Phosphorylation-dependent trafficking of GluR2-containing AMPA receptors in the nucleus accumbens plays a critical role in the reinstatement of cocaine seeking

PKMζ alters oxycodone-taking in a dose- and sex-dependent manner

Opioid use disorder involves disruptions to glutamate homeostasis and dendritic spine density in the reward system. PKMζ is an atypical isoform of protein kinase C that is expressed exclusively in neurons and plays a role in postsynaptic glutamate signaling and dendritic spine maturation. As opioid use leads to alterations in glutamate transmission and dendritic spine density, we hypothesized that PKMζ deletion would alter opioid-taking behaviors. The current study examined two doses of oxycodone self-administration in male and female mice with constitutive deletion of PKMζ compared to wildtype controls. At a dose of 0.25 mg/kg/infusion, PKMζ deletion significantly potentiated oxycodone self-administration in both male and female mice. However, increases in motivation for oxycodone, as indicated by increased breakpoint on a progressive ratio schedule, were only seen in male PKMζ knockout mice and not females. When we examined a lower dose of oxycodone, 0.125 mg/kg/infusion, PKMζ knockout led to increases in oxycodone self-administration only in female mice. Additionally, female PKMζ knockout mice exhibited higher breakpoints on a progressive ratio schedule at this dose compared to all other groups. In addition to the self-administration studies, we also examined locomotor sensitization in response to experimenter administered oxycodone. PKMζ KO decreased oxycodone induced locomotion in males and potentiated oxycodone sensitization in females. Together, these results suggest that PKMζ acts to dampen oxycodone taking in both sexes, but females may be more sensitive to its effects.

Circulating ovarian hormones interact with protein interacting with C kinase (PICK1) within the medial prefrontal cortex to influence cocaine seeking in female mice

Protein interacting with C kinase 1 (PICK1) is an AMPA receptor binding protein that works in conjunction with glutamate receptor interacting protein (GRIP) to balance the number of GluA2-containing AMPARs in the synapse. In male mice, disrupting PICK1 in the medial prefrontal cortex (mPFC) leads to a decrease in cue-induced cocaine seeking and disrupting GRIP in the mPFC has the opposing effect, consistent with other evidence that removal of GluA2-containing AMPARs potentiates reinstatement. However, PICK1 does not seem to play the same role in female mice, as knockdown of either PICK1 or GRIP in the mPFC leads to similar increases in cue-induced cocaine seeking. These previous findings indicate that the role of PICK1 in the prefrontal cortex is sex specific. The goal of the current study was to examine whether ovarian hormones contribute to the effect of prefrontal PICK1 knockdown on reinstatement of cocaine seeking. While we replicated the increased cue-induced cocaine seeking in prefrontal PICK1 knockdown sham mice, we did not see any difference between the GFP control mice and PICK1 knockdowns following ovariectomy. However, this effect was driven primarily by an increase in cocaine seeking in ovariectomized GFP control mice while there was no effect ovariectomy in PICK1 knockdown mice. Taken together, these findings suggest that circulating ovarian hormones interact with the effects of PICK1 on cue-induced reinstatement.

A mechanistic overview of approaches for the treatment of psychostimulant dependence

Psychostimulant use disorder is a major health issue around the world with enormous individual, family-related and societal consequences, yet there are no effective pharmacological treatments available. In this review, a target-based overview of pharmacological treatments toward psychostimulant addiction will be presented. We will go through therapeutic approaches targeting different aspects of psychostimulant addiction with focus on three major areas; 1) drugs targeting signalling, and metabolism of the dopamine system, 2) drugs targeting either AMPA receptors or metabotropic glutamate receptors of the glutamate system and 3) drugs targeting the severe side-effects of quitting long-term psychostimulant use. For each of these major modes of intervention, findings from pre-clinical studies in rodents to clinical trials in humans will be listed, and future perspectives of the different treatment strategies as well as their potential side-effects will be discussed. Pharmaceuticals modulating the dopamine system, such as antipsychotics, DAT-inhibitors, and disulfiram, have shown some promising results. Cognitive enhancers have been found to increase aspects of behavioural control, and drugs targeting the glutamate system such as modulators of metabotropic glutamate receptors and AMPA receptors have provided interesting changes in relapse behaviour. Furthermore, CRF-antagonists directed toward alleviating the symptoms of the withdrawal stage have been examined with interesting resulting changes in behaviour. There are promising results investigating therapeutics for psychostimulant addiction, but further preclinical work and additional human studies with a more stratified patient selection are needed to prove sufficient evidence of efficacy and tolerability.

Three Binding Conformations of BIO124 in the Pocket of the PICK1 PDZ Domain

The PDZ family has drawn attention as possible drug targets because of the domains’ wide ranges of function and highly conserved binding pockets. The PICK1 PDZ domain has been proposed as a possible drug target because the interactions between the PICK1 PDZ domain and the GluA2 subunit of the AMPA receptor have been shown to progress neurodegenerative diseases. BIO124 has been identified as a sub µM inhibitor of the PICK1–GluA2 interaction. Here, we use all-atom molecular dynamics simulations to reveal the atomic-level interaction pattern between the PICK1 PDZ domain and BIO124. Our simulations reveal three unique binding conformations of BIO124 in the PICK1 PDZ binding pocket, referred to here as state 0, state 1, and state 2. Each conformation is defined by a unique hydrogen bonding network and a unique pattern of hydrophobic interactions between BIO124 and the PICK1 PDZ domain. Interestingly, each conformation of BIO124 results in different dynamic changes to the PICK1 PDZ domain. Unlike states 1 and 2, state 0 induces dynamic coupling between BIO124 and the αA helix. Notably, this dynamic coupling with the αA helix is similar to what has been observed in other PDZ–ligand complexes. Our analysis indicates that the interactions formed between BIO124 and I35 may be the key to inducing dynamic coupling with the αA helix. Lastly, we suspect that the conformational shifts observed in our simulations may affect the stability and thus the overall effectiveness of BIO124. We propose that a physically larger inhibitor may be necessary to ensure sufficient interactions that permit stable binding between a drug and the PICK1 PDZ domain.

The Scaffold Protein PICK1 as a Target in Chronic Pain

Well-tolerated and effective drugs for treating chronic pain conditions are urgently needed. Most chronic pain patients are not effectively relieved from their pain and suffer from debilitating drug side effects. This has not only drastic negative consequences for the patients' quality of life, but also constitute an enormous burden on society. It is therefore of great interest to explore new potent targets for effective pain treatment with fewer side effects and without addiction liability. A critical component of chronic pain conditions is central sensitization, which involves the reorganization and strengthening of synaptic transmission within nociceptive pathways. Such changes are considered as maladaptive and depend on changes in the surface expression and signaling of AMPA-type glutamate receptors (AMPARs). The PDZ-domain scaffold protein PICK1 binds the AMPARs and has been suggested to play a key role in these maladaptive changes. In the present paper, we review the regulation of AMPARs by PICK1 and its relation to pain pathology. Moreover, we highlight other pain-relevant PICK1 interactions, and we evaluate various compounds that target PICK1 and have been successfully tested in pain models. Finally, we evaluate the potential on-target side effects of interfering with the action of PICK1 action in CNS and beyond. We conclude that PICK1 constitutes a valid drug target for the treatment of inflammatory and neuropathic pain conditions without the side effects and abuse liability associated with current pain medication.

mTOR regulates cocaine-induced behavioural sensitization through the SynDIG1-GluA2 interaction in the nucleus accumbens

Behavioral sensitization is a progressive increase in locomotor or stereotypic behaviours in response to drugs. It is believed to contribute to the reinforcing properties of drugs and to play an important role in relapse after cessation of drug abuse. However, the mechanism underlying this behaviour remains poorly understood. In this study, we showed that mTOR signaling was activated during the expression of behavioral sensitization to cocaine and that intraperitoneal or intra-nucleus accumbens (NAc) treatment with rapamycin, a specific mTOR inhibitor, attenuated cocaine-induced behavioural sensitization. Cocaine significantly modified brain lipid profiles in the NAc of cocaine-sensitized mice and markedly elevated the levels of phosphatidylinositol-4-monophosphates (PIPs), including PIP, PIP_2, and PIP₃. The behavioural effect of cocaine was attenuated by intra-NAc administration of LY294002, an AKT-specific inhibitor, suggesting that PIPs may contribute to mTOR activation in response to cocaine. An RNA-sequencing analysis of the downstream effectors of mTOR signalling revealed that cocaine significantly decreased the expression of SynDIG1, a known substrate of mTOR signalling, and decreased the surface expression of GluA2. In contrast, AAV-mediated SynDIG1 overexpression in NAc attenuated intracellular GluA2 internalization by promoting the SynDIG1-GluA2 interaction, thus maintaining GluA2 surface expression and repressing cocaine-induced behaviours. In conclusion, NAc SynDIG1 may play a negative regulatory role in cocaine-induced behavioural sensitization by regulating synaptic surface expression of GluA2.

Upregulation of Src Family Tyrosine Kinases in the Rat Striatum by Adenosine A2A Receptors

Adenosine A_2A receptors are G_olf-coupled receptors and are predominantly expressed in the striatum of mammalian brains. As a mostly postsynaptic receptor, A_2A receptors are implicated in the regulation of a variety of intracellular signaling pathways in striatopallidal output neurons and are linked to the pathogenesis of various neuropsychiatric and neurological disorders. This study investigated the possible role of A_2A receptors in the modulation of the Src family kinase (SFK) in the adult rat striatum. In acutely prepared striatal slices, adding the A_2A receptor agonist PSB-0777 induced a significant increase in phosphorylation of SFKs at a conserved autophosphorylation site (Y416) in the caudate putamen (CPu). This increase was also seen in the nucleus accumbens (NAc). Another A_2A agonist CGS-21680 showed the similar ability to elevate SFK Y416 phosphorylation in the striatum. Treatment with the A_2A receptor antagonist KW-6002 blocked the effect of PSB-0777 on SFK Y416 phosphorylation. In addition, PSB-0777 enhanced kinase activity of two key SFK members (Src and Fyn) immunoprecipitated from the striatum. These data demonstrate a positive linkage from A_2A receptors to the SFK signaling pathway in striatal neurons. Activation of A_2A receptors leads to the upregulation of phosphorylation of SFKs (Src and Fyn) at an activation-associated autophosphorylation site and kinase activity of these SFK members.

Repeated exposure to cocaine during adolescence enhances the rewarding threshold for cocaine-conditioned place preference in adulthood

Previous studies have shown that adolescent exposure to cocaine increases drug use in adulthood, albeit incubation of cocaine seeking was found to be attenuated in rats trained to self-administer cocaine during adolescence. We here hypothesize that adolescent exposure to cocaine could alter the rewarding properties of the psychostimulant in adulthood. By employing two of the most widely used animal-experimental-preclinical models to investigate drug addiction, we evaluated whether contingent versus non-contingent cocaine self-administration during adolescence modulates its rewarding threshold in adulthood evaluated by conditioned place preference (CPP). Cocaine self-administration during adolescence increases the rewarding threshold in adulthood; CPP for cocaine was observed at the higher (20 mg/kg), but not at the lower (10 mg/kg), dose employed. Rats exposed to either contingent or non-contingent cocaine during adolescence exhibited the same behavior in the CPP paradigm suggesting that, under our experimental conditions, cocaine rewarding properties are shaped by the psychostimulant itself and not by its motivational effects. From a mechanistic standpoint, the preference for the 20 mg/kg cocaine-paired side in a CPP paradigm appears to depend, at least partially, upon the formation of GluA2-lacking Ca²⁺ -permeable AMPA receptors and the consequent increase of αCaMKII activity in the NAc, both of which are instead reduced when the 10 mg/kg dose was used. In conclusion, contingent or non-contingent cocaine exposure during adolescence desensitizes adult animals to a rewarding dose of cocaine (10 mg/kg) elevating the rewarding threshold necessary (20 mg/kg) to drive conditioned place preference, an effect that may predispose to higher consumption of cocaine during adulthood.

Sex-specific role for prefrontal cortical protein interacting with C kinase 1 in cue-induced cocaine seeking

Disruption of prefrontal glutamate receptor interacting protein (GRIP), which anchors GluA2-containing AMPA receptors (AMPARs) into the synaptic membrane, potentiates cue-induced cocaine seeking in both males and females. Protein interacting with C kinase 1 (PICK1) plays an opposing role to that of GRIP, removing AMPARs from the synapse. Consistent with our hypothesis that disruption of PICK1 in the mPFC would lead to a decrease in addiction-like behaviour, we found that conditional deletion of PICK1 in the mPFC attenuates cue-induced cocaine seeking in male mice. However, prefrontal PICK1 deletion had the opposite effect in females, leading to an increase in cue-induced reinstatement of cocaine seeking. We did not see any effects of PICK1 knockdown on sucrose taking or seeking, suggesting the sex-specific effects do not generalise to natural reinforcers. These findings suggest the role of PICK1 in the prefrontal cortex of females may not be consistent with its accepted role in males. To determine whether these sex differences were influenced by gonadal hormones, we gonadectomised a cohort of males and found that removal of circulating androgens eliminated the effect of prefrontal PICK1 knockdown. As there was no effect of gonadectomy on its own on any of the behavioural measures collected, our results suggest that androgens may be involved in compensatory downstream effects of PICK1 knockdown. Taken together, these results highlight the need for consideration of sex as a biological variable when examining mechanisms underlying all behaviours, as convergent sex differences can reveal different mechanisms where behavioural sex differences do not exist.

Contribution of AMPA Receptor-Mediated LTD in LA/BLA-CeA Pathway to Comorbid Aversive and Depressive Symptoms in Neuropathic Pain

Comorbid anxiety and depressive symptoms in chronic pain are a common health problem, but the underlying mechanisms remain unclear. Previously, we have demonstrated that sensitization of the CeA neurons via decreased GABAergic inhibition contributes to anxiety-like behaviors in neuropathic pain rats. In this study, by using male Sprague Dawley rats, we reported that the CeA plays a key role in processing both sensory and negative emotional-affective components of neuropathic pain. Bilateral electrolytic lesions of CeA, but not lateral/basolateral nucleus of the amygdala (LA/BLA), abrogated both pain hypersensitivity and aversive and depressive symptoms of neuropathic rats induced by spinal nerve ligation (SNL). Moreover, SNL rats showed structural and functional neuroplasticity manifested as reduced dendritic spines on the CeA neurons and enhanced LTD at the LA/BLA-CeA synapse. Disruption of GluA2-containing AMPAR trafficking and endocytosis from synapses using synthetic peptides, either pep2-EVKI or Tat-GluA2(3Y), restored the enhanced LTD at the LA/BLA-CeA synapse, and alleviated the mechanical allodynia and comorbid aversive and depressive symptoms in neuropathic rats, indicating that the endocytosis of GluA2-containing AMPARs from synapses is probably involved in the LTD at the LA/BLA-CeA synapse and the comorbid aversive and depressive symptoms in neuropathic pain in SNL-operated rats. These data provide a novel mechanism for elucidating comorbid aversive and depressive symptoms in neuropathic pain and highlight that structural and functional neuroplasticity in the amygdala may be important as a promising therapeutic target for comorbid negative emotional-affective disorders in chronic pain.SIGNIFICANCE STATEMENT Several studies have demonstrated the high comorbidity of negative affective disorders in patients with chronic pain. Understanding the affective aspects related to chronic pain may facilitate the development of novel therapies for more effective management. Here, we unravel that the CeA plays a key role in processing both sensory and negative emotional-affective components of neuropathic pain, and LTD at the amygdaloid LA/BLA-CeA synapse mediated by GluA2-containing AMPAR endocytosis underlies the comorbid aversive and depressive symptoms in neuropathic pain. This study provides a novel mechanism for elucidating comorbid aversive and depressive symptoms in neuropathic pain and highlights that structural and functional neuroplasticity in the amygdala may be important as a promising therapeutic target for comorbid negative emotional-affective disorders in chronic pain.

Topiramate-chitosan nanoparticles prevent morphine reinstatement with no memory impairment: Dopaminergic and glutamatergic molecular aspects in rats

Besides their clinical application, chronic misuse of opioids has often been associated to drug addiction due to their addictive properties, underlying neuroadaptations of AMPA glutamate-receptor-dependent synaptic plasticity. Topiramate (TPM), an AMPAR antagonist, has been used to treat psychostimulants addiction, despite its harmful effects on memory. This study aimed to evaluate the effects of a novel topiramate nanosystem on molecular changes related to morphine reinstatement. Rats were previously exposed to morphine in conditioned place preference (CPP) paradigm and treated with topiramate-chitosan nanoparticles (TPM-CS-NP) or non-encapsulated topiramate in solution (S-TPM) during CPP extinction; following memory performance evaluation, they were re-exposed to morphine reinstatement. While morphine-CPP extinction was comparable among all experimental groups, TPM-CS-NP treatment prevented morphine reinstatement, preserving memory performance, which was impaired by both morphine-conditioning and S-TPM treatment. In the NAc, morphine increased D1R, D2R, D3R, DAT, GluA1 and MOR immunoreactivity. It also increased D1R, DAT, GluA1 and MOR in the dorsal hippocampus. TPM-CS-NP treatment decreased D1R, D3R and GluA1 and increased DAT in the NAc, decreasing GluA1 and increasing D2 and DAT in the dorsal hippocampus. Taken together, we may infer that TPM-CS-NP treatment was able to prevent the morphine reinstatement without memory impairment. Therefore, TPM-CS-NP may be considered an innovative therapeutic tool due to its property to prevent opioid reinstatement because it acts modifying both dopaminergic and glutamatergic neurotransmission, which are commonly related to morphine addiction.

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.

Noninvasive Brain Stimulation Rescues Cocaine-Induced Prefrontal Hypoactivity and Restores Flexible Behavior

BACKGROUND

To obtain desirable goals, individuals must predict the outcome of specific choices, use that information to direct appropriate actions, and adjust behavior accordingly in changing environments (behavioral flexibility). Substance use disorders are marked by impairments in behavioral flexibility along with decreased prefrontal cortical function that limits the efficacy of treatment strategies. Restoring prefrontal hypoactivity, ideally in a noninvasive manner, is an intriguing target for improving flexible behavior and treatment outcomes.

METHODS

A behavioral flexibility task was used in Long-Evans male rats (n = 97) in conjunction with electrophysiology, optogenetics, and a novel rat model of transcranial alternating current stimulation (tACS) to examine the prelimbic cortex (PrL) to nucleus accumbens (NAc) core circuit in behavioral flexibility and determine whether tACS can restore cocaine-induced neural and cognitive dysfunction.

RESULTS

Optogenetic inactivation revealed that the PrL-NAc core circuit is necessary for the ability to learn strategies to flexibly shift behavior. Cocaine self-administration history caused aberrant PrL-NAc core neural encoding and deficits in flexibility. Optogenetics that selectively activated the PrL-NAc core pathway prior to learning rescued cocaine-induced cognitive flexibility deficits. Remarkably, tACS prior to learning the task reestablished adaptive signaling in the PrL-NAc circuit and restored flexible behavior in a relatively noninvasive and frequency-specific manner.

CONCLUSIONS

We establish a role of NAc core-projecting PrL neurons in behavioral flexibility and provide a novel noninvasive brain stimulation method in rats to rescue cocaine-induced frontal hypofunction and restore flexible behavior, supporting a role of tACS as a therapeutic to treat cognitive deficits in substance use disorders.

Targeting receptor complexes: a new dimension in drug discovery

Targeting receptor proteins, such as ligand-gated ion channels and G protein-coupled receptors, has directly enabled the discovery of most drugs developed to modulate receptor signalling. However, as the search for novel and improved drugs continues, an innovative approach - targeting receptor complexes - is emerging. Receptor complexes are composed of core receptor proteins and receptor-associated proteins, which have profound effects on the overall receptor structure, function and localization. Hence, targeting key protein-protein interactions within receptor complexes provides an opportunity to develop more selective drugs with fewer side effects. In this Review, we discuss our current understanding of ligand-gated ion channel and G protein-coupled receptor complexes and discuss strategies for their pharmacological modulation. Although such strategies are still in preclinical development for most receptor complexes, they exemplify how receptor complexes can be drugged, and lay the groundwork for this nascent area of research.

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

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

A high-affinity, bivalent PDZ domain inhibitor complexes PICK1 to alleviate neuropathic pain

Maladaptive plasticity involving increased expression of AMPA-type glutamate receptors is involved in several pathologies, including neuropathic pain, but direct inhibition of AMPARs is associated with side effects. As an alternative, we developed a cell-permeable, high-affinity (~2 nM) peptide inhibitor, Tat-P₄ -(C5)₂ , of the PDZ domain protein PICK1 to interfere with increased AMPAR expression. The affinity is obtained partly from the Tat peptide and partly from the bivalency of the PDZ motif, engaging PDZ domains from two separate PICK1 dimers to form a tetrameric complex. Bivalent Tat-P₄ -(C5)₂ disrupts PICK1 interaction with membrane proteins on supported cell membrane sheets and reduce the interaction of AMPARs with PICK1 and AMPA-receptor surface expression in vivo. Moreover, Tat-P₄ -(C5)₂ administration reduces spinal cord transmission and alleviates mechanical hyperalgesia in the spared nerve injury model of neuropathic pain. Taken together, our data reveal Tat-P₄ -(C5)₂ as a novel promising lead for neuropathic pain treatment and expand the therapeutic potential of bivalent inhibitors to non-tandem protein-protein interaction domains.

Administration of a novel high affinity PICK1 PDZ domain inhibitor attenuates cocaine seeking in rats

Protein interacting with C kinase-1 (PICK1) regulates intra-cellular trafficking of GluA2-containing AMPA receptors, a process known to play a critical role in cocaine-seeking behavior. This suggests that PICK1 may represent a molecular target for developing novel pharmacotherapies to treat cocaine craving-induced relapse. Emerging evidence indicates that inhibition of PICK1 attenuates the reinstatement of cocaine-seeking behavior, an animal model of relapse. Here, we show that systemic administration of TAT-P4-(DATC5)2, a novel high-affinity peptide inhibitor of the PICK1 PDZ domain, dose-dependently attenuated the reinstatement of cocaine seeking in rats at doses that did not produce operant learning deficits or suppress locomotor activity. We also show that systemic TAT-P4-(DATC5)2 penetrated the brain where it was visualized in the nucleus accumbens shell. Consistent with these effects, infusions of TAT-P4-(DATC5)2 directly into the accumbens shell reduced cocaine, but not sucrose, seeking. The effects of TAT-P4-(DATC5)2 on cocaine seeking are likely due, in part, to inhibition of PICK1 in medium spiny neurons (MSNs) of the accumbens shell as TAT-P4-(DATC5)2 was shown to accumulate in striatal neurons and bind PICK1. Taken together, these findings highlight a novel role for PICK1 in the reinstatement of cocaine seeking and support future studies examining the efficacy of peptide inhibitors of PICK1 in animal and human models of cocaine relapse.

Understanding Addiction Using Animal Models

Drug addiction is a neuropsychiatric disorder with grave personal consequences that has an extraordinary global economic impact. Despite decades of research, the options available to treat addiction are often ineffective because our rudimentary understanding of drug-induced pathology in brain circuits and synaptic physiology inhibits the rational design of successful therapies. This understanding will arise first from animal models of addiction where experimentation at the level of circuits and molecular biology is possible. We will review the most common preclinical models of addictive behavior and discuss the advantages and disadvantages of each. This includes non-contingent models in which animals are passively exposed to rewarding substances, as well as widely used contingent models such as drug self-administration and relapse. For the latter, we elaborate on the different ways of mimicking craving and relapse, which include using acute stress, drug administration or exposure to cues and contexts previously paired with drug self-administration. We further describe paradigms where drug-taking is challenged by alternative rewards, such as appetitive foods or social interaction. In an attempt to better model the individual vulnerability to drug abuse that characterizes human addiction, the field has also established preclinical paradigms in which drug-induced behaviors are ranked by various criteria of drug use in the presence of negative consequences. Separation of more vulnerable animals according to these criteria, along with other innate predispositions including goal- or sign-tracking, sensation-seeking behavior or impulsivity, has established individual genetic susceptibilities to developing drug addiction and relapse vulnerability. We further examine current models of behavioral addictions such as gambling, a disorder included in the DSM-5, and exercise, mentioned in the DSM-5 but not included yet due to insufficient peer-reviewed evidence. Finally, after reviewing the face validity of the aforementioned models, we consider the most common standardized tests used by pharmaceutical companies to assess the addictive potential of a drug during clinical trials.

The role of PI3K-mediated AMPA receptor changes in post-conditioning of propofol in brain protection

BACKGROUND

We aimed to study the role of amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) glutamate receptor 2 (GluR2) subunit trafficking, and activity changes in short-term neuroprotection provided by propofol post-conditioning. We also aimed to determine the role of phosphoinositide-3-kinase (PI3K) in the regulation of these processes.

METHODS

Rats underwent 1 h of focal cerebral ischemia followed by 23 h of reperfusion were randomly divided into 6 groups (n = 36 per group): sham- operation (S), ischemia-reperfusion (IR), propofol (P group, propofol 20 mg/kg/h at the onset of reperfusion for 2 h after 60 min of occlusion), and LY294002 (PI3K non-selective antagonist) + sham (L + S, LY294002 of 1.5 mg/kg was infused 30 min before sham operation), LY294002+ ischemia-reperfusion (L + IR, LY294002 of 1.5 mg/kg was infused 30 min before middle cerebral artery occlusion), LY294002 + IR + propofol (L + P, LY294002 of 1.5 mg/kg was infused 30 min before middle cerebral artery occlusion and propofol 20 mg/kg/h at the onset of reperfusion for 2 h after 60 min of occlusion).

RESULTS

Compared with group IR, rats in group P had significant lower neurologic defect scores and infarct volume. Additionally, consistent with enhanced expression of PI3K-AMPAR GluR2 subunit complex substances in ipsilateral hippocampus, GluR2 subunits showed increased levels in both the plasma and postsynaptic membranes of neurons, while pGluR2 expression was reduced in group P. Furthermore, LY294002, the PI3K non-selective antagonist, blocked those effects.

CONCLUSION

These observations demonstrated that propofol post-conditioning revealed acute neuroprotective role against transient MCAO in rats. The short-term neuroprotective effect was contributed by enhanced GluR2 subunits trafficking to membrane and postsynaptic membranes of neurons, as well as down-regulated the expression of pGluR2 in damaged hippocampus. Finally, the above-mentioned protective mechanism might be contributed by increased combination of PI3K to AMPAR GluR2 subunit, thus maintained the expression and activation of AMPAR GluR2 in the ipsilateral hippocampus.

Reversing Cocaine-Induced Plasticity with Zeta Inhibitory Peptide

Cocaine-induced plasticity persists during abstinence and is thought to underlie cue-evoked craving. Reversing this plasticity could provide an opportunity for therapeutic intervention. Converging evidence suggest that zeta inhibitory peptide (ZIP) eliminates memories for experience-dependent behaviors, including conditioned drug associations. However, the effect of ZIP on reward seeking and drug-induced plasticity is unknown. The current study examined the effect of ZIP administration in the nucleus accumbens on reinstatement (RI) of cocaine seeking, a rodent model of relapse. We demonstrate that intra-accumbal ZIP administration blocks cocaine-primed RI in rats when administered 24 h or 1 week before testing. These effects of ZIP on drug seeking are specific, as we did not see any effect of ZIP on RI of sucrose seeking. ZIP is a synthetic compound designed to inhibit the atypical PKC, PKMζ, a protein implicated in learning and memory. However, recent evidence from PKMζ-knock-out (KO) mice suggests that ZIP may function through alternative mechanisms. In support of this, we found that ZIP was able to block cue-induced RI in PKMζ-KO mice. One possible mechanism underlying addictive phenotypes is the ability of cocaine to block further plasticity. We hypothesized that ZIP may be working to reverse this anaplasticity. Although ZIP has no effect on accumbal LTD in slices from naive or yoked saline mice, it is able to restore both NMDA-dependent and mGluR5-dependent LTD in animals after cocaine self-administration and withdrawal. These findings demonstrate that intra-accumbal ZIP persistently reverses cocaine-induced behavioral and synaptic plasticity in male and female rodents.SIGNIFICANCE STATEMENT Zeta-inhibitory peptide (ZIP) has been shown to disrupt memory maintenance for experience-dependent behaviors. We examined the effect of ZIP infused into the nucleus accumbens on the reinstatement (RI) of cocaine seeking. We found that intra-accumbal ZIP blocked RI of cocaine seeking 24 h and 1 week later. This effect was specific to RI of cocaine seeking as ZIP did not disrupt RI of food seeking. In conjunction with these behavioral studies we examined the ability of ZIP to reverse cocaine-induced deficits in LTD. We found that ZIP was able to rescue two forms of LTD in cocaine-experienced mice. These studies demonstrate that ZIP is able to reverse cocaine-induced behavioral and synaptic plasticity in a persistent manner.

A potential role for microglia in stress- and drug-induced plasticity in the nucleus accumbens: A mechanism for stress-induced vulnerability to substance use disorder

Stress is an important risk factor for the development of substance use disorder (SUD). Exposure to both stress and drugs abuse lead to changes in synaptic plasticity and stress-induced alterations in synaptic plasticity may contribute to later vulnerability to SUD. Recent developmental neuroscience studies have identified microglia as regulators of synaptic plasticity. As both stress and drugs of abuse lead to microglial activation, we propose this as a potential mechanism underlying their ability to change synaptic plasticity. This review focuses on three components of synaptic plasticity: spine density, brain-derived neurotrophic factor (BDNF) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor expression. Their roles in addiction, stress, and development will be reviewed, as well as possible mechanisms by which microglia could regulate their function. Potential links between stress, vulnerability to addiction, and microglial activity will be explored.

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

Despite evidence that morphine-related pathologies reflect adaptations in NAc glutamate signaling, substantial gaps in basic information remain. The current study examines the impact of non-contingent acute, repeated, and withdrawal-inducing morphine dosing regimens on glutamate transmission in D1- or D2-MSNs in the nucleus accumbens shell (NAcSh) and core (NAcC) sub-regions in hopes of identifying excitatory plasticity that may contribute to unique facets of opioid addiction-related behavior. Following an acute morphine injection (10 mg/kg), average miniature excitatory postsynaptic current (mEPSC) amplitude mediated by AMPA-type glutamate receptors was increased at D1-MSNs in the both the NAcShl and NAcC, whereas only the frequency of events was elevated at D2-MSNs in the NAcSh. In contrast, spontaneous somatic withdrawal induced by escalating dose of repeated morphine twice per day (20, 40, 60, 80, 100 mg/kg) enhanced mEPSC frequency specifically at D2-MSNs in the NAcSh. Similar to previous findings, excitatory drive was elevated at NAcSh D1-MSNs after 10-14 days home cage abstinence. Following abstinence, an acute drug re-exposure produced a rapid and enduring endocytosis of GluA2-containing AMPARs at D1-MSNs in the shell, that when blocked by an intra-NAc shell infusion of the Tat-GluA23Y peptide, increased reinstatement of morphine place preference-a phenomenon distinctly different than effects previously found with cocaine. The present study is the first to directly identify unique circuit specific adaptations in NAc glutamate synaptic transmission associated with morphine-related acute reward and somatic withdrawal as well as post-abstinence short-term plasticity. Moreover, while differing classes of abused drugs (i.e., psychostimulants and opioids) produce seemingly similar bidirectional plasticity in the NAc following drug re-exposure, our findings indicate this plasticity has distinct behavioral consequences.

PDZ Domains as Drug Targets

Protein-protein interactions within protein networks shape the human interactome, which often is promoted by specialized protein interaction modules, such as the postsynaptic density-95 (PSD-95), discs-large, zona occludens 1 (ZO-1) (PDZ) domains. PDZ domains play a role in several cellular functions, from cell-cell communication and polarization, to regulation of protein transport and protein metabolism. PDZ domain proteins are also crucial in the formation and stability of protein complexes, establishing an important bridge between extracellular stimuli detected by transmembrane receptors and intracellular responses. PDZ domains have been suggested as promising drug targets in several diseases, ranging from neurological and oncological disorders to viral infections. In this review, the authors describe structural and genetic aspects of PDZ-containing proteins and discuss the current status of the development of small-molecule and peptide modulators of PDZ domains. An overview of potential new therapeutic interventions in PDZ-mediated protein networks is also provided.

Glutamate receptor interacting protein acts within the prefrontal cortex to blunt cocaine seeking

Glutamate receptor interacting protein (GRIP) is a neuronal scaffolding protein that anchors GluA2-containing AMPA receptors to the cell membrane. GRIP plays a critical role in activity-dependent synaptic plasticity, including that which occurs after drug exposure. Given that cocaine administration alters glutamate receptor trafficking within the prefrontal cortex (PFC), a better understanding of the role of receptor trafficking proteins could lead to a more complete understanding of addictive phenotypes. AMPA receptor trafficking in general, and GRIP specifically, is known to play a role in cocaine seeking and conditioned reward in the nucleus accumbens, but its role in the PFC has not been characterized. The current study demonstrates that conditional deletion of GRIP1 in the medial prefrontal cortex increases the motivation for cocaine and potentiates cue-induced reinstatement of cocaine seeking in male and female mice. As no effects of PFC GRIP1 deletion were seen in reinstatement of food seeking, strategy set-shifting, or reversal learning the effects on cocaine seeking are not related to generalized alterations in cognitive function. While disrupting GRIP1 might be expected to lead to decreased AMPA transmission, our electrophysiological data indicate an increase in sEPSC amplitude in the prefrontal cortex and a corresponding decrease in paired pulse facilitation in the nucleus accumbens. Taken together this suggests a strengthening of the PFC to NAc input following prefrontal GRIP1 deletion that may mediate the enhanced drug seeking behavior.

Synaptic Depotentiation and mGluR5 Activity in the Nucleus Accumbens Drive Cocaine-Primed Reinstatement of Place Preference

Understanding the neurobiological processes that incite drug craving and drive relapse has the potential to help target efforts to treat addiction. The NAc serves as a critical substrate for reward and motivated behavior, in part due to alterations in excitatory synaptic strength within cortical-accumbens pathways. The present studies investigated a causal link between cocaine-induced reinstatement of conditioned place preference and rapid reductions of cocaine-dependent increases in NAc shell synaptic strength in male mice. Cocaine-conditioned place preference behavior and ex vivo whole-cell electrophysiology showed that cocaine-primed reinstatement and synaptic depotentiation were disrupted by inhibiting AMPAR internalization via intra-NAc shell infusion of a Tat-GluA23Y peptide. Furthermore, reinstatement was driven by an mGluR5-dependent reduction in AMPAR signaling. Intra-NAc shell infusion of the mGluR5 antagonist MTEP blocked cocaine-primed reinstatement and corresponding depotentiation, whereas infusion of the mGluR5 agonist CHPG itself promoted reinstatement and depotentiated synaptic strength in the NAc shell. Optogenetic examination of circuit-specific plasticity showed that inhibition of infralimbic cortical input to the NAc shell blocked cocaine-primed reinstatement, whereas low-frequency stimulation (10 Hz) of this pathway in the absence of cocaine triggered a reduction in synaptic strength akin to that observed with cocaine, and was sufficient to promote reinstatement in the absence of a cocaine challenge. These data support a model in which mGluR5-mediated reduction in GluA2-containing AMPARs at NAc shell synapses receiving input from the infralimbic cortex is a critical factor in triggering reinstatement of cocaine-primed conditioned approach behavior.SIGNIFICANCE STATEMENT These studies identified a sequence of neural events whereby reexposure to cocaine activates a signaling cascade that alters synaptic strength in the NAc shell and triggers a behavioral response driven by a drug-associated memory.

Amphetamine activates non-receptor tyrosine kinase Fyn and stimulates ERK phosphorylation in the rat striatum in vivo

The psychostimulant amphetamine (AMPH) has an impact on a variety of cellular activities in striatal neurons, although underlying signaling mechanisms are incompletely understood. The Src family kinase (SFK) is among key signaling molecules enriched in striatal neurons and is involved in the regulation of a set of discrete downstream targets. Given the likelihood that AMPH may regulate SFKs, we investigated and characterized the effect of AMPH on SFK phosphorylation and enzymatic activity in rat striatal neurons in vivo. We found that AMPH elevated SFK Y416 phosphorylation in striatal slices and the adult rat striatum. This elevation was concentration- and time-dependent and occurred in all subdivisions of the striatum, including the caudate putamen and nucleus accumbens (core and shell). The dopamine D₁ receptor antagonist SCH23390 blocked the effect of AMPH. Between Fyn and Src, AMPH elevated phosphorylation of immunoprecipitated Fyn but not Src and increased Fyn kinase activity in the striatum. In parallel with SFKs, striatal ERK phosphorylation was increased by AMPH. This increase in ERK phosphorylation was reduced by the SFK inhibitor PP2. These results demonstrate that AMPH is able to activate SFKs (mainly Fyn) in striatal neurons via a D₁ receptor-dependent mechanism. Activated SFKs participate in processing the concomitant ERK response to AMPH.

PKMζ in the nucleus accumbens acts to dampen cocaine seeking

The constitutively active, atypical protein kinase C, protein kinase M-ζ (PKMζ), is exclusively expressed in the brain and its expression increases following exposure to drugs of abuse. However, the limitations of currently available tools have made it difficult to examine the role of PKMζ in cocaine self-administration and relapse. The current study demonstrates that constitutive deletion of PKMζ potentiates cue-induced reinstatement of cocaine seeking and increases both food and cocaine self-administration, without affecting cue-driven food seeking in both male and female mice. Conditional deletion of PKMζ within the nucleus accumbens recapitulated the increase in cocaine taking and seeking seen in the constitutive knockout mice, but only in male animals. Site-specific knockdown of PKMζ in the nucleus accumbens had no effect on cocaine taking or seeking in female mice. Additionally, neither male nor female mice exhibited any alterations in food self-administration or cue-induced reinstatement of food seeking following accumbal deletion of PKMζ. Taken together these results indicate that PKMζ may act to dampen cocaine taking and seeking. Furthermore, these results indicate that PKMζ is playing divergent roles in reward seeking in males and females.

Extinction and Reinstatement of Cocaine-seeking in Self-administering Mice is Associated with Bidirectional AMPAR-mediated Plasticity in the Nucleus Accumbens Shell

Experience-dependent synaptic plasticity is an important component of both learning and motivational disturbances found in addicted individuals. Here, we investigated the role of cocaine experience-dependent plasticity at excitatory synapses in the nucleus accumbens shell (NAcSh) in relapse-related behavior in mice with a history of volitional cocaine self-administration. Using an extinction/reinstatement paradigm of cocaine-seeking behavior, we demonstrate that cocaine-experienced mice with extinguished cocaine-seeking behavior show potentiation of synaptic strength at excitatory inputs onto NAcSh medium spiny neurons (MSNs). Conversely, we found that exposure to various distinct types of reinstating stimuli (cocaine, cocaine-associated cues, yohimbine "stress") after extinction can produce a relative depotentiation of NAcSh synapses that is strongly associated with the magnitude of cocaine-seeking behavior exhibited in response to these challenges. Furthermore, we show that these effects are due to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-specific mechanisms that differ depending on the nature and context of the reinstatement-inducing stimuli. Together, our findings identify common themes as well as differential mechanisms that are likely important for the ability of diverse environmental stimuli to drive relapse to addictive-like cocaine-seeking behavior.

PICK1-Deficient Mice Exhibit Impaired Response to Cocaine and Dysregulated Dopamine Homeostasis

Protein interacting with C-kinase 1 (PICK1) is a widely expressed scaffold protein known to interact via its PSD-95/discs-large/ZO-1 (PDZ)-domain with several membrane proteins including the dopamine (DA) transporter (DAT), the primary target for cocaine's reinforcing actions. Here, we establish the importance of PICK1 for behavioral effects observed after both acute and repeated administration of cocaine. In PICK1 knock-out (KO) mice, the acute locomotor response to a single injection of cocaine was markedly attenuated. Moreover, in support of a role for PICK1 in neuroadaptive changes induced by cocaine, we observed diminished cocaine intake in a self-administration paradigm. Reduced behavioral effects of cocaine were not associated with decreased striatal DAT distribution and most likely not caused by the ∼30% reduction in synaptosomal DA uptake observed in PICK1 KO mice. The PICK1 KO mice demonstrated preserved behavioral responses to DA receptor agonists supporting intact downstream DA receptor signaling. Unexpectedly, we found a prominent increase in striatal DA content and levels of striatal tyrosine hydroxylase (TH) in PICK1 KO mice. Chronoamperometric recordings showed enhanced DA release in PICK1 KO mice, consistent with increased striatal DA pools. Viral-mediated knock-down (KD) of PICK1 in cultured dopaminergic neurons increased TH expression, supporting a direct cellular effect of PICK1. In summary, in addition to demonstrating a key role of PICK1 in mediating behavioral effects of cocaine, our data reveal a so far unappreciated role of PICK1 in DA homeostasis that possibly involves negative regulation of striatal TH levels.

Opioid and Psychostimulant Plasticity: Targeting Overlap in Nucleus Accumbens Glutamate Signaling

Commonalities in addictive behavior, such as craving, stimuli-driven drug seeking, and a high propensity for relapse following abstinence, have pushed for a unified theory of addiction that encompasses most abused substances. This unitary theory has recently been challenged - citing distinctions in structural neural plasticity, biochemical signaling, and neural circuitry to argue that addiction to opioids and psychostimulants is behaviorally and neurobiologically distinct. Recent more selective examination of drug-induced plasticity has highlighted that these two drug classes promote an overall reward circuitry signaling overlap through modifying excitatory synapses in the nucleus accumbens - a key constituent of the reward system. We discuss adaptations in presynaptic/postsynaptic and extrasynaptic glutamate signaling produced by opioids and psychostimulants, and their relevance to circuit remodeling and addiction-related behavior - arguing that these core neural adaptations are important targets for developing pharmacotherapies to treat addiction to multiple drugs.

Uridine attenuates morphine-induced conditioned place preference and regulates glutamate/GABA levels in mPFC of mice

Several lines of evidence suggest that uridine, as a neuromodulator, plays an important role in drug addiction. We previously found that uridine circumvents morphine-induced behavioral sensitization by decreasing the extracellular dopamine levels in the dorsal striatum. In the present study, the effects of uridine on morphine-induced conditioned place preference (CPP) and the possible roles of glutamate and GABA in the stress-induced reinstatement of CPP were investigated. First, the effects of uridine (1, 10 and 100mg/kg, i.p.) on the four defined phases - acquisition, expression, extinction and reinstatement (drug priming and restraint stress) - of morphine-induced CPP were studied. The results showed that pretreatment with uridine significantly blocked the acquisition and expression phases of CPP. Additionally, uridine also facilitated CPP extinction and inhibited stress-induced CPP reinstatement, although it failed to affect drug-induced CPP reinstatement. Since glutamatergic and GABAergic systems are both involved in CPP reinstatement, the extracellular levels of glutamate and GABA in the mPFC during the stress-induced CPP reinstatement were determined using in vivo microdialysis. The results showed that uridine attenuated the stress-induced glutamate increase in the mPFC without influencing the basal glutamate levels, and increased the levels of extracellular GABA in the mPFC both under normal physiological conditions and after the stress stimulus. Thus, our results indicate that uridine depresses the stress-induced reinstatement of CPP, simultaneously regulating glutamatergic and GABAergic neurotransmission in the mPFC. The present work provides further understanding of the role of uridine in morphine-induced neurobehavioral changes.

Contrasting the Role of xCT and GLT-1 Upregulation in the Ability of Ceftriaxone to Attenuate the Cue-Induced Reinstatement of Cocaine Seeking and Normalize AMPA Receptor Subunit Expression

Long-term treatment with ceftriaxone attenuates the reinstatement of cocaine seeking while increasing the function of the glutamate transporter 1 (GLT-1) and system xC- (Sxc) in the nucleus accumbens core (NAc). Sxc contributes the majority of nonsynaptic extracellular glutamate in the NAc, while GLT-1 is responsible for the majority of glutamate uptake. Here we used antisense to decrease the expression of GLT-1 and xCT (a catalytic subunit of Sxc) to determine the relative importance of both proteins in mediating the ability of ceftriaxone to prevent cue-induced reinstatement of cocaine seeking and normalize glutamatergic proteins in the NAc of rats. Intra-NAc xCT knockdown prevented ceftriaxone from attenuating reinstatement and from upregulating GLT-1 and resulted in increased surface expression of AMPA receptor subunits GluA1 and GluA2. Intra-NAc GLT-1 knockdown also prevented ceftriaxone from attenuating reinstatement and from upregulating xCT expression, without affecting GluA1 and GluA2 expression. In the absence of cocaine or ceftriaxone treatment, xCT knockdown in the NAc increased the expression of both GluA1 and GluA2 without affecting GLT-1 expression while GLT-1 knockdown had no effect. PCR and immunoprecipitation of GLT-1 revealed that ceftriaxone does not upregulate GLT-1 and xCT through a transcriptional mechanism, and their coregulation by ceftriaxone is not mediated by physical interaction. These data support important and distinct roles for xCT and GLT-1 in the actions of ceftriaxone and add to a body of literature finding evidence for coregulation of these transporters. Our results also point to xCT expression and subsequent basal glutamate levels as being a key mediator of AMPA receptor expression in the NAc.SIGNIFICANCE STATEMENT Ceftriaxone attenuates the reinstatement of cocaine, alcohol, and heroin seeking. The mechanism of action of this behavioral effect has been attributed to glutamate transporter 1 (GLT-1) and xCT (a catalytic subunit of Sxc)/Sxc upregulation in the nucleus accumbens core. Here we used an antisense strategy to knock down GLT-1 or xCT in the nucleus accumbens core and examined the behavioral and molecular consequences. While upregulation of both xCT and GLT-1 are essential to the ability of ceftriaxone to attenuate cue-induced reinstatement of cocaine seeking, each protein uniquely affects the expression of other glutamate receptor and transporter proteins. We also report that reducing basal glutamate levels through the manipulation of xCT expression increases the surface expression of AMPA receptor subunits, providing insight to the mechanism by which cocaine alters AMPA surface expression.

Involvement of AMPA/Kainate Glutamate Receptor in the Extinction and Reinstatement of Morphine-Induced Conditioned Place Preference: A Behavioral and Molecular Study

Glutamate receptors in mesolimbic areas such as the nucleus accumbens, ventral tegmental area, prefrontal cortex (PFC), and hippocampus (HIP) are a component of the mechanisms of drug-induced reward and can modulate the firing pattern of dopaminergic neurons in the reward system. In addition, several lines of study have indicated that cAMP response element-binding protein (CREB) and c-fos have important role in morphine-induced conditioned place preference (CPP) induced by drugs of abuse, such as morphine, cocaine, nicotine, and alcohol. Therefore, in the present study, we investigated the changes in phosphorylated CREB (p-CREB) and c-fos induction within the nucleus accumbens (NAc), HIP, and PFC after intracerebroventricular (ICV) administration of different doses of CNQX or vehicle during extinction period or reinstatement of morphine-induced CPP. In all groups, the CPP procedure was done; afterward, the conditioning scores were recorded by Ethovision software. After behavioral test recording, we dissected out the NAc, HIP, and PFC regions and measured the p-CREB/CREB ratio and c-fos level by Western blot analysis. Our results showed that administration of CNQX significantly shortened the extinction of morphine CPP. Besides, ICV microinjection of CNQX following extinction period decreased the reinstatement of morphine CPP in extinguished rats. In molecular section, in treatment group, all mentioned factors were dose-dependently decreased in comparison with vehicle group (DMSO) after ICV microinjection of different doses of CNQX but not in pre-extinction microinjection. These findings suggested that antagonism of AMPA receptor decreased p-CREB/CREB ratio and c-fos level in the PFC, NAc, and HIP. Modulation of the drug memory reconsolidation may be useful for faster extinction of drug-induced reward and attenuation of drug-seeking behavior.

Integrins in synapse regulation

Integrins are a large family of extracellular matrix (ECM) receptors. In the developing and adult brain, many integrins are present at high levels at synapses. The tetrapartite structure of synapses - which comprises presynaptic and postsynaptic neurons, the ECM and glial processes - places synaptic integrins in an excellent position to sense dynamic changes in the synaptic environment and use this information to coordinate further changes in synapse structure and function that will shape neural circuit properties. Recent developments in our understanding of the cellular and physiological roles of integrins, which range from control of neural process outgrowth and synapse formation to regulation of synaptic plasticity and memory, enable us to attempt a synthesis of synaptic integrin function.

The good and bad news about glutamate in drug addiction

In 1998 we published a perspective review describing how drug-induced neuroadaptations might serve towards understanding drug craving. We proposed experimental perspectives to help discern data relevant to long-lasting brain changes, and to distinguish dopamine-related changes that were largely pharmacological from glutamatergic changes that were based on drug-environment associations. These perspectives are embedded in drug abuse research, and the last 18 years has witnessed marked development in understanding addiction-associated corticostriatal glutamate plasticity. Here we propose three new perspectives on how the field might approach integrating and using the emerging data on glutamatergic adaptations. (1) Consider adaptations produced in kind across drug classes as most useful towards understanding shared characteristics of addiction, such as relapse. (2) Consider how drug-induced changes in glia and the extracellular matrix may contribute to synaptic alterations. (3) Make measurements not only at late withdrawal, but also during drug seeking events to capture transient changes that mediate active drug seeking that are shared across drug classes.

Disrupting GluA2 phosphorylation potentiates reinstatement of cocaine seeking

Addiction is associated with changes in synaptic plasticity mediated, in part, by alterations in the trafficking and stabilization of AMPA receptors at synapses within the nucleus accumbens. Exposure to cocaine can lead to protein kinase C-mediated phosphorylation of GluA2 AMPA subunits and this phosphorylation event leads to the internalization of GluA2-containing AMPARs, which are calcium-impermeable. However, it is not clear whether this internalization is necessary for the expression of addictive phenotypes. Utilizing a mouse with a point mutation within the GluA2 subunit c-terminus, the current study demonstrates that disrupting PKC-mediated GluA2 phosphorylation potentiates reinstatement of both cue-induced cocaine seeking and cocaine conditioned reward without affecting operant learning, food self-administration or cocaine sensitization. Electrophysiological recordings revealed increased GluA2-mediated AMPA transmission as evidenced by increased sEPSC amplitude without any changes in sEPSC frequency or rectification. In support of this increase in GluA2 activity mediating the augmented cocaine reinstatement, we found that accumbal overexpression of GluA2 recapitulated this behavioral effect in wildtype mice while not altering reinstatement behavior in the GluA2 K882A knock-in mice. In addition, disrupting GluA2 phosphorylation was associated with blunted long-term depression in the nucleus accumbens, mimicking the anaplasticity seen following cocaine self-administration. Taken together these results indicate that disrupting GluA2 phosphorylation and increasing GluA2-mediated transmission in the nucleus accumbens leads to increased vulnerability to cocaine relapse. Further, these results indicate that modulating GluA2-containing AMPAR trafficking can contribute to addictive phenotypes in the absence of alterations in GluA2-lacking receptors. These results highlight the GluA2 phosphorylation site as a novel target for the development of cocaine addiction therapeutics.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

Signals from the Fourth Dimension Regulate Drug Relapse

Despite the enormous societal burden of alcohol and drug addiction and abundant research describing drug-induced maladaptive synaptic plasticity, there are few effective strategies for treating substance use disorders. Recent awareness that synaptic plasticity involves astroglia and the extracellular matrix is revealing new possibilities for understanding and treating addiction. We first review constitutive corticostriatal adaptations that are elicited by and shared between all abused drugs from the perspective of tetrapartite synapses, and integrate recent discoveries regarding cell type-specificity in striatal neurons. Next, we describe recent discoveries that drug-seeking is associated with transient synaptic plasticity that requires all four synaptic elements and is shared across drug classes. Finally, we prognosticate how considering tetrapartite synapses can provide new treatment strategies for addiction.

A Critical Role for the GluA1 Accessory Protein, SAP97, in Cocaine Seeking

A growing body of evidence indicates that the transport of GluA1 subunit-containing calcium-permeable AMPA receptors (CP-AMPARs) to synapses in subregions of the nucleus accumbens promotes cocaine seeking. Consistent with these findings, the present results show that administration of the CP-AMPAR antagonist, Naspm, into the caudal lateral core or caudal medial shell of the nucleus accumbens attenuated cocaine priming-induced reinstatement of drug seeking. Moreover, viral-mediated overexpression of 'pore dead' GluA1 subunits (via herpes simplex virus (HSV) GluA1-Q582E) in the lateral core or medial shell attenuated the reinstatement of cocaine seeking. The overexpression of wild-type GluA1 subunits (via HSV GluA1-WT) in the medial shell, but not the lateral core, enhanced the reinstatement of cocaine seeking. These results indicate that activation of GluA1-containing AMPARs in subregions of the nucleus accumbens reinstates cocaine seeking. SAP97 and 4.1N are proteins involved in GluA1 trafficking to and stabilization in synapses; SAP97-GluA1 interactions also influence dendritic growth. We next examined potential roles of SAP97 and 4.1N in cocaine seeking. Viral-mediated expression of a microRNA that reduces SAP97 protein expression (HSV miSAP97) in the medial accumbens shell attenuated cocaine seeking. In contrast, a virus that overexpressed a dominant-negative form of a 4.1N C-terminal domain (HSV 4.1N-CTD), which prevents endogenous 4.1N binding to GluA1 subunits, had no effect on cocaine seeking. These results indicate that the GluA1 subunit accessory protein SAP97 may represent a novel target for pharmacotherapeutic intervention in the treatment of cocaine craving.

ADAR2-dependent GluA2 editing regulates cocaine seeking

Activation of AMPA receptors (AMPARs) in the nucleus accumbens is necessary for the reinstatement of cocaine-seeking behavior, an animal model of drug craving and relapse. AMPARs are tetrameric protein complexes that consist of GluA1-4 subunits, of which GluA2 imparts calcium permeability. Adenosine deaminase acting on RNA 2 (ADAR2) is a nuclear enzyme that is essential for editing GluA2 pre-mRNA at Q/R site 607. Unedited GluA2(Q) subunits form calcium-permeable AMPARs (CP-AMPARs), whereas edited GluA2(R) subunits form calcium-impermeable channels (CI-AMPARs). Emerging evidence suggests that the reinstatement of cocaine seeking is associated with increased synaptic expression of CP-AMPARs in the nucleus accumbens. However, the role of GluA2 Q/R site editing and ADAR2 in cocaine seeking is unclear. In the present study, we investigated the effects of forced cocaine abstinence on GluA2 Q/R site editing and ADAR2 expression in the nucleus accumbens. Our results demonstrate that 7 days of cocaine abstinence is associated with decreased GluA2 Q/R site editing and reduced ADAR2 expression in the accumbens shell, but not core, of cocaine-experienced rats compared with yoked saline controls. To examine the functional significance of ADAR2 and GluA2 Q/R site editing in cocaine seeking, we used viral-mediated gene delivery to overexpress ADAR2b in the accumbens shell. Increased ADAR2b expression in the shell attenuated cocaine priming-induced reinstatement of drug seeking and was associated with increased GluA2 Q/R site editing and surface expression of GluA2-containing AMPARs. Taken together, these findings support the novel hypothesis that an increased contribution of accumbens shell CP-AMPARs containing unedited GluA2(Q) promotes cocaine seeking. Therefore, CP-AMPARs containing unedited GluA2(Q) represent a novel target for cocaine addiction pharmacotherapies.

Gender differences in spatial learning, synaptic activity, and long-term potentiation in the hippocampus in rats: molecular mechanisms

In tests of spatial ability, males outperform females both in rats and in humans. The mechanism underlying this gender differential learning ability and memory in spatial tasks remains unknown. Long-term potentiation (LTP) in the hippocampus is considered the basis for spatial learning and memory. The aims of this work were (a) to assess spatial learning and memory in male and female rats in the radial and Morris mazes; (b) to assess whether basal synaptic activity and LTP in the hippocampus are different in male and female rats; and (c) to identify the molecular mechanisms responsible for the gender differences in LTP. We analyzed in young male and female rats (a) performance in spatial tasks in the radial and Morris water mazes; (b) basal synaptic activity in hippocampal slices; and (c) LTP and some mechanisms modulating its magnitude. The results reported allow us to conclude that female rats show larger AMPA receptor-mediate synaptic responses under basal conditions, likely due to enhanced phosphorylation of GluR2 in Ser880 and increased amounts of GluR2-containing AMPA receptors in postsynaptic densities. In contrast, the magnitude of tetanus-induced LTP was lower in females than in males. This is due to reduced activation of soluble guanylate cyclase and the formation of cGMP, leading to lower activation of cGMP-dependent protein kinase and phosphorylation of GluR1 in Ser845, which results in lower insertion of AMPA receptors in the synaptic membrane and a lower magnitude of LTP. These mechanisms may contribute to the reduced performance of females in the radial and Morris water mazes.

A novel UCS memory retrieval-extinction procedure to inhibit relapse to drug seeking

We recently reported that a conditioned stimulus (CS) memory retrieval-extinction procedure decreases reinstatement of cocaine and heroin seeking in rats and heroin craving in humans. Here we show that non-contingent cocaine or methylphenidate injections (UCS retrieval) 1 h before the extinction sessions decreases cocaine-priming-induced reinstatement, spontaneous recovery, and renewal of cocaine seeking in rats. Unlike the CS-based memory retrieval-extinction procedure, the UCS memory retrieval manipulation decreases renewal and reinstatement of cocaine seeking in the presence of cocaine cues that were not present during extinction training and also decreases cocaine seeking when the procedure commences after 28 days of abstinence. The inhibitory effect of the UCS retrieval manipulation on cocaine-priming-induced reinstatement is mediated by regulation of AMPA-receptor endocytosis in the basolateral amygdala. The UCS memory retrieval-extinction procedure has superior relapse prevention characteristics than the CS memory retrieval-extinction procedure and could be a promising method for decreasing relapse in human addicts.

Cue-based therapies for treating drug addiction have proven to be only partially effective. Here the authors demonstrate a new memory retrieval based treatment protocol for drug addiction that results in long-lasting inhibition of drug seeking behavior in rodents.

Cocaine-seeking is associated with PKC-dependent reduction of excitatory signaling in accumbens shell D2 dopamine receptor-expressing neurons

Stimulation of D1-like dopamine receptors (D1DRs) or D2-like dopamine receptors (D2DRs) in the nucleus accumbens (NAc) shell reinstates cocaine seeking in rats, an animal model of relapse. D2DRs and D1DRs activate protein kinase C (PKC) and recent studies indicate that activation of PKC in the NAc plays an important role in the reinstatement of drug seeking induced by a systemic cocaine priming injection. In the present study, pharmacological inhibition of PKC in the NAc shell attenuated cocaine seeking induced by intra-accumbens shell microinjection of a D2DR agonist, but not a D1DR agonist. D1DRs and D2DRs are primarily expressed on different accumbens medium spiny (MSN) neurons. Neuronal signaling and activity were assessed in these two populations of NAc neurons with transgenic mice expressing fluorescent labels under the control of D1DR and D2DR promoters. Following the extinction of cocaine self-administration, bath application of a PKC inhibitor produced similar effects on single evoked excitatory and inhibitory post-synaptic currents in D1DR- and D2DR-positive MSNs in the NAc shell. However, inhibition of PKC preferentially improved the ability of excitatory, but not inhibitory, synapses to sustain responding to brief train of stimuli specifically in D2DR-positive MSNs. This effect did not appear to involve modulation of presynaptic release mechanisms. Taken together, these findings indicate that the reinstatement of cocaine seeking is at least partially due to D2DR-dependent increases in PKC signaling in the NAc shell, which reduce excitatory synaptic efficacy in D2DR-expressing MSNs.

Episodic sucrose intake during food restriction increases synaptic abundance of AMPA receptors in nucleus accumbens and augments intake of sucrose following restoration of ad libitum feeding

Weight-loss dieting often leads to loss of control, rebound weight gain, and is a risk factor for binge pathology. Based on findings that food restriction (FR) upregulates sucrose-induced trafficking of glutamatergic AMPA receptors to the nucleus accumbens (NAc) postsynaptic density (PSD), this study was an initial test of the hypothesis that episodic "breakthrough" intake of forbidden food during dieting interacts with upregulated mechanisms of synaptic plasticity to increase reward-driven feeding. Ad libitum (AL) fed and FR subjects consumed a limited amount of 10% sucrose, or had access to water, every other day for 10 occasions. Beginning three weeks after return of FR rats to AL feeding, when 24-h chow intake and rate of body weight gain had normalized, subjects with a history of sucrose intake during FR consumed more sucrose during a four week intermittent access protocol than the two AL groups and the group that had access to water during FR. In an experiment that substituted noncontingent administration of d-amphetamine for sucrose, FR subjects displayed an enhanced locomotor response during active FR but a blunted response, relative to AL subjects, during recovery from FR. This result suggests that the enduring increase in sucrose consumption is unlikely to be explained by residual enhancing effects of FR on dopamine signaling. In a biochemical experiment which paralleled the sucrose behavioral experiment, rats with a history of sucrose intake during FR displayed increased abundance of pSer845-GluA1, GluA2, and GluA3 in the NAc PSD relative to rats with a history of FR without sucrose access and rats that had been AL throughout, whether they had a history of episodic sucrose intake or not. A history of FR, with or without a history of sucrose intake, was associated with increased abundance of GluA1. A terminal 15-min bout of sucrose intake produced a further increase in pSer845-GluA1 and GluA2 in subjects with a history of sucrose intake during FR. Generally, neither a history of sucrose intake nor a terminal bout of sucrose intake affected AMPA receptor abundance in the NAc PSD of AL subjects. Together, these results are consistent with the hypothesis, but the functional contribution of increased synaptic incorporation of AMPA receptors remains to be established.

Group I metabotropic glutamate receptor-mediated activation of PKC gamma in the nucleus accumbens core promotes the reinstatement of cocaine seeking

Emerging evidence indicates that type I metabotropic glutamate receptors (mGluRs) in the nucleus accumbens play a critical role in cocaine seeking. The present study sought to determine the role of accumbens core mGluR1, mGluR5 and protein kinase C (PKC) in cocaine priming-induced reinstatement of drug seeking. Here, we show that intra-accumbens core administration of the mGluR1/5 agonist DHPG (250 μM) promoted cocaine seeking in rats. Consistent with these results, administration of an mGluR1 (50.0 μM YM 298198) or mGluR5 (9.0 μM MPEP) antagonist directly into the accumbens core prior to a priming injection of cocaine (10 mg/kg) attenuated the reinstatement of drug seeking. mGluR1/5 stimulation activates a signaling cascade including PKC. Intracore microinjection of PKC inhibitors (10 μM Ro 31-8220 or 30.0 μM chelerythrine) also blunted cocaine seeking. In addition, cocaine priming-induced reinstatement of drug seeking was associated with increased phosphorylation of PKCγ, but not PKCα or PKCβII, in the core. There were no effects of pharmacological inhibition of mGluR1, mGluR5 or PKC in the accumbens core on sucrose seeking. Together, these findings indicate that mGluR1 and mGluR5 activation in the accumbens core promotes cocaine seeking and that these effects are reinforcer specific. Furthermore, stimulation of mGluR1 and mGluR5 in the accumbens core may regulate cocaine seeking, in part, through activation of PKCγ.

Novel RNA modifications in the nervous system: form and function

Modified RNA molecules have recently been shown to regulate nervous system functions. This mini-review and associated mini-symposium provide an overview of the types and known functions of novel modified RNAs in the nervous system, including covalently modified RNAs, edited RNAs, and circular RNAs. We discuss basic molecular mechanisms involving RNA modifications as well as the impact of modified RNAs and their regulation on neuronal processes and disorders, including neural fate specification, intellectual disability, neurodegeneration, dopamine neuron function, and substance use disorders.