The effects of methylphenidate on novel object exploration in weanling and periadolescent rats.

Methylphenidate (Ritalin) is used in the treatment of attention-deficit hyperactivity disorder. Surprisingly, little research has been conducted on the effects of methylphenidate during early development. Therefore, the present study was conducted to examine the effects of methylphenidate on object exploration in developing rats. Male and female weanling (21-day-old) and periadolescent (34-day-old) Sprague-Dawley rats were tested after acute or chronic treatment with methylphenidate. In weanling rats, chronic methylphenidate (5.0 mg/kg) increased locomotor activity and disrupted novel object exploration. In periadolescent rats, methylphenidate disrupted exploration of the novel object, but had no effect on locomotor activity at any dose tested. Periadolescent rats appear to be less sensitive to methylphenidate-induced changes in activity compared to weanling animals, whereas methylphenidate disrupted novel object exploration in both ages. Our results suggest that methylphenidate may alter recognition memory and/or reactivity to or preference for novelty.