Different effects of 5-HT receptor agonists on operant response in rats under DRL 10-s and DRL 30-s schedules

5-hydroxytryptamine (5-HT ) is thought to be involved in a wide range of behavioral functions. Based on binding evidence, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), 1-(2,5-dimethoxy-4-indophenyl)-2-aminopropane (DOI), and m-chloro-phenyl-biguanide (m-CPBG) are selective 5-HT1a, 5-HT2, and 5-HT3 receptor agonists, respectively. The present study examined the effects of these 5-HT receptor agonists on operant behavior maintained on the differential reinforcement for low rate response 10-second (DRL 10-s) and 30-second (DRL 30-s) schedules of reinforcement. Water-deprived rats were trained to press a lever in response to DRL 10-s and DRL 30-s schedules. After a stable baseline was set, each subject was then repeatedly challenged using one drug with different doses through peripheral administration. The dose ranges were 0.025, 0.05, and 0.1 mg/kg (SC) for 8-OH-DPAT; 0.5, 1, and 2 mg/kg (SC) for DOI; and 1.3 and 9 mg/kg (IP) for m-CPBG. The overall results of the present work indicate that distinct profiles of operant response on DRL 10-s and DRL 30-s schedules were produced by 8-OH-DPAT, DOI, and, m-CPBG, based on quantitative and qualitative data analyses. All three 5-HT receptor agonists caused the number of responses to the DRL 10-s schedule to decrease significantly in a dose-related fashion. The operant performance on the DRL 10-s schedule was more sensitive to drug treatment than was that on the DRL 30-s schedule. Analyses of inter-response time (IRT) distributions revealed that different time bins were shifted by each of these three agents. The current data indicate that 8-OH-DPAT, DOI and m-CPBG can significantly alter operant response maintained on a DRL schedule. The distinct operant performance for each drug is believed to be derived from drug activation of its own specific 5-HT subtype receptors.