Pharmacokinetics and pharmacodynamics of methylphenidate enantiomers in rats

We investigated the relationships between methylphenidate (MPD) enantiomers and endogenous dopamine (DA) levels in striatal extracellular fluid, and that between DA level and locomotor activity, after intravenous administration of racemic MPD (2,5 or 10 mg/kg dose) or the individual enantiomers (2.5 mg/kg dose) to rats. MPD and DA levels in the extracellular fluid were measured by in vivo brain microdialysis. The maximum levels of MPD enantiomers in the striatal extracellular fluid were obtained within 15 min after administration. On the other hand, the mean maximum DA levels after administration of 2–10 mg/kg dose of racemic MPD were obtained within 10 min with values in the range of 3.0- to 8.6- fold higher than the basal DA level. The maximum DA level (4.2-fold of the basal level) after administration of (+)-MPD was greater than that (2.2-fold) of the same dose of (−)-MPD. A clockwise hysteresis was observed between MPD concentration and DA level in the extracellular fluid after MPD administration. Locomotor activity after administration of (+)-MPD was also greater than (−)-MPD. From these results, it was shown that the locomotor activity induced by MPD may be related to the increase of DA level in the extracellular fluid, and the degree of increase of the DA level by (+)-MPD was greater than that of the (−)-isomer.