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

Alleviating pain with delta opioid receptor agonists: evidence from experimental models

The use of opioids for the relief of pain and headache disorders has been studied for years. Nowadays, particularly because of its ability to produce analgesia in various pain models, delta opioid receptor (DOPr) emerges as a promising target for the development of new pain therapies. Indeed, their potential to avoid the unwanted effects commonly observed with clinically used opioids acting at the mu opioid receptor (MOPr) suggests that DOPr agonists could be a therapeutic option. In this review, we discuss the use of opioids in the management of pain in addition to describing the evidence of the analgesic potency of DOPr agonists in animal models.

The effects of estrogen on temporomandibular joint pain as influenced by trigeminal caudalis neurons

The signs and symptoms of persistent temporomandibular joint (TMJ)/muscle disorder (TMJD) pain suggest the existence of a central neural dysfunction or a problem of pain amplification. The etiology of chronic TMJD is not known; however, female sex hormones have been identified as significant risk factors. Converging lines of evidence indicate that the junctional region between the trigeminal subnucleus caudalis (Vc) and the upper cervical spinal cord, termed the Vc/C1-2 region, is the primary site for the synaptic integration of sensory input from TMJ nociceptors. In this paper, the mechanisms behind the estrogen effects on the processing of nociceptive inputs by neurons in the Vc/C1-2 region reported by human and animal studies are reviewed. The Vc/C1-2 region has direct connections to endogenous pain and autonomic control pathways, which are modified by estrogen status and are suggested to be critical for somatomotor and autonomic reflex responses of TMJ-related sensory signals.

Mapping of Morphine-Induced OPRM1 Gene Expression Pattern in the Adult Zebrafish Brain

Morphine is a potent analgesic opiate commonly used in treating pain, and it is also a substance of abuse and highly addictive. Hence, it is vital to discover the action sites of morphine in the brain to increase its efficacy of treatment. In the present study, we aimed at identifying comprehensive neuroanatomical locations that are sensitive to morphine in the adult zebrafish (Danio rerio). We performed in situ hybridization to localize the mu opioid receptor (oprm1) gene and to map the morphine sensitive brain areas using neuronal PAS domain-containing protein 4a (npas4a), an early gene marker. Real-time PCR was used to detect changes in mRNA levels of oprm1 and npas4a in control and acute morphine treated fish (2 mg/L; 20 min). Intense positive oprm1 signals were seen in the telencephalon, preoptic area, habenula, hypothalamic area and periventricular gray zone of the optic tectum. Acute morphine exposure significantly increased oprm1 and npas4a mRNA levels in the medial zone of dorsal telencephalon (Dm), ventral region of the ventral telencephalon (Vv), preoptic area, and in the hypothalamus but a decrease in oprm1 and npas4a signals in the dorsal habenula. This study provides a detailed map of oprm1 localization in the brain, which includes previously unreported oprm1 in the habenula of teleost. Presence of oprm1 in multiple brain sites implies multiple action targets of morphine and potential brain functions which could include reward, cognitive and negative emotions.

Opioids depress breathing through two small brainstem sites

The rates of opioid overdose in the United States quadrupled between 1999 and 2017, reaching a staggering 130 deaths per day. This health epidemic demands innovative solutions that require uncovering the key brain areas and cell types mediating the cause of overdose— opioid-induced respiratory depression. Here, we identify two primary changes to murine breathing after administering opioids. These changes implicate the brainstem’s breathing circuitry which we confirm by locally eliminating the µ-Opioid receptor. We find the critical brain site is the preBötzinger Complex, where the breathing rhythm originates, and use genetic tools to reveal that just 70–140 neurons in this region are responsible for its sensitivity to opioids. Future characterization of these neurons may lead to novel therapies that prevent respiratory depression while sparing analgesia.

Opioids such as morphine or fentanyl are powerful substances used to relieve pain in medical settings. However, taken in too high a dose they can depress breathing – in other words, they can lead to slow, shallow breaths that cannot sustain life. In the United States, where the misuse of these drugs has been soaring in the past decades, about 130 people die each day from opioid overdose. Pinpointing the exact brain areas and neurons that opioids act on to depress breathing could help to create safer painkillers that do not have this deadly effect. While previous studies have proposed several brain regions that could be involved, they have not been able to confirm these results, or determine which area plays the biggest role.

Opioids influence the brain of animals (including humans) by attaching to proteins known as opioid receptors that are present at the surface of neurons. Here, Bachmutsky et al. genetically engineered mice that lack these receptors in specific brain regions that control breathing. The animals were then exposed to opioids, and their breathing was closely monitored.

The experiments showed that two small brain areas were responsible for breathing becoming depressed under the influence of opioids. The region with the most critical impact also happens to be where the breathing rhythms originate. There, a small group of 50 to 140 neurons were used by opioids to depress breathing. Crucially, these cells were not necessary for the drugs’ ability to relieve pain.

Overall, the work by Bachmutsky et al. highlights a group of neurons whose role in creating breathing rhythms deserves further attention. It also opens the possibility that targeting these neurons would help to create safer painkillers.

The effect of activated κ-opioid receptor (κ-OR) on the role of calcium sensing receptor (CaSR) in preventing hypoxic pulmonary hypertension development

κ-opioid receptor (κ-OR) plays a key role in preventing hypoxic pulmonary hypertension (HPH) development after activated by exogenous agonist U50,488H. Calcium sensing receptor (CaSR) activation induces HPH by promoting vasoconstriction and vascular remodeling. The activated κ-OR is reported to inhibit the expression of CaSR in pulmonary artery smooth muscle cells (PASMCs). Thus, in this study, we aimed to explore the effect of activated κ-OR on the role of CaSR in preventing HPH development. An HPH rat model was constructed using Sprague-Dawley rats. Changes in mean pulmonary arterial pressure (mPAP) and right ventricular pressure (RVP) mediated by κ-OR agonist U50,488H and CaSR inhibitor NPS2143 were observed. The effects of CaSR agonist spermine and inhibitor NPS2143 on pulmonary artery tension were tested. The expression and localization of κ-OR and CaSR were measured in isolated PASMCs. A cell-counting kit-8 assay was performed to evaluate the effect of spermine in PASMC proliferation. Expression of proliferating cell nuclear antigen (PCNA), Erk, and p-Erk was evaluated by western blot analysis. Results showed that κ-OR and CaSR were co-expressed and colocalized in PASMCs under normoxic and hypoxic conditions. Interactions between κ-OR and CaSR were also observed. Spermine improved vasoconstriction in the pulmonary artery in HPH rats, which was abolished by U50,488H. RVP and mPAP were significantly increased in HPH rats under CaSR stimulation, but were significantly reduced when the rats were pretreated with U50,488H and NPS2143 (P < 0.01). Spermine treatment significantly promoted PASMC proliferation, which was significantly inhibited by U50,488H, p38 inhibitor SB203580, JNK inhibitor SP600125, Erk inhibitor SCH772984, and MEK inhibitor U0126, especially Erk inhibitor (P < 0.01). Spermine significantly increased PCNA and P-Erk expression in hypoxic conditions, which was inhibited by U50,488H and NPS2143. κ-OR stimulation prevented HPH development via the CaSR/MAPK signaling pathway.

Learned avoidance requires VTA KOR-mediated reductions in dopamine

Proper learning from an aversive experience is essential for survival, yet it is an aberrant process in a wide range of mental disorders, as well as dopaminergic neurodegenerative disease. While the mesolimbic dopamine system is known to be essential for reward learning, the characterization of a potential pattern of dopamine signaling that guides avoidance remains unknown. Aversive stimuli may directly modulate dopamine signaling through the dynorphin/kappa opioid receptor (KOR) system, as kappa opioid receptors are expressed in this neural circuit and their activation is aversive in both rodents and humans. Ventral tegmental area (VTA) KORs are ideally positioned to directly shape aversion-induced reductions in dopamine signaling, but their role in this process has received little consideration. To determine the necessity of VTA KOR activity in the regulation of dopamine signaling and avoidance, we tested the effects of VTA KOR blockade on real time dopaminergic responses to aversive stimuli and learned avoidance in male Sprague-Dawley rats. We found that blockade of VTA KORs attenuated aversion-induced reductions in dopamine, and this treatment also prevented avoidance following the aversive experience. To determine whether aversion-induced reductions in striatal dopamine are necessary for avoidance, we tested avoidance following treatment with an intra nucleus accumbens D2 receptor agonist. This treatment also prevented avoidance and is consistent with the view that aversion-induced reductions in dopamine reduce dopamine signaling at high affinity D2 receptors and disinhibit an aversion-sensitive striatal output circuit to promote avoidance.

Cocaine Dysregulates Dynorphin Modulation of Inhibitory Neurotransmission in the Ventral Pallidum in a Cell-Type-Specific Manner

Cocaine-driven changes in the modulation of neurotransmission by neuromodulators are poorly understood. The ventral pallidum (VP) is a key structure in the reward system, in which GABA neurotransmission is regulated by opioid neuropeptides, including dynorphin. However, it is not known whether dynorphin acts differently on different cell types in the VP and whether its effects are altered by withdrawal from cocaine. Here, we trained wild-type, D1-Cre, A2A-Cre, or vGluT2-Cre:Ai9 male and female mice in a cocaine conditioned place preference protocol followed by 2 weeks of abstinence, and then recorded GABAergic synaptic input evoked either electrically or optogenetically onto identified VP neurons before and after applying dynorphin. We found that after cocaine CPP and abstinence dynorphin attenuated inhibitory input to VPGABA neurons through a postsynaptic mechanism. This effect was absent in saline mice. Furthermore, this effect was seen specifically on the inputs from nucleus accumbens medium spiny neurons expressing either the D1 or the D2 dopamine receptor. Unlike its effect on VPGABA neurons, dynorphin surprisingly potentiated the inhibitory input on VPvGluT2 neurons, but this effect was abolished after cocaine CPP and abstinence. Thus, dynorphin has contrasting influences on GABA input to VPGABA and VPvGluT2 neurons and these influences are affected differentially by cocaine CPP and abstinence. Collectively, our data suggest a role for dynorphin in withdrawal through its actions in the VP. As VPGABA and VPvGluT2 neurons have contrasting effects on drug-seeking behavior, our data may indicate a complex role for dynorphin in withdrawal from cocaine.SIGNIFICANCE STATEMENT The ventral pallidum consists mainly of GABAergic reward-promoting neurons, but it also encloses a subgroup of aversion-promoting glutamatergic neurons. Dynorphin, an opioid neuropeptide abundant in the ventral pallidum, shows differential modulation of GABA input to GABAergic and glutamatergic pallidal neurons and may therefore affect both the rewarding and aversive aspects of withdrawal. Indeed, abstinence after repeated exposure to cocaine alters dynorphin actions in a cell-type-specific manner; after abstinence dynorphin suppresses the inhibitory drive on the "rewarding" GABAergic neurons but ceases to modulate the inhibitory drive on the "aversive" glutamatergic neurons. This reflects a complex role for dynorphin in cocaine reward and abstinence.

Neurochemical organization of the ventral striatum's olfactory tubercle

The ventral striatum is a collection of brain structures, including the nucleus accumbens, ventral pallidum and the olfactory tubercle (OT). While much attention has been devoted to the nucleus accumbens, a comprehensive understanding of the ventral striatum and its contributions to neurological diseases requires an appreciation for the complex neurochemical makeup of the ventral striatum's other components. This review summarizes the rich neurochemical composition of the OT, including the neurotransmitters, neuromodulators and hormones present. We also address the receptors and transporters involved in each system as well as their putative functional roles. Finally, we end with briefly reviewing select literature regarding neurochemical changes in the OT in the context of neurological disorders, specifically neurodegenerative disorders. By overviewing the vast literature on the neurochemical composition of the OT, this review will serve to aid future research into the neurobiology of the ventral striatum.

The globus pallidus as a target for neuropeptides and endocannabinoids participating in central activities

The globus pallidus in the basal ganglia plays an important role in movement regulation. Neuropeptides and endocannabinoids are neuronal signalling molecules that influence the functions of the whole brain. Endocannabinoids, enkephalin, substance P, neurotensin, orexin, somatostatin and pituitary adenylate cyclase-activating polypeptides are richly concentrated in the globus pallidus. Neuropeptides and endocannabinoids exert excitatory or inhibitory effects in the globus pallidus mainly by modulating GABAergic, glutamatergic and dopaminergic neurotransmission, as well as many ionic mechanisms. Pallidal neuropeptides and endocannabinoids are associated with the pathophysiology of a number of neurological disorders, such as Parkinson's disease, Huntington's disease, schizophrenia, and depression. The levels of neuropeptides and endocannabinoids and their receptors in the globus pallidus change in neurological diseases. It has been demonstrated that spontaneous firing activity of globus pallidus neurons is closely related to the manifestations of Parkinson's disease. Therefore, the neuropeptides and endocannabinoids in the globus pallidus may function as potential targets for treatment in some neurological diseases. In this review, we highlight the morphology and function of neuropeptides and endocannabinoids in the globus pallidus and their involvement in neurological diseases.

Chronic opioid pretreatment potentiates the sensitization of fear learning by trauma

Despite the large comorbidity between PTSD and opioid use disorders, as well as the common treatment of physical injuries resulting from trauma with opioids, the ability of opioid treatments to subsequently modify PTSD-related behavior has not been well studied. Using the stress-enhanced fear learning (SEFL) model for PTSD, we characterized the impact of chronic opioid regimens on the sensitization of fear learning seen following traumatic stress in mice. We demonstrate for the first time that chronic opioid pretreatment is able to robustly augment associative fear learning. Highlighting aversive learning as the cognitive process mediating this behavioral outcome, these changes were observed after a considerable period of drug cessation, generalized to learning about multiple aversive stimuli, were not due to changes in stimulus sensitivity or basal anxiety, and correlated with a marker of synaptic plasticity within the basolateral amygdala. Additionally, these changes were not observed when opioids were given after the traumatic event. Moreover, we found that neither reducing the frequency of opioid administration nor bidirectional manipulation of acute withdrawal impacted the subsequent enhancement in fear learning seen. Given the fundamental role of associative fear learning in the generation and progression of PTSD, these findings are of direct translational relevance to the comorbidity between opioid dependence and PTSD, and they are also pertinent to the use of opioids for treating pain resulting from traumas involving physical injuries.

Prenatal exposure to valproic acid reduces social responses and alters mRNA levels of opioid receptor and pre-pro-peptide in discrete brain regions of adolescent and adult male rats

Altered social behaviours are a hallmark of several psychiatric and developmental disorders. Clinical and preclinical data have demonstrated that prenatal exposure to valproic acid (VPA), an anti-epileptic and mood stabiliser, is associated with impaired social responses, and thus provides a useful model for the evaluation of neurobiological mechanisms underlying altered social behaviours. The opioid system is widely recognised to regulate and modulate social behaviours, however few studies have examined if the endogenous opioid system is altered in animal models of social impairment. The present study examined social behavioural responses of adolescent and adult male rats prenatally exposed to VPA, and the expression of mRNA encoding opioid receptors and pre-pro-peptides in discrete brain regions. Adolescent and adult rats prenatally exposed to VPA spent less time engaging in social behaviours in the direct social interaction test and exhibited reduced sociability and social novelty preference in the 3-chamber sociability test, compared to saline-treated counterparts. The VPA-exposed adolescent rats exhibited significantly reduced kappa opioid receptor (oprk1) and pre-pro-dynorphin (pdyn) mRNA expression in the cerebral cortex, and reduced oprk1 and nociceptin/orphanin FQ (oprl1) mRNA expression in the hypothalamus. Adult rats prenatally exposed to VPA exhibited decreased mRNA expression of oprk1 and pdyn in hypothalamus, reduced pro-opiomelanocortin(pomc) in the striatum and an increase in delta opioid receptor (oprd1) mRNA in the amygdaloid cortex, when compared to saline-treated counterparts. Mu opioid receptor (oprm1) mRNA expression did not differ between saline and VPA-exposed rats in any region examined. The data demonstrate that impaired social behaviours in adolescent and adult rats prenatally exposed to VPA is accompanied by altered mRNA expression of opioid receptors and pre-pro-peptides in a region specific manner. In particular, both adolescent and adult VPA-exposed rats exhibit reduced oprk1-pdyn mRNA expression in several brain regions, which are associated with deficits in social behavioural responding in the model.

Escalated Oxycodone Self-Administration and Punishment: Differential Expression of Opioid Receptors and Immediate Early Genes in the Rat Dorsal Striatum and Prefrontal Cortex

Opioid use disorder (OUD) is characterized by compulsive drug taking despite adverse life consequences. Here, we sought to identify neurobiological consequences associated with the behavioral effects of contingent footshocks administered after escalation of oxycodone self-administration. To reach these goals, Sprague-Dawley rats were trained to self-administer oxycodone for 4 weeks and were then exposed to contingent electric footshocks. This paradigm helped to dichotomize rats into two distinct behavioral phenotypes: (1) those that reduce lever pressing (shock-sensitive) and (2) others that continue lever pressing (shock-resistant) for oxycodone during contingent punishment. The rats were euthanized at 2-h after the last oxycodone plus footshock session. The dorsal striata and prefrontal cortices were dissected for use in western blot and RT-qPCR analyses. All oxycodone self-administration rats showed significant decreased expression of Mu and Kappa opioid receptor proteins only in the dorsal striatum. However, expression of Delta opioid receptor protein was decreased in both brain regions. RT-qPCR analyses documented significant decreases in the expression of c-fos, fosB, fra2, junB, egr1, and egr2 mRNAs in the dorsal striatum (but not in PFC) of the shock-sensitive rats. In the PFC, junD expression was reduced in both phenotypes. However, egr3 mRNA expression was increased in the PFC of only shock-resistant rats. These results reveal that, similar to psychostimulants and alcohol, footshocks can dichotomize rats that escalated their intake of oxycodone into two distinct behavioral phenotypes. These animals also show significant differences in the mRNA expression of immediate early genes, mainly, in the dorsal striatum. The increases in PFC egr3 expression in the shock-resistant rats suggest that Egr3 might be involved in the persistence of oxycodone-associated memory under aversive conditions. This punishment-driven model may help to identify neurobiological substrates of persistent oxycodone taking and abstinence in the presence of adverse consequences.

Differential impact of two critical respiratory centres in opioid-induced respiratory depression in awake mice

KEY POINTS

The main cause of death from opioid overdose is respiratory depression due to the activation of µ-opioid receptors (MORs). We conditionally deleted MORs from neurons in two key areas of the brainstem respiratory circuitry (the Kölliker-Fuse nucleus (KF) and pre-Bötzinger complex (preBötC)) to determine their role in opioid-induced respiratory disturbances in adult, awake mice. Deletion of MORs from KF neurons attenuated respiratory rate depression at all doses of morphine. Deletion of MORs from preBötC neurons attenuated rate depression at the low dose, but had no effect on rate following high doses of morphine. Instead, high doses of morphine increased the occurrence of apnoeas. The results indicate that opioids affect distributed key areas of the respiratory network in a dose-dependent manner and countering the respiratory effects of high dose opioids via the KF may be an effective approach to combat overdose.

ABSTRACT

The primary cause of death from opioid overdose is respiratory failure. High doses of opioids cause severe rate depression and increased risk of fatal apnoea, which correlate with increasing irregularities in breathing pattern. µ-Opioid receptors (MORs) are widely distributed throughout the brainstem respiratory network, but the mechanisms underlying respiratory depression are poorly understood. The medullary pre-Bötzinger complex (preBötC) and the pontine Kölliker-Fuse nucleus (KF) are considered critical for inducing opioid-related respiratory disturbances. We used a conditional knockout approach to investigate the roles and relative contribution of MORs in KF and preBötC neurons in opioid-induced respiratory depression in awake adult mice. The results revealed dose-dependent and region-specific opioid effects on the control of both respiratory rate and pattern. Respiratory depression induced by an anti-nociceptive dose of morphine was significantly attenuated following deletion of MORs from either the KF or the preBötC, suggesting cumulative network effects on respiratory rate control at low opioid doses. Deletion of MORs from KF neurons also relieved rate depression at near-maximal respiratory depressant doses of morphine. Meanwhile, deletion of MORs from the preBötC had no effect on rate following administration of high doses of morphine. Instead, a severe ataxic breathing pattern emerged with many apnoeas. We conclude that opioids affect distributed areas of the respiratory network and opioid-induced respiratory depression cannot be attributed to only one area in isolation. However, countering the effects of near maximal respiratory depressant doses of opioids in the KF may be a powerful approach to combat opioid overdose.

Opioids differentially modulate two synapses important for pain processing in the amygdala

BACKGROUND AND PURPOSE

Pain is a subjective experience involving sensory discriminative and emotionally aversive components. Consistent with its role in pain processing and emotions, the amygdala modulates the aversive component of pain. The laterocapsular region of the central nucleus of the amygdala (CeLC) receives nociceptive information from the parabrachial nucleus (PB) and polymodal, including nociceptive, inputs from the basolateral nucleus of the amygdala (BLA). Opioids are strong analgesics and reduce both the sensory discriminative and the affective component of pain. However, it is unknown whether opioids regulate activity at the two nociceptive inputs to the amygdala.

EXPERIMENTAL APPROACH

Using whole-cell electrophysiology, optogenetics, and immunohistochemistry, we investigated whether opioids inhibit the rat PB-CeLC and BLA-CeLC synapses.

KEY RESULTS

Opioids inhibited glutamate release at the PB-CeLC and BLA-CeLC synapses. Opioid inhibition is via the μ-receptor at the PB-CeLC synapse, while at the BLA-CeLC synapse, inhibition is via μ-receptors in all neurons and via δ-receptors and κ-receptors in a subset of neurons.

CONCLUSIONS AND IMPLICATIONS

Agonists of μ-receptors inhibited two of the synaptic inputs carrying nociceptive information into the laterocapsular amygdala. Therefore, μ-receptor agonists, such as morphine, will inhibit glutamate release from PB and BLA in the CeLC, and this could serve as a mechanism through which opioids reduce the affective component of pain and pain-induced associative learning. The lower than expected regulation of BLA synaptic outputs by δ-receptors does not support the proposal that opioid receptor subtypes segregate into subnuclei of brain regions.

Modulatory effect of opioid ligands on status epilepticus and the role of nitric oxide pathway

Epilepsy is a chronic disorder that causes unprovoked, recurrent seizures. Status epilepticus (SE) is a medical emergency associated with significant morbidity and mortality. Morphine has been the cornerstone of pain controlling medicines for a long time. In addition to the analgesic and opioid responses, morphine has also revealed anticonvulsant effects in different epilepsy models including pentylenetetrazole (PTZ)-induced seizures threshold. Some authors suggest that nitric oxide (NO) pathway interactions of morphine explain the reason for its pro or anticonvulsant activities. To induce SE, injection of a single dose of lithium chloride (127 mg/kg, intraperitoneal (i.p.)) 20 h before pilocarpine (60 mg/kg, i.p.) was used. Administration of morphine (15 mg/kg, i.p.) inhibited the SE and decreased the mortality in rats when injected 30 min before pilocarpine. On the other hand, injection of L-N^G-nitro arginine methyl ester (L-NAME, a nonselective NO synthase (NOS) blocker; 10 mg/kg, i.p.), 7-nitroindazole (7-NI, a neuronal NOS (nNOS) blocker; 30 mg/kg, i.p.), and aminoguanidine (AG, an inducible NOS (iNOS) blocker; 50 mg/kg, i.p.) 15 min before morphine, significantly reversed inhibitory effect of morphine on SE. Subsequently, measurement of nitrite metabolite levels in the hippocampus of SE-induced rats displayed high levels of nitrite metabolite for the control group. However, after injection of morphine in SE-induced rats, nitrite metabolite levels reduced. In conclusion, these findings demonstrated that NO pathway (both nNOS and iNOS) interactions are involved in the anticonvulsant effects of morphine on the SE signs and mortality rate induced by lithium-pilocarpine in rats.

Comparison of pre-pulse inhibition, tactile discrimination learning and barrel cortical neural response in adult male rats following chronic exposure to morphine, methadone and buprenorphine

Chronic exposure to opioids is the most common treatment plan to reduce the pain. In this study, the stereotyped behaviors and cognitive functions related to different types of tactile and auditory inputs were investigated in the rats following chronic exposure to the morphine, methadone, and buprenorphine. Here, three addicted groups received morphine, methadone, and buprenorphine while the control rats received saline for 21 days. Our results demonstrated that the opioid-treated groups showed stereotyped behaviors including grooming and rearing. In the behavioral level, prepulse inhibition and preference indices were not changed significantly in the opioids-treated groups compared to those of the saline group as two criteria for acoustic startle reflex and tactile discrimination, respectively. In the neuronal level, chronic morphine and methadone treatment changed the response properties of the barrel cortical neurons to the whisker deflections in the experimental groups compared to the saline group. Thus, it was concluded that the excitatory receptive fields of neurons in the barrel cortex can be changed as a result of chronic exposure to morphine and methadone.

Differential disruption of conditioned ejaculatory preference in the male rat based on different sensory modalities by micro-infusions of naloxone to the medial preoptic area or ventral tegmental area

RATIONALE

Male rats trained to associate a neutral odor or rodent jacket on a female with their post-ejaculatory reward state display a preference to ejaculate with females bearing the odor or jacket. This conditioned ejaculatory preference (CEP) can be shifted by systemic administration of the opioid antagonist naloxone (NAL) during training, such that NAL-trained males distribute their ejaculations to females without the cue, relative to saline (SAL)-trained males.

OBJECTIVE

The present study examined two brain sites, the medial preoptic area (mPOA) or ventral tegmental area (VTA), where the opioid reward state might be induced.

METHODS

Sexually naïve Long-Evans males were implanted with bilateral guide cannula aimed at either site before they underwent multi-ejaculatory conditioning trials at 4-day intervals with sexually receptive females that bore either an almond odor or rodent tethering jacket. Infusions of NAL (1 μl/side) or SAL (1 μl/side) were made prior to each conditioning trial. All males were infused with SAL prior to a final open-field choice test with two sexually receptive females, one scented and the other unscented, or one jacketed and the other unjacketed.

RESULTS

Males previously conditioned with SAL in either region showed significant CEP. In contrast, prior infusions of NAL to the mPOA shifted the preference towards the unfamiliar female, whereas prior infusions to the VTA abolished CEP for the odor. Subsequent detection of Fos protein induced by the cue showed that, relative to SAL-treated males, prior experience with NAL in the mPOA suppressed Fos in both the mPOA and VTA, whereas prior experience with NAL in to the VTA suppressed Fos in the VTA alone.

CONCLUSIONS

Opioid antagonism in the mPOA produces a state of non-reward whereas in the VTA, it produces a state in which the odor does not acquire incentive properties.

µ-opioid receptor-mediated downregulation of midline thalamic pathways to basal and central amygdala

Brain µ-opioid receptors (MOR) mediate reward and help coping with pain, social rejection, anxiety and depression. The dorsal midline thalamus (dMT) integrates visceral/emotional signals and biases behavior towards aversive or defensive states through projections to the amygdala. While a dense MOR expression in the dMT has been described, the exact cellular and synaptic mechanisms of µ-opioidergic modulation in the dMT-amygdala circuitry remain unresolved. Here, we hypothesized that MORs are important negative modulators of dMT-amygdala excitatory networks. Using retrograde tracers and targeted channelrhodopsin expression in combination with patch-clamp electrophysiology, we found that projections of dMT neurons onto both basal amygdala principal neurons (BA PN) and central amygdala (CeL) neurons are attenuated by stimulation of somatic or synaptic MORs. Importantly, dMT efferents to the amygdala drive feedforward excitation of centromedial amygdala neurons (CeM), which is dampened by MOR activation. This downregulation of excitatory activity in dMT-amygdala networks puts the µ-opioid system in a position to ameliorate aversive or defensive behavioral states associated with stress, withdrawal, physical pain or social rejection.

Generation of a MOR-CreER knock-in mouse line to study cells and neural circuits involved in mu opioid receptor signaling

Mu opioid receptor (MOR) is involved in various brain functions, such as pain modulation, reward processing, and addictive behaviors, and mediates the main pharmacologic effects of morphine and other opioid compounds. To gain genetic access to MOR-expressing cells, and to study physiological and pathological roles of MOR signaling, we generated a MOR-CreER knock-in mouse line, in which the stop codon of the Oprm1 gene was replaced by a DNA fragment encoding a T2A peptide and tamoxifen (Tm)-inducible Cre recombinase. We show that the MOR-CreER allele undergoes Tm-dependent recombination in a discrete subtype of neurons that express MOR in the adult nervous system, including the olfactory bulb, cerebral cortex, striosome compartments in the striatum, hippocampus, amygdala, thalamus, hypothalamus, interpeduncular nucleus, superior and inferior colliculi, periaqueductal gray, parabrachial nuclei, cochlear nucleus, raphe nuclei, pontine and medullary reticular formation, ambiguus nucleus, solitary nucleus, spinal cord, and dorsal root ganglia. The MOR-CreER mouse line combined with a Cre-dependent adeno-associated virus vector enables robust gene manipulation in the MOR-enriched striosomes. Furthermore, Tm treatment during prenatal development effectively induces Cre-mediated recombination. Thus, the MOR-CreER mouse is a powerful tool to study MOR-expressing cells with conditional gene manipulation in developing and mature neural tissues.

Effects of the Alpha-1 Antagonist Prazosin on KOR Agonist-Induced Reinstatement of Alcohol Seeking

BACKGROUND

Stress is associated with relapse to alcohol seeking during abstinence, but the processes underlying this relationship are poorly understood. Noradrenaline is a key transmitter in stress responses and in stress-induced drug seeking. The alpha-1 adrenoceptor antagonist prazosin has been investigated as a treatment for alcoholism and for chronic stress disorders that are frequently comorbid with alcoholism. In rats, we previously showed that prazosin blocks reinstatement of alcohol seeking induced by footshock and yohimbine stressors and reduces yohimbine-induced brain activation. The role of alpha-1 adrenoceptors in reinstatement induced by other stressors is not known. Our most recent work is on the role of kappa opioid receptors in stress-induced reinstatement of alcohol seeking and have reported that the selective kappa opioid receptor agonist U50,488 induces reinstatement and neuronal activation in stress- and relapse-related brain regions. Here we determine the involvement of alpha-1 receptors in reinstatement and brain activation induced by U50,488.

METHODS

We trained male Long-Evans rats to self-administer alcohol (12% w/v), extinguished alcohol-reinforced responding, and then determined the effects of prazosin (1 mg/kg) on U50,488 (2.5 mg/kg)-induced reinstatement and regional Fos expression.

RESULTS

Prazosin blocked U50,488-induced reinstatement and decreased U50,488-induced Fos expression in the orbitofrontal cortex, nucleus accumbens core, ventral bed nucleus of the stria terminalis, central and basolateral amygdalar nuclei and ventral tegmental area.

CONCLUSIONS

These findings suggest that prazosin may reduce U50,488-induced relapse by inhibiting activity in 1 or more of these brain areas.

Transcriptomics Analysis of Porcine Caudal Dorsal Root Ganglia in Tail Amputated Pigs Shows Long-Term Effects on Many Pain-Associated Genes

Tail amputation by tail docking or as an extreme consequence of tail biting in commercial pig production potentially has serious implications for animal welfare. Tail amputation causes peripheral nerve injury that might be associated with lasting chronic pain. The aim of this study was to investigate the short- and long-term effects of tail amputation in pigs on caudal DRG gene expression at different stages of development, particularly in relation to genes associated with nociception and pain. Microarrays were used to analyse whole DRG transcriptomes from tail amputated and sham-treated pigs 1, 8, and 16 weeks following tail treatment at either 3 or 63 days of age (8 pigs/treatment/age/time after treatment; n = 96). Tail amputation induced marked changes in gene expression (up and down) compared to sham-treated intact controls for all treatment ages and time points after tail treatment. Sustained changes in gene expression in tail amputated pigs were still evident 4 months after tail injury. Gene correlation network analysis revealed two co-expression clusters associated with amputation: Cluster A (759 down-regulated) and Cluster B (273 up-regulated) genes. Gene ontology (GO) enrichment analysis identified 124 genes in Cluster A and 61 genes in Cluster B associated with both “inflammatory pain” and “neuropathic pain.” In Cluster A, gene family members of ion channels e.g., voltage-gated potassium channels (VGPC) and receptors e.g., GABA receptors, were significantly down-regulated compared to shams, both of which are linked to increased peripheral nerve excitability after axotomy. Up-regulated gene families in Cluster B were linked to transcriptional regulation, inflammation, tissue remodeling, and regulatory neuropeptide activity. These findings, demonstrate that tail amputation causes sustained transcriptomic expression changes in caudal DRG cells involved in inflammatory and neuropathic pain pathways.

Lesions of the Patch Compartment of Dorsolateral Striatum Disrupt Stimulus-Response Learning

The striatum mediates habit formation and reward association. The striatum can be divided into the patch and matrix compartment, which are two distinct regions that sub-serve different aspects of behavior. The patch compartment may mediate reward-related behaviors, while the matrix compartment may mediate adaptive motor functions. Previous studies indicate that enhanced relative activation of the patch versus matrix compartment is associated with inflexible behaviors, such as stereotypy. Habitual behaviors are also inflexible in nature, but whether enhanced activation of the patch compartment contributes to habitual behavior is not known. The goal of the current study was to examine the role of patch compartment in the development of habit formation. We used dermorphin-saporin to ablate neurons of the patch compartment in the dorsolateral striatum prior to training animals to self-administer sucrose on a random interval schedule of reinforcement. Our data showed that patch compartment lesions in the dorsolateral striatum reduced the reinstatement of sucrose self-administration after sucrose devaluation, indicating that destruction of this region prevented the development of habitual behavior. Additionally, in animals with patch compartment lesions in the DLS that did not develop habitual behavior, activation of the dorsolateral striatum and sensorimotor cortex was diminished, while activity in the dorsomedial striatum and prefrontal cortex was increased, suggesting less engagement of regions that mediate habitual behaviors and heightened engagement of regions that mediate goal-directed behaviors occurs with reduced habit formation. These data indicate that the dorsolateral patch compartment may mediate habit formation by altering information flow through basal ganglia circuits.

Testosterone modulation of ethanol effects on the �‑opioid receptor kinetics in castrated rats

The present investigation was conducted to evaluate the effects of testosterone on ethanol-induced alterations of µ-opioid receptor binding kinetics in specific brain regions of castrated rats. Male Sprague Dawley rats (100-124 g) adapted to a 12-h light/dark cycle were used. Animals were castrated under pentobarbital anesthesia. After a recovery period of 14 days, ethanol [3 g/kg as 22.5% solution in saline via intraperitoneal injection (i.p.)], testosterone [2.5 mg in 0.2 ml of olive oil via subcutaneous injection (s.c.) in the dorsal neck region] or the combination of ethanol and testosterone were administered to rats at 9:00 a.m. The control group was injected i.p. with 2 ml saline and s.c. with 0.2 ml olive oil for 7 days. Animals were sacrificed by decapitation at 2 h after the final injection. The brains were immediately removed, and the cortex, hippocampus, hypothalamus and midbrain were dissected. In an attempt to elucidate the mechanism involved in the hormonal modulation of the effects of ethanol and testosterone on the endogenous opioid system, the binding kinetics of the µ-opioid receptors were determined. The results obtained in the present study assisted in identifying the regulatory role of testosterone on ethanol-induced changes on µ-opioid receptor binding kinetics.

Dual enkephalinase inhibitor PL265: a novel topical treatment to alleviate corneal pain and inflammation

Ocular pain is a core symptom of inflammatory or traumatic disorders affecting the anterior segment. To date, the management of chronic ocular pain remains a therapeutic challenge in ophthalmology. The main endogenous opioids (enkephalins) play a key role in pain control but exhibit only transient analgesic effects due to their rapid degradation. The aim of this study was to explore the antinociceptive and anti-inflammatory effects of topical administration of PL265 (a dual enkephalinase inhibitor) on murine models of corneal pain. On healthy corneas, chronic PL265 topical administration did not alter corneal integrity nor modify corneal mechanical and chemical sensitivity. Then, on murine models of corneal pain, we showed that repeated instillations of PL265 (10 mM) significantly reduced corneal mechanical and chemical hypersensitivity. PL265-induced corneal analgesia was completely antagonized by naloxone methiodide, demonstrating that PL265 antinociceptive effects were mediated by peripheral corneal opioid receptors. Moreover, flow cytometry (quantification of CD11b+ cells) and in vivo confocal microscopy analysis revealed that instillations of PL265 significantly decreased corneal inflammation in a corneal inflammatory pain model. Chronic PL265 topical administration also decreased Iba1 and neuronal injury marker (ATF3) staining in the nucleus of primary sensory neurons of ipsilateral trigeminal ganglion. These results open a new avenue for ocular pain treatment based on the enhancement of endogenous opioid peptides' analgesic effects in tissues of the anterior segment of the eye. Dual enkephalinase inhibitor PL265 seems to be a promising topical treatment for safe and effective alleviation of ocular pain and inflammation.

Opioid induced adrenal insufficiency: what is new?

PURPOSE OF REVIEW

Despite the declaration of an opioid epidemic, opioid use remains prevalent. Side-effects of chronic opioid use continue to be problematic. Opioid-induced endocrinopathies have been well documented, yet opioid-induced adrenal insufficiency (OIAI) remains underappreciated. This review summarizes what is currently known regarding the prevalence, predictive factors for the development and effect of treatment of OIAI.

RECENT FINDINGS

Although several case reports have highlighted the development of adrenal crisis among those receiving chronic opioids, only a few studies have systematically assessed patients for OIAI. The heterogeneity of these small studies presents challenges when trying to assess prevalence of or potential risk factors for OIAI. The estimated prevalence of OIAI among those treated with chronic opioids ranges from 8.3 to 29% and is more likely in those receiving higher doses of opioids. Reduced health-related quality of life variables and altered pain perception has been associated with lower cortisol levels; however, the effect of glucocorticoid replacement on the parameters remains unknown.

SUMMARY

Further research is critical to better identify those at greatest risk and guide optimal management of OIAI. Frontline providers should remain vigilant for possibility of OIAI among chronic opioid users.

Dezocine Alleviates Morphine-Induced Dependence in Rats

BACKGROUND

Opioid dependence is a major public health issue without optimal therapeutics. This study investigates the potential therapeutic effect of dezocine, a nonaddictive opioid, in opioid dependence in rat models.

METHODS

Dezocine was administered intraperitoneally to a morphine-dependent rat model to investigate its effect on withdrawal and conditioned place preference (CPP). Effect of dezocine on morphine withdrawal syndrome and CPP was analyzed using 2-way analysis of variance (ANOVA) followed by Tukey's post hoc test. Buprenorphine and vehicle solution containing 20% (v/v) dimethyl sulfoxide were used for positive and negative control, respectively. The astrocytes activation in nucleus accumbens was assessed by immunofluorescence assay of glial fibrillary acidic protein. Effect of dezocine and buprenorphine on the internalization of κ opioid receptor (KOR) was investigated using Neuro2A expressing KOR fused to red fluorescent protein tdTomato (KOR-tdT). Buprenorphine and dezocine were screened against 44 G-protein-coupled receptors, ion channels, and transporter proteins using radioligand-binding assay to compare the molecular targets.

RESULTS

The mean withdrawal score was reduced in rats treated with 1.25 mg·kg dezocine compared to vehicle-treated control animals starting from the day 1 (mean difference: 7.8; 95% confidence interval [CI], 6.35-9.25; P < .0001 by 2-way ANOVA). Significance was observed at all treatment days, including day 7 (mean difference: 2.13; 95% CI, 0.68-3.58; P < .001 by 2-way ANOVA). Furthermore, dezocine inhibited the reinstatement of morphine-induced CPP (mean difference: 314; 95% CI, 197.9-430.1; P < .0001 by 2-way ANOVA) compared to the control group. Chronic morphine administration induced astrocytes activation in nucleus accumbens, which was attenuated by dezocine. Dezocine blocked the agonist-induced KOR internalization in vitro, 1 of the mechanisms involved in the downstream signaling and development of opioid dependence. Dezocine had affinity to norepinephrine and serotonin transporters and sigma-1 receptor, whereas buprenorphine showed no activity against these targets.

CONCLUSIONS

Dezocine could potentially be used to alleviate opioid dependence. Due to the unique molecular target profile different from buprenorphine, it might have important value in studying the mechanisms of morphine dependence and developing novel therapeutic approaches.

Synapse-specific opioid modulation of thalamo-cortico-striatal circuits

The medial thalamus (MThal), anterior cingulate cortex (ACC) and striatum play important roles in affective-motivational pain processing and reward learning. Opioids affect both pain and reward through uncharacterized modulation of this circuitry. This study examined opioid actions on glutamate transmission between these brain regions in mouse. Mu-opioid receptor (MOR) agonists potently inhibited MThal inputs without affecting ACC inputs to individual striatal medium spiny neurons (MSNs). MOR activation also inhibited MThal inputs to the pyramidal neurons in the ACC. In contrast, delta-opioid receptor (DOR) agonists disinhibited ACC pyramidal neuron responses to MThal inputs by suppressing local feed-forward GABA signaling from parvalbumin-positive interneurons. As a result, DOR activation in the ACC facilitated poly-synaptic (thalamo-cortico-striatal) excitation of MSNs by MThal inputs. These results suggest that opioid effects on pain and reward may be shaped by the relative selectivity of opioid drugs to the specific circuit components.

A Motivational and Neuropeptidergic Hub: Anatomical and Functional Diversity within the Nucleus Accumbens Shell

The mesocorticolimbic pathway is canonically known as the "reward pathway." Embedded within the center of this circuit is the striatum, a massive and complex network hub that synthesizes motivation, affect, learning, cognition, stress, and sensorimotor information. Although striatal subregions collectively share many anatomical and functional similarities, it has become increasingly clear that it is an extraordinarily heterogeneous region. In particular, the nucleus accumbens (NAc) medial shell has repeatedly demonstrated that the rules dictated by more dorsal aspects of the striatum do not apply or are even reversed in functional logic. These discrepancies are perhaps most easily captured when isolating the functions of various neuromodulatory peptide systems within the striatum. Endogenous peptides are thought to play a critical role in modulating striatal signals to either amplify or dampen evoked behaviors. Here we describe the anatomical-functional backdrop upon which several neuropeptides act within the NAc to modulate behavior, with a specific emphasis on nucleus accumbens medial shell and stress responsivity. Additionally, we propose that, as the field continues to dissect fast neurotransmitter systems within the NAc, we must also provide considerable contextual weight to the roles local peptides play in modulating these circuits to more comprehensively understand how this important subregion gates motivated behaviors.

Gabapentin completely neutralized the acute morphine activation in the rat hypothalamus: a c-Fos study

Aim: The molecular mechanism of gabapentin (GBP)–morphine combinational function and its neuro-anatomical sites of action to prevent, to neutralize morphine side effects and also the enhancement its analgesic effect of morphine is unknown. Methods: Morphine (10 mg/kg), saline, co-injection: GBP (150 mg/kg) with morphine (10 mg/kg) were injected by intraperitoneal injection in rats under deep anaesthesia. C-Fos immunohistochemistry technique was used to locate c-Fos expression in rat hypothalamus. Results: Gabapentin in combination with morphine significantly (p < 0.01) attenuated the acute morphine induced c-Fos immunoreactive neuron in hypothalamus. Conclusion: GBP neutralized the morphine sensitization in rat hypothalamus. GBP might neuromodulate and or antagonize the receptor regulatory machinery of morphine sensitization circuit which might work for drug discovery of morphine abuse.

Differential suppression of the ipsi- and contralateral nociceptive reflexes in the neonatal rat spinal cord by agonists of µ-, δ- and κ-opioid receptors

Nociceptive discharges caused by the unilateral tissue damage are processed in the spinal cord by both ipsi- and contralateral neuronal circuits. The mechanisms of the neurotransmitter control of this bilateral excitation spread is poorly understood. Spinally administered opiates are known to suppress nociceptive transmission and nociceptive withdrawal reflexes. Here we investigated whether three major types of opioid receptors are involved in the bilateral control of the spinal nociceptive sensorimotor processing. Effects of the µ-, δ- and κ-opioid receptor agonists on the ipsi- and contralateral nociceptive reflexes were studied by recording slow ventral root potentials in an isolated spinal cord preparation of the new-born rat. Absolute levels of expression of the opioid genes were analyzed by the droplet digital PCR. Ipsi- and contralateral slow ventral root potentials were most strongly suppressed by the µ-opioid receptor agonist DAMGO, by 63% and 85%, followed by the κ-opioid receptor agonist U-50488H, by 44% and 73%, and δ-opioid receptor agonist leucine-enkephalin, by 27% and 49%, respectively. All these agonists suppressed stronger contra- than ipsilateral responses. Naloxone prevented effects of the agonists indicating that they act through opioid receptors, which, as we show, are expressed in the neonatal spinal cord at the levels similar to those in adults. Thus, opioid receptor agonists suppress the segmental nociceptive reflexes. Stronger contralateral effects suggest that the endogenous opioid system regulates sensorimotor processing in the spinal commissural pathways. These effects of opioids may be relevant for treatment of symmetric clinical pain symptoms caused by unilateral tissue injury.

Methiopropamine, a methamphetamine analogue, produces neurotoxicity via dopamine receptors

Methiopropamine (MPA) is structurally categorized as a thiophene ring-based methamphetamine (MA) derivative. Although abusive potential of MPA was recognized, little is known about the neurotoxic potential of MPA up to now. We investigated whether MPA induces dopaminergic neurotoxicity, and whether MPA activates a specific dopamine receptor. Here, we observed that treatment with MPA resulted in dopaminergic neurotoxicity in a dose-dependent manner. MPA treatment potentiated oxidative parameters (i.e., increases in the level of reactive oxygen species, 4-hydroxynonenal, and protein carbonyl), M1 phenotype-related microglial activity, and pro-apoptotic property (i.e., increases in Bax- and cleaved caspase-3-expressions, while a decrease in Bcl-2-expression). Moreover, treatment with MPA resulted in significant impairments in dopaminergic parameters [i.e., changes in dopamine level, dopamine turnover rate, tyrosine hydroxylase (TH) levels, dopamine transporter (DAT) expression, and vesicular monoamine transporter-2 (VMAT-2) expression], and in behavioral deficits. Both dopamine D1 receptor antagonist SCH23390 and D2 receptor antagonist sulpiride protected from these neurotoxic consequences. Therefore, our results suggest that dopamine D1 and D2 receptors simultaneously mediate MPA-induced dopaminergic neurodegeneration in mice via oxidative burdens, microgliosis, and pro-apoptosis.

Progress in brain cannabinoid CB2 receptor research: From genes to behavior

The type 2 cannabinoid receptor (CB2R) was initially regarded as a peripheral cannabinoid receptor. However, recent technological advances in gene detection, alongside the availability of transgenic mouse lines, indicate that CB2Rs are expressed in both neurons and glial cells in the brain under physiological and pathological conditions, and are involved in multiple functions at cellular and behavioral levels. Brain CB2Rs are inducible and neuroprotective via up-regulation in response to various insults, but display species differences in gene and receptor structures, CB2R expression, and receptor responses to various CB2R ligands. CB2R transcripts also differ between the brain and spleen. In the brain, CB2A is the major transcript isoform, while CB2A and CB2B transcripts are present at higher levels in the spleen. These new findings regarding brain versus spleen CB2R isoforms may in part explain why early studies failed to detect brain CB2R gene expression. Here, we review evidence supporting the expression and function of brain CB2R from gene and receptor levels to cellular functioning, neural circuitry, and animal behavior.

The paraventricular thalamic nucleus: A key hub of neural circuits underlying drug addiction

Drug addiction is a chronic relapsing brain disease characterized by compulsive, out-of-control drug use and the appearance of negative somatic and emotional consequences when drug access is prevented. The limited efficacy of treatment urges researchers toward a deeper understanding of the neural mechanism of drug addiction. Brain circuits that regulate reward and motivation are considered to be the neural substrate of drug addiction. An increasing body of literature indicates that the paraventricular thalamic nucleus (PVT) could serve as a key node in the neurocircuits that control goal-directed behaviors. In this review, we summarize the anatomical and functional evidence that the PVT regulates drug-related behaviors. The PVT receives extensive inputs from the brainstem and hypothalamus, and is reciprocally connected with the limbic system. Neurons in the PVT are recruited by drug exposure as well as cues and context associated with drug taking. Pathway-specific perturbation studies have begun to decipher the precise role of PVT circuits in drug-related behaviors. We also highlight recent findings about the involvement of neural plasticity of the PVT pathways in drug addiction and provide perspectives on future studies.

An Integrated Neuronal Model of Claustral Function in Timing the Synchrony Between Cortical Areas

It has been suggested that the function of the claustrum (CL) may be to orchestrate and integrate the activity of the different cortical areas that are involved in a particular function by boosting the synchronized oscillations that occur between these areas. We propose here a model of how this may be done, thanks to the unique synaptic morphology of the CL and its excitatory and inhibitory connections with most cortical areas. Using serial visual search as an example, we describe how the functional anatomy of the claustral connections can potentially execute the sequential activation of the representations of objects that are being processed serially. We also propose that cross-frequency coupling (CFC) between low frequency signals from CL and higher frequency oscillations in the cortical areas will be an efficient means of CL modulating neural activity across multiple brain regions in synchrony. This model is applicable to the wide range of functions one performs, from simple object recognition to reading and writing, listening to or performing music, etc.

The relationship between the claustrum and endopiriform nucleus: A perspective towards consensus on cross-species homology

With the emergence of interest in studying the claustrum, a recent special issue of the Journal of Comparative Neurology dedicated to the claustrum (Volume 525, Issue 6, pp. 1313-1513) brought to light questions concerning the relationship between the claustrum (CLA) and a region immediately ventral known as the endopiriform nucleus (En). These structures have been identified as separate entities in rodents but appear as a single continuous structure in primates. During the recent Society for Claustrum Research meeting, a panel of experts presented data pertaining to the relationship of these regions and held a discussion on whether the CLA and En should be considered (a) separate unrelated structures, (b) separate nuclei within the same formation, or (c) subregions of a continuous structure. This review article summarizes that discussion, presenting comparisons of the cytoarchitecture, neurochemical profiles, genetic markers, and anatomical connectivity of the CLA and En across several mammalian species. In rodents, we conclude that the CLA and the dorsal endopiriform nucleus (DEn) are subregions of a larger complex, which likely performs analogous computations and exert similar effects on their respective cortical targets (e.g., sensorimotor versus limbic). Moving forward, we recommend that the field retain the nomenclature currently employed for this region but should continue to examine the delineation of these structures across different species. Using thorough descriptions of a variety of anatomical features, this review offers a clear definition of the CLA and En in rodents, which provides a framework for identifying homologous structures in primates.

Nucleus Accumbens Modulation in Reward and Aversion

The nucleus accumbens (NAc) is a key node of the brain’s circuitry that is responsible for translating motivation into action. It has been implicated in playing critical roles in virtually all forms of adaptive and pathological motivated behaviors. It is subject to modulation by a broad array of inputs that influence NAc activity in complex ways that are still poorly understood. Here, we briefly review current knowledge about the behavioral consequences of NAc modulation, focusing on recent studies that use novel techniques developed and implemented over the last decade.

Chronic immobilization stress primes the hippocampal opioid system for oxycodone-associated learning in female but not male rats

In adult female, but not male, Sprague Dawley rats, chronic immobilization stress (CIS) increases mossy fiber (MF) Leu-Enkephalin levels and redistributes delta- and mu-opioid receptors (DORs and MORs) in hippocampal CA3 pyramidal cells and GABAergic interneurons to promote excitation and learning processes following subsequent opioid exposure. Here, we demonstrate that CIS females, but not males, acquire conditioned place preference (CPP) to oxycodone and that CIS "primes" the hippocampal opioid system in females for oxycodone-associated learning. In CA3b, oxycodone-injected (Oxy) CIS females relative to saline-injected (Sal) CIS females exhibited an increase in the cytoplasmic and total densities of DORs in pyramidal cell dendrites so that they were similar to Sal- and Oxy-CIS males. Consistent with our earlier studies, Sal- and Oxy-CIS females but not CIS males had elevated DOR densities in MF-CA3 dendritic spines, which we have previously shown are important for opioid-mediated long-term potentiation. In the dentate gyrus, Oxy-CIS females had more DOR-labeled interneurons than Sal-CIS females. Moreover, Sal- and Oxy-CIS females compared to both groups of CIS males had elevated levels of DORs and MORs in GABAergic interneuron dendrites, suggesting capacity for greater synthesis or storage of these receptors in circuits important for opioid-mediated disinhibition. However, more plasmalemmal MORs were on large parvalbumin-containing dendrites of Oxy-CIS males compared to Sal-CIS males, suggesting a limited ability for increased granule cell disinhibition. These results suggest that low levels of DORs in MF-CA3 synapses and hilar GABAergic interneurons may contribute to the attenuation of oxycodone CPP in males exposed to CIS.

Rostral and Caudal Ventral Tegmental Area GABAergic Inputs to Different Dorsal Raphe Neurons Participate in Opioid Dependence

Both the ventral tegmental area (VTA) and dorsal raphe nucleus (DRN) are involved in affective control and reward-related behaviors. Moreover, the neuronal activities of the VTA and DRN are modulated by opioids. However, the precise circuits from the VTA to DRN and how opioids modulate these circuits remain unknown. Here, we found that neurons projecting from the VTA to DRN are primarily GABAergic. Rostral VTA (rVTA) GABAergic neurons preferentially innervate DRN GABAergic neurons, thus disinhibiting DRN serotonergic neurons. Optogenetic activation of this circuit induces aversion. In contrast, caudal VTA (cVTA) GABAergic neurons mainly target DRN serotonergic neurons, and activation of this circuit promotes reward. Importantly, μ-opioid receptors (MOPs) are selectively expressed at rVTA→DRN GABAergic synapses, and morphine depresses the synaptic transmission. Chronically elevating the activity of the rVTA→DRN pathway specifically interrupts morphine-induced conditioned place preference. This opioid-modulated inhibitory circuit may yield insights into morphine reward and dependence pathogenesis.

Electrophysiological Actions of N/OFQ

Whilst the nociceptin/orphanin FQ (N/OFQ) receptor (NOP) has similar intracellular coupling mechanisms to opioid receptors, it has distinct modulatory effects on physiological functions such as pain. These actions range from agonistic to antagonistic interactions with classical opioids within the spinal cord and brain, respectively. Understanding the electrophysiological actions of N/OFQ has been crucial in ascertaining the mechanisms by which these agonistic and antagonistic interactions occur. These similarities and differences between N/OFQ and opioids are due to the relative location of NOP versus opioid receptors on specific neuronal elements within these CNS regions. These mechanisms result in varied cellular actions including postsynaptic modulation of ion channels and presynaptic regulation of neurotransmitter release.

Activation of enkephalinergic (Enk) interneurons in the central amygdala (CeA) buffers the behavioral effects of persistent pain

Enk neurons in CeA modulate the activity of the amygdala projection neurons and it is very likely that changes of Enk signaling cause the heightened anxiety that accompanies chronic pain. We use chemogenetics and transgenic mice to investigate the effects of acute and continuous activation of the amygdala Enk neurons on persistent pain and anxiodepressive-like behavior in mice. Enk-cre mice were injected bilaterally into the CeA with cre-activated AAV-DREADD/Gq/mCherry, while neuropathic pain was induced by sciatic nerve constriction. A single injection of DREADD's ligand CNO decreased the anxiety-like behavior in both, uninjured mice and in mice with neuropathic pain and produced robust analgesia that lasted for 24 h. Furthermore, the activation of Enk neurons by the DREADD ligand led to increased c-Fos expression in PKC-δ interneurons of the CeA and in non-serotonergic neurons in the ventrolateral periaqueductal gray (vlPAG), a brain structure that is an essential part of the descending pain inhibitory system. Next, we added CNO to the drinking water of the experimental mice for 14 days in order to assess the effects of continuous activation of CeA Enk interneurons on anxiodepressive-like behavior, which is affected by chronic pain. The prolonged activation of the CeA Enk interneurons reduced neohypophagia in the novelty suppressed feeding test and increased ΔFosB (a marker for sustained neuronal activation) in the vlPAG of mice with chronic pain. All together, the results of our experiments point to an important role of the CeA Enk neurons in the control of both nociception and emotion. Activation of Enk neurons resulted in sustained analgesia accompanied by anxiolysis and antidepressant effects. Very likely, these effects of CeA Enk neurons are result of the activation of vlPAG, a brain region that is essential not only for descending inhibition of pain but it is also a core element in the resilience to stress.

Pharmacologically evoked apnoeas. Receptors and nervous pathways involved

This review analyses the knowledge about the incidence of transient apnoeic spells, induced by substances which activate vagal chemically sensitive afferents. It considers the specificity and expression of appropriate receptors, and relevant research on pontomedullary circuits contributing to a cessation of respiration. Insight is gained into an excitatory drive of 5-HT_1A serotonin receptors in overcoming opioid-induced respiratory inhibition.

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.

Untangling the complexity of opioid receptor function

Mu opioid receptor agonists are among the most powerful analgesic medications but also among the most addictive. The current opioid crisis has energized a quest to develop opioid analgesics that are devoid of untoward effects. Since their discovery in the 1970's, there have been major advances in our understanding of the endogenous opioid systems that these drugs target. Yet many questions remain and the development of non-addictive opioid analgesics has not been achieved. However, access to new molecular, genetic and computational tools have begun to elucidate the structural dynamics of opioid receptors, the scaffolding that links them to intracellular signaling cascades, their cellular trafficking and the distinct ways that various opioid drugs modify them. This mini-review highlights some of the chemical and pharmacological findings and new perspectives that have arisen from studies using these tools. They reveal multiple layers of complexity of opioid receptor function, including a spatiotemporal specificity in opioid receptor-induced cellular signaling, ligand-directed biased signaling, allosteric modulation of ligand interactions, heterodimerization of different opioid receptors, and the existence of slice variants with different ligand specificity. By untangling these layers, basic research into the chemistry and pharmacology of opioid receptors is guiding the way towards deciphering the mysteries of tolerance and physical dependence that have plagued the field and is providing a platform for the development of more effective and safer opioids.

Sex Differences in the Rat Hippocampal Opioid System After Oxycodone Conditioned Place Preference

Although opioid addiction has risen dramatically, the role of gender in addiction has been difficult to elucidate. We previously found sex-dependent differences in the hippocampal opioid system of Sprague-Dawley rats that may promote associative learning relevant to drug abuse. The present studies show that although female and male rats acquired conditioned place preference (CPP) to the mu-opioid receptor (MOR) agonist oxycodone (3 mg/kg, I.P.), hippocampal opioid circuits were differentially altered. In CA3, Leu-Enkephalin-containing mossy fibers had elevated levels in oxycodone CPP (Oxy) males comparable to those in females and sprouted in Oxy-females, suggesting different mechanisms for enhancing opioid sensitivity. Electron microscopy revealed that in Oxy-males delta opioid receptors (DORs) redistributed to mossy fiber-CA3 synapses in a manner resembling females that we previously showed is important for opioid-mediated long-term potentiation. Moreover, in Oxy-females DORs redistributed to CA3 pyramidal cell spines, suggesting the potential for enhanced plasticity processes. In Saline-injected (Sal) females, dentate hilar parvalbumin-containing basket interneuron dendrites had fewer MORs, however plasmalemmal and total MORs increased in Oxy-females. In dentate hilar GABAergic dendrites that contain neuropeptide Y, Sal-females compared to Sal-males had higher plasmalemmal DORs, and near-plasmalemmal DORs increased in Oxy-females. This redistribution of MORs and DORs within hilar interneurons in Oxy-females would potentially enhance disinhibition of granule cells via two different circuits. Together, these results indicate that oxycodone CPP induces sex-dependent redistributions of opioid receptors in hippocampal circuits in a manner facilitating opioid-associative learning processes and may help explain the increased susceptibility of females to opioid addiction acquisition and relapse.

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.