Synaptic plasticity and addiction

Coupled mixed model for joint genetic analysis of complex disorders with two independently collected data sets

BACKGROUND

In the last decade, Genome-wide Association studies (GWASs) have contributed to decoding the human genome by uncovering many genetic variations associated with various diseases. Many follow-up investigations involve joint analysis of multiple independently generated GWAS data sets. While most of the computational approaches developed for joint analysis are based on summary statistics, the joint analysis based on individual-level data with consideration of confounding factors remains to be a challenge.

RESULTS

In this study, we propose a method, called Coupled Mixed Model (CMM), that enables a joint GWAS analysis on two independently collected sets of GWAS data with different phenotypes. The CMM method does not require the data sets to have the same phenotypes as it aims to infer the unknown phenotypes using a set of multivariate sparse mixed models. Moreover, CMM addresses the confounding variables due to population stratification, family structures, and cryptic relatedness, as well as those arising during data collection such as batch effects that frequently appear in joint genetic studies. We evaluate the performance of CMM using simulation experiments. In real data analysis, we illustrate the utility of CMM by an application to evaluating common genetic associations for Alzheimer's disease and substance use disorder using datasets independently collected for the two complex human disorders. Comparison of the results with those from previous experiments and analyses supports the utility of our method and provides new insights into the diseases. The software is available at https://github.com/HaohanWang/CMM .

Neurogranin, Encoded by the Schizophrenia Risk Gene NRGN, Bidirectionally Modulates Synaptic Plasticity via Calmodulin-Dependent Regulation of the Neuronal Phosphoproteome

BACKGROUND

Neurogranin (Ng), encoded by the schizophrenia risk gene NRGN, is a calmodulin-binding protein enriched in the postsynaptic compartments, and its expression is reduced in the postmortem brains of patients with schizophrenia. Experience-dependent translation of Ng is critical for encoding contextual memory, and Ng regulates developmental plasticity in the primary visual cortex during the critical period. However, the overall impact of Ng on the neuronal signaling that regulates synaptic plasticity is unknown.

METHODS

Altered Ng expression was achieved via virus-mediated gene manipulation in mice. The effect on long-term potentiation (LTP) was accessed using spike timing-dependent plasticity protocols. Quantitative phosphoproteomics analyses led to discoveries in significant phosphorylated targets. An identified candidate was examined with high-throughput planar patch clamp and was validated with pharmacological manipulation.

RESULTS

Ng bidirectionally modulated LTP in the hippocampus. Decreasing Ng levels significantly affected the phosphorylation pattern of postsynaptic density proteins, including glutamate receptors, GTPases, kinases, RNA binding proteins, selective ion channels, and ionic transporters, some of which highlighted clusters of schizophrenia- and autism-related genes. Hypophosphorylation of NMDA receptor subunit Grin2A, one significant phosphorylated target, resulted in accelerated decay of NMDA receptor currents. Blocking protein phosphatase PP2B activity rescued the accelerated NMDA receptor current decay and the impairment of LTP mediated by Ng knockdown, implicating the requirement of synaptic PP2B activity for the deficits.

CONCLUSIONS

Altered Ng levels affect the phosphorylation landscape of neuronal proteins. PP2B activity is required for mediating the deficit in synaptic plasticity caused by decreasing Ng levels, revealing a novel mechanistic link of a schizophrenia risk gene to cognitive deficits.

Drug overdose deaths, addiction neuroscience and the challenges of translation

In this article, we argue that the rapid rise in drug overdose deaths in America is a tragedy that draws attention to fundamental conceptual and experimental problems in addiction science that have significant human consequences. Despite enormous economic investment, political support and claims to have revolutionised addiction medicine, neurobiological models are yet to produce a treatment for substance addiction. This is partly, we claim, because neurobiology is unable to explain essential features of addiction and relapse that neurobehavioral models of addiction are better placed to investigate. We show how addiction neuroscience turned to long-term memory to explain the chronicity of addiction and persistent relapses long after neurochemical traces have left the body. The turn to memory may in time help to close the translational gap facing addiction medicine, but it is our view in this article that the primary value of memory theory lays in its potential to create new critical friendships between biological and social sciences that are attuned to the lived experience and suffering of stigmatised people. The value of the memory turn may rest upon the capacity of these critical friendships to wean addiction science off its long-term dependence on disease concepts of human distress.

Classical Psychedelics as Therapeutics in Psychiatry - Current Clinical Evidence and Potential Therapeutic Mechanisms in Substance Use and Mood Disorders

Classical psychedelics, primarily psilocybin and lysergic acid diethylamide (LSD), have been used and extensively studied in Western medicine as part of substance-assisted psychotherapy in the 1950s and 1960s. Modern clinical research is currently gaining momentum and provides new evidence for the safety and efficacy of classical psychedelics (primarily psilocybin, but also LSD and ayahuasca) in the treatment of different psychiatric conditions, including substance use and mood disorders.In this review article, we outline common pathological mechanisms of substance use disorders (SUD) and unipolar depression. Next, the current literature on the effects of psychedelics is summarized in order to generate hypotheses regarding their potential therapeutic mechanisms of action in treating these psychiatric conditions. Finally, we review and discuss clinical trials published since 2011 investigating the effects of psychedelics in SUD and depression.While results from those modern clinical trials are promising, most of them do not meet the methodological requirements to allow firm conclusions on the clinical efficacy of psychedelics. Larger, blinded, randomized controlled trials (RCT) with clearly defined patient groups and well-defined primary endpoints are needed. Additionally, the therapeutic mechanisms of classical psychedelics are currently unknown. This review presents hypotheses derived from preclinical and human studies that need to be tested in future trials to better understand the clinical potential of psychedelic substances in modern psychiatry.

Looking for Novelty in an "Old" Receptor: Recent Advances Toward Our Understanding of GABAARs and Their Implications in Receptor Pharmacology

Diverse populations of GABA_A receptors (GABA_ARs) throughout the brain mediate fast inhibitory transmission and are modulated by various endogenous ligands and therapeutic drugs. Deficits in GABA_AR signaling underlie the pathophysiology behind neurological and neuropsychiatric disorders such as epilepsy, anxiety, and depression. Pharmacological intervention for these disorders relies on several drug classes that target GABA_ARs, such as benzodiazepines and more recently neurosteroids. It has been widely demonstrated that subunit composition and receptor stoichiometry impact the biophysical and pharmacological properties of GABA_ARs. However, current GABA_AR-targeting drugs have limited subunit selectivity and produce their therapeutic effects concomitantly with undesired side effects. Therefore, there is still a need to develop more selective GABA_AR pharmaceuticals, as well as evaluate the potential for developing next-generation drugs that can target accessory proteins associated with native GABA_ARs. In this review, we briefly discuss the effects of benzodiazepines and neurosteroids on GABA_ARs, their use as therapeutics, and some of the pitfalls associated with their adverse side effects. We also discuss recent advances toward understanding the structure, function, and pharmacology of GABA_ARs with a focus on benzodiazepines and neurosteroids, as well as newly identified transmembrane proteins that modulate GABA_ARs.

Gray and white matter morphology in substance use disorders: a neuroimaging systematic review and meta-analysis

Substance use disorders (SUDs) are characterized by a compulsion to seek and consume one or more substances of abuse, with a perceived loss of control and a negative emotional state. Prolonged substance use seems to be associated with morphological changes of multiple neural circuits, in particular the frontal-striatal and limbic pathways. Such neuroadaptations are evident across several substance disorders, but may vary depending on the type of substance, consumption severity and/or other unknown factors. We therefore identified studies investigating the effects of SUDs using volumetric whole-brain voxel-based morphometry (VBM) in gray (GM) and white matter (WM). We performed a systematic review and meta-analysis of VBM studies using the anatomic likelihood estimation (ALE) method implemented in GingerALE (PROSPERO pre-registration CRD42017071222 ). Sixty studies met inclusion criteria and were included in the final quantitative meta-analysis, with a total of 614 foci, 94 experiments and 4938 participants. We found convergence and divergence in brain regions and volume effects (higher vs. lower volume) in GM and WM depending on the severity of the consumption pattern and type of substance used. Convergent pathology was evident across substances in GM of the insula, anterior cingulate cortex, putamen, and thalamus, and in WM of the thalamic radiation and internal capsule bundle. Divergent pathology between occasional use (cortical pathology) and addiction (cortical-subcortical pathology) provides evidence of a possible top-down neuroadaptation. Our findings indicate particular brain morphometry alterations in SUDs, which may inform our understanding of disease progression and ultimately therapeutic approaches.

Sirtuin 1 inhibitor EX527 suppresses morphine-induced behavioral sensitization

Morphine addiction is categorized as a chronic recurrent brain disease which always results in mental disturbance, concomitant diseases and early death. Recent evidence suggested that Sirtuin 1 (SIRT1) played a crucial role in learning, memory and reward, nevertheless, its role in morphine addiction is still unclear. We explored whether SIRT1 in the ventrolateral orbital cortex (VLO) is associated with morphine addiction and its potential mechanism. We applied the morphine-induced behavioral sensitization paradigm to investigate whether microinjection of EX527, a SIRT1 inhibitor, into the VLO could affect the rat behaviors. Furthermore, we focused on the expression of extracellular signal-regulated protein kinases (ERK) and brain-derived neurotrophic factor (BDNF), potential downstream targets of SIRT1. Microinjecting EX527 into the VLO significantly suppressed morphine-induced behavioral sensitization. We found that the expression of SIRT1, phosphorylated ERK (p-ERK) and BDNF in the VLO were markedly up-regulated by morphine administrations in expression phase. These positive changes were significantly inhibited by microinjecting EX527 into the VLO. These results suggest that SIRT1 in the VLO may mediate morphine-induced behavioral sensitization and the overexpression of SIRT1, p-ERK and BDNF could be the potential mechanism. Taken together, the results of our research provide evidence to support that SIRT1 play an important role in morphine vulnerability and microinjecting EX527 into the VLO could significantly suppress morphine addiction in rats.

Opioid-induced structural and functional plasticity of medium-spiny neurons in the nucleus accumbens

Opioid Use Disorder (OUD) is a chronic relapsing clinical condition with tremendous morbidity and mortality that frequently persists, despite treatment, due to an individual's underlying psychological, neurobiological, and genetic vulnerabilities. Evidence suggests that these vulnerabilities may have neurochemical, cellular, and molecular bases. Key neuroplastic events within the mesocorticolimbic system that emerge through chronic exposure to opioids may have a determinative influence on behavioral symptoms associated with OUD. In particular, structural and functional alterations in the dendritic spines of medium spiny neurons (MSNs) within the nucleus accumbens (NAc) and its dopaminergic projections from the ventral tegmental area (VTA) are believed to facilitate these behavioral sequelae. Additionally, glutamatergic neurons from the prefrontal cortex, the basolateral amygdala, the hippocampus, and the thalamus project to these same MSNs, providing an enriched target for synaptic plasticity. Here, we review literature related to neuroadaptations in NAc MSNs from dopaminergic and glutamatergic pathways in OUD. We also describe new findings related to transcriptional, epigenetic, and molecular mechanisms in MSN plasticity in the different stages of OUD.

Targeting presynaptic H3 heteroreceptor in nucleus accumbens to improve anxiety and obsessive-compulsive-like behaviors

Anxiety commonly co-occurs with obsessive-compulsive disorder (OCD). Both of them are closely related to stress. However, the shared neurobiological substrates and therapeutic targets remain unclear. Here we report an amelioration of both anxiety and OCD via the histamine presynaptic H3 heteroreceptor on glutamatergic afferent terminals from the prelimbic prefrontal cortex (PrL) to the nucleus accumbens (NAc) core, a vital node in the limbic loop. The NAc core receives direct hypothalamic histaminergic projections, and optogenetic activation of hypothalamic NAc core histaminergic afferents selectively suppresses glutamatergic rather than GABAergic synaptic transmission in the NAc core via the H3 receptor and thus produces an anxiolytic effect and improves anxiety- and obsessive-compulsive-like behaviors induced by restraint stress. Although the H3 receptor is expressed in glutamatergic afferent terminals from the PrL, basolateral amygdala (BLA), and ventral hippocampus (vHipp), rather than the thalamus, only the PrL- and not BLA- and vHipp-NAc core glutamatergic pathways among the glutamatergic afferent inputs to the NAc core is responsible for co-occurrence of anxiety- and obsessive-compulsive-like behaviors. Furthermore, activation of the H3 receptor ameliorates anxiety and obsessive-compulsive-like behaviors induced by optogenetic excitation of the PrL-NAc glutamatergic afferents. These results demonstrate a common mechanism regulating anxiety- and obsessive-compulsive-like behaviors and provide insight into the clinical treatment strategy for OCD with comorbid anxiety by targeting the histamine H3 receptor in the NAc core.

Neuromorphic Engineering: From Biological to Spike-Based Hardware Nervous Systems

The human brain is a sophisticated, high-performance biocomputer that processes multiple complex tasks in parallel with high efficiency and remarkably low power consumption. Scientists have long been pursuing an artificial intelligence (AI) that can rival the human brain. Spiking neural networks based on neuromorphic computing platforms simulate the architecture and information processing of the intelligent brain, providing new insights for building AIs. The rapid development of materials engineering, device physics, chip integration, and neuroscience has led to exciting progress in neuromorphic computing with the goal of overcoming the von Neumann bottleneck. Herein, fundamental knowledge related to the structures and working principles of neurons and synapses of the biological nervous system is reviewed. An overview is then provided on the development of neuromorphic hardware systems, from artificial synapses and neurons to spike-based neuromorphic computing platforms. It is hoped that this review will shed new light on the evolution of brain-like computing.

Increased Response to 3,4-Methylenedioxymethamphetamine (MDMA) Reward and Altered Gene Expression in Zebrafish During Short- and Long-Term Nicotine Withdrawal

An interactive effect between nicotine and 3,4-methylenedioxymethamphetamine (MDMA) has been reported but the mechanism underlying such interaction is not completely understood. This study used zebrafish to explore gene expression changes associated with altered sensitivity to the rewarding effects of MDMA following 2-week exposure to nicotine and 2-60 days of nicotine withdrawal. Reward responses to MDMA were assessed using a conditioned place preference (CPP) paradigm and gene expression was evaluated using quantitative real-time PCR of mRNA from whole brain samples from drug-treated and control adult zebrafish. Zebrafish pre-exposed for 2 weeks to nicotine showed increased conditioned place preference in response to low-dose, 0.1 mg/kg, MDMA compared to un-exposed fish at 2, 7, 30 and 60 days withdrawal. Pre-exposure to nicotine for 2 weeks induced a significant increase of c-Fos and vasopressin receptor expression but a decrease of D₃ dopaminergic and oxytocin receptor expression at 2 days of withdrawal. C-Fos mRNA increased also at 7, 30, 60 days of withdrawal. Nicotine pre-exposed zebrafish submitted to MDMA-induced CPP showed an increase in expression of p35 at day 2, α4 at day 30, vasopressin at day 7 and D₃ dopaminergic receptor at day 7, 30 and 60. These gene alterations could account for the altered sensitivity to the rewarding effects of MDMA in nicotine pre-exposed fish, suggesting that zebrafish have an altered ability to modulate behaviour as a function of reward during nicotine withdrawal.

Neuronal activity regulates alternative exon usage

Neuronal activity-regulated gene transcription underlies plasticity-dependent changes in the molecular composition and structure of neurons. A large number of genes regulated by different neuronal plasticity inducing pathways have been identified, but altered gene expression levels represent only part of the complexity of the activity-regulated transcriptional program. Alternative splicing, the differential inclusion and exclusion of exonic sequence in mRNA, is an additional mechanism that is thought to define the activity-dependent transcriptome. Here, we present a genome wide microarray-based survey to identify exons with increased expression levels at 1, 4 or 8 h following neuronal activity in the murine hippocampus provoked by generalized seizures. We used two different bioinformatics approaches to identify alternative activity-induced exon usage and to predict alternative splicing, ANOSVA (ANalysis Of Splicing VAriation) which we here adjusted to accommodate data from different time points and FIRMA (Finding Isoforms using Robust Multichip Analysis). RNA sequencing, in situ hybridization and reverse transcription PCR validate selected activity-dependent splicing events of previously described and so far undescribed activity-regulated transcripts, including Homer1a, Homer1d, Ania3, Errfi1, Inhba, Dclk1, Rcan1, Cda, Tpm1 and Krt75. Taken together, our survey significantly adds to the comprehensive understanding of the complex activity-dependent neuronal transcriptomic signature. In addition, we provide data sets that will serve as rich resources for future comparative expression analyses.

Methamphetamine use alters human plasma extracellular vesicles and their microRNA cargo: An exploratory study

Methamphetamine (MA) is the largest drug threat across the globe, with health effects including neurotoxicity and cardiovascular disease. Recent studies have begun to link microRNAs (miRNAs) to the processes related to MA use and addiction. Our studies are the first to analyse plasma EVs and their miRNA cargo in humans actively using MA (MA-ACT) and control participants (CTL). In this cohort we also assessed the effects of tobacco use on plasma EVs. We used vesicle flow cytometry to show that the MA-ACT group had an increased abundance of EV tetraspanin markers (CD9, CD63, CD81), but not pro-coagulant, platelet-, and red blood cell-derived EVs. We also found that of the 169 plasma EV miRNAs, eight were of interest in MA-ACT based on multiple statistical criteria. In smokers, we identified 15 miRNAs of interest, two that overlapped with the eight MA-ACT miRNAs. Three of the MA-ACT miRNAs significantly correlated with clinical features of MA use and target prediction with these miRNAs identified pathways implicated in MA use, including cardiovascular disease and neuroinflammation. Together our findings indicate that MA use regulates EVs and their miRNA cargo, and support that further studies are warranted to investigate their mechanistic role in addiction, recovery, and recidivism.

SynaptoPAC, an optogenetic tool for induction of presynaptic plasticity

Optogenetic manipulations have transformed neuroscience in recent years. While sophisticated tools now exist for controlling the firing patterns of neurons, it remains challenging to optogenetically define the plasticity state of individual synapses. A variety of synapses in the mammalian brain express presynaptic long-term potentiation (LTP) upon elevation of presynaptic cyclic adenosine monophosphate (cAMP), but the molecular expression mechanisms as well as the impact of presynaptic LTP on network activity and behavior are not fully understood. In order to establish optogenetic control of presynaptic cAMP levels and thereby presynaptic potentiation, we developed synaptoPAC, a presynaptically targeted version of the photoactivated adenylyl cyclase bPAC. In cultures of hippocampal granule cells of Wistar rats, activation of synaptoPAC with blue light increased action potential-evoked transmission, an effect not seen in hippocampal cultures of non-granule cells. In acute brain slices of C57BL/6N mice, synaptoPAC activation immediately triggered a strong presynaptic potentiation at mossy fiber synapses in CA3, but not at Schaffer collateral synapses in CA1. Following light-triggered potentiation, mossy fiber transmission decreased within 20 min, but remained enhanced still after 30 min. The optogenetic potentiation altered the short-term plasticity dynamics of release, reminiscent of presynaptic LTP. Our work establishes synaptoPAC as an optogenetic tool that enables acute light-controlled potentiation of transmitter release at specific synapses in the brain, facilitating studies of the role of presynaptic potentiation in network function and animal behavior in an unprecedented manner. Read the Editorial Highlight for this article on page 270.

Identifiability of a Binomial Synapse

Synapses are highly stochastic transmission units. A classical model describing this stochastic transmission is called the binomial model, and its underlying parameters can be estimated from postsynaptic responses to evoked stimuli. The accuracy of parameter estimates obtained via such a model-based approach depends on the identifiability of the model. A model is said to be structurally identifiable if its parameters can be uniquely inferred from the distribution of its outputs. However, this theoretical property does not necessarily imply practical identifiability. For instance, if the number of observations is low or if the recording noise is high, the model's parameters can only be loosely estimated. Structural identifiability, which is an intrinsic property of a model, has been widely characterized; but practical identifiability, which is a property of both the model and the experimental protocol, is usually only qualitatively assessed. Here, we propose a formal definition for the practical identifiability domain of a statistical model. For a given experimental protocol, this domain corresponds to the set of parameters for which the model is correctly identified as the ground truth compared to a simpler alternative model. Considering a model selection problem instead of a parameter inference problem allows to derive a non-arbitrary criterion for practical identifiability. We apply our definition to the study of neurotransmitter release at a chemical synapse. Our contribution to the analysis of synaptic stochasticity is three-fold: firstly, we propose a quantitative criterion for the practical identifiability of a statistical model, and compute the identifiability domains of different variants of the binomial release model (uni or multi-quantal, with or without short-term plasticity); secondly, we extend the Bayesian Information Criterion (BIC), a classically used tool for model selection, to models with correlated data (which is the case for most models of chemical synapses); finally, we show that our approach allows to perform data free model selection, i.e., to verify if a model used to fit data was indeed identifiable even without access to the data, but having only access to the fitted parameters.

Drug-Induced Conditioned Place Preference and Its Practical Use in Substance Use Disorder Research

The conditioned place preference (CPP) paradigm is a well-established model utilized to study the role of context associations in reward-related behaviors, including both natural rewards and drugs of abuse. In this review article, we discuss the basic history, various uses, and considerations that are tied to this technique. There are many potential takeaway implications of this model, including negative affective states, conditioned drug effects, memory, and motivation, which are all considered here. We also discuss the neurobiology of CPP including relevant brain regions, molecular signaling cascades, and neuromodulatory systems. We further examine some of our prior findings and how they integrate CPP with self-administration paradigms. Overall, by describing the fundamentals of CPP, findings from the past few decades, and implications of using CPP as a research paradigm, we have endeavored to support the case that the CPP method is specifically advantageous for studying the role of a form of Pavlovian learning that associates drug use with the surrounding environment.

Glucose Overload Inhibits Glutamatergic Synaptic Transmission: A Novel Role for CREB-Mediated Regulation of Synaptotagmins 2 and 4

Glucose metabolism derangement is critically involved in the age-related memory loss but the underlying molecular mechanisms are still poorly understood. In a mouse model of type 1 diabetes we found memory impairment associated with inhibition of the transcription factor CREB and alteration of pre- and post-synaptic protein expression in the hippocampus. Accordingly, glucose excess negatively affected activity-dependent CREB phosphorylation and CREB-mediated mRNA expression of synaptic proteins in hippocampal primary neurons. Specifically, glucose excess inhibited the activity-dependent recruitment of CREB on the regulatory sequences of synaptotagmin (SYT) 2 and 4 promoters and the expression of SYT4 protein. As a result, high glucose affected both the frequency of miniature excitatory postsynaptic currents and NMDA receptor-mediated currents in autaptic hippocampal neuronal cultures. Collectively, our findings highlight novel mechanisms underlying hyperglycaemia-related memory loss, including CREB-dependent downregulation of synaptotagmin expression.

Experience-dependent plasticity in an innate social behavior is mediated by hypothalamic LTP

Modification of instinctive behaviors occurs through experience, yet the mechanisms through which this happens have remained largely unknown. Recent studies have shown that potentiation of aggression, an innate behavior, can occur through repeated winning of aggressive encounters. Here, we show that synaptic plasticity at a specific excitatory input to a hypothalamic cell population is correlated with, and required for, the expression of increasingly higher levels of aggressive behavior following aggressive experience. We additionally show that the amplitude and persistence of long-term potentiation at this synapse are influenced by serum testosterone, administration of which can normalize individual differences in the expression of intermale aggression among genetically identical mice.

All animals can perform certain survival behaviors without prior experience, suggesting a “hard wiring” of underlying neural circuits. Experience, however, can alter the expression of innate behaviors. Where in the brain and how such plasticity occurs remains largely unknown. Previous studies have established the phenomenon of “aggression training,” in which the repeated experience of winning successive aggressive encounters across multiple days leads to increased aggressiveness. Here, we show that this procedure also leads to long-term potentiation (LTP) at an excitatory synapse, derived from the posteromedial part of the amygdalohippocampal area (AHiPM), onto estrogen receptor 1-expressing (Esr1⁺) neurons in the ventrolateral subdivision of the ventromedial hypothalamus (VMHvl). We demonstrate further that the optogenetic induction of such LTP in vivo facilitates, while optogenetic long-term depression (LTD) diminishes, the behavioral effect of aggression training, implying a causal role for potentiation at AHiPM→VMHvl^Esr1 synapses in mediating the effect of this training. Interestingly, ∼25% of inbred C57BL/6 mice fail to respond to aggression training. We show that these individual differences are correlated both with lower levels of testosterone, relative to mice that respond to such training, and with a failure to exhibit LTP after aggression training. Administration of exogenous testosterone to such nonaggressive mice restores both behavioral and physiological plasticity. Together, these findings reveal that LTP at a hypothalamic circuit node mediates a form of experience-dependent plasticity in an innate social behavior, and a potential hormone-dependent basis for individual differences in such plasticity among genetically identical mice.

Cocaine self-administration abolishes endocannabinoid-mediated long-term depression of glutamatergic synapses in the ventral tegmental area

Drugs of abuse, including cocaine, alter the mechanisms underpinning synaptic plasticity, including long-term potentiation of glutamatergic synapses in the mesolimbic system. These effects are thought to underlie addictive behaviors. In the ventral tegmental area (VTA), glutamatergic synapses also exhibit long-term depression (LTD), a type of plasticity that weakens synaptic strength. This form of synaptic plasticity is induced by low-frequency stimulation and mediated by endocannabinoid (eCB) signaling, which also modulates addictive behaviors. However, it remains unknown whether eCB-LTD in the VTA could be altered by cocaine use. Therefore, the goal of the present study was to examine the impact of cocaine self-administration on eCB-LTD of glutamatergic synapses onto VTA dopaminergic (DA) neurons. To that end, male rats underwent cocaine (0.75 mg/kg/infusion) or saline self-administration under the fixed ratio 1 schedule for 6-9 days. One day after the last self-administration session, the magnitude of eCB-LTD was examined using ex vivo whole-cell recordings of putative VTA DA neurons from naïve rats and rats with saline or cocaine self-administration. The results revealed that cocaine self-administration abolished eCB-LTD. The cocaine-induced blockade of eCB-LTD in the VTA was mediated by an impaired function of presynaptic CB1 receptors. Collectively, these findings indicate that cocaine exposure blunts eCB-mediated synaptic plasticity in midbrain DA neurons. This effect could be one of the cellular mechanisms that mediate, at least in part, addictive behaviors.

The Emerging Role of ATP-Dependent Chromatin Remodeling in Memory and Substance Use Disorders

Long-term memory formation requires coordinated regulation of gene expression and persistent changes in cell function. For decades, research has implicated histone modifications in regulating chromatin compaction necessary for experience-dependent changes to gene expression and cell function during memory formation. Recent evidence suggests that another epigenetic mechanism, ATP-dependent chromatin remodeling, works in concert with the histone-modifying enzymes to produce large-scale changes to chromatin structure. This review examines how histone-modifying enzymes and chromatin remodelers restructure chromatin to facilitate memory formation. We highlight the emerging evidence implicating ATP-dependent chromatin remodeling as an essential mechanism that mediates activity-dependent gene expression, plasticity, and cell function in developing and adult brains. Finally, we discuss how studies that target chromatin remodelers have expanded our understanding of the role that these complexes play in substance use disorders.

Crisis, what crisis? Addiction neuroscience and the challenges of translation

In this article we interrogate the claim that there is an opioid crisis: a dramatic rise in drug overdose fatalities in the United States over the past two decades that is also spreading to other countries.  The usual argument is that this crisis is largely explained by errant prescription practices leading to an oversupply of opioids, leading to addiction, premature mortality and drug overdose deaths, both among those prescribed opioids for pain relief, and those obtaining them on the illegal market.  We argue, that this view is highly problematic and that it is likely to entrench deeper problems with how substance addiction has been perceived and known. In this article, we develop an alternative picture of the addiction crisis based on four years of research and collaboration with addiction neuroscientists. Drug overdose deaths, we claim, are symptoms of what we term the ‘structural distribution of social despair.’ We argue that this is compounded by a translation crisis at the heart of addiction neuroscience. For all its dominance, the ‘dopamine hypothesis’ of addiction that shaped understandings for some three decades, has still not produced a single effective treatment. However, this translation crisis also represents an opportunity for ‘the memory turn’ in addiction neuroscience as it seeks to translate its emerging conception of addiction as a problem of memory into effective forms of treatment. We conclude by arguing that, for the ‘memory turn’ to underpin effective interventions into ‘the opioid crisis’, a new relation between neuroscientists and social scientists of addiction is needed, one that proceeds from the lived experience of human beings.

SCH23390 Reduces Methamphetamine Self-Administration and Prevents Methamphetamine-Induced Striatal LTD

Extended-access methamphetamine self-administration results in unregulated intake of the drug; however, the role of dorsal striatal dopamine D1-like receptors (D1Rs) in the reinforcing properties of methamphetamine under extended-access conditions is unclear. Acute (ex vivo) and chronic (in vivo) methamphetamine exposure induces neuroplastic changes in the dorsal striatum, a critical region implicated in instrumental learning. For example, methamphetamine exposure alters high-frequency stimulation (HFS)-induced long-term depression in the dorsal striatum; however, the effect of methamphetamine on HFS-induced long-term potentiation (LTP) in the dorsal striatum is unknown. In the current study, dorsal striatal infusion of SCH23390, a D1R antagonist, prior to extended-access methamphetamine self-administration reduced methamphetamine addiction-like behavior. Reduced behavior was associated with reduced expression of PSD-95 in the dorsal striatum. Electrophysiological findings demonstrate that superfusion of methamphetamine reduced basal synaptic transmission and HFS-induced LTP in dorsal striatal slices, and SCH23390 prevented this effect. These results suggest that alterations in synaptic transmission and synaptic plasticity induced by acute methamphetamine via D1Rs could assist with methamphetamine-induced modification of corticostriatal circuits underlying the learning of goal-directed instrumental actions and formation of habits, mediating escalation of methamphetamine self-administration and methamphetamine addiction-like behavior.

Axonal mechanisms mediating γ-aminobutyric acid receptor type A (GABA-A) inhibition of striatal dopamine release

Axons of dopaminergic neurons innervate the striatum where they contribute to movement and reinforcement learning. Past work has shown that striatal GABA tonically inhibits dopamine release, but whether GABA-A receptors directly modulate transmission or act indirectly through circuit elements is unresolved. Here, we use whole-cell and perforated-patch recordings to test for GABA-A receptors on the main dopaminergic neuron axons and branching processes within the striatum of adult mice. Application of GABA depolarized axons, but also decreased the amplitude of axonal spikes, limited propagation and reduced striatal dopamine release. The mechanism of inhibition involved sodium channel inactivation and shunting. Lastly, we show the positive allosteric modulator diazepam enhanced GABA-A currents on dopaminergic axons and directly inhibited release, but also likely acts by reducing excitation from cholinergic interneurons. Thus, we reveal the mechanisms of GABA-A receptor modulation of dopamine release and provide new insights into the actions of benzodiazepines within the striatum.

Role of BRD4 phosphorylation in the nucleus accumbens in relapse to cocaine-seeking behavior in mice

Cocaine addiction is a chronic relapsing brain disorder characterized by compulsive drug seeking. Preliminary study suggested that bromodomain-containing protein 4 (BRD4), an epigenetic reader protein, participates in cocaine-induced reward and neuroplasticity. However, the exact role of BRD4 in cocaine addiction, particularly cocaine relapse, remains elusive. In this study, we found that BRD4 phosphorylation in the nucleus accumbens (NAc) was closely related to the maintenance of cocaine reinforcement and relapse in different cocaine exposure paradigms. Cocaine significantly increased the binding of phosphorylated BRD4 (pBRD4) at the promoter of Gria2 and Bdnf genes in the NAc. (+)JQ1, a selective BRD4 inhibitor, markedly reduced the reinforcement and reinstatement of cocaine-seeking behaviors, which was accompanied by the decreased expressions of GRIA2 and BDNF. Furthermore, chromatin immunoprecipitation assay showed that (+)JQ1 clearly attenuated cocaine-enhanced binding of pBRD4 at the promotor of Gria2 and Bdnf genes. Blockade of casein kinase II significantly attenuated BRD4 phosphorylation and cocaine relapse-like behaviors, suggesting the important role of pBRD4 in modulating cocaine effect. Together, our findings suggest that BRD4 phosphorylation in the NAc modulates multiple addiction-related behaviors of cocaine and particularly relapse to cocaine-seeking behaviors. Inhibition of BRD4 activity may be a novel target against cocaine addiction and relapse.

Fluid Biomarkers for Synaptic Dysfunction and Loss

Synapses are the site for brain communication where information is transmitted between neurons and stored for memory formation. Synaptic degeneration is a global and early pathogenic event in neurodegenerative disorders with reduced levels of pre- and postsynaptic proteins being recognized as a core feature of Alzheimer's disease (AD) pathophysiology. Together with AD, other neurodegenerative and neurodevelopmental disorders show altered synaptic homeostasis as an important pathogenic event, and due to that, they are commonly referred to as synaptopathies. The exact mechanisms of synapse dysfunction in the different diseases are not well understood and their study would help understanding the pathogenic role of synaptic degeneration, as well as differences and commonalities among them and highlight candidate synaptic biomarkers for specific disorders. The assessment of synaptic proteins in cerebrospinal fluid (CSF), which can reflect synaptic dysfunction in patients with cognitive disorders, is a keen area of interest. Substantial research efforts are now directed toward the investigation of CSF synaptic pathology to improve the diagnosis of neurodegenerative disorders at an early stage as well as to monitor clinical progression. In this review, we will first summarize the pathological events that lead to synapse loss and then discuss the available data on established (eg, neurogranin, SNAP-25, synaptotagmin-1, GAP-43, and α-syn) and emerging (eg, synaptic vesicle glycoprotein 2A and neuronal pentraxins) CSF biomarkers for synapse dysfunction, while highlighting possible utilities, disease specificity, and technical challenges for their detection.

More than Smoke and Patches: The Quest for Pharmacotherapies to Treat Tobacco Use Disorder

Tobacco use is a persistent public health issue. It kills up to half its users and is the cause of nearly 90% of all lung cancers. The main psychoactive component of tobacco is nicotine, primarily responsible for its abuse-related effects. Accordingly, most pharmacotherapies for smoking cessation target nicotinic acetylcholine receptors (nAChRs), nicotine's major site of action in the brain. The goal of the current review is twofold: first, to provide a brief overview of the most commonly used behavioral procedures for evaluating smoking cessation pharmacotherapies and an introduction to pharmacokinetic and pharmacodynamic properties of nicotine important for consideration in the development of new pharmacotherapies; and second, to discuss current and potential future pharmacological interventions aimed at decreasing tobacco use. Attention will focus on the potential for allosteric modulators of nAChRs to offer an improvement over currently approved pharmacotherapies. Additionally, given increasing public concern for the potential health consequences of using electronic nicotine delivery systems, which allow users to inhale aerosolized solutions as an alternative to smoking tobacco, an effort will be made throughout this review to address the implications of this relatively new form of nicotine delivery, specifically as it relates to smoking cessation. SIGNIFICANCE STATEMENT: Despite decades of research that have vastly improved our understanding of nicotine and its effects on the body, only a handful of pharmacotherapies have been successfully developed for use in smoking cessation. Thus, investigation of alternative pharmacological strategies for treating tobacco use disorder remains active; allosteric modulators of nicotinic acetylcholine receptors represent one class of compounds currently under development for this purpose.

Involvement of the endogenous opioid system in the beneficial influence of physical exercise on amphetamine-induced addiction parameters

Psychostimulant drugs addiction is a chronic public health problem and individuals remain susceptible to relapses increasing public expenses even after withdrawal and treatment. Our research group has focused on finding new therapies to be employed in drug addiction treatment, suggesting the physical exercise as a promising tool. This way, it is necessary to know the mechanisms involved in the beneficial influences of physical exercise observing the pathway that could be explored in drug addiction treatment. Male Wistar rats were conditioned with amphetamine (AMPH) following the conditioned place preference (CPP) protocol and subsequently submitted to swimming for 5 weeks (1 h per day, 5 days per week). Half of the animals were injected with Naloxone (0.3 mg/mL/kg body weight, i.p.) 5 min prior each physical exercise day. After AMPH-CPP re-exposure, our outcomes showed that physical exercise, in addition to minimizing the relapse behavior in the CPP, it increased D1R, D2R and DAT in the Ventral Tegmental Area (VTA), but not in the Nucleus accumbens (NAc). Interestingly, while naloxone inhibited the partial beneficial influence of the exercise on drug-relapse behavior, exercise-induced changes in the dopaminergic system were not observed in the group administered with naloxone as well. Based on these evidences, besides reinforcing the beneficial influence of the physical exercise on AMPH-induced drug addiction, we propose the involvement of endogenous opioid system activation, not as a single one, but as a possible mechanism of action resulting from the physical activity practice, thus characterizing an important therapeutic approach, which may contribute to drug withdrawal consequently preventing relapse.

Potential of Glial Cell Modulators in the Management of Substance Use Disorders

The pervasive and devastating nature of substance use disorders underlies the need for the continued development of novel pharmacotherapies. We now know that glia play a much greater role in neuronal processes than once believed.  The various types of glial cells (e.g., astrocytes, microglial, oligodendrocytes) participate in numerous functions that are crucial to healthy central nervous system function. Drugs of abuse have been shown to interact with glia in ways that directly contribute to the pharmacodynamic effects responsible for their abuse potential. Through their effect upon glia, drugs of abuse also alter brain function resulting in behavioral changes associated with substance use disorders. Therefore, drug-induced changes in glia and inflammation within the central nervous system (neuroinflammation) have been investigated to treat various aspects of drug abuse and dependence. This article presents a brief overview of the effects of each of the major classes of addictive drugs on glia. Next, the paper reviews the pre-clinical and clinical studies assessing the effects that glial modulators have on abuse-related behavioral effects, such as pleasure, withdrawal, and motivation. There is a strong body of pre-clinical literature demonstrating the general effectiveness of several glia-modulating drugs in models of reward and relapse. Clinical studies have also yielded promising results, though not as robust. There is still much to disentangle regarding the integration between addictive drugs and glial cells. Improved understanding of the relationship between glia and the pathophysiology of drug abuse should allow for more precise exploration in the development and testing of glial-directed treatments for substance use disorders.

Morphine-element interactions - The influence of selected chemical elements on neural pathways associated with addiction

Addiction is a pressing social problem worldwide and opioid dependence can be considered the strongest and most difficult addiction to treat. Mesolimbic and mesocortical dopaminergic pathways play an important role in modulation of cognitive processes and decision making and, therefore, changes in dopamine metabolism are considered the central basis for the development of dependence. Disturbances caused by excesses or deficiency of certain elements have a significant impact on the functioning of the central nervous system (CNS) both in physiological conditions and in pathology and can affect the cerebral reward system and therefore, may modulate processes associated with the development of addiction. In this paper we review the mechanisms of interactions between morphine and zinc, manganese, chromium, cadmium, lead, fluoride, their impact on neural pathways associated with addiction, and on antinociception and morphine tolerance and dependence.

Neuronal Adenylyl Cyclase Targeting Central Plasticity for the Treatment of Chronic Pain

Chronic pain is a major health problem and the effective treatment for chronic pain is still lacking. The recent crisis created by the overuse of opioids for pain treatment has clearly shown the need for non-addictive novel pain medicine. Conventional pain medicines usually inhibit peripheral nociceptive transmission and reduce central transmission, especially pain-related excitatory transmission. For example, both opioids and gabapentin produce analgesic effects by inhibiting the release of excitatory transmitters and reducing neuronal excitability. Here, we will review recent studies of central synaptic plasticity contributing to central sensitization in chronic pain. Neuronal selective adenylyl cyclase subtype 1 (AC1) is proposed to be a key intracellular protein that causes both presynaptic and postsynaptic forms of long-term potentiation (LTP). Inhibiting the activity of AC1 by selective inhibitor NB001 blocks behavioral sensitization and injury-related anxiety in animal models of chronic pain. We propose that inhibiting injury-related LTPs will provide new mechanisms for designing novel medicines for the treatment of chronic pain and its related emotional disorders.

Addiction associated N40D mu-opioid receptor variant modulates synaptic function in human neurons

The OPRM1 A118G single nucleotide polymorphism (SNP rs1799971) gene variant encoding the N40D µ-opioid receptor (MOR) has been associated with dependence on opiates and other drugs of abuse but its mechanism is unknown. The frequency of G-allele carriers is ~40% in Asians, ~16% in Europeans, and ~3% in African-Americans. With opioid abuse-related deaths rising at unprecedented rates, understanding these mechanisms may provide a path to therapy. Here we generated homozygous N40D subject-specific induced inhibitory neuronal cells (iNs) from seven human-induced pluripotent stem (iPS) cell lines from subjects of European descent (both male and female) and probed the impact of N40D MOR regulation on synaptic transmission. We found that D40 iNs exhibit consistently stronger suppression (versus N40) of spontaneous inhibitory postsynaptic currents (sIPSCs) across multiple subjects. To mitigate the confounding effects of background genetic variation on neuronal function, the regulatory effects of MORs on synaptic transmission were recapitulated in two sets of independently engineered isogenic N40D iNs. In addition, we employed biochemical analysis and observed differential N-linked glycosylation of human MOR N40D. This study identifies neurophysiological and molecular differences between human MOR variants that may predict altered opioid responsivity and/or dependence in this subset of individuals.

Excitability, synaptic balance, and addiction: The homeostatic dynamics of ionotropic glutamatergic receptors in VTA after cocaine exposure

Glutamatergic AMPA and NMDA receptors in the ventral tegmental area (VTA) are central for cocaine first exposure and posterior craving maintenance. However, the exact rules that coordinate the synaptic dynamics of these receptors in dopaminergic VTA neurons and behavioral outcomes are poorly understood. Additionally, synaptic homeostatic plasticity is present in response to chronic excitability changes in neuronal circuits, adjusting the strength of synapses to stabilize the firing rate. Despite having correspondent mechanisms, little is known about the relationship between continuous cocaine exposure and homeostatic synaptic changes in the VTA neurons. Here, we assess the role of homeostatic mechanisms in the neurobiology of cocaine addiction by providing a brief overview of the parallels between cocaine-induced synaptic potentiation and long-term synaptic adaptations, focusing on the regulation of GluA1- and GluN1- containing receptors.

Sex Differences and the Role of Estradiol in Mesolimbic Reward Circuits and Vulnerability to Cocaine and Opiate Addiction

Although both men and women become addicted to drugs of abuse, women transition to addiction faster, experience greater difficulties remaining abstinent, and relapse more often than men. In both humans and rodents, hormonal cycles are associated with females' faster progression to addiction. Higher concentrations and fluctuating levels of ovarian hormones in females modulate the mesolimbic reward system and influence reward-directed behavior. For example, in female rodents, estradiol (E2) influences dopamine activity within the mesolimbic reward system such that drug-directed behaviors that are normally rewarding and reinforcing become enhanced when circulating levels of E2 are high. Therefore, neuroendocrine interactions, in part, explain sex differences in behaviors motivated by drug reward. Here, we review sex differences in the physiology and function of the mesolimbic reward system in order to explore the notion that sex differences in response to drugs of abuse, specifically cocaine and opiates, are the result of molecular neuroadaptations that differentially develop depending upon the hormonal state of the animal. We also reconsider the notion that ovarian hormones, specifically estrogen/estradiol, sensitize target neurons thereby increasing responsivity when under the influence of either cocaine or opiates or in response to exposure to drug-associated cues. These adaptations may ultimately serve to guide the motivational behaviors that underlie the factors that cause women to be more vulnerable to cocaine and opiate addiction than men.

Neuromorphic Processing of Pressure Signal Using Integrated Sensor-Synaptic Device Capable of Selective and Reversible Short- and Long-Term Plasticity Operation

To mimic the tactile sensing properties of the human skin, signals from tactile sensors need to be processed in an efficient manner. The integration of the tactile sensor with a neuromorphic device can potentially address this issue, as the neuromorphic device has both signal processing and memory capability through which parallel and efficient processing of information is possible. In this article, an intelligent haptic perception device (IHPD) is presented that combines pressure sensing with an organic electrochemical transistor-based synaptic device into a simple device architecture. More importantly, the IHPD is capable of rapid and reversible switching between short-term plasticity (STP) and long-term plasticity (LTP) operation through which accelerated learning, processing of new information, and distinctive operation of STP and LTP are possible. Various types of pressure information such as magnitude, rate, and duration were processed utilizing STP by which error-tolerant perception was demonstrated. Meanwhile, memorization and learning of pressure through a stepwise change in a conductive state was demonstrated using LTP. These demonstrations present unique approaches to process and learn tactile information, which can potentially be utilized in various electronic skin applications in the future.

7,8-Dihydroxyflavone Attenuates Alcohol-Related Behavior in Rat Models of Alcohol Consumption via TrkB in the Ventral Tegmental Area

Alcohol use disorder (AUD) is a ubiquitous substance use disorder in the world, of which neural mechanisms remain unclear. Alcohol consumption induces neuro-adaptations in the dopaminergic system originating from the ventral tegmental area (VTA), an important brain region for the reward function in AUD. Endogenous brain-derived neurotrophic factor (BDNF)-TrkB implicated in the development of neuroplasticity, including long-term potentiation of GABAergic synapses (LTP _GABA ). We previously found that ethanol blocks LTP _GABA in the VTA, either in vivo or in vitro. 7,8-dihydroflavone (7,8-DHF), a BDNF-mimicking small compound, was recently found to penetrate the blood-brain barrier to mimic the biological role of BDNF-TrkB. In this study, we demonstrate that repeated ethanol consumption (including intermittent and continuous ethanol exposure) results in low expression of BDNF in rat VTA. The amount of ethanol intake enhances significantly in rats with intermittent ethanol exposure after 72 h abstinence. Withdrawal signs emerge in rats with continuous ethanol exposure within 3 days after abstinence. Using behavioral tests, intraperitoneal injection of 7,8-DHF can reduce excessive ethanol consumption and preference as well as withdrawal signs in rats with repeated ethanol exposure. Interestingly, microinjection of K252a, an antagonist of TrkB, into the VTA blocks the effects of 7,8-DHF on ethanol-related behaviors. Furthermore, direct microinjection of BDNF into the VTA mimics the effect of 7,8-DHF on ethanol related behaviors. Taken together, 7,8-DHF attenuates alcohol-related behaviors in rats undergoing alcohol consumption via TrkB in the VTA. Our findings suggest BDNF-TrkB in VTA is a part of regulating signals for opposing neural adaptations in AUD, and 7,8-DHF may serve as a potential candidate for treating alcoholism.

Stress induces insertion of calcium-permeable AMPA receptors in the OFC-BLA synapse and modulates emotional behaviours in mice

Stress increases the risk of neuropsychiatric disorders, such as major depression. Exposure to stress has been reported to induce various neuronal changes, such as alterations in synaptic transmission and structure. However, a causal link between stress-induced neural circuit alterations and changes in emotional behaviours is not well understood. In the present study, we focused on a projection pathway from the orbitofrontal cortex (OFC) to the basolateral nucleus of the amygdala (BLA) as a crucial circuit for negative emotions and examined the effect of stress on OFC-BLA excitatory synaptic transmission using optogenetic and whole-cell patch-clamp methods in mice. As a stress-inducing procedure, we used repeated tail-shock, which increased stress-related behaviours. We found greater α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/N-methyl-D-aspartate current ratios and insertion of calcium-permeable AMPA receptors (AMPARs) in the OFC-BLA synapse after stress. These stress-induced synaptic and behavioural changes were reduced by a blockade of protein kinase A, which plays a principal role in stress-induced targeting of AMPARs into the synaptic membrane. To examine a possible causal relationship between alterations in synaptic transmission in the OFC-BLA pathway and stress-related behaviour, we performed optogenetic activation or chemogenetic inactivation of OFC-BLA transmission in mice. We found that optogenetic activation and chemogenetic inactivation of OFC-BLA transmission increased and decreased stress-related behaviour, respectively. In conclusion, we have demonstrated that stress altered the postsynaptic properties of the OFC-BLA pathway. These synaptic changes might be one of the underlying mechanisms of stress-induced behavioural alterations.

Dynamic changes in cerebral and peripheral markers of glutamatergic signaling across the human sleep-wake cycle

Sleep and brain glutamatergic signaling are homeostatically regulated. Recovery sleep following prolonged wakefulness restores efficient functioning of the brain, possibly by keeping glutamatergic signaling in a homeostatic range. Evidence in humans and mice suggested that metabotropic glutamate receptors of subtype-5 (mGluR5) contribute to the brain's coping mechanisms with sleep deprivation. Here, proton magnetic resonance spectroscopy in 31 healthy men was used to quantify the levels of glutamate (Glu), glutamate-to-glutamine ratio (GLX), and γ-amino-butyric-acid (GABA) in basal ganglia (BG) and dorsolateral prefrontal cortex on 3 consecutive days, after ~8 (baseline), ~32 (sleep deprivation), and ~8 hours (recovery sleep) of wakefulness. Simultaneously, mGluR5 availability was quantified with the novel radioligand for positron emission tomography, [18F]PSS232, and the blood levels of the mGluR5-regulated proteins, fragile X mental retardation protein (FMRP) and brain-derived neurotrophic factor (BDNF) were determined. The data revealed that GLX (p = 0.03) in BG (for Glu: p < 0.06) and the serum concentration of FMRP (p < 0.04) were increased after sleep loss. Other brain metabolites (GABA, N-acetyl-aspartate, choline, glutathione) and serum BDNF levels were not altered by sleep deprivation (pall > 0.6). By contrast, the night without sleep enhanced whole-brain, BG, and parietal cortex mGluR5 availability, which was normalized by recovery sleep (pall < 0.05). The findings provide convergent multimodal evidence that glutamatergic signaling is affected by sleep deprivation and recovery sleep. They support a role for mGluR5 and FMRP in sleep-wake regulation and warrant further studies to investigate their causality and relevance for regulating human sleep in health and disease. Clinical Trial Registration: www.clinicaltrials.gov (study identifier: NCT03813082).

TAAR1 agonists attenuate extended-access cocaine self-administration and yohimbine-induced reinstatement of cocaine-seeking

BACKGROUND AND PURPOSE

The trace amine-associated receptor 1 (TAAR1) negatively modulates dopamine transmission. Our previous studies demonstrated that TAAR1 agonists attenuated cue- and drug-induced cocaine-seeking and increased the elasticity of the cocaine demand curve, in the short-access cocaine self-administration model. Compulsive use of cocaine, which is an essential criterion of cocaine use disorder, can be induced by extended access to cocaine self-administration.

EXPERIMENTAL APPROACH

To characterize the role of TAAR1 in compulsive cocaine use, we evaluated the effects of activation of TAAR1 on cocaine intake, cocaine binge and cue-induced cocaine-seeking using the extended-access cocaine self-administration model in adult male Sprague-Dawley rats. We also investigated the role of TAAR1 in stress-triggered cocaine relapse by using the α₂ -adrenoceptor antagonist yohimbine-induced reinstatement of cocaine-seeking.

KEY RESULTS

The selective TAAR1 partial agonist RO5263397 attenuated cocaine intake and did not develop tolerance during the 10-day extended-access cocaine self-administration. RO5263397 reduced a 12-h binge intake of cocaine after forced abstinence. RO5263397 also decreased cue-induced cocaine-seeking after prolonged abstinence from extended-access cocaine self-administration. Furthermore, RO5263397 and the selective TAAR1 full agonist RO5166017 reduced yohimbine-induced reinstatement of cocaine-seeking behaviour.

CONCLUSION AND IMPLICATIONS

Activation of TAAR1 attenuated extended-access cocaine self-administration and stress-induced cocaine reinstatement. These results suggest that TAAR1 agonists are promising pharmacological interventions to treat cocaine use disorder and relapse.

Pharmacogenetics of drug dependence: Polymorphisms of genes involved in glutamate neurotransmission

Multiple studies provide evidence to support dysfunction of glutamate neurotransmission in the pathogenesis of drug dependence. Pharmacogenetic investigation of glutamate-related genes has provided further support for the involvement of this neurotransmitter in the risk of, and consequences of, drug abuse and dependence. This paper aims to provide a brief review of these association studies. Findings involving single nucleotide polymorphisms (SNPs) in glutamate receptor genes (GRIN, GRIA) and glutamate transporter genes (SLC1A, SLC17A) are reviewed as potential risk factors. As yet a clear perspective of the functional consequences and interactions of the various reported findings is lacking.

Memory engrams: Recalling the past and imagining the future

In 1904, Richard Semon introduced the term "engram" to describe the neural substrate for storing memories. An experience, Semon proposed, activates a subset of cells that undergo off-line, persistent chemical and/or physical changes to become an engram. Subsequent reactivation of this engram induces memory retrieval. Although Semon's contributions were largely ignored in his lifetime, new technologies that allow researchers to image and manipulate the brain at the level of individual neurons has reinvigorated engram research. We review recent progress in studying engrams, including an evaluation of evidence for the existence of engrams, the importance of intrinsic excitability and synaptic plasticity in engrams, and the lifetime of an engram. Together, these findings are beginning to define an engram as the basic unit of memory.

Altered Corticolimbic Control of the Nucleus Accumbens by Long-term Δ9-Tetrahydrocannabinol Exposure

BACKGROUND

The decriminalization and legalization of cannabis and the expansion of availability of medical cannabis in North America have led to an increase in cannabis use and the availability of high-potency strains. Cannabis potency is determined by the concentration of Δ⁹-tetrahydrocannabinol (Δ⁹-THC), a psychoactive constituent that activates cannabinoid CB₁ and CB₂ receptors. The use of high-potency cannabis is associated with cannabis use disorder and increased susceptibility to psychiatric illness. The nucleus accumbens (NAc) is part of a brain reward circuit affected by Δ⁹-THC through modulation of glutamate afferents arising from corticolimbic brain areas implicated in drug addiction and psychiatric disorders. Moreover, brain imaging studies show alterations in corticolimbic and NAc properties in human cannabis users.

METHODS

Using in vitro electrophysiology and optogenetics, we examined how Δ⁹-THC alters corticolimbic input to the NAc in rats.

RESULTS

We found that long-term exposure to Δ⁹-THC weakens prefrontal cortex glutamate input to the NAc shell and strengthens input from basolateral amygdala and ventral hippocampus. Further, whereas long-term exposure to Δ⁹-THC had no effect on net strength of glutamatergic input to NAc shell arising from midbrain dopamine neurons, it alters fundamental properties of these synapses.

CONCLUSIONS

Long-term exposure to Δ⁹-THC shifts control of the NAc shell from cortical to limbic input, likely contributing to cognitive and psychiatric dysfunction that is associated with cannabis use.

Functional Consequences of Calcium-Dependent Synapse-to-Nucleus Communication: Focus on Transcription-Dependent Metabolic Plasticity

In the nervous system, calcium signals play a major role in the conversion of synaptic stimuli into transcriptional responses. Signal-regulated gene transcription is fundamental for a range of long-lasting adaptive brain functions that include learning and memory, structural plasticity of neurites and synapses, acquired neuroprotection, chronic pain, and addiction. In this review, we summarize the diverse mechanisms governing calcium-dependent transcriptional regulation associated with central nervous system plasticity. We focus on recent advances in the field of synapse-to-nucleus communication that include studies of the signal-regulated transcriptome in human neurons, identification of novel regulatory mechanisms such as activity-induced DNA double-strand breaks, and the identification of novel forms of activity- and transcription-dependent adaptations, in particular, metabolic plasticity. We summarize the reciprocal interactions between different kinds of neuroadaptations and highlight the emerging role of activity-regulated epigenetic modifiers in gating the inducibility of signal-regulated genes.

The role of 6-acetylmorphine in heroin-induced reward and locomotor sensitization in mice

We have previously demonstrated that heroin's first metabolite, 6-acetylmorphine (6-AM), is an important mediator of heroin's acute effects. However, the significance of 6-AM to the rewarding properties of heroin still remains unknown. The present study therefore aimed to examine the contribution of 6-AM to heroin-induced reward and locomotor sensitization. Mice were tested for conditioned place preference (CPP) induced by equimolar doses of heroin or 6-AM (1.25-5 μmol/kg). Psychomotor activity was recorded during the CPP conditioning sessions for assessment of drug-induced locomotor sensitization. The contribution of 6-AM to heroin reward and locomotor sensitization was further examined by pretreating mice with a 6-AM specific antibody (anti-6-AM mAb) 24 hours prior to the CPP procedure. Both heroin and 6-AM induced CPP in mice, but heroin generated twice as high CPP scores compared with 6-AM. Locomotor sensitization was expressed after repeated exposure to 2.5 and 5 μmol/kg heroin or 6-AM, but not after 1.25 μmol/kg, and we found no correlation between the expression of CPP and the magnitude of locomotor sensitization for either opioid. Pretreatment with anti-6-AM mAb suppressed both heroin-induced and 6-AM-induced CPP and locomotor sensitization. These findings provide evidence that 6-AM is essential for the rewarding and sensitizing properties of heroin; however, heroin caused stronger reward compared with 6-AM. This may be explained by the higher lipophilicity of heroin, providing more efficient drug transfer to the brain, ensuring rapid increase in the brain 6-AM concentration.

Spermidine, a positive modulator of the NMDA receptor, facilitates extinction and prevents the reinstatement of morphine-induced conditioned place preference in mice

RATIONALE

Individuals with opioid use disorders often relapse into drug-seeking behavior after recalling memories linked to the drug use experience. Improving extinction efficacy has been used as a strategy to treat substance use disorders and suppress relapse. Although N-methyl-D-aspartate receptor (NMDAr) agonists facilitate acquisition, consolidation, and extinction, no study has addressed whether spermidine (SPD), a natural polyamine ligand of the NMDA receptor, facilitates the extinction and reinstatement of morphine-induced conditioned place preference (CPP).

OBJECTIVES AND METHODS

The aim of the present study was to investigate the effect of SPD, an NMDAr agonist, on the extinction and reinstatement of morphine-induced CPP in mice. Adult male albino Swiss mice received saline (0.9% NaCl) or morphine (5 mg/kg) intraperitoneally (i.p.) and were respectively confined to a black or a white compartment for 30 min for four consecutive days for CPP induction. SPD (10-30 mg/kg, i.p.) or ifenprodil (NMDAr antagonist, 0.1-1 mg/kg, i.p.) were injected 15 min before extinction training.

RESULTS

SPD and ifenprodil facilitated the extinction of morphine-induced CPP. SPD treatment during the extinction period impaired reinstatement induced by a priming dose of morphine (1.25 mg/kg). Ifenprodil (0.1 mg/kg) prevented the facilitatory effect of spermidine on the extinction of morphine-induced CPP but did not prevent reinstatement induced by morphine.

CONCLUSIONS

These results suggest that SPD facilitated the extinction of morphine-induced CPP by modulating the polyamine binding site of the NMDA receptor. Our findings reveal important effects of SPD and ifenprodil on the re-exposure-induced decrease in morphine-induced CPP, which may be promising for developing novel pharmacological strategies to treat opioid use disorder.

Physical exercise modifies behavioral and molecular parameters related to opioid addiction regardless of training time

Addiction is a devastating worldwide disorder that requires effective and innovative therapies. Physical exercise could be useful in addiction treatment because it shares a common neural circuit with addictive drugs. Based on this, molecular adaptations consequent to time of exercise in opioid exposed animals were evaluated. Rats were designed as sedentary (SED) or exercised (EXE). This last group was separated to perform three different periods of swimming: short-term (S-EXE), medium-term (M-EXE) and long-term (L-EXE) for 14, 28 and 42 days, respectively. On the last exercising week, one-half of the animals from SED and all animals from S-, M- and l-EXE were concomitantly exposed to morphine-conditioned place preference (CPP) paradigm and y-maze task for behavioral assessments followed by molecular assays in both Nucleus accumbens (NAc) and hippocampus. Between SED groups, morphine conditioning showed drug-CPP and increased dopamine transporter (DAT), dopamine receptor type-1 (D1R), type-2 (D2R) and glucocorticoid receptor (GR) in both brain areas in relation to saline group. Besides the small morphine-CPP in relation to SED group, all periods decreased DAT, D1R, and GR immunoreactivity in NAc, DAT and D1R in hippocampus, while D2R in both brain areas and GR in hippocampus were primarily decreased by L-EXE. Our findings show that even a short-term exercise modifies behaviors related to drug withdrawal, changing DA targets and GR, which are closely linked to addiction. Therefore, our outcomes involving physical exercise are interesting to perform a possible clinical trial, thus expanding the knowledge about drug addiction.

Activation of AMPK-dependent autophagy in the nucleus accumbens opposes cocaine-induced behaviors of mice

Cocaine is a strong central nervous system stimulant, which can induce drug addiction. Previous studies have reported that cocaine-induced autophagy is involved in neuroinflammation and cell death. However, the role of autophagy in psychomotor sensitivity to cocaine has not been explored. Here, we reported that D₁ receptor -CaMKII-AMPK-FoxO3a signaling pathway was involved in acute cocaine application-induced autophagy in the nucleus accumbens (NAc) both in vitro and in vivo. Furthermore, we found that knockdown of the ATG5 gene in the NAc augmented behavioral response to cocaine, and induction of autophagy in the NAc with rapamycin attenuated cocaine-induced behavioral response, which was coincident with the alterations of dendritic spine density in neurons of NAc. These results suggest that cocaine exposure leads to the induction of autophagy, which is a protective mechanism against behavioral response to cocaine of male mice.

Variance analysis as a tool to predict the mechanism underlying synaptic plasticity

BACKGROUND

The strength of synaptic transmission onto a neuron depends on the number of functional vesicle release sites (N), the probability of vesicle release (P_r), and the quantal size (Q). Statistical tools based on the quantal model of synaptic transmission can be used to acquire information on which of these parameters is the source of plasticity. However, quantal analysis depends on assumptions that may not be met at central synapses.

NEW METHOD

We examined the merit of quantal analysis to extract the mechanisms underlying synaptic plasticity by applying binomial statistics on the variance in amplitude of postsynaptic currents evoked at Schaffer collateral-CA1 (Sc-CA1) synapses in mouse hippocampal slices. We extend this analysis by combining the conventional inverse square of the coefficient of variation (1/CV²) with the variance-to-mean ratio (VMR).

RESULTS

This method can be used to assess the relative, but not absolute, contribution of N, P_r and Q to synaptic plasticity. The changes in 1/CV² and VMR values correctly reflect experimental modifications of N, P_r and Q at Sc-CA1 synapses.

COMPARISON WITH EXISTING METHODS

While the 1/CV² depends on N and P_r, but is independent of Q, the VMR is dependent on P_r and Q, but not on N. Combining both allows for a rapid assessment of the mechanism underlying synaptic plasticity without the need for additional electrophysiological experiments.

CONCLUSION

Combining the 1/CV² with the VMR allows for a reliable prediction of the relative contribution of changes in N, P_r and Q to synaptic plasticity.