Distinct mu, delta, and kappa opioid receptor mechanisms underlie low sociability and depressive-like behaviors during heroin abstinence

Low doses of acetaminophen produce antidepressive-like effects through the opioid system in mice

Acetaminophen (paracetamol) is one of the most popular analgesics for the management of fever and pain but few reports have investigated its antidepressant-like effect. Moreover, the role of the opioidergic pathway has been indicated in depression pathophysiology. This study aimed to examine the involvement of the opioid receptors in the antidepressant-like effect of acetaminophen after acute and sub-chronic administration using mice forced swimming test (FST). Our finding showed that administration of acetaminophen (50 and 100mg/kg, i.p.) 30min before the FST produced an antidepressant effect which was reduced by naloxone (1mg/kg, i.p., a nonselective opioid receptor antagonist). Moreover, we observed that acetaminophen in higher doses (200 and 400mg/kg) was ineffective. Also, the response of the non-effective dose of acetaminophen (25mg/kg) was potentiated by the non-effective dose of morphine (0.1mg/kg) in the FST that was antagonized by naloxone. Also, in contrast to morphine (10mg/kg), acetaminophen (100mg/kg, i.p.) induced neither tolerance to the anti-immobility behavior nor withdrawal syndrome after repeated administration. In addition, RT-PCR showed that hippocampal mu- and kappa-opioid receptor mRNA expression increased in mice after repeated administration of acetaminophen; however, morphine therapy for 6 days did not affect kappa-opioid receptor expression. Our findings demonstrated that acetaminophen in lower doses but not high doses revealed an antidepressant-like activity without inducing tolerance and withdrawal syndromes. Moreover, the observed effect of acetaminophen may be via altering the opioid system, particularly hippocampal mu- and kappa-receptors.

Neuroscience in addiction research

The prevention and treatment of addiction (moderate to severe substance use disorder-SUD) have remained challenging because of the dynamic and complex interactions between multiple biological and social determinants that shape SUD. The pharmacological landscape is ever changing and the use of multiple drugs is increasingly common, requiring an unraveling of pharmacological interactions to understand the effects. There are different stages in the trajectory from drug use to addiction that are characterized by distinct cognitive and emotional features. These are directed by different neurobiological processes that require identification and characterization including those that underlie the high co-morbidity with other disorders. Finally, there is substantial individual variability in the susceptibility to develop SUD because there are multiple determinants, including genetics, sex, developmental trajectories and times of drug exposures, and psychosocial and environmental factors including commercial determinants that influence drug availability. Elucidating how these factors interact to determine risk is essential for identifying the biobehavioral basis of addiction and developing prevention and treatment strategies. Basic research is tasked with addressing each of these challenges. The recent proliferation of technological advances that allow for genetic manipulation, visualization of molecular reactions and cellular activity in vivo, multiscale whole brain mapping across the life span, and the mining of massive data sets including multimodality human brain imaging are accelerating our ability to understand how the brain functions and how drugs influence it. Here, we highlight how the application of these tools to the study of addiction promises to illuminate its neurobiological basis and guide strategies for prevention and treatment.

Cold nociception as a measure of hyperalgesia during spontaneous heroin withdrawal in mice

Opioids are powerful analgesic drugs that are used clinically to treat pain. However, chronic opioid use causes compensatory neuroadaptations that result in greater pain sensitivity during withdrawal, known as opioid withdrawal-induced hyperalgesia (OWIH). Cold nociception tests are commonly used in humans, but preclinical studies often use mechanical and heat stimuli to measure OWIH. Thus, further characterization of cold nociception stimuli is needed in preclinical models. We assessed three cold nociception tests-thermal gradient ring (5-30 °C, 5-50 °C, 15-40 °C, and 25-50 °C), dynamic cold plate (4 °C to -1 °C at -1 °C/min, -1 °C to 4 °C at +1 °C/min), and stable cold plate (10 °C, 6 °C, and 2 °C)-to measure hyperalgesia in a mouse protocol of heroin dependence. On the thermal gradient ring, mice in the heroin withdrawal group preferred warmer temperatures, and the results depended on the ring's temperature range. On the dynamic cold plate, heroin withdrawal increased the number of nociceptive responses, with a temperature ramp from 4 °C to -1 °C yielding the largest response. On the stable cold plate, heroin withdrawal increased the number of nociceptive responses, and a plate temperature of 2 °C yielded the most significant increase in responses. Among the three tests, the stable cold plate elicited the most robust change in behavior between heroin-dependent and nondependent mice and had the highest throughput. To pharmacologically characterize the stable cold plate test, we used μ-opioid and non-opioid receptor-targeting drugs that have been previously shown to reverse OWIH in mechanical and heat nociception assays. The full μ-opioid receptor agonist methadone and μ-opioid receptor partial agonist buprenorphine decreased OWIH, whereas the preferential μ-opioid receptor antagonist naltrexone increased OWIH. Two N-methyl-d-aspartate receptor antagonists (ketamine, MK-801), a corticotropin-releasing factor 1 receptor antagonist (R121919), a β2-adrenergic receptor antagonist (butoxamine), an α2-adrenergic receptor agonist (lofexidine), and a 5-hydroxytryptamine-3 receptor antagonist (ondansetron) had no effect on OWIH. These data demonstrate that the stable cold plate at 2 °C yields a robust, reliable, and concise measure of OWIH that is sensitive to opioid agonists.

Chronic Morphine Leaves a Durable Fingerprint on Whole-Brain Functional Connectivity

BACKGROUND

Opioid use disorder is a chronic relapsing disorder. The brain adapts to opioids that are taken for pain treatment or recreational use so that abstinence becomes a true challenge for individuals with opioid use disorder. Studying brain dysfunction at this stage is difficult, and human neuroimaging has provided highly heterogeneous information.

METHODS

Here, we took advantage of an established mouse model of morphine abstinence together with functional magnetic resonance imaging to investigate whole-brain functional connectivity (FC) first at rest and then in response to an acute morphine challenge during image acquisition.

RESULTS

Hierarchical clustering of seed pair correlation coefficients showed modified FC in abstinent animals, brainwide and regardless of the condition. Seed-to-voxel analysis and random forest classification, performed on data at rest, indicated that the retrosplenial cortex (a core component of the default mode network) and the amygdala (a major aversion center) are the best markers of abstinence, thus validating the translatability of the study. Seed pair network clustering confirmed disruption of a retrosplenial cortex-centered network, reflecting major reorganization of brain FC. The latter analysis also identified a persistent but unreported morphine signature in abstinent mice at rest, which involves cortical and midbrain components and characterizes the enduring morphine footprint. Finally, dynamic FC analysis revealed that the intrascanner acute morphine challenge modified FC faster and more broadly in abstinent animals, demonstrating brainwide adaptations of FC reactivity to an acute opioid challenge.

CONCLUSIONS

This study used a unique experimental design to demonstrate that a prior history of chronic opioid exposure leaves a durable pharmacological signature on brain communication, with implications for pain management and recovery from opioid use disorder.

Mu Opioid Receptor-Expressing Neurons in the Dorsal Raphe Nucleus Are Involved in Reward Processing and Affective Behaviors

BACKGROUND

Mu opioid receptors (MORs) are key for reward processing, mostly studied in dopaminergic pathways. MORs are also expressed in the dorsal raphe nucleus (DRN), which is central for the modulation of reward and mood, but MOR function in the DRN remains underexplored. Here, we investigated whether MOR-expressing neurons of the DRN (DRN-MOR neurons) participate in reward and emotional responses.

METHODS

We characterized DRN-MOR neurons anatomically using immunohistochemistry and functionally using fiber photometry in responses to morphine and rewarding/aversive stimuli. We tested the effect of opioid uncaging on the DRN on place conditioning. We examined the effect of DRN-MOR neuron optostimulation on positive reinforcement and mood-related behaviors. We mapped their projections and selected DRN-MOR neurons projecting to the lateral hypothalamus for a similar optogenetic experimentation.

RESULTS

DRN-MOR neurons form a heterogeneous neuronal population essentially composed of GABAergic (gamma-aminobutyric acidergic) and glutamatergic neurons. Calcium activity of DRN-MOR neurons was inhibited by rewarding stimuli and morphine. Local photo-uncaging of oxymorphone in the DRN produced conditioned place preference. DRN-MOR neuron optostimulation triggered real-time place preference and was self-administered, promoted social preference, and reduced anxiety and passive coping. Finally, specific optostimulation of DRN-MOR neurons projecting to the lateral hypothalamus recapitulated the reinforcing effects of total DRN-MOR neuron stimulation.

CONCLUSIONS

Our data show that DRN-MOR neurons respond to rewarding stimuli and that their optoactivation has reinforcing effects and promotes positive emotional responses, an activity which is partially mediated by their projections to the lateral hypothalamus. Our study also suggests a complex regulation of DRN activity by MOR opioids, involving mixed inhibition/activation mechanisms that fine-tune DRN function.

Mu Opioid Receptor-Positive Neurons in the Dorsal Raphe Nucleus Are Impaired by Morphine Abstinence

BACKGROUND

Chronic opioid exposure leads to hedonic deficits and enhanced vulnerability to addiction, which are observed and even strengthen after a period of abstinence, but the underlying circuit mechanisms are poorly understood. In this study, using both molecular and behavioral approaches, we tested the hypothesis that neurons expressing mu opioid receptors (MORs) in the dorsal raphe nucleus (DRN) are involved in addiction vulnerability associated with morphine abstinence.

METHODS

MOR-Cre mice were exposed to chronic morphine and then went through spontaneous withdrawal for 4 weeks, a well-established mouse model of morphine abstinence. We studied DRN-MOR neurons of abstinent mice using 1) viral translating ribosome affinity for transcriptome profiling, 2) fiber photometry to measure neuronal activity, and 3) an opto-intracranial self-stimulation paradigm applied to DRN-MOR neurons to assess responses related to addiction vulnerability including persistence to respond, motivation to obtain the stimulation, self-stimulation despite punishment, and cue-induced reinstatement.

RESULTS

DRN-MOR neurons of abstinent animals showed a downregulation of genes involved in ion conductance and MOR-mediated signaling, as well as altered responding to acute morphine. Opto-intracranial self-stimulation data showed that abstinent animals executed more impulsive-like and persistent responses during acquisition and scored higher on addiction-like criteria.

CONCLUSIONS

Our data suggest that protracted abstinence to chronic morphine leads to reduced MOR function in DRN-MOR neurons and abnormal self-stimulation of these neurons. We propose that DRN-MOR neurons have partially lost their reward-facilitating properties, which in turn may lead to increased propensity to perform addiction-related behaviors.

Nucleus accumbens D1/D2 circuits control opioid withdrawal symptoms in mice

The nucleus accumbens (NAc) is the most promising target for drug use disorder treatment. Deep brain stimulation (DBS) of NAc is effective for drug use disorder treatment. However, the mechanisms by which DBS produces its therapeutic effects remain enigmatic. Here, we define a behavioral cutoff criterion to distinguish depressive-like behaviors and non-depressive-like behaviors in mice after morphine withdrawal. We identified a basolateral amygdala (BLA) to NAc D1 medium spiny neuron (MSN) pathway that controls depressive-like behaviors after morphine withdrawal. Furthermore, the paraventricular nucleus of thalamus (PVT) to NAc D2 MSN pathway controls naloxone-induced acute withdrawal symptoms. Optogenetically induced long-term potentiation with κ-opioid receptor (KOR) antagonism enhanced BLA to NAc D1 MSN signaling and also altered the excitation/inhibition balance of NAc D2 MSN signaling. We also verified that a new 50 Hz DBS protocol reversed morphine withdrawal-evoked abnormal plasticity in NAc. Importantly, this refined DBS treatment effectively alleviated naloxone-induced withdrawal symptoms and depressive-like behaviors and prevented stress-induced reinstatement. Taken together, the results demonstrated that input- and cell type-specific synaptic plasticity underlies morphine withdrawal, which may lead to novel targets for the treatment of opioid use disorder.

Analysis of heroin effects on calcium channels in rat cardiomyocytes based on transcriptomics and metabolomics

Heroin can cause damage to many human organs, possibly leading to different types of arrhythmias and abnormal electrophysiological function of the heart muscle and the steady state of calcium-ion channels. We explored cardiomyocytes treated with heroin and the effect on calcium-ion channels. Transcriptomics and metabolomics were used to screen for differential genes and metabolite alterations after heroin administration to jointly analyze the effect of heroin on calcium channels in cardiomyocytes. Cardiomyocytes from primary neonatal rats were cultured in vitro and were treated with different concentrations of heroin to observe the changes in morphology and spontaneous beat frequency and rhythm by a patch clamp technique. Transcriptomic studies selected a total of 1,432 differentially expressed genes, 941 upregulated and 491 downregulated genes in rat cardiomyocytes from the control and drug intervention groups. Gene Ontology functional enrichment showed that 1,432 differential genes selected by the two groups were mainly involved in the regulation of the multicellular organismal process, response to external stimulus, myofibril, inflammatory response, muscle system process, cardiac muscle contraction, etc. Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis indicated that these genes were mainly concentrated in cardiac muscle contraction, osteoclast differentiation, adrenergic signaling in cardiomyocytes, dilated cardiomyopathy, hypertrophic cardiomyopathy, and other important pathways. Metabolomic testing further suggested that cardiomyocyte metabolism was severely affected after heroin intervention. After the treatment with heroin, the L-type calcium channel current I-V curve was up-shifted, the peak value was significantly lower than that of the control group, action potential duration 90 was significantly increased in the action potential, resting potential negative value was lowered, and action potential amplitude was significantly decreased in cardiomyocytes. In this study, heroin could cause morphological changes in primary cardiomyocytes of neonatal rats and electrophysiological function. Heroin can cause myocardial contraction and calcium channel abnormalities, damage the myocardium, and change the action potential and L-type calcium channel.

Oxycodone withdrawal induces HDAC1/HDAC2-dependent transcriptional maladaptations in the reward pathway in a mouse model of peripheral nerve injury

The development of physical dependence and addiction disorders due to misuse of opioid analgesics is a major concern with pain therapeutics. We developed a mouse model of oxycodone exposure and subsequent withdrawal in the presence or absence of chronic neuropathic pain. Oxycodone withdrawal alone triggered robust gene expression adaptations in the nucleus accumbens, medial prefrontal cortex and ventral tegmental area, with numerous genes and pathways selectively affected by oxycodone withdrawal in mice with peripheral nerve injury. Pathway analysis predicted that histone deacetylase (HDAC) 1 is a top upstream regulator in opioid withdrawal in nucleus accumbens and medial prefrontal cortex. The novel HDAC1/HDAC2 inhibitor, Regenacy Brain Class I HDAC Inhibitor (RBC1HI), attenuated behavioral manifestations of oxycodone withdrawal, especially in mice with neuropathic pain. These findings suggest that inhibition of HDAC1/HDAC2 may provide an avenue for patients with chronic pain who are dependent on opioids to transition to non-opioid analgesics.

Advances in the characterization of negative affect caused by acute and protracted opioid withdrawal using animal models

Opioid use disorder (OUD) is a chronic brain disease which originates from long-term neuroadaptations that develop after repeated opioid consumption and withdrawal episodes. These neuroadaptations lead among other things to the development of a negative affect, which includes loss of motivation for natural rewards, higher anxiety, social deficits, heightened stress reactivity, an inability to identify and describe emotions, physical and/or emotional pain, malaise, dysphoria, sleep disorders and chronic irritability. The urge for relief from this negative affect is one of major causes of relapse, and thus represents a critical challenge for treatment and relapse prevention. Animal models of negative affect induced by opioid withdrawal have recapitulated the development of a negative emotional state with signs such as anhedonia, increased anxiety responses, increased despair-like behaviour and deficits in social interaction. This research has been critical to determine neurocircuitry adaptations during chronic opioid administration or upon withdrawal. In this review, we summarize the recent literature of rodent models of (i) acute withdrawal, (ii) protracted abstinence from passive administration of opioids, (iii) withdrawal or protracted abstinence from opioid self-administration. Finally, we describe neurocircuitry involved in acute withdrawal and protracted abstinence. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

All Hands on Deck: We Need Multiple Approaches To Uncover the Neuroscience behind the Opioid Overdose Crisis

Opioid use disorder (OUD) affects millions of people throughout the United States, yet there are only three Food and Drug Administration-approved pharmacological treatments. Though these treatments have been shown to be effective, the number of overdose deaths continues to rise. The increase of fentanyl, fentanyl analogs, and adulterants in the illicit drug supply has further complicated treatment strategies. Preclinical researchers strive to model OUD to better understand this complicated disorder, and this research is a critical enabler for the development of novel treatments. As a result, there are many different preclinical models of OUD. Often, researchers form strong opinions on what they believe to be the "best" model to mimic the human condition. Here, we argue that researchers should be supportive of multiple models to promote new perspectives and discoveries and always consider the trends in human opioid use when designing preclinical studies. We describe the benefits of contingent and noncontingent models as well as models of opioid withdrawal and how each of these can help illuminate different components of OUD.

Inhibition of dorsal raphe GABAergic neurons blocks hyperalgesia during heroin withdrawal

Opioid withdrawal signs, such as hyperalgesia, are manifestations of opioid use disorder that may contribute to opioid seeking and taking. We have previously identified an association between dorsal raphe (DR) neurons and the expression of hyperalgesia during spontaneous heroin withdrawal. Here, we found that chemogenetic inhibition of DR neurons decreased hyperalgesia during spontaneous heroin withdrawal in male and female C57/B6 mice. By neuroanatomy, we identified three major subtypes of DR neurons expressing μ-opioid receptors (MOR) that were activated in hyperalgesia during spontaneous withdrawal, those expressing vesicular GABA transporter (VGaT), glutamate transporter 3 (VGluT3), or co-expressing VGluT3 and tryptophan hydroxylase (TPH). In contrast, we identified a small population of DR-MOR neurons expressing solely TPH, which were not activated in hyperalgesia during spontaneous withdrawal. Collectively, these findings indicate a role of the DR in hyperalgesia during spontaneous heroin withdrawal mediated, in part, by the activation of local MOR-GABAergic, MOR-glutamatergic and MOR-co-releasing glutamatergic-serotonergic neurons. We found that  specific chemogenetic inhibition of DR-VGaT neurons blocked hyperalgesia during spontaneous heroin withdrawal in male and female mice. Collectively, these findings indicate that DR-GABAergic neurons play a role in the expression of hyperalgesia during spontaneous heroin withdrawal.

Upregulation of dynorphin/kappa opioid receptor system in the dorsal hippocampus contributes to morphine withdrawal-induced place aversion

Aversive emotion of opioid withdrawal generates motivational state leading to compulsive drug seeking and taking. Kappa opioid receptor (KOR) and its endogenous ligand dynorphin have been shown to participate in the regulation of aversive emotion. In the present study, we investigated the role of dynorphin/KOR system in the aversive emotion following opioid withdrawal in acute morphine-dependent mice. We found that blockade of KORs before pairing by intracerebroventricular injection of KOR antagonist norBNI (20, 40 μg) attenuated the development of morphine withdrawal-induced conditioned place aversion (CPA) behavior. We further found that morphine withdrawal increased dynorphin A expression in the dorsal hippocampus, but not in the amygdala, prefrontal cortex, nucleus accumbens, and thalamus. Microinjection of norBNI (20 μg) into the dorsal hippocampus significantly decreased morphine withdrawal-induced CPA behavior. We further found that p38 MAPK was significantly activated in the dorsal hippocampus after morphine withdrawal, and the activation of p38 MAPK was blocked by pretreatment with norBNI. Accordingly, microinjection of p38 MAPK inhibitor SB203580 (5 μg) into the dorsal hippocampus significantly decreased morphine withdrawal-produced CPA behavior. This study demonstrates that upregulation of dynorphin/KOR system in the dorsal hippocampus plays a critical role in the formation of aversive emotion associated with morphine withdrawal, suggesting that KOR antagonists may have therapeutic value for the treatment of opioid withdrawal-induced mood-related disorders.

Flavonoids as G Protein-coupled Receptors Ligands: New Potential Therapeutic Natural Drugs

G protein coupled receptors (GPCRs) are among the largest family of cell surface receptors found in the human genome. They govern a wide range of physiological responses in both health and diseases, making them one of the potential targeted surface receptors for pharmaceuticals. Flavonoids can modulate GPCRs activity by acting as allosteric ligands. They can either enhance or reduce the GPCR's effect. Emerging research shows that individual flavonoids or mixtures of flavonoids from plant extracts can have relevant pharmacological effects against a number of diseases, particularly by influencing GPCRs. In the present review, we are considering to give a comprehensive overview of flavonoids and related compounds that exhibit GPCRs activity and to further explore which beneficial structural features. Molecular docking was used to strengthen experimental evidence and describe flavonoid-GPCRs interactions at molecular level.

Modulation of 5-HT release by dynorphin mediates social deficits during opioid withdrawal

Social isolation during opioid withdrawal is a major contributor to the current opioid addiction crisis. We find that sociability deficits during protracted opioid withdrawal in mice require activation of kappa opioid receptors (KORs) in the nucleus accumbens (NAc) medial shell. Blockade of release from dynorphin (Pdyn)-expressing dorsal raphe neurons (DRPdyn), but not from NAcPdyn neurons, prevents these deficits in prosocial behaviors. Conversely, optogenetic activation of DRPdyn neurons reproduced NAc KOR-dependent decreases in sociability. Deletion of KORs from serotonin (5-HT) neurons, but not from NAc neurons or dopamine (DA) neurons, prevented sociability deficits during withdrawal. Finally, measurements with the genetically encoded GRAB5-HT sensor revealed that during withdrawal KORs block the NAc 5-HT release that normally occurs during social interactions. These results define a neuromodulatory mechanism that is engaged during protracted opioid withdrawal to induce maladaptive deficits in prosocial behaviors, which in humans contribute to relapse.

Habenular Neurons Expressing Mu Opioid Receptors Promote Negative Affect in a Projection-Specific Manner

BACKGROUND

The mu opioid receptor (MOR) is central to hedonic balance and produces euphoria by engaging reward circuits. MOR signaling may also influence aversion centers, notably the habenula (Hb), where the receptor is highly dense. Our previous data suggest that the inhibitory activity of MOR in the Hb may limit aversive states. To investigate this hypothesis, we tested whether neurons expressing MOR in the Hb (Hb-MOR neurons) promote negative affect.

METHODS

Using Oprm1-Cre knockin mice, we combined tracing and optogenetics with behavioral testing to investigate consequences of Hb-MOR neuron stimulation for approach/avoidance (real-time place preference), anxiety-related responses (open field, elevated plus maze, and marble burying), and despair-like behavior (tail suspension).

RESULTS

Optostimulation of Hb-MOR neurons elicited avoidance behavior, demonstrating that these neurons promote aversive states. Anterograde tracing showed that, in addition to the interpeduncular nucleus, Hb-MOR neurons project to the dorsal raphe nucleus. Optostimulation of Hb-MOR/interpeduncular nucleus terminals triggered avoidance and despair-like responses with no anxiety-related effect, whereas light-activation of Hb-MOR/dorsal raphe nucleus terminals increased levels of anxiety with no effect on other behaviors, revealing 2 dissociable pathways controlling negative affect.

CONCLUSIONS

Together, the data demonstrate that Hb neurons expressing MOR facilitate aversive states via 2 distinct Hb circuits, contributing to despair-like behavior (Hb-MOR/interpeduncular nucleus) and anxiety (Hb-MOR/dorsal raphe nucleus). The findings support the notion that inhibition of these neurons by either endogenous or exogenous opioids may relieve negative affect, a mechanism that would have implications for hedonic homeostasis and addiction.

NEURONAL CORRELATES OF HYPERALGESIA AND SOMATIC SIGNS OF HEROIN WITHDRAWAL IN MALE AND FEMALE MICE

Opioid withdrawal involves the manifestation of motivational and somatic symptoms. However, the brain structures that are involved in the expression of different opioid withdrawal signs remain unclear. We induced opioid dependence by repeatedly injecting escalating heroin doses in male and female C57BL/6J mice. We assessed hyperalgesia during spontaneous heroin withdrawal and somatic signs of withdrawal that was precipitated by the preferential µ-opioid receptor antagonist naloxone. Heroin-treated mice exhibited significantly higher hyperalgesia and somatic signs than saline-treated mice. Following behavioral assessment, we measured regional changes in brain activity by automated the counting of c-Fos expression (a marker of cellular activity). Using Principal Component Analysis, we determined the association between behavior (hyperalgesia and somatic signs of withdrawal) and c-Fos expression in different brain regions. Hyperalgesia was associated with c-Fos expression in the lateral hypothalamus, central nucleus of the amygdala, ventral tegmental area, parabrachial nucleus, dorsal raphe, and locus coeruleus. Somatic withdrawal was associated with c-Fos expression in the paraventricular nucleus of the thalamus, lateral habenula, dorsal raphe, and locus coeruleus. Thus, hyperalgesia and somatic withdrawal signs were each associated with c-Fos expression in unique sets of brain areas. The expression of c-Fos in the dorsal raphe and locus coeruleus was associated with both hyperalgesia and somatic withdrawal. Understanding common neurobiological mechanisms of acute and protracted opioid withdrawal may help identify new targets for treating this salient aspect of opioid use disorder.SIGNIFICANCE STATEMENTThe public impact of the opioid crisis has prompted an effort to understand the neurobiological mechanisms of opioid use disorder (OUD). The need to avoid withdrawal symptoms is hypothesized to drive compulsive drug-taking and -seeking in OUD. Thus, understanding the mechanisms of acute and protracted opioid withdrawal may help identify new targets for treating this salient aspect of OUD. We reported brain structures that are associated with the expression of hyperalgesia and somatic signs of opioid withdrawal in male and female heroin-dependent mice. Hyperalgesia during spontaneous opioid withdrawal and somatic withdrawal resulted in c-Fos expression in autonomic and limbic brain regions. The expression of c-Fos in the dorsal raphe and locus coeruleus were associated with both hyperalgesia and somatic withdrawal.

Goofballing of Opioid and Methamphetamine: The Science Behind the Deadly Cocktail

Globally, millions of people suffer from various substance use disorders (SUD), including mono-and polydrug use of opioids and methamphetamine. Brain regions such as the cingulate cortex, infralimbic cortex, dorsal striatum, nucleus accumbens, basolateral and central amygdala have been shown to play important roles in addiction-related behavioral changes. Clinical and pre-clinical studies have characterized these brain regions and their corresponding neurochemical changes in numerous phases of drug dependence such as acute drug use, intoxication, craving, withdrawal, and relapse. At present, many studies have reported the individual effects of opioids and methamphetamine. However, little is known about their combined effects. Co-use of these drugs produces effects greater than either drug alone, where one decreases the side effects of the other, and the combination produces a prolonged intoxication period or a more desirable intoxication effect. An increasing number of studies have associated polydrug abuse with poorer treatment outcomes, drug-related deaths, and more severe psychopathologies. To date, the pharmacological treatment efficacy for polydrug abuse is vague, and still at the experimental stage. This present review discusses the human and animal behavioral, neuroanatomical, and neurochemical changes underlying both morphine and methamphetamine dependence separately, as well as its combination. This narrative review also delineates the recent advances in the pharmacotherapy of mono- and poly drug-use of opioids and methamphetamine at clinical and preclinical stages.

Amygdalar κ-opioid receptor-dependent upregulating glutamate transporter 1 mediates depressive-like behaviors of opioid abstinence

Opiates produce a strong rewarding effect, but abstinence from opiate use emerges with severe negative emotions. Depression is one of the most frequent emotion disorders associated with opiate abstinence, which is thought to be a main cause for relapse. However, neurobiological bases of such an aversive emotion processing are poorly understood. Here, we find that morphine abstinence activates κ-opioid receptors (KORs) by increasing endogenous KOR ligand dynorphin expression in the amygdala, which in turn facilitates glutamate transporter 1 (GLT1) expression by activation of p38 mitogen-activated protein kinase (MAPK). Upregulation of GLT1 expression contributes to opiate-abstinence-elicited depressive-like behaviors through modulating amygdalar glutamatergic inputs to the nucleus accumbens (NAc). Intra-amygdala injection of GLT1 inhibitor DHK or knockdown of GLT1 expression in the amygdala significantly suppresses morphine-abstinence-induced depressive-like behaviors. Pharmacological and pharmacogenetic activation of amygdala-NAc projections prevents morphine-abstinence-induced behaviors. Overall, our study provides key molecular and circuit insights into the mechanisms of depression associated with opiate abstinence.

Gray areas: Neuropeptide circuits linking the Edinger-Westphal and Dorsal Raphe nuclei in addiction

The circuitry of addiction comprises several neural networks including the midbrain - an expansive region critically involved in the control of motivated behaviors. Midbrain nuclei like the Edinger-Westphal (EW) and dorsal raphe (DR) contain unique populations of neurons that synthesize many understudied neuroactive molecules and are encircled by the periaqueductal gray (PAG). Despite the proximity of these special neuron classes to the ventral midbrain complex and surrounding PAG, functions of the EW and DR remain substantially underinvestigated by comparison. Spanning approximately -3.0 to -5.2 mm posterior from bregma in the mouse, these various cell groups form a continuum of neurons that we refer to collectively as the subaqueductal paramedian zone. Defining how these pathways modulate affective behavioral states presents a difficult, yet conquerable challenge for today's technological advances in neuroscience. In this review, we cover the known contributions of different neuronal subtypes of the subaqueductal paramedian zone. We catalogue these cell types based on their spatial, molecular, connectivity, and functional properties and integrate this information with the existing data on the EW and DR in addiction. We next discuss evidence that links the EW and DR anatomically and functionally, highlighting the potential contributions of an EW-DR circuit to addiction-related behaviors. Overall, we aim to derive an integrated framework that emphasizes the contributions of EW and DR nuclei to addictive states and describes how these cell groups function in individuals suffering from substance use disorders. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

Mechanisms Underlying the Anti-Suicidal Treatment Potential of Buprenorphine

Death by suicide is a global epidemic with over 800 K suicidal deaths worlwide in 2012. Suicide is the 10th leading cause of death among Americans and more than 44 K people died by suicide in 2019 in the United States. Patients with chronic pain, including, but not limited to, those with substance use disorders, are particularly vulnerable. Chronic pain patients have twice the risk of death by suicide compared to those without pain, and 50% of chronic pain patients report that they have considered suicide at some point due to their pain. The kappa opioid system is implicated in negative mood states including dysphoria, depression, and anxiety, and recent evidence shows that chronic pain increases the function of this system in limbic brain regions important for affect and motivation. Additionally, dynorphin, the endogenous ligand that activates the kappa opioid receptor is increased in the caudate putamen of human suicide victims. A potential treatment for reducing suicidal ideation and suicidal attempts is buprenorphine. Buprenorphine, a partial mu opioid agonist with kappa opioid antagonist properties, reduced suicidal ideation in chronic pain patients with and without an opioid use disorder. This review will highlight the clinical and preclinical evidence to support the use of buprenorphine in mitigating pain-induced negative affective states and suicidal thoughts, where these effects are at least partially mediated via its kappa antagonist properties.

Facilitating mGluR4 activity reverses the long-term deleterious consequences of chronic morphine exposure in male mice

Understanding the neurobiological underpinnings of abstinence from drugs of abuse is critical to allow better recovery and ensure relapse prevention in addicted subjects. By comparing the long-term transcriptional consequences of morphine and cocaine exposure, we identified the metabotropic glutamate receptor subtype 4 (mGluR4) as a promising pharmacological target in morphine abstinence. We evaluated the behavioral and molecular effects of facilitating mGluR4 activity in abstinent mice. Transcriptional regulation of marker genes of medium spiny neurons (MSNs) allowed best discriminating between 4-week morphine and cocaine abstinence in the nucleus accumbens (NAc). Among these markers, Grm4, encoding mGluR4, displayed down-regulated expression in the caudate putamen and NAc of morphine, but not cocaine, abstinent mice. Chronic administration of the mGluR4 positive allosteric modulator (PAM) VU0155041 (2.5 and 5 mg/kg) rescued social behavior, normalized stereotypies and anxiety and blunted locomotor sensitization in morphine abstinent mice. This treatment improved social preference but increased stereotypies in cocaine abstinent mice. Finally, the beneficial behavioral effects of VU0155041 treatment in morphine abstinent mice were correlated with restored expression of key MSN and neural activity marker genes in the NAc. This study reports that chronic administration of the mGluR4 PAM VU0155041 relieves long-term deleterious consequences of morphine exposure. It illustrates the neurobiological differences between opiate and psychostimulant abstinence and points to pharmacological repression of excessive activity of D2-MSNs in the NAc as a promising therapeutic lever in drug addiction.

The role of social isolation in opioid addiction

Humans are social animals: social isolation hurts people both psychologically and physically. Strong, positive social bonds help people to live longer and healthier lives compared with their more isolated peers. Opioid use disorder is associated with feelings of social isolation, an increased risk of suicide and, at the community level, lower social capital. I propose a psychobiological mechanistic explanation that contributes to the association between opioid use and social isolation. The endogenous opioid system plays a central role in the formation and maintenance of social bonds across the life span and has been investigated primarily through the framework of the brain opioid theory of social attachment. In primates, maternal-infant bonding and social play are both impaired by the administration of naltrexone (an opioid antagonist), and in humans, the chronic use of opioids appears to be particularly (relative to other drugs) corrosive to close relationships. Social isolation may play a role in the development and exacerbation of opioid use disorder. Taken together, work on the brain’s opioid system suggests a possible mechanistic basis for bidirectional causal links between social isolation and opioid use disorder. Evaluation of this hypothesis would benefit from longitudinal psychosocial and neuropsychopharmacological investigations.

Pleasure, addiction, and hypocretin (orexin)

The hypocretins/orexins were discovered in 1998. Within 2 years, this led to the discovery of the cause of human narcolepsy, a 90% loss of hypothalamic neurons containing these peptides. Further work demonstrated that these neurons were not simply linked to waking. Rather these neurons were active during pleasurable behaviors in waking and were silenced by aversive stimulation. This was seen in wild-type mice, rats, cats, and dogs. It was also evident in humans, with increased Hcrt release during pleasurable activities and decreased release, to the levels seen in sleep, during pain. We found that human heroin addicts have, on average, an increase of 54% in the number of detectable Hcrt neurons compared to "control" human brains and that these Hcrt neurons are substantially smaller than those in control brains. We found that in mice, chronic morphine administration induced the same changes in Hcrt neuron number and size. Our studies in the mouse allowed us to determine the specificity, dose response relations, time course of the change in the number of Hcrt neurons, and that the increased number of Hcrt neurons after opiates was not due to neurogenesis. Furthermore, we found that it took a month or longer for these anatomical changes in the mouse brain to return to baseline. Human narcoleptics, despite their prescribed use of several commonly addictive drugs, do not show significant evidence of dose escalation or substance use disorder. Similarly, mice in which the peptide has been eliminated are resistant to addiction. These findings are consistent with the concept that an increased number of Hcrt neurons may underlie and maintain opioid or cocaine use disorders.

Descended Social Anxiety Disorder and Craving in Women Heroin Dependence Through Exercise Alerts Plasma Oxytocin Levels

Purpose: This study explored the association between peripheral blood oxytocin (OT) and social anxiety disorder (SAD) and cue-induced cravings in female heroin addicts. The effect of exercise on alleviation of SAD and OT levels was also explored. Methods: A total of 72 females with heroin dependence were assigned to three groups based on SAD severity. The three groups were Non-SAD control, SAD control, and SAD exercise groups. Subjects in the SAD exercise group underwent aerobic exercise and resistance training for 8 weeks (60 min/day, 5 days/week). Enzyme-linked immunosorbent assay analysis and Liebowitz Social Anxiety Scale (LSAS) scores were used to determine plasma OT concentration and SAD, respectively. Cue-induced craving was assessed using Visual Analog Scale (VAS) and Desires for Drug Questionnaire (DDQ). Mixed-effect analysis of variance and Pearson correlation analysis were used to explore the effect and correlation between different parameters. Results: OT levels in the SAD exercise group were significantly high after exercise (p < 0.01). LSAS, VAS, and DDQ ("Desire and Intention" and "Negative reinforcement") scores in the SAD exercise group were significantly lower after exercise (p < 0.01). Plasma OT level was negatively correlated with LSAS score (r = -0.534, p < 0.001), VAS score (r = -0.609, p < 0.001), "Desire and Intention" score (r = -0.555, p < 0.001), and "Negative reinforcement" score (r = -0.332, p < 0.01) and positively correlated with the "control" score (r = 0.258, p < 0.05). LSAS was positively correlated with VAS score (r = 0.588, p < 0.001) and "Desire and Intention" score (r = 0.282, p < 0.05). Conclusions: The findings of the present study indicate that plasma OT is a potential peripheral biomarker for prediction of the severity of social anxiety in female heroin withdrawal patients. Aerobic exercise combined with resistance training plus incremental load for 8 weeks can increase plasma OT levels and significantly reduce severity of SAD and cue-induced cravings in female heroin addicts.

Changes in opioid receptors, opioid peptides and morphine antinociception in mice subjected to early life stress

Recent studies have shown that the endogenous opioid system is considerably affected by early life stress such as child abuse. Here, we investigated whether early life stress changes the endogenous opioid receptors and their peptides, and if such stress impacts morphine antinociception. We used mice affected by maternal separation and social isolation (MSSI) as an early life stress model. In the tail-flick test, 10-week-old MSSI mice showed a significant decrease in morphine antinociception compared to age-matched control mice. The number of c-Fos-positive cells increased in the periaqueductal gray (PAG), nucleus accumbens, and thalamus of control mice after the morphine injections, whereas hardly any positive cells were detected in the same areas of MSSI mice. The expression of μ- and κ-opioid receptor (MOR and KOR, respectively) messenger RNA (mRNA) was significantly decreased in the PAG of MSSI mice, whereas KOR expression was significantly increased in the amygdala of MSSI mice. The expression of δ-opioid receptor (DOR) mRNA was significantly reduced in the PAG and rostral ventromedial medulla of MSSI mice compared to control mice. Moreover, the lack of morphine antinociception was observed in 18-week-old MSSI mice. Our findings suggest that the supraspinal opioid system may be affected by early life stress exposure, and that this exposure may impact morphine antinociception.

Long-lasting pseudo-social aggressive behavior in opiate-withdrawn mice

Poor sociability and aggressive behavior are major clinical features of opiate use disorders that may contribute to the establishment and maintenance of these harmful diseases. The present study investigated the long-term effects of chronic morphine administration and withdrawal upon social and aggressive behavior as well as the interrelationship between these two behaviors. Thus, social behavior was measured in C57BL/6J male mice 7, 21, 35 and 49 days after cessation of escalating morphine doses (20-100 mg/kg, i.p.) administered during 6 consecutive days, using the three-chamber task for sociability (i.e., preference for an unfamiliar conspecific vs. an object) and social novelty preference (i.e., preference for a novel vs. a familiar conspecific). Moreover, aggressive biting behavior towards an unfamiliar conspecific was assessed throughout the three-chamber tests. Opiate withdrawal increased both social approach and aggressive biting behavior. Moreover, in morphine-withdrawn, but not in control, mice social approach and aggressive behavior followed a similar time-course and positively correlated one with each other, suggesting that social interest was mainly driven by aggressiveness. Aggressive biting behavior was still elevated 49 days after the last morphine administration, revealing that opiate withdrawal is followed by long-lasting aggressiveness. Throughout, opiate withdrawal did not affect social novelty preference, ruling out a role for olfactory or social discrimination dysfunction in the elevated social approach and aggressive behavior. The present findings of very long-lasting aggressive behavior and aggression-driven social approach in opiate-withdrawn mice might help understanding the behavioral and brain underpinnings of poor sociability and aggressiveness commonly observed in opiate use disorders patients.

Very low dose naltrexone in opioid detoxification: a double-blind, randomized clinical trial of efficacy and safety

Withdrawal syndrome is one of the initial focuses of opioid detoxification. Very low dose naltrexone (VLNTX) has been found to reduce opioid tolerance and dependence in animal and human clinical studies. The aim of this study was to determine the safety and efficacy of VLNTX during early stages of detoxification. In a multi-arm parallel, double-blind, randomized controlled trial, 63 opioid-dependent male participants referring to Imam Reza Rehabilitation Center were allocated to three equal groups using block randomization method. They received 0.125 mg, 0.250 mg of VLNTX or placebo daily for 10 days, together with the routine clonidine-based protocol. Self-reported and observer ratings of withdrawal severity and adverse events were measured on the 1st, 4th and 10th day of treatment. Runny eyes (p = 0.006), anxiety (p = 0.031) and dehydration (p = 0.014) were reduced during the whole 10 days in the 0.125 mg VLNTX-treated group compared to placebo. Only drowsiness (p = 0.043) and dysphoric mood (p < 0.001) were reduced in the 0.250 mg VLNTX-treated group. Results of 1st, 4th, and 10th-day assessment showed that most symptoms reductions were for the 0.125 mg VLNTX and the placebo group in the 1st and 4th days, respectively. On the 10th day, there was not any significant difference between 0.250 mg VLNTX-treated group and placebo group. No adverse effect was observed. In the starting days of detoxification, VLNTX can reduce the withdrawal symptoms, but the efficacy declined by passing time. Further studies are needed to test the utility of this new therapeutic approach.

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.

The Opioid-Addicted Tetrapartite Synapse

Opioid administration in preclinical models induces long-lasting adaptations in reward and habit circuitry. The latest research demonstrates that in the nucleus accumbens, opioid-induced excitatory synaptic plasticity involves presynaptic and postsynaptic elements as well as adjacent astroglial processes and the perisynaptic extracellular matrix. We outline opioid-induced modifications within each component of the tetrapartite synapse and provide a neurobiological perspective on how these adaptations converge to produce addiction-related behaviors in rodent models. By incorporating changes observed at each of the excitatory synaptic compartments into a unified framework of opioid-induced glutamate dysregulation, we highlight new avenues for restoring synaptic homeostasis that might limit opioid craving and relapse vulnerability.

Mu opioid receptors in the medial habenula contribute to naloxone aversion

The medial habenula (MHb) is considered a brain center regulating aversive states. The mu opioid receptor (MOR) has been traditionally studied at the level of nociceptive and mesolimbic circuits, for key roles in pain relief and reward processing. MOR is also densely expressed in MHb, however, MOR function at this brain site is virtually unknown. Here we tested the hypothesis that MOR in the MHb (MHb-MOR) also regulates aversion processing. We used chnrb4-Cre driver mice to delete the Oprm1 gene in chnrb4-neurons, predominantly expressed in the MHb. Conditional mutant (B4MOR) mice showed habenula-specific reduction of MOR expression, restricted to chnrb4-neurons (50% MHb-MORs). We tested B4MOR mice in behavioral assays to evaluate effects of MOR activation by morphine, and MOR blockade by naloxone. Locomotor, analgesic, rewarding, and motivational effects of morphine were preserved in conditional mutants. In contrast, conditioned place aversion (CPA) elicited by naloxone was reduced in both naïve (high dose) and morphine-dependent (low dose) B4MOR mice. Further, physical signs of withdrawal precipitated by either MOR (naloxone) or nicotinic receptor (mecamylamine) blockade were attenuated. These data suggest that MORs expressed in MHb B4-neurons contribute to aversive effects of naloxone, including negative effect and aversive effects of opioid withdrawal. MORs are inhibitory receptors, therefore we propose that endogenous MOR signaling normally inhibits chnrb4-neurons of the MHb and moderates their known aversive activity, which is unmasked upon receptor blockade. Thus, in addition to facilitating reward at several brain sites, tonic MOR activity may also limit aversion within the MHb circuitry.

The Negative Affect of Protracted Opioid Abstinence: Progress and Perspectives From Rodent Models

Opioid use disorder (OUD) is characterized by the development of a negative emotional state that develops after a history of long-term exposure to opioids. OUD represents a true challenge for treatment and relapse prevention. Human research has amply documented emotional disruption in individuals with an opioid substance use disorder, at both behavioral and brain activity levels; however, brain mechanisms underlying this particular facet of OUD are only partially understood. Animal research has been instrumental in elucidating genes and circuits that adapt to long-term opioid use or are modified by acute withdrawal, but research on long-term consequences of opioid exposure and their relevance to the negative affect of OUD remains scarce. In this article, we review the literature with a focus on two questions: 1) Do we have behavioral models in rodents, and what do they tell us? and 2) What do we know about the neuronal populations involved? Behavioral rodent models have successfully recapitulated behavioral signs of the OUD-related negative affect, and several neurotransmitter systems were identified (i.e., serotonin, dynorphin, corticotropin-releasing factor, oxytocin). Circuit mechanisms driving the negative mood of prolonged abstinence likely involve the 5 main reward-aversion brain centers (i.e., nucleus accumbens, bed nucleus of the stria terminalis, amygdala, habenula, and raphe nucleus), all of which express mu opioid receptors and directly respond to opioids. Future work will identify the nature of these mu opioid receptor-expressing neurons throughout reward-aversion networks, characterize their adapted phenotype in opioid abstinent animals, and hopefully position these primary events in the broader picture of mu opioid receptor-associated brain aversion networks.

Opioid receptors: drivers to addiction?

Drug addiction is a worldwide societal problem and public health burden, and results from recreational drug use that develops into a complex brain disorder. The opioid system, one of the first discovered neuropeptide systems in the history of neuroscience, is central to addiction. Recently, opioid receptors have been propelled back on stage by the rising opioid epidemics, revolutions in G protein-coupled receptor research and fascinating developments in basic neuroscience. This Review discusses rapidly advancing research into the role of opioid receptors in addiction, and addresses the key questions of whether we can kill pain without addiction using mu-opioid-receptor-targeting opiates, how mu- and kappa-opioid receptors operate within the neurocircuitry of addiction and whether we can bridge human and animal opioid research in the field of drug abuse.

Behavioural and metabolomic changes from chronic dietary exposure to low-level deoxynivalenol reveal impact on mouse well-being

The mycotoxin deoxynivalenol (DON) has a high global prevalence in grain-based products. Biomarkers of exposure are detectable in most humans and farm animals. Considering the acute emetic and chronic anorexigenic toxicity of DON, maximum levels for food and feed have been implemented by food authorities. The tolerable daily intake (TDI) is 1 µg/kg body weight (bw)/day for the sum of DON and its main derivatives, which was based on the no-observed adverse-effect level (NOAEL) of 100 µg DON/kg bw/day for anorexic effects in rodents. Chronic exposure to a low-DON dose can, however, also cause inflammation and imbalanced neurotransmitter levels. In the present study, we therefore investigated the impact of a 2-week exposure at the NOAEL in mice by performing behavioural experiments, monitoring brain activation by c-Fos expression, and analysing changes in the metabolomes of brain and serum. We found that DON affected neuronal activity and innate behaviour in both male and female mice. Metabolite profiles were differentiable between control and treated mice. The behavioural changes evidenced at NOAEL reduce the safety margin to the established TDI and may be indicative of a risk for human health.

The opioid system in stress-induced memory disorders: From basic mechanisms to clinical implications in post-traumatic stress disorder and Alzheimer's disease

Cognitive and emotional impairment are a serious consequence of stress exposure and are core features of neurological and psychiatric conditions that involve memory disorders. Indeed, acute and chronic stress are high-risk factors for the onset of post-traumatic stress disorder (PTSD) and Alzheimer's disease (AD), two devastating brain disorders associated with memory dysfunction. Besides the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis, stress response also involves the activation of the opioid system in brain regions associated with stress regulation and memory processing. In this context, it is possible that stress-induced memory disorders may be attributed to alterations in the interaction between the neuroendocrine stress system and the opioid system. In this review, we: (1) describe the effects of acute and chronic stress on memory, and the modulatory role of the opioid system, (2) discuss the contribution of the opioid system to the pathophysiology of PTSD and AD, and (3) present evidence of current and potential therapies that target the opioid receptors to treat PTSD- and AD-associated symptoms.

Molecular Imaging of Opioid and Dopamine Systems: Insights Into the Pharmacogenetics of Opioid Use Disorders

Opioid use in the United States has steadily risen since the 1990s, along with staggering increases in addiction and overdose fatalities. With this surge in prescription and illicit opioid abuse, it is paramount to understand the genetic risk factors and neuropsychological effects of opioid use disorder (OUD). Polymorphisms disrupting the opioid and dopamine systems have been associated with increased risk for developing substance use disorders. Molecular imaging studies have revealed how these polymorphisms impact the brain and contribute to cognitive and behavioral differences across individuals. Here, we review the current molecular imaging literature to assess how genetic variations in the opioid and dopamine systems affect function in the brain's reward, cognition, and stress pathways, potentially resulting in vulnerabilities to OUD. Continued research of the functional consequences of genetic variants and corresponding alterations in neural mechanisms will inform prevention and treatment of OUD.

Heteromerization Modulates mu Opioid Receptor Functional Properties in vivo

Mu opioid receptors modulate a large number of physiological functions. They are in particular involved in the control of pain perception and reward properties. They are also the primary molecular target of opioid drugs and mediate their beneficial analgesic effects, euphoric properties as well as negative side effects such as tolerance and physical dependence. Importantly, mu opioid receptors can physically associate with another receptor to form a novel entity called heteromer that exhibits specific ligand binding, signaling, and trafficking properties. As reviewed here, in vivo physical proximity has now been evidenced for several receptor pairs, subsequent impact of heteromerization on native mu opioid receptor signaling and trafficking identified and a link to behavioral changes established. Selective targeting of heteromers as a tool to modulate mu opioid receptor activity is therefore attracting growing interest and raises hopes for innovative therapeutic strategies.

Involvement of opioid system in behavioral despair induced by social isolation stress in mice

Social isolation stress (SIS) as a type of chronic stress could induce depressive- and anxiety-like behaviors. Our study evaluates the role of opioid system on negative behavioral impacts of SIS in male NMRI mice. We investigated effects of morphine, a nonselective opioid receptor (OR) agonist, naltrexone (NLX), an OR antagonist, naltrindole (NLT), a delta opioid receptor (DOR) antagonist, SNC80, a DOR agonist, U-69593, a kappa opioid receptor (KOR) agonist, nor-Binaltorphimine, a selective KOR antagonist and cyprodime hydrochloride a selective mu opioid receptor (MOR) antagonist on depressive- and anxiety-like behaviors. Using RT-PCR we evaluated ORs gene expression in mice brain. Our findings showed that SIS induced anxiety- and depressive-like behavior in the forced swimming test, open field test, splash test and hole-board test. Moreover, administration of SNC-80 significantly mitigated anxiety- and depressive-like behaviors. NLT decreased grooming-activity in the splash test. Excitingly, administration of agents affecting KOR failed to alter the negative effects of SIS. RT-PCR demonstrated that MOR and KOR gene expression decreased in socially isolated mice; however, SIS did not affect DORs expression. Our findings suggest that SIS at least in part, probably via altering endogenous opioids particularly MORs and KORs but not DORs mediated negative impacts on behavior; also, it could be concluded that DORs might be considered as a novel target for studying depression and anxiety.

Focusing on the Opioid System for Addiction Biomarker Discovery

Substance use disorders (SUD) and behavioral addictions are devastating conditions that impose a severe burden on all societies, and represent difficult challenges for clinicians. Therefore, biomarkers are urgently needed to help predict vulnerability, clinical course, and response to treatment. Here, we elaborate on the potential for addiction biomarker discovery of the opioid system, particularly within the emerging framework aiming to probe opioid function in peripheral tissues. Mu, delta, and kappa opioid receptors all critically regulate neurobiological and behavioral processes that define addiction, and are also targeted by major pharmacotherapies used in the management of patients with SUD. We propose that opioid biomarkers may have the potential to improve and guide diagnosis and therapeutic decisions in the addiction field.

Buprenorphine-naloxone treatment responses differ between young adults with heroin and prescription opioid use disorders

BACKGROUND AND OBJECTIVES

Opioid use disorder among young adults is rising sharply with an increase in morbidity and mortality. This study examined differences in treatment response to a fixed dose of buprenorphine-naloxone between heroin (HU) and prescriptions opioids (POU) users.

METHODS

Eighty opioid dependent young adults (M = 22 years) were treated with buprenorphine-naloxone 16-4 mg/day for 8 weeks. Differences between HU (N = 17) and POU (N = 63) on changes in weekly opioid use, opioid craving, withdrawal, and depression symptoms were analyzed with mixed-effects regression models.

RESULTS

The HU had an overall mean proportion of weekly opioid use of .32 (SD = .14) compared to POU's weekly mean of .24 (SD = .15) showing a significant main effect (Z = 2.21, p = .02). Depressive symptoms (CES-D scores) were elevated at baseline for both groups (HU: M = 23.1, SD = 11.9; PO: M = 22.2, SD = 9.4), but only POU improved significantly to a score of 9.88 (SD = 7.4) compared to HU's score of 18.58 (SD = 10.3) at week 8 (Z = 2.24, p = .02). There were no significant differences in treatment retention, craving, or withdrawal symptoms.

DISCUSSION AND CONCLUSIONS

Treatment response to 16-4 mg/day of buprenorphine-naloxone was significantly diminished for heroin users relative to opioid prescription users in weekly opioid use. Heroin users also had persistent depressive symptoms suggesting the need for close monitoring.

SCIENTIFIC SIGNIFICANCE

These data suggest that young heroin users might require higher doses of buprenorphine. (Am J Addict 2017;26:838-844).

Differential behavioral and molecular alterations upon protracted abstinence from cocaine versus morphine, nicotine, THC and alcohol

Unified theories of addiction are challenged by differing drug-seeking behaviors and neurobiological adaptations across drug classes, particularly for narcotics and psychostimulants. We previously showed that protracted abstinence to opiates leads to despair behavior and social withdrawal in mice, and we identified a transcriptional signature in the extended amygdala that was also present in animals abstinent from nicotine, Δ9-tetrahydrocannabinol (THC) and alcohol. Here we examined whether protracted abstinence to these four drugs would also share common behavioral features, and eventually differ from abstinence to the prototypic psychostimulant cocaine. We found similar reduced social recognition, increased motor stereotypies and increased anxiety with relevant c-fos response alterations in morphine, nicotine, THC and alcohol abstinent mice. Protracted abstinence to cocaine, however, led to strikingly distinct, mostly opposing adaptations at all levels, including behavioral responses, neuronal activation and gene expression. Together, these data further document the existence of common hallmarks for protracted abstinence to opiates, nicotine, THC and alcohol that develop within motivation/emotion brain circuits. In our model, however, these do not apply to cocaine, supporting the notion of unique mechanisms in psychostimulant abuse.

Kappa opioid receptor antagonism and chronic antidepressant treatment have beneficial activities on social interactions and grooming deficits during heroin abstinence

Addiction is a chronic brain disorder that progressively invades all aspects of personal life. Accordingly, addiction to opiates severely impairs interpersonal relationships, and the resulting social isolation strongly contributes to the severity and chronicity of the disease. Uncovering new therapeutic strategies that address this aspect of addiction is therefore of great clinical relevance. We recently established a mouse model of heroin addiction in which, following chronic heroin exposure, 'abstinent' mice progressively develop a strong and long-lasting social avoidance phenotype. Here, we explored and compared the efficacy of two pharmacological interventions in this mouse model. Because clinical studies indicate some efficacy of antidepressants on emotional dysfunction associated with addiction, we first used a chronic 4-week treatment with the serotonergic antidepressant fluoxetine, as a reference. In addition, considering prodepressant effects recently associated with kappa opioid receptor signaling, we also investigated the kappa opioid receptor antagonist norbinaltorphimine (norBNI). Finally, we assessed whether fluoxetine and norBNI could reverse abstinence-induced social avoidance after it has established. Altogether, our results show that two interspaced norBNI administrations are sufficient both to prevent and to reverse social impairment in heroin abstinent animals. Therefore, kappa opioid receptor antagonism may represent a useful approach to alleviate social dysfunction in addicted individuals.

Roles of the µ-opioid receptor and its related signaling pathways in the pathogenesis of premenstrual syndrome liver-qi stagnation

The present study aimed to investigate the roles of the µ-opioid receptor (MOR) and its related signaling pathways in the pathogenesis of premenstrual syndrome (PMS) liver-qi stagnation, along with the therapeutic effects of the Shu-Yu capsule in treating the condition. A PMS liver-qi stagnation rat model was established using a chronic restraint stress method. The protein expression level of MOR within rat hippocampal tissue was detected via western blot analysis and cyclic adenosine monophosphate (cAMP) levels within the supernatant of a rat hippocampal cell culture were determined by ELISA. The western blot analysis indicated that the hippocampal expression level of MOR was significantly elevated in the PMS liver-qi stagnation model group. However, subsequent treatment with a Shu-Yu capsule was found to significantly decrease the level of MOR expression. In addition, in vitro experiments were performed, whereby primary hippocampal neurons were treated with model rat serum. It was observed that the level of MOR expression was significantly elevated, while brain-derived neurotrophic factor (BDNF) and cAMP levels in the culture supernatant were significantly decreased. These effects were reversed by treatment with serum from the Shu-Yu capsule-treated rats. Furthermore, when treated with the MOR activator DAMGO, the following were significantly decreased in the primary neurons: Phosphorylation levels of cAMP response element binding protein and extracellular signal-regulated protein kinases (ERK); BDNF expression; and cAMP content in the culture supernatant. These effects were reversed in primary neurons treated with DAMGO and Shu-Yu-containing rat serum. Collectively, the data suggest that increased MOR expression and activation of the cAMP/ERK signaling pathway in the hippocampus may be involved in the pathogenesis of PMS liver-qi stagnation. Furthermore, the efficacy of the Shu-Yu capsule in treating the condition may be via its regulation of MOR receptor signaling.

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.

Temporal and spatial dynamics of peripheral afferent-evoked activity in the dorsal horn recorded in rat spinal cord slices

In the present study, multi-electrode array recording was used to examine dorsal horn activity following stimulation of primary afferents in a rat dorsal root attached-spinal cord slice preparation. The multi-electrode array probe was placed under the dorsal horn slice and local field potentials evoked by stimulation on the dorsal root were analyzed. Three kinds of dorsal root-evoked responses were identified. In lamina II_o, local field potentials exhibited P1 (peak latency 1.46±0.08ms), N1 (2.77±0.18ms, n=12), N2 (7.31±0.48ms), N3 (12.12±0.73ms) and P2(18.30±0.80ms) waves. In lamina II_i local field potentials exhibited P (1.99±0.10ms), N1 (3.35±0.17ms) and N2 (8.58±0.44ms) waves. In laminae III-VI, local field potentials exhibited P1 (3.01±0.07ms), P2 (7.02±0.21ms) and N waves (22.57±0.79ms). Sweep spread was calculated by two dimensional current source density (2D-CSD) analysis. Both α-amino-3-hydroxy-5-methylisoxazole-4-propionic a/kainate and N-methyl-d-aspartate-type glutamate receptors participated in this neuronal circuitry. Morphine diminished local field potentials. Gabapentin diminished the negative components in lamina II and P2 component in lamina II_o, but increased the positive components in lamina II_i and laminae III-VI. The present study revealed that functional dorsal horn activity was preserved in the spinal cord slice preparation. Glutamatergic synapses were crucially involved in information processing. Opioid interneurons and gabapentin may play a modulatory role in regulating signal flows in the dorsal horn. Taken together, these results identify a spatio-temporal profile of dorsal horn activity evoked by dorsal root stimulation, and implicate glutamatergic and opioidergic receptors and gabapentin in this activity.