Psychostimulants given in adolescence modulate their effects in adulthood using the open field and the wheel-running assays.

Acute and chronic methylphenidate (MPD) were given to adults treated with MPD only in adulthood (adult I) and to adults that had been treated repeatedly during adolescence and adulthood (adult II). Two locomotor activity assays, the open field and the running wheel, were used in a dose response experiment to assess whether methylphenidate (MPD) treatment during adolescence would affect responses to MPD in adulthood. Each experiment lasted 11 days as follows: saline control on experimental day 1 (ED 1), followed by a single daily dose of saline, 0.6, 2.5, or 10mg/kg MPD for 6 days (ED 2 to ED 7), 3 washout days with no drug administration (ED 8 to ED 10), and saline or MPD challenge on ED 11 at a dose identical to that given on ED 2 to ED 7. Acute MPD elicited characteristic dose response increases in locomotion in both experimental assays of adult I and adult II groups. Adult I and adult II rats tested in the open field assay exhibited sensitization to 2.5mg/kg MPD and tolerance to 10mg/kg MPD, while in the wheel-running assay all the three MPD doses elicited sensitization in both adult I and adult II rats. MPD treatment in adolescence did not change the baseline activity when animal reached adulthood. However, the responses to MPD in adult II rat groups were significantly different from the adult I group. Similar observations were noted during washout days. At the low and moderate MPD treatment both experimental assay exhibited similar observations while following the high dose of MPD treatment, the open field assay indicated that tolerance to MPD was expressed, while the wheel-running assay indicated that behavioral sensitization was developed. The distinction between the two assays and adult I and II differences are discussed.