Role of mGluR5 neurotransmission in reinstated cocaine-seeking

Blockade of mGluR5 in nucleus accumbens modulates calcium sensor proteins, facilitates extinction, and attenuates reinstated morphine place preference in rats

Numerous findings confirm that the metabotropic glutamate receptors (mGluRs) are involved in the conditioned place preference (CPP) induced by morphine. Here we focused on the role of mGluR5 in the nucleus accumbens (NAc) as a main site of glutamate action on the rewarding effects of morphine. Firstly, we investigated the effects of intra-NAc administrating mGluR5 antagonist 3-((2-Methyl-1,3-thiazol-4-yl) ethynyl) pyridine hydrochloride (MTEP; 1, 3, and 10 μg/μl saline) on the extinction and the reinstatement phase of morphine CPP. Moreover, to determine the downstream signaling cascades of mGluR5 in morphine CPP, the protein levels of stromal interaction molecules (STIM1 and 2) in the NAc and hippocampus (HPC) were measured by western blotting. The behavioral data indicated that the mGluR5 blockade by MTEP at the high doses of 3 and 10 μg facilitated the extinction of morphine-induced CPP and attenuated the reinstatement to morphine in extinguished rats. Molecular results showed that the morphine led to increased levels of STIM proteins in the HPC and increased the level of STIM1 without affecting STIM2 in the NAc. Furthermore, intra-NAc microinjection of MTEP (10 μg) in the reinstatement phase decreased STIM1 in the NAc and HPC and reduced the STIM2 in the HPC. Collectively, our data show that morphine could facilitate brain reward function in part by increasing glutamate-mediated transmission through activation of mGluR5 and modulation of STIM proteins. Therefore, these results highlight the therapeutic potential of mGluR5 antagonists in morphine use disorder.

The importance of translationally evaluating steroid hormone contributions to substance use

Clinically, women appear to be more susceptible to certain aspects of substance use disorders (SUDs). The steroid hormones 17β-estradiol (E2) and progesterone (Pg) have been linked to women-specific drug behaviors. Here, we review clinical and preclinical studies investigating how cycling ovarian hormones affect nicotine-, cocaine-, and opioid-related behaviors. We also highlight gaps in the literature regarding how synthetic steroid hormone use may influence drug-related behaviors. In addition, we explore how E2 and Pg are known to interact in brain reward pathways and provide evidence of how these interactions may influence drug-related behaviors. The synthesis of this review demonstrates the critical need to study women-specific factors that may influence aspects of SUDs, which may play important roles in addiction processes in a sex-specific fashion. It is important to understand factors that impact women's health and may be key to moving the field forward toward more efficacious and individualized treatment strategies.

mGlu5 inhibition in the basolateral amygdala prevents estrous cycle-dependent changes in cue-induced cocaine seeking

Drug associated cues are a common relapse trigger for individuals recovering from cocaine use disorder. Sex and ovarian hormones influence patterns of cocaine use and relapse vulnerability, with studies indicating that females show increased cue-induced craving and relapse vulnerability compared to males. In a rodent model of cocaine craving and relapse vulnerability, cue-induced cocaine seeking behavior following weeks of withdrawal from extended-access cocaine self-administration is higher in females in the estrus stage of the reproductive (estrous) cycle (Estrus Females) compared to both Males and females in all other stages (Non-Estrus Females). However, the neuronal substrates and cellular mechanisms underlying these sex differences is not fully understood. One region that contributes to both sex differences in behavioral responding and cue-induced cocaine seeking is the basolateral amygdala (BLA), while one receptor known to play a critical role in mediating cocaine seeking behavior is metabotropic glutamate receptor 5 (mGlu5). Here we assessed the effects of BLA mGlu5 inhibition following prolonged withdrawal from cocaine self-administration on observed estrous cycle-dependent changes in cue-induced cocaine seeking behavior. We found that BLA microinjections of the mGlu5 antagonist MTEP selectively reduced the enhanced cue-induced cocaine seeking normally observed in Estrus Females while having no effect on cocaine seeking in Males and Non-Estrus Females. These findings identify a unique interaction between cocaine-exposure, estrous cycle fluctuations and BLA mGlu5-dependent transmission on cue-induced cocaine seeking behavior.

The interaction of membrane estradiol receptors and metabotropic glutamate receptors in adaptive and maladaptive estradiol-mediated motivated behaviors in females

Estrogen receptors were initially identified as intracellular, ligand-regulated transcription factors that result in genomic change upon ligand binding. However, rapid estrogen receptor signaling initiated outside of the nucleus was also known to occur via mechanisms that were less clear. Recent studies indicate that these traditional receptors, estrogen receptor α and estrogen receptor β, can also be trafficked to act at the surface membrane. Signaling cascades from these membrane-bound estrogen receptors (mERs) can rapidly alter cellular excitability and gene expression, particularly through the phosphorylation of CREB. A principal mechanism of neuronal mER action has been shown to occur through glutamate-independent transactivation of metabotropic glutamate receptors (mGlu), which elicits multiple signaling outcomes. The interaction of mERs with mGlu has been shown to be important in many diverse functions in females, including driving motivated behaviors. Experimental evidence suggests that a large part of estradiol-induced neuroplasticity and motivated behaviors, both adaptive and maladaptive, occurs through estradiol-dependent mER activation of mGlu. Herein we will review signaling through estrogen receptors, both "classical" nuclear receptors and membrane-bound receptors, as well as estradiol signaling through mGlu. We will focus on how the interactions of these receptors and their downstream signaling cascades are involved in driving motivated behaviors in females, discussing a representative adaptive motivated behavior (reproduction) and maladaptive motivated behavior (addiction).

The Homer1 family of proteins at the crossroad of dopamine-glutamate signaling: An emerging molecular "Lego" in the pathophysiology of psychiatric disorders. A systematic review and translational insight

Once considered only scaffolding proteins at glutamatergic postsynaptic density (PSD), Homer1 proteins are increasingly emerging as multimodal adaptors that integrate different signal transduction pathways within PSD, involved in motor and cognitive functions, with putative implications in psychiatric disorders. Regulation of type I metabotropic glutamate receptor trafficking, modulation of calcium signaling, tuning of long-term potentiation, organization of dendritic spines' growth, as well as meta- and homeostatic plasticity control are only a few of the multiple endocellular and synaptic functions that have been linked to Homer1. Findings from preclinical studies, as well as genetic studies conducted in humans, suggest that both constitutive (Homer1b/c) and inducible (Homer1a) isoforms of Homer1 play a role in the neurobiology of several psychiatric disorders, including psychosis, mood disorders, neurodevelopmental disorders, and addiction. On this background, Homer1 has been proposed as a putative novel target in psychopharmacological treatments. The aim of this review is to summarize and systematize the growing body of evidence on Homer proteins, highlighting the role of Homer1 in the pathophysiology and therapy of mental diseases.

Ovarian Hormones Regulate Nicotine Consumption and Accumbens Glutamatergic Plasticity in Female Rats

Women report greater cigarette cravings during the menstrual cycle phase with higher circulating levels of 17β-estradiol (E2), which is metabolized to estrone (E1). Both E2 and E1 bind to estrogen receptors (ERs), which have been highly studied in the breast, uterus, and ovary. Recent studies have found that ERs are also located on GABAergic medium spiny neurons (MSNs) within the nucleus accumbens core (NAcore). Glutamatergic plasticity in NAcore MSNs is altered following nicotine use; however, it is unknown whether estrogens impact this neurobiological consequence. To test the effect of estrogen on nicotine use, we ovariectomized (OVX) female rats that then underwent nicotine self-administration acquisition and compared them to ovary-intact (sham) rats. The OVX animals then received either sesame oil (vehicle), E2, or E1+E2 supplementation for 4 or 20 d before nicotine sessions. While both ovary-intact and OVX females readily discriminated levers, OVX females consumed less nicotine than sham females. Further, neither E2 nor E1+E2 increased nicotine consumption back to sham levels following OVX, regardless of the duration of the treatment. OVX also rendered NAcore MSNs in a potentiated state following nicotine self-administration, which was reversed by 4 d of systemic E2 treatment. Finally, we found that E2 and E1+E2 increased ERα mRNA in the NAcore, but nicotine suppressed this regardless of hormone treatment. Together, these results show that estrogens regulate nicotine neurobiology, but additional factors may be required to restore nicotine consumption to ovary-intact levels.

Sex differences in cocaine-associated memory: The interplay between CB1, mGluR5, and estradiol

We know surprisingly little about the sex differences in the neurobiology of cocaine addiction, except females are more susceptible to the rewarding effects of cocaine than their male counterparts. Only a handful of recent studies have examined the neurobiology of cocaine-induced conditioned place preference (CPP) memory among female rodents. We contribute to this emerging line of research by documenting sex differences in cocaine-associated memory and illustrating the underlying signaling pathways in five experiments. Rimonabant (Rim), a cannabinoid CB₁ antagonist and inverse agonist, exerted a facilitating effect for low-dose cocaine and an impairing effect for high-dose cocaine CPP memory in male mice, as in our previous study, but not in female mice. Nor did we observe the effect exist among CB₁ knockout male mice, which indicated that the CB₁ receptors played a mediating role. We also found that the metabotropic glutamate receptor 5 (mGluR5) was located in the same signaling pathway as CB₁ in male mice. To clarify the mechanisms behind the sex differences, we used ovariectomized (OVX) female mice with estradiol benzoate (EB) replacement. In the OVX female mice, we showed that Rim-alone and EB-alone, but not Rim-and-EB-combined, facilitated the low-dose cocaine CPP memory. Moreover, 4-hydroxytamoxifen (4-OHT), an estrogen receptor (ER) antagonist, blocked Rim's and EB's facilitating effect. Finally, 2-methyl-6-(phenylethynyl)pyridine (MPEP), an mGluR5 antagonist, partially blocked EB's facilitating effect. In sum, we identified sex-specific effects of Rim on cocaine-induced CPP memory and the respective signaling pathways: mGluR5-CB₁ for male mice and ER-mGluR5-CB₁ for female mice. These findings may have merits for the development of sex-specific treatment for cocaine addiction.

Extinction vs. Abstinence: A Review of the Molecular and Circuit Consequences of Different Post-Cocaine Experiences

The intravenous cocaine self-administration model is widely used to characterize the neurobiology of cocaine seeking. When studies are aimed at understanding relapse to cocaine-seeking, a post-cocaine abstinence period is imposed, followed by “relapse” tests to assess the ability of drug-related stimuli (“primes”) to evoke the resumption of the instrumental response previously made to obtain cocaine. Here, we review the literature on the impact of post-cocaine abstinence procedures on neurobiology, finding that the prelimbic and infralimbic regions of the prefrontal cortex are recruited by extinction training, and are not part of the relapse circuitry when extinction training does not occur. Pairing cocaine infusions with discrete cues recruits the involvement of the NA, which together with the dorsal striatum, is a key part of the relapse circuit regardless of abstinence procedures. Differences in molecular adaptations in the NA core include increased expression of GluN1 and glutamate receptor signaling partners after extinction training. AMPA receptors and glutamate transporters are similarly affected by abstinence and extinction. Glutamate receptor antagonists show efficacy at reducing relapse following extinction and abstinence, with a modest increase in efficacy of compounds that restore glutamate homeostasis after extinction training. Imaging studies in humans reveal cocaine-induced adaptations that are similar to those produced after extinction training. Thus, while instrumental extinction training does not have face validity, its use does not produce adaptations distinct from human cocaine users.

Restoring glutamate homeostasis in the nucleus accumbens via endocannabinoid-mimetic drug prevents relapse to cocaine seeking behavior in rats

Impaired glutamate homeostasis is a key characteristic of the neurobiology of drug addiction in rodent models and contributes to the vulnerability to relapse to drug seeking. Although disrupted astrocytic and presynaptic regulation of glutamate release has been considered to constitute with impaired glutamate homeostasis in rodent model of drug relapse, the involvement of endocannabinoids (eCBs) in this neurobiological process has remained largely unknown. Here, using cocaine self-administration in rats, we investigated the role of endocannabinoids in impaired glutamate homeostasis in the core of nucleus accumbens (NAcore), which was indicated by augmentation of spontaneous synaptic glutamate release, downregulation of metabotropic glutamate receptor 2/3 (mGluR2/3), and mGluR5-mediated astrocytic glutamate release. We found that the endocannabinoid, anandamide (AEA), rather than 2-arachidonoylglycerol elicited glutamate release through presynaptic transient receptor potential vanilloid 1 (TRPV1) and astrocytic cannabinoid type-1 receptors (CB1Rs) in the NAcore of saline-yoked rats. In rats with a history of cocaine self-administration and extinction training, AEA failed to alter synaptic glutamate release in the NAcore, whereas CB1R-mediated astrocytic glutamate release by AEA remained functional. In order to induce increased astrocytic glutamate release via exogenous AEA, (R)-methanandamide (methAEA, a metabolically stable form of AEA) was chronically infused in the NAcore via osmotic pumps during extinction training. Restoration of mGluR2/3 function and mGluR5-mediated astrocytic glutamate release was observed after chronic methAEA infusion. Additionally, priming or cue-induced reinstatement of cocaine seeking was inhibited in methAEA-infused rats. These results demonstrate that enhancing endocannabinoid signaling is a potential pathway to restore glutamate homeostasis and may represent a promising therapeutic strategy for preventing cocaine relapse.

Negative Modulation of the Metabotropic Glutamate Receptor Type 5 as a Potential Therapeutic Strategy in Obesity and Binge-Like Eating Behavior

Obesity is a multifactorial disease, which in turn contributes to the onset of comorbidities, such as diabetes and atherosclerosis. Moreover, there are only few options available for treating obesity, and most current pharmacotherapy causes severe adverse effects, while offering minimal weight loss. Literature shows that metabotropic glutamate receptor 5 (mGluR5) modulates central reward pathways. Herein, we evaluated the effect of VU0409106, a negative allosteric modulator (NAM) of mGluR5 in regulating feeding and obesity parameters. Diet-induced obese C57BL/6 mice were treated for 14 days with VU0409106, and food intake, body weight, inflammatory/hormonal levels, and behavioral tests were performed. Our data suggest reduction of feeding, body weight, and adipose tissue inflammation in mice treated with high-fat diet (HFD) after chronic treatment with VU0409106. Furthermore, a negative modulation of mGluR5 also reduces binge-like eating, the most common type of eating disorder. Altogether, our results pointed out mGluR5 as a potential target for treating obesity, as well as related disorders.

mGlu5 function in the nucleus accumbens core during the incubation of methamphetamine craving

Many studies have demonstrated that negative allosteric modulators (NAM) of metabotropic glutamate receptor 5 (mGlu5) reduce cocaine and methamphetamine seeking in extinction-reinstatement animal models of addiction. Less is known about effects of mGlu5 NAMs in abstinence models, particularly for methamphetamine. We used the incubation of drug craving model, in which cue-induced craving progressively intensifies after withdrawal from drug self-administration, to conduct the first studies of the following aspects of mGlu5 function in the rat nucleus accumbens (NAc) core during abstinence from methamphetamine self-administration: 1) functionality of the major form of synaptic depression in NAc medium spiny neurons, which is induced postsynaptically via mGlu5 and expressed presynaptically via cannabinoid type 1 receptors (CB1Rs), 2) mGlu5 surface expression and physical associations between mGlu5, Homer proteins, and diacylglycerol lipase-α, and 3) the effect of systemic and intra-NAc core administration of the mGlu5 NAM 3-((2-methyl-4-)ethynyl)pyridine (MTEP) on expression of incubated methamphetamine craving. We found that mGlu5/CB1R-dependent synaptic depression was lost during the rising phase of methamphetamine incubation but then recovered, in contrast to its persistent impairment during the plateau phase of incubation of cocaine craving. Furthermore, whereas the cocaine-induced impairment was accompanied by reduced mGlu5 levels and mGlu5-Homer associations, this was not the case for methamphetamine. Systemic MTEP reduced incubated methamphetamine seeking, but also reduced inactive hole nose-pokes and locomotion, while intra-NAc core MTEP had no significant effects. These findings provide the first insight into the role of mGlu5 in the incubation of methamphetamine craving and reveal differences from incubation of cocaine craving.

Dysregulation of Decision Making Related to Metabotropic Glutamate 5, but Not Midbrain D3, Receptor Availability Following Cocaine Self-administration in Rats

BACKGROUND

Compulsive patterns of drug use are thought to be the consequence of drug-induced adaptations in the neural mechanisms that enable behavior to be flexible. Neuroimaging studies have found evidence of robust alterations in glutamate and dopamine receptors within brain regions that are known to be critical for decision-making processes in cocaine-dependent individuals, and these changes have been argued to be the consequence of persistent drug use. The causal relationships among drug-induced alterations, cocaine taking, and maladaptive decision-making processes, however, are difficult to establish in humans.

METHODS

We assessed decision making in adult male rats using a probabilistic reversal learning task and used positron emission tomography with the [¹¹C]-(+)-PHNO and [¹⁸F]FPEB radioligands to quantify regional dopamine D_2/3 and metabotropic glutamate 5 (mGlu5) receptor availability, respectively, before and after 21 days of cocaine or saline self-administration. Tests of motivation and relapse-like behaviors were also conducted.

RESULTS

We found that self-administration of cocaine, but not of saline, disrupted behavior in the probabilistic reversal learning task measured by selective impairments in negative-outcome updating and also increased cortical mGlu5 receptor availability following 2 weeks of forced abstinence. D_2/3 and, importantly, midbrain D₃ receptor availability was not altered following 2 weeks of abstinence from cocaine. Notably, the degree of the cocaine-induced increase in cortical mGlu5 receptor availability was related to the degree of disruption in negative-outcome updating.

CONCLUSIONS

These findings suggest that cocaine-induced changes in mGlu5 signaling may be a mechanism by which disruptions in negative-outcome updating emerge in cocaine-dependent individuals.

Cannabinoids and the endocannabinoid system in reward processing and addiction: from mechanisms to interventions

The last decades have seen a major gain in understanding the action of cannabinoids and the endocannabinoid system in reward processing and the development of addictive behavior. Cannabis-derived psychoactive compounds such as Δ⁹-tetrahydrocannabinol and synthetic cannabinoids directly interact with the reward system and thereby have addictive properties. Cannabinoids induce their reinforcing properties by an increase in tonic dopamine levels through a cannabinoid type 1 (CB₁) receptor–dependent mechanism within the ventral tegmental area. Cues that are conditioned to cannabis smoking can induce drug-seeking responses (ie, craving) by eliciting phasic dopamine events. A dopamine-independent mechanism involved in drug-seeking responses involves an endocannabinoid/glutamate interaction within the corticostriatal part of the reward system. In conclusion, pharmacological blockade of endocannabinoid signaling should lead to a reduction in drug craving and subsequently should reduce relapse behavior in addicted individuals. Indeed, there is increasing preclinical evidence that targeting the endocannabinoid system reduces craving and relapse, and allosteric modulators at CB₁ receptors and fatty acid amide hydrolase inhibitors are in clinical development for cannabis use disorder. Cannabidiol, which mainly acts on CB₁ and CB₂ receptors, is currently being tested in patients with alcohol use disorder and opioid use disorder.


Sex differences in the glutamate system: Implications for addiction

Clinical and preclinical research have identified sex differences in substance use and addiction-related behaviors. Historically, substance use disorders are more prevalent in men than women, though this gap is closing. Despite this difference, women appear to be more susceptible to the effects of many drugs and progress to substance abuse treatment more quickly than men. While the glutamate system is a key regulator of addiction-related behaviors, much of the work implicating glutamate signaling and glutamatergic circuits has been conducted in men and male rodents. An increasing number of studies have identified sex differences in drug-induced glutamate alterations as well as sex and estrous cycle differences in drug seeking behaviors. This review will describe sex differences in the glutamate system with an emphasis on implications for substance use disorders, highlighting the gaps in our current understanding of how innate and drug-induced alterations in the glutamate system may contribute to sex differences in addiction-related behaviors.

Overexpression of ASIC1A in the nucleus accumbens of rats potentiates cocaine-seeking behavior

Acid-sensing ion channels (ASICs) are abundantly expressed in the nucleus accumbens core (NAcore), a region of the mesolimbocortical system that has an established role in regulating drug-seeking behavior. Previous work shows that a single dose of cocaine reduced the AMPA-to-NMDA ratio in Asic1a-/- mice, an effect observed after withdrawal in wild-type mice, whereas ASIC1A overexpression in the NAcore of rats decreases cocaine self-administration. However, whether ASIC1A overexpression in the NAcore alters measures of drug-seeking behavior after the self-administration period is unknown. To examine this issue, the ASIC1A subunit was overexpressed in male Sprague-Dawley rats by injecting them with adeno-associated virus, targeted at the NAcore, after completion of 2 weeks of cocaine or food self-administration. After 21 days of homecage abstinence, rats underwent a cue-/context-driven drug/food-seeking test, followed by extinction training and then drug/food-primed, cued, and cued + drug/food-primed reinstatement tests. The results indicate that ASIC1A overexpression in the NAcore enhanced cue-/context-driven cocaine seeking, cocaine-primed reinstatement, and cued + cocaine-primed reinstatement but had no effect on food-seeking behavior, indicating a selective effect for ASIC1A in the processes underlying extinction and cocaine-seeking behavior.

Endocannabinoid LTD in Accumbal D1 Neurons Mediates Reward-Seeking Behavior

The nucleus accumbens (NAc) plays a key role in drug-related behavior and natural reward learning. Synaptic plasticity in dopamine D1 and D2 receptor medium spiny neurons (MSNs) of the NAc and the endogenous cannabinoid (eCB) system have been implicated in reward seeking. However, the precise molecular and physiological basis of reward-seeking behavior remains unknown. We found that the specific deletion of metabotropic glutamate receptor 5 (mGluR5) in D1-expressing MSNs (D1^miRmGluR5 mice) abolishes eCB-mediated long-term depression (LTD) and prevents the expression of drug (cocaine and ethanol), natural reward (saccharin), and brain-stimulation-seeking behavior. In vivo enhancement of 2-arachidonoylglycerol (2-AG) eCB signaling within the NAc core restores both eCB-LTD and reward-seeking behavior in D1^miRmGluR5 mice. The data suggest a model where the eCB and glutamatergic systems of the NAc act in concert to mediate reward-seeking responses.

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mGluR5-D1-CB1-induced eCB-LTD mediates drugs of abuse and natural reward seeking

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eCB-LTD in D2-MSNs plays no important role in processing of reward-seeking responses

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Loss of eCB-LTD is a consequence of higher MAGL activity and lower CB1R expression

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Acute drug administration stops craving for alternative rewards on following days

Sequential cocaine-alcohol self-administration produces adaptations in rat nucleus accumbens core glutamate homeostasis that are distinct from those produced by cocaine self-administration alone

There are currently no FDA-approved medications to reduce cocaine relapse. The majority of preclinical studies aimed at identifying the neurobiology underlying relapse involve the self-administration of cocaine alone, whereas many, if not a majority, of cocaine users engage in polysubstance use. Here we developed a rat model of sequential cocaine and alcohol self-administration to test the hypothesis that this combination produces distinct neuroadaptations relative to those produced by cocaine alone. Male rats underwent intravenous cocaine self-administration (2 h/day) followed by 6 h access to unsweetened alcohol (20% v/v) for 12 days. After extinction training, we assessed surface expression of the glutamate transporter GLT-1 and glutamate efflux in the nucleus accumbens (NA) core during the reinstatement of cocaine-seeking. We also tested the ability of ceftriaxone to attenuate the reinstatement of cocaine-seeking and assessed reinstatement-induced Fos expression in several regions critical for reinstatement. Alcohol consumption did not alter cocaine intake, nor did access to cocaine alter alcohol consumption. However, we noted significant changes in glutamate homeostasis in the NA core of cocaine + alcohol rats relative to rats consuming cocaine alone, such as increased surface GLT-1 expression and a lack of increase in glutamate efflux during reinstatement of cocaine-seeking. A history of cocaine + alcohol also altered patterns of reinstatement-induced Fos expression. These changes likely account for the inability of ceftriaxone to attenuate cocaine relapse in cocaine + alcohol rats, while it does so in rats consuming only cocaine. As such glutamate neuroadaptations are targeted by medications to reduce cocaine relapse, preclinical models should consider polysubstance use.

mGlu5 in GABAergic neurons modulates spontaneous and psychostimulant-induced locomotor activity

RATIONALE

A role of group I metabotropic glutamate receptor 5 (mGlu5) in regulating spontaneous locomotion and psychostimulant-induced hyperactivity has been proposed.

OBJECTIVES

This study aims to determine if mGlu5 in GABAergic neurons regulates spontaneous or psychostimulant-induced locomotion.

METHODS

We generated mice specifically lacking mGlu5 in forebrain GABAergic neuron by crossing DLX-Cre mice with mGlu5^flox/flox mice to generate DLX-mGlu5 KO mice. The locomotion of adult mice was examined in the open-field assay (OFA) and home cage setting. The effects of the mGlu5 antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP), cocaine, and methylphenidate on acute motor behaviors in DLX-mGlu5 KO and littermate control mice were assessed in OFA. Striatal synaptic plasticity of these mice was examined with field potential electrophysiological recordings.

RESULTS

Deleting mGlu5 from forebrain GABAergic neurons results in failure to induce long-term depression (LTD) in the dorsal striatum and absence of habituated locomotion in both novel and familiar settings. In a familiar environment (home cage), DLX-mGlu5 KO mice were hyperactive. In the OFA, DLX-mGlu5 KO mice exhibited initial hypo-activity, and then gradually increased their locomotion with time, resulting in no habituation response. DLX-mGlu5 KO mice exhibited almost no locomotor response to MPEP (40 mg/kg), while the same dose elicited hyperlocomotion in control mice. The DLX-mGlu5 KO mice also showed reduced hyperactivity response to cocaine, while they retained normal hyperactivity response to methylphenidate, albeit with delayed onset.

CONCLUSION

mGlu5 in forebrain GABAergic neurons is critical to trigger habituation upon the initiation of locomotion as well as to mediate MPEP-induced hyperlocomotion and modulate psychostimulant-induced hyperactivity.

The cognitive cost of reducing relapse to cocaine-seeking with mGlu5 allosteric modulators

RATIONALE

Cocaine use disorder (CUD) remains difficult to treat with no FDA-approved medications to reduce relapse. Antagonism of metabotropic glutamate receptor 5 (mGlu5) has been demonstrated to decrease cocaine-seeking but may also further compromise cognitive function in long-term cocaine users.

OBJECTIVES

Here we assessed the effect of repeated administration of negative or positive allosteric modulators (NAM or PAM) of mGlu5 on both cognitive performance and (context+cue)-primed cocaine-seeking after prolonged abstinence (≥ 45 days).

METHODS

Adult male Sprague-Dawley rats underwent 6 days of short-access (1 h/day) and 12 days of long-access (6 h/day) cocaine self-administration. Rats were then trained and tested in a delayed match-to-sample (DMS) task to establish baseline working memory performance over a 5-day block of testing. Next, rats received daily systemic administration of the mGlu5 NAM 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine hydrochloride (MTEP; 3 mg/kg), the mGlu5 PAM 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB; 30 mg/kg) or vehicle prior to DMS testing during a block of 5 days, followed by a 5-day washout DMS testing block.

RESULTS

MTEP and CDPPB decreased drug-seeking in response to cocaine-associated cues after prolonged abstinence. However, repeated treatment with MTEP impaired working memory, while CDPPB had no effects on performance.

CONCLUSIONS

These results emphasize the relevance of evaluating cognitive function within the context of investigating pharmacotherapies to treat CUD. Further research is needed to determine how two mechanistically different pharmacological compounds can exert the same behavioral effects to reduce cocaine-seeking.

Mechanisms underlying the efficacy of exercise as an intervention for cocaine relapse: a focus on mGlu5 in the dorsal medial prefrontal cortex

RATIONALE

Exercise shows promise as a treatment option for addiction; but in order to prevent relapse, it may need to be introduced early in the course of treatment.

OBJECTIVE

We propose that exercise, by upregulating dorsal medial prefrontal cortex (dmPFC)-nucleus accumbens (NAc) transmission, offsets deficits in pathways targeting glutamate, BDNF, and dopamine during early abstinence, and in doing so, normalizes neuroadaptations that underlie relapse.

METHODS

We compared the effects of exercise (wheel running, 2-h/day) during early (days 1-7), late (days 8-14), and throughout abstinence (days 1-14) to sedentary conditions on cocaine-seeking and gene expression in the dmPFC and NAc core of male rats tested following 24-h/day extended-access cocaine (up to 96 infusions/day) or saline self-administration and protracted abstinence (15 days). Based on these data, we then used site-specific manipulation to determine whether dmPFC metabotropic glutamate receptor5 (mGlu5) underlies the efficacy of exercise.

RESULTS

Exercise initiated during early, but not late abstinence, reduced cocaine-seeking; this effect was strongly associated with dmPFC Grm5 expression (gene encoding mGlu5), and modestly associated with dmPFC Grin1 and Bdnf-IV expression. Activation of mGlu5 in the dmPFC during early abstinence mimicked the efficacy of early-initiated exercise; however, inhibition of these receptors prior to the exercise sessions did not block its efficacy indicating that there may be redundancy in the mechanisms through which exercise reduces cocaine-seeking.

CONCLUSION

These findings indicate that addiction treatments, including exercise, should be tailored for early versus late phases of abstinence since their effectiveness will vary over abstinence due to the dynamic nature of the underlying neuroadaptations.

Withdrawal From Cocaine Self-administration Alters the Regulation of Protein Translation in the Nucleus Accumbens

BACKGROUND

Cue-induced cocaine craving incubates during abstinence from cocaine self-administration. Expression of incubation ultimately depends on elevation of homomeric GluA1 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors in the nucleus accumbens (NAc). This adaptation requires ongoing protein translation for its maintenance. Aberrant translation is implicated in central nervous system diseases, but nothing is known about glutamatergic regulation of translation in the drug-naïve NAc or after incubation.

METHODS

NAc tissue was obtained from drug-naïve rats and from rats after 1 or >40 days of abstinence from extended-access cocaine or saline self-administration. Newly translated proteins were labeled using 35S-Met/Cys or puromycin. We compared basal overall translation and its regulation by metabotropic glutamate receptor 1 (mGlu1), mGlu5, and N-methyl-D-aspartate receptors (NMDARs) in drug-naïve, saline control, and cocaine rats, and we compared GluA1 and GluA2 translation by immunoprecipitating puromycin-labeled proteins.

RESULTS

In all groups, overall translation was unaltered by mGlu1 blockade (LY367385) but increased by mGlu5 blockade (MTEP). NMDAR blockade (AVP) increased overall translation in drug-naïve and saline control rats but not in cocaine/late withdrawal rats. Cocaine/late withdrawal rats exhibited greater translation of GluA1 (but not GluA2), which was not further affected by NMDAR blockade.

CONCLUSIONS

Our results suggest that increased GluA1 translation contributes to the elevated homomeric GluA1 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor levels in the NAc that mediate incubation. Additional contributions to incubation-related plasticity may result from loss of the braking influence on translation normally exerted by NMDARs. Apart from elucidating incubation-related adaptations, we found a suppressive effect of mGlu5 on NAc translation regardless of drug exposure, which is opposite to results obtained in the hippocampus and points to heterogeneity of translational regulation between brain regions.

The effects of ceftriaxone on cue-primed reinstatement of cocaine-seeking in male and female rats: estrous cycle effects on behavior and protein expression in the nucleus accumbens

RATIONALE

Effective pharmacological treatments to prevent cocaine relapse remain elusive. In male rats, ceftriaxone attenuates the reinstatement of cocaine-seeking while increasing glutamate transporter-1 (GLT-1) and xCT expression in the nucleus accumbens core (NAc). Despite reported sex differences in cocaine relapse, these effects have not yet been confirmed in female rats.

OBJECTIVE

We investigated the effects of ceftriaxone on cue-primed reinstatement and cocaine-induced alterations in glutamatergic proteins in the NAc of female rats. Potential interactions between estrous phase and treatment were also assessed.

METHOD

Male and female rats self-administered cocaine in the presence of discrete cues for 12 days, followed by 2-3 weeks of extinction. Ceftriaxone or vehicle was administered daily for a minimum of 6 days immediately preceding a cue-primed reinstatement test.

RESULTS

Total cocaine intake was greater in females than in males, but reinstatement behavior was similar. Ceftriaxone attenuated reinstatement in both sexes and was accompanied by increased expression of GLT-1a and xCT in the NAc. However, ceftriaxone attenuated reinstatement only when females were tested during met-, di-, and proestrus phases and not during estrus. A significant increase in AMPA receptor subunit GluA1 surface expression was also observed during estrus, potentially influencing reinstatement.

CONCLUSION

These findings extend the beneficial effects of ceftriaxone on persistent cocaine-seeking from males to females, increasing its potential as a pharmacological treatment for preventing relapse. The effects of estrus on GluA1 expression and reinstatement observed here indicate that females may need additional interventions during some phases of the menstrual cycle.

mGluR5 antagonism inhibits cocaine reinforcement and relapse by elevation of extracellular glutamate in the nucleus accumbens via a CB1 receptor mechanism

Metabotropic glutamate receptor 5 (mGluR5) antagonism inhibits cocaine self-administration and reinstatement of drug-seeking behavior. However, the cellular and molecular mechanisms underlying this action are poorly understood. Here we report a presynaptic glutamate/cannabinoid mechanism that may underlie this action. Systemic or intra-nucleus accumbens (NAc) administration of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) dose-dependently reduced cocaine (and sucrose) self-administration and cocaine-induced reinstatement of drug-seeking behavior. The reduction in cocaine-taking and cocaine-seeking was associated with a reduction in cocaine-enhanced extracellular glutamate, but not cocaine-enhanced extracellular dopamine (DA) in the NAc. MPEP alone, when administered systemically or locally into the NAc, elevated extracellular glutamate, but not DA. Similarly, the cannabinoid CB1 receptor antagonist, rimonabant, elevated NAc glutamate, not DA. mGluR5s were found mainly in striatal medium-spiny neurons, not in astrocytes, and MPEP-enhanced extracellular glutamate was blocked by a NAc CB1 receptor antagonist or N-type Ca⁺⁺ channel blocker, suggesting that a retrograde endocannabinoid-signaling mechanism underlies MPEP-induced glutamate release. This interpretation was further supported by our findings that genetic deletion of CB1 receptors in CB1-knockout mice blocked both MPEP-enhanced extracellular glutamate and MPEP-induced reductions in cocaine self-administration. Together, these results indicate that the therapeutic anti-cocaine effects of mGluR5 antagonists are mediated by elevation of extracellular glutamate in the NAc via an endocannabinoid-CB1 receptor disinhibition mechanism.

The Winding Road to Relapse: Forging a New Understanding of Cue-Induced Reinstatement Models and Their Associated Neural Mechanisms

In drug addiction, cues previously associated with drug use can produce craving and frequently trigger the resumption of drug taking in individuals vulnerable to relapse. Environmental stimuli associated with drugs or natural reinforcers can become reliably conditioned to increase behavior that was previously reinforced. In preclinical models of addiction, these cues enhance both drug self-administration and reinstatement of drug seeking. In this review, we will dissociate the roles of conditioned stimuli as reinforcers from their modulatory or discriminative functions in producing drug-seeking behavior. As well, we will examine possible differences in neurobiological encoding underlying these functional differences. Specifically, we will discuss how models of drug addiction and relapse should more systematically evaluate these different types of stimuli to better understand the neurobiology underlying craving and relapse. In this way, behavioral and pharmacotherapeutic interventions may be better tailored to promote drug use cessation outcomes and long-term abstinence.

Behavioral and Neurochemical Phenotyping of Mice Incapable of Homer1a Induction

Immediate early and constitutively expressed products of the Homer1 gene regulate the functional assembly of post-synaptic density proteins at glutamatergic synapses to influence excitatory neurotransmission and synaptic plasticity. Earlier studies of Homer1 gene knock-out (KO) mice indicated active, but distinct, roles for IEG and constitutively expressed Homer1 gene products in regulating cognitive, emotional, motivational and sensorimotor processing, as well as behavioral and neurochemical sensitivity to cocaine. More recent characterization of transgenic mice engineered to prevent generation of the IEG form (a.k.a Homer1a KO) pose a critical role for Homer1a in cocaine-induced behavioral and neurochemical sensitization of relevance to drug addiction and related neuropsychiatric disorders. Here, we extend our characterization of the Homer1a KO mouse and report a modest pro-depressant phenotype, but no deleterious effects of the KO upon spatial learning/memory, prepulse inhibition, or cocaine-induced place-conditioning. As we reported previously, Homer1a KO mice did not develop cocaine-induced behavioral or neurochemical sensitization within the nucleus accumbens; however, virus-mediated Homer1a over-expression within the nucleus accumbens reversed the sensitization phenotype of KO mice. We also report several neurochemical abnormalities within the nucleus accumbens of Homer1a KO mice that include: elevated basal dopamine and reduced basal glutamate content, Group1 mGluR agonist-induced glutamate release and high K⁺-stimulated release of dopamine and glutamate within this region. Many of the neurochemical anomalies exhibited by Homer1a KO mice are recapitulated upon deletion of the entire Homer1 gene; however, Homer1 deletion did not affect NAC dopamine or alter K⁺-stimulated neurotransmitter release within this region. These data show that the selective deletion of Homer1a produces a behavioral and neurochemical phenotype that is distinguishable from that produced by deletion of the entire Homer1 gene. Moreover, the data indicate a specific role for Homer1a in regulating cocaine-induced behavioral and neurochemical sensitization of potential relevance to the psychotogenic properties of this drug.

Accumbens Mechanisms for Cued Sucrose Seeking

Many studies support a perspective that addictive drugs usurp brain circuits used by natural rewards, especially for the dopamine-dependent reinforcing qualities of both drugs and natural rewards. Reinstated drug seeking in animal models of relapse relies on glutamate spillover from cortical terminals synapsing in the nucleus accumbens core (NAcore) to stimulate metabotropic glutamate receptor5 (mGluR5) on neuronal nitric oxide synthase (nNOS) interneurons. Contrasting the release of dopamine that is shared by sucrose and drugs of abuse, reinstated sucrose seeking does not induce glutamate spillover. We hypothesized that pharmacologically promoting glutamate spillover in the NAcore would mimic cocaine-induced adaptations and potentiate cued reinstatement of sucrose seeking. Inducing glutamate spillover by blocking astroglial glutamate transporters (GLT-1) had no effect on reinstated sucrose seeking. However, glutamate release probability is negatively regulated by presynaptic mGluR2/3, and sucrose reinstatement was potentiated following mGluR2/3 blockade. Potentiated sucrose reinstatement by mGluR2/3 blockade was reversed by antagonizing mGluR5, but reinstated sucrose seeking in the absence of mGluR2/3 blockade was not affected by blocking mGluR5. In cocaine-trained rodents mGluR5 stimulation reinstates drug seeking by activating nNOS, but activating mGluR5 did not promote reinstated sucrose seeking, nor was potentiated reinstatement after mGluR2/3 blockade reduced by blocking nNOS. However, chemogenetic activation of nNOS interneurons in the NAcore reinstated sucrose seeking. These data indicate that dysregulated presynaptic mGluR2/3 signaling is a possible site of shared signaling in drug seeking and potentiated reinstated sucrose seeking, but that downregulated glutamate transport and subsequent activation of nNOS by synaptic glutamate spillover is not shared.

Corticostriatal plasticity, neuronal ensembles, and regulation of drug-seeking behavior

The idea that interconnected neuronal ensembles code for specific behaviors has been around for decades; however, recent technical improvements allow studying these networks and their causal role in initiating and maintaining behavior. In particular, the role of ensembles in drug-seeking behaviors in the context of addiction is being actively investigated. Concurrent with breakthroughs in quantifying ensembles, research has identified a role for synaptic glutamate spillover during relapse. In particular, the transient relapse-associated changes in glutamatergic synapses on accumbens neurons, as well as in adjacent astroglia and extracellular matrix, are key elements of the synaptic plasticity encoded by drug use and the metaplasticity induced by drug-associated cues that precipitate drug-seeking behaviors. Here, we briefly review the recent discoveries related to ensembles in the addiction field and then endeavor to link these discoveries with drug-induced striatal plasticity and cue-induced metaplasticity toward deeper neurobiological understandings of drug seeking.

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.

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.

Corticostriatal circuitry in regulating diseases characterized by intrusive thinking

Intrusive thinking triggers clinical symptoms in many neuropsychiatric disorders. Using drug addiction as an exemplar disorder sustained in part by intrusive thinking, we explore studies demonstrating that impairments in corticostriatal circuitry strongly contribute to intrusive thinking. Neuroimaging studies have long implicated this projection in cue-induced craving to use drugs, and preclinical models show that marked changes are produced at corticostriatal synapses in the nucleus accumbens during a relapse episode. We delineate an accumbens microcircuit that mediates cue-induced drug seeking becoming an intrusive event. This microcircuit harbors many potential therapeutic targets. We focus on preclinical and clinical studies, showing that administering N-acetylcysteine restores uptake of synaptic glutamate by astroglial glutamate transporters and thereby inhibits intrusive thinking. We posit that because intrusive thinking is a shared endophenotype in many disorders, N-acetylcysteine has positive effects in clinical trials for a variety of neuropsychiatric disorders, including drug addiction, gambling, trichotillomania, and depression.

Cocaine self-administration, extinction training and drug-induced relapse change metabotropic glutamate mGlu5 receptors expression: Evidence from radioligand binding and immunohistochemistry assays

Several behavioral findings highlight the importance of glutamatergic transmission and its metabotropic receptor type 5 (mGlu₅) in the controlling of cocaine reward and seeking behaviors. The molecular or neurochemical nature of such interactions is not well recognized, so in the present paper we determine if cocaine self-administration and extinction/reinstatement models with the yoked triad control procedure alter mGlu₅ receptor density in rats. [³H]MPEP was used to evaluate mGlu₅ receptors density and affinity in selected brain structures, while immunofluorescence analysis was used to detect changes in mGlu₅ receptors' brain location. Cocaine self-administration and yoked cocaine delivery evoked a significant elevation in mGlu₅ receptors' density in the dorsal striatum, while receptor protein expression was importantly elevated in the substantia nigra and reduced in the nucleus accumbens shell. Cocaine administration followed by 10 extinction training sessions resulted in biphasic mGlu₅ receptor density changes in the prefrontal cortex-nucleus accumbens pathway. mGlu₅ receptors' up-regulation was noted for cocaine self-administration and extinction training in the hippocampus and in yoked cocaine controls following drug abstinence in the dorsal striatum. A cocaine priming dose (but not a saline priming) resulted in a significant decrease of mGlu₅ receptors' density in the nucleus accumbens of rats previously treated with the drug and in the hippocampus of rats previously self-administered cocaine. The latter decrease in mGlu₅ receptors' density and protein expression in the hippocampus was parallel to an increase in [³H]MPEP affinity and opposite to a rise observed after single cocaine administration (ip) to drug-naïve yoked saline controls. Additionally, we also observed a significant elevation in the protein expression of the tested receptors in the limbic cortex in both cocaine groups. The present results shown modality dependent and brain-region specific changes in mGlu₅ receptors' localization and membrane specific binding.

Estradiol Facilitation of Cocaine Self-Administration in Female Rats Requires Activation of mGluR5

In comparison to men, women initiate drug use at earlier ages and progress from initial use to addiction more rapidly. This heightened intake and vulnerability to drugs of abuse is regulated in part by estradiol, although the signaling mechanisms by which this occurs are not well understood. Recent findings indicate that within the nucleus accumbens core, estradiol induces structural plasticity via membrane-localized estrogen receptor α, functionally coupled to metabotropic glutamate receptor subtype 5 (mGluR5). Hence, we sought to determine whether mGluR5 activation was essential for estradiol-mediated enhancement of cocaine self-administration. Ovariectomized (OVX) female rats were allowed to freely self-administer cocaine under extended access conditions (6 h/d) for 10 consecutive days. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) or vehicle was administered before estradiol (or oil), on a 2 d on/2 d off schedule throughout the extended access period. MPEP treatment prevented the estradiol-dependent enhancement of cocaine self-administration in OVX females. In a separate experiment, potentiation of mGluR5 function with the positive allosteric modulator 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (in the absence of estradiol treatment) failed to increase cocaine self-administration. These data suggest that mGluR5 activation is necessary for estradiol-mediated enhancement of responses to cocaine, but that direct mGluR5 activation is insufficient to mimic the female response to estradiol. Building on previous studies in male animals, these findings further highlight the therapeutic potential of mGluR5 antagonism in the treatment of addiction and suggest that there may be added therapeutic benefit in females.

Long-Lasting Impairment of mGluR5-Activated Intracellular Pathways in the Striatum After Withdrawal of Cocaine Self-Administration

BACKGROUND

Cocaine addiction continues to be a major heath concern, and despite public health intervention there is a lack of efficient pharmacological treatment options. A newly identified potential target are the group I metabotropic glutamate receptors, with allosteric modulators showing particular promise.

METHODS

We evaluated the capacity of group I metabotropic glutamate receptors to induce functional responses in ex vivo striatal slices from rats with (1) acute cocaine self-administration, (2) chronic cocaine self-administration, and (3) 60 days cocaine self-administration withdrawal by Western blot and extracellular recordings of synaptic transmission.

RESULTS

We found that striatal group I metabotropic glutamate receptors are the principal mediator of the mGluR1/5 agonist (RS)-3,5-dihydroxyphenylglycine-induced cAMP responsive-element binding protein phosphorylation. Both acute and chronic cocaine self-administration blunted group I metabotropic glutamate receptor effects on cAMP responsive-element binding protein phosphorylation in the striatum, which correlated with the capacity to induce long-term depression, an effect that was maintained 60 days after chronic cocaine self-administration withdrawal. In the nucleus accumbens, the principal brain region mediating the rewarding effects of drugs, chronic cocaine self-administration blunted group I metabotropic glutamate receptor stimulation of extracellular signal-regulated protein kinases 1/2 and cAMP responsive-element binding protein. Interestingly, the group I metabotropic glutamate receptor antagonist/inverse-agonist, 2-methyl-6-(phenylethynyl)pyridine hydrochloride, led to a specific increase in cAMP responsive-element binding protein phosphorylation after chronic cocaine self-administration, specifically in the nucleus accumbens, but not in the striatum.

CONCLUSIONS

Prolonged cocaine self-administration, through withdrawal, leads to a blunting of group I metabotropic glutamate receptor responses in the striatum. In addition, specifically in the accumbens, group I metabotropic glutamate receptor signaling to cAMP responsive-element binding protein shifts from an agonist-induced to an antagonist-induced cAMP responsive-element binding protein phosphorylation.

Opposite Effects of mGluR1a and mGluR5 Activation on Nucleus Accumbens Medium Spiny Neuron Dendritic Spine Density

The group I metabotropic glutamate receptors (mGluR1a and mGluR5) are important modulators of neuronal structure and function. Although these receptors share common signaling pathways, they are capable of having distinct effects on cellular plasticity. We investigated the individual effects of mGluR1a or mGluR5 activation on dendritic spine density in medium spiny neurons in the nucleus accumbens (NAc), which has become relevant with the potential use of group I mGluR based therapeutics in the treatment of drug addiction. We found that systemic administration of mGluR subtype-specific positive allosteric modulators had opposite effects on dendritic spine densities. Specifically, mGluR5 positive modulation decreased dendritic spine densities in the NAc shell and core, but was without effect in the dorsal striatum, whereas increased spine densities in the NAc were observed with mGluR1a positive modulation. Additionally, direct activation of mGluR5 via CHPG administration into the NAc also decreased the density of dendritic spines. These data provide insight on the ability of group I mGluRs to induce structural plasticity in the NAc and demonstrate that the group I mGluRs are capable of producing not just distinct, but opposing, effects on dendritic spine density.

The individual and combined effects of phenmetrazine and mgluR2/3 agonist LY379268 on the motivation to self-administer cocaine

BACKGROUND

The US Food and Drug Administration has not approved a treatment for cocaine addiction, possibly due in part to the fact that repeated cocaine use results in dysregulation of multiple neurotransmitter systems, including glutamate and dopamine, and an emergence of increased negative affective states and heightening motivation to take cocaine despite negative consequences. We used a combination therapy approach to assess whether modulation of both glutamate and dopamine transmission would reduce the motivation to self- administer cocaine compared to modulation of either system alone.

METHODS

The metabotropic glutamate 2/3 receptor agonist, LY379268, and the monoamine releaser, phenmetrazine, were used to assess their individual and combined ability to decrease the reinforcing efficacy of cocaine because they modulate glutamate and dopamine levels, respectively. Cocaine breakpoints and cocaine intake was assessed, using a progressive ratio schedule, at baseline in three groups based on dose of cocaine (0.19, 0.38, 0.75mg/kg/infusion), and following LY379268 (0.03 or 0.30mg/kg; i.p.), phenmetrazine (25mg/kg/day; osmotic minipump), and a combination of the two drugs.

RESULTS

LY379268 and phenmetrazine alone reduced breakpoints for all doses of cocaine. The combination of the two drugs showed a concerted effect in reducing breakpoints for all doses of cocaine, with the lowest dose of cocaine reduced by as much as 70%.

CONCLUSIONS

These data support combination therapy of dopamine and glutamate systems as an effective means to reduce the motivation to take cocaine since a combination of drugs can address neurobiological dysfunction in multiple neurotransmitter systems compared to therapies using single drugs.

Protein kinase Cɛ activity regulates mGluR5 surface expression in the rat nucleus accumbens

Type 5 metabotropic glutamate receptors (mGluR5) activate protein kinase C (PKC) via coupling to G_αq/11 protein signaling. We have previously demonstrated that the epsilon isoform of PKC (PKCɛ) is a critical downstream target of mGluR5 in regulating behavioral and biochemical responses to alcohol. Recent evidence suggests that PKC-mediated phosphorylation of mGluR5 can lead to receptor desensitization and internalization. We therefore sought to examine the specific involvement of PKCɛ in the regulation of mGluR5 surface expression in the nucleus accumbens (NAc), a key regulator of alcohol-associated behaviors. Coronal brain sections from male Wistar rats were analyzed for either colocalization of mGluR5 and PKCɛ via immunohistochemistry or changes in mGluR5 surface expression and PKCɛ phosphorylation following local application of PKCɛ translocation activator or inhibitor peptides and/or an orthosteric mGluR5 agonist. We observed colocalization of mGluR5 and PKCɛ in the NAc. We also showed that intra-NAc infusion of the PKCɛ translocation inhibitor ɛV1-2 increased mGluR5 surface expression under baseline conditions. Stimulation of mGluR5 with an orthosteric agonist DHPG, dose dependently increased ERK1/2 and PKCɛ phosphorylation as well as mGluR5 internalization in acute NAc slices. Finally, we observed that activation of PKCɛ translocation with Tat-ΨɛRACK peptide mediates agonist-independent mGluR5 internalization, whereas PKCɛ translocation inhibitor ɛV1-2 prevents agonist-dependent internalization of mGluR5 in NAc slice preparations. These findings suggest that the subcellular localization of mGluR5 in the NAc is regulated by PKCɛ under basal and stimulation conditions, which may influence the role of mGluR5-PKCɛ signaling in alcohol-related behaviors. © 2016 Wiley Periodicals, Inc.

Corticostriatal circuitry in regulating diseases characterized by intrusive thinking

Intrusive thinking triggers clinical symptoms in many neuropsychiatric disorders. Using drug addiction as an exemplar disorder sustained in part by intrusive thinking, we explore studies demonstrating that impairments in corticostriatal circuitry strongly contribute to intrusive thinking. Neuroimaging studies have long implicated this projection in cue-induced craving to use drugs, and preclinical models show that marked changes are produced at corticostriatal synapses in the nucleus accumbens during a relapse episode. We delineate an accumbens microcircuit that mediates cue-induced drug seeking becoming an intrusive event. This microcircuit harbors many potential therapeutic targets. We focus on preclinical and clinical studies, showing that administering N-acetylcysteine restores uptake of synaptic glutamate by astroglial glutamate transporters and thereby inhibits intrusive thinking. We posit that because intrusive thinking is a shared endophenotype in many disorders, N-acetylcysteine has positive effects in clinical trials for a variety of neuropsychiatric disorders, including drug addiction, gambling, trichotillomania, and depression.

Gq-DREADD Selectively Initiates Glial Glutamate Release and Inhibits Cue-induced Cocaine Seeking

BACKGROUND

Glial cells of the central nervous system directly influence neuronal activity by releasing neuroactive small molecules, including glutamate. Long-lasting cocaine-induced reductions in extracellular glutamate in the nucleus accumbens core (NAcore) affect synaptic plasticity responsible for relapse vulnerability.

METHODS

We transduced NAcore astrocytes with an adeno-associated virus vector expressing hM3D designer receptor exclusively activated by a designer drug (DREADD) under control of the glial fibrillary acidic protein promoter in 62 male Sprague Dawley rats, 4 dominant-negative soluble N-ethylmaleimide-sensitive factor attachment protein receptor mice, and 4 wild-type littermates. Using glutamate biosensors, we measured NAcore glutamate levels following intracranial or systemic administration of clozapine N-oxide (CNO) and tested the ability of systemic CNO to inhibit reinstated cocaine or sucrose seeking following self-administration and extinction training.

RESULTS

Administration of CNO in glial fibrillary acidic protein-hM3D-DREADD transfected animals increased NAcore extracellular glutamate levels in vivo. The glial origin of released glutamate was validated by an absence of CNO-mediated release in mice expressing a dominant-negative soluble N-ethylmaleimide-sensitive factor attachment protein receptor variant in glia. Also, CNO-mediated release was relatively insensitive to N-type calcium channel blockade. Systemic administration of CNO inhibited cue-induced reinstatement of cocaine seeking in rats extinguished from cocaine but not sucrose self-administration. The capacity to inhibit reinstated cocaine seeking was prevented by systemic administration of the group II metabotropic glutamate receptor antagonist LY341495.

CONCLUSIONS

DREADD-mediated glutamate gliotransmission inhibited cue-induced reinstatement of cocaine seeking by stimulating release-regulating group II metabotropic glutamate receptor autoreceptors to inhibit cue-induced synaptic glutamate spillover.

Main path and byways: non-vesicular glutamate release by system xc(-) as an important modifier of glutamatergic neurotransmission

System xc(-) is a cystine/glutamate antiporter that exchanges extracellular cystine for intracellular glutamate. Cystine is intracellularly reduced to cysteine, a building block of GSH. As such, system xc(-) can regulate the antioxidant capacity of cells. Moreover, in several brain regions, system xc(-) is the major source of extracellular glutamate. As such this antiporter is able to fulfill key physiological functions in the CNS, while evidence indicates it also plays a role in certain brain pathologies. Since the transcription of xCT, the specific subunit of system xc(-), is enhanced by the presence of reactive oxygen species and inflammatory cytokines, system xc(-) could be involved in toxic extracellular glutamate release in neurological disorders that are associated with increased oxidative stress and neuroinflammation. System xc(-) has also been reported to contribute to the invasiveness of brain tumors and, as a source of extracellular glutamate, could participate in the induction of peritumoral seizures. Two independent reviews (Pharmacol. Rev. 64, 2012, 780; Antioxid. Redox Signal. 18, 2013, 522), approached from a different perspective, have recently been published on the functions of system xc(-) in the CNS. In this review, we highlight novel achievements and insights covering the regulation of system xc(-) as well as its involvement in emotional behavior, cognition, addiction, neurological disorders and glioblastomas, acquired in the past few years. System xc(-) constitutes an important source of extrasynaptic glutamate in the brain. By modulating the tone of extrasynaptic metabotropic or ionotropic glutamate receptors, it affects excitatory neurotransmission, the threshold for overexcitation and excitotoxicity and, as a consequence, behavior. This review describes the current knowledge of how system xc(-) is regulated and involved in physiological as well as pathophysiological brain functioning.

mGluR5 in the nucleus accumbens shell regulates morphine-associated contextual memory through reactive oxygen species signaling

Emerging evidence indicates that metabotropic glutamate receptor 5 (mGluR5) critically modulates drug and drug-related behaviors. However, the role of mGluR5 in the opiate-induced contextual memory remains unclear. Here, we found that microinfusion of the mGluR5 antagonist 3-((2-Methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP) into the nucleus accumbens (NAc) shell, but not into the core, significantly attenuated the expression of morphine conditioned place preference (CPP) in rats. Following the expression of morphine CPP, the protein level of membrane mGluR5 was selectively increased in the NAc shell. In primary striatal neurons, we observed that treatment with the mGluR5 agonist CHPG increased the phosphorylation level of extracellular signal-regulated kinase (ERK), which was dependent on the mGluR5-inositol-1,4,5-trisphosphate-reactive oxygen species (ROS) pathway. Moreover, the microinjection of the ROS scavenger Tempol into the NAc shell of rats blocked the expression of morphine CPP. Further, the administration of t-BOOH, a ROS donor, into the NAc shell rescued the retrieval impairment of morphine CPP produced by MTEP. Our previous study demonstrated that the expression of morphine CPP increased the phosphorylation of ERK selectively in the NAc shell. Thus, results of the present study suggest that mGluR5 in the NAc shell, but not in the core, is essential for the retrieval of morphine contextual memory, which is mediated at least in part, through the ROS/ERK signaling pathway. Uncovering the molecular basis of opiate contextual memory will benefit the development of new therapeutic approaches for the treatment of opiate addiction.

Differential regulation of mGlu5 R and ΜOPr by priming- and cue-induced reinstatement of cocaine-seeking behaviour in mice

The key problem for the treatment of drug addiction is relapse to drug use after abstinence that can be triggered by drug-associated cues, re-exposure to the drug itself and stress. Understanding the neurobiological mechanisms underlying relapse is essential in order to develop effective pharmacotherapies for its prevention. Given the evidence implicating the metabotropic glutamate receptor 5 (mGlu5 R), μ-opioid receptor (MOPr), κ-opioid receptor (ΚOPr) and oxytocin receptor (OTR) systems in cocaine addiction and relapse, our aim was to assess the modulation of these receptors using a mouse model of cue- and priming-induced reinstatement of cocaine seeking. Male mice were trained to self-administer cocaine (1 mg/kg/infusion, i.v.) and were randomized into different groups: (1) cocaine self-administration; (2) cocaine extinction; (3) cocaine-primed (10 mg/kg i.p.); or (4) cue-induced reinstatement of cocaine seeking. Mice undergoing the same protocols but receiving saline instead of cocaine were used as controls. Quantitative autoradiography of mGlu5 R, MOPr, KOPr and OTR showed a persistent cocaine-induced upregulation of the mGlu5 R and OTR in the lateral septum and central amygdala, respectively. Moreover, a downregulation of mGlu5 R and MOPr was observed in the basolateral amygdala and striatum, respectively. Further, we showed that priming- but not cue-induced reinstatement upregulates mGlu5 R and MOPr binding in the nucleus accumbens core and basolateral amygdala, respectively, while cue- but not priming-induced reinstatement downregulates MOPr binding in caudate putamen and nucleus accumbens core. This is the first study to provide direct evidence of reinstatement-induced receptor alterations that are likely to contribute to the neurobiological mechanisms underpinning relapse to cocaine seeking.

Cocaine exposure alters dopaminergic modulation of prefronto-accumbens transmission

In the nucleus accumbens (NAc), dopamine transmission modulates glutamatergic input from the prefrontal cortex (PFC). This neuromodulatory action of dopamine can be disrupted by repeated exposure to psychostimulants such as cocaine. However, it is unclear whether this modulation depends on the precise timing of transmission at the same medium spiny neurons (MSNs) and if so, then whether this timing related modulation is also influenced by cocaine experience. Here, combining cocaine self-administration and in vivo extracellular recordings in anesthetized rats, we show that dopamine efflux in the NAc evoked by electrically stimulating the ventral tegmental area (VTA) exerted timing-dependent regulation of the excitatory accumbens response to stimulation of the medial prefrontal cortex (mPFC), and also that this modulation was blunted following prolonged abstinence from cocaine self-administration. These data indicate that dopaminergic timing-dependent dysregulation of mPFC-NAc glutamatergic transmission is implicated in cocaine addiction and might contribute to vulnerability to drug relapse after prolonged abstinence.

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.

Glutamate transporter GLT-1 mediates N-acetylcysteine inhibition of cocaine reinstatement

Both pre-clinical and clinical studies indicate that N-acetylcysteine (NAC) may be useful in treating relapse to addictive drug use. Cocaine self-administration in rats reduces both cystine-glutamate exchange and glutamate transport via GLT-1 in the nucleus accumbens, and NAC treatment normalizes these two glial processes critical for maintaining glutamate homeostasis. However, it is not known if one or both of these actions by NAC is needed to inhibit relapse to cocaine seeking. To determine whether the restoration of GLT-1 and/or cystine-glutamate exchange is required for NAC to inhibit cue-induced reinstatement of cocaine seeking, we utilized the rat self-administration/extinction/reinstatement model of cocaine relapse. Rats were pre-treated in the nucleus accumbens with vivo-morpholino antisense oligomers targeting either GLT-1 or xCT (catalytic subunit of the cystine-glutamate exchanger) overlapping with daily NAC administration during extinction (100 mg/kg, i.p. for the last 5 days). Rats then underwent cue-induced reinstatement of active lever pressing in the absence of NAC, to determine if preventing NAC-induced restoration of one or the other protein was sufficient to block the capacity of chronic NAC to inhibit reinstatement. The vivo-morpholino suppression of xCT reduced cystine-glutamate exchange but did not affect NAC-induced reduction of reinstated cocaine seeking. In contrast, suppressing NAC-induced restoration of GLT-1 not only prevented NAC from inhibiting reinstatement, but augmented the capacity of cues to reinstate cocaine seeking. We hypothesized that the increased reinstatement after inhibiting NAC induction of GLT-1 resulted from increased extracellular glutamate, and show that augmented reinstatement is prevented by blocking mGluR5. Restoring GLT-1, not cystine-glutamate exchange, is a key mechanism whereby daily NAC reduces cue-induced cocaine reinstatement.

Blockade of mGluR5 in the nucleus accumbens shell but not core attenuates heroin seeking behavior in rats

AIM

Glutamatergic neurotransmission in the nucleus accumbens (NAc) is crucial for the relapse to heroin seeking. The aim of this study was to determine whether mGluR5 in the NAc core or shell involved in heroin seeking behavior in rats.

METHODS

Male SD rats were self-administered heroin under a fixed-ratio 1 (FR1) reinforcement schedule for 14 d, and subsequently withdrawn for 2 weeks. The selective mGluR5 antagonist 2-methyl-6-phenylethynyl-pyridine (MPEP, 5, 15 and 50 nmol per side) was then microinjected into the NAc core or shell 10 min before a heroin-seeking test induced by context, cues or heroin priming.

RESULTS

Microinjection of MPEP into the NAc shell dose-dependently decreased the heroin seeking induced by context, cues or heroin priming. In contrast, microinjection of MPEP into the NAc core did not alter the heroin seeking induced by cues or heroin priming. In addition, microinjection with MPEP (15 nmol per side) in the NAc shell reversed both the percentage of open arms entries (OE%) and the percentage of time spent in open arms (OT%) after heroin withdrawal. Microinjection of MPEP (50 nmol per side) in the striatum as a control location did not affect the heroin seeking behavior. Microinjection of MPEP in the 3 locations did not change the locomotion activities.

CONCLUSION

Blockade of mGluR5 in NAc shell in rats specifically suppresses the relapse to heroin-seeking and anxiety-like behavior, suggesting that mGluR5 antagonists may be a potential candidate for the therapy of heroin addiction.

Withdrawal from cocaine self-administration and yoked cocaine delivery dysregulates glutamatergic mGlu5 and NMDA receptors in the rat brain

In human addicts and in animal models, chronic cocaine use leads to numerous alterations in glutamatergic transmission, including its receptors. The present study focused on metabotropic glutamatergic receptors type 5 (mGluR₅) and N-methyl-D-aspartate receptor subunits (NMDAR: GluN1, GluN2A, GluN2B) proteins during cocaine self-administration and after 10-day of extinction training in rats. To discriminate the contingent from the non-contingent cocaine delivery, we employed the “yoked”-triad control procedure. Protein expression in rat prefrontal cortex, nucleus accumbens, hippocampus, and dorsal striatum was determined. We also examined the Homer1b/c protein, a member of the postsynaptic density protein family that links NMDAR to mGluR₅. Our results revealed that cocaine self-administration selectively increased GluN1 and GluN2A subunit in the rat hippocampus and dorsal striatum, respectively, while mGluR₅ protein expression was similarly increased in the dorsal striatum of both experimental groups. Withdrawal from both contingent and non-contingent cocaine delivery induced parallel increases in prefrontal cortical GluN2A protein expression, hippocampal mGluR₅, and GluN1 protein expression as well as in accumbal GluN1 subunit expression, while the mGluR₅ expression was reduced in the prefrontal cortex. Extinction training in animals with a history of cocaine self-administration resulted in an elevation of the hippocampal GluN2A/GluN2B subunits and accumbal mGluR₅, and in a 50 % decrease of mGluR₅ protein expression in the dorsal striatum. The latter reduction was associated with Homer1b/1c protein level decrease. Our results showed that both contingent and non-contingent cocaine administration produces numerous, brain region specific, alterations in the mGluR₅, NMDA, and Homer1b/1c protein expression which are dependent on the modality of cocaine administration.

Nicotinic, glutamatergic and dopaminergic synaptic transmission and plasticity in the mesocorticolimbic system: focus on nicotine effects

Cigarette smoking is currently the leading cause of preventable deaths and disability throughout the world, being responsible for about five million premature deaths/year. Unfortunately, fewer than 10% of tobacco users who try to stop smoking actually manage to do so. The main addictive agent delivered by cigarette smoke is nicotine, which induces psychostimulation and reward, and reduces stress and anxiety. The use of new technologies (including optogenetics) and the development of mouse models characterised by cell-specific deletions of receptor subtype genes or the expression of gain-of-function nAChR subunits has greatly increased our understanding of the molecular mechanisms and neural substrates of nicotine addiction first revealed by classic electrophysiological, neurochemical and behavioural approaches. It is now becoming clear that various aspects of nicotine dependence are mediated by close interactions of the glutamatergic, dopaminergic and γ-aminobutyric acidergic systems in the mesocorticolimbic system. This review is divided into two parts. The first provides an updated overview of the circuitry of the ventral tegmental area, ventral striatum and prefrontal cortex, the neurotransmitter receptor subtypes expressed in these areas, and their physiological role in the mesocorticolimbic system. The second will focus on the molecular, functional and behavioural mechanisms involved in the acute and chronic effects of nicotine on the mesocorticolimbic system.