Dependence of adolescent novelty-seeking behavior on response phenotype and effects of apparatus scaling

Adult rats have been phenotyped as high (HRs) or low (LRs) responders to novelty, the former associated with high-risk behaviors. Data indicating that adolescent rodents exhibit increased novelty-seeking relative to adults are equivocal, and phenotypic variations in adolescent novel stimulus reactivity are unknown. To determine whether novelty-seeking differs between adolescent and adult rats, reflecting phenotypic variations within each age, activities in an inescapable novel environment and novel object exploration were measured. Adolescents moved further, faster, and more continuously than adults, and exhibited a greater variability and range of activity in the novel environment. Both adolescent and adult LRs habituated to the environment by the second trial, but HRs maintained increased activity throughout 8 trials. In the free-choice paradigm, adolescents approached the novel object more frequently and spent more time in proximity to the object than adults. Similar results were obtained using arenas and objects of the same size or scaled to animal size. Results confirm that novelty-seeking by adolescents is greater than by adults, a behavior attributed primarily to the response magnitude exhibited by adolescents with the HR phenotype.