Benefit, Burdens and Brain Correlates of Heightened Reward Sensitivity in Adolescence

Increased exploration, risk-taking and reward-seeking are the hallmarks of adolescence. These and related behaviors prepare the young to transition from the parent’s nest to independent living. Adolescence is also a period of heightened structural and functional brain reorganization, particularly within the mesocorticolimbic dopamine system, frontal cortex, striatum and amygdala - an interconnected network of brain regions that supports reward-directed behavior. The goal of this dissertation to further our understanding of ways in which the adolescent brain interacts with rewards. In Chapter 2, I present the results of a set of experiments demonstrating that adolescent rats do not differ from adults in a simple form of stimulus-reward learning but are willing to invest greater effort to obtain larger rewards. While previous studies have focused on the role of neurodevelopmental changes in the dopamine system and within the striatum in heightened reward seeking in adolescence, our data suggest that synaptic remodeling within the frontal cortex and amygdala may also contribute to enhanced reward sensitivity. Chapter 3 explores the long-term consequences of prescription drug exposure during the adolescent period of heightened reward sensitivity in rats. The data suggest that adolescent exposure to both fluoxetine and methylphenidate impairs learning and cognitive flexibility in adulthood in male rats long after the drug administration has been terminated. Adolescent methylphenidate exposure has the direst consequences, impairing both the initial learning and reversal of reward contingencies. The data also reveal sex differences: while females take longer to learn the task, they are also less vulnerable to the negative effects of drug exposure. In Chapter 5, I compare adult and adolescent humans in their approaches to solving the credit assignment problem (i.e., the problem of discovering which choices are responsible for rewards, introduced in Chapter 4). The data provide preliminary evidence for enhanced contingent credit assignment in adolescence. While adults integrate information about their previous decisions and past outcomes to guide their subsequent choices, adolescents are more narrowly focused on the most recent choice-reward history. In Chapter 6, I discuss the implications of the present work and offer cautious advice on drug abuse prevention and improvements in pedagogical practice