Gene expression is altered in the lateral hypothalamus upon activation of the mu opioid receptor

Functional genomic mechanisms of opioid action and opioid use disorder: a systematic review of animal models and human studies

In the past two decades, over-prescription of opioids for pain management has driven a steep increase in opioid use disorder (OUD) and death by overdose, exerting a dramatic toll on western countries. OUD is a chronic relapsing disease associated with a lifetime struggle to control drug consumption, suggesting that opioids trigger long-lasting brain adaptations, notably through functional genomic and epigenomic mechanisms. Current understanding of these processes, however, remain scarce, and have not been previously reviewed systematically. To do so, the goal of the present work was to synthesize current knowledge on genome-wide transcriptomic and epigenetic mechanisms of opioid action, in primate and rodent species. Using a prospectively registered methodology, comprehensive literature searches were completed in PubMed, Embase, and Web of Science. Of the 2709 articles identified, 73 met our inclusion criteria and were considered for qualitative analysis. Focusing on the 5 most studied nervous system structures (nucleus accumbens, frontal cortex, whole striatum, dorsal striatum, spinal cord; 44 articles), we also conducted a quantitative analysis of differentially expressed genes, in an effort to identify a putative core transcriptional signature of opioids. Only one gene, Cdkn1a, was consistently identified in eleven studies, and globally, our results unveil surprisingly low consistency across published work, even when considering most recent single-cell approaches. Analysis of sources of variability detected significant contributions from species, brain structure, duration of opioid exposure, strain, time-point of analysis, and batch effects, but not type of opioid. To go beyond those limitations, we leveraged threshold-free methods to illustrate how genome-wide comparisons may generate new findings and hypotheses. Finally, we discuss current methodological development in the field, and their implication for future research and, ultimately, better care.

Apolipoprotein D as a Potential Biomarker in Neuropsychiatric Disorders

Neuropsychiatric disorders (NDs) are a diverse group of pathologies, including schizophrenia or bipolar disorders, that directly affect the mental and physical health of those who suffer from them, with an incidence that is increasing worldwide. Most NDs result from a complex interaction of multiple genes and environmental factors such as stress or traumatic events, including the recent Coronavirus Disease (COVID-19) pandemic. In addition to diverse clinical presentations, these diseases are heterogeneous in their pathogenesis, brain regions affected, and clinical symptoms, making diagnosis difficult. Therefore, finding new biomarkers is essential for the detection, prognosis, response prediction, and development of new treatments for NDs. Among the most promising candidates is the apolipoprotein D (Apo D), a component of lipoproteins implicated in lipid metabolism. Evidence suggests an increase in Apo D expression in association with aging and in the presence of neuropathological processes. As a part of the cellular neuroprotective defense machinery against oxidative stress and inflammation, changes in Apo D levels have been demonstrated in neuropsychiatric conditions like schizophrenia (SZ) or bipolar disorders (BPD), not only in some brain areas but in corporal fluids, i.e., blood or serum of patients. What is not clear is whether variation in Apo D quantity could be used as an indicator to detect NDs and their progression. This review aims to provide an updated view of the clinical potential of Apo D as a possible biomarker for NDs.

Exposure to Morphine and Cocaine Modify the Transcriptomic Landscape in Zebrafish Embryos

Morphine and other opioid analgesics are the drugs of election to treat moderate-to-severe pain, and they elicit their actions by binding to the opioid receptors. Cocaine is a potent inhibitor of dopamine, serotonin, and noradrenaline reuptake, as it blocks DAT, the dopamine transporter, causing an increase in the local concentration of these neurotransmitters in the synaptic cleft. The molecular effects of these drugs have been studied in specific brain areas or nuclei, but the systemic effects in the whole organism have not been comprehensively analyzed. This study aims to analyze the transcriptomic changes elicited by morphine (10 uM) and cocaine (15 uM) in zebrafish embryos. An RNAseq assay was performed with tissues extracts from zebrafish embryos treated from 5 hpf (hours post fertilization) to 72 hpf, and the most representative deregulated genes were experimentally validated by qPCR. We have found changes in the expression of genes related to lipid metabolism, chemokine receptor ligands, visual system, hemoglobins, and metabolic detoxification pathways. Besides, morphine and cocaine modified the global DNA methylation pattern in zebrafish embryos, which would explain the changes in gene expression elicited by these two drugs of abuse.

Mechanism of opioid addiction and its intervention therapy: Focusing on the reward circuitry and mu-opioid receptor

Opioid abuse and addiction have become a global pandemic, posing tremendous health and social burdens. The rewarding effects and the occurrence of withdrawal symptoms are the two mainstays of opioid addiction. Mu-opioid receptors (MORs), a member of opioid receptors, play important roles in opioid addiction, mediating both the rewarding effects of opioids and opioid withdrawal syndrome (OWS). The underlying mechanism of MOR-mediated opioid rewarding effects and withdrawal syndrome is of vital importance to understand the nature of opioid addiction and also provides theoretical basis for targeting MORs to treat drug addiction. In this review, we first briefly introduce the basic concepts of MORs, including their structure, distribution in the nervous system, endogenous ligands, and functional characteristics. We focused on the brain circuitry and molecular mechanism of MORs-mediated opioid reward and withdrawal. The neuroanatomical and functional elements of the neural circuitry of the reward system underlying opioid addiction were thoroughly discussed, and the roles of MOR within the reward circuitry were also elaborated. Furthermore, we interrogated the roles of MORs in OWS, along with the structural basis and molecular adaptions of MORs-mediated withdrawal syndrome. Finally, current treatment strategies for opioid addiction targeting MORs were also presented.

The Lipocalin Apolipoprotein D Functional Portrait: A Systematic Review

Apolipoprotein D is a chordate gene early originated in the Lipocalin protein family. Among other features, regulation of its expression in a wide variety of disease conditions in humans, as apparently unrelated as neurodegeneration or breast cancer, have called for attention on this gene. Also, its presence in different tissues, from blood to brain, and different subcellular locations, from HDL lipoparticles to the interior of lysosomes or the surface of extracellular vesicles, poses an interesting challenge in deciphering its physiological function: Is ApoD a moonlighting protein, serving different roles in different cellular compartments, tissues, or organisms? Or does it have a unique biochemical mechanism of action that accounts for such apparently diverse roles in different physiological situations? To answer these questions, we have performed a systematic review of all primary publications where ApoD properties have been investigated in chordates. We conclude that ApoD ligand binding in the Lipocalin pocket, combined with an antioxidant activity performed at the rim of the pocket are properties sufficient to explain ApoD association with different lipid-based structures, where its physiological function is better described as lipid-management than by long-range lipid-transport. Controlling the redox state of these lipid structures in particular subcellular locations or extracellular structures, ApoD is able to modulate an enormous array of apparently diverse processes in the organism, both in health and disease. The new picture emerging from these data should help to put the physiological role of ApoD in new contexts and to inspire well-focused future research.

Effect of the spinal apelin‑APJ system on the pathogenesis of chronic constriction injury‑induced neuropathic pain in rats

Apelin is hypothesized to serve a dual function in pain processing. Spinal administration of apelin induces hyperalgesia, while opioid receptors are implicated in the antinociceptive effects of apelin in acute nociceptive models. However, whether the apelin-apelin receptor (APJ) system is involved in neuropathic pain remains to be elucidated. The present study aimed to evaluate the impact and mechanism of the spinal apelin-APJ system in neuropathic pain. Chronic constriction injury (CCI) of the sciatic nerve produced sustained spinal apelin and APJ upregulation, which was associated with mechanical allodynia and heat hyperalgesia development in the hind-paw plantar surface. Immunofluorescence demonstrated that apelin and APJ were localized to the superficial dorsal horns. In order to further clarify the function of the apelin-APJ system, a single intrathecal administration of ML221, an APJ antagonist, was used; this transiently reduced CCI-induced pain hypersensitivity. However, apelin-13 (the isoform which binds most strongly to APJ) exhibited no effect on the nociceptive response, suggesting an essential role for the spinal apelin-APJ system in neuropathic pain sensitization. The present study demonstrated that a single application of ML221 alleviated mechanical allodynia and heat hyperalgesia 7 days following CCI, in a dose-dependent manner. Intraspinal delivery of ML221, at the onset of and in fully-established neuropathic pain, persistently attenuated CCI-induced pain hypersensitivity, indicating that the apelin-APJ system was involved in initiating and maintaining pain. It was demonstrated, using immunoblotting, that intrathecal ML221 downregulated phosphorylated extracellular signal-related kinase (ERK) in the rat spinal cord dorsal horn, suggesting that the effect of apelin on neuropathic pain may be mediated via ERK signaling. The results of the present study suggested that the spinal apelin-APJ system may drive neuropathic pain. Inhibition of APJ may provide novel pharmacological interventions for neuropathic pain.

Mu Opioid Receptors in Gamma-Aminobutyric Acidergic Forebrain Neurons Moderate Motivation for Heroin and Palatable Food

BACKGROUND

Mu opioid receptors (MORs) are central to pain control, drug reward, and addictive behaviors, but underlying circuit mechanisms have been poorly explored by genetic approaches. Here we investigate the contribution of MORs expressed in gamma-aminobutyric acidergic forebrain neurons to major biological effects of opiates, and also challenge the canonical disinhibition model of opiate reward.

METHODS

We used Dlx5/6-mediated recombination to create conditional Oprm1 mice in gamma-aminobutyric acidergic forebrain neurons. We characterized the genetic deletion by histology, electrophysiology, and microdialysis; probed neuronal activation by c-Fos immunohistochemistry and resting-state functional magnetic resonance imaging; and investigated main behavioral responses to opiates, including motivation to obtain heroin and palatable food.

RESULTS

Mutant mice showed MOR transcript deletion mainly in the striatum. In the ventral tegmental area, local MOR activity was intact, and reduced activity was only observed at the level of striatonigral afferents. Heroin-induced neuronal activation was modified at both sites, and whole-brain functional networks were altered in live animals. Morphine analgesia was not altered, and neither was physical dependence to chronic morphine. In contrast, locomotor effects of heroin were abolished, and heroin-induced catalepsy was increased. Place preference to heroin was not modified, but remarkably, motivation to obtain heroin and palatable food was enhanced in operant self-administration procedures.

CONCLUSIONS

Our study reveals dissociable MOR functions across mesocorticolimbic networks. Thus, beyond a well-established role in reward processing, operating at the level of local ventral tegmental area neurons, MORs also moderate motivation for appetitive stimuli within forebrain circuits that drive motivated behaviors.

Induction of antinociceptive tolerance to the chronic intrathecal administration of apelin-13 in rat

Pain represents a major contributing factor to the individual's quality of life. Although pain killers as opioids, endogenous or exogenous peptides can decrease pain perception, the chronic use of them leads to antinociceptive tolerance. It has been demonstrated that neuropeptide apelin has potent antinoceptive effect. However, the possibility of the induction of its antinociceptive tolerance has not yet been clarified. The tail-flick test was used to assess the nociceptive threshold. All experiments were carried out on male Wistar rats which received intrathecal apelin for 7days. To determine the role of apelin and opioid receptors on the development of apelin analgesic tolerance, their receptor antagonists (F-13 A and naloxone, respectively) were injected simultaneously with apelin. The lumbar spinal cord was assayed to determine apelin receptor levels by the western blotting method. Plasma corticosterone levels were assayed using ELISA. Results showed that apelin (3μg/rat) induced strong thermal antinociception. In addition, chronic apelin produced tolerance to its antinociceptive effect and down regulated spinal apelin receptor. F-13 A and naloxone could inhibit apelin tolerance development. The corticosterone levels did not change following drug administration. Taken together, the data indicated that apelin like other analgesic drugs leads to the induction of side effects such as analgesic tolerance which is mediated partly via the apelin and opioid receptors activation.

The Effect of Aerobic Exercise Training on Plasma Apelin Levels and Pain Threshold in T1DM Rats

Background

Diabetes mellitus (types 1 and 2) leads to secondary complications such as neuropathy, which reduce a patient’s quality of life. Apelin and its receptor, APJ, have been shown to have antinociceptive effects and to decrease blood glucose levels.

Objectives

The present experimental study was conducted in Iran and investigated the role of apelin, which is used to manage type 1 diabetes mellitus, during exercise training.

Materials and Methods

Male Wistar rats (n = 36) were assigned by simple random allocation to six groups (n = 6): non-diabetic (ND), diabetic (D), sedentary non-diabetic (SND), sedentary diabetic (SD), exercise non-diabetic (END), and exercise diabetic (ED). Diabetes was induced by a single subcutaneous injection of streptozotocin (50 mg/kg). Exercise training consisted of treadmill running 60 minutes/day × 5 days/week for 10 weeks. The tail-flick test was used to assess the thermal pain threshold, and an apelin enzyme immunoassay kit was utilized to assess plasma apelin levels.

Results

Plasma apelin level was higher (0.3 vs. 0.1, P < 0.0001) and the tail-flick latency was lower (2.2 vs. 3.8, P < 0.0001) in the D group than in the ND group. After the training program, plasma apelin levels decreased in the exercise groups, and the tail-flick latency increased in the ED group. No correlation was found between apelin blood concentrations and tail-flick latency following the training program in the ED group.

Conclusions

These findings suggest that apelin does not play any significant role in regulating the pain threshold in type 1 diabetes mellitus during exercise training.

Whole-genome expression profile in zebrafish embryos after chronic exposure to morphine: identification of new genes associated with neuronal function and mu opioid receptor expression

BACKGROUND

A great number of studies have investigated changes induced by morphine exposure in gene expression using several experimental models. In this study, we examined gene expression changes during chronic exposure to morphine during maturation and differentiation of zebrafish CNS.

RESULTS

Microarray analysis showed 254 genes whose expression was identified as different by at least 1.3 fold change following chronic morphine exposure as compared to controls. Of these, several novel genes (grb2, copb2, otpb, magi1b, grik-l, bnip4 and sox19b) have been detected for the first time in an experimental animal model treated with morphine. We have also identified a subset of genes (dao.1, wls, bnip4 and camk1γb) differentially expressed by chronic morphine exposure whose expression is related to mu opioid receptor gene expression. Altered expression of copb2, bnip4, sox19b, otpb, dao.1, grik-l and wls is indicative of modified neuronal development, CNS patterning processes, differentiation and dopaminergic neurotransmission, serotonergic signaling pathway, and glutamatergic neurotransmission. The deregulation of camk1γb signaling genes suggests an activation of axonogenesis and dendritogenesis.

CONCLUSIONS

Our study identified different functional classes of genes and individual candidates involved in the mechanisms underlying susceptibility to morphine actions related to CNS development. These results open new lines to study the treatment of pain and the molecular mechanisms involved in addiction. We also found a set of zebrafish-specific morphine-induced genes, which may be putative targets in human models for addiction and pain processes.

Differential Changes in Expression of Stress- and Metabolic-Related Neuropeptides in the Rat Hypothalamus during Morphine Dependence and Withdrawal

Chronic morphine treatment and naloxone precipitated morphine withdrawal activates stress-related brain circuit and results in significant changes in food intake, body weight gain and energy metabolism. The present study aimed to reveal hypothalamic mechanisms underlying these effects. Adult male rats were made dependent on morphine by subcutaneous implantation of constant release drug pellets. Pair feeding revealed significantly smaller weight loss of morphine treated rats compared to placebo implanted animals whose food consumption was limited to that eaten by morphine implanted pairs. These results suggest reduced energy expenditure of morphine-treated animals. Chronic morphine exposure or pair feeding did not significantly affect hypothalamic expression of selected stress- and metabolic related neuropeptides - corticotropin-releasing hormone (CRH), urocortin 2 (UCN2) and proopiomelanocortin (POMC) compared to placebo implanted and pair fed animals. Naloxone precipitated morphine withdrawal resulted in a dramatic weight loss starting as early as 15–30 min after naloxone injection and increased adrenocorticotrophic hormone, prolactin and corticosterone plasma levels in morphine dependent rats. Using real-time quantitative PCR to monitor the time course of relative expression of neuropeptide mRNAs in the hypothalamus we found elevated CRH and UCN2 mRNA and dramatically reduced POMC expression. Neuropeptide Y (NPY) and arginine vasopressin (AVP) mRNA levels were transiently increased during opiate withdrawal. These data highlight that morphine withdrawal differentially affects expression of stress- and metabolic-related neuropeptides in the rat hypothalamus, while relative mRNA levels of these neuropeptides remain unchanged either in rats chronically treated with morphine or in their pair-fed controls.

Supraspinal antinociceptive effect of apelin-13 in a mouse visceral pain model

Apelin, as the endogenous ligand of the APJ receptor, is a novel identified neuropeptide whose biological functions are not fully understood. APJ receptor mRNA was found in several brain regions related to descending control system of pain, such as amygdala, hypothalamus and dorsal raphe nucleus (DRN). The present study was designed to determine whether supraspinal apelin-13 may produce antinociceptive effect observed in the acetic acid-induced writhing test, a model of visceral pain. Apelin-13 not only significantly produced preemptive antinociception at the dose of 0.3, 0.5, 1 and 3 μg/mouse when injected intracerebroventricularly (i.c.v.) before acetic acid, but also significantly induced antinociception at a dose of 0.5, 1 and 3 μg/mouse when injected i.c.v. after acetic acid. And i.c.v. apelin-13 did not influence 30-min locomotor activity counts in mice. Intrathecal (i.t.) administration of apelin-13 (1 and 3 μg/mouse) significantly decreased the number of writhes, however, intraperitoneal (i.p.) injection of apelin-13 (10-100 μg/mouse) had no effect on the number of writhes in the writhing test. The specific APJ receptor antagonist apelin-13(F13A), no-specific opioid receptor antagonist naloxone and μ-opioid receptor antagonist β-funaltrexamine hydrochloride (β-FNA) could significantly antagonize the antinociceptive effect of i.c.v. apelin-13, suggesting APJ receptor and μ-opioid receptor are involved in this process. Central low dose of apelin-13 (0.3 μg/mouse, i.c.v.) could significantly potentiate the analgesic potencies of modest and even relatively ineffective doses of morphine administrated at supraspinal level. This enhanced antinociceptive effect was reversed by naloxone, suggesting that the potentiated analgesic response is mediated by opioid-responsive neurons.

Identification of genes regulated in the mouse extended amygdala by excessive ethanol drinking associated with dependence

Alcoholism is characterized by a progressive loss of control over ethanol intake. The purpose of this study was to identify transcriptional changes selectively associated with excessive ethanol drinking in dependent mice, as opposed to non-dependent mice maintaining a stable voluntary consumption or mice solely undergoing forced intoxication. We measured expression levels of 106 candidate genes in the extended amygdala, a key brain structure for the development of drug addiction. Cluster analysis identified 17 and 15 genes selectively induced or repressed, respectively, under conditions of excessive drinking. These genes belong to signaling pathways involved in neurotransmission and transcriptional regulation.

Effect of centrally administered apelin-13 on gastric emptying and gastrointestinal transit in mice

Apelin, as the endogenous ligand for the APJ, regulates many biological functions, including blood pressure, neuroendocrine, drinking behavior, food intake and colonic motility. The present study was designed to investigate the effect of central apelin-13 on gastric emptying and gastrointestinal transit in mice. Intracerebroventricular (i.c.v.) injection of apelin-13 (3 and 10 μg/mouse) decreased gastric emptying rate by 10.9% and 17.1%. This effect was significantly antagonized by the APJ receptor antagonist apelin-13(F13A) and the opioid receptor antagonist naloxone, respectively. However, intraperitoneal (i.p.) injection of apelin-13 (10-100 μg/mouse) did not affect gastric emptying. Apelin-13 (0.3, 1 and 3 μg/mouse, i.c.v.) inhibited gastrointestinal transit by 16.8%, 23.4% and 19.2%. Apelin-13(F13A) and naloxone could also reverse this antitransit effect induced by apelin-13. Taken together, these results suggest that i.c.v. injected apelin-13 inhibits gastric emptying and gastrointestinal transit and it seems that APJ receptor and opioid receptor might be involved in these processes.

Impaired emotional-like behavior and serotonergic function during protracted abstinence from chronic morphine

BACKGROUND

Opiate abuse is a chronic relapsing disorder, and maintaining prolonged abstinence remains a major challenge. Protracted abstinence is characterized by lowered mood, and clinical studies show elevated comorbidity between addiction and depressive disorders. At present, their relationship remains unclear and has been little studied in animal models. Here we investigated emotional alterations during protracted abstinence, in mice with a history of chronic morphine exposure.

METHODS

C57BL6J mice were exposed to a chronic intermittent escalating morphine regimen (20-100 mg/kg). Physical dependence (naloxone-precipitated withdrawal), despair-related behaviors (tail suspension test), and social behaviors were examined after 1 or 4 weeks of abstinence. Stress hormones and forebrain bioamine levels were analyzed at the end of morphine regimen and after 4 weeks of abstinence. Finally, we examined the effects of chronic fluoxetine during abstinence on morphine-induced behavioral deficits.

RESULTS

Acute naloxone-induced withdrawal was clearly measurable after 1 week, and became undetectable after 4 weeks. In contrast, social and despair-related behaviors were unchanged after 1 week, but low sociability and despair-like behavior became significant after 4 weeks. Chronic morphine regimen increased both corticosterone levels and forebrain serotonin turnover, but only serotonergic activity in the dorsal raphe remained impaired after 4 weeks. Remarkably, chronic fluoxetine prevented depressive-like behavioral deficits in 4-week abstinent mice.

CONCLUSIONS

During protracted abstinence, the immediate consequences of morphine exposure attenuate, whereas fluoxetine-sensitive emotional alterations strengthen with time. Our study establishes a direct link between morphine abstinence and depressive-like symptoms and strongly suggests that serotonin dysfunction represents a main mechanism contributing to mood disorders in opiate abstinence.

Intracerebroventricular administration of apelin-13 inhibits distal colonic transit in mice

Apelin is a novel bioactive peptide as the endogenous ligand for the orphan G-protein-coupled receptor (GPCR), APJ, a receptor distributed in various tissues such as the hypothalamus and the gastrointestinal tract. Recent reports showed that apelin regulated many biological functions, including blood pressure, neuroendocrine, drinking behavior and food intake. However, the role of apelin in regulating gastrointestinal motility remains unknown. The present study aimed to investigate the actions of intracerebroventricularly administered apelin-13 on colonic transit as well as the actions of apelin-13 on the contraction of isolated distal colon in vitro. Intracerebroventricular (i.c.v.) injection of apelin-13 (0.3, 0.5, 1 and 3 μg/mouse) dose-dependently inhibited fecal pellet output and bead expulsion. This effect was significantly antagonized by the APJ receptor antagonist apelin-13(F13A), indicating an APJ receptor-mediated mechanism. Furthermore, naloxone could also reverse the inhibitory effect of apelin-13 on fecal pellet output and bead expulsion, suggesting the involvement of opioid receptors in the suppressive effect of apelin-13 on distal colon transit. However, apelin-13 (10⁻⁸-10⁻⁶ M) did not affect distal colonic contractions in vitro.

Regulator of G protein-signaling proteins and addictive drugs

Regulator of G protein-signaling (RGS) proteins are a family of more than 30 intracellular proteins that negatively modulate intracellular signaling of receptors in the G protein-coupled receptor family. This family includes receptors for opioids, cannabinoids, and dopamine that mediate the acute effects of addictive drugs or behaviors and chronic effects leading to the development of addictive disease. Members of the RGS protein family, by negatively modulating receptor signaling, influence the intracellular processes that lead to addiction. In turn, addictive drugs control the expression levels of several RGS proteins. This review will consider the distribution and mechanisms of action of RGS proteins, particularly the R4 and R7 families that have been implicated in the actions of addictive drugs, how knowledge of these proteins is contributing to an understanding of addictive processes, and whether specific RGS proteins could provide targets for the development of medications to manage and/or treat addiction.

Gene expression profiling following short-term and long-term morphine exposure in mice uncovers genes involved in food intake

Addictive drugs including opioids activate signal transduction pathways that regulate gene expression in the brain. However, changes in CNS gene expression following morphine exposure are poorly understood. We determined changes in gene expression following short- and long-term morphine treatment in the hypothalamus and pituitary using genome-wide DNA microarray analysis and confirmed those alterations in gene expression by real-time reverse transcriptase polymerase chain reaction (RT-PCR) analysis. In the hypothalamus, short-term morphine administration up-regulated (at least twofold) 39 genes and down-regulated six genes. Long-term morphine treatment up-regulated 35 genes and down-regulated 51 genes. In the pituitary, short-term morphine administration up-regulated 110 genes and down-regulated 29 genes. Long-term morphine treatment up-regulated 85 genes and down-regulated 37 pituitary genes. Microarray analysis uncovered several genes involved in food intake (neuropeptide Y, agouti-related protein, and cocaine and amphetamine-regulated transcript) whose expression was strongly altered by morphine exposure in either the hypothalamus or pituitary. Subsequent RT-PCR analysis confirmed similar regulation in expression of these genes in the hypothalamus and pituitary. Finally, we found functional correlation between morphine-induced alterations in food intake and regulation of genes involved in this process. Changes in genes related to food intake may uncover new pathways related to some of the physiological effects of opioids.

Inhibition of heat shock protein 90 attenuates adenylate cyclase sensitization after chronic morphine treatment

Cellular adaptations to chronic opioid treatment result in enhanced responsiveness of adenylate cyclase and an increase in forskolin- or agonist-stimulated cAMP production. It is, however, not known whether chaperone molecules such as heat shock proteins contribute to this adenylate cyclase sensitization. Here, we report that treatment of cells with geldanamycin, an inhibitor of heat shock protein 90 (Hsp90), led to effective attenuation of morphine-induced adenylate cyclase sensitization. In SK-N-SH human neuroblastoma cells, morphine significantly increased RNA transcript and protein levels of type I adenylate cyclase, leading to sensitization. Whole-genome tiling array analysis revealed that cAMP response element-binding protein, an important mediator for cellular adaptation to morphine, associated with the proximal promoter of Hsp90AB1 not only in SK-N-SH cells but also in rat PC12 and human embryonic kidney cells. Hsp90AB1 transcript and protein levels increased significantly during morphine treatment, and co-application of geldanamycin (0.1-10 nM) effectively suppressed the increase in forskolin-activated adenylate cyclase activation by 56%. Type I adenylate cyclase, but not Hsp90AB1, underwent significant degradation during geldanamycin treatment. These results indicate that Hsp90 is a new pharmacological target for the suppression of adenylate cyclase sensitization induced by chronic morphine treatment.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (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).

Supraspinal administration of apelin-13 induces antinociception via the opioid receptor in mice

The effect of apelin-13 on pain modulation at the supraspinal level was investigated in mice using the tail immersion test. Intracerebroventricular (i.c.v.) administration of apelin-13 (0.3, 0.5, 0.8 and 3 microg/mouse) produced a dose- and time-related antinociceptive effect. This effect was significantly antagonized by the APJ receptor antagonist apelin-13(F13A), indicating an APJ receptor-mediated mechanism. Furthermore, naloxone, beta-funaltrexamine and naloxonazine, could reverse the analgesic effect. However, naltrindole or nor-binaltorphimine could not reverse the effect, suggesting that mu opioid receptor (primarily mu(1) opioid receptor subtype) is involved in the analgesic response evoked by apelin-13. Moreover, i.c.v. administration of apelin-13 potentiated the analgesic effect induced by morphine (i.c.v., 5 microg/kg) and this potentiated effect can be also reversed by naloxone.

Transcriptome analysis identifies genes with enriched expression in the mouse central extended amygdala

The central extended amygdala (EAc) is an ensemble of highly interconnected limbic structures of the anterior brain, and forms a cellular continuum including the bed nucleus of the stria terminalis (BNST), the central nucleus of the amygdala (CeA) and the nucleus accumbens shell (AcbSh). This neural network is a key site for interactions between brain reward and stress systems, and has been implicated in several aspects of drug abuse. In order to increase our understanding of EAc function at the molecular level, we undertook a genome-wide screen (Affymetrix) to identify genes whose expression is enriched in the mouse EAc. We focused on the less-well known BNST-CeA areas of the EAc, and identified 121 genes that exhibit more than twofold higher expression level in the EAc compared with whole brain. Among these, 43 genes have never been described to be expressed in the EAc. We mapped these genes throughout the brain, using non-radioactive in situ hybridization, and identified eight genes with a unique and distinct rostro-caudal expression pattern along AcbSh, BNST and CeA. Q-PCR analysis performed in brain and peripheral organ tissues indicated that, with the exception of one (Spata13), all these genes are predominantly expressed in brain. These genes encode signaling proteins (Adora2, GPR88, Arpp21 and Rem2), a transcription factor (Limh6) or proteins of unknown function (Rik130, Spata13 and Wfs1). The identification of genes with enriched expression expands our knowledge of EAc at a molecular level, and provides useful information to toward genetic manipulations within the EAc.

Opioid receptors: from binding sites to visible molecules in vivo

Opioid drugs such as heroin interact directly with opioid receptors whilst other addictive drugs, including marijuana, alcohol and nicotine indirectly activate endogenous opioid systems to contribute to their rewarding properties. The opioid system therefore plays a key role in addiction neurobiology and continues to be a primary focus for NIDA-supported research. Opioid receptors and their peptide ligands, the endorphins and enkephalins, form an extensive heterogeneous network throughout the central and peripheral nervous system. In addition to reward, opioid drugs regulate many functions such that opioid receptors are targets of choice in several physiological, neurological and psychiatric disorders. Because of the multiplicity and diversity of ligands and receptors, opioid receptors have served as an optimal model for G protein coupled receptor (GPCR) research. The isolation of opioid receptor genes opened the way to molecular manipulations of the receptors, both in artificial systems and in vivo, contributing to our current understanding of the diversity of opioid receptor biology at the behavioral, cellular and molecular levels. This review will briefly summarize some aspects of current knowledge that has accumulated since the very early characterization of opioid receptor genes. Importantly, we will identify a number of research directions that are likely to develop during the next decade.