mGlu5 receptors are involved in mechanisms of activity-dependent synaptic plasticity, and are targeted by drugs developed for the treatment of CNS disorders. We report that mGlu3 receptors, which are traditionally linked to the control of neurotransmitter release, support mGlu5 receptor signaling in neurons and largely contribute to the robust mGlu5 receptor-mediated polyphosphoinositide hydrolysis in the early postnatal life. In cortical pyramidal neurons, mGlu3 receptor activation potentiated mGlu5 receptor-mediated somatic Ca mobilization, and mGlu3 receptor-mediated long-term depression in the prefrontal cortex required the endogenous activation of mGlu5 receptors. The interaction between mGlu3 and mGlu5 receptors was also relevant to mechanisms of neuronal toxicity, with mGlu3 receptors shaping the influence of mGlu5 receptors on excitotoxic neuronal death. These findings shed new light into the complex role played by mGlu receptors in physiology and pathology, and suggest reconsideration of some of the current dogmas in the mGlu receptor field.

Stress can precipitate or worsen symptoms of many psychiatric disorders by dysregulating glutamatergic function within the prefrontal cortex (PFC). Previous studies suggest that antagonists of group II metabotropic glutamate (mGlu) receptors (mGlu2 and mGlu3) reduce stress-induced anhedonia through actions in the PFC, but the mechanisms by which these receptors act are not known. We now report that activation of mGlu3 induces long-term depression (LTD) of excitatory transmission in the PFC at inputs from the basolateral amygdala. Our data suggest mGlu3-LTD is mediated by postsynaptic AMPAR internalization in PFC pyramidal cells, and we observed a profound impairment in mGlu3-LTD following a single, 20-min restraint stress exposure. Finally, blocking mGlu3 activation in vivo prevented the stress-induced maladaptive changes to amygdalo-cortical physiology and motivated behavior. These data demonstrate that mGlu3 mediates stress-induced physiological and behavioral impairments and further support the potential for mGlu3 modulation as a treatment for stress-related psychiatric disorders.

Alcohol use disorder (AUD) affects all sexes, however women who develop AUD may be particularly susceptible to cravings and other components of the disease. While many brain regions are involved in AUD etiology, proper function of the prefrontal cortex (PFC) is particularly important for top-down craving management and the moderation of drinking behaviors. Essential regulation of PFC output is provided by local inhibitory interneurons, yet the effects of chronic drinking on interneuron physiology remain poorly understood, particularly in female individuals. To address this gap, we generated fluorescent reporter transgenic mice to label the two major classes of interneuron in deep layer prelimbic PFC, based on expression of parvalbumin (PV-IN) or somatostatin (SST-IN). We then interrogated PV-IN and SST-IN membrane and synaptic physiology in a rodent model of binge drinking. Beginning in late adolescence, mice received 3-4 weeks of intermittent access (IA) ethanol. One day after the last drinking session, adaptations to PV-IN and SST-IN intrinsic physiology were observed in male mice but not in female mice. Furthermore, IA ethanol precipitated diametrically opposing changes to PV-IN synaptic physiology based on sex. IA ethanol decreased excitatory synaptic strength onto PV-INs from female mice and potentiated excitatory transmission onto PV-INs male mice. In contrast, decreased synaptic strength onto SST-INs was observed following IA ethanol in all groups of mice. Together, these findings illustrate novel sex differences in drinking-related PFC pathophysiology. Discovering means to restore PV-IN and SST-IN dysfunction following extended drinking provides opportunities for developing new treatments for all AUD patients.

BACKGROUND Excitatory synaptic transmission in the nucleus accumbens (NAc) is a key biological substrate underlying behavioral responses to psychostimulants and susceptibility to relapse. Recent studies have demonstrated that cocaine induces changes in glutamatergic signaling at distinct inputs to the NAc. However, consequences of cocaine experience on synaptic transmission from the midline nuclei of the thalamus (mThal) to the NAc have yet to be reported. METHODS To examine synapses from specific NAc core inputs, we recorded light-evoked excitatory postsynaptic currents (EPSCs) following viral-mediated expression of channelrhodopsin-2 in the mThal, prefrontal cortex (PFC), or basolateral amygdala (BLA) from acute brain slices. To identify NAc medium spiny neuron (MSN) subtypes we utilized mice expressing tdTomato driven by the promoter for the dopamine receptor subtype 1 (D1). We recorded N-methyl-D-aspartate receptor (NMDAR) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) properties to evaluate synaptic adaptations induced by cocaine experience, a 5-day cocaine exposure followed by 2-weeks of abstinence. RESULTS We determine that excitatory inputs to the NAc core display differential NMDAR properties and cocaine experience uniquely alters AMPAR and NMDAR properties at mThal-D1(+), mThal-D1(−), and PFC-D1(+) synapses, but not PFC-D1(−) synapses. Finally, at mThal-D1(+) synapses, we demonstrate that cocaine enhances GluN2C/D function and NMDAR-dependent synaptic plasticity. CONCLUSIONS Our results identify contrasting cocaine-induced AMPAR and NMDAR modifications at mThal- and PFC-NAc core synapses. These changes include an enhancement of NMDAR function and plasticity at mThal-D1(+) synapses. Incorporation of GluN2C/D-containing NMDARs most likely underlies these phenomena and represents a potential therapeutic target for psychostimulant use disorders.

Highlights d mGlu 2 and mGlu 3 negative allosteric modulators (NAMs) stimulate prefrontal cortex d mGlu 2 regulates thalamocortical glutamate release probability d mGlu 3 directs prefrontal cortex postsynaptic plasticity d mGlu 2 and mGlu 3 NAMs independently decrease passive coping and reverse anhedonia

Stress can precipitate or worsen symptoms of many psychiatric illnesses. Dysregulation of the prefrontal cortex (PFC) glutamate system may underlie these disruptions and restoring PFC glutamate signaling has emerged as a promising avenue for the treatment of stress disorders. Recently, we demonstrated that activation of metabotropic glutamate receptor subtype 3 (mGlu3) induces a postsynaptic form of long-term depression (LTD) that is dependent on the activity of another subtype, mGlu5. Stress exposure disrupted this plasticity, but the underlying signaling mechanisms and involvement in higher-order cognition have not yet been investigated. Acute stress was applied by 20-minutes restraint and early reversal learning was evaluated in an operant-based food-seeking task. We employed whole-cell patch-clamp recordings of layer 5 prelimbic (PL)-PFC pyramidal cells to examine mGlu3-LTD and several mechanistically distinct mGlu5-dependent functions. Acute stress impaired both mGlu3-LTD and early reversal learning. Interestingly, potentiating mGlu5 signaling with the mGlu5 positive allosteric modulator (PAM) VU0409551 rescued stress-induced deficits in both mGlu3-LTD and reversal learning. Other aspects of PL-PFC mGlu5 function were not disrupted following stress; however, signaling downstream of mGlu5-Homer interactions, phosphoinositide-3-kinase (PI3K), Akt, and glycogen synthase kinase 3β was implicated in these phenomena. These findings demonstrate that acute stress disrupts early reversal learning and PL-PFC-dependent synaptic plasticity and that potentiating mGlu5 function can restore these impairments. These findings provide a framework through which modulating coordinated mGlu3/mGlu5 signaling may confer benefits for the treatment of stress-related psychiatric disorders.

Cocaine abuse remains a public health concern for which pharmacotherapies are largely ineffective. Comorbidities between cocaine abuse, depression, and anxiety support the development of novel treatments targeting multiple symptom clusters. Selective negative allosteric modulators (NAMs) targeting the metabotropic glutamate receptor 5 (mGlu 5 ) subtype are currently in clinical trials for the treatment of multiple neuropsychiatric disorders and have shown promise in preclinical models of substance abuse. However, complete blockade or inverse agonist activity by some full mGlu 5 NAM chemotypes demonstrated adverse effects, including psychosis in humans and psychotomimetic-like effects in animals, suggesting a narrow therapeutic window. Development of partial mGlu 5 NAMs, characterized by their submaximal but saturable levels of blockade, may represent a novel approach to broaden the therapeutic window. To understand potential therapeutic vs adverse effects in preclinical behavioral assays, we examined the partial mGlu 5 NAMs, M-5MPEP and Br-5MPEPy, in comparison with the full mGlu 5 NAM MTEP across models of addiction and psychotomimetic-like activity. M-5MPEP, Br-5MPEPy, and MTEP dose-dependently decreased cocaine self-administration and attenuated the discriminative stimulus effects of cocaine. M-5MPEP and Br-5MPEPy also demonstrated antidepressant-and anxiolytic-like activity. Dose-dependent effects of partial and full mGlu 5 NAMs in these assays corresponded with increasing in vivo mGlu 5 occupancy, demonstrating an orderly occupancy-to-efficacy relationship. PCP-induced hyperlocomotion was potentiated by MTEP, but not by M-5MPEP and Br-5MPEPy. Further, MTEP, but not M-5MPEP, potentiated the discriminative-stimulus effects of PCP. The present data suggest that partial mGlu 5 NAM activity is sufficient to produce therapeutic effects similar to full mGlu 5 NAMs, but with a broader therapeutic index.

This review is an introduction to addiction, the reward circuitry, and laboratory addiction models. Addiction is a chronic disease hallmarked by a state of compulsive drug seeking that persists despite negative consequences. Most of the advances in addiction research have centered on the canonical and contemporary drugs of abuse, however, addictions to other activities and stimuli also exist. Substances of abuse have the potential to induce long-lasting changes in the brain at the behavioral, circuit and synaptic levels. Addiction-related behavioral changes involve initiation, escalation and obsession to drug seeking and much of the current research is focused on mapping these manifestations to specific neural pathways. Drug abuse is well known to recruit components of the mesolimbic dopamine system, including the nucleus accumbens and ventral tegmental area. In addition, altered function of a wide variety of brain regions is tightly associated with specific manifestations of drug abuse. These regions peripheral to the mesolimbic pathway likely play a role in specific observed comorbidities and endophenotypes that can facilitate, or be caused by, substance abuse. Alterations in synaptic structure, function and connectivity, as well as epigenetic and genetic mechanisms are thought to underlie the pathologies of addiction. In preclinical models, these persistent changes are studied at the levels of molecular pharmacology and biochemistry, ex vivo and in vivo electrophysiology, radiography and behavior. Coordinating research efforts across these disciplines and examining cell type- and circuit-specific phenomena are crucial components for translating preclinical findings to viable medical interventions that effectively treat addiction and related disorders.