analgesic effect of morphine and fentanyl microinjected into the periaqueductal gray in both groups of mice. Our findings indicate that MORs expressed at primary afferent terminals in the spinal cord contribute to the supraspinal opioid analgesic effect. These presynaptic MORs in the spinal cord may serve as an interface between ascending inhibition and descending modulation that are involved in opioid analgesia.

Radiosynthesis of [11C]LY2795050 for Preclinical and Clinical PET Imaging Using Cu(II)-Mediated Cyanation

Copper-mediated ¹¹C-cyanation reactions have enabled the synthesis of PET radiotracers from a range of readily available precursors and avoid the need to use more toxic Pd catalysts. In this work we adapt our recently developed ¹¹C-cyanation of arylpinacolboronate (BPin) esters for the cGMP synthesis of [¹¹C]LY2795050, a selective antagonist radiotracer for the kappa opioid receptor (KOR). [¹¹C]LY2795050 was synthesized in 6 ± 1% noncorrected radiochemical yield (based on [¹¹C]HCN, n = 3) using an automated synthesis module. Quality control testing confirmed the suitability of doses for preclinical and clinical PET imaging (radiochemical purity >99%; specific activity >900 mCi/μmol; residual Cu < 0.1 μg/mL). PET imaging was conducted in rodent and nonhuman primates, showing good brain uptake of [¹¹C]LY2795050 and the expected distribution of KOR. Analogous imaging with [¹¹C]carfentanil (a selective mu opioid receptor (MOR) radiotracer) revealed the anticipated regional differences in MOR and KOR distribution in the primate brain.

Molecular Adaptations in the Rat Dorsal Striatum and Hippocampus Following Abstinence-Induced Incubation of Drug Seeking After Escalated Oxycodone Self-Administration

Repeated exposure to the opioid agonist, oxycodone, can lead to addiction. Here, we sought to identify potential neurobiological consequences of withdrawal from escalated and non-escalated oxycodone self-administration in rats. To reach these goals, we used short-access (ShA) (3 h) and long-access (LgA) (9 h) exposure to oxycodone self-administration followed by protracted forced abstinence. After 31 days of withdrawal, we quantified mRNA and protein levels of opioid receptors in the rat dorsal striatum and hippocampus. Rats in the LgA, but not the ShA, group exhibited escalation of oxycodone SA, with distinction of two behavioral phenotypes of relatively lower (LgA-L) and higher (LgA-H) oxycodone takers. Both LgA, but not ShA, phenotypes showed time-dependent increases in oxycodone seeking during the 31 days of forced abstinence. Rats from both LgA-L and LgA-H groups also exhibited decreased levels of striatal mu opioid receptor protein levels in comparison to saline and ShA rats. In contrast, mu opioid receptor mRNA expression was increased in the dorsal striatum of LgA-H rats. Moreover, hippocampal mu and kappa receptor protein levels were both increased in the LgA-H phenotype. Nevertheless, hippocampal mu receptor mRNA levels were decreased in the two LgA groups whereas kappa receptor mRNA expression was decreased in ShA and LgA oxycodone groups. Decreases in striatal mu opioid receptor protein expression in the LgA rats may serve as substrates for relapse to drug seeking because these changes occur in rats that showed incubation of oxycodone seeking.

G protein signaling-biased agonism at the κ-opioid receptor is maintained in striatal neurons

Biased agonists of G protein-coupled receptors may present a means to refine receptor signaling in a way that separates side effects from therapeutic properties. Several studies have shown that agonists that activate the κ-opioid receptor (KOR) in a manner that favors G protein coupling over β-arrestin2 recruitment in cell culture may represent a means to treat pain and itch while avoiding sedation and dysphoria. Although it is attractive to speculate that the bias between G protein signaling and β-arrestin2 recruitment is the reason for these divergent behaviors, little evidence has emerged to show that these signaling pathways diverge in the neuronal environment. We further explored the influence of cellular context on biased agonism at KOR ligand-directed signaling toward G protein pathways over β-arrestin-dependent pathways and found that this bias persists in striatal neurons. These findings advance our understanding of how a G protein-biased agonist signal differs between cell lines and primary neurons, demonstrate that measuring [³⁵S]GTPγS binding and the regulation of adenylyl cyclase activity are not necessarily orthogonal assays in cell lines, and emphasize the contributions of the environment to assessing biased agonism.

The Delta Opioid Receptor in Pain Control

Nowadays, the delta opioid receptor (DOPr) represents a promising target for the treatment of chronic pain and emotional disorders. Despite the fact that they produce limited antinociceptive effects in healthy animals and in most acute pain models, DOPr agonists have shown efficacy in various chronic pain models. In this chapter, we review the progresses that have been made over the last decades in understanding the role played by DOPr in the control of pain. More specifically, the distribution of DOPr within the central nervous system and along pain pathways is presented. We also summarize the literature supporting a role for DOPr in acute, tonic, and chronic pain models, as well as the mechanisms regulating its activity under specific conditions. Finally, novel compounds that have make their way to clinical trials are discussed.

Prefrontal Cortical Kappa Opioid Receptors Attenuate Responses to Amygdala Inputs

Kappa opioid receptors (KORs) have been implicated in anxiety and stress, conditions that involve activation of projections from the basolateral amygdala (BLA) to the medial prefrontal cortex (mPFC). Although KORs have been studied in several brain regions, their role on mPFC physiology and on BLA projections to the mPFC remains unclear. Here, we explored whether KORs modify synaptic inputs from the BLA to the mPFC using in vivo electrophysiological recordings with electrical and optogenetic stimulation. Systemic administration of the KOR agonist U69,593 inhibited BLA-evoked synaptic responses in the mPFC without altering hippocampus-evoked responses. Intra-mPFC U69,593 inhibited electrical and optogenetic BLA-evoked synaptic responses, an effect blocked by the KOR antagonist nor-BNI. Bilateral intra-mPFC injection of the KOR antagonist nor-BNI increased center time in the open field test, suggesting an anxiolytic effect. The data demonstrate that mPFC KORs negatively regulate glutamatergic synaptic transmission in the BLA-mPFC pathway and anxiety-like behavior. These findings provide a framework whereby KOR signaling during stress and anxiety can regulate the flow of emotional state information from the BLA to the mPFC.

Dynorphin activation of kappa opioid receptor reduces neuronal excitability in the paraventricular nucleus of mouse thalamus

It has been reported that kappa opioid receptor (KOR) is expressed in the paraventricular nucleus of thalamus (PVT), a brain region associated with arousal, drug reward and stress. Although intra-PVT infusion of KOR agonist was found to inhibit drug-seeking behavior, it is still unclear whether endogenous KOR agonists directly regulate PVT neuron activity. Here, we investigated the effect of the endogenous KOR agonist dynorphin-A (Dyn-A) on the excitability of mouse PVT neurons at different developmental ages. We found Dyn-A strongly inhibited PVT neurons through a direct postsynaptic hyperpolarization. Under voltage-clamp configuration, Dyn-A evoked an obvious outward current in majority of neurons tested in anterior PVT (aPVT) but only in minority of neurons in posterior PVT (pPVT). The Dyn-A current was abolished by KOR antagonist nor-BNI, Ba(2+) and non-hydrolyzable GDP analogue GDP-β-s, indicating that Dyn-A activates KOR and opens G-protein-coupled inwardly rectifying potassium channels in PVT neurons. More interestingly, by comparing Dyn-A currents in aPVT neurons of mice at various ages, we found Dyn-A evoked significant larger current in aPVT neurons from mice around prepuberty and early puberty stage. In addition, KOR activation by Dyn-A didn't produce obvious desensitization, while mu opioid receptor (MOR) activation induced obvious desensitization of mu receptor itself and also heterologous desensitization of KOR in PVT neurons. Together, our findings indicate that Dyn-A activates KOR and inhibits aPVT neurons in mice at various ages especially around puberty, suggesting a possible role of KOR in regulating aPVT-related brain function including stress response and drug-seeking behavior during adolescence.

Direct hypothalamic and indirect trans-pallidal, trans-thalamic, or trans-septal control of accumbens signaling and their roles in food intake

Due in part to the increasing incidence of obesity in developed nations, recent research aims to elucidate neural circuits that motivate humans to overeat. Earlier research has described how the nucleus accumbens shell (AcbSh) motivates organisms to feed by activating neuronal populations in the lateral hypothalamus (LH). However, more recent research suggests that the LH may in turn communicate with the AcbSh, both directly and indirectly, to re-tune the motivation to consume foods with homeostatic and food-related sensory signals. Here, we discuss the functional and anatomical evidence for an LH to AcbSh connection and its role in eating behaviors. The LH appears to modulate Acb activity directly, using neurotransmitters such as hypocretin/orexin or melanin concentrating hormone (MCH). The LH also indirectly regulates AcbSh activity through certain subcortical "relay" regions, such as the lateral septum (LS), ventral pallidum (VP), and paraventricular thalamus, using a variety of neurotransmitters. This review aims to summarize studies on these topics and outline a model by which LH circuits processing energy balance can modulate AcbSh neural activity to regulate feeding behavior.

Fluorescent knock-in mice to decipher the physiopathological role of G protein-coupled receptors

G protein-coupled receptors (GPCRs) modulate most physiological functions but are also critically involved in numerous pathological states. Approximately a third of marketed drugs target GPCRs, which places this family of receptors in the main arena of pharmacological pre-clinical and clinical research. The complexity of GPCR function demands comprehensive appraisal in native environment to collect in-depth knowledge of receptor physiopathological roles and assess the potential of therapeutic molecules. Identifying neurons expressing endogenous GPCRs is therefore essential to locate them within functional circuits whereas GPCR visualization with subcellular resolution is required to get insight into agonist-induced trafficking. Both remain frequently poorly investigated because direct visualization of endogenous receptors is often hampered by the lack of appropriate tools. Also, monitoring intracellular trafficking requires real-time visualization to gather in-depth knowledge. In this context, knock-in mice expressing a fluorescent protein or a fluorescent version of a GPCR under the control of the endogenous promoter not only help to decipher neuroanatomical circuits but also enable real-time monitoring with subcellular resolution thus providing invaluable information on their trafficking in response to a physiological or a pharmacological challenge. This review will present the animal models and discuss their contribution to the understanding of the physiopathological role of GPCRs. We will also address the drawbacks associated with this methodological approach and browse future directions.

Identification of neuropeptide receptors expressed by melanin-concentrating hormone neurons

Melanin-concentrating hormone (MCH) is a 19-amino-acid cyclic neuropeptide that acts in rodents via the MCH receptor 1 (MCHR1) to regulate a wide variety of physiological functions. MCH is produced by a distinct population of neurons located in the lateral hypothalamus (LH) and zona incerta (ZI), but MCHR1 mRNA is widely expressed throughout the brain. The physiological responses and behaviors regulated by the MCH system have been investigated, but less is known about how MCH neurons are regulated. The effects of most classical neurotransmitters on MCH neurons have been studied, but those of most neuropeptides are poorly understood. To gain insight into how neuropeptides regulate the MCH system, we investigated which neuropeptide receptors are expressed by MCH neurons by using double in situ hybridization. In all, 20 receptors, selected based on either a suspected interaction with the MCH system or demonstrated high expression levels in the LH and ZI, were tested to determine whether they are expressed by MCH neurons. Overall, 11 neuropeptide receptors were found to exhibit significant colocalization with MCH neurons: nociceptin/orphanin FQ opioid receptor (NOP), MCHR1, both orexin receptors (ORX), somatostatin receptors 1 and 2 (SSTR1, SSTR2), kisspeptin recepotor (KissR1), neurotensin receptor 1 (NTSR1), neuropeptide S receptor (NPSR), cholecystokinin receptor A (CCKAR), and the κ-opioid receptor (KOR). Among these receptors, six have never before been linked to the MCH system. Surprisingly, several receptors thought to regulate MCH neurons displayed minimal colocalization with MCH, suggesting that they may not directly regulate the MCH system.

Expression of mu opioid receptor in dorsal diencephalic conduction system: new insights for the medial habenula

The habenular complex, encompassing medial (MHb) and lateral (LHb) divisions, is a highly conserved epithalamic structure involved in the dorsal diencephalic conduction system (DDC). These brain nuclei regulate information flow between the limbic forebrain and the mid- and hindbrain, integrating cognitive with emotional and sensory processes. The MHb is also one of the strongest expression sites for mu opioid receptors (MORs), which mediate analgesic and rewarding properties of opiates. At present however, anatomical distribution and function of these receptors have been poorly studied in MHb pathways. Here we took advantage of a newly generated MOR-mcherry knock-in mouse line to characterize MOR expression sites in the DDC. MOR-mcherry fluorescent signal is weak in the LHb, but strong expression is visible in the MHb, fasciculus retroflexus (fr) and interpeduncular nucleus (IPN), indicating that MOR is mainly present in the MHb-IPN pathway. MOR-mcherry cell bodies are detected both in basolateral and apical parts of MHb, where the receptor co-localizes with cholinergic and substance P (SP) neurons, respectively, representing two main MHb neuronal populations. MOR-mcherry is expressed in most MHb-SP neurons, and is present in only a subpopulation of MHb-cholinergic neurons. Intense diffuse fluorescence detected in lateral and rostral parts of the IPN further suggests that MOR-mcherry is transported to terminals of these SP and cholinergic neurons. Finally, MOR-mcherry is present in septal regions projecting to the MHb, and in neurons of the central and intermediate IPN. Together, this study describes MOR expression in several compartments of the MHb-IPN circuitry. The remarkably high MOR density in the MHb-IPN pathway suggests that these receptors are in a unique position to mediate analgesic, autonomic and reward responses.

Neurokinin B signaling in the female rat: a novel link between stress and reproduction

Acute systemic stress disrupts reproductive function by inhibiting pulsatile gonadotropin secretion. The underlying mechanism involves stress-induced suppression of the GnRH pulse generator, the functional unit of which is considered to be the hypothalamic arcuate nucleus kisspeptin/neurokinin B/dynorphin A neurons. Agonists of the neurokinin B (NKB) receptor (NK3R) have been shown to suppress the GnRH pulse generator, in a dynorphin A (Dyn)-dependent fashion, under hypoestrogenic conditions, and Dyn has been well documented to mediate several stress-related central regulatory functions. We hypothesized that the NKB/Dyn signaling cascade is required for stress-induced suppression of the GnRH pulse generator. To investigate this ovariectomized rats, iv administered with Escherichia coli lipopolysaccharide (LPS) following intracerebroventricular pretreatment with NK3R or κ-opioid receptor (Dyn receptor) antagonists, were subjected to frequent blood sampling for hormone analysis. Antagonism of NK3R, but not κ-opioid receptor, blocked the suppressive effect of LPS challenge on LH pulse frequency. Neither antagonist affected LPS-induced corticosterone secretion. Hypothalamic arcuate nucleus NKB neurons project to the paraventricular nucleus, the major hypothalamic source of the stress-related neuropeptides CRH and arginine vasopressin (AVP), which have been implicated in the stress-induced suppression of the hypothalamic-pituitary-gonadal axis. A separate group of ovariectomized rats was, therefore, used to address the potential involvement of central CRH and/or AVP signaling in the suppression of LH pulsatility induced by intracerebroventricular administration of a selective NK3R agonist, senktide. Neither AVP nor CRH receptor antagonists affected the senktide-induced suppression of the LH pulse; however, antagonism of type 2 CRH receptors attenuated the accompanying elevation of corticosterone levels. These data indicate that the suppression of the GnRH pulse generator by acute systemic stress requires hypothalamic NKB/NK3R signaling and that any involvement of CRH therewith is functionally upstream of NKB.

Virus-mediated shRNA knockdown of prodynorphin in the rat nucleus accumbens attenuates depression-like behavior and cocaine locomotor sensitization

Dynorphins, endogenous opioid peptides that arise from the precursor protein prodynorphin (Pdyn), are hypothesized to be involved in the regulation of mood states and the neuroplasticity associated with addiction. The current study tested the hypothesis that dynorphin in the nucleus accumbens (NAcc) mediates such effects. More specifically, we examined whether knockdown of Pdyn within the NAcc in rats would alter the expression of depressive-like and anxiety-like behavior, as well as cocaine locomotor sensitization. Wistar rats were injected with adeno-associated viral (AAV) vectors encoding either a Pdyn-specific short hairpin RNA (AAV-shPdyn) or a scrambled shRNA (AAV-shScr) as control. Four weeks later, rats were tested for anxiety-like behavior in the elevated plus maze test and depressive-like behavior in the forced swim test (FST). Finally, rats received one daily injection of saline or cocaine (20 mg/kg, i.p.), followed by assessment of locomotion for 4 consecutive days. Following 3 days of abstinence, the rats completed 2 additional daily cocaine/saline locomotor trials. Pdyn knockdown in the NAcc led to a significant reduction in depressive-like behavior in the FST, but had no effect on anxiety-like behavior in the elevated plus maze. Pdyn knockdown did not alter baseline locomotor behavior, the locomotor response to acute cocaine, or the initial sensitization of the locomotor response to cocaine over the first 4 cocaine treatment days. However, following 3 days abstinence the locomotor response to the cocaine challenge returned to their original levels in the AAV-shPdyn rats while remaining heightened in the AAV-shScr rats. These results suggest that dynorphin in a very specific area of the nucleus accumbens contributes to depressive-like states and may be involved in neuroadaptations in the NAcc that contribute to the development of cocaine addiction as a persistent and lasting condition.

Intrinsic properties and neuropharmacology of midline paraventricular thalamic nucleus neurons

Neurons in the midline and intralaminar thalamic nuclei are components of an interconnected brainstem, limbic and prefrontal cortex neural network that is engaged during arousal, vigilance, motivated and addictive behaviors, and stress. To better understand the cellular mechanisms underlying these functions, here we review some of the recently characterized electrophysiological and neuropharmacological properties of neurons in the paraventricular thalamic nucleus (PVT), derived from whole cell patch clamp recordings in acute rat brain slice preparations. PVT neurons display firing patterns and ionic conductances (I_T and I_H) that exhibit significant diurnal change. Their resting membrane potential (RMP) is maintained by various ionic conductances that include inward rectifier (Kir), hyperpolarization-activated nonselective cation (HCN) and TWIK-related acid sensitive (TASK) K⁺ channels. Firing patterns are regulated by high voltage-activated (HVA) and low voltage-activated (LVA) Ca²⁺ conductances. Moreover, transient receptor potential (TRP)-like nonselective cation channels together with Ca²⁺- and Na⁺-activated K⁺ conductances (K_Ca; K_Na) contribute to unique slow afterhyperpolarizing potentials (sAHPs) that are generally not detectable in lateral thalamic or reticular thalamic nucleus neurons. The excitability of PVT neurons is also modulated by activation of neurotransmitter receptors associated with afferent pathways to PVT and other thalamic midline nuclei. We report on receptor-mediated actions of GABA, glutamate, monoamines and several neuropeptides: arginine vasopressin, gastrin-releasing peptide, thyrotropin releasing hormone and the orexins (hypocretins). This review represents an initial survey of intrinsic and transmitter-sensitive ionic conductances that are deemed to be unique to this population of midline thalamic neurons, information that is fundamental to an appreciation of the role these thalamic neurons may play in normal central nervous system (CNS) physiology and in CNS disorders that involve the dorsomedial thalamus.

A mu-delta opioid receptor brain atlas reveals neuronal co-occurrence in subcortical networks

Opioid receptors are G protein-coupled receptors (GPCRs) that modulate brain function at all levels of neural integration, including autonomic, sensory, emotional and cognitive processing. Mu (MOR) and delta (DOR) opioid receptors functionally interact in vivo, but whether interactions occur at circuitry, cellular or molecular levels remains unsolved. To challenge the hypothesis of MOR/DOR heteromerization in the brain, we generated redMOR/greenDOR double knock-in mice and report dual receptor mapping throughout the nervous system. Data are organized as an interactive database offering an opioid receptor atlas with concomitant MOR/DOR visualization at subcellular resolution, accessible online. We also provide co-immunoprecipitation-based evidence for receptor heteromerization in these mice. In the forebrain, MOR and DOR are mainly detected in separate neurons, suggesting system-level interactions in high-order processing. In contrast, neuronal co-localization is detected in subcortical networks essential for survival involved in eating and sexual behaviors or perception and response to aversive stimuli. In addition, potential MOR/DOR intracellular interactions within the nociceptive pathway offer novel therapeutic perspectives.

Opioids induce dissociable forms of long-term depression of excitatory inputs to the dorsal striatum

As prescription opioid analgesic abuse rates rise, so does the need to understand the long-term effects of opioid exposure on brain function. The dorsal striatum is an important site for drug-induced neuronal plasticity. We found that exogenously applied and endogenously released opioids induced long-term depression (OP-LTD) of excitatory inputs to the dorsal striatum in mice and rats. Mu and delta OP-LTD, although both being presynaptically expressed, were dissociable in that they summated, differentially occluded endocannabinoid-LTD and inhibited different striatal inputs. Kappa OP-LTD showed a unique subregional expression in striatum. A single in vivo exposure to the opioid analgesic oxycodone disrupted mu OP-LTD and endocannabinoid-LTD, but not delta or kappa OP-LTD. These data reveal previously unknown opioid-mediated forms of long-term striatal plasticity that are differentially affected by opioid analgesic exposure and are likely important mediators of striatum-dependent learning and behavior.

Co-localization of hypocretin-1 and leucine-enkephalin in hypothalamic neurons projecting to the nucleus of the solitary tract and their effect on arterial pressure

Experiments were done to investigate whether hypothalamic hypocretin-1 (hcrt-1; orexin-A) neurons that sent axonal projections to cardiovascular responsive sites in the nucleus of the solitary tract (NTS) co-expressed leucine-enkephalin (L-Enk), and to determine the effects of co-administration of hcrt-1 and D-Ala2,D-Leu5-Enkephalin (DADL) into NTS on mean arterial pressure (MAP) and heart rate. In the first series, in the Wistar rat the retrograde tract-tracer fluorogold (FG) was microinjected (50nl) into caudal NTS sites at which L-glutamate (0.25 M; 10 nl) elicited decreases in MAP and where fibers hcrt-1 immunoreactive fibers were observed that also contained L-Enk immunoreactivity. Of the number of hypothalamic hcrt-1 immunoreactive neurons identified ipsilateral to the NTS injection site (1207 ± 78), 32.3 ± 2.3% co-expressed L-Enk immunoreactivity and of these, 2.6 ± 1.1% were retrogradely labeled with FG. Hcrt-1/L-Enk neurons projecting to NTS were found mainly within the perifornical region. In the second series, the region of caudal NTS found to contain axons that co-expressed hcrt-1 and L-Enk immunoreactivity was microinjected with a combination of hcrt-1 and DADL in α-chloralose anesthetized Wistar rats. Microinjection of DADL into NTS elicited depressor and bradycardia responses similar to those elicited by microinjection of hcrt-1. An hcrt-1 injection immediately after the DADL injection elicited an almost twofold increase in the magnitude of the depressor and bradycardia responses compared to those elicited by hcrt-1 alone. Prior injections of the non-specific opioid receptor antagonist naloxone or the specific opioid δ-receptor antagonist ICI 154,129 significantly attenuated the cardiovascular responses to the combined hcrt-1-DADL injections. Taken together, these data suggest that activation of hypothalamic-opioidergic neuronal systems contribute to the NTS hcrt-1 induced cardiovascular responses, and that this descending hypothalamo-medullary pathway may represent the anatomical substrate by which hcrt-1/L-Enk neurons function in the coordination of autonomic-cardiovascular responses during different behavioral states.

Immunohistochemistry detection of kappa-opioid receptors in human skin

The imbalance of p- and kappa-opioid receptors in the skin or central nervous system is currently deemed to be one of the reasons of chronic pruritus. A number of studies demonstrated a positive effect of system agonists of kappa-opioid receptors in the treatment of uremic pruritus, nodular pruritus, paraneoplastic and cholestatic pruritus. This research demonstrates an expression of kappa-opioid receptors in human skin (basal keratinocytes, dendritic cells, epidermal melanocytes and fibroblasts of the upper dermis) detected with the use of different immunochemistry methods.

Olfaction under metabolic influences

Recently published work and emerging research efforts have suggested that the olfactory system is intimately linked with the endocrine systems that regulate or modify energy balance. Although much attention has been focused on the parallels between taste transduction and neuroendocrine controls of digestion due to the novel discovery of taste receptors and molecular components shared by the tongue and gut, the equivalent body of knowledge that has accumulated for the olfactory system, has largely been overlooked. During regular cycles of food intake or disorders of endocrine function, olfaction is modulated in response to changing levels of various molecules, such as ghrelin, orexins, neuropeptide Y, insulin, leptin, and cholecystokinin. In view of the worldwide health concern regarding the rising incidence of diabetes, obesity, and related metabolic disorders, we present a comprehensive review that addresses the current knowledge of hormonal modulation of olfactory perception and how disruption of hormonal signaling in the olfactory system can affect energy homeostasis.

Mu-opioid receptors and dietary protein stimulate a gut-brain neural circuitry limiting food intake

Intestinal gluconeogenesis is involved in the control of food intake. We show that mu-opioid receptors (MORs) present in nerves in the portal vein walls respond to peptides to regulate a gut-brain neural circuit that controls intestinal gluconeogenesis and satiety. In vitro, peptides and protein digests behave as MOR antagonists in competition experiments. In vivo, they stimulate MOR-dependent induction of intestinal gluconeogenesis via activation of brain areas receiving inputs from gastrointestinal ascending nerves. MOR-knockout mice do not carry out intestinal gluconeogenesis in response to peptides and are insensitive to the satiety effect induced by protein-enriched diets. Portal infusions of MOR modulators have no effect on food intake in mice deficient for intestinal gluconeogenesis. Thus, the regulation of portal MORs by peptides triggering signals to and from the brain to induce intestinal gluconeogenesis are links in the satiety phenomenon associated with alimentary protein assimilation.

Central kappa opioid receptors modulate salt appetite in rats

The role of the central opioid system in the control of water and salt intake is complex, with both stimulatory and inhibitory effects having been observed. The aim of the present study was to investigate the participation of the central κ-opioid receptors in the control of salt appetite. Male Wistar rats were submitted to two different experimental protocols: sodium deficit produced by the diuretic, furosemide, and brain angiotensinergic stimulation in rats under normal sodium balance. Lateral ventricle (LV) injections of Nor-binaltorphimine (Nor-BNI) at different doses (5, 10 and 20 nmol) inhibited hypertonic saline solution (1.5%) intake in sodium-depleted rats. The salt appetite induced by an LV injection of angiotensin II (Ang II) (10 ng) was also blocked by Nor-BNI injections into the LV, while no significant change was observed in water intake. Furthermore, the decrease in salt intake seems not to have been due to a general inhibition of locomotor activity or to any change in palatability, since central administration of Nor-BNI failed to modify the intake of a 0.1% saccharin solution when the animals were submitted to a "dessert test" or to induce any significant locomotor deficit in the open-field test. Also the central administration of Nor-BNI was unable to modify blood pressure in sodium-depleted animals. The present results suggest that activation of endogenous κ-opioid receptors modulates salt appetite induced by sodium depletion and by central angiotensinergic stimulation in rats.

The dynorphin/κ-opioid receptor system and its role in psychiatric disorders

The dynorphin/κ-opioid receptor system has been implicated in the pathogenesis and pathophysiology of several psychiatric disorders. In the present review, we present evidence indicating a key role for this system in modulating neurotransmission in brain circuits that subserve mood, motivation, and cognitive function. We overview the pharmacology, signaling, post-translational, post-transcriptional, transcriptional, epigenetic and cis regulation of the dynorphin/κ-opioid receptor system, and critically review functional neuroanatomical, neurochemical, and pharmacological evidence, suggesting that alterations in this system may contribute to affective disorders, drug addiction, and schizophrenia. We also overview the dynorphin/κ-opioid receptor system in the genetics of psychiatric disorders and discuss implications of the reviewed material for therapeutics development.

Multiple opioid receptors mediate the hypotensive response induced by central 5-HT(3) receptor stimulation

The aim of the present work was to investigate the role of brain μ, κ and δ opioid receptors in the central serotonergic mechanisms regulating blood pressure in rats. The data obtained show that: (1) pharmacological activation of central 5-HT(3) receptors yields a significant decrease in blood pressure; (2) the blockade of those receptors by a selective antagonist induces an acute hypertensive response; (3) the pharmacological blockade of central opioid receptors by three different opioid antagonists exhibiting variable degrees of selectivity to μ, κ and δ opioid receptors always suppressed the hypotensive response induced by central 5-HT(3) receptor stimulation; (4) the blockade of opioid receptors by the same opioid antagonists that impaired the hypotensive effect of central 5-HT(3) receptor stimulation failed to modify blood pressure in animals not submitted to pharmacological manipulations of central 5-HT(3) receptor function. It is shown that a 5-HT(3) receptor-dependent mechanism seems to be part of the brain serotonergic system that contributes to cardiovascular regulation since the hypertensive response observed after ondansetron administration indicates that central 5-HT(3) receptors exert a tonic inhibitory drive on blood pressure. Furthermore, the data obtained here clearly indicate that the hypotensive response observed after pharmacological stimulation of central 5-HT(3) receptors depends on the functional integrity of brain μ, κ and δ opioid receptors, suggesting that a functional interaction between serotonergic and opiatergic pathways in the brain is part of the complex, multifactorial system that regulates blood pressure in the central nervous system.

Ovarian hormones influence corticotropin releasing factor receptor colocalization with delta opioid receptors in CA1 pyramidal cell dendrites

Stress interacts with addictive processes to increase drug use, drug seeking, and relapse. The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect and likely plays a critical role in the interaction between stress and drug addiction. Our prior studies demonstrate that the stress-related neuropeptide corticotropin-releasing factor (CRF) and the delta-opioid receptor (DOR) colocalize in interneuron populations in the hilus of the dentate gyrus and stratum oriens of CA1 and CA3. While independent ultrastructural studies of DORs and CRF receptors suggest that each receptor is found in CA1 pyramidal cell dendrites and dendritic spines, whether DORs and CRF receptors colocalize in CA1 neuronal profiles has not been investigated. Here, hippocampal sections of adult male and proestrus female Sprague-Dawley rats were processed for dual label pre-embedding immunoelectron microscopy using well-characterized antisera directed against the DOR for immunoperoxidase and against the CRF receptor for immunogold. DOR-immunoreactivity (-ir) was found presynaptically in axons and axon terminals as well as postsynaptically in somata, dendrites and dendritic spines in stratum radiatum of CA1. In contrast, CRF receptor-ir was predominantly found postsynaptically in CA1 somata, dendrites, and dendritic spines. CRF receptor-ir frequently was observed in DOR-labeled dendritic profiles and primarily was found in the cytoplasm rather than at or near the plasma membrane. Quantitative analysis of CRF receptor-ir colocalization with DOR-ir in pyramidal cell dendrites revealed that proestrus females and males show comparable levels of CRF receptor-ir per dendrite and similar cytoplasmic density of CRF receptor-ir. In contrast, proestrus females display an increased number of dual-labeled dendritic profiles and an increased membrane density of CRF receptor-ir in comparison to males. We further examined the functional consequences of CRF receptor-ir colocalization with DOR-ir in the same neuron using the hormone responsive neuronal cell line NG108-15, which endogenously expresses DORs, and assayed intracellular cAMP production in response to CRF receptor and DOR agonists. Results demonstrated that short-term application of DOR agonist SNC80 inhibited CRF-induced cAMP accumulation in NG108-15 cells transfected with the CRF receptor. These studies provide new insights on opioid-stress system interaction in the hippocampus of both males and females and establish potential mechanisms through which DOR activation may influence CRF receptor activity.

Kappa opioids and the modulation of pain

BACKGROUND AND RATIONALE

Pain is a complex sensory experience, involving cognitive factors, environment (setting, society, and culture), experience, and gender and is modulated significantly by the central nervous system (CNS). The mechanisms by which opioid analgesics work are understood, but this class of drugs is not ideal as either an analgesic or anti-hyperalgesic. Accordingly, considerable effort continues to be directed at improved understanding of nociceptor function and development of selective analgesics that do not have the unwanted effects associated with opioid analgesics.

OBJECTIVE

The purpose of this paper is to provide a review of the role of KOP receptors in the modulation of pain and highlight several chemotypes currently being explored as peripherally restricted KOP ligands.

RESULTS

A growing body of literature has shown that KOP receptors are implicated in a variety of behavioral pain models. Several different classes of peripherally restricted peptidic and nonpeptidic KOP agonists have been identified and show utility in treating painful conditions.

CONCLUSION

The pharmacological profile of KOP agonists in visceral pain models suggest that peripherally restricted KOP agonists are potentially useful for a variety of peripheral pain states. Further, clinical investigation of peripherally restricted KOP agonists will help to clarify the painful conditions where KOP agonists will be most effective.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

Spirocyclic delta opioid receptor agonists for the treatment of pain: discovery of N,N-diethyl-3-hydroxy-4-(spiro[chromene-2,4'-piperidine]-4-yl) benzamide (ADL5747)

Selective, nonpeptidic delta opioid receptor agonists have been the subject of great interest as potential novel analgesic agents. The discoveries of BW373U86 (1) and SNC80 (2) contributed to the rapid expansion of research in this field. However, poor drug-like properties and low therapeutic indices have prevented clinical evaluation of these agents. Doses of 1 and 2 similar to those required for analgesic activity produce convulsions in rodents and nonhuman primates. Recently, we described a novel series of potent, selective, and orally bioavailable delta opioid receptor agonists. The lead derivative, ADL5859 (4), is currently in phase II proof-of-concept studies for the management of pain. Further structure activity relationship exploration has led to the discovery of ADL5747 (36), which is approximately 50-fold more potent than 4 in an animal model of inflammatory pain. On the basis of its favorable efficacy, safety, and pharmacokinetic profile, 36 was selected as a clinical candidate for the treatment of pain.

Food deprivation increases the mRNA expression of micro-opioid receptors in the ventral medial hypothalamus and arcuate nucleus

Activation of micro-opioid receptors makes animals hyperphagic and increases their preference for a high-fat diet. Previous studies have suggested that this receptor population plays a role in mediating the hyperphagia that is associated with food deprivation. In this paper, we tested the hypothesis that food deprivation will increase the expression of micro-opioid receptors in the ventral medial hypothalamus and arcuate nucleus (VMH/ARC). Food deprivation resulted in a significant increase in the mRNA expression of micro-opioid receptors in the VMH/ARC and the lateral hypothalamus (LH) after 48 h of fasting but not after 24 or 12 h of fasting in either the light or dark. We did not observe a change in the mRNA expression of kappa- or delta-opioid receptors after food deprivation. When food-deprived animals were given a choice between a low-fat diet and a high-fat diet, they were hyperphagic and consumed significantly more of the high-fat diet. When the micro-opioid receptors were blocked with beta-funaltrexamine (selective mu-opioid receptor antagonist), prior to giving food-deprived animals access to both a low-fat and high-fat diet, it significantly decreased the percentage of high-fat diet consumed. These data demonstrate that hypothalamic micro-opioid receptors may contribute to the hyperphagia and increased preference for a high-fat diet that is associated with food deprivation.

Anxiolytic- and antidepressant-like activities of H-Dmt-Tic-NH-CH(CH2-COOH)-Bid (UFP-512), a novel selective delta opioid receptor agonist

Knockout and pharmacological studies have shown that delta opioid peptide (DOP) receptor signalling regulates emotional responses. In the present study, the in vitro and in vivo pharmacological profile of the DOP ligand, H-Dmt-Tic-NH-CH(CH2-COOH)-Bid (UFP-512) was investigated. In receptor binding experiments performed on membranes of CHO cells expressing the human recombinant opioid receptors, UFP-512 displayed very high affinity (pKi 10.20) and selectivity (>150-fold) for DOP sites. In functional studies ([35S]GTP gamma S binding in CHOhDOP membranes and electrically stimulated mouse vas deferens) UFP-512 behaved as a DOP selective full agonist showing potency values more than 100-fold higher than DPDPE. In vivo, in the mouse forced swimming test, UFP-512 reduced immobility time both after intracerebroventricular (i.c.v.) and intraperitoneal (i.p.) administration. Similar effects were recorded in rats. Moreover, UFP-512 evoked anxiolytic-like effects in the mouse elevated plus maze and light-dark aversion assays. All these in vivo actions of UFP-512 were fully prevented by the selective DOP antagonist naltrindole (3 mg/kg, s.c.). In conclusion, the present findings demonstrate that UFP-512 behaves as a highly potent and selective agonist at DOP receptors and corroborate the proposal that the selective activation of DOP receptors elicits robust anxiolytic- and antidepressant-like effects in rodents.

Effects of delta-opioid receptor stimulation and inhibition on hippocampal survival in a rat model of forebrain ischaemia

BACKGROUND

It has been reported that delta-opioid (DOP) receptor agonists may be neuroprotective in the central nervous system. However, the DOP agonist [d-Ala(2), d-Leu(5)]enkephalin (DADLE) does not produce neuroprotection in severe forebrain ischaemia. The aim of this study was to examine the effects of DADLE on hippocampal neurone survival against less severe forebrain ischaemia.

METHODS

Intraperitoneal injection of DADLE (0 or 16 mg kg(-1)) in male Sprague-Dawley rats was performed 30 min before ischaemia. Severe (10 min), moderate (8 min), or mild (6 min) forebrain ischaemia was produced by bilateral carotid occlusion combined with hypotension (35 mm Hg) under isoflurane (1.5%) anaesthesia. Naltrindole (10 mg kg(-1)) (DOP antagonist) was administered 30 min before DADLE in order to confirm DOP receptor activation in the neuroprotective efficacy of DADLE. Naltrindole alone was also administered 30 min before ischaemia to examine endogenous DOP agonism as a self-protecting mechanism against ischaemia. All animals were evaluated neurologically and histologically after a 1 week recovery period.

RESULTS

DADLE improved neurone survival in hippocampal CA3 and dentate gyrus (DG) sectors. CA1 neurones were not protected against moderate and mild ischaemia. Naltrindole abolished DADLE neuroprotection in the CA3 and DG after both moderate and mild ischaemia. Interestingly, regardless of co-administration of DADLE, naltrindole significantly worsened neuronal injury in the CA1 region after mild ischaemia.

CONCLUSIONS

These results suggest that DADLE provides limited neuroprotection to relatively ischaemia-resistant regions but not to selectively vulnerable regions. This was probably mediated by DOP stimulation. Pre-ischaemic treatment with a DOP antagonist, regardless of co-administration of DADLE, worsened neuronal damage at the selectively vulnerable regions only after mild forebrain ischaemia. These data suggest that DOP activation with endogenous DOP ligand may be involved in self-protecting ischaemia-sensitive regions of the brain.

Increased expression of mu opioid receptors in animals susceptible to diet-induced obesity

Stimulation of mu opioid receptors preferentially increases the intake of a high fat diet. In this paper we investigated whether there was a difference in the expression of mu opioid receptors between animals susceptible (Osborne-Mendel) or resistant (S5B/Pl) to obesity induced by eating a high fat diet. Immunohistochemical studies demonstrated that Osborne-Mendel rats eating a chow diet had an increased number of mu opioid receptors in the arcuate nucleus when compared to S5B/Pl rats. These immunohistochemical findings were supported by Real Time-PCR which demonstrated that the mRNA level of mu opioid receptors was also increased in the hypothalamus of Osborne-Mendel rats compared to S5B/Pl rats. Low doses of the mu opioid receptor agonist DAMGO [d-Ala(2)-N-Me-Phe(4)-Glycol(5)]-enkephalin administered to Osborne-Mendel rats caused a significant increase in the preference for a diet high in fat. The same doses of DAMGO switched the diet preference of S5B/Pl rats to high fat but did not significantly increase food intake. The combination of these findings suggests that the increased levels of hypothalamic mu opioid receptors in Osborne-Mendel rats may contribute to their preference for a diet high in fat and increase their susceptibility to becoming obese.

Butorphanol dependence increases hippocampal kappa-opioid receptor gene expression

Butorphanol is a synthetic opioid agonist/antagonist analgesic agent, which exerts its effects mainly via kappa-opioid receptors. Characterizations of the gene expression levels of the mRNA for and protein levels of the kappa-opioid receptor in different brain regions of rats are essential for investigating possible mechanisms in the development of physical dependence on and withdrawal from butorphanol. Animals were rendered dependent by intracerebroventricular (i.c.v.) infusion of butorphanol (26 nmol/microl/hr) via osmotic minipumps for 3 days. Rats were sacrificed immediately (dependent group) or 7 hr after discontinuation of i.c.v. butorphanol infusion (withdrawal group). Expression levels of the mRNA for the kappa-opioid receptor, as detected by reverse transcription-polymerase chain reaction followed by Southern blot analysis, were significantly increased in the cerebral cortex, striatum, and midbrain, including thalamus, hippocampus, and pons, in animals dependent on butorphanol. In both dependent and withdrawal groups, Western blot analysis of kappa-opioid receptor protein levels showed significant increases in the amygdaloid nucleus, paraventricular thalamus, and thalamus. However, in the withdrawal group, there were significant decreases in the hippocampus and cortical regions, including the frontal, parietal, and temporal cortex. Regional changes in the mRNA for and protein levels of the kappa-opioid receptor focus attention on highly special roles for this receptor in the development of physical dependence on and the expression of withdrawal from butorphanol dependence.