Evaluation of the pharmacological similarities between phenylpropanolamine and amphetamine: effects on...

In an effort to determine the degree to which the repeated administration of phenylpropanolamine (PPA) results in the development of tolerance to its disruptive effects on operant responding as well as cross-tolerance to the effects of acutely administered amphetamine, water-deprived rats were first trained on a fixed-ratio 5 (FR-5) schedule for water presentation. Dose-response curves for the effects of PPA and amphetamine (administered IP, 15 min presession) were then determined (ED50 = 35.0 and 2.6 mg/kg, respectively) followed by the chronic administration of 40.0 mg/kg PPA (administered IP, 15 min prior to each session). When responding returned to prechronic rates, the dose-response curves were redetermined for both PPA (ED50 = 220 mg/kg) and amphetamine (ED50 = 4.8 mg/kg). In a second set of rats, trained under similar conditions, it was observed that pretreatment with alpha-methyltyrosine (AMT, 100 mg/kg IP, 2 h presession) antagonized the disruptive effects of both PPA and amphetamine, whereas pretreatment with reserpine (0.31 mg/kg, IP, 12 h presession) antagonized the disruptive effects of PPA, but exacerbated the disruptive effects of amphetamine. In a separate experiment, the repeated administration of PPA 100 mg/kg or 200 mg/kg IP resulted in no long-lasting depletions of rat striatal dopamine, serotonin, or 3,4-dihydroxyphenylacetic acid (DOPAC) concentrations. These observations indicate that PPA and amphetamine share a similar mechanism of action to the degree that cross-tolerance develops, but which is nonetheless dissociable with respect to their differential sensitivity to antagonists and their neurotoxic efficacy.