Associations between behavior, hormones, and Fos responses to novelty differ in pre- and post-pubertal grass rats

As animals progress from one developmental stage to the next, the nature of the challenges they face can change in systematic ways, as do the mechanisms that enable them to deal effectively with them. Here we examined the changes in the behavioral patterns and neuroendocrine mechanisms associated with exposure to a novel environment before and after the transition from a pre- to a post-pubertal stage of development in the unstriped Nile grass rat (Arvicanthis niloticus), a murid rodent that appears to live in family groups in the wild. We introduced juvenile (28 days old) and adult (60 days old) grass rats to a novel glass aquarium where they were kept for 1 h; controls were maintained in their home cages during this time. Animals were then killed, blood was sampled, and plasma corticosterone and testosterone were measured. Brains were collected and processed for immunohistochemical detection of Fos. Although behavior in the novel environment did not differ as a function of age, corticosterone secretion and Fos expression in a variety of stress-related brain regions were increased by the manipulation to a greater extent in the juveniles compared to the adults. The data suggest a pattern of development in which a novel environment that elicits the same levels of exploratory behaviors in young and adult animals triggers a greater response in stress-related brain regions as well as corticosterone secretion in the more vulnerable young ones.