Frontal cortex genetic ablation of metabotropic glutamate receptor subtype 3 (mGlu3) impairs postsynaptic plasticity and modulates affective behaviors.

Clinical and translational studies suggest that prefrontal cortex (PFC) dysregulation is a hallmark feature of several affective disorders. Thus, investigating the mechanisms involved in the regulation of PFC function and synaptic plasticity could aid in developing new medications. In recent years, the mGlu2 and mGlu3 subtypes of metabotropic glutamate (mGlu) receptors have emerged as exciting potential targets for the treatment of affective disorders, as mGlu2/3 antagonists exert antidepressant-like effects across many rodent models. Several recent studies suggest that presynaptic mGlu2 receptors may contribute to these effects by regulating excitatory transmission at synapses from the thalamus to the PFC. Interestingly, we found that mGlu3 receptors also inhibit excitatory drive to the PFC but act by inducing long-term depression (LTD) at amygdala-PFC synapses. It remains unclear, however, whether blockade of presynaptic, postsynaptic, or glial mGlu3 receptors contribute to long-term effects on PFC circuit function and antidepressant-like effects of mGlu2/3 antagonists. To address these outstanding questions, we leveraged transgenic Grm3fl/fl mice and viral-mediated gene transfer to genetically ablate mGlu3 receptors from pyramidal cells in the frontal cortex of adult mice of all sexes. Consistent with a role for mGlu3 in PFC pyramidal cells, mGlu3-dependent amygdala-cortical LTD was eliminated following mGlu3 receptor knockdown. Furthermore, knockdown mice displayed a modest, task-specific anxiolytic phenotype and decreased passive coping behaviors. These studies reveal that postsynaptic mGlu3 receptors are critical for mGlu3-dependent LTD and provide convergent genetic evidence suggesting that modulating cortical mGlu3 receptors may provide a promising new approach for the treatment of mood disorders.

mGlu2 and mGlu3 Negative Allosteric Modulators Divergently Enhance Thalamocortical Transmission and Exert Rapid Antidepressant-like Effects.

Non-selective antagonists of metabotropic glutamate receptor subtypes 2 (mGlu2) and 3 (mGlu3) exert rapid antidepressant-like effects by enhancing prefrontal cortex (PFC) glutamate transmission; however, the receptor subtype contributions and underlying mechanisms remain unclear. Here, we leveraged newly developed negative allosteric modulators (NAMs), transgenic mice, and viral-assisted optogenetics to test the hypothesis that selective inhibition of mGlu2 or mGlu3 potentiates PFC excitatory transmission and confers antidepressant efficacy in preclinical models. We found that systemic treatment with an mGlu2 or mGlu3 NAM rapidly activated biophysically unique PFC pyramidal cell ensembles. Mechanistic studies revealed that mGlu2 and mGlu3 NAMs enhance thalamocortical transmission and inhibit long-term depression by mechanistically distinct presynaptic and postsynaptic actions. Consistent with these actions, systemic treatment with either NAM decreased passive coping and reversed anhedonia in two independent chronic stress models, suggesting that both mGlu2 and mGlu3 NAMs induce antidepressant-like effects through related but divergent mechanisms of action.

Mechanisms underlying prelimbic prefrontal cortex mGlu3/mGlu5-dependent plasticity and reversal learning deficits following acute stress.

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-min 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 3ß 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.

Metabotropic glutamate receptor subtype 3 gates acute stress-induced dysregulation of amygdalo-cortical function.

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 amydalo-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.