Drug-driven AMPA receptor redistribution mimicked by selective dopamine neuron stimulation

Mother's little helper turned a foe: Alprazolam use, misuse, and abuse

Benzodiazepines are effective in managing anxiety and related disorders when used properly (short-term). Their inappropriate use, however, carries significant risks, involving amnesia, rebound insomnia, rebound anxiety, depression, dependence, abuse, addiction, and an intense and exceedingly prolonged withdrawal, among other complications. Benzodiazepines also amplify the effects of opioids and, consequently, have been implicated in approximately 30 % of opioid overdose deaths. Despite their unfavorable profile, sharp increases in medical and non-medical use of benzodiazepines have been steadily reported worldwide. Alprazolam (Xanax®), a potent, short-acting benzodiazepine, is among the most prescribed and abused anxiolytics in the United States. This medication is commonly co-abused with opioids, increasing the likelihood for oversedation, overdose, and death. Notwithstanding these risks, it is surprising that research investigating how benzodiazepines, such as alprazolam, interact with opioids is severely lacking in clinical and preclinical settings. This review therefore aims to present our current knowledge of benzodiazepine use and misuse, with an emphasis on alprazolam when data is available, and particularly in populations at higher risk for developing substance use disorders. Additionally, the potential mechanism(s) surrounding tolerance, dependence and abuse liability are discussed. Despite their popularity, our understanding of how benzodiazepines and opioids interact is less than adequate. Therefore, it is now more important than ever to understand the short- and long-term consequences of benzodiazepine/alprazolam use.

Forskolin reverses the O-GlcNAcylation dependent decrease in GABAAR current amplitude at hippocampal synapses possibly at a neurosteroid site on GABAARs

GABAergic transmission is influenced by post-translational modifications, like phosphorylation, impacting channel conductance, allosteric modulator sensitivity, and membrane trafficking. O-GlcNAcylation is a post-translational modification involving the O-linked attachment of β-N-acetylglucosamine on serine/threonine residues. Previously we reported an acute increase in O-GlcNAcylation elicits a long-term depression of evoked GABAAR inhibitory postsynaptic currents (eIPSCs) onto hippocampal principal cells. Importantly, O-GlcNAcylation and phosphorylation can co-occur or compete for the same residue; whether they interact in modulating GABAergic IPSCs is unknown. We tested this by recording IPSCs from hippocampal principal cells and pharmacologically increased O-GlcNAcylation, before or after increasing serine phosphorylation using the adenylate cyclase activator, forskolin. Although forskolin had no significant effect on baseline eIPSC amplitude, we found that a prior increase in O-GlcNAcylation unmasks a forskolin-dependent increase in eIPSC amplitude, reversing the O-GlcNAc-induced eIPSC depression. Inhibition of adenylate cyclase or protein kinase A did not prevent the potentiating effect of forskolin, indicating serine phosphorylation is not the mechanism. Surprisingly, increasing O-GlcNAcylation also unmasked a potentiating effect of the neurosteroids 5α-pregnane-3α,21-diol-20-one (THDOC) and progesterone on eIPSC amplitude in about half of the recorded cells, mimicking forskolin. Our findings show that under conditions of heightened O-GlcNAcylation, the neurosteroid site on synaptic GABAARs is possibly accessible to agonists, permitting strengthening of synaptic inhibition.

Forskolin reverses the O-GlcNAcylation dependent decrease in GABAAR current amplitude at hippocampal synapses possibly at a neurosteroid site on GABAARs

GABAergic transmission is influenced by post-translational modifications, like phosphorylation, impacting channel conductance, allosteric modulator sensitivity, and membrane trafficking. O-GlcNAcylation is a post-translational modification involving the O-linked attachment of β-N-acetylglucosamine on serine/threonine residues. Previously we reported an acute increase in O-GlcNAcylation elicits a long-term depression of evoked GABAAR inhibitory post synaptic currents (eIPSCs) onto hippocampal principal cells. Importantly, O-GlcNAcylation and phosphorylation can co-occur or compete for the same residue; whether they interact in modulating GABAergic IPSCs is unknown. We tested this by recording IPSCs from hippocampal principal cells and pharmacologically increased O-GlcNAcylation, before or after increasing serine phosphorylation using the adenylate cyclase activator, forskolin. Although forskolin had no significant effect on baseline eIPSC amplitude, we found that a prior increase in O-GlcNAcylation unmasks a forskolin-dependent increase in eIPSC amplitude, reversing the O-GlcNAc-induced eIPSC depression. Inhibition of adenylate cyclase or protein kinase A did not prevent the potentiating effect of forskolin, indicating serine phosphorylation is not the mechanism. Surprisingly, increasing O-GlcNAcylation also unmasked a potentiating effect of the neurosteroids 5α-pregnane-3α,21-diol-20-one (THDOC) and progesterone on eIPSC amplitude, mimicking forskolin. Our findings show under conditions of heightened O-GlcNAcylation, the neurosteroid site on synaptic GABAARs is accessible to agonists, permitting strengthening of synaptic inhibition.

Forskolin reverses the O-GlcNAcylation dependent decrease in GABAAR current amplitude at hippocampal synapses possibly through a neurosteroid site on GABAARs

GABAergic transmission is influenced by post-translational modifications, like phosphorylation, impacting channel conductance, allosteric modulator sensitivity, and membrane trafficking. O-GlcNAcylation is a post-translational modification involving the O-linked attachment of β-N-acetylglucosamine on serine/threonine residues. Previously we reported an acute increase in O-GlcNAcylation elicits a long-term depression of evoked GABAAR inhibitory post synaptic currents (eIPSCs) onto hippocampal principal cells. Importantly O-GlcNAcylation and phosphorylation can co-occur or compete for the same residue; whether they interact in modulating GABAergic IPSCs is unknown. We tested this by recording IPSCs from hippocampal principal cells and pharmacologically increased O-GlcNAcylation, before or after increasing serine phosphorylation using the adenylate cyclase activator, forskolin. Although forskolin had no significant effect on baseline eIPSC amplitude, we found that a prior increase in O-GlcNAcylation unmasks a forskolin-dependent increase in eIPSC amplitude, reversing the O-GlcNAc-induced eIPSC depression. Inhibition of adenylate cyclase or protein kinase A did not prevent the potentiating effect of forskolin, indicating serine phosphorylation is not the mechanism. Surprisingly, increasing O-GlcNAcylation also unmasked a potentiating effect of the neurosteroids 5α-pregnane-3α,21-diol-20-one (THDOC) and progesterone on eIPSC amplitude, mimicking forskolin. Our findings show under conditions of heightened O-GlcNAcylation, the neurosteroid site on synaptic GABAARs is accessible to agonists, permitting strengthening of synaptic inhibition.

Mechanisms and Functions of Activity-Regulated Cytoskeleton-Associated Protein in Synaptic Plasticity

Activity-regulated cytoskeleton-associated protein (Arc) is one of the most important regulators of cognitive functions in the brain regions. As a hub protein, Arc plays different roles in modulating synaptic plasticity. Arc supports the maintenance of long-term potentiation (LTP) by regulating actin cytoskeletal dynamics, while it guides the endocytosis of AMPAR in long-term depression (LTD). Moreover, Arc can self-assemble into capsids, leading to a new way of communicating among neurons. The transcription and translation of the immediate early gene Arc are rigorous procedures guided by numerous factors, and RNA polymerase II (Pol II) is considered to regulate the precise timing dynamics of gene expression. Since astrocytes can secrete brain-derived neurotrophic factor (BDNF) and L-lactate, their unique roles in Arc expression are emphasized. Here, we review the entire process of Arc expression and summarize the factors that can affect Arc expression and function, including noncoding RNAs, transcription factors, and posttranscriptional regulations. We also attempt to review the functional states and mechanisms of Arc in modulating synaptic plasticity. Furthermore, we discuss the recent progress in understanding the roles of Arc in the occurrence of major neurological disorders and provide new thoughts for future research on Arc.

Intermittent access to oxycodone decreases dopamine uptake in the nucleus accumbens core during abstinence

A major obstacle in treating opioid use disorder is the persistence of drug seeking or craving during periods of abstinence, which is believed to contribute to relapse. Dopamine transmission in the mesolimbic pathway is posited to contribute to opioid reinforcement, but the processes by which dopamine influences drug seeking have not been completely elucidated. To examine whether opioid seeking during abstinence is associated with alterations in dopamine transmission, female and male rats self-administered oxycodone under an intermittent access schedule of reinforcement. Following self-administration, rats underwent a forced abstinence period, and cue-induced seeking tests were conducted to assess oxycodone seeking. One day following the final seeking test, rats were sacrificed to perform ex vivo fast scan cyclic voltammetry and western blotting in the nucleus accumbens. Rats displayed reduced dopamine uptake rate on abstinence day 2 and abstinence day 15, compared to oxycodone-naïve rats. Further, on abstinence day 15, rats had reduced phosphorylation of the dopamine transporter. Additionally, local application of oxycodone to the nucleus accumbens reduced dopamine uptake in oxycodone-naïve rats and in rats during oxycodone abstinence, on abstinence day 2 and abstinence day 15. These observations suggest that abstinence from oxycodone results in dysfunctional dopamine transmission, which may contribute to sustained oxycodone seeking during abstinence.

Incubation of cocaine craving coincides with changes in dopamine terminal neurotransmission

Relapse to drug use is one of the major challenges in treating substance use disorders. Exposure to drug-related cues and contexts triggers drug craving, which drives cocaine seeking, and increases the probability of relapse. Clinical and animal studies have shown a progressive intensification of cocaine seeking and craving that develops over the course of abstinence, a phenomenon commonly referred to as incubation of cocaine craving. Although the neurobiology underlying incubation of cocaine craving has been examined – particularly within the context of glutamate plasticity– the extent to which increased cocaine craving engenders mesolimbic dopamine (DA) changes has received relatively little attention. To assess whether incubation of cocaine craving is associated with alterations in DA terminal neurotransmission in the nucleus accumbens core (NAc), we used ex vivo fast scan cyclic voltammetry in female and male rats to assess DA dynamics following short access, long access, or intermittent access to cocaine self-administration followed by 28 days of abstinence. Results indicated that both long access and intermittent access to cocaine produced robust incubation of cocaine craving, which was associated with increases in cocaine potency. In addition, intermittent access self-administration also produced a robust increase in DA uptake rate at baseline. In contrast, short access to cocaine did not engender incubation of cocaine craving, nor produce changes in DA neurotransmission. Together these observations indicate that incubation of cocaine craving coincides with changes in DA transmission, suggesting that underlying changes in mesolimbic DA signaling may contribute to the progressive intensification of drug craving that occurs across periods of abstinence.

Dual action of ketamine confines addiction liability

Ketamine is used clinically as an anaesthetic and a fast-acting antidepressant, and recreationally for its dissociative properties, raising concerns of addiction as a possible side effect. Addictive drugs such as cocaine increase the levels of dopamine in the nucleus accumbens. This facilitates synaptic plasticity in the mesolimbic system, which causes behavioural adaptations and eventually drives the transition to compulsion^1-4. The addiction liability of ketamine is a matter of much debate, in part because of its complex pharmacology that among several targets includes N-methyl-D-aspartic acid (NMDA) receptor (NMDAR) antagonism^5,6. Here we show that ketamine does not induce the synaptic plasticity that is typically observed with addictive drugs in mice, despite eliciting robust dopamine transients in the nucleus accumbens. Ketamine nevertheless supported reinforcement through the disinhibition of dopamine neurons in the ventral tegmental area (VTA). This effect was mediated by NMDAR antagonism in GABA (γ-aminobutyric acid) neurons of the VTA, but was quickly terminated by type-2 dopamine receptors on dopamine neurons. The rapid off-kinetics of the dopamine transients along with the NMDAR antagonism precluded the induction of synaptic plasticity in the VTA and the nucleus accumbens, and did not elicit locomotor sensitization or uncontrolled self-administration. In summary, the dual action of ketamine leads to a unique constellation of dopamine-driven positive reinforcement, but low addiction liability.

Application of optogenetics and in vivo imaging approaches for elucidating the neurobiology of addiction

The neurobiology of addiction has been an intense topic of investigation for more than 50 years. Over this time, technological innovation in methods for studying brain function rapidly progressed, leading to increasingly sophisticated experimental approaches. To understand how specific brain regions, cell types, and circuits are affected by drugs of abuse and drive behaviors characteristic of addiction, it is necessary both to observe and manipulate neural activity in addiction-related behavioral paradigms. In pursuit of this goal, there have been several key technological advancements in in vivo imaging and neural circuit modulation in recent years, which have shed light on the cellular and circuit mechanisms of addiction. Here we discuss some of these key technologies, including circuit modulation with optogenetics, in vivo imaging with miniaturized single-photon microscopy (miniscope) and fiber photometry, and how the application of these technologies has garnered novel insights into the neurobiology of addiction.

Ibudilast attenuates cocaine self-administration and prime- and cue-induced reinstatement of cocaine seeking in rats

Ibudilast is a non-selective phosphodiesterase (PDE) inhibitor and glial cell modulator which has shown great promise for the treatment of drug and alcohol use disorders in recent clinical studies. However, it is unknown whether and how ibudilast affects cocaine seeking behavior. Here we show that systemic administration of ibudilast dose-dependently reduced cocaine self-administration under fixed- and progressive-ratio reinforcement schedules in rats and shifted cocaine dose-response curves downward. In addition, ibudilast decreased cocaine prime- and cue-induced reinstatement of cocaine seeking. These results indicate that ibudilast was effective in reducing the reinforcing effects of cocaine and relapse to cocaine seeking. Chronic cocaine exposure induces cAMP-related neuroadaptations in the reward circuitry of the brain. To investigate potential mechanisms for ibudilast-induced attenuation of cocaine self-administration, we recorded from ventral tegmental area (VTA) dopamine neurons in ex vivo midbrain slices prepared from rats that had undergone saline and cocaine self-administration. We found cocaine self-administration led to a decrease in inhibitory postsynaptic currents (IPSCs), an increase in the AMPAR/NMDAR ratio, and an increase in the excitation to inhibition (E/I) ratio. Ibudilast pretreatments enhanced GABAergic inhibition and did not further change cocaine-induced potentiation of excitation, leading to normalization of the E/I ratio. Restoration of the balance between excitation and inhibition in VTA dopamine neurons may contribute to the attenuation of cocaine self-administration by ibudilast.

Corticostriatal Activity Driving Compulsive Reward Seeking

BACKGROUND

Activation of the mesolimbic dopamine system is positively reinforcing. After repeated activation, some individuals develop compulsive reward-seeking behavior, which is a core symptom of addiction. However, the underlying neural mechanism remains elusive.

METHODS

We trained mice in a seek-take chain, rewarded by optogenetic dopamine neuron self-stimulation. After compulsivity was evaluated, AMPA/NMDA ratio was measured at three distinct corticostriatal pathways confirmed by retrograde labeling and anterograde synaptic connectivity. Fiber photometry method and chemogenetics were used to parse the contribution of orbitofrontal cortex afferents onto the dorsal striatum (DS) during the behavioral task. We established a causal link between DS activity and compulsivity using optogenetic inhibition.

RESULTS

Mice that persevered when seeking was punished exhibited an increased AMPA/NMDA ratio selectively at orbitofrontal cortex to DS synapses. In addition, an activity peak of spiny projection neurons in the DS at the moment of signaled reward availability was detected. Chemogenetic inhibition of orbitofrontal cortex neurons curbed the activity peak and reduced punished reward seeking, as did optogenetic hyperpolarization of spiny projection neurons time-locked to the cue predicting reward availability.

CONCLUSIONS

Our results suggest that compulsive individuals display stronger neuronal activity in the DS during the cue predicting reward availability even when at the risk of punishment, nurturing further compulsive reward seeking.

Single Exposure to Cocaine Impairs Reinforcement Learning by Potentiating the Activity of Neurons in the Direct Striatal Pathway in Mice

Plasticity in the glutamatergic synapses on striatal medium spiny neurons (MSNs) is not only essential for behavioral adaptation but also extremely vulnerable to drugs of abuse. Modulation on these synapses by even a single exposure to an addictive drug may interfere with the plasticity required by behavioral learning and thus produce impairment. In the present work, we found that the negative reinforcement learning, escaping mild foot-shocks by correct nose-poking, was impaired by a single in vivo exposure to 20 mg/kg cocaine 24 h before the learning in mice. Either a single exposure to cocaine or reinforcement learning potentiates the glutamatergic synapses on MSNs expressing the striatal dopamine 1 (D1) receptor (D1-MSNs). However, 24 h after the cocaine exposure, the potentiation required for reinforcement learning was disrupted. Specific manipulation of the activity of striatal D1-MSNs in D1-cre mice demonstrated that activation of these MSNs impaired reinforcement learning in normal D1-cre mice, but inhibition of these neurons reversed the reinforcement learning impairment induced by cocaine. The results suggest that cocaine potentiates the activity of direct pathway neurons in the dorsomedial striatum and this potentiation might disrupt the potentiation produced during and required for reinforcement learning.

Drug-Evoked Synaptic Plasticity of Excitatory Transmission in the Ventral Tegmental Area

Cocaine leads to a strong euphoria, which is at the origin of its recreational use. Past the acute effects, the drug leaves traces in the brain that persist long after it has been cleared from the body. These traces eventually shape behavior such that drug use may become compulsive, and addiction develops. Here, we discuss cocaine-evoked synaptic plasticity of glutamatergic transmission onto dopamine (DA) neurons of the ventral tegmental area (VTA) as one of the earliest traces after a first injection of cocaine. We review the literature that has examined the induction requirements, as well as the expression mechanism of this form of plasticity, and ask the question about its functional significance.

Acute cocaine exposure occludes long-term depression in ventral tegmental area GABA neurons

The ventral tegmental area (VTA) in the midbrain is essential in incentive salience of reward behavior. Drugs of abuse increase midbrain dopamine cell activity and/or dopamine levels, and can alter endogenous VTA glutamate plasticity, leading to addiction or dependence. VTA dopamine cells are regulated by local inhibitory GABA cells, which exhibit a form of pre-synaptic cannabinoid receptor 1-dependent long-term depression of their glutamatergic inputs. Our current aim was to determine cocaine's influence on VTA GABA cell glutamate plasticity and circuity. Using whole cell voltage-clamp electrophysiology in VTA slices of GAD67-GFP knock-in mice, we recorded excitatory inputs on VTA GABA cells. Acute and chronic injections of cocaine were sufficient to occlude long-term depression. The plasticity could be reversed to the naïve state however, as long-term depression was again observed following a 7-day abstinence from acute cocaine exposure. Furthermore, chronic cocaine decreased AMPA/NMDA ratios at glutamate synapses onto VTA GABA cells, compared to vehicle injection controls, the opposite change noted in dopamine cells. Collectively, our data suggest the cellular mechanism of cocaine-mediated synaptic modification that may result in dependence/withdrawal could involve changes in glutamate input to VTA GABA circuitry in addition to VTA dopamine cells. Therefore VTA GABA cells may also play a role, possibly in a synergistic manner with the dopamine circuit, in cocaine-induced changes to the VTA reward pathway than previously known.

Accumbens Cholinergic Interneurons Mediate Cue-Induced Nicotine Seeking and Associated Glutamatergic Plasticity

Nicotine, the primary addictive substance in tobacco, is widely abused. Relapse to cues associated with nicotine results in increased glutamate release within nucleus accumbens core (NAcore), modifying synaptic plasticity of medium spiny neurons (MSNs), which contributes to reinstatement of nicotine seeking. However, the role of cholinergic interneurons (ChIs) within the NAcore in mediating these neurobehavioral processes is unknown. ChIs represent less than 1% of the accumbens neuronal population and are activated during drug seeking and reward-predicting events. Thus, we hypothesized that ChIs may play a significant role in mediating glutamatergic plasticity that underlies nicotine-seeking behavior. Using chemogenetics in transgenic rats expressing Cre under the control of the choline acetyltransferase (ChAT) promoter, ChIs were bidirectionally manipulated before cue-induced reinstatement. Following nicotine self-administration and extinction, ChIs were activated or inhibited before a cue reinstatement session. Following reinstatement, whole-cell electrophysiology from NAcore MSNs was used to assess changes in plasticity, measured via AMPA/NMDA (A/N) ratios. Chemogenetic inhibition of ChIs inhibited cued nicotine seeking and resulted in decreased A/N, relative to control animals, whereas activation of ChIs was unaltered, demonstrating that ChI inhibition may modulate plasticity underlying cue-induced nicotine seeking. These results demonstrate that ChI neurons play an important role in mediating cue-induced nicotine reinstatement and underlying synaptic plasticity within the NAcore.

Opioid-Induced Molecular and Cellular Plasticity of Ventral Tegmental Area Dopamine Neurons

Opioid drugs are highly valued as potent analgesics; however, there are significant risks associated with long-term use because of their abuse liability. Opioids cause changes in ventral tegmental area (VTA) gene expression and cell activity that have been linked to addiction-related behaviors in rodent models. Here, we focus on VTA dopamine (DA) neurons and review the cellular, structural, and synaptic plasticity changes induced by acute and chronic opioid exposure. We also discuss many avenues for future research including determination of whether opioid neuroadaptations are specific for subpopulations of VTA DA neurons. A better understanding of the molecular adaptations within the cells and circuits that drive opioid abuse is crucial for the development of better treatments for substance use disorders and to create novel, safer pain-relieving therapeutics.

Evolution of in vivo dopamine monitoring techniques

The brain dopamine system is central to numerous behavioral processes, including movement, learning, and motivation. Accordingly, disruptions of this neural system underlie numerous neurological and psychiatric disorders. Current understanding of how dopamine neurotransmission contributes to behavior and its dysfunction has been driven by technological advancements that permit spatiotemporally-defined measurements of dopaminergic signaling in behaving animals. In this review, we will discuss the evolution of in vivo neural monitoring technologies for measuring dopamine neuron function. We focus on the dopamine system for two reasons: (1) the central role of dopamine neurotransmission in normal behavior and disease, and (2) dopamine neuron measurements have long been at the forefront of in vivo neural monitoring technologies. We will provide a brief overview of standard techniques for monitoring dopamine function, including electrophysiology, microdialysis, and voltammetry. Then, we will discuss recent advancements in optical technologies using genetically-encoded fluorescent proteins (GEFPs), including a critical evaluation of their advantages and limitations.

Psychostimulant-Induced Adaptations in Nucleus Accumbens Glutamatergic Transmission

Carrying different aspects of emotional and motivational signals, glutamatergic synaptic projections from multiple limbic and paralimbic brain regions converge to the nucleus accumbens (NAc), in which these arousing signals are processed and prioritized for behavioral output. In animal models of drug addiction, some key drug-induced alterations at NAc glutamatergic synapses underlie important cellular and circuit mechanisms that promote subsequent drug taking, seeking, and relapse. With the focus of cocaine, we review changes at NAc glutamatergic synapses that occur after different drug procedures and abstinence durations, and the behavioral impact of these changes.

The transition to compulsion in addiction

Compulsion is a cardinal symptom of drug addiction (severe substance use disorder). However, compulsion is observed in only a small proportion of individuals who repeatedly seek and use addictive substances. Here, we integrate accounts of the neuropharmacological mechanisms that underlie the transition to compulsion with overarching learning theories, to outline how compulsion develops in addiction. Importantly, we emphasize the conceptual distinctions between compulsive drug-seeking behaviour and compulsive drug-taking behaviour (that is, use). In the latter, an individual cannot stop using a drug despite major negative consequences, possibly reflecting an imbalance in frontostriatal circuits that encode reward and aversion. By contrast, an individual may compulsively seek drugs (that is, persist in seeking drugs despite the negative consequences of doing so) when the neural systems that underlie habitual behaviour dominate goal-directed behavioural systems, and when executive control over this maladaptive behaviour is diminished. This distinction between different aspects of addiction may help to identify its neural substrates and new treatment strategies.

The SERT Met172 Mouse: An Engineered Model To Elucidate the Contributions of Serotonin Signaling to Cocaine Action

Cocaine abuse and addiction remain highly prevalent and, unfortunately, poorly treated. It is well-known that essential aspects of cocaine's addictive actions involve the drug's ability to block the presynaptic dopamine (DA) transporter (DAT), thereby elevating extracellular levels of DA in brain circuits that subserve reward, reinforcement, and habit. Less well appreciated are the multiple DA-independent actions of cocaine, activities that we and others believe contribute key pieces to the puzzle of cocaine addiction, treatment, and relapse. In particular, a significant body of work points to altered serotonin (5-HT) signaling as one such component, not surprising given that, relative to DAT, cocaine acts as potently to block the 5-HT transporter (SERT) as to block DAT, and thereby elevates extracellular 5-HT levels throughout the brain when reward-eliciting DA elevations occur. To elucidate the contribution of SERT antagonism to the actions of cocaine, we engineered a mouse model that significantly reduces cocaine potency at SERT without disrupting the expression or function of SERT in vivo. In this short Perspective, we review the rationale for development of the SERT Met172 model, the studies that document the pharmacological impact of the Ile172Met substitution in vitro and in vivo, and our findings with the model that demonstrate serotonergic contributions to the genetic, physiological, and behavioral actions of cocaine.

Glucagon-Like Peptide-1 Receptor Agonist Treatment Does Not Reduce Abuse-Related Effects of Opioid Drugs

Dependence on opioids and the number of opioid overdose deaths are serious and escalating public health problems, but medication-assisted treatments for opioid addiction remain inadequate for many patients. Glucagon-like pepide-1 (GLP-1) is a gut hormone and neuropeptide with actions in peripheral tissues and in the brain, including regulation of blood glucose and food intake. GLP-1 analogs, which are approved diabetes medications, can reduce the reinforcing and rewarding effects of alcohol, cocaine, amphetamine, and nicotine in rodents. Investigations on effects of GLP-1 analogs on opioid reward and reinforcement have not been reported. We assessed the effects of the GLP-1 receptor agonist Exendin-4 (Ex4) on opioid-related behaviors in male mice, i.e., morphine-conditioned place preference (CPP), intravenous self-administration (IVSA) of the short-acting synthetic opioid remifentanil, naltrexone-precipitated morphine withdrawal, morphine analgesia (male and female mice), and locomotor activity. Ex4 treatment had no effect on morphine-induced CPP, withdrawal, or hyperlocomotion. Ex4 failed to decrease remifentanil self-administration, if anything reinforcing effects of remifentanil appeared increased in Ex4-treated mice relative to saline. Ex4 did not significantly affect analgesia. In contrast, Ex4 dose dependently decreased oral alcohol self-administration, and suppressed spontaneous locomotor activity. Taken together, Ex4 did not attenuate the addiction-related behavioral effects of opioids, indicating that GLP-1 analogs would not be useful medications in the treatment of opioid addiction. This difference between opioids and other drug classes investigated to date may shed light on the mechanism of action of GLP-1 receptor treatment in the addictive effects of alcohol, central stimulants, and nicotine.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Dopamine neurons projecting to medial shell of the nucleus accumbens drive heroin reinforcement

The dopamine (DA) hypothesis posits the increase of mesolimbic dopamine levels as a defining commonality of addictive drugs, initially causing reinforcement, eventually leading to compulsive consumption. While much experimental evidence from psychostimulants supports this hypothesis, it has been challenged for opioid reinforcement. Here, we monitor genetically encoded DA and calcium indicators as well as cFos in mice to reveal that heroin activates DA neurons located in the medial part of the VTA, preferentially projecting to the medial shell of the nucleus accumbens (NAc). Chemogenetic and optogenetic manipulations of VTA DA or GABA neurons establish a causal link to heroin reinforcement. Inhibition of DA neurons blocked heroin self-administration, while heroin inhibited optogenetic self-stimulation of DA neurons. Likewise, heroin occluded the self-inhibition of VTA GABA neurons. Together, these experiments support a model of disinhibition of a subset of VTA DA neurons in opioid reinforcement.

Role of VTA dopamine neurons and neuroligin 3 in sociability traits related to nonfamiliar conspecific interaction

Atypical habituation and aberrant exploration of novel stimuli have been related to the severity of autism spectrum disorders (ASDs), but the underlying neuronal circuits are unknown. Here we show that chemogenetic inhibition of dopamine (DA) neurons of the ventral tegmental area (VTA) attenuates exploration toward nonfamiliar conspecifics and interferes with the reinforcing properties of nonfamiliar conspecific interaction in mice. Exploration of nonfamiliar stimuli is associated with the insertion of GluA2-lacking AMPA receptors at excitatory synapses on VTA DA neurons. These synaptic adaptations persist upon repeated exposure to social stimuli and sustain conspecific interaction. Global or VTA DA neuron-specific loss of the ASD-associated synaptic adhesion molecule neuroligin 3 alters the behavioral response toward nonfamiliar conspecifics and the reinforcing properties of conspecific interaction. These behavioral deficits are accompanied by an aberrant expression of AMPA receptors and an occlusion of synaptic plasticity. Altogether, these findings link impaired exploration of nonfamiliar conspecifics to VTA DA neuron dysfunction in mice.

Individuals with autism spectrum disorder have alteration in social and novelty behaviors. Here, Bellone and colleagues show that chemogenetic inhibition of mouse dopamine neurons in the ventral tegmental area can blunt exploration towards unfamiliar conspecifics, and that these behavioral deficits are recapitulated in mice lacking neuroligin3 gene product.

Addiction-related interactions of pregabalin with morphine in mice and humans: reinforcing and inhibiting effects

The gabapentinoid pregabalin is a rapid-acting anxiolytic and analgesic, possibly suitable in supervised opioid detoxification. However, clinicians have been cautious in using it because of its unknown addictive risk and rising number of mortalities after pregabalin self-medication in opioid abusers. Here, we studied interactions of pregabalin and morphine on reward functions of the dopamine system in mice and the efficacy of pregabalin on withdrawal in opioid addicts. After the treatment of mice with pregabalin and morphine, we used electrophysiology to study neuroplasticity in midbrain slices, self-administration and conditioned place preference tests to investigate the rewarding potential of pregabalin and naloxone-precipitated morphine withdrawal to evaluate opioid withdrawal symptoms. Further, we ran a pilot single-blind, randomized, controlled trial (34 heroin addicts) to evaluate the efficacy and safety of pregabalin in the treatment of opioid withdrawal syndrome. Pregabalin alone did not induce glutamate receptor neuroplasticity of dopamine neurons in the ventral tegmental area, but pre-treatment with pregabalin suppressed morphine-induced neuroplasticity, hyperlocomotion and morphine self-administration. Pregabalin administration after chronic morphine exposure failed to induce any rewarding effects. Instead, pregabalin suppressed withdrawal symptoms in both morphine-treated mice and opioid addicts and was well tolerated. Intriguingly, pregabalin administration after a low dose of morphine strongly facilitated ventral tegmental area neuroplasticity and led to increased conditioned place preference. Pregabalin appears to have the efficacy to counteract both reinforcing and withdrawal effects of opioids, but it also has a potentiating effect when given to mice with existing opioid levels.

Prenatal nicotine exposure alters postsynaptic AMPA receptors and glutamate neurotransmission within the laterodorsal tegmentum (LDT) of juvenile mice

Despite dissemination of information regarding the harm on fetal development of smoking while pregnant, the number of pregnancies associated with nicotine exposure appears to have stagnated. Presence of nicotine during neural formulation is associated with a higher susceptibility of drug dependence, suggesting an altered development of neurons in circuits involved in saliency and motivation. The laterodorsal tegmental nucleus (LDT) plays a role in coding stimuli valence via afferents to mesolimbic nuclei. Accordingly, alterations in development of neural mechanisms in the LDT could be involved in vulnerability to drug dependency. Therefore, we examined the effect of prenatal nicotine exposure (PNE) on glutamatergic functioning of LDT neurons in mouse brain slices using whole-cell, patch clamp concurrent with fluorescence-based calcium imaging. PNE was associated with larger amplitudes of AMPA-induced currents, and greater AMPA-mediated rises in intracellular calcium. AMPA/NMDA ratios and the AMPA-current rectification index were lower and higher, respectively, consistent with changes in the functionality of AMPA receptors in the PNE, which was substantiated by a greater inhibition of evoked and spontaneous glutamatergic synaptic events by a selective inhibitor of GluA2-lacking AMPA receptors. Paired pulse ratios showed a decreased probability of glutamate release from presynaptic inputs, and fluorescent imaging indicated a decreased action potential-dependent calcium increase associated with PNE. When taken together, our data suggest that PNE alters LDT glutamatergic functioning, which could alter output to mesolimbic targets. Such an alteration could play a role in altered coding of relevancy of drug stimuli that could enhance risk for development of drug dependency.

Convergent Neuronal Plasticity and Metaplasticity Mechanisms of Stress, Nicotine, and Alcohol

Stress and tobacco smoking are risk factors for alcoholism, but the underlying neural mechanisms are not well understood. Although stress, nicotine, and alcohol have broad, individual effects in the brain, some of their actions converge onto the same mechanisms and circuits. Stress and nicotine augment alcohol-related behaviors, in part via modulation of alcohol-evoked neuronal plasticity and metaplasticity mechanisms. Stress modulates alcohol-evoked plasticity via the release of signaling molecules that influence synaptic transmission. Nicotine also activates some of the same signaling molecules, cells, and circuits, producing a convergence of both stress and nicotine onto common plasticity mechanisms that influence alcohol self-administration. We describe several forms of alcohol-induced plasticity, including classic Hebbian plasticity at glutamatergic synapses, and we highlight less appreciated forms, such as non-Hebbian and GABAergic synaptic plasticity. Risk factors such as stress and nicotine initiate lasting neural changes that modify subsequent alcohol-induced synaptic plasticity and increase the vulnerability to alcohol addiction.

PDE4 Inhibition Restores the Balance Between Excitation and Inhibition in VTA Dopamine Neurons Disrupted by Repeated In Vivo Cocaine Exposure

Phosphodiesterase type 4 (PDE4) is a family of enzymes that selectively degrade intracellular cAMP. PDE4 inhibitors have been shown to regulate the rewarding and reinforcing effects of cocaine, but the underlying mechanisms remain poorly understood. Here we show that pretreatments with the PDE4 inhibitor rolipram attenuated cocaine-induced locomotor sensitization in mice. Repeated cocaine exposure in vivo caused a decrease in inhibitory postsynaptic currents (IPSCs) and an increase in the AMPAR/NMDAR ratio in ventral tegmental area (VTA) dopamine neurons in midbrain slices ex vivo. Cocaine exposure disrupted the balance between excitation and inhibition as shown by an increase in the excitation to inhibition (E/I) ratio. Rolipram pretreatments in vivo prevented cocaine-induced reductions in GABAergic inhibition but did not further increase cocaine-induced potentiation of excitation, leading to the restoration of a balance between excitation and inhibition and normalization of the E/I ratio. In support of this idea, we found that repeated cocaine exposure led to an increase in the single-unit action potential firing rate in vivo in VTA dopamine neurons, which was blocked by rolipram pretreatments. These results suggest that repeated cocaine exposure in vivo disrupts the balance between excitation and inhibition in VTA dopamine neurons, while PDE4 inhibition reestablishes the balance between excitation and inhibition through distinct mechanisms.

Hypocretin/Orexin and Plastic Adaptations Associated with Drug Abuse

Dopamine neurons in the ventral tegmental area (VTA) are a critical part of the neural circuits that underlie reward learning and motivation. Dopamine neurons send dense projections throughout the brain and recent observations suggest that both the intrinsic properties and the functional output of dopamine neurons are dependent on projection target and are subject to neuromodulatory influences. Lateral hypothalamic hypocretin (also termed orexin) neurons project to the VTA and contain both hypocretin and dynorphin peptides in the same dense core vesicles suggesting they may be co-released. Hypocretin peptides act at excitatory Gαq protein-coupled receptors and dynorphin acts at inhibitory Gαi/o protein-coupled receptors, which are both expressed on subpopulations of dopamine neurons. This review describes a role for neuromodulation of dopamine neurons and the influence on motivated behaviour in response to natural and drug rewards.

A single dose of cocaine potentiates glutamatergic synaptic transmission onto locus coeruleus neurons

The brainstem locus coeruleus (LC), the primary norepinephrinergic (NE) nucleus in the brain, has been implicated in the abuse of drugs such as opioids. However, whether and how the LC-NE system is involved in cocaine addiction remains elusive. Here, we demonstrated cocaine-evoked synaptic plasticity of glutamatergic transmission onto LC neurons as one of the earliest traces occurring after a single injection of cocaine. Twenty-four hours after mice were injected intraperitoneally with cocaine, the evoked α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) mediated synaptic transmission onto LC neurons were strongly potentiated without major effect on N-methyl-d-aspartate receptor (NMDAR) mediated synaptic transmission. Compared with saline-pretreated mice, AMPAR-mediated excitatory postsynaptic currents (EPSCs) of cocaine-pretreated mice showed a marked inward rectification, demonstrating the insertion of GluR2-lacking AMPARs to plasma membrane. In addition, the single injection of cocaine did not affect presynaptic glutamate release probability measured by paired pulse ratio. Furthermore, we found that the cocaine-induced potentiation of AMPAR EPSCs could be blocked by prazosin, an inhibitor of α1-adrenoreceptor (AR), indicating that cocaine increases AMPAR transmission via α1-ARs. These results reveal that LC-NE serves as an initial target of drug intake.

Interactions of Cathinone NPS with Human Transporters and Receptors in Transfected Cells

Pharmacological assays carried out in transfected cells have been very useful for describing the mechanism of action of cathinone new psychoactive substances (NPS). These in vitro characterizations provide fast and reliable information on psychoactive substances soon after they emerge for recreational use. Well-investigated comparator compounds, such as methamphetamine, 3,4-methylenedioxymethamphetamine, cocaine, and lysergic acid diethylamide, should always be included in the characterization to enhance the translation of the in vitro data into clinically useful information. We classified cathinone NPS according to their pharmacology at monoamine transporters and receptors. Cathinone NPS are monoamine uptake inhibitors and most induce transporter-mediated monoamine efflux with weak to no activity at pre- or postsynaptic receptors. Cathinones with a nitrogen-containing pyrrolidine ring emerged as NPS that are extremely potent transporter inhibitors but not monoamine releasers. Cathinones exhibit clinically relevant differences in relative potencies at serotonin vs. dopamine transporters. Additionally, cathinone NPS have more dopaminergic vs. serotonergic properties compared with their non-β-keto amphetamine analogs, suggesting more stimulant and reinforcing properties. In conclusion, in vitro pharmacological assays in heterologous expression systems help to predict the psychoactive and toxicological effects of NPS.

Epac Signaling Is Required for Cocaine-Induced Change in AMPA Receptor Subunit Composition in the Ventral Tegmental Area

Exchange protein directly activated by cAMP (Epac) and protein kinase A (PKA) are intracellular receptors for cAMP. Although PKA and its downstream effectors have been studied extensively in the context of drug addiction, whether and how Epac regulates cellular and behavioral effects of drugs of abuse remain essentially unknown. Epac is known to regulate AMPA receptor (AMPAR) trafficking. Previous studies have shown that a single cocaine exposure in vivo leads to an increase in GluA2-lacking AMPARs in dopamine neurons of the ventral tegmental area (VTA). We tested the hypothesis that Epac mediates cocaine-induced changes in AMPAR subunit composition in the VTA. We report that a single cocaine injection in vivo in wild-type mice leads to inward rectification of EPSCs and renders EPSCs sensitive to a GluA2-lacking AMPAR blocker in VTA dopamine neurons. The cocaine-induced increase in GluA2-lacking AMPARs was absent in Epac2-deficient mice but not in Epac1-deficient mice. In addition, activation of Epac with the selective Epac agonist 8-CPT-2Me-cAMP (8-CPT) recapitulated the cocaine-induced increase in GluA2-lacking AMPARs, and the effects of 8-CPT were mediated by Epac2. We also show that conditioned place preference to cocaine was impaired in Epac2-deficient mice and in mice in which Epac2 was knocked down in the VTA but was not significantly altered in Epac1-deficient mice. Together, these results suggest that Epac2 is critically involved in the cocaine-induced change in AMPAR subunit composition and drug-cue associative learning.

SIGNIFICANCE STATEMENT

Addictive drugs, such as cocaine, induce long-lasting adaptions in the reward circuits of the brain. A single intraperitoneal injection of cocaine leads to changes in the composition and property of the AMPAR that carries excitatory inputs to dopamine neurons. Here, we provide evidence that exchange protein directly activated by cAMP (Epac), a cAMP sensor protein, is required for the cocaine-induced changes of the AMPAR. We found that the effects of cocaine were mimicked by activation of Epac but were blocked by genetic deletion of Epac. Furthermore, cocaine-cue associative learning was impaired in mice lacking Epac. These findings uncovered a critical role of Epac in regulating the cellular and behavioral actions of cocaine.

Diversity of Dopaminergic Neural Circuits in Response to Drug Exposure

Addictive substances are known to increase dopaminergic signaling in the mesocorticolimbic system. The origin of this dopamine (DA) signaling originates in the ventral tegmental area (VTA), which sends afferents to various targets, including the nucleus accumbens, the medial prefrontal cortex, and the basolateral amygdala. VTA DA neurons mediate stimuli saliency and goal-directed behaviors. These neurons undergo robust drug-induced intrinsic and extrinsic synaptic mechanisms following acute and chronic drug exposure, which are part of brain-wide adaptations that ultimately lead to the transition into a drug-dependent state. Interestingly, recent investigations of the differential subpopulations of VTA DA neurons have revealed projection-specific functional roles in mediating reward, aversion, and stress. It is now critical to view drug-induced neuroadaptations from a circuit-level perspective to gain insight into how differential dopaminergic adaptations and signaling to targets of the mesocorticolimbic system mediates drug reward. This review hopes to describe the projection-specific intrinsic characteristics of these subpopulations, the differential afferent inputs onto these VTA DA neuron subpopulations, and consolidate findings of drug-induced plasticity of VTA DA neurons and highlight the importance of future projection-based studies of this system.

The differential effects of alprazolam and oxazepam on methamphetamine self-administration in rats

BACKGROUND

Methamphetamine is the second most commonly used illicit drug in the world, and despite recent attempts by the Drug Enforcement Administration to combat this epidemic, methamphetamine use is still on the rise. As methamphetamine use increases so does polydrug use, particularly that involving methamphetamine and benzodiazepines. The present study was designed to examine the effects of two benzodiazepines on methamphetamine self-administration.

METHODS

Five doses of methamphetamine (0.0075, 0.015, 0.03, 0.09, and 0.12mg/kg/infusion) were tested, producing an inverted U-shaped dose-response curve. Rats were then pretreated with oxazepam, alprazolam, or vehicle prior to methamphetamine self-administration. To determine if the effects of these drugs were due to the GABAA receptor and/or translocator protein (TSPO), we also pretreated rats with an antagonist for the benzodiazepine-binding site on the GABAA receptor (i.e., flumazenil) and a TSPO antagonist (i.e., PK11195) prior to alprazolam or oxazepam administration.

RESULTS

Oxazepam significantly reduced methamphetamine self-administration as demonstrated by a downward shift of the dose-response curve. In contrast, alprazolam significantly enhanced methamphetamine self-administration as evidenced by a leftward shift of the dose-response curve. Flumazenil completely blocked the effects of alprazolam on methamphetamine self-administration. When administered individually, both flumazenil and PK11195 partially reversed the effects of oxazepam on methamphetamine self-administration. However, when these two antagonists were combined, the effects of oxazepam were completely reversed.

CONCLUSIONS

The GABAA receptor is responsible for the alprazolam-induced enhancement of methamphetamine self-administration, while the activation of both the GABAA receptor and TSPO are responsible for the oxazepam-induced reduction of methamphetamine self-administration.

The Emergence of a Circuit Model for Addiction

Addiction is a disease of altered behavior. Addicts use drugs compulsively and will continue to do so despite negative consequences. Even after prolonged periods of abstinence, addicts are at risk of relapse, particularly when cues evoke memories that are associated with drug use. Rodent models mimic many of the core components of addiction, from the initial drug reinforcement to cue-associated relapse and continued drug intake despite negative consequences. Rodent models have also enabled unprecedented mechanistic insight into addiction, revealing plasticity of glutamatergic synaptic transmission evoked by the strong activation of mesolimbic dopamine-a defining feature of all addictive drugs-as a neural substrate for these drug-adaptive behaviors. Cell type-specific optogenetic manipulations have allowed both identification of the relevant circuits and design of protocols to reverse drug-evoked plasticity and to establish links of causality with drug-adaptive behaviors. The emergence of a circuit model for addiction will open the door for novel therapies, such as deep brain stimulation.

Morphine-induced synaptic plasticity in the VTA is reversed by HDAC inhibition

Dopamine (DA) dysfunction originating from the ventral tegmental area (VTA) occurs as a result of synaptic abnormalities following consumption of drugs of abuse and underlies behavioral plasticity associated with drug abuse. Drugs of abuse can cause changes in gene expression through epigenetic mechanisms in the brain that underlie some of the lasting neuroplasticity and behavior associated with addiction. Here we investigated the function of histone acetylation and histone deacetylase (HDAC)2 in the VTA in recovery of morphine-induced synaptic modifications following a single in vivo exposure to morphine. Using a combination of immunohistochemistry, Western blot, and whole cell patch-clamp recording in rat midbrain slices, we show that morphine increased HDAC2 activity in VTA DA neurons and reduced histone H3 acetylation at lysine 9 (Ac-H3K9) in the VTA 24 h after the injection. Morphine-induced synaptic changes at glutamatergic synapses involved endocannabinoid signaling to reduce GABAergic synaptic strength onto VTA DA neurons. Both plasticities were recovered by in vitro incubation of midbrain slices with a class I-specific HDAC inhibitor (HDACi), CI-994, through an increase in acetylation of histone H3K9. Interestingly, HDACi incubation also increased levels of Ac-H3K9 and triggered GABAergic and glutamatergic plasticities in DA neurons of saline-treated rats. Our results suggest that acute morphine-induced changes in VTA DA activity and synaptic transmission engage HDAC2 activity locally in the VTA to maintain synaptic modifications through histone hypoacetylation.

Sufficiency of Mesolimbic Dopamine Neuron Stimulation for the Progression to Addiction

The factors causing the transition from recreational drug consumption to addiction remain largely unknown. It has not been tested whether dopamine (DA) is sufficient to trigger this process. Here we use optogenetic self-stimulation of DA neurons of the ventral tegmental area (VTA) to selectively mimic the defining commonality of addictive drugs. All mice readily acquired self-stimulation. After weeks of abstinence, cue-induced relapse was observed in parallel with a potentiation of excitatory afferents onto D1 receptor-expressing neurons of the nucleus accumbens (NAc). When the mice had to endure a mild electric foot shock to obtain a stimulation, some stopped while others persevered. The resistance to punishment was associated with enhanced neural activity in the orbitofrontal cortex (OFC) while chemogenetic inhibition of the OFC reduced compulsivity. Together, these results show that stimulating VTA DA neurons induces behavioral and cellular hallmarks of addiction, indicating sufficiency for the induction and progression of the disease.

Orexin Signaling in the VTA Gates Morphine-Induced Synaptic Plasticity

Dopamine neurons in the ventral tegmental area (VTA) are a key target of addictive drugs, and neuroplasticity in this region may underlie some of the core features of addiction. From the very first exposure, all drugs of abuse induce synaptic plasticity in the VTA. However, it is not well understood how this diverse group of drugs brings about common synaptic change. Orexin (also known as hypocretin) is a lateral hypothalamic neuropeptide released into the VTA that promotes drug-seeking behaviors and potentiates excitatory synaptic transmission onto VTA dopamine neurons. Here we show that signaling at orexin receptor type 1 (OxR1) in the VTA is required for morphine-induced plasticity of dopamine neurons. Systemic or intra-VTA administration of the OxR1 antagonist SB 334867 in rats blocked a morphine-induced increase in the AMPAR/NMDAR ratio, an increase in presynaptic glutamate release, and a postsynaptic change in AMPAR number or function, including a switch in subunit composition. Furthermore, SB 334867 blocked a morphine-induced decrease in presynaptic GABA release, and a morphine-induced shift in the balance of excitatory and inhibitory synaptic inputs to dopamine neurons. These findings identify a novel role for orexin in morphine-induced plasticity in the VTA and provide a mechanism by which orexin can gate the output of dopamine neurons.

Modulation of the glutamatergic transmission by Dopamine: a focus on Parkinson, Huntington and Addiction diseases

Dopamine (DA) plays a major role in motor and cognitive functions as well as in reward processing by regulating glutamatergic inputs. In particular in the striatum the release of DA rapidly influences synaptic transmission modulating both AMPA and NMDA receptors. Several neurodegenerative and neuropsychiatric disorders, including Parkinson, Huntington and addiction-related diseases, manifest a dysregulation of glutamate and DA signaling. Here, we will focus our attention on the mechanisms underlying the modulation of the glutamatergic transmission by DA in striatal circuits.

Brain tissue responses to neural implants impact signal sensitivity and intervention strategies

Implantable biosensors are valuable scientific tools for basic neuroscience research and clinical applications. Neurotechnologies provide direct readouts of neurological signal and neurochemical processes. These tools are generally most valuable when performance capacities extend over months and years to facilitate the study of memory, plasticity, and behavior or to monitor patients’ conditions. These needs have generated a variety of device designs from microelectrodes for fast scan cyclic voltammetry (FSCV) and electrophysiology to microdialysis probes for sampling and detecting various neurochemicals. Regardless of the technology used, the breaching of the blood–brain barrier (BBB) to insert devices triggers a cascade of biochemical pathways resulting in complex molecular and cellular responses to implanted devices. Molecular and cellular changes in the microenvironment surrounding an implant include the introduction of mechanical strain, activation of glial cells, loss of perfusion, secondary metabolic injury, and neuronal degeneration. Changes to the tissue microenvironment surrounding the device can dramatically impact electrochemical and electrophysiological signal sensitivity and stability over time. This review summarizes the magnitude, variability, and time course of the dynamic molecular and cellular level neural tissue responses induced by state-of-the-art implantable devices. Studies show that insertion injuries and foreign body response can impact signal quality across all implanted central nervous system (CNS) sensors to varying degrees over both acute (seconds to minutes) and chronic periods (weeks to months). Understanding the underlying biological processes behind the brain tissue response to the devices at the cellular and molecular level leads to a variety of intervention strategies for improving signal sensitivity and longevity.

Signaling to the microtubule cytoskeleton: an unconventional role for CaMKII

Synaptic plasticity is a hallmark of the nervous system and is thought to be integral to higher brain functions such as learning and memory. Calcium, acting as a second messenger, and the calcium/calmodulin dependent kinase CaMKII are key regulators of neuronal plasticity. Given the importance of the actin and microtubule (MT) cytoskeleton in dendritic spine morphology, composition and plasticity, it is not surprising that many regulators of these cytoskeletal elements are downstream of the CaMKII pathway. In this review, we discuss the emerging role of calcium and CaMKII in the regulation of MTs and cargo unloading during synaptic plasticity.

It's MORe exciting than mu: crosstalk between mu opioid receptors and glutamatergic transmission in the mesolimbic dopamine system

Opioids selective for the G protein-coupled mu opioid receptor (MOR) produce potent analgesia and euphoria. Heroin, a synthetic opioid, is considered one of the most addictive substances, and the recent exponential rise in opioid addiction and overdose deaths has made treatment development a national public health priority. Existing medications (methadone, buprenorphine, and naltrexone), when combined with psychosocial therapies, have proven efficacy in reducing aspects of opioid addiction. Unfortunately, these medications have critical limitations including those associated with opioid agonist therapies (e.g., sustained physiological dependence and opioid withdrawal leading to high relapse rates upon discontinuation), non-adherence to daily dosing, and non-renewal of monthly injection with extended-release naltrexone. Furthermore, current medications fail to ameliorate key aspects of addiction such as powerful conditioned associations that trigger relapse (e.g., cues, stress, the drug itself). Thus, there is a need for developing novel treatments that target neural processes corrupted with chronic opioid use. This requires a basic understanding of molecular and cellular mechanisms underlying effects of opioids on synaptic transmission and plasticity within reward-related neural circuits. The focus of this review is to discuss how crosstalk between MOR-associated G protein signaling and glutamatergic neurotransmission leads to immediate and long-term effects on emotional states (e.g., euphoria, depression) and motivated behavior (e.g., drug-seeking, relapse). Our goal is to integrate findings on how opioids modulate synaptic release of glutamate and postsynaptic transmission via α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptors in the nucleus accumbens and ventral tegmental area with the clinical (neurobehavioral) progression of opioid dependence, as well as to identify gaps in knowledge that can be addressed in future studies.

Interference with AMPA receptor endocytosis: effects on behavioural and neurochemical correlates of amphetamine sensitization in male rats

BACKGROUND

Behavioural sensitization has been linked to drug craving in both clinical and preclinical studies of addiction. Increased motor activity is accompanied by enhanced dopamine (DA) release, particularly in the nucleus accumbens (NAcc). The neural bases of sensitization are linked to alterations in synaptic connections that also underlie learning and memory. The present study uses an "interference" peptide, Tat-GluA2(3Y), that blocks long-term depression (LTD) at glutamatergic synapses by disrupting the endocytosis of α- amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPARs), to explore the role of this form of synaptic plasticity in the induction and maintenance of sensitization.

METHODS

Rats were given 5 injections of d-amphetamine (d-AMPH, 1.0 mg/kg, intraperitoneal) every second day. Tat-GluA2(3Y), was administered by 2 different routes (intravenously and intracerebrally to the ventral tegmental area [VTA] or to the NAcc) before each injection of d-AMPH. After a 14-day drug-free period, expression of behavioural sensitization was evoked by a challenge injection of d-AMPH (0.5 mg/kg, intraperitoneal). Dopamine efflux in the NAcc was measured by high-pressure liquid chromatography with electrochemical detection analyses of brain dialysates on days 1, 9 and 24 of the intravenous peptide experiment.

RESULTS

Systemic administration of Tat-GluA2(3Y) during the induction phase blocked maintenance of behavioural sensitization and attenuated the maintenance of neurochemical sensitization. Intra-VTA infusion of Tat-GluA2(3Y) before each administration of d-AMPH did not affect induction, but inhibited maintenance and subsequent expression of sensitization, whereas intra-NAcc infusion of the peptide did not affect induction or maintenance of sensitization.

LIMITATIONS

The relevance of behavioural sensitization in rodents is related to the development of craving and does not provide direct measures of drug reinforcement.

CONCLUSION

These findings confirm that drug-induced neuroplasticity is labile and may be subject to disruption at a time when long-lasting associations between drug reward and contextual stimuli are formed. Furthermore, the unique ability of Tat-GluA2(3Y) to block maintenance of behavioural sensitization implicates LTD in the consolidation of essential associative memories. Tat-GluA2(3Y) has the unique ability to disrupt functional neuroadaptations triggered by repeated psychostimulant exposure and therefore may protect against the development of craving and drug seeking behaviours.

Illicit dopamine transients: reconciling actions of abused drugs

Phasic increases in brain dopamine are required for cue-directed reward seeking. Although compelling within the framework of appetitive behavior, the view that illicit drugs hijack reward circuits by hyperactivating these dopamine transients is inconsistent with established psychostimulant pharmacology. However, recent work reclassifying amphetamine (AMPH), cocaine, and other addictive dopamine-transporter inhibitors (DAT-Is) supports transient hyperactivation as a unifying hypothesis of abused drugs. We argue here that reclassification also identifies generating burst firing by dopamine neurons as a keystone action. Unlike natural rewards, which are processed by sensory systems, drugs act directly on the brain. Consequently, to mimic natural rewards and exploit reward circuits, dopamine transients must be elicited de novo. Of available drug targets, only burst firing achieves this essential outcome.

VTA GABA neurons modulate specific learning behaviors through the control of dopamine and cholinergic systems

The mesolimbic reward system is primarily comprised of the ventral tegmental area (VTA) and the nucleus accumbens (NAc) as well as their afferent and efferent connections. This circuitry is essential for learning about stimuli associated with motivationally-relevant outcomes. Moreover, addictive drugs affect and remodel this system, which may underlie their addictive properties. In addition to dopamine (DA) neurons, the VTA also contains approximately 30% γ-aminobutyric acid (GABA) neurons. The task of signaling both rewarding and aversive events from the VTA to the NAc has mostly been ascribed to DA neurons and the role of GABA neurons has been largely neglected until recently. GABA neurons provide local inhibition of DA neurons and also long-range inhibition of projection regions, including the NAc. Here we review studies using a combination of in vivo and ex vivo electrophysiology, pharmacogenetic and optogenetic manipulations that have characterized the functional neuroanatomy of inhibitory circuits in the mesolimbic system, and describe how GABA neurons of the VTA regulate reward and aversion-related learning. We also discuss pharmacogenetic manipulation of this system with benzodiazepines (BDZs), a class of addictive drugs, which act directly on GABA_A receptors located on GABA neurons of the VTA. The results gathered with each of these approaches suggest that VTA GABA neurons bi-directionally modulate activity of local DA neurons, underlying reward or aversion at the behavioral level. Conversely, long-range GABA projections from the VTA to the NAc selectively target cholinergic interneurons (CINs) to pause their firing and temporarily reduce cholinergic tone in the NAc, which modulates associative learning. Further characterization of inhibitory circuit function within and beyond the VTA is needed in order to fully understand the function of the mesolimbic system under normal and pathological conditions.

AMPA receptors in post-mortem brains of Cloninger type 1 and 2 alcoholics: a whole-hemisphere autoradiography study

Dysfunction of the brain glutamate system has been associated with alcoholism. Ionotropic glutamatergic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPARs) play an important role in both neurotransmission and post-synaptic plasticity. Alterations in AMPAR densities may also play a role in the neurobiological changes associated with alcoholism. In the present study, [(3)H] AMPA binding density was evaluated in the nucleus accumbens (NAc), frontal cortex, anterior cingulate cortex (ACC), dentate gyrus and hippocampus of Cloninger type 1 (n=9) and 2 (n=8) alcoholics, and compared with non-alcoholic control subjects (n=10) by post-mortem whole-hemisphere autoradiography. The [(3)H] AMPA binding density was significantly higher in the ACC of early onset type 2 alcoholics when compared with controls (p=0.011). There was also a significant negative correlation between [(3)H] AMPA binding and previously published results of dopamine transporter (DAT) density in the ACC in these same brain samples (R=-0.95, p=0.001). Although preliminary, and from a relatively small diagnostic group, the present results help to further explain the pathology of alcohol dependence and impulsive behaviour in type 2 alcoholics.