A COMPARISON OF THE MOTOR-ACTIVATING EFFECTS OF ACUTE AND CHRONIC EXPOSURE TO AMPHETAMINE AND METHYLPHENIDATE

Acute exposure to methylphenidate (0.0, 5.0, 10.0, or 20.0 mg/kg) or a mphetamine (0.0, 0.5, 1.0, 2.0, or 4.0 mg/kg) dose-dependently increas ed horizontal activity. The amphetamine-induced increase in activity w as progressively augmented with repeated exposures over 7 days. In con trast, methylphenidate (20.0 mg/kg)-induced increases in activity beca me smaller with repeated exposures. Subthreshold doses of methylphenid ate (1.0 or 5.0 mg/kg) were ineffective in stimulating motor activity even after 7 daily exposures. These findings suggest that, although se nsitization develops with chronic amphetamine treatment, the consequen ce of chronic exposure to methylphenidate is tolerance. These data are discussed in terms of the different mechanisms through which methylph enidate and amphetamine affect central dopamine release.