PROJECT ABSTRACT Opioid addiction is pervasive and widespread, affecting roughly three million U.S. adults. Currently available opioid addiction treatments, such as opioid replacement therapy, fail to slow the growing opioid pandemic and maintain the risk of addiction and overdose. Moreover, available opioid addiction treatments require long-term commitment to treatment with little evidence of long-lasting abstinence. Lastly, opioid replacement therapy is unable to alleviate addiction-induced cognitive impairments. A deeper understanding of the neural and cognitive systems that underlie addiction is necessary for the development of better targeted treatments for opioid addiction. The rodent model of opioid addiction exhibits behavioral markers analogous to those induced in human opioid addiction. Hence, this is a reliable and feasible model for studies of the neural correlates of addiction-related maladaptive behaviors. An emerging body of research suggests that the evolutionarily conserved lateral habenula in rodents is highly implicated in addiction. The lateral habenula is unique in that it directly regulates dopaminergic and serotonergic structures, both of which exhibit dysfunction in addiction. However, with traditional electrophysiological methods of recording lateral habenula neural activity, it has been difficult to clearly assess responses of large populations of neurons. More recent advances in imaging technology have allowed for week-long monitoring of individual neuron calcium dynamics, easing the feasibility of studying the lateral habenula neural responses. Serotonin agonists, such as psilocybin, have shown promising results in reducing the rates of relapse in alcohol and nicotine addiction and improving cognitive function in unhealthy adults. Importantly, lateral habenula hyperactivity is known to drive aversion and is present in withdrawal. Serotonergic agonists have also been shown to quiet lateral habenula activity, suggesting a potential unexplored treatment avenue. Hence, with the use of calcium imaging, I hypothesize that lateral habenula neuron dynamics will shift to a hyperactive state following morphine withdrawal, and that these neural signatures will correlate with decreased performance on cognitive tasks. Additionally, I hypothesize that psilocybin treatment will reinstate baseline lateral habenula activity and improve cognitive performance. The proposed series of experiments will fill the gap in understanding the neural circuitry that drives maladaptive decisions during opiate withdrawal, as well as the behavioral and neural effect of a novel treatment for opiate addiction.