Relevance and plasticity of inhibitory metabotropic signaling in reward circuits

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PROJECT SUMMARY Drugs of abuse share an ability to enhance dopamine (DA) neurotransmission from the ventral tegmental area (VTA) to downstream targets, including the medial prefrontal cortex (mPFC) and nucleus accumbens (NAc). Direct stimulation of VTA DA neurons is reinforcing and sufficient to trigger the array of molecular, cellular, and behavioral adaptations that define addiction. The actions of drugs of abuse are opposed by inhibitory G protein signaling pathways in the reward circuitry. Psychostimulants can weaken inhibitory G protein signaling in VTA DA neurons and layer 5/6 pyramidal neurons of the prelimbic cortex (PLC), via a selective reduction in the cell surface expression of G protein-gated inwardly rectifying K+ (GIRK) channels. Genetic suppression of GIRK channel activity in drug-naïve mice evokes some of the cellular adaptations and behavioral outcomes typically associated with repeated drug exposure, supporting the outlook that GIRK channels are critical and exploitable contributors to innate addiction barriers. The goals of this project are to use new tools and approaches to gain refined insights into the mechanisms mediating the recruitment of GIRK-dependent signaling by drugs of abuse, and to investigate the therapeutic potential associated with enhancing GIRK channel activity in a neuron- and/or channel subtype-specific fashion. There are two specific aims: (1) To probe the recruitment and therapeutic potential of GIRK-dependent feedback to VTA DA neurons. Loss-of-function mutants, including mice lacking GIRK channels in DA neurons, suggest that the unique GIRK channel subtype found in VTA DA neurons is a key regulator of behavioral sensitivity to drugs of abuse. This premise will be tested using intersectional viral manipulations to enhance or suppress GIRK-dependent signaling selectively in VTA DA neurons, followed by behavioral assessments in non-contingent and response-contingent tests involving cocaine. In parallel, the therapeutic potential of novel activators of the unique VTA DA neuron GIRK channel subtype will be evaluated. Lastly, an in vivo optogenetic approach will be used to test whether phasic VTA DA activity is sufficient to engage and suppress GIRK-dependent signaling in VTA DA neurons. (2) To reveal the mechanisms and relevance of a GIRK-dependent feedforward inhibitory circuit in the PLC. Although work in the initial project period established that GIRK-dependent signaling in layer 5/6 PLC pyramidal neurons is an addiction barrier, how and when this barrier is engaged is unclear. Optogenetic and chemogenetic approaches will be used to test the working model that phasic activation of VTA DA neurons evokes a feedforward inhibitory circuit involving the D1R-dependent activation of layer 5/6 PLC GABA interneurons, which tempers the parallel DA-dependent activation of adjacent pyramidal neurons. The proposed studies also test the predictions that repeated engagement of this feedforward circuit is sufficient to trigger the suppression of GIRK channel activity in layer 5/6 PLC pyramidal neurons, and that strengthening GIRK-dependent signaling in these neurons confers resilience to the addictive effects of cocaine.