Time-dependent changes in chloride homeostasis are a mechanism of plasticity critical for learning

ABSTRACT The mechanisms underlying how rewards are learned, and how they translate into behavior have been a subject of interest to researchers and scientists since Pavlov’s classical conditioning experiments in the 1890s. Recently, Gamma Amino Butyric Acid (GABA) neurons in the Ventral Tegmental Area (VTA) have emerged as key modulators of reward learning, and potential therapeutic targets for addressing addiction, depression, and other neuropsychiatric disorders. Previous studies have shown that GABA neurons in the VTA undergo a form of experience-dependent plasticity that involves the downregulation of the anion transporter, KCC2. This reduction of KCC2 levels has been found to increase the excitability of GABA neurons by decreasing the efficacy of GABAA receptor function. In the context of drugs of abuse and aversive stimuli, this mechanism has been studied in great detail. However, the impact of this mechanism on specific reward-related pathways and its contribution to naturalistic reward learning remains unknown. In my thesis work thus far, I have examined whether modifications in KCC2 serve as an innate mechanism during reward learning that allows the brain to establish enduring associations between contextual cues and appetitive stimuli. I first characterized the expression of KCC2 throughout the course of cue-reward associative learning and showed that KCC2 downregulation shifts VTA GABA neurons towards excitability at critical time points during reward learning. My data also demonstrate that KCC2-mediated plasticity drives increased synchronized activity between VTA GABA neurons ex vivo. Subsequently, I demonstrate that VTA GABA neurons exhibit enhanced synchronization in vivo, and it is concurrent with the same time points when KCC2 downregulation occurs. This F99/K00 proposal comprises two aims outlined in the following research plan. In Aim 1, I will examine whether this form of plasticity occurs in vivo, and consequently leads to enhanced synchronized network activity. To achieve this, I will combine multi-unit silicon recordings with pharmacological and genetic approaches to manipulate levels of KCC2 and examine neural network dynamics during reward learning. Aim 2 delineates my aspirations to identify and obtain a postdoctoral position. During the K00 phase, I plan to investigate the molecular, cellular, and neurocircuit-specific mechanisms shaping susceptibility or resilience to common neuropsychiatric disorders. To this end, my sponsors and I have crafted a thorough plan for my F99 (predoctoral) and K00 (postdoctoral) phases. The F99/K00 will support my goals for becoming an independent researcher by facilitating the successful completion of my PhD and bolstering a seamless transition into a strong postdoctoral position.