Cell-type and region-specific nucleus accumbens AMPAR plasticity associated with morphine reward, reinstatement, and spontaneous withdrawal

Comparing withdrawal- and anxiety-like behaviors following oral and subcutaneous oxycodone administration in C57BL/6 mice

Excessive prescribing and misuse of prescription opioids, such as oxycodone, significantly contributed to the current opioid crisis. Although oxycodone is typically consumed orally by humans, parenteral routes of administration have primarily been used in preclinical models of oxycodone dependence. To address this issue, more recent studies have used oral self-administration procedures to study oxycodone seeking and withdrawal in rodents. Behavioral differences, however, following oral oxycodone intake versus parenteral oxycodone administration remain unclear. Thus, the goal of the current studies was to compare anxiety- and withdrawal-like behaviors using established opioid dependence models of either home cage oral intake of oxycodone (0.5 mg/ml) or repeated subcutaneous (s.c.) injections of oxycodone (10 mg/kg) in male and female mice. Here, mice received 10 days of oral or s.c. oxycodone administration, and following 72 h of forced abstinence, anxiety- and withdrawal-like behaviors were measured using elevated zero maze, open field, and naloxone-induced precipitated withdrawal procedures. Global withdrawal scores were increased to a similar degree following oral and s.c. oxycodone use, while both routes of oxycodone administration had minimal effects on anxiety-like behaviors. When examining individual withdrawal-like behaviors, mice receiving s.c. oxycodone exhibited more paw tremors and jumps during naloxone-induced precipitated withdrawal compared with oral oxycodone mice. These results indicate that both models of oxycodone administration are sufficient to elevate global withdrawal scores, but, when compared with oral consumption, s.c. oxycodone injections yielded more pronounced effects on some withdrawal-like behaviors.

Treadmill exercise training inhibits morphine CPP by reversing morphine effects on GABA neurotransmission in D2-MSNs of the accumbens-pallidal pathway in male mice

Relapse is a major challenge in the treatment of drug addiction, and exercise has been shown to decrease relapse to drug seeking in animal models. However, the neural circuitry mechanisms by which exercise inhibits morphine relapse remain unclear. In this study, we report that 4-week treadmill training prevented morphine conditioned place preference (CPP) expression during abstinence by acting through the nucleus accumbens (NAc)-ventral pallidum (VP) pathway. We found that neuronal excitability was reduced in D2-dopamine receptor-expressing medium spiny neurons (D2-MSNs) following repeated exposure to morphine and forced abstinence. Enhancing the excitability of NAc D2-MSNs via treadmill training decreased the expression of morphine CPP. We also found that the effects of treadmill training were mediated by decreasing enkephalin levels and that restoring opioid modulation of GABA neurotransmission in the VP, which increased neurotransmitter release from NAc D2-MSNs to VP, decreased morphine CPP. Our findings suggest the inhibitory effect of exercise on morphine CPP is mediated by reversing morphine-induced neuroadaptations in the NAc-to-VP pathway.

Targeting the cytoskeleton as a therapeutic approach to substance use disorders

Substance use disorders (SUD) are chronic relapsing disorders governed by continually shifting cycles of positive drug reward experiences and drug withdrawal-induced negative experiences. A large body of research points to plasticity within systems regulating emotional, motivational, and cognitive processes as drivers of continued compulsive pursuit and consumption of substances despite negative consequences. This plasticity is observed at all levels of analysis from molecules to networks, providing multiple avenues for intervention in SUD. The cytoskeleton and its regulatory proteins within neurons and glia are fundamental to the structural and functional integrity of brain processes and are potentially the major drivers of the morphological and behavioral plasticity associated with substance use. In this review, we discuss preclinical studies that provide support for targeting the brain cytoskeleton as a therapeutic approach to SUD. We focus on the interplay between actin cytoskeleton dynamics and exposure to cocaine, methamphetamine, alcohol, opioids, and nicotine and highlight preclinical studies pointing to a wide range of potential therapeutic targets, such as nonmuscle myosin II, Rac1, cofilin, prosapip 1, and drebrin. These studies broaden our understanding of substance-induced plasticity driving behaviors associated with SUD and provide new research directions for the development of SUD therapeutics.

The Paraventricular Thalamic Nucleus and Its Projections in Regulating Reward and Context Associations

The paraventricular thalamic nucleus (PVT) is a brain region that mediates aversive and reward-related behaviors as shown in animals exposed to fear conditioning, natural rewards, or drugs of abuse. However, it is unknown whether manipulations of the PVT, in the absence of external factors or stimuli (e.g., fear, natural rewards, or drugs of abuse), are sufficient to drive reward-related behaviors. Additionally, it is unknown whether drugs of abuse administered directly into the PVT are sufficient to drive reward-related behaviors. Here, using behavioral as well as pathway and cell-type specific approaches, we manipulate PVT activity as well as the PVT-to-nucleus accumbens shell (NAcSh) neurocircuit to explore reward phenotypes. First, we show that bath perfusion of morphine (10 µM) caused hyperpolarization of the resting membrane potential, increased rheobase, and decreased intrinsic membrane excitability in PVT neurons that project to the NAcSh. Additionally, we found that direct injections of morphine (50 ng) in the PVT of mice were sufficient to generate conditioned place preference (CPP) for the morphine-paired chamber. Mimicking the inhibitory effect of morphine, we employed a chemogenetic approach to inhibit PVT neurons that projected to the NAcSh and found that pairing the inhibition of these PVT neurons with a specific context evoked the acquisition of CPP. Lastly, using brain slice electrophysiology, we found that bath-perfused morphine (10 µM) significantly reduced PVT excitatory synaptic transmission on both dopamine D1 and D2 receptor-expressing medium spiny neurons in the NAcSh, but that inhibiting PVT afferents in the NAcSh was not sufficient to evoke CPP.

The different cell-specific mechanisms of voluntary exercise and forced exercise in the nucleus accumbens

Physical inactivity is a global epidemic. People who take the initiative to exercise will feel pleasure during the exercise process and stick with it for a long time, while people who passively ask for exercise will feel pain and cannot stick with it. However, the neural mechanisms underlying voluntary and forced exercise remain unclear. Here, we report that voluntary running increased the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSC) but decreased membrane excitability in D1R-MSNs, whereas D2R-MSNs did not change in mEPSC and membrane excitability. Forced running increased the frequency of mEPSC and membrane excitability in D2R-MSNs, but D1R-MSNs did not change, which may be the mechanism by which forced exercise has a non-rewarding effect. These findings provide new insights into how voluntary and forced exercise mediate reward and non-reward effects.

Neuroplasticity of the extended amygdala in opioid withdrawal and prolonged opioid abstinence

Opioid use disorder is characterized by excessive use of opioids, inability to control its use, a withdrawal syndrome upon discontinuation of opioids, and long-term likelihood of relapse. The behavioral stages of opioid addiction correspond with affective experiences that characterize the opponent process view of motivation. In this framework, active involvement is accompanied by positive affective experiences which gives rise to "reward craving," whereas the opponent process, abstinence, is associated with the negative affective experiences that produce "relief craving." Relief craving develops along with a hypersensitization to the negatively reinforcing aspects of withdrawal during abstinence from opioids. These negative affective experiences are hypothesized to stem from neuroadaptations to a network of affective processing called the "extended amygdala." This negative valence network includes the three core structures of the central nucleus of the amygdala (CeA), the bed nucleus of the stria terminalis (BNST), and the nucleus accumbens shell (NAc shell), in addition to major inputs from the basolateral amygdala (BLA). To better understand the major components of this system, we have reviewed their functions, inputs and outputs, along with the associated neural plasticity in animal models of opioid withdrawal. These models demonstrate the somatic, motivational, affective, and learning related models of opioid withdrawal and abstinence. Neuroadaptations in these stress and motivational systems are accompanied by negative affective and aversive experiences that commonly give rise to relapse. CeA neuroplasticity accounts for many of the aversive and fear-related effects of opioid withdrawal via glutamatergic plasticity and changes to corticotrophin-releasing factor (CRF)-containing neurons. Neuroadaptations in BNST pre-and post-synaptic GABA-containing neurons, as well as their noradrenergic modulation, may be responsible for a variety of aversive affective experiences and maladaptive behaviors. Opioid withdrawal yields a hypodopaminergic and amotivational state and results in neuroadaptive increases in excitability of the NAc shell, both of which are associated with increased vulnerability to relapse. Finally, BLA transmission to hippocampal and cortical regions impacts the perception of conditioned aversive effects of opioid withdrawal by higher executive systems. The prevention or reversal of these varied neuroadaptations in the extended amygdala during opioid withdrawal could lead to promising new interventions for this life-threatening condition.

Morphine exposure during adolescence induces enduring social changes dependent on adolescent stage of exposure, sex, and social test

Drug exposure during adolescence, when the 'reward' circuitry of the brain is developing, can permanently impact reward-related behavior. Epidemiological studies show that opioid treatment during adolescence, such as pain management for a dental procedure or surgery, increases the incidence of psychiatric illness including substance use disorders. Moreover, the opioid epidemic currently in the United States is affecting younger individuals raising the impetus to understand the pathogenesis of the negative effects of opioids. One reward-related behavior that develops during adolescence is social behavior. We previously demonstrated that social development occurs in rats during sex-specific adolescent periods: early to mid-adolescence in males (postnatal day (P)30-40) and pre-early adolescence in females (P20-30). We thus hypothesized that morphine exposure during the female critical period would result in adult sociability deficits in females, but not males, and morphine administered during the male critical period would result in adult sociability deficits in males, but not females. We found that morphine exposure during the female critical period primarily resulted in deficits in sociability in females, while morphine exposure during the male critical period primarily resulted in deficits in sociability primarily in males. However, depending on the test performed and the social parameter measured, social alterations could be found in both sexes that received morphine exposure at either adolescent stage. These data indicate that when drug exposure occurs during adolescence, and how the endpoint data are measured, will play a large role in determining the effects of drug exposures on social development.

Adaptations in Nucleus Accumbens Neuron Subtypes Mediate Negative Affective Behaviors in Fentanyl Abstinence

BACKGROUND

Opioid discontinuation generates a withdrawal syndrome marked by increased negative affect. Increased symptoms of anxiety and dysphoria during opioid discontinuation are significant barriers to achieving long-term abstinence in opioid-dependent individuals. While adaptations in the nucleus accumbens are implicated in opioid abstinence syndrome, the precise neural mechanisms are poorly understood. Additionally, our current knowledge is limited to changes following natural and semisynthetic opioids, despite recent increases in synthetic opioid use and overdose.

METHODS

We used a combination of cell subtype-specific viral labeling and electrophysiology in male and female mice to investigate structural and functional plasticity in nucleus accumbens medium spiny neuron (MSN) subtypes after fentanyl abstinence. We characterized molecular adaptations after fentanyl abstinence with subtype-specific RNA sequencing and weighted gene co-expression network analysis. We used viral-mediated gene transfer to manipulate the molecular signature of fentanyl abstinence in D1-MSNs.

RESULTS

Here, we show that fentanyl abstinence increases anxiety-like behavior, decreases social interaction, and engenders MSN subtype-specific plasticity in both sexes. D1-MSNs, but not D2-MSNs, exhibit dendritic atrophy and an increase in excitatory drive. We identified a cluster of coexpressed dendritic morphology genes downregulated selectively in D1-MSNs that are transcriptionally coregulated by E2F1. E2f1 expression in D1-MSNs protects against loss of dendritic complexity, altered physiology, and negative affect-like behaviors caused by fentanyl abstinence.

CONCLUSIONS

Our findings indicate that fentanyl abstinence causes unique structural, functional, and molecular changes in nucleus accumbens D1-MSNs that can be targeted to alleviate negative affective symptoms during abstinence.

Differential Patterns of Synaptic Plasticity in the Nucleus Accumbens Caused by Continuous and Interrupted Morphine Exposure

Opioid exposure and withdrawal both cause adaptations in brain circuits that may contribute to abuse liability. These adaptations vary in magnitude and direction following different patterns of opioid exposure, but few studies have systematically manipulated the pattern of opioid administration while measuring neurobiological impact. In this study, we compared cellular and synaptic adaptations in the nucleus accumbens shell caused by morphine exposure that was either continuous or interrupted by daily bouts of naloxone-precipitated withdrawal. At the behavioral level, continuous morphine administration caused psychomotor tolerance, which was reversed when the continuity of morphine action was interrupted by naloxone-precipitated withdrawal. Using ex vivo slice electrophysiology in female and male mice, we investigated how these patterns of morphine administration altered intrinsic excitability and synaptic plasticity of medium spiny neurons (MSNs) expressing the D1 or D2 dopamine receptor. We found that morphine-evoked adaptations at excitatory synapses were predominately conserved between patterns of administration, but there were divergent effects on inhibitory synapses and the subsequent balance between excitatory and inhibitory synaptic input. Overall, our data suggest that continuous morphine administration produces adaptations that dampen the output of D1-MSNs, which are canonically thought to promote reward-related behaviors. Interruption of otherwise continuous morphine exposure does not dampen D1-MSN functional output to the same extent, which may enhance behavioral responses to subsequent opioid exposure. Our findings support the hypothesis that maintaining continuity of opioid administration could be an effective therapeutic strategy to minimize the vulnerability to opioid use disorders.SIGNIFICANCE STATEMENT Withdrawal plays a key role in the cycle of addiction to opioids like morphine. We studied how repeated cycles of naloxone-precipitated withdrawal from otherwise continuous opioid exposure can change brain function of the nucleus accumbens, which is an important brain region for reward and addiction. Different patterns of opioid exposure caused unique changes in communication between neurons in the nucleus accumbens, and the nature of these changes depended on the type of neuron being studied. The specific changes in communication between neurons caused by repeated cycles of withdrawal may increase vulnerability to opioid use disorders. This highlights the importance of reducing or preventing the experience of withdrawal during opioid treatment.

Glutamate receptor-interacting protein 1 in D1- and D2-dopamine receptor-expressing medium spiny neurons differentially regulates cocaine acquisition, reinstatement, and associated spine plasticity

BACKGROUND

The nucleus accumbens (NAc) is involved in the expression of cocaine addictive phenotypes, including acquisition, extinction, and reinstatement. In the NAc, D1-medium spiny neurons (MSNs) encode cocaine reward, whereas D2-MSNs encode aversive responses in drug addiction. Glutamate receptor-interacting protein 1 (GRIP1) is known to be associated with cocaine addiction, but the role of GRIP1 in D1-MSNs and D2-MSNs of the NAc in cocaine acquisition and reinstatement remains unknown.

METHODS

A conditioned place preference apparatus was used to establish cocaine acquisition, extinction, and reinstatement in mouse models. GRIP1 expression was evaluated using Western blotting. Furthermore, GRIP1-siRNA and GRIP1 overexpression lentivirus were used to interfere with GRIP1 in the NAc. After the behavioral test, green fluorescent protein immunostaining of brain slices was used to detect spine density.

RESULTS

GRIP1 expression decreased during cocaine acquisition and reinstatement. GRIP1-siRNA enhanced cocaine-induced CPP behavior in acquisition and reinstatement and regulated associated spine plasticity. Importantly, the decreased GRIP1 expression that mediated cocaine acquisition and reinstatement was mainly driven by the interference of the GRIP1-GluA2 interaction in D1-MSNs and could be blocked by the interference of the GRIP1-GluA2 interaction in D2-MSNs. Interference with the GRIP1-GluA2 interaction in D1- and D2-MSNs decreased spine density in D1- and D2-MSNs, respectively.

CONCLUSION

GRIP1 in D1- and D2-MSNs of the NAc differentially modulates cocaine acquisition and reinstatement. GRIP1 downregulation in D1-MSNs has a positive effect on cocaine acquisition and reinstatement, while GRIP1 downregulation in D2-MSNs has a negative effect. Additionally, GRIP1 downregulation in D1-MSNs plays a leading role in cocaine acquisition and reinstatement.

Single nucleus transcriptomic analysis of rat nucleus accumbens reveals cell type-specific patterns of gene expression associated with volitional morphine intake

Opioid exposure is known to cause transcriptomic changes in the nucleus accumbens (NAc). However, no studies to date have investigated cell type-specific transcriptomic changes associated with volitional opioid taking. Here, we use single nucleus RNA sequencing (snRNAseq) to comprehensively characterize cell type-specific alterations of the NAc transcriptome in rats self-administering morphine. One cohort of male Brown Norway rats was injected with acute morphine (10 mg/kg, i.p.) or saline. A second cohort of rats was allowed to self-administer intravenous morphine (1.0 mg/kg/infusion) for 10 consecutive days. Each morphine-experienced rat was paired with a yoked saline control rat. snRNAseq libraries were generated from NAc punches and used to identify cell type-specific gene expression changes associated with volitional morphine taking. We identified 1106 differentially expressed genes (DEGs) in the acute morphine group, compared to 2453 DEGs in the morphine self-administration group, across 27 distinct cell clusters. Importantly, we identified 1329 DEGs that were specific to morphine self-administration. DEGs were identified in novel clusters of astrocytes, oligodendrocytes, and D1R- and D2R-expressing medium spiny neurons in the NAc. Cell type-specific DEGs included Rgs9, Celf5, Oprm1, and Pde10a. Upregulation of Rgs9 and Celf5 in D2R-expressing neurons was validated by RNAscope. Approximately 85% of all oligodendrocyte DEGs, nearly all of which were associated with morphine taking, were identified in two subtypes. Bioinformatic analyses identified cell type-specific upstream regulatory mechanisms of the observed transcriptome alterations and downstream signaling pathways, including both novel and previously identified molecular pathways. These findings show that volitional morphine taking is associated with distinct cell type-specific transcriptomic changes in the rat NAc and highlight specific striatal cell populations and novel molecular substrates that could be targeted to reduce compulsive opioid taking.

Anterior cingulate cortex and its projections to the ventral tegmental area regulate opioid withdrawal, the formation of opioid context associations and context-induced drug seeking

Clinical evidence suggests that there are correlations between activity within the anterior cingulate cortex (ACC) following re-exposure to drug-associated contexts and drug craving. However, there are limited data contributing to our understanding of ACC function at the cellular level during re-exposure to drug-context associations as well as whether the ACC is directly related to context-induced drug seeking. Here, we addressed this issue by employing our novel behavioral procedure capable of measuring the formation of drug-context associations as well as context-induced drug-seeking behavior in male mice (8–12 weeks of age) that orally self-administered oxycodone. We found that mice escalated oxycodone intake during the long-access training sessions and that conditioning with oxycodone was sufficient to evoke conditioned place preference (CPP) and drug-seeking behaviors. Additionally, we found that thick-tufted, but not thin-tufted pyramidal neurons (PyNs) in the ACC as well as ventral tegmental area (VTA)-projecting ACC neurons had increased intrinsic membrane excitability in mice that self-administered oxycodone compared to controls. Moreover, we found that global inhibition of the ACC or inhibition of VTA-projecting ACC neurons was sufficient to significantly reduce oxycodone-induced CPP, drug seeking, and spontaneous opioid withdrawal. These results demonstrate a direct role of ACC activity in mediating context-induced opioid seeking among other behaviors, including withdrawal, that are associated with the DSM-V criteria of opioid use disorder.

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

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

Postsynaptic signaling at glutamatergic synapses as therapeutic targets

Dysregulation of glutamatergic synapses plays an important role in the pathogenesis of neurological diseases. In addition to mediating excitatory synaptic transmission, postsynaptic glutamate receptors interact with various membrane and intracellular proteins. They form structural and/or signaling synaptic protein complexes and thereby play diverse postsynaptic functions. Recently, several postsynaptic protein complexes have been associated with various neurological diseases and hence, have been characterized as important therapeutic targets. Moreover, novel small molecules and therapeutic peptides targeting and modulating the activities of these protein complexes have been discovered, some of which have advanced through preclinical translational research and/or clinical studies. This article describes the recent investigation of eight key protein complexes associated with the postsynaptic ionotropic and metabotropic glutamate receptors as therapeutic targets for central nervous system diseases.

Extended amygdala, conditioned withdrawal and memory consolidation

Opioid withdrawal can be associated to environmental cues through classical conditioning. Exposure to these cues can precipitate a state of conditioned withdrawal in abstinent subjects, and there are suggestions that conditioned withdrawal can perpetuate the addiction cycle in part by promoting the storage of memories. This review discusses evidence supporting the hypothesis that conditioned withdrawal facilitates memory consolidation by activating a neurocircuitry that involves the extended amygdala. Specifically, the central amygdala, the bed nucleus of the stria terminalis, and the nucleus accumbens shell interact functionally during withdrawal, mediate expression of conditioned responses, and are implicated in memory consolidation. From this perspective, the extended amygdala could be a neural pathway by which drug-seeking behaviour performed during a state of conditioned withdrawal is more likely to become habitual and persistent.

Oxytocin prevents cue-induced reinstatement of oxycodone seeking: Involvement of DNA methylation in the hippocampus

Oxycodone is one of the most commonly used analgesics in the clinic. However, long-term use can contribute to drug dependence. Accumulating evidence of changes in DNA methylation after opioid relapse has provided insight into mechanisms underlying drug-associated memory. The neuropeptide oxytocin is reported to be a potential treatment for addiction. The present study sought to identify changes in global and synaptic gene methylation after cue-induced reinstatement of oxycodone conditioned place preference (CPP) and the effect of oxytocin. We analyzed hippocampal mRNA of synaptic genes and also synaptic density in response to oxycodone CPP. We determined the mRNA levels of DNA methyltransferases (Dnmts) and ten-eleven translocations (Tets), observed global 5-methylcytosine (5-mC) and 5-hydroxymethylcytosine (5-hmC) levels, and measured DNA methylation status of four synaptic genes implicated in learning and memory (Arc, Dlg1, Dlg4, and Syn1). Both synaptic density and the transcription of 15 hippocampal synaptic genes significantly increased following cue-induced reinstatement of oxycodone CPP. Oxycodone relapse was also related to markedly decreased 5-mC levels and decreased transcription of Dnmt1, Dnmt3a, and Dnmt3b; in contrast, 5-hmC levels and the transcription of Tet1 and Tet3 were increased. Oxycodone exposure induced DNA hypomethylation at the exons of the Arc, Dlg1, Dlg4, and Syn1 genes. Intracerebroventricular (ICV) administration of oxytocin (2.5 μg/μl) specifically blocked oxycodone relapse, possibly by inhibition of Arc, Dlg1, Dlg4, and Syn1 hypomethylation in oxycodone-treated rats. Together, these data indicate the occurrence of epigenetic changes in the hippocampus following oxycodone relapse and the potential role of oxytocin in oxycodone addiction.

Anterior cingulate cortex is necessary for spontaneous opioid withdrawal and withdrawal-induced hyperalgesia in male mice

The anterior cingulate cortex (ACC) is implicated in many pathologies, including depression, anxiety, substance-use disorders, and pain. There is also evidence from brain imaging that the ACC is hyperactive during periods of opioid withdrawal. However, there are limited data contributing to our understanding of ACC function at the cellular level during opioid withdrawal. Here, we address this issue by performing ex vivo electrophysiological analysis of thick-tufted, putative dopamine D2 receptor expressing, layer V pyramidal neurons in the ACC (ACC L5 PyNs) in a mouse model of spontaneous opioid withdrawal. We found that escalating doses of morphine (20, 40, 60, 80, and 100 mg/kg, i.p. on days 1-5, respectively) injected twice daily into male C57BL/6 mice evoked withdrawal behaviors and an associated withdrawal-induced mechanical hypersensitivity. Brain slices prepared 24 h following the last morphine injection showed increases in ACC L5 thick-tufted PyN-intrinsic membrane excitability, increases in membrane resistance, reductions in the rheobase, and reductions in HCN channel-mediated currents (I_H). We did not observe changes in intrinsic or synaptic properties on thin-tufted, dopamine D1-receptor-expressing ACC L5 PyNs recorded from male Drd1a-tdTomato transgenic mice. In addition, we found that chemogenetic inhibition of the ACC blocked opioid-induced withdrawal and withdrawal-induced mechanical hypersensitivity. These results demonstrate that spontaneous opioid withdrawal alters neuronal properties within the ACC and that ACC activity is necessary to control behaviors associated with opioid withdrawal and withdrawal-induced mechanical hypersensitivity. The ability of the ACC to regulate both withdrawal behaviors and withdrawal-induced mechanical hypersensitivity suggests overlapping mechanisms between two seemingly distinguishable behaviors. This commonality potentially suggests that the ACC is a locus for multiple withdrawal symptoms.

Differential expression of H19, BC1, MIAT1, and MALAT1 long non-coding RNAs within key brain reward regions after repeated morphine treatment

Morphine-induced analgesic tolerance and dependence are significant limits of pain control; however, the exact molecular mechanisms underlying morphine tolerance and dependence have remained unclear. The role of long non-coding RNAs (lncRNAs) in morphine tolerance and dependence is yet to be determined. We aimed to explore the association of specific lncRNAs expression in key brain reward regions after repeated injection of morphine. Male Wistar rats received subcutaneous injections of twice-daily morphine (10 mg/kg) or saline (1 mL/kg) for eight days. On day 8 of the repeated injections, induction of morphine analgesic tolerance and dependence was confirmed through a hotplate test and a naloxone-precipitated withdrawal analysis, respectively. Expression of H19, BC1, MIAT1, and MALAT1 lncRNAs was determined from the midbrain, striatum, hypothalamus, prefrontal cortex (PFC), and hippocampus by real-time PCR on day 8 of the repeated injections. The H19 expression was significantly different between morphine-treated and control saline-treated rats in all investigated areas except for the hippocampus. The BC1 expression significantly altered in the midbrain, hypothalamus, and hippocampus, but not in the striatum and PFC after repeated morphine treatment. The MIAT1 and MALAT1 expression site-specifically altered in the midbrain, hypothalamus, and striatum; however, no significant changes were detected in their expression in the PFC and hippocampus after repeated morphine treatment. We conclude that alterations in the expression of these lncRNAs in the brain reward regions especially in the midbrain, striatum and hypothalamus may have critical roles in the development of morphine dependence and tolerance, which need to be considered in future researches.

Optogenetically-inspired neuromodulation: Translating basic discoveries into therapeutic strategies

Optogenetic tools allow for the selective activation, inhibition or modulation of genetically-defined neural circuits with incredible temporal precision. Over the past decade, application of these tools in preclinical models of psychiatric disease has advanced our understanding the neural circuit basis of maladaptive behaviors in these disorders. Despite their power as an investigational tool, optogenetics cannot yet be applied in the clinical for the treatment of neurological and psychiatric disorders. To date, deep brain stimulation (DBS) is the only clinical treatment that can be used to achieve circuit-specific neuromodulation in the context of psychiatric. Despite its increasing clinical indications, the mechanisms underlying the therapeutic effects of DBS for psychiatric disorders are poorly understood, which makes optimization difficult. We discuss the variety of optogenetic tools available for preclinical research, and how these tools have been leveraged to reverse-engineer the mechanisms underlying DBS for movement and compulsive disorders. We review studies that have used optogenetics to induce plasticity within defined basal ganglia circuits, to alter neural circuit function and evaluate the corresponding effects on motor and compulsive behaviors. While not immediately applicable to patient populations, the translational power of optogenetics is in inspiring novel DBS protocols by providing a rationale for targeting defined neural circuits to ameliorate specific behavioral symptoms, and by establishing optimal stimulation paradigms that could selectively compensate for pathological synaptic plasticity within these defined neural circuits.

Endogenous opiates and behavior: 2019

This paper is the forty-second consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2019 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Cues conditioned to withdrawal and negative reinforcement: Neglected but key motivational elements driving opioid addiction

Opioid use disorder (OUD) is a debilitating disorder that affects millions of people. Neutral cues can acquire motivational properties when paired with the positive emotional effects of drug intoxication to stimulate relapse. However, much less research has been devoted to cues that become conditioned to the aversive effects of opioid withdrawal. We argue that environmental stimuli promote motivation for opioids when cues are paired with withdrawal (conditioned withdrawal) and generate opioid consumption to terminate conditioned withdrawal (conditioned negative reinforcement). We review evidence that cues associated with pain drive opioid consumption, as patients with chronic pain may misuse opioids to escape physical and emotional pain. We highlight sex differences in withdrawal-induced stress reactivity and withdrawal cue processing and discuss neurocircuitry that may underlie withdrawal cue processing in dependent individuals. These studies highlight the importance of studying cues associated with withdrawal in dependent individuals and point to areas for exploration in OUD research.

Glutamatergic Systems and Memory Mechanisms Underlying Opioid Addiction

Glutamate is the main excitatory neurotransmitter in the brain and is of critical importance for the synaptic and circuit mechanisms that underlie opioid addiction. Opioid memories formed over the course of repeated drug use and withdrawal can become powerful stimuli that trigger craving and relapse, and glutamatergic neurotransmission is essential for the formation and maintenance of these memories. In this review, we discuss the mechanisms by which glutamate, dopamine, and opioid signaling interact to mediate the primary rewarding effects of opioids, and cover the glutamatergic systems and circuits that mediate the expression, extinction, and reinstatement of opioid seeking over the course of opioid addiction.

Fragility of reward vs antifragility of defense brain systems in drug dependence

Drug dependence is a debilitating disorder, affecting 30 million people worldwide. In this short review we discuss about the plasticity changes in the reward and defense brain systems induced by early-life psychosocial stressful experiences. Such changes may render persons more vulnerable to illicit drugs use, facilitating behaviors of abuse and development of addiction. We propose that underlying plasticity changes render brain reward system as increasingly fragile because of tolerance and other physiological effects that reduce responsiveness with repeated use. In contrast, we propose that brain defense system makes maintain antifragile mechanisms that generate more robust responses with the prolonged consumption of drugs. Investigating the underlying mechanisms of these brain plasticity changes may advance the development of more efficacious pharmacologic and psychotherapeutic approaches to rehabilitate patients and more efficacious prevention policies to protect children from stressful experiences.

The ethanol metabolite acetic acid activates mouse nucleus accumbens shell medium spiny neurons

Although ethanol consumption leads to an array of neurophysiological alterations involving the neural circuits for reward, the underlying mechanisms remain unclear. Acetic acid is a major metabolite of ethanol with high bioactivity and potentially significant pharmacological importance in regulating brain function. Yet, the impact of acetic acid on reward circuit function has not been well explored. Given the rewarding properties associated with ethanol consumption, we investigated the acute effects of ethanol and/or acetic acid on the neurophysiological function of medium spiny neurons of the nucleus accumbens shell, a key node in the mammalian reward circuit. We find that acetic acid, but not ethanol, provided a rapid and robust boost in neuronal excitability at physiologically relevant concentrations, whereas both compounds enhanced glutamatergic synaptic activity. These effects were consistent across both sexes in C57BL/6J mice. Overall, our data suggest acetic acid is a promising candidate mediator for ethanol effects on mood and motivation that deserves further investigation.NEW & NOTEWORTHY Ethanol consumption disrupts many neurophysiological processes leading to alterations in behavior and physiological function. The possible involvement of acetic acid, produced via ethanol metabolism, has been insufficiently explored. Here, we demonstrate that acetic acid contributes to rapid neurophysiological alterations in the accumbens shell. These findings raise the interesting possibility that ethanol may serve as a prodrug-generating acetic acid as a metabolite-that may influence ethanol consumption-associated behaviors and physiological responses by altering neurophysiological function.

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.

Progress in opioid reward research: From a canonical two-neuron hypothesis to two neural circuits

Opioid abuse and related overdose deaths continue to rise in the United States, contributing to the national opioid crisis in the USA. The neural mechanisms underlying opioid abuse and addiction are still not fully understood. This review discusses recent progress in basic research dissecting receptor mechanisms and circuitries underlying opioid reward and addiction. We first review the canonical GABA-dopamine neuron hypothesis that was upheld for half a century, followed by major findings challenging this hypothesis. We then focus on recent progress in research evaluating the role of the mesolimbic and nigrostriatal dopamine circuitries in opioid reward and relapse. Based on recent findings that activation of dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) is equally rewarding and that GABA neurons in the rostromedial tegmental nucleus (RMTg) and the substantia nigra pars reticula (SNr) are rich in mu opioid receptors and directly synapse onto midbrain DA neurons, we proposed that the RTMg→VTA → ventrostriatal and SNr → SNc → dorsostriatal pathways may act as the two major neural substrates underlying opioid reward and abuse. Lastly, we discuss possible integrations of these two pathways during initial opioid use, development of opioid abuse and maintenance of compulsive opioid seeking.

Bidirectional Dysregulation of AMPA Receptor-Mediated Synaptic Transmission and Plasticity in Brain Disorders

AMPA receptors (AMPARs) are glutamate-gated ion channels that mediate the majority of fast excitatory synaptic transmission throughout the brain. Changes in the properties and postsynaptic abundance of AMPARs are pivotal mechanisms in synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission. A wide range of neurodegenerative, neurodevelopmental and neuropsychiatric disorders, despite their extremely diverse etiology, pathogenesis and symptoms, exhibit brain region-specific and AMPAR subunit-specific aberrations in synaptic transmission or plasticity. These include abnormally enhanced or reduced AMPAR-mediated synaptic transmission or plasticity. Bidirectional reversal of these changes by targeting AMPAR subunits or trafficking ameliorates drug-seeking behavior, chronic pain, epileptic seizures, or cognitive deficits. This indicates that bidirectional dysregulation of AMPAR-mediated synaptic transmission or plasticity may contribute to the expression of many brain disorders and therefore serve as a therapeutic target. Here, we provide a synopsis of bidirectional AMPAR dysregulation in animal models of brain disorders and review the preclinical evidence on the therapeutic targeting of AMPARs.

Decreased Neuronal Excitability in Medial Prefrontal Cortex during Morphine Withdrawal is associated with enhanced SK channel activity and upregulation of small GTPase Rac1

Rationale: Neuroadaptations in the medial prefrontal cortex (mPFC) and Nucleus Accumbens (NAc) play a role in the disruption of control-reward circuits in opioid addiction. Small Conductance Calcium-Activated Potassium (SK) channels in the mPFC have been implicated in neuronal excitability changes during morphine withdrawal. However, the mechanism that modulates SK channels during withdrawal is still unknown.

Methods: Rats were exposed for one week to daily morphine injections (10 mg·kg^-1 s.c.) followed by conditional place preference (CPP) assessment. One week after withdrawal, electrophysiological, morphological and molecular biological methods were applied to investigate the effects of morphine on SK channels in mPFC, including infralimbic (IL), prelimbic (PrL) cortices and NAc (core and shell). We verified the hypothesis that Rac1, a member of Rho family of small GTPases, implicated in SK channel regulation, modulate SK channel neuroadaptations during opiate withdrawal.

Results: One week after morphine withdrawal, the neuronal excitability of layer 5 pyramidal neurons in IL was decreased, but not in PrL. Whereas, the excitability was increased in NAc-shell, but not in NAc-core. In mPFC, the expression of the SK3 subunit was enhanced after one-week of withdrawal compared to controls. In the IL, Rac1 signaling was increased during withdrawal, and the Rac1 inhibitor NSC23766 disrupted SK current, which increased neuronal firing. Suppression of Rac1 inhibited morphine-induced CPP and expression of SK channels in IL.

Conclusions: These findings highlight the potential value of SK channels and the upstream molecule Rac1, which may throw light on the therapeutic mechanism of neuromodulation treatment for opioid dependence.

Homeostatic regulation of reward via synaptic insertion of calcium-permeable AMPA receptors in nucleus accumbens

The incentive effects of food and related cues are determined by stimulus properties and the internal state of the organism. Enhanced hedonic reactivity and incentive motivation in energy deficient subjects have been demonstrated in animal models and humans. Defining the neurobiological underpinnings of these state-based modulatory effects could illuminate fundamental mechanisms of adaptive behavior, as well as provide insight into maladaptive consequences of weight loss dieting and the relationship between disturbed eating behavior and substance abuse. This article summarizes research of our laboratory aimed at identifying neuroadaptations induced by chronic food restriction (FR) that increase the reward magnitude of drugs and associated cues. The main findings are that FR decreases basal dopamine (DA) transmission, upregulates signaling downstream of the D1 DA receptor (D1R), and triggers synaptic incorporation of calcium-permeable AMPA receptors (CP-AMPARs) in the nucleus accumbens (NAc). Selective antagonism of CP-AMPARs decreases excitatory postsynaptic currents in NAc medium spiny neurons of FR rats and blocks the enhanced rewarding effects of d-amphetamine and a D1R, but not a D2R, agonist. These results suggest that FR drives CP-AMPARs into the synaptic membrane of D1R-expressing MSNs, possibly as a homeostatic response to reward loss. FR subjects also display diminished aversion for contexts associated with LiCl treatment and centrally infused cocaine. An encompassing, though speculative, hypothesis is that NAc synaptic incorporation of CP-AMPARs in response to food scarcity and other forms of sustained reward loss adaptively increases incentive effects of reward stimuli and, at the same time, diminishes responsiveness to aversive stimuli that have potential to interfere with goal pursuit.

Morphine Differentially Alters the Synaptic and Intrinsic Properties of D1R- and D2R-Expressing Medium Spiny Neurons in the Nucleus Accumbens

Exposure to opioids reshapes future reward and motivated behaviors partially by altering the functional output of medium spiny neurons (MSNs) in the nucleus accumbens shell. Here, we investigated how morphine, a highly addictive opioid, alters synaptic transmission and intrinsic excitability on dopamine D1-receptor (D1R) expressing and dopamine D2-receptor (D2R) expressing MSNs, the two main output neurons in the nucleus accumbens shell. Using whole-cell electrophysiology recordings, we show, that 24 h abstinence following repeated non-contingent administration of morphine (10 mg/kg, i.p.) in mice reduces the miniature excitatory postsynaptic current (mEPSC) frequency and miniature inhibitory postsynaptic current (mIPSC) frequency on D2R-MSNs, with concomitant increases in D2R-MSN intrinsic membrane excitability. We did not observe any changes in synaptic or intrinsic changes on D1R-MSNs. Last, in an attempt to determine the integrated effect of the synaptic and intrinsic alterations on the overall functional output of D2R-MSNs, we measured the input-output efficacy by measuring synaptically-driven action potential firing. We found that both D1R-MSN and D2R-MSN output was unchanged following morphine treatment.