Opioid-induced microglia reactivity modulates opioid reward, analgesia, and behavior

The influence of drug class on reward in substance use disorders

In the United States, the societal costs associated with drug use surpass $500 billion annually. The rewarding and reinforcing properties that drive the use of these addictive substances are typically examined concerning the neurobiological effects responsible for their abuse potential. In this review, terms such as "abuse potential," "drug," and "addictive properties" are used due to their relevance to the methodological, theoretical, and conceptual framework for understanding the phenomenon of drug-taking behavior and the associated body of preclinical and clinical literature. The use of these terms is not intended to cast aspersions on individuals with substance use disorders (SUD). Understanding what motivates substance use has been a focus of SUD research for decades. Much of this corpus of work has focused on the shared effects of each drug class to increase dopaminergic transmission within the central reward pathways of the brain, or the "reward center." However, the precise influence of each drug class on dopamine signaling, and the extent thereof, differs considerably. Furthermore, the aforementioned substances have effects on several neurobiological targets that mediate and modulate their addictive properties. The current manuscript sought to review the influence of drug class on the rewarding effects of each of the major pharmacological classes of addictive drugs (i.e., psychostimulants, opioids, nicotine, alcohol, and cannabinoids). Our review suggests that even subtle differences in drug effects can result in significant variability in the subjective experience of the drug, altering rewarding and other reinforcing effects. Additionally, this review will argue that reward (i.e., the attractive and motivational property of a stimulus) alone is not sufficient to explain the abuse liability of these substances. Instead, abuse potential is best examined as a function of both positive and negative reinforcing drug effects (i.e., stimuli that the subject will work to attain and stimuli that the subject will work to end or avoid, respectively). Though reward is central to drug use, the factors that motivate and maintain drug taking are varied and complex, with much to be elucidated.

Long-access heroin self-administration induces region specific reduction of grey matter volume and microglia reactivity in the rat

In opioid use disorder (OUD) patients, a decrease in brain grey matter volume (GMV) has been reported. It is unclear whether this is the consequence of prolonged exposure to opioids or is a predisposing causal factor in OUD development. To investigate this, we conducted a structural MRI longitudinal study in NIH Heterogeneous Stock rats exposed to heroin self-administration and age-matched naïve controls housed in the same controlled environment. Structural MRI scans were acquired before (MRI1) and after (MRI2) a prolonged period of long access heroin self-administration resulting in escalation of drug intake. Heroin intake resulted in reduced GMV in various cortical and sub-cortical brain regions. In drug-naïve controls no difference was found between MRI1 and MRI2. Notably, the degree of GMV reduction in the medial prefrontal cortex (mPFC) and the insula positively correlated with the amount of heroin consumed and the escalation of heroin use. In a preliminary gene expression analysis, we identified a number of transcripts linked to immune response and neuroinflammation. This prompted us to hypothesize a link between changes in microglia homeostasis and loss of GMV. For this reason, we analyzed the number and morphology of microglial cells in the mPFC and insula. The number of neurons and their morphology was also evaluated. The primary motor cortex, where no GMV change was observed, was used as negative control. We found no differences in the number of neurons and microglia cells following heroin. However, in the same regions where reduced GMV was detected, we observed a shift towards a rounder shape and size reduction in microglia, suggestive of their homeostatic change towards a reactive state. Altogether these findings suggest that escalation of heroin intake correlates with loss of GMV in specific brain regions and that this phenomenon is linked to changes in microglial morphology.

Genome-wide association study reveals multiple loci for analgesia and opioid consumption behaviors associated with heroin vulnerability in outbred rats

The increased prevalence of opioid use disorder (OUD) has made it imperative to disentangle the biological mechanisms contributing to individual differences in susceptibility to OUD. OUD shows strong heritability, however genetic variants contributing toward vulnerability remain poorly defined. We performed a genome-wide association study (GWAS) using over 850 male and female heterogeneous stock rats to identify genes underlying behaviors associated with OUD such as analgesia, as well as heroin-taking, refraining and seeking behaviors. By using an animal model of OUD, we were able to identify genetic variants associated with distinct OUD behaviors while maintaining a uniform environment, an experimental design not easily achieved in humans. Furthermore, we applied an animal model capturing individual variation in OUD propensity to assess if GWAS results were associated with OUD vulnerable versus resilient behavioral phenotypes. Our findings confirm the heritability of several OUD-like behaviors, including overall phenotype. We identified several genetic variants associated with basal analgesia prior to heroin experience, heroin consumption, escalation of intake, and motivation to obtain heroin. Ets2 , a regulator of microglia functional plasticity, and its eQTL PCP4 were identified for heroin consumption, and were associated with an OUD vulnerable phenotype through phenotype wide association study analysis. Furthermore, the coding variant Phd1l2 and the eQTL MMP15 for break point are both known mediators of addiction-related behaviors, and correlated with OUD vulnerability. These findings identify novel genetic markers related to individual differences in the susceptibility to OUD-relevant behaviors.

A natural small molecule aspidosperma-type alkaloid, hecubine, as a new TREM2 activator for alleviating lipopolysaccharide-induced neuroinflammation in vitro and in vivo

Neuroinflammation and oxidative stress play a crucial role in the pathogenesis of neurodegenerative diseases, including Alzheimer's disease. The triggering receptor expressed on myeloid cells 2 (TREM2), highly expressed by microglia in the central nervous system (CNS), can modulate neuroinflammatory responses. Currently, there are no approved drugs specifically targeting TREM2 for CNS diseases. Aspidosperma alkaloids have shown potential as anti-inflammatory and neuroprotective agents. This study aimed to elucidate the potential therapeutic effect of Hecubine, a natural aspidosperma-type alkaloid, as a TREM2 activator in lipopolysaccharide (LPS)-stimulated neuroinflammation in in vitro and in vivo models. In this study, molecular docking and cellular thermal shift assay (CTSA) were employed to investigate the interaction between Hecubine and TREM2. Enzyme-linked immunosorbent assay (ELISA), quantitative PCR, immunofluorescence, Western blotting, and shRNA gene knockdown were used to assess the anti-neuroinflammatory and antioxidant effects of Hecubine in microglial cells and zebrafish. Our results revealed that Hecubine directly interacted with TREM2, leading to its activation. Knockdown of TREM2 mRNA expression significantly abolished the anti-inflammatory and antioxidant effects of Hecubine on LPS-stimulated proinflammatory mediators (NO, TNF-α, IL-6, and IL-1β) and oxidative stress in microglia cells. Furthermore, Hecubine upregulated Nrf2 expression levels while downregulating TLR4 signaling expression levels both in vivo and in vitro. Silencing TREM2 upregulated TLR4 and downregulated Nrf2 signaling pathways, mimicking the effect of Hecubine, further supporting TREM2 as the drug target by which Hecubine inhibits neuroinflammation. In conclusion, this is the first study to identify a small molecule, namely Hecubine directly targeting TREM2 to mediate anti-neuroinflammation and anti-oxidative effects, which serves as a potential therapeutic agent for the treatment of neural inflammation-associated CNS diseases.

Microglial Nrf2/HO-1 signaling gates remifentanil-induced hyperalgesia via suppressing TRPV4-mediated M1 polarization

Remifentanil-induced hyperalgesia (RIH) represents a significant clinical challenge due to the widespread use of opioids in pain management. However, the molecular and cellular mechanisms underlying RIH remain elusive. This study aimed to unravel the role of spinal cord microglia, focusing on the Nrf2/HO-1 signaling pathway and TRPV4 channels in the development of RIH. We used both in vivo and in vitro models to investigate the activation state of spinal cord microglia, the expression of TRPV4 channels, and the modulation of the Nrf2/HO-1 pathway under remifentanil exposure. In addition, we evaluated the potential therapeutic effects of dexmedetomidine, a perioperative α2-adrenergic agonist, on RIH and its related molecular pathways. Our results revealed a prominent role of spinal cord microglia in RIH, demonstrating an apparent microglial M1 polarization and increased TRPV4 channel expression. A notable observation was the downregulation of the Nrf2/HO-1 pathway, which was associated with increased neuroinflammation and mechanical allodynia. By upregulating or overexpressing Nrf2, we confirmed its ability to inhibit TRPV4 and thereby attenuate RIH-associated mechanical allodynia, M1 polarization, and neuroinflammation. Encouragingly, dexmedetomidine demonstrated therapeutic potential by positively modulating the Nrf2-TRPV4 nexus, attenuating mechanical allodynia, and reducing microglial inflammation. Our research highlights the critical role of spinal cord microglia in RIH mediated by the Nrf2-TRPV4 axis. The ability of dexmedetomidine to modulate this axis suggests its potential as an adjunctive therapy to remifentanil in mitigating RIH. Further studies are imperative to explore the broader implications and practical applicability of our findings.

Long-access heroin self-administration induces region specific reduction of grey matter volume and microglia reactivity in the rat

In opioid use disorder (OUD) patients, a decrease in brain grey matter volume (GMV) has been reported. It is unclear whether this is the consequence of prolonged exposure to opioids or is a predisposing causal factor in OUD development. To investigate this, we conducted a structural MRI longitudinal study in NIH Heterogeneous Stock rats exposed to heroin self-administration and age-matched naïve controls housed in the same controlled environment. Structural MRI scans were acquired before (MRI 1 ) and after (MRI 2 ) a prolonged period of long access heroin self-administration resulting in escalation of drug intake. Heroin intake resulted in reduced GMV in various cortical and sub-cortical brain regions. In drug-naïve controls no difference was found between MRI 1 and MRI 2 . Notably, the degree of GMV reduction in the medial prefrontal cortex (mPFC) and the insula positively correlated with the amount of heroin consumed and the escalation of heroin use. In a preliminary gene expression analysis, we identified a number of transcripts linked to immune response and neuroinflammation. This prompted us to hypothesize a link between changes in microglia homeostasis and loss of GMV. For this reason, we analyzed the number and morphology of microglial cells in the mPFC and insula. The number of neurons and their morphology was also evaluated. The primary motor cortex, where no GMV change was observed, was used as negative control. We found no differences in the number of neurons and microglia cells following heroin. However, in the same regions where reduced GMV was detected, we observed a shift towards a rounder shape and size reduction in microglia, suggestive of their homeostatic change towards a reactive state. Altogether these findings suggest that escalation of heroin intake correlates with loss of GMV in specific brain regions and that this phenomenon is linked to changes in microglial morphology.

Microglia in neuroimmunopharmacology and drug addiction

Drug addiction is a chronic and debilitating disease that is considered a global health problem. Various cell types in the brain are involved in the progression of drug addiction. Recently, the xenobiotic hypothesis has been proposed, which frames substances of abuse as exogenous molecules that are responded to by the immune system as foreign "invaders", thus triggering protective inflammatory responses. An emerging body of literature reveals that microglia, the primary resident immune cells in the brain, play an important role in the progression of addiction. Repeated cycles of drug administration cause a progressive, persistent induction of neuroinflammation by releasing microglial proinflammatory cytokines and their metabolic products. This contributes to drug addiction via modulation of neuronal function. In this review, we focus on the role of microglia in the etiology of drug addiction. Then, we discuss the dynamic states of microglia and the correlative and causal evidence linking microglia to drug addiction. Finally, possible mechanisms of how microglia sense drug-related stimuli and modulate the addiction state and how microglia-targeted anti-inflammation therapies affect addiction are reviewed. Understanding the role of microglia in drug addiction may help develop new treatment strategies to fight this devastating societal challenge.

Opioids, microglia, and temporal lobe epilepsy

A lack of treatment options for temporal lobe epilepsy (TLE) demands an urgent quest for new therapies to recover neuronal damage and reduce seizures, potentially interrupting the neurotoxic cascades that fuel hyper-excitability. Endogenous opioids, along with their respective receptors, particularly dynorphin and kappa-opioid-receptor, present as attractive candidates for controlling neuronal excitability and therapeutics in epilepsy. We perform a critical review of the literature to evaluate the role of opioids in modulating microglial function and morphology in epilepsy. We find that, in accordance with anticonvulsant effects, acute opioid receptor activation has unique abilities to modulate microglial activation through toll-like 4 receptors, regulating downstream secretion of cytokines. Abnormal activation of microglia is a dominant feature of neuroinflammation, and inflammatory cytokines are found to aggravate TLE, inspiring the challenge to alter microglial activation by opioids to suppress seizures. We further evaluate how opioids can modulate microglial activation in epilepsy to enhance neuroprotection and reduce seizures. With controlled application, opioids may interrupt inflammatory cycles in epilepsy, to protect neuronal function and reduce seizures. Research on opioid-microglia interactions has important implications for epilepsy and healthcare approaches. However, preclinical research on opioid modulation of microglia supports a new therapeutic pathway for TLE.

Neuroimaging of opioid exposure: a review of preclinical animal models to inform addiction research

Opioid use results in thousands of overdose deaths each year. To address this crisis, we need a better understanding of the neurobiological mechanisms that drive opioid abuse. The noninvasive imaging tools positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and manganese-enhanced magnetic resonance imaging (MEMRI) can be used to identify how brain activity responds to acute opioid exposure and adapts to chronic drug treatment. These techniques can be performed in humans and animal models, and brain networks identified in animals closely map to the human brain. Animal models have the advantage of being able to systematically examine the independent effects of opioid exposure in a controlled environment accounting for the complex factors that drive opioid misuse in humans. This review synthesizes literature that utilized noninvasive neuroimaging tools (PET, fMRI, and MEMRI) measuring brain activity correlates in animals to understand the neurobiological consequences of exposure to abused opioids. A PubMed search in September 2023 identified 25 publications. These manuscripts were divided into 4 categories based on the route and duration of drug exposure (acute/chronic, active/passive administration). Within each category, the results were generally consistent across drug and imaging protocols. These papers cover a 20-year range and highlight the advancements in neuroimaging methodology during that time. These advances have enabled researchers to achieve greater resolution of brain regions altered by opioid exposure and to identify patterns of brain activation across regions (i.e., functional connectivity) and within subregions of structures. After describing the existing literature, we suggest areas where additional research is needed.

Extracellular vesicles-Mediators of opioid use disorder?

Opioid use disorder (OUD) is a growing health emergency in the United States leading to an epidemic of overdose deaths. OUD is recognized as an addictive brain disorder resulting in psychological, cognitive and behavioural dysfunction. These observed clinical dysfunctions are a result of cellular changes that occur in the brain. Derangements in inflammation, neurogenesis and synaptic plasticity are observed in the brains of OUD patients. The mechanisms of these derangements are unclear; however, extracellular vesicles (EVs), membrane bound particles containing protein, nucleotides and lipids are currently being investigated as agents that invoke these cellular changes. The primary function of EVs is to facilitate intercellular communication by transfer of cargo (protein, nucleotides and lipids) between cells; however, changes in this cargo have been observed in models of OUD suggesting that EVs may be agents promoting the observed cellular derangements. This review summarizes evidence that altered cargo of EVs, specifically protein and miRNA, in models of OUD promote impairments in neurons, astrocytes and microglial cells. These findings support the premise that opioids alter EVs to detrimentally affect neuro-cellular function resulting in the observed addictive, psychological and neurocognitive deficits in OUD patients.

Single nucleus transcriptomics of ventral midbrain identifies glial activation associated with chronic opioid use disorder

Dynamic interactions of neurons and glia in the ventral midbrain mediate reward and addiction behavior. We studied gene expression in 212,713 ventral midbrain single nuclei from 95 individuals with history of opioid misuse, and individuals without drug exposure. Chronic exposure to opioids was not associated with change in proportions of glial and neuronal subtypes, however glial transcriptomes were broadly altered, involving 9.5 - 6.2% of expressed genes within microglia, oligodendrocytes, and astrocytes. Genes associated with activation of the immune response including interferon, NFkB signaling, and cell motility pathways were upregulated, contrasting with down-regulated expression of synaptic signaling and plasticity genes in ventral midbrain non-dopaminergic neurons. Ventral midbrain transcriptomic reprogramming in the context of chronic opioid exposure included 325 genes that previous genome-wide studies had linked to risk of substance use traits in the broader population, thereby pointing to heritable risk architectures in the genomic organization of the brain's reward circuitry.

Cannabidiol Analogue for the Treatment of Morphine-Induced Addiction by Targeting PKM2

Opioid addiction is a chronically relapsing disorder that causes critical public health problems. Currently, there is a lack of effective drug treatment. Herein, one cannabidiol derivative, CIAC001, was discovered as an effective agent for treating morphine-induced addiction. In vitro, CIAC001 exhibited significantly improved anti-neuroinflammatory activity with lower toxicity. In vivo, CIAC001 ameliorated the morphine-induced withdrawal reaction, behavioral sensitization, and conditional position preference by inhibiting morphine-induced microglia activation and neuroinflammation. Target fishing for CIAC001 by activity-based protein profiling led to the identification of pyruvate kinase M2 (PKM2) as the target protein. CIAC001 bound to the protein-protein interface of the PKM2 dimer and promoted the tetramerization of PKM2. Moreover, CIAC001 exhibited an anti-neuroinflammatory effect by reversing the decrease of the PKM2 tetramer and inhibiting the nuclear translocation of PKM2. In summary, this study identified CIAC001 as a lead compound in targeting PKM2 to treat morphine-induced addiction.

Developing non-opioid therapeutics to alleviate pain among persons with opioid use disorder: a review of the human evidence

The opioid crisis remains a major public health concern, causing significant morbidity and mortality worldwide. Pain is frequently observed among individuals with opioid use disorder (OUD), and the current opioid agonist therapies (OAT) have limited efficacy in addressing the pain needs of this population. We reviewed the most promising non-opioid analgesic therapies for opioid-dependent individuals synthesising data from randomised controlled trials in the Medline database from December 2022 to March 2023. Ketamine, gabapentin, serotoninergic antidepressants, and GABAergic drugs were found to be the most extensively studied non-opioid analgesics with positive results. Additionally, we explored the potential of cannabinoids, glial activation inhibitors, psychedelics, cholecystokinin antagonists, alpha-2 adrenergic agonists, and cholinergic drugs. Methodological improvements are required to advance the development of novel analgesic strategies and establish their safety profile for opioid-dependent populations. We highlight the need for greater integration of experimental pain methods and abuse liability assessments, more granular assessments of prior opioid exposure, greater uniformity of pain types within study samples, and a particular focus on individuals with OUD receiving OAT. Finally, future research should investigate pharmacokinetic interactions between OAT and various non-opioid analgesics and perform reverse translation basic experiments, particularly with methadone and buprenorphine, which remain the standard OUD treatment.

Phenytoin Decreases Pain-like Behaviors and Improves Opioid Analgesia in a Rat Model of Neuropathic Pain

Neuropathic pain remains a clinical challenge due to its complex and not yet fully understood pathomechanism, which result in limited analgesic effectiveness of the management offered, particularly for patients with acute, refractory neuropathic pain states. In addition to the introduction of several modern therapeutic approaches, such as neuromodulation or novel anti-neuropathic drugs, significant efforts have been made in the repurposing of well-known substances such as phenytoin. Although its main mechanism of action occurs at sodium channels in excitable and non-excitable cells and is well documented, how the drug affects the disturbed neuropathic interactions at the spinal cord level and how it influences morphine-induced analgesia have not been clarified, both being crucial from a clinical perspective. We demonstrated that single and repeated systemic administrations of phenytoin decreased tactile and thermal hypersensitivity in an animal model of neuropathic pain. Importantly, we observed an increase in the antinociceptive effect on thermal stimuli with repeated administrations of phenytoin. This is the first study to report that phenytoin improves morphine-induced antinociceptive effects and influences microglia/macrophage activity at the spinal cord and dorsal root ganglion levels in a neuropathic pain model. Our findings support the hypothesis that phenytoin may represent an effective strategy for neuropathic pain management in clinical practice, particularly when combination with opioids is needed.

Pharmacological and nutritional therapy of children and adults with chronic intestinal pseudo-obstruction

INTRODUCTION

Chronic intestinal pseudo-obstruction (CIPO) is a rare, heterogeneous and severe form of gastrointestinal dysmotility.

AREAS COVERED

Pertinent literature on pediatric and adult CIPO management has been assessed via PubMed, Scopus, and EMBASE from inception to June 2022. Prokinetics, aimed at restoring intestinal propulsion (e.g. orthopramides and substituted benzamides, acetyl cholinesterase inhibitors, serotonergic agents and others), have been poorly tested and the available data showed only partial efficacy. Moreover, some prokinetic agents (e.g. orthopramides and substituted benzamides) can cause major side effects. The CIPO-related small intestinal bacterial overgrowth requires treatment preferably via poorly absorbable antibiotics to avoid bacterial resistance. Apart from opioids, which worsen gut motility, analgesics should be considered to manage visceral pain, which might dominate the clinical manifestations. Nutritional support, via modified oral feeding, enteral or parenteral nutrition, is key to halt CIPO-related malnutrition.

EXPERT OPINION

There have been significant roadblocks preventing the development of CIPO treatment. Nonetheless, the considerable advancement in neurogastroenterology and pharmacological agents cast hopes to test the actual efficacy of new prokinetics via well-designed clinical trials. Adequate dietary strategies and supplementation remain of crucial importance. Taken together, novel pharmacological and nutritional options are expected to provide adequate treatments for these patients.

Exploring Novel Therapeutic Targets in the Common Pathogenic Factors in Migraine and Neuropathic Pain

Migraine and neuropathic pain (NP) are both painful, disabling, chronic conditions which exhibit some symptom similarities and are thus considered to share a common etiology. The calcitonin gene-related peptide (CGRP) has gained credit as a target for migraine management; nevertheless, the efficacy and the applicability of CGRP modifiers warrant the search for more effective therapeutic targets for pain management. This scoping review focuses on human studies of common pathogenic factors in migraine and NP, with reference to available preclinical evidence to explore potential novel therapeutic targets. CGRP inhibitors and monoclonal antibodies alleviate inflammation in the meninges; targeting transient receptor potential (TRP) ion channels may help prevent the release of nociceptive substances, and modifying the endocannabinoid system may open a path toward discovery of novel analgesics. There may exist a potential target in the tryptophan-kynurenine (KYN) metabolic system, which is closely linked to glutamate-induced hyperexcitability; alleviating neuroinflammation may complement a pain-relieving armamentarium, and modifying microglial excitation, which is observed in both conditions, may be a possible approach. Those are several potential analgesic targets which deserve to be explored in search of novel analgesics; however, much evidence remains missing. This review highlights the need for more studies on CGRP modifiers for subtypes, the discovery of TRP and endocannabinoid modulators, knowledge of the status of KYN metabolites, the consensus on cytokines and sampling, and biomarkers for microglial function, in search of innovative pain management methods for migraine and NP.

Current and emerging pharmacotherapies for opioid dependence treatments in adults: a comprehensive update

INTRODUCTION

Opioid use disorder (OUD) is characterized by compulsive opioid seeking and taking, intense drug craving, and intake of opioids despite negative consequences. The prevalence of OUDs has now reached an all-time high, in parallel with peak rates of fatal opioid-related overdoses, where 15 million individuals worldwide meet the criteria for OUD. Further, in 2020, 120,000 opioid-related deaths were reported worldwide with over 75,000 of those deaths occurring within the United States.

AREAS COVERED

In this review, we highlight pharmacotherapies utilized in patients with OUDs, including opioid replacement therapies, and opioid antagonists utilized for opioid overdoses and deterrent of opioid use. We also highlight newer treatments, such as those targeting the neuroimmune system, which are potential new directions for research given the recently established role of opioids in activating neuroinflammatory pathways, as well as over the counter remedies, including kratom, that may mitigate withdrawal.

EXPERT OPINION

To effectively treat OUDs, a deeper understanding of the current therapeutics being utilized, their additive effects, and the added involvement of the neuroimmune system are essential. Additionally, a complete understanding of opioid-induced neuronal alterations and therapeutics that target these abnormalities - including the neuroimmune system - is required to develop effective treatments for OUDs.

Crotalphine Modulates Microglia M1/M2 Phenotypes and Induces Spinal Analgesia Mediated by Opioid-Cannabinoid Systems

Pain is a worldwide public health problem and its treatment is still a challenge since clinically available drugs do not completely reverse chronic painful states or induce undesirable effects. Crotalphine is a 14 amino acids synthetic peptide that induces a potent and long-lasting analgesic effect on acute and chronic pain models, peripherally mediated by the endogenous release of dynorphin A and the desensitization of the transient receptor potential ankyrin 1 (TRPA1) receptor. However, the effects of crotalphine on the central nervous system (CNS) and the signaling pathway have not been investigated. Thus, the central effect of crotalphine was evaluated on the partial sciatic nerve ligation (PSNL)-induced chronic neuropathic pain model. Crotalphine (100 µg/kg, p.o.)-induced analgesia on the 14th day after surgery lasting up to 24 h after administration. This effect was prevented by intrathecal administration of CB1 (AM251) or CB2 (AM630) cannabinoid receptor antagonists. Besides that, crotalphine-induced analgesia was reversed by CTOP, nor-BNI, and naltrindole, antagonists of mu, kappa, and delta-opioid receptors, respectively, and also by the specific antibodies for β-endorphin, dynorphin-A, and met-enkephalin. Likewise, the analgesic effect of crotalphine was blocked by the intrathecal administration of minocycline, an inhibitor of microglial activation and proliferation. Additionally, crotalphine decreased the PSNL-induced IL-6 release in the spinal cord. Importantly, in vitro, crotalphine inhibited LPS-induced CD86 expression and upregulated CD206 expression in BV-2 cells, demonstrating a polarization of microglial cells towards the M2 phenotype. These results demonstrated that crotalphine, besides activating opioid and cannabinoid analgesic systems, impairs central neuroinflammation, confirming the neuromodulatory mechanism involved in the crotalphine analgesic effect.

Substance use, microbiome and psychiatric disorders

Accumulating evidence from several studies has shown association between substance use, dysregulation of the microbiome and psychiatric disorders such as depression, anxiety, and psychosis. Many of the abused substances such as cocaine and alcohol have been shown to alter immune signaling pathways and cause inflammation in both the periphery and the central nervous system (CNS). In addition, these substances of abuse also alter the composition and function of the gut microbiome which is known to play important roles such as the synthesis of neurotransmitters and metabolites, that affect the CNS homeostasis and consequent behavioral outcomes. The emerging interactions between substance use, microbiome and CNS neurochemical alterations could contribute to the development of psychiatric disorders. This review provides an overview of the associative effects of substance use such as alcohol, cocaine, methamphetamine, nicotine and opioids on the gut microbiome and psychiatric disorders involving anxiety, depression and psychosis. Understanding the relationship between substance use, microbiome and psychiatric disorders will provide insights for potential therapeutic targets, aimed at mitigating these adverse outcomes.

A Glitch in the Matrix: The Role of Extracellular Matrix Remodeling in Opioid Use Disorder

Opioid use disorder (OUD) and deaths from drug overdoses have reached unprecedented levels. Given the enormous impact of the opioid crisis on public health, a more thorough, in-depth understanding of the consequences of opioids on the brain is required to develop novel interventions and pharmacological therapeutics. In the brain, the effects of opioids are far reaching, from genes to cells, synapses, circuits, and ultimately behavior. Accumulating evidence implicates a primary role for the extracellular matrix (ECM) in opioid-induced plasticity of synapses and circuits, and the development of dependence and addiction to opioids. As a network of proteins and polysaccharides, including cell adhesion molecules, proteases, and perineuronal nets, the ECM is intimately involved in both the formation and structural support of synapses. In the human brain, recent findings support an association between altered ECM signaling and OUD, particularly within the cortical and striatal circuits involved in cognition, reward, and craving. Furthermore, the ECM signaling proteins, including matrix metalloproteinases and proteoglycans, are directly involved in opioid seeking, craving, and relapse behaviors in rodent opioid models. Both the impact of opioids on the ECM and the role of ECM signaling proteins in opioid use disorder, may, in part, depend on biological sex. Here, we highlight the current evidence supporting sex-specific roles for ECM signaling proteins in the brain and their associations with OUD. We emphasize knowledge gaps and future directions to further investigate the potential of the ECM as a therapeutic target for the treatment of OUD.