Effects of extinction, pimozide, SCH 23390, and metoclopramide on food-rewarded operant responding of rats

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Abstract
The similarity in the pattern of responding produced by extinction and dopamine (DA) receptor blockers has led to the suggestion that DA neurons may participate in the usual effects of reward on behaviour. The purpose of the present study was to evaluate the effect of receptor-subtype specific DA antagonists on food-rewarded operant responding. Rats were trained to lever press for food on a variable interval 30-s schedule. They then received one of the following treatments prior to testing on the next 5 days: saline, nonreinforcement, the DA receptor blocker pimozide (0.5 or 1.0 mg/kg), the D1 receptor blocker SCH 23390 (0.01, 0.05, 0.1 mg/kg), and the D2 receptor blocker metoclopramide (1.0, 5.0, 10.0 mg/kg). Nonreinforcement resulted in both intra- and intersession declines in responding. The drugs produced dose-dependent decreases in overall responding. Additionally, both doses of pimozide and the higher doses of SCH 23390 and metoclopramide altered intrasession patterns of responding when compared to saline, with their greatest effect being in the latter portion of the session. Intersession declines were seen with the highest doses of SCH 23390 and metoclopramide and control studies showed that these declines could not be attributed to a buildup of the drug with repeated dosing. It was concluded that both D1 and D2 receptors participate in the control of behaviour by reward.