Effects of 5-HT3 antagonists on fixed-interval behavior in rats

The effects of 5-hydroxytryptamine3 (5-HT3) antagonists were studied in rats responding under a fixed-interval (FI) schedule of food presentation. BRL 43694, GR 38032F, ICS-205,930 and zacopride had no effect on FI responding up to 10.0 mg/kg IP. Response rate decreases were observed only at high doses (30.0 or 56.0 mg/kg); rate increases were not observed. In comparison, caffeine (1.0-10.0 mg/kg) and cocaine (3.0-18.0 mg/kg) increased and lorazepam (0.3 mg/kg) decreased FI responding. The effects of caffeine, cocaine, and lorazepam were unchanged when coadministered with the 5-HT3 antagonists. The results show that 5-HT3 antagonists, when given alone or with other CNS-active drugs, have little effect on FI responding of rats.