Defining the role of a prefrontal-midbrain circuit in exploratory behavior

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ABSTRACT Investigating how neural circuits mediate natural behavior is a critical to our understanding of the brain. Exploratory behaviors are necessary for rodent survival in the natural world and ubiquitous in freely-moving rodent experiments. One neural circuit that may mediate natural exploratory behaviors are the projections from prelimbic cortex (PL, part of rodent prefrontal cortex) to ventral tegmental area (VTA). Stimulation of PL-VTA projections causes increased velocity but is not rewarding, despite the well-known rewarding effect of direct stimulation of VTA-dopamine neurons. Taken together with the fact that VTA has been implicated in exploration, my central hypothesis is that PL-VTA neurons mediate natural exploratory behaviors through downstream effects on a subpopulation of non-dopaminergic VTA neurons. To investigate this hypothesis, I will first develop an unbiased, machine-learning-based method to quantify untrained, freely-moving mouse behaviors (Experiment 1.1). My method will leverage multi-view, high resolution video, supervised body part tracking, and unsupervised machine learning-based postural clustering methods. I will use this method to identify which specific behaviors (rearing, walking, grooming, sniffing, etc.) are affected by optogenetic stimulation or inhibition of PL-VTA cells in the presence and absence of rewards (Experiment 1.2). Preliminary data demonstrates the feasibility of my behavioral quantification method and suggests that PL-VTA stimulation increases exploratory behaviors. Next, to identify the genetically- and target-defined VTA cells that are preferentially synapsed onto by PL projections, I will use a combination of optogenetics, in vitro electrophysiology, transgenic mice, and retrograde tracing. First, I will use transgenic mice to measure the functional strength of PL input specifically onto VTA cells that are dopaminergic, GABAergic, or glutamatergic (Experiment 2.1). In a different set of mice, I will use retrograde tracing to identify if PL synapses preferentially onto VTA cells that project to nucleus accumbens or amygdala (Experiment 2.2). After learning which genetic and target-defined VTA subpopulations receive PL input, I will use retrograde tracing in combination with transgenic labeling to identify specifically which genetically and projection-defined VTA subpopulation receives PL input (Experiment 2.3). Finally, I will use a retrograde cre-dependent virus to selectively infect this VTA subpopulation with ChR2 and use my behavioral quantification pipeline to investigate whether in vivo stimulation of this subpopulation recapitulates the exploratory effects of PL-VTA stimulation (Experiment 2.4). Thus, the project proposed will develop a novel method for quantifying naturalistic behavior and integrate this computational method with diverse experimental methods, including in vivo and in vitro optogenetics, natural behavior, and synaptic physiology. These experiments will contribute novel information on how the PL-VTA circuit mediates naturalistic motor output, independent of the reinforcement effects of VTA-dopamine activity.