Effects of methylphenidate, pentobarbital, and reserpine on behavior controlled by a schedule of interresponse time reinforcement

Summary

Three rats, deprived of water, were given separate injections of methylphenidate and pentobarbital after each had received extensive training on a DRL schedule of reinforcement, modified by the inclusion of a limited-hold. Methylphenidate (2.5, 5.0, and 10.0 mg/kg) increased response rates at each of these dosages; pentobarbital increased response rates at dosages of 2.5 mg/kg and 5 mg/kg but had little effect on overall response rate at 10 mg/kg though, at this dose, performance was characterized by periods of rapid responding alternating with periods of inactivity. The rate-increasing effects of the two drugs were found to be qualitatively different.

Several mixtures of the two drugs induced rate increases greater than those found when each drug was given separately. However, pentobarbital (10 mg/kg) reduced the rate-increasing effects of methylphenidate obtained when the latter was given alone at a dosage of 10 mg/kg.

Chronic pretreatment with reserpine (0.3 mg/kg for 12 days), given to two animals, progressively reduced rate of responding. Methylphenidate antagonized the effects of reserpine as an increasing function of dosage but the resulting pattern of behavior more closely resembled the effects of methylphenidate given alone than performance recorded in the absence of any drug treatment.