Characterizing the Development of Reward Evaluation Across Adolescence in Female and Male Long-Evans Rats

Abstract

Adolescence is a developmental stage describing the transition from childhood to adulthood, that is a heightened period of sensitivity to influence from the environment due to extensive physical, physiological, and psychological maturation. Adolescents, both human and rat, have been observed to evaluate reward differently than other ages, however little is known about how reward evaluation may change throughout adolescence, and how these developmental trajectories may vary between reward types and between sexes. My thesis work focused on characterizing changes in sensitivity to and motivation for reward across adolescence in female and male Long-Evans rats. In chapter 2, I used a free-access drinking task to compare how testing conditions (within- group or between-group) influenced intake of diluted sweetened condensed milk and found that previous test experience was necessary to facilitate a peak in adolescent male rats intake normalized for body mass as reported in previous research. However, this peak did not occur in female rats, and when intake was not normalized per body mass the peak was not found in male rats either. In chapter 3, I used an operant conditioning paradigm to investigate social motivation across adolescence and found that male rats overall were more socially directed than female rats when using a fixed ratio, however there was a lack of sex differences when using a progressive ratio, except for male rats at postnatal day (P) 40. At P40 male rats were less socially motivated than P40 female rats, and less socially motivated than male rats at younger and older ages. Overall, my thesis work suggests that the development of reward processing is dynamic across the adolescent period, and that peaks in reward evaluation depend on sex and procedure.