The anti-nociceptive dose of difenamizole, morphine, aminopyrine and aspirin was studied for effects on behavior maintained by schedule of positive reinforcement. Male, albino rats were trained to press a lever for food pellets on a fixed-ratio (FR) 10 and 30 schedule of reinforcement or a differential reinforcement of low rates of responding (DRL) schedule. Difenamizole (200 and 400 mg/kg, p.o.) produced a dose-related decrease in the response under FR-10 schedule. The response rate decrease observed under the FR-10 schedule was similar to that resulting from the oral administration of 400 mg/kg of aminopyrine. Response in the FR-30 schedule was not affected by any dose of difenamizole (100~400 mg/kg, p.o.) and aminopyrine (200~400 mg/kg, p.o.). In the response maintained by the DRL schedule, the overall response rate and the mean interresponse time were not altered significantly by most doses of difenamizole, aminopyrine and aspirin given, however, food reinforcement was decreased significantly with ingestion of these drugs. Morphine (20 mg/kg, p.o.) shortened the mean interresponse time and increased the response in DRL schedule. These results suggest that the central action of difenamizole is similar to that produced by aminopyrine, but not that produced by morphine.
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