Mu, delta, and kappa opioid receptor mRNA expression in the rat CNS: an in situ hybridization study

Influence of dynorphin on estradiol- and cervical stimulation-induced prolactin surges in ovariectomized rats

Prolactin is an anterior pituitary hormone necessary for fertility, pregnancy maintenance, lactation, and aspects of maternal behavior. In rodents, there is a surge of prolactin on the afternoon of proestrus, and a semi-circadian pattern of prolactin surges during early pregnancy, with a diurnal and nocturnal surge every day. Both of these patterns can be replicated in ovariectomized rats. A prior study demonstrated that central antagonism of κ-opioid receptors, the target of dynorphin, largely abolished the nocturnal prolactin surge in pregnant rats. We build on this to determine whether dynorphin, perhaps from the arcuate population that co-express kisspeptin, neurokinin B, and dynorphin (KNDy neurons), also contributes to the estradiol- or cervical stimulation-induced surges in ovariectomized rats. Ovariectomized rats were treated with either estradiol or cervical stimulation to induce prolactin surge(s). Blood samples were taken around the expected surge time to determine the effect of either acute κ-opioid receptor antagonism or previous chemical ablation of the KNDy population on prolactin levels. Dynorphin antagonism does significantly disrupt the nocturnal prolactin surge, but it does not contribute to the estradiol-induced surge. Chemical ablation of KNDy neurons had opposite effects; ablation of 40 % of the KNDy neurons had no impact on the nocturnal prolactin surge, while a somewhat larger ablation significantly reduced the size of the estradiol-induced surge. We conclude that dynorphin is likely a controlling factor for the nocturnal surge induced by cervical stimulation, and that other KNDy neuron products must play a role in the estradiol-induced surge.

Ablation of μ opioid receptor-expressing GABA neurons in rostromedial tegmental nucleus increases ethanol consumption and regulates ethanol-related behaviors

There has been increasing interest in the rostromedial tegmental nucleus (RMTg), given its potential regulatory role in many aversion-related behaviors. The RMTg contains mostly GABAergic neurons, sends a dense inhibitory projection to dopamine neurons in the midbrain, and is rich with μ-opioid receptors (MOR). Like most addictive drugs, ethanol has both aversive and rewarding properties. However, the cellular mechanisms underlying the effects of ethanol, particularly the aversive effect that limits its intake are not well understood. Recent studies have linked aversion with synaptic inhibition of dopamine neurons in the ventral tegmental area. To determine a potential role that the RMTg plays in the effect of ethanol, in this study, we employed a neurotoxin, dermorphin-saporin (DS), to lesion RMTg neurons prior to assessing ethanol-related behaviors. Rats were infused with DS bilaterally into the RMTg. This manipulation substantially increased the intake and preference for ethanol but not sucrose. It also reduced the number of neurons with MOR and glutamic acid decarboxylase 67 immunoreactivity within the RMTg. These changes did not occur after intra-RMTg infusion of blank saporin or vehicle. Importantly, intra-RMTg DS infusion significantly enhanced expression of conditioned place preference induced by ethanol (2 g/kg, i.p.), and slowed the extinction process. These results suggest that MOR-expressing GABAergic neurons in the RMTg contribute significantly to the regulation of ethanol consumption and related behaviors.

Noise-induced hearing loss: Neuropathic pain via Ntrk1 signaling

Severe noise-induced damage to the inner ear leads to auditory nerve fiber degeneration thereby reducing the neural input to the cochlear nucleus (CN). Paradoxically, this leads to a significant increase in spontaneous activity in the CN which has been linked to tinnitus, hyperacusis and ear pain. The biological mechanisms that lead to an increased spontaneous activity are largely unknown, but could arise from changes in glutamatergic or GABAergic neurotransmission or neuroinflammation. To test this hypothesis, we unilaterally exposed rats for 2h to a 126dB SPL narrow band noise centered at 12kHz. Hearing loss measured by auditory brainstem responses exceeded 55dB from 6 to 32kHz. The mRNA from the exposed CN was harvested at 14 or 28days post-exposure and qRT-PCR analysis was performed on 168 genes involved in neural inflammation, neuropathic pain and glutamatergic or GABAergic neurotransmission. Expression levels of mRNA of Slc17a6 and Gabrg3, involved in excitation and inhibition respectively, were significantly increased at 28days post-exposure, suggesting a possible role in the CN spontaneous hyperactivity associated with tinnitus and hyperacusis. In the pain and inflammatory array, noise exposure upregulated mRNA expression levels of four pain/inflammatory genes, Tlr2, Oprd1, Kcnq3 and Ntrk1 and decreased mRNA expression levels of two more genes, Ccl12 and Il1β. Pain/inflammatory gene expression changes via Ntrk1 signaling may induce sterile inflammation, neuropathic pain, microglial activation and migration of nerve fibers from the trigeminal, cuneate and vestibular nuclei into the CN. These changes could contribute to somatic tinnitus, hyperacusis and otalgia.

Methadone Reverses Analgesic Tolerance Induced by Morphine Pretreatment

BACKGROUND

Opiates such as morphine are the most powerful analgesics, but their protracted use is restrained by the development of tolerance to analgesic effects. Recent works suggest that tolerance to morphine might be due to its inability to promote mu opioid receptor endocytosis, and the co-injection of morphine with a mu opioid receptor internalizing agonist like [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]enkephalin reduces tolerance to morphine. So far, no studies have been conducted to evaluate the ability of methadone to reduce morphine tolerance in morphine-pretreated animals, a treatment sequence that could be encountered in opiate rotation protocol. We investigated the ability of methadone (a mu opioid receptor internalizing agonist used in therapy) to reverse morphine tolerance and the associated cellular mechanisms in the periaqueductal gray matter, a region involved in pain control.

METHODS

We measured analgesic response following a challenge dose of morphine in the hot plate test and investigated regulation of mu opioid receptor (coupling and endocytosis) and some cellular mechanisms involved in tolerance such as adenylate cyclase superactivation and changes in N-methyl-d-aspartate receptor subunits expression and phosphorylation state.

RESULTS

A chronic treatment with morphine promoted tolerance to its analgesic effects and was associated with a lack of mu opioid receptor endocytosis, adenylate cyclase overshoot, NR2A and NR2B downregulation, and phosphorylation of NR1. We reported that a methadone treatment in morphine-treated mice reversed morphine tolerance to analgesia by promoting mu opioid receptor endocytosis and blocking cellular mechanisms of tolerance.

CONCLUSIONS

Our data might lead to rational strategies to tackle opiate tolerance in the frame of opiate rotation.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

κ-Opioid Receptor Is Colocalized in GnRH and KNDy Cells in the Female Ovine and Rat Brain

Kisspeptin-neurokinin B-dynorphin (KNDy) cells of the hypothalamus are a key component in the neuroendocrine regulation of GnRH secretion. Evidence in sheep and other species suggests that dynorphin released by KNDy cells inhibits pulsatile GnRH secretion by acting upon κ-opioid receptors (KOR). However, the precise anatomical location and neurochemical phenotype of KOR-expressing cells in sheep remain unknown. To this end, we determined the distribution of KOR mRNA and protein in the brains of luteal phase ewes, using an ovine specific KOR mRNA probe for in situ hybridization and an antibody whose specificity we confirmed by Western blot analyses and blocking peptide controls. KOR cells were observed in a number of regions, including the preoptic area (POA); anterior hypothalamic area; supraoptic and paraventricular nuclei; ventromedial, dorsomedial, and lateral hypothalamus; and arcuate nucleus. Next, we determined whether KOR is colocalized in KNDy and/or GnRH cells. Dual-label immunofluorescence and confocal analysis of the KNDy population showed a high degree of colocalization, with greater than 90% of these neurons containing KOR. Surprisingly, GnRH cells also showed high levels of colocalization in sheep, ranging from 74.4% to 95.4% for GnRH cells in the POA and medial basal hypothalamus, respectively. Similarly, 97.4% of GnRH neurons in the POA of ovariectomized, steroid-primed female rats also contained immunoreactive KOR protein. These findings suggest that the inhibitory effects of dynorphin on pulsatile GnRH secretion may occur either indirectly by actions upon KOR within the KNDy population and/or directly via the activation of KOR on GnRH cells.

Nociceptin/Orphanin FQ Receptor Structure, Signaling, Ligands, Functions, and Interactions with Opioid Systems

The NOP receptor (nociceptin/orphanin FQ opioid peptide receptor) is the most recently discovered member of the opioid receptor family and, together with its endogenous ligand, N/OFQ, make up the fourth members of the opioid receptor and opioid peptide family. Because of its more recent discovery, an understanding of the cellular and behavioral actions induced by NOP receptor activation are less well developed than for the other members of the opioid receptor family. All of these factors are important because NOP receptor activation has a clear modulatory role on mu opioid receptor-mediated actions and thereby affects opioid analgesia, tolerance development, and reward. In addition to opioid modulatory actions, NOP receptor activation has important effects on motor function and other physiologic processes. This review discusses how NOP pharmacology intersects, contrasts, and interacts with the mu opioid receptor in terms of tertiary structure and mechanism of receptor activation; location of receptors in the central nervous system; mechanisms of desensitization and downregulation; cellular actions; intracellular signal transduction pathways; and behavioral actions with respect to analgesia, tolerance, dependence, and reward. This is followed by a discussion of the agonists and antagonists that have most contributed to our current knowledge. Because NOP receptors are highly expressed in brain and spinal cord and NOP receptor activation sometimes synergizes with mu receptor-mediated actions and sometimes opposes them, an understanding of NOP receptor pharmacology in the context of these interactions with the opioid receptors will be crucial to the development of novel therapeutics that engage the NOP receptor.

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Interhemispheric connections between the infralimbic and entorhinal cortices: The endopiriform nucleus has limbic connections that parallel the sensory and motor connections of the claustrum

We have previously shown that the claustrum is part of an interhemispheric circuit that interconnects somesthetic-motor and visual-motor cortical regions. The role of the claustrum in processing limbic information, however, is poorly understood. Some evidence suggests that the dorsal endopiriform nucleus (DEn), which lies immediately ventral to the claustrum, has connections with limbic cortical areas and should be considered part of a claustrum-DEn complex. To determine whether DEn has similar patterns of cortical connections as the claustrum, we used anterograde and retrograde tracing techniques to elucidate the connectivity of DEn. Following injections of retrograde tracers into DEn, labeled neurons appeared bilaterally in the infralimbic (IL) cortex and ipsilaterally in the entorhinal and piriform cortices. Anterograde tracer injections in DEn revealed labeled terminals in the same cortical regions, but only in the ipsilateral hemisphere. These tracer injections also revealed extensive longitudinal projections throughout the rostrocaudal extent of the nucleus. Dual retrograde tracer injections into IL and lateral entorhinal cortex (LEnt) revealed intermingling of labeled neurons in ipsilateral DEn, including many double-labeled neurons. In other experiments, anterograde and retrograde tracers were separately injected into IL of each hemisphere of the same animal. This revealed an interhemispheric circuit in which IL projects bilaterally to DEn, with the densest terminal labeling appearing in the contralateral hemisphere around retrogradely labeled neurons that project to IL in that hemisphere. By showing that DEn and claustrum have parallel sets of connections, these results suggest that DEn and claustrum perform similar functions in processing limbic and sensorimotor information, respectively. J. Comp. Neurol. 525:1363-1380, 2017. © 2016 Wiley Periodicals, Inc.

Dynorphin inhibits basal forebrain cholinergic neurons by pre- and postsynaptic mechanisms

KEY POINTS

The basal forebrain is an important component of the ascending arousal system and may be a key site through which the orexin neurons promote arousal. It has long been known that orexin-A and -B excite basal forebrain cholinergic neurons, but orexin-producing neurons also make the inhibitory peptide dynorphin. Using whole-cell recordings in brain slices, we found that dynorphin-A directly inhibits basal forebrain cholinergic neurons via κ-opioid receptors, and decreases afferent excitatory synaptic input to these neurons. While the effects of dynorphin-A and orexin-A desensitize over multiple applications, co-application of dynorphin-A and orexin-A produces a sustained response that reverses depending on the membrane potential of basal forebrain cholinergic neurons. At -40 mV the net effect of the co-application is inhibition by dynorphin-A, whereas at -70 mV the excitatory response to orexin-A prevails.

ABSTRACT

The basal forebrain (BF) is an essential component of the ascending arousal systems and may be a key site through which the orexin (also known as hypocretin) neurons drive arousal and promote the maintenance of normal wakefulness. All orexin neurons also make dynorphin, and nearly all brain regions innervated by the orexin neurons express kappa opiate receptors, the main receptor for dynorphin. This is remarkable because orexin excites target neurons including BF neurons, but dynorphin has inhibitory effects. We identified the sources of dynorphin input to the magnocellular preoptic nucleus and substantia innominata (MCPO/SI) in mice and determined the effects of dynorphin-A on MCPO/SI cholinergic neurons using patch-clamp recordings in brain slices. We found that the orexin neurons are the main source of dynorphin input to the MCPO/SI region, and dynorphin-A inhibits MCPO/SI cholinergic neurons through κ-opioid receptors by (1) activation of a G protein-coupled inwardly rectifying potassium current, (2) inhibition of a voltage-gated Ca(2+) current and (3) presynaptic depression of the glutamatergic input to these neurons. The responses both to dynorphin-A and to orexin-A desensitize, but co-application of dynorphin-A and orexin-A produces a sustained response. In addition, the polarity of the response to the co-application depends on the membrane potential of BF neurons; at -40 mV the net effect of the co-application is inhibition by dynorphin-A, whereas at -70 mV the excitatory response to orexin-A prevails. This suggests that depending on their state of activation, BF cholinergic neurons can be excited or inhibited by signals from the orexin neurons.

Delta Opioid Pharmacology in Relation to Alcohol Behaviors

Delta opioid receptors (DORs) are heavily involved in alcohol-mediated processes in the brain. In this chapter we provide an overview of studies investigating how alcohol directly impacts DOR pharmacology and of early studies indicating DOR modulation of alcohol behavior. We will offer a brief summary of the different animal species used in alcohol studies investigating DORs followed by a broader overview of the types of alcohol behaviors modulated by DORs. We will highlight a small set of studies investigating the relationship between alcohol and DORs in analgesia. We will then provide an anatomical overview linking DOR expression in specific brain regions to different alcohol behaviors. In this section, we will provide two models that try to explain how endogenous opioids acting at DORs may influence alcohol behaviors. Next, we will provide an overview of studies investigating certain new aspects of DOR pharmacology, including the formation of heteromers and biased signaling. Finally, we provide a short overview of the genetics of the DORs in relation to alcohol use disorders (AUDs) and a short statement on the potential of using DOR-based therapeutics for treatment of AUDs.

Delta Opioid Receptors: Learning and Motivation

Delta opioid receptor (DOR) displays a unique, highly conserved, structure and an original pattern of distribution in the central nervous system, pointing to a distinct and specific functional role among opioid peptide receptors. Over the last 15 years, in vivo pharmacology and genetic models have allowed significant advances in the understanding of this role. In this review, we will focus on the involvement of DOR in modulating different types of hippocampal- and striatal-dependent learning processes as well as motor function, motivation, and reward. Remarkably, DOR seems to play a key role in balancing hippocampal and striatal functions, with major implications for the control of cognitive performance and motor function under healthy and pathological conditions.

Generation of a KOR-Cre knockin mouse strain to study cells involved in kappa opioid signaling

The kappa opioid receptor (KOR) has numerous important roles in the nervous system including the modulation of mood, reward, pain, and itch. In addition, KOR is expressed in many non-neuronal tissues. However, the specific cell types that express KOR are poorly characterized. Here, we report the development of a KOR-Cre knockin allele, which provides genetic access to cells that express KOR. In this mouse, Cre recombinase (Cre) replaces the initial coding sequence of the Opkr1 gene (encoding the kappa opioid receptor). We demonstrate that the KOR-Cre allele mediates recombination by embryonic day 14.5 (E14.5). Within the brain, KOR-Cre shows expression in numerous areas including the cerebral cortex, nucleus accumbens and striatum. In addition, this allele is expressed in epithelium and throughout many regions of the body including the heart, lung, and liver. Finally, we reveal that KOR-Cre mediates recombination of a subset of bipolar and amacrine cells in the retina. Thus, the KOR-Cre mouse line is a valuable new tool for conditional gene manipulation to enable the study of KOR.

Sex Differences in Kappa Opioid Receptor Function and Their Potential Impact on Addiction

Behavioral, biological, and social sequelae that lead to drug addiction differ between men and women. Our efforts to understand addiction on a mechanistic level must include studies in both males and females. Stress, anxiety, and depression are tightly linked to addiction, and whether they precede or result from compulsive drug use depends on many factors, including biological sex. The neuropeptide dynorphin (DYN), an endogenous ligand at kappa opioid receptors (KORs), is necessary for stress-induced aversive states and is upregulated in the brain after chronic exposure to drugs of abuse. KOR agonists produce signs of anxiety, fear, and depression in laboratory animals and humans, findings that have led to the hypothesis that drug withdrawal-induced DYN release is instrumental in negative reinforcement processes that drive addiction. However, these studies were almost exclusively conducted in males. Only recently is evidence available that there are sex differences in the effects of KOR activation on affective state. This review focuses on sex differences in DYN and KOR systems and how these might contribute to sex differences in addictive behavior. Much of what is known about how biological sex influences KOR systems is from research on pain systems. The basic molecular and genetic mechanisms that have been discovered to underlie sex differences in KOR function in pain systems may apply to sex differences in KOR function in reward systems. Our goals are to discuss the current state of knowledge on how biological sex contributes to KOR function in the context of pain, mood, and addiction and to explore potential mechanisms for sex differences in KOR function. We will highlight evidence that the function of DYN-KOR systems is influenced in a sex-dependent manner by: polymorphisms in the prodynorphin (pDYN) gene, genetic linkage with the melanocortin-1 receptor (MC1R), heterodimerization of KORs and mu opioid receptors (MORs), and gonadal hormones. Finally, we identify several gaps in our understanding of “if” and “how” DYN and KORs modulate addictive behavior in a sex-dependent manner. Future work may address these gaps by building on the mechanistic studies outlined in this review. Ultimately this will enable the development of novel and effective addiction treatments tailored to either males or females.

Structural and functional interactions between six-transmembrane μ-opioid receptors and β2-adrenoreceptors modulate opioid signaling

The primary molecular target for clinically used opioids is the μ-opioid receptor (MOR). Besides the major seven-transmembrane (7TM) receptors, the MOR gene codes for alternatively spliced six-transmembrane (6TM) isoforms, the biological and clinical significance of which remains unclear. Here, we show that the otherwise exclusively intracellular localized 6TM-MOR translocates to the plasma membrane upon coexpression with β₂-adrenergic receptors (β₂-ARs) through an interaction with the fifth and sixth helices of β₂-AR. Coexpression of the two receptors in BE(2)-C neuroblastoma cells potentiates calcium responses to a 6TM-MOR ligand, and this calcium response is completely blocked by a selective β₂-antagonist in BE(2)-C cells, and in trigeminal and dorsal root ganglia. Co-administration of 6TM-MOR and β₂-AR ligands leads to substantial analgesic synergy and completely reverses opioid-induced hyperalgesia in rodent behavioral models. Together, our results provide evidence that the heterodimerization of 6TM-MOR with β₂-AR underlies a molecular mechanism for 6TM cellular signaling, presenting a unique functional responses to opioids. This signaling pathway may contribute to the hyperalgesic effects of opioids that can be efficiently blocked by β₂-AR antagonists, providing a new avenue for opioid therapy.

Role of dorsal hippocampal orexin-1 receptors in memory restoration induced by morphine sensitization phenomenon

The present study was examined the blockade of CA1 orexin-1 receptors (OX1Rs) of the dorsal hippocampus in the induction or expression phase on morphine sensitization-induced memory restoration using the Morris water maze (MWM) apparatus. Results showed that pre-training administration of morphine (5mg/kg, s.c.) increases escape latency and traveled distance, while does not alter swimming speed. This supports the impairing effect of morphine on the spatial memory acquisition in male adult rats. Also, in the retrieval session (probe trial) this treatment decreased the time spent in the target quadrant. Moreover, morphine-induced sensitization (15 or 20mg/kg, s.c.; once daily for 3days and followed by 5days no drug treatment) restored the memory acquisition/retrieval deficit which had been induced by pre-training administration of morphine (5mg/kg, s.c.). Intra-CA1 microinjection of subthreshold doses of SB-334867 (OX1Rs antagonist; 10, 20 and 40nmol/rat), 5min before morphine (20mg/kg/day×3days, s.c.; induction phase for morphine sensitization) did not alter restoration of memory acquisition/retrieval produced by the morphine sensitization phenomenon. In contrast, microinjection of subthreshold doses of SB-334867 (10, 20 and 40nmol/rat) into the CA1 region in the training session, 5min prior to morphine (5mg/kg, s.c.; expression phase for morphine sensitization) blocked the spatial memory acquisition/retrieval in morphine-sensitized rats. In conclusion, these findings show that morphine sensitization reverses morphine-induced amnesia. Furthermore, the blockade of CA1 OX1Rs in the expression phase, but not in the induction phase, disrupts memory restoration induced by morphine sensitization.

Activation of κ Opioid Receptors in Cutaneous Nerve Endings by Conorphin-1, a Novel Subtype-Selective Conopeptide, Does Not Mediate Peripheral Analgesia

Selective activation of peripheral κ opioid receptors (KORs) may overcome the dose-limiting adverse effects of conventional opioid analgesics. We recently developed a vicinal disulfide-stabilized class of peptides with subnanomolar potency at the KOR. The aim of this study was to assess the analgesic effects of one of these peptides, named conorphin-1, in comparison with the prototypical KOR-selective small molecule agonist U-50488, in several rodent pain models. Surprisingly, neither conorphin-1 nor U-50488 were analgesic when delivered peripherally by intraplantar injection at local concentrations expected to fully activate the KOR at cutaneous nerve endings. While U-50488 was analgesic when delivered at high local concentrations, this effect could not be reversed by coadministration with the selective KOR antagonist ML190 or the nonselective opioid antagonist naloxone. Instead, U-50488 likely mediated its peripheral analgesic effect through nonselective inhibition of voltage-gated sodium channels, including peripheral sensory neuron isoforms NaV1.8 and NaV1.7. Our study suggests that targeting the KOR in peripheral sensory nerve endings innervating the skin is not an alternative analgesic approach.

Agonist Binding and Desensitization of the μ-Opioid Receptor Is Modulated by Phosphorylation of the C-Terminal Tail Domain

Sustained activation of G protein-coupled receptors can lead to a rapid decline in signaling through acute receptor desensitization. In the case of the μ-opioid receptor (MOPr), this desensitization may play a role in the development of analgesic tolerance. It is understood that phosphorylation of MOPr promotes association with β-arrestin proteins, which then facilitates desensitization and receptor internalization. Agonists that induce acute desensitization have been shown to induce a noncanonical high-affinity agonist binding state in MOPr, conferring a persistent memory of prior receptor activation. In the current study, live-cell confocal imaging was used to investigate the role of receptor phosphorylation in agonist binding to MOPr. A phosphorylation cluster in the C-terminal tail of MOPr was identified as a mediator of agonist-induced affinity changes in MOPr. This site is unique from the primary phosphorylation cluster responsible for β-arrestin binding and internalization. Electrophysiologic measurements of receptor function suggest that both phosphorylation clusters may play a parallel role during acute receptor desensitization. Desensitization was unaffected by alanine mutation of either phosphorylation cluster, but was largely eliminated when both clusters were mutated. Overall, this work suggests that there are multiple effects of MOPr phosphorylation that appear to regulate MOPr function: one affecting β-arrestin binding and a second affecting agonist binding.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Role of steroid hormones and morphine treatment in the modulation of opioid receptor gene expression in brain structures in the female rat

This study determined the effects of acute treatment with morphine on the expression of the Oprm1, Oprk1, and Oprd1 genes (which encode μ, κ, and δ receptors, respectively) in the striatum, hypothalamus, and periaqueductal gray (PAG) in ovariectomized female rats treated with estrogen. Ovariectomized female rats were divided into five equal groups. Two groups received estrogen (50 µg/kg, 54 h before testing) and saline (ES group) or 3.5 mg/kg morphine (EM group) 2 h before euthanasia. The SS group received saline solution 54 and 2 h before the experiments. The SM group received saline 54 h and 3.5 mg/kg morphine 2 h before the experiments. The W group remained undisturbed. The genes expression were evaluated. Oprm1 and Oprk1 expression were activated, respectively, in the hypothalamus and PAG and in the striatum and PAG by morphine only in estrogen-treated animals. Oprd1 expression in the hypothalamus and PAG was activated by morphine in both estrogen-treated and -nontreated animals. The Oprm1 and Oprk1 gene response to morphine might depend on estrogen, whereas the Oprd1 gene response to morphine might not depend on estrogen, supporting the hypothesis of a functional role for ovarian hormones in opioid receptor-mediated functional adaptations in the female brain.

Dual effect of morphine in long-term social memory in rat

BACKGROUND AND PURPOSE

Bimodal dose-response relationships have been demonstrated in animals and humans following morphine administration. We examined if systemic administration of morphine, in extremely low (μg) and high (mg, analgesic) doses, changed the learning process.

EXPERIMENTAL APPROACH

In the social learning test, an adult rat investigates a juvenile. The juvenile is submitted to a second encounter after a few days and investigation by the adult should be reduced. Morphine was administered before the first encounter between rats, and the critical test was performed 24, 72 or 168 h later, when animals were re-exposed to each other, in the absence of morphine.

KEY RESULTS

Low doses of morphine, comparable with endogenous brain concentrations, enhanced long-term memory recognition; while high doses did the reverse, indicating the adult failed to recognize the juvenile. Recognition of a familiar rat appeared to be mediated within the brain accessory olfactory bulb (AOB) by an opioid system intrinsic to the olfactory system through μ-opioid receptors (MORs). At this supraspinal site, the PLC/PKC signalling pathway was activated by extremely low morphine doses.

CONCLUSIONS AND IMPLICATIONS

Morphine treatment administration may either disrupt or facilitate social memory, depending on the dose, extending to memory formation the bimodal effects of morphine previously shown in pain. Social memory formation elicited by extremely low morphine doses, was mediated within the AOB by an opioid system, intrinsic to the olfactory system through MORs.

The opioid receptors as targets for drug abuse medication

The endogenous opioid system is largely expressed in the brain, and both endogenous opioid peptides and receptors are present in areas associated with reward and motivation. It is well known that this endogenous system plays a key role in many aspects of addictive behaviours. The present review summarizes the modifications of the opioid system induced by chronic treatment with drugs of abuse reported in preclinical and clinical studies, as well as the action of opioid antagonists and agonists on the reinforcing effects of drugs of abuse, with therapeutic perspectives. We have focused on the effects of chronic psychostimulants, alcohol and nicotine exposure. Taken together, the changes in both opioid peptides and opioid receptors in different brain structures following acute or chronic exposure to these drugs of abuse clearly identify the opioid system as a potential target for the development of effective pharmacotherapy for the treatment of addiction and the prevention of relapse.

Acute morphine affects the rat circadian clock via rhythms of phosphorylated ERK1/2 and GSK3β kinases and Per1 expression in the rat suprachiasmatic nucleus

BACKGROUND AND PURPOSE

Opioids affect the circadian clock and may change the timing of many physiological processes. This study was undertaken to investigate the daily changes in sensitivity of the circadian pacemaker to an analgesic dose of morphine, and to uncover a possible interplay between circadian and opioid signalling.

EXPERIMENTAL APPROACH

A time-dependent effect of morphine (1 mg·kg(-1) , i.p.) applied either during the day or during the early night was followed, and the levels of phosphorylated ERK1/2, GSK3β, c-Fos and Per genes were assessed by immunohistochemistry and in situ hybridization. The effect of morphine pretreatment on light-induced pERK and c-Fos was examined, and day/night difference in activity of opioid receptors was evaluated by [(35) S]-GTPγS binding assay.

KEY RESULTS

Morphine stimulated a rise in pERK1/2 and pGSK3β levels in the suprachiasmatic nucleus (SCN) when applied during the day but significantly reduced both kinases when applied during the night. Morphine at night transiently induced Period1 but not Period2 in the SCN and did not attenuate the light-induced level of pERK1/2 and c-Fos in the SCN. The activity of all three principal opioid receptors was high during the day but decreased significantly at night, except for the δ receptor. Finally, we demonstrated daily profiles of pERK1/2 and pGSK3β levels in the rat ventrolateral and dorsomedial SCN.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that the phase-shifting effect of opioids may be mediated via post-translational modification of clock proteins by means of activated ERK1/2 and GSK3β.

Dual κ-agonist/μ-antagonist opioid receptor modulation reduces levodopa-induced dyskinesia and corrects dysregulated striatal changes in the nonhuman primate model of Parkinson disease

OBJECTIVE

Effective medical management of levodopa-induced dyskinesia (LID) remains an unmet need for patients with Parkinson disease (PD). Changes in opioid transmission in the basal ganglia associated with LID suggest a therapeutic opportunity. Here we determined the impact of modulating both mu and kappa opioid receptor signaling using the mixed agonist/antagonist analgesic nalbuphine in reducing LID and its molecular markers in the nonhuman primate model.

METHODS

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated macaques with advanced parkinsonism and reproducible LID received a range of nalbuphine doses or saline subcutaneously as: (1) monotherapy, (2) acute coadministration with levodopa, and (3) chronic coadministration for 1 month. Animals were assessed by blinded examiners for motor disability and LID severity using standardized rating scales. Plasma levodopa levels were determined with and without nalbuphine, and postmortem brain samples were subjected to Western blot analyses.

RESULTS

Nalbuphine reduced LID in a dose-dependent manner by 48% (p < 0.001) without compromising the anti-PD effect of levodopa or changing plasma levodopa levels. There was no tolerance to the anti-LID effect of nalbuphine given chronically. Nalbuphine coadministered with levodopa was well tolerated and did not cause sedation. Nalbuphine monotherapy had no effect on motor disability. Striatal tissue analyses showed that nalbuphine cotherapy blocks several molecular correlates of LID, including overexpression of ΔFosB, prodynorphin, dynorphin A, cyclin-dependent kinase 5, and increased phosphorylation of DARPP-32 at threonine-34.

INTERPRETATION

Nalbuphine reverses the molecular milieu in the striatum associated with LID and is a safe and effective anti-LID agent in the primate model of PD. These findings support repurposing this analgesic for the treatment of LID.

Types of skin afferent fibers and spinal opioid receptors that contribute to touch-induced inhibition of heart rate changes evoked by noxious cutaneous heat stimulation

Background

In anesthetized rats and conscious humans, a gentle touch using a soft disc covered with microcones (with a texture similar to that of a finger), but not with a flat disc, inhibits nociceptive somatocardiac reflexes. Such an inhibitory effect is most reliably evoked when touch is applied to the skin ipsilateral and closest to nociceptive inputs. However, the mechanism of this inhibition is not completely elucidated. We aimed to clarify the types of cutaneous afferent fibers and spinal opioid receptors that contribute to antinociceptive effects of microcone touch.

Results

The present study comprised two experiments with urethane-anesthetized rats. In the first experiment, unitary activity of skin afferent fibers was recorded from the saphenous nerve, and responses to a 10-min touch using a microcone disc and a flat disc (control) were compared. Greater discharge rate during microcone touch was observed in low-threshold mechanoreceptive Aδ and C afferent units, whereas many Aβ afferents responded similarly to the two types of touch. In the second experiment, the effect of an intrathecal injection of opioid receptor antagonists on the inhibitory effects of microcone touch on heart rate responses to noxious heat stimulation was examined. The magnitude of the heart rate response was significantly reduced by microcone touch in rats that received saline or naltrindole (δ-opioid receptor antagonist) injections. However, such an inhibition was not observed in rats that received naloxone (non-selective opioid receptor antagonist) or Phe-Cys-Tyr-Trp-Orn-Thr-Pen-Thr-NH₂ (CTOP; μ-opioid receptor antagonist) injections.

Conclusions

Microcone touch induced greater responses of low-threshold mechanoreceptive Aδ and C afferent units than control touch. The antinociceptive effect of microcone touch was abolished by intrathecal injection of μ-opioid receptor antagonist. These results suggest that excitation of low-threshold mechanoreceptive Aδ and C afferents produces the release of endogenous μ-opioid ligands in the spinal cord, resulting in the inhibition of nociceptive transmission that contributes to somatocardiac reflexes.

Morphological elucidation of basal ganglia circuits contributing reward prediction

Electrophysiological studies in monkeys have shown that dopaminergic neurons respond to the reward prediction error. In addition, striatal neurons alter their responsiveness to cortical or thalamic inputs in response to the dopamine signal, via the mechanism of dopamine-regulated synaptic plasticity. These findings have led to the hypothesis that the striatum exhibits synaptic plasticity under the influence of the reward prediction error and conduct reinforcement learning throughout the basal ganglia circuits. The reinforcement learning model is useful; however, the mechanism by which such a process emerges in the basal ganglia needs to be anatomically explained. The actor–critic model has been previously proposed and extended by the existence of role sharing within the striatum, focusing on the striosome/matrix compartments. However, this hypothesis has been difficult to confirm morphologically, partly because of the complex structure of the striosome/matrix compartments. Here, we review recent morphological studies that elucidate the input/output organization of the striatal compartments.

Determining pharmacological selectivity of the kappa opioid receptor antagonist LY2456302 using pupillometry as a translational biomarker in rat and human

BACKGROUND

Selective kappa opioid receptor antagonism is a promising experimental strategy for the treatment of depression. The kappa opioid receptor antagonist, LY2456302, exhibits ~30-fold higher affinity for kappa opioid receptors over mu opioid receptors, which is the next closest identified pharmacology.

METHODS

Here, we determined kappa opioid receptor pharmacological selectivity of LY2456302 by assessing mu opioid receptor antagonism using translational pupillometry in rats and humans.

RESULTS

In rats, morphine-induced mydriasis was completely blocked by the nonselective opioid receptor antagonist naloxone (3mg/kg, which produced 90% mu opioid receptor occupancy), while 100 and 300 mg/kg LY2456302 (which produced 56% and 87% mu opioid receptor occupancy, respectively) only partially blocked morphine-induced mydriasis. In humans, fentanyl-induced miosis was completely blocked by 50mg naltrexone, and LY2456302 dose-dependently blocked miosis at 25 and 60 mg (minimal-to-no blockade at 4-10mg).

CONCLUSIONS

We demonstrate, for the first time, the use of translational pupillometry in the context of receptor occupancy to identify a clinical dose of LY2456302 achieving maximal kappa opioid receptor occupancy without evidence of significant mu receptor antagonism.

Orexin/hypocretin role in reward: implications for opioid and other addictions

Addiction is a devastating disorder that affects 15.3 million people worldwide. While prevalent, few effective treatments exist. Orexin receptors have been proposed as a potential target for anti-craving medications. Orexins, also known as hypocretins, are neuropeptides produced in neurons of the lateral and dorsomedial hypothalamus and perifornical area, which project widely throughout the brain. The absence of orexins in rodents and humans leads to narcolepsy. However, orexins also have an established role in reward seeking. This review will discuss some of the original studies describing the roles of the orexins in reward seeking as well as specific works that were presented at the 2013 International Narcotics Research Conference. Orexin signalling can promote drug-induced plasticity of glutamatergic synapses onto dopamine neurons of the ventral tegmental area (VTA), a brain region implicated in motivated behaviour. Additional evidence suggests that orexin signalling can also promote drug seeking by initiating an endocannabinoid-mediated synaptic depression of GABAergic inputs to the VTA, and thereby disinhibiting dopaminergic neurons. Orexin neurons co-express the inhibitory opioid peptide dynorphin. It has been proposed that orexin in the VTA may not mediate reward per se, but rather occludes the 'anti-reward' effects of dynorphin. Finally, orexin signalling in the prefrontal cortex and the central amygdala is implicated in reinstatement of reward seeking. This review will highlight recent work describing the role of orexin signalling in cellular processes underlying addiction-related behaviours and propose novel hypotheses for the mechanisms by which orexin signalling may impart drug seeking.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Synthesis and κ-opioid receptor activity of furan-substituted salvinorin A analogues

The neoclerodane diterpene salvinorin A, found in the leaves of Salvia divinorum, is a potent κ-opioid receptor agonist, making it an attractive scaffold for development into a treatment for substance abuse. Although several successful semisynthetic studies have been performed to elucidate structure-activity relationships, the lack of analogues with substitutions to the furan ring of salvinorin A has prevented a thorough understanding of its role in binding to the κ-opioid receptor. Herein we report the synthesis of several salvinorin A derivatives with modified furan rings. Evaluation of these compounds in a functional assay indicated that sterically less demanding substitutions are preferred, suggesting the furan ring is bound in a congested portion of the binding pocket. The most potent of the analogues successfully reduced drug-seeking behavior in an animal model of drug-relapse without producing the sedation observed with other κ-opioid agonists.

Preprodynorphin-expressing neurons constitute a large subgroup of somatostatin-expressing GABAergic interneurons in the mouse neocortex

Dynorphins, leumorphin, and neoendorphins are preprodynorphin (PPD)-derived peptides and ligands for κ-opioid receptors. Using an antibody to PPD C-terminal, we investigated the chemical and molecular characteristics of PPD-expressing neurons in mouse neocortex. PPD-immunopositive neuronal somata were distributed most frequently in layer 5 and less frequently in layers 2-4 and 6 throughout neocortical regions. Combined labeling of immunofluorescence and fluorescent mRNA signals revealed that almost all PPD-immunopositive neurons expressed glutamic acid decarboxylase but not vesicular glutamate transporter, indicating their γ-aminobutyric acid (GABA)ergic characteristics, and that PPD-immunopositive neurons accounted for 15% of GABAergic interneurons in the primary somatosensory area. As GABAergic interneurons were divided into several groups by specific markers, we further examined the chemical characteristics of PPD-expressing neurons by the double immunofluorescence labeling method. More than 95% of PPD-immunopositive neurons were also somatostatin (SOM)-immunopositive in the primary somatosensory, primary motor, orbitofrontal, and primary visual areas, but only 24% were SOM-immunopositive in the medial prefrontal cortex. In the primary somatosensory area, PPD-immunopositive neurons constituted 50%, 79%, 55%, and 17% of SOM-immunopositive neurons in layers 2-3, 4, 5, and 6, respectively. Although SOM-expressing neurons contained calretinin-, neuropeptide Y-, nitric oxide synthase-, and reelin-expressing neurons as subgroups, only reelin immunoreactivity was detected in many PPD-immunopositive neurons. These results indicate that PPD-expressing neurons constitute a large subgroup of SOM-expressing cortical interneurons, and the PPD/SOM-expressing GABAergic neurons might serve not only as inhibitory elements in the local cortical circuit, but also as modulators for cortical neurons expressing κ-opioid and/or SOM receptors.

Cannabinoid CB2 receptors modulate midbrain dopamine neuronal activity and dopamine-related behavior in mice

Cannabinoid CB2 receptors (CB2Rs) have been recently reported to modulate brain dopamine (DA)-related behaviors; however, the cellular mechanisms underlying these actions are unclear. Here we report that CB2Rs are expressed in ventral tegmental area (VTA) DA neurons and functionally modulate DA neuronal excitability and DA-related behavior. In situ hybridization and immunohistochemical assays detected CB2 mRNA and CB2R immunostaining in VTA DA neurons. Electrophysiological studies demonstrated that activation of CB2Rs by JWH133 or other CB2R agonists inhibited VTA DA neuronal firing in vivo and ex vivo, whereas microinjections of JWH133 into the VTA inhibited cocaine self-administration. Importantly, all of the above findings observed in WT or CB1(-/-) mice are blocked by CB2R antagonist and absent in CB2(-/-) mice. These data suggest that CB2R-mediated reduction of VTA DA neuronal activity may underlie JWH133's modulation of DA-regulated behaviors.

Differential expressions of the alternatively spliced variant mRNAs of the µ opioid receptor gene, OPRM1, in brain regions of four inbred mouse strains

The µ opioid receptor gene, OPRM1, undergoes extensive alternative pre-mRNA splicing in rodents and humans, with dozens of alternatively spliced variants of the OPRM1 gene. The present studies establish a SYBR green quantitative PCR (qPCR) assay to more accurately quantify mouse OPRM1 splice variant mRNAs. Using these qPCR assays, we examined the expression of OPRM1 splice variant mRNAs in selected brain regions of four inbred mouse strains displaying differences in µ opioid-induced tolerance and physical dependence: C56BL/6J, 129P3/J, SJL/J and SWR/J. The complete mRNA expression profiles of the OPRM1 splice variants reveal marked differences of the variant mRNA expression among the brain regions in each mouse strain, suggesting region-specific alternative splicing of the OPRM1 gene. The expression of many variants was also strain-specific, implying a genetic influence on OPRM1 alternative splicing. The expression levels of a number of the variant mRNAs in certain brain regions appear to correlate with strain sensitivities to morphine analgesia, tolerance and physical dependence in four mouse strains.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

Frequency-dependent, cell type-divergent signaling in the hippocamposeptal projection

Hippocampal oscillations are critical for information processing, and are strongly influenced by inputs from the medial septum. Hippocamposeptal neurons provide direct inhibitory feedback from the hippocampus onto septal cells, and are therefore likely to also play an important role in the circuit; these neurons fire at either low or high frequency, reflecting hippocampal network activity during theta oscillations or ripple events, respectively. Here, we optogenetically target the long-range GABAergic projection from the hippocampus to the medial septum in rats, and thereby simulate hippocampal input onto downstream septal cells in an acute slice preparation. In response to optogenetic activation of hippocamposeptal fibers at theta and ripple frequencies, we elicit postsynaptic GABAergic responses in a subset (24%) of septal cells, most predominantly in fast-spiking cells. In addition, in another subset of septal cells (19%) corresponding primarily to cholinergic cells, we observe a slow hyperpolarization of the resting membrane potential and a decrease in input resistance, particularly in response to prolonged high-frequency (ripple range) stimulation. This slow response is partially sensitive to GIRK channel and D2 dopamine receptor block. Our results suggest that two independent populations of septal cells distinctly encode hippocampal feedback, enabling the septum to monitor ongoing patterns of activity in the hippocampus.

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.

Butorphanol suppression of histamine itch is mediated by nucleus accumbens and septal nuclei: a pharmacological fMRI study

Opioid receptors in the central nervous system are important modulators of itch transmission. In this study, we examined the effect of mixed-action opioid butorphanol on histamine itch, cowhage itch and heat pain in healthy volunteers. Using functional MRI, we investigated significant changes in cerebral perfusion to identify the critical brain centers mediating the antipruritic effect of butorphanol. Butorphanol suppressed the itch induced experimentally with histamine, reduced the intensity of cowhage itch by approximately 35%, and did not affect heat pain sensitivity. In comparison with the placebo, butorphanol produced a bilateral deactivation of claustrum, insula and putamen, areas activated during itch processing. Analysis of cerebral perfusion patterns of brain processing of itch vs. itch inhibition under the effect of the drug, revealed that the reduction of cowhage itch by butorphanol was correlated with changes in cerebral perfusion in the midbrain, thalamus, S1, insula and cerebellum. The suppression of histamine itch by butorphanol was paralleled by the activation of nucleus accumbens and septal nuclei, structures expressing high levels of kappa opioid receptors. In conclusion, important relays of the mesolimbic circuit were involved in the inhibition of itch by butorphanol and could represent potential targets for the development of antipruritic therapy.

Specific regions display altered grey matter volume in μ-opioid receptor knockout mice: MRI voxel-based morphometry

BACKGROUND AND PURPOSE

μ Opioid receptor knockout (MOP-KO) mice display several behavioural differences from wild-type (WT) littermates including differential responses to nociceptive stimuli. Brain structural changes have been tied to behavioural alterations noted in transgenic mice with targeting of different genes. Hence, we assess the brain structure of MOP-KO mice.

EXPERIMENTAL APPROACH

Magnetic resonance imaging (MRI) voxel-based morphometry (VBM) and histological methods were used to identify structural differences between extensively backcrossed MOP-KO mice and WT mice.

KEY RESULTS

MOP-KO mice displayed robust increases in regional grey matter volume in olfactory bulb, several hypothalamic nuclei, periaqueductal grey (PAG) and several cerebellar areas, most confirmed by VBM analysis. The largest increases in grey matter volume were detected in the glomerular layer of the olfactory bulb, arcuate nucleus of hypothalamus, ventrolateral PAG (VLPAG) and cerebellar regions including paramedian and cerebellar lobules. Histological analyses confirm several of these results, with increased VLPAG cell numbers and increased thickness of the olfactory bulb granule cell layer and cerebellar molecular and granular cell layers.

CONCLUSIONS AND IMPLICATIONS

MOP deletion causes previously undescribed structural changes in specific brain regions, but not in all regions with high MOP receptor densities (e.g. thalamus, nucleus accumbens) or that exhibit adult neurogenesis (e.g. hippocampus). Volume differences in hypothalamus and PAG may reflect behavioural changes including hyperalgesia. Although the precise relationship between volume change and MOP receptor deletion was not determined from this study alone, these findings suggest that levels of MOP receptor expression may influence a broader range of neural structure and function in humans than previously supposed.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Short term morphine exposure in vitro alters proliferation and differentiation of neural progenitor cells and promotes apoptosis via mu receptors

BACKGROUND

Chronic morphine treatment inhibits neural progenitor cell (NPC) progression and negatively effects hippocampal neurogenesis. However, the effect of acute opioid treatment on cell development and its influence on NPC differentiation and proliferation in vitro is unknown. We aim to investigate the effect of a single, short term exposure of morphine on the proliferation, differentiation and apoptosis of NPCs and the mechanism involved.

METHODS

Cell cultures from 14-day mouse embryos were exposed to different concentrations of morphine and its antagonist naloxone for 24 hours and proliferation, differentiation and apoptosis were studied. Proliferating cells were labeled with bromodeoxyuridine (BrdU) and cell fate was studied with immunocytochemistry.

RESULTS

Cells treated with morphine demonstrated decreased BrdU expression with increased morphine concentrations. Analysis of double-labeled cells showed a decrease in cells co-stained for BrdU with nestin and an increase in cells co-stained with BrdU and neuron-specific class III β-tubuline (TUJ1) in a dose dependent manner. Furthermore, a significant increase in caspase-3 activity was observed in the nestin- positive cells. Addition of naloxone to morphine-treated NPCs reversed the anti-proliferative and pro-apoptotic effects of morphine.

CONCLUSIONS

Short term morphine exposure induced inhibition of NPC proliferation and increased active caspase-3 expression in a dose dependent manner. Morphine induces neuronal and glial differentiation and decreases the expression of nestin- positive cells. These effects were reversed with the addition of the opioid antagonist naloxone. Our results demonstrate the effects of short term morphine administration on the proliferation and differentiation of NPCs and imply a mu-receptor mechanism in the regulation of NPC survival.

Recent advances on the δ opioid receptor: from trafficking to function

Within the opioid family of receptors, δ (DOPrs) and μ opioid receptors (MOPrs) are typical GPCRs that activate canonical second-messenger signalling cascades to influence diverse cellular functions in neuronal and non-neuronal cell types. These receptors activate well-known pathways to influence ion channel function and pathways such as the map kinase cascade, AC and PI3K. In addition new information regarding opioid receptor-interacting proteins, downstream signalling pathways and resultant functional effects has recently come to light. In this review, we will examine these novel findings focusing on the DOPr and, in doing so, will contrast and compare DOPrs with MOPrs in terms of differences and similarities in function, signalling pathways, distribution and interactions. We will also discuss and clarify issues that have recently surfaced regarding the expression and function of DOPrs in different cell types and analgesia.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Dopamine D3 receptor dysfunction prevents anti-nociceptive effects of morphine in the spinal cord

Dopamine (DA) modulates spinal reflexes, including nociceptive reflexes, in part via the D3 receptor subtype. We have previously shown that mice lacking the functional D3 receptor (D3KO) exhibit decreased paw withdrawal latencies from painful thermal stimuli. Altering the DA system in the CNS, including D1 and D3 receptor systems, reduces the ability of opioids to provide analgesia. Here, we tested if the increased pain sensitivity in D3KO might result from a modified μ-opioid receptor (MOR) function at the spinal cord level. As D1 and D3 receptor subtypes have competing cellular effects and can form heterodimers, we tested if the changes in MOR function may be mediated in D3KO through the functionally intact D1 receptor system. We assessed thermal paw withdrawal latencies in D3KO and wild type (WT) mice before and after systemic treatment with morphine, determined MOR and phosphorylated MOR (p-MOR) protein expression levels in lumbar spinal cords, and tested the functional effects of DA and MOR receptor agonists in the isolated spinal cord. In vivo, a single morphine administration (2 mg/kg) increased withdrawal latencies in WT but not D3KO, and these differential effects were mimicked in vitro, where morphine modulated spinal reflex amplitudes (SRAs) in WT but not D3KO. Total MOR protein expression levels were similar between WT and D3KO, but the ratio of pMOR/total MOR was higher in D3KO. Blocking D3 receptors in the isolated WT cord precluded morphine's inhibitory effects observed under control conditions. Lastly, we observed an increase in D1 receptor protein expression in the lumbar spinal cord of D3KO. Our data suggest that the D3 receptor modulates the MOR system in the spinal cord, and that a dysfunction of the D3 receptor can induce a morphine-resistant state. We propose that the D3KO mouse may serve as a model to study the onset of morphine resistance at the spinal cord level, the primary processing site of the nociceptive pathway.