ANALYSIS OF DRUG EFFECTS ON MULTIPLE FIXED RATIO 33-FIXED INTERVAL 5 MIN IN PIGEONS

Several drugs were tested for their effects on the performance of pigeons in a schedule of positive reinforcement (Multiple Fixed Ratio 33-Fixed Interval 5 min). The experiments consisted of three successive cross-over designs, each involving a pair of drugs: amphetamine versus scopolamine; phenobarbital versus chlordiazepoxide; chlorpromazine versus haloperidol. Diazepam was tested alone at the end of the experiments. Amphetamine induced an enhancement of overall FI response rates at lower doses, while higher doses were followed by a reduction or a reversal of effect in most animals. The response enhancement was not necessarily associated to an alteration of the characteristic FI scalloped pattern (quantified by means of quarter-life measurements), while higher doses caused a marked enhancement of responding in the early portion of the interval. Scopolamine provoked a dose-dependent reduction of response rate and of quarter-life. Marked effects on the latter appeared, in general, at doses higher than those sufficient to depress response rate. Nevertheless, this drug was the most potent of all agents tested in regard to attenuation of the FI curvature. On the other hand, the discrimination between the FI and FR components of the schedule, based on visual stimuli, was unaffected. Phenobarbital provoked a marked dose-dependent enhancement of response rate and a reduction of quarter-life. Chlordiazepoxide gave a clear-cut, dose-dependent reduction of response rate in some animals, while negligible or non-systematic changes were observed in others. Diazepam provoked a wide variety of changes in opposite directions. With both benzodiazepine derivatives no significant average deviations of response rates from control baselines were obtained, while significant changes in quarter-life were observed. Again, the latter were in the direction of an increased proportion of early responses. The well-known properties of chlorpromazine, which gives a reduction of response rate and of quarter-life, were confirmed. However, a rather large dose (5 mg/kg) was necessary to obtain clear-cut, significant effects. On the contrary, haloperidol gave a major depression of response rate already at a dose of 0.05 mg/kg, but left the response distribution unaffected at all doses tested (0.05-0.4 mg/kg). © 1969 Springer-Verlag.