| Adolescence is a period of development characterized by rapid hormonal and bodily changes, as well as more gradual transitions in psychosocial and cognitive abilities. Mouse models of behavior are important tools for elucidating the basic biological processes that underlie this critical period of development. In this dissertation, I developed a new model of adolescent sociability using inbred mouse strains. In the first part of the work, I identified two inbred strains that exhibit a substantial difference in the tendency to approach one other. Early-adolescent C57BL/6J (B6) mice expressed greater levels of approach behavior towards conspecifics and produced a unique repertoire of ultrasonic vocalizations relative to mice from the BALB/cJ (BALB) strain. Adapting a conditioned place-preference procedure, in the second part of the work, I identified 'social reward' and 'isolation aversion' as proximate mechanisms that may contribute to the strain difference in social approach behavior. Following general principles of conditioning, B6 mice spent more time in an otherwise neutral environment when the environment had been associated with social contact, while BALB mice were behaviorally indifferent to the same social conditioning procedure. Finally, using a modified cue-conditioned, fear learning paradigm, I demonstrated that B6 mice can acquire salient information about environmental threats by observing conspecifics exposed to a conditioning contingency, whereas BALB mice do not express this type of socially mediated acquisition of fear. Employing a series of control experiments for each behavioral paradigm, I systematically eliminated a contributory role for factors such as maternal care, sexual or general exploratory motivation, learning abilities or anxiety in the strain-dependent social responses of adolescent BALB and B6 mice. Thus, enhanced responses of social motivation, social reward and social learning in adolescent mice all segregated with the B6 genetic background. Taken together, these experiments underscore the BALB-B6 mouse model of adolescent sociability as a promising tool for identifying genetic and neural substrates that constitute the mammalian social brain. |
