Enantioselective behavioral effects of threo-methylphenidate in rats.

The relative potency of d- and l-threo-methylphenidate (d-MPH and l-MPH) was evaluated using three behavioral paradigms for rats: Responding maintained by a fixed-interval schedule of reinforcement (FI), responding maintained by a concurrent variable-interval schedule of reinforcement (Conc VI VI), and consumption of sweetened condensed milk during a 15-min free-access period. In each case the potency of the d-MPH enantiomer greatly exceeded that of the l-MPH enantiomer. Temporal control of responding was reduced (FI) choice responding was equalized for most rats (Conc VI VI), and milk consumption was suppressed by d-MPH and dl-MPH.