PROJECT SUMMARY The 2019 National Survey on Drug Use and Health (NSDUH) reported that over 20 million Americans suffered from a substance use disorder (SUD) in 2018. Yet, only 3.7 million people in need received treatment, and up to 60% of people that received treatment relapsed within the first year of recovery. While genetics and environmental factors can greatly increase a person’s risk for developing a SUD, the specific neural circuits underlying SUDs still need to be identified. Thus, I believe that the field requires a multidisciplinary approach, including pharmacology, circuit neuroscience, genetics, and behavioral neuroscience, to understand how the brain is affected by SUDs. Although the neural mechanisms underlying SUDs are poorly understood, disruption in associative learning is thought to play a role. Associative learning refers to the ability to associate rewarding and aversive stimuli with prior cues that predict the occurrence of these stimuli. Identifying the neural mechanisms that regulate associative learning therefore holds promise for understanding substance use disorders. During my PhD I have studied the neural mechanisms that govern cue-associated reward learning in mice. Specifically, my thesis project investigates co-regulation between two neuromodulators, dopamine (DA) and acetylcholine (ACh), during associative reward learning. My preliminary findings outlined in Aim 1 show that 1) striatal DA and ACh are concurrently modulated by rewards and reward predicting cues, 2) cue evoked changes in DA and ACh are highly correlated with each other and 3) correlated with behavior. Together, these findings lead me to my hypothesis that striatal DA and ACh are co- regulating each other to drive associative reward learning. I will test my hypothesis in Aim 2 of this proposal. I am confident that these experiments will reveal novel mechanisms underlying associative learning and I will build upon the knowledge and skill sets obtained from this study to address how dysregulation in associative learning contributes to substance use disorders as described in Aim 3.