Comparison between the effects of propranolol and chlordiazepoxide on timing behaviour in the rat

Ten rats were trained to lever press for food reward on a schedule of differential reinforcement of low rates of response with a 20-s criterion (DRL 20). Ten more were trained on a new schedule of punishment, designed to be comparable to DRL 20 - differential punishment of high rates of response (DPH 20). Under this schedule, responses with a latency of 20 s or more earned food rewards, while those of less than 20 s were followed by food reward and brief electric footshock. After 42 sessions, rats on each schedule showed temporal discrimination in the distribution of inter-response times. The effects on these baselines of the anxiolytic chlordiazepoxide (CDP; 1 mg/kg IP) and the beta-blocker propranolol (2, 5 and 10 mg/kg IP) were investigated. Both drugs reduced numbers of responses reaching criterion (criterion responses) in DPH, CDP increasing total responses. CDP acted similarly under DRL, but propranolol only affected performance at the highest dose, which reduced criterion responses, probably because of changes in total responding. Each drug increased response bursts. It is concluded that propranolol can exert a disinhibitory action in these schedules, although with some differences from that of the benzodiazepine CDP.