Development of oxytocin, vasopressin V1a, and mu-opioid receptor expression in the rat brain
Across species, the juvenile period is characterized by increased social interaction with peers and heightened novelty-seeking behavior, as compared to any other life stage. These behaviors are likely to be highly adaptive during this developmental phase. Still, an excessive novelty-seeking phenotype may predispose individuals to risk-taking and substance abuse, while too little social engagement and low novelty-seeking are characteristics of neuropsychiatry disorders such as autism. The over-arching aim of this dissertation research has been to elucidate the neural mechanisms underlying juvenile social novelty-seeking behavior. Central activation of oxytocin, vasopressin V1a, and µ-opioid receptors (OTR, V1aR, and MOR, respectively) have been implicated in the regulation of adult social behavior, but our understanding of the expression and function of OTR, V1aR, and MORs in the juvenile brain is incomplete. Therefore, in Studies 1 and 2, age differences in binding density of OTR, V1aR, and MOR throughout the rat brain were identified using receptor autoradiography. Next, in Study 3, I established the social novelty preference test, a new paradigm designed to assess the preference of juvenile rats to interact with either a novel or a familiar (cage mate) conspecific. Using this social novelty preference test, in Studies 3, 4, and 5, the functional involvement of OTR, V1aR, and MOR in the regulation of juvenile social novelty preference was characterized using both intracerebroventricular and local in-vivo pharmacological manipulations. The results of these experiments demonstrate that both OTR and MOR activation in the brain are involved in the regulation of juvenile social novelty preference, particularly acting within the nucleus accumbens. Finally, in Study 5, I investigated the impact of social isolation on juvenile social novelty preference. My findings show that social isolation potently reduces social novelty preference, which, in turn, can be restored by MOR activation in the nucleus accumbens. Taken together, this body of work significantly advances our understanding of the neural systems underlying juvenile social novelty preference, and suggests that both oxytocin and opioid systems in the brain may be potential clinical targets for restoring social novelty-seeking behavior in neurodevelopmental disorders, such as autism.