Lack of mu-opioid receptor leads to an increase in the NMDA receptor subunit mRNA expression and NMDA-induced convulsion

Receptor-centric solutions for the opioid epidemic: Making the opioid user impervious to overdose death

This mini-review presents the view that contemporary approaches to stem the tide of opioid overdose deaths are insufficient to make a significant impact. Instead, a focus on the opioid receptor as the ultimate molecular target to directly abolish opioid overdose death is explored. After identifying the key brainstem neurons that control respiration which are inhibited by opioid drugs, genetic techniques targeting the mu opioid receptor are detailed which prevent opioid overdose. This receptor-centric solution for the opioid overdose epidemic can be realized with existing technology and, with sufficient effort, could enter clinical trials within 5 to 10 years.

μ opioid receptor, social behaviour and autism spectrum disorder: reward matters

The endogenous opioid system is well known to relieve pain and underpin the rewarding properties of most drugs of abuse. Among opioid receptors, the μ receptor mediates most of the analgesic and rewarding properties of opioids. Based on striking similarities between social distress, physical pain and opiate withdrawal, μ receptors have been proposed to play a critical role in modulating social behaviour in humans and animals. This review summarizes experimental data demonstrating such role and proposes a novel model, the μ opioid receptor balance model, to account for the contribution of μ receptors to the subtle regulation of social behaviour. Interestingly, μ receptor null mice show behavioural deficits similar to those observed in patients with autism spectrum disorder (ASD), including severe impairment in social interactions. Therefore, after a brief summary of recent evidence for blunted (social) reward processes in subjects with ASD, we review here arguments for altered μ receptor function in this pathology. This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Autistic-like syndrome in mu opioid receptor null mice is relieved by facilitated mGluR4 activity

The etiology of Autism Spectrum Disorders (ASDs) remains largely unknown. Identifying vulnerability genes for autism represents a major challenge in the field and allows the development of animal models for translational research. Mice lacking the mu opioid receptor gene (Oprm1(-/-)) were recently proposed as a monogenic mouse model of autism, based on severe deficits in social behavior and communication skills. We confirm this hypothesis by showing that adult Oprm1(-/-) animals recapitulate core and multiple comorbid behavioral symptoms of autism and also display anatomical, neurochemical, and genetic landmarks of the disease. Chronic facilitation of mGluR4 signaling, which we identified as a novel pharmacological target in ASDs in these mice, was more efficient in alleviating behavioral deficits than the reference molecule risperidone. Altogether, our data provide first evidence that disrupted mu opioid receptor signaling is sufficient to trigger a comprehensive autistic syndrome, maybe through blunted social reward processes, and this mouse model opens promising avenues for therapeutic innovation.

Effects of LiCl/pilocarpine-induced status epilepticus on rat brain mu and benzodiazepine receptor binding: regional and ontogenetic studies

Neurochemical studies document involvement of benzodiazepine (BDZ) and mu opioid receptors in seizure development and their possible age-related role during epileptogenesis. To study developmental changes of this role LiCl/pilocarpine status epilepticus (SE) was induced in P12, P25 and/or adult rats. This SE leads to epilepsy in all adult and subpopulation of immature rats. Using in vitro autoradiography, benzodiazepine (BDZ) and mu opioid receptor binding was evaluated 1 week (early phase of epileptogenesis) and 3 months (chronic phase) after SE in 27 brain structures involved in seizure generation and spread (amygdala, hippocampus, basal ganglia and thalamic nuclei). The pattern of receptor binding changes was related to the age at SE, interval after SE and to brain structures. Enhanced BDZ binding was found 1 week after SE in many cortical areas in P12 and also in the amygdala complex and dentate gyrus in both P12 and P25. No changes of BDZ binding occurred in adults at that time, but 3 months after SE a decrease of binding appeared in all evaluated areas in both adult and P25 but not P12 rats. This decrease did not reflect neuronal loss. mu opioid receptors were less significantly affected but clear tendency to decrease binding occurred in adult rats in various cortical, amygdala and thalamic regions early after SE. Changes were less expressed in immature rats. Our data support the hypothesis that age-related changes of receptor properties may participate in different functional consequences of SE including epileptogenesis (more common in older age groups) and behavioral changes.

Autoradiographic analysis of GABAA receptors in mu-opioid receptor knockout mice

Anatomical evidence indicates that gamma-aminobutyric acid (GABA)-ergic and opioidergic systems are closely linked and act on the same neurons. However, the regulatory mechanisms between GABAergic and opioidergic system have not been well characterized. In the present study, we investigated whether there are changes in GABA(A) receptors in mice lacking mu-opioid receptor gene. The GABA(A) receptor binding was carried out by autoradiography using [(3)H]-muscimol (GABA(A)), [(3)H]-flunitrazepam (FNZ, native type 1 benzodiazepine) and [(35)S]-t-butylbicyclophosphorothionate (TBPS, binding to GABA(A)-gated chloride channels) in brain slices of wild type and mu-opioid receptor knockout mice. The binding of [(3)H]-FNZ in mu-opioid receptor knockout mice was significantly higher than that of the wild type controls in most of the cortex and hippocampal CA1 and CA2 formations. mu-Opioid receptor knockout mice show significantly lower binding of [(35)S]-TBPS than that of the wild type mice in few of the cortical areas including ectorhinal cortex layers I, III, and V, but not in the hippocampus. There was no significant difference in binding of [(3)H]-muscimol between mu-opioid receptor knockout and wild type mice in the cortex and hippocampus. These data indicate that there are specific regional changes in GABA(A) receptor binding sites in mu-opioid receptor knockout mice. These data also suggest that there are compensatory up-regulation of benzodiazepine binding site of GABA(A) receptors in the cortex and hippocampus and down-regulation of GABA-gated chloride channel binding site of GABA(A) receptors in the cortex of the mu-opioid receptor knockout mice.

No Association of Single Nucleotide Polymorphisms in the ?-Opioid Receptor Subunit Gene with Idiopathic Generalized Epilepsy

We have investigated the reported association (p = 0.019) between the A118G single nucleotide polymorphism (SNP) of the opioid receptor micro subunit gene (OPRM1) and idiopathic absence epilepsy (IAE). Five SNPs, including A118G, were investigated by association studies in a sample of 240 probands with idiopathic generalized epilepsy (IGE), including 110 with IAE, and 257 controls. No significant association was found for A118G with IGE or IAE. The difference between the two studies was in the control samples that had significantly different allele frequencies (p = 0.00005), suggesting that population stratification may explain the earlier significant association with IAE. In the current study, none of the other four SNPs was significantly associated with IGE or IAE. Our results provide no support for association of A118G with either IAE or IGE and also exclude association in our sample of a small-to-moderate gene effect with IGE from a large part of OPRM1.

A lack of μ-opioid receptors modulates the expressions of neuropeptide Y and substance P mRNA

The present study was undertaken to investigate changes in the expressions of neuropeptide Y (NPY) and substance P (SP) in mice lacking mu-opioid receptors. In an in situ hybridization study, in which we compared wild type and mu-opioid receptor knockout mice, NPY mRNA levels were found to be lower in the caudate-putamen and nucleus accumbens of mu-opioid receptor knockout mice. In addition, SP mRNA levels were lower in the ventromedial hypothalamic nucleus of mu-opioid receptor knockout mice. Our findings suggest that a lack of mu-opioid receptors modulates basal NPY mRNA levels in striatal regions and SP mRNA levels in the ventromedial hypothalamic nucleus of the mouse, and that these changes are due to compensatory modulation in the brain.

Deletion of the mu opioid receptor results in impaired acquisition of Pavlovian context fear

The mu opioid receptor may constitute a critical component of a negative feedback system that regulates Pavlovian fear conditioning. We investigated context fear conditioning acquisition and expression in mu opioid receptor knockout mice (on an inbred, C57 genetic background). We discovered that the mu receptor knockout results in an unexpected and significant deficit in context fear acquisition. Mice lacking the mu receptor showed normal fear acquisition when subjected to a 1-day fear conditioning protocol but evinced deficient fear learning when acquisition was conducted across 5 days. The knockout mice showed normal reactivity to footshock in both fear conditioning protocols. Finally, we confirmed the effectiveness of the receptor deletion in the C57 strain by subjecting the mice to a test of morphine analgesia in the hot-plate assay. As has been seen with mixed genetic background, the receptor deletion resulted in a complete lack of analgesic response to 10 mg/kg morphine. Surprisingly, mice with a single copy of the mu receptor gene (heterozygous knockouts) showed intact sensitivity to morphine but a significant deficit in Pavlovian fear conditioning. The results indicate that deletion of the mu receptor gene impairs fear conditioning and that the conditioning and analgesia effects of heterozygous deletion are dissociable. The conditioning deficit seen in this line of mice may be related to impairment in hippocampus function.

Passage à la chronicité d’une toux : quels mécanismes ?

INTRODUCTION

In some situations such as post-virus or post whooping cough, a non productive subacute cough may occur without apparent local inflammation, epithelium abnormalities or bronchoconstriction. This subacute or chronic cough represents a real syndrome (cough disease) due to the central nervous system (CNS) and its ortho and parasympathic outputs. At the CNS level, functional disturbancies and neosynaptogenesis can be described, with the intervention of the NMDA-type glutamatergic receptors.

STATE OF ART

The neurons located in the expiratory area of the breathing center (Pre-Boetzinger complex of the lower brainstem) present exagerated responses to stimuli, due to the repetitive stimulation of the NMDA receptors; this phenomenon is similar to long-term-potentiation (LTP), the molecular basis of learning, memory and neosynaptogenesis. The cough reflex is thus amplified and rapidly chronic and would justify any pharmacological intervention at the NMDA-receptors level.

PERSPECTIVES

More recently 5TH4 receptors have been implied in the control of respiration; an overexpression of these receptors in the Pre-Boetzinger area could contribute to an increase of the cough reflex.

CONCLUSION

The present review aims at summarizing the main rationale target to pharmacologically block the chronic cough.

Neurofilament proteins and cAMP pathway in brains of mu-, delta- or kappa-opioid receptor gene knock-out mice: effects of chronic morphine administration

Opiate addiction is associated with abnormalities of neurofilament (NF) proteins and upregulation of cAMP signaling in the brain, which may modulate neuronal plasticity. This study investigated, using gene-targeted mice lacking mu-, delta- or kappa-opioid receptors, the role of these receptors in modulating the basal activity and the chronic effects of morphine on both intracellular targets. In WT mice, chronic treatment (5 days) with morphine (20-100 mg/kg) resulted in decreases in the immunodensity of neurofilament (NF)-L in the cerebral cortex (14-23%). In contrast, chronic morphine did not decrease NF-L in cortices of mu-, delta-, and kappa-KO mice, suggesting the involvement of the three types of opioid receptors in this effect of morphine. Also, the marked increase in phosphorylated NF-H induced by chronic morphine in WT mice (two-fold) was abolished in mu -KO mice. In cortex and/or striatum of mu-, delta- and kappa-KO mice, the basal immunodensities of Galphai1/2 proteins, the catalytic isoform (Calpha) of protein kinase A (PKA) and the total content of cAMP response element-binding protein (CREB, the nuclear target of PKA) were not different from those of WT mice. In contrast, phosphorylated CREB (the active form of this transcription factor) was reduced in cortex and/or striatum (23-26%) of mu- and delta-KO mice, but not in kappa-KO animals. These results suggest that the endogenous opioid tone acting on mu-/delta-receptors tonically stimulate CREB activation in the brain. In cortex and/or striatum of WT mice, chronic morphine did not induce upregulation of the main components of the cAMP signaling pathway. In contrast, chronic morphine treatment in mu-KO mice, but not in delta- or kappa-KO, resulted in a paradoxical upregulation of Galphai1/2 (12-19%), PKA (19-21%,) and phosphorylated CREB (21-73%), but not total CREB, in cortex and/or striatum. The induction of heterologous receptor adaptations in mu-KO mice may explain this paradoxical effect of morphine.

Altered emotional behaviors and the expression of 5-HT1A and M1 muscarinic receptors in ?-opioid receptor knockout mice

Anxiety and depression alterations have been reported in micro-opioid receptor knockout mice after exon 2 disruption. However, emotional behaviors, such as novelty and emergence responses have not been reported in micro-opioid receptor knockout mice due to the disruptions of exon 2 and 3. Here, we report that mu-opioid receptor knockout mice, with deletion of exon 2 and 3, display significant emotional behavior changes; they showed less anxiety in the elevated plus maze and emergence tests, reduced response to novel stimuli in the novelty test, and less depressive-like behavior in the forced-swim test. Analysis of the compensatory mechanism in mu-opioid receptor knockout mice revealed that the M1 mRNA levels were reduced in the cortex, caudate putamen, nucleus accumbens, and hippocampus, and that M1 receptor levels were reduced in the nucleus accumbens, CA1, and the dentate gyrus of the hippocampus, versus the wild-type. However, 5-HT1A receptor levels were significantly elevated in the cerebral cortex and in the hypothalamus of mu-opioid receptor knockout mice versus the wild-type. These aberrant emotional behavioral phenotypes are possibly related to M1 and 5-HT1A receptor alterations in the micro-opioid receptor knockout mice. Overall, our study suggests that micro-opioid receptor may play a role in the modification of emotional responses to novelty, anxiety, and depression.

Impaired water maze learning performance in μ-opioid receptor knockout mice

Previous study has demonstrated that the lack of mu-opioid receptor decreased LTP in the dentate gyrus of the hippocampus, suggesting the possibility that the lack of mu-opioid receptor may accompany a change in learning and memory. However, no behavioral study has been undertaken to correlate LTP deficits with spatial memory impairment in mu-opioid receptor knockout mice. Therefore, the present study investigated the hypothesis that mu-opioid receptors contribute to learning and memory by using the Morris water maze, and comparing responses in wild type and mu-opioid receptor gene knockout mice. Our results indicated that mu-opioid receptor knockout mice showed a significant spatial memory impairment compared to wild type in the Morris water maze. This result suggests that the expression of mu-opioid receptor plays an important role in spatial learning and memory examined by Morris water maze.

Regional specific increases of [3H]AMPA binding and mRNA expression of AMPA receptors in the brain of mu-opioid receptor knockout mice

Previous pharmacological studies have indicated the possible existence of functional interactions between opioidergic and glutamatergic neurons in the CNS. In the present study, [(3)H]AMPA binding and the expression of mRNAs encoding flip and flop variants of three subtypes of AMPA glutamate receptor GluR1-3 were examined by in situ hybridization technique in order to investigate whether there is a change in the AMPA receptor system of mice lacking the mu-opioid receptor. In the mu-opioid receptor knockout mice, [(3)H]AMPA binding was increased in the hippocampal CA1 and dentate gyrus, cortex, and caudate putamen compared with that of the wild-type animals. The expression of GluR1 flip mRNA was increased in the cortex and caudate putamen of mu-opioid receptor knockout mice. The expression of GluR1 flop mRNA was increased in the cortex, caudate putamen, and hippocampal CA1 layer of mu-opioid receptor knockout mice. The expression of GluR2 flip mRNA was decreased in the hippocampal dentate gyrus of mu-opioid receptor knockout mice. The expression of GluR2 flop was not altered in any regions studied. The expression of GluR3 flip was increased in the cortical area and caudate putamen of mu-opioid receptor knockout mice. The expression of GluR3 flop was increased in the cortical area, hippocampal CA3 area, and caudate putamen of mu-opioid receptor knockout mice. These results indicate that [(3)H]AMPA binding and the expression of GluR1-3 mRNA were increased in a region and subunit specific manner, and suggest that changes in the AMPA receptor system are accompanied by the absence of mu-opioid receptor gene.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).