Elucidating the role of the VTA-NAc medial shell projection in motivation, extinction, and reinstatement

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PROJECT SUMMARY Reward association and motivation are key processes guiding goal-directed behaviors. These functions are modulated by mesolimbic dopamine circuitry, which encompasses dopamine neurons projecting from the ventral tegmental area (VTA) to the nucleus accumbens (NAc). VTA-NAc dopamine projections broadly facilitate the learning of action- and cue-outcome associations and regulate the extinction and reinstatement of goal-directed behaviors. Further, an emerging body of research suggests that specific subregions of the NAc shell integrate midbrain dopaminergic signals with sensory and limbic inputs to drive actions involved in the maintenance, extinction, and reinstatement of reward-seeking behaviors. However, little is known about how isolated VTA dopamine inputs to the NAc medial shell contribute to these behaviors. The primary goal of this proposal is to dissect the neural circuitry of isolated VTA projections to the NAc medial shell in motivated behavior. To define the role of a specific VTA dopamine projection to the NAc medial shell, we identified a genetic marker, cerebellin- 4 (Cbln4), that displays restricted expression within a subpopulation of VTA dopamine neurons. Using viral- mediated circuit mapping in Cbln4-IRES-CRE (Cbln4iCre) mice, I found that VTACbln4 neurons project to the NAc medial shell region but not lateral shell or core regions. Additionally, in vivo optical stimulation of this circuit is sufficient to facilitate the maintenance of reward-seeking behaviors in the absence of primary reward reinforcement. Further, stimulation of this population is sufficient to reinstate an extinguished reward-seeking behavioral response. Although highly informative, these studies did not examine the role of endogenous neural activity or dopamine signaling in the VTACbln4-Nac medial shell circuit for its functions in modulating motivated behavior. Two specific aims are proposed using the Cbln4iCre mouse line with viral-mediated targeting strategies to dissect the VTACbln4-Nac medial shell circuit. Aim 1 will anatomically and functionally characterize the VTACbln4- Nac medial shell dopamine projection using high resolution quantitative image analysis and a genetically encoded dopamine sensor coupled with circuit-specific slice imaging. Aim 2 uses in vivo fiber photometry recording to determine the neural activity of VTACbln4 neurons during the acquisition, extinction, and cue-induced reinstatement of food-motivated operant responding. Further, this aim examines the necessity of endogenous neural activity and dopamine production in the VTACbln4-Nac medial shell circuit in determining the expression of these behaviors using chemogenetic inhibition and cell-type specific CRISPR/SaCas9-mediated gene knockout approaches. This study will define how isolated projections within mesolimbic dopamine circuitry regulate distinct aspects of motivated behavior and provide insight into how disruptions in these circuits contribute to substance use and obsessive-compulsive disorders. The proposed studies will facilitate my training in in vivo fiber photometry, ex vivo slice imaging, and computational data analysis methods, as well as prepare me to pursue a career as an independent researcher.