PROJECT SUMMARY Approximately 29.5 million people over the age of 12 in the US had alcohol use disorder (AUD) according to the 2021 National Survey on Drug Use and Health. Chronic alcohol use can lead to the emergence of cognitive deficits, particularly working memory, which persist during alcohol withdrawal and abstinence increasing the odds of relapse. Approved treatments for alcohol use disorder are not designed to treat alcohol-induced cognitive dysfunction, so understanding potential mechanisms is necessary to develop new treatments. The nucleus reuniens of the thalamus (RE) is a ventral midline thalamic nucleus that plays a key role in cognitive function, such as spatial working memory, attention, and behavioral flexibility. The RE is also bidirectionally connected to the medial prefrontal cortex (mPFC) and the hippocampus, which have been highly studied in AUD and cognition. Despite its interconnectedness, the RE is understudied in the alcohol field. In my preliminary studies, I induced alcohol dependence using chronic intermittent ethanol (CIE) vapor exposure (4 cycles) in male and female C57BL/6J mice. Following CIE, mice were either tested on a T-maze delayed alternation spatial working memory task or sacrificed for whole cell patch clamp electrophysiology to measure intrinsic excitability in the RE. Mice used in the T-maze task were trained prior to CIE and tested following 2 and 4 cycles, to track cognitive decline across the development of dependence. Both male and female mice exhibited significant deficits in performance after CIE. Despite this similar behavioral phenotype, CIE had divergent effects on intrinsic excitability, causing an increase in firing in females and a decrease in males. I also found that firing in the RE in the absence of alcohol exposure appeared to be sex-dependent, where males have significantly greater excitability than females. These results indicated that the RE could be an exciting new target for the study of chronic alcohol- induced cognitive deficits. The overall hypothesis of this research proposal is that chronic alcohol exposure causes sex-dependent functional adaptations to the RE and that regulating RE activation will improve cognitive performance. To achieve this, the proposal uses a multi-technical approach to study the RE in vivo and in slice physiology. Aim 1 of this proposal will determine if chronic alcohol exposure alters the function of the RE using fiber photometry during performance on a spatial working memory task and whether normalizing aberrant activity using optogenetics will improve cognitive performance in CIE-exposed mice. Aim 2 of this proposal will characterize the effect of CIE exposure on the physiology of RE neurons involved in mPFC and hippocampal circuitry by recording intrinsic excitability and synaptic transmission from neurons that project to the mPFC, the hippocampus, or both. The results of these experiments will serve as landmark studies for the role of the RE in alcoh...