Cyclic-ratio schedule analysis of a serotonin agonist and depletor on consummatory behaviour

Four male Wistar rats, 180 days old at the start of the experiment, at 85% of their free-feeding body weight were trained to respond on a geometric cyclic-ratio schedule comprised of the following ratio values: 2, 4, 8, 16, 32 and 64, for 0.1 ml of 5% sucrose reinforcement. The response functions (response rates plotted against reinforcement rates) were linear and of negative slope over the range of ratio values from 2 to 16. IP administration of 1.0 mg/kg 5-MeODMT reduced the x- and y-intercepts of the linear portion of the response function without altering the slope relative to 2.0 ml/kg 0.85% saline IP. This was interpreted as a perceived palatability effect. IP administration of 100 mg/kg pCPA elevated the reinforcement rate intercept but also decreased the slope of the response function. This finding was interpreted as an increase in the perceived palatability of the reinforcer, coupled with a decrease in motivation at higher schedule conditions, possibly due to peripheral effects of pCPA.