Abstract
Early treatment with methylphenidate has a persistent effect on the affective (i.e., anxiety- and depressive-like) behaviors of adult rats and mice. Interestingly, age at methylphenidate exposure appears to be a critical determinant influencing the expression of affective behaviors. In the present study, we exposed rats to methylphenidate during the preweanling period (i.e., PD 11-PD 20) because this ontogenetic period is analogous to early childhood in humans (an age associated with increasing methylphenidate usage). Rats were injected with methylphenidate (0, 2, or 5mg/kg) from PD 11 to PD 20 and reactivity to rewarding and aversive stimuli were measured in early adulthood. Specifically, novelty-induced CPP, sucrose preference, and elevated plus maze behavior were assessed on PD 60. Early treatment with 2 or 5mg/kg methylphenidate increased total time spent in the white compartment of the CPP chamber. This methylphenidate-induced effect occurred regardless of exposure condition. Performance on the elevated plus maze was also impacted by early methylphenidate exposure, because rats treated with 5mg/kg methylphenidate spent more time in the closed compartment of the elevated plus maze than vehicle controls. Early methylphenidate exposure did not alter sucrose preference. These data indicate that exposing rats to methylphenidate during the preweanling period differentially affects anxiety-like behavior depending on the type of anxiety-provoking stimulus. Specifically, early methylphenidate exposure decreased aversion to a bright white room when measured on a novelty-induced CPP task, whereas methylphenidate caused a long-term increase in anxiety when measured on the elevated plus maze.