B25-P: A Novel Circuit Controlling Motivation

To maximize rewards, one must learn what specifically causes those rewards. This learning process is disrupted in numerous psychological disorders, including depression and substance use. Understanding the systems responsible for these processes is key to developing future treatments for psychological diseases. The serotonin and dopamine systems play critical roles in learning and motivation. While these systems are individually well-understood, the interactions between them are understudied. The purpose of this study was to examine how interactions between the serotonin and dopamine systems influence learning about reward-predictive cues (associative learning) and motivation to obtain rewards (instrumental learning). Serotonergic neurons originate in the dorsal-raphe nucleus (DRN) and interact with dopamine circuits in two key sites: the ventral-tegmental area (VTA) and the nucleus accumbens (NAc). We independently manipulated each of these circuits using chemogenetic methods that increase their activity in both male and female rats. We then assessed the role of each circuit in the rate of learning about reward-predictive cues and found no significant differences in associative learning following manipulation of either circuit, suggesting that activating these circuits does not alter the ability to learn reward-predictive cues. We assessed each circuit’s control of instrumental learning using a fixed ratio task and found that activation of DRN projections to the NAc reduced instrumental learning. Lastly, we examined the role of each circuit in motivation using a progressive ratio task and found that activation of DRN projections to the NAc pathway decreases motivation. Together, these results implicate DRN projections to the NAc, but not VTA, in motivation to obtain rewards and learn about action-outcome relationships, but not to learn about cues that predict rewards. These results suggest that targeting DRN projections to the NAc could have therapeutic utility in diseases characterized by lack of motivation, and also explain a key neurobiological system involved in cognition.

Authors: Emily A. Bogdanski, Sarah J. Loberger, Zoe L. Platow

Faculty Advisor: Matthew S. McMurray, Psychology

Graduate Student Advisor: Danielle N. Tapp, Psychology