Differential effects of dopamine D1 and D2 receptor antagonists on conditioned orienting behavior in the rat

Abstract

Brain dopamine (DA) systems are known to be important in regulation of behavior conditioned to\r\nappetitive stimuli. Nevertheless, despite a large body of evidence showing behavioral deficits in the\r\noperant conditioning paradigm produced by DA receptor blockade, there have been relatively few\r\nstudies directly assessing behavioral changes in classical conditioning paradigm under this drug\r\ntreatment. By employing an appetitive Pavlovian conditioning task, the present work investigated the\r\neffects of selective D1\r\n and D2\r\n receptor antagonists on the expression and acquisition of the conditioned\r\norienting response (COR) and food-cup approach. SCH23390 (0, 0.05, and 0.10 mg/kg) and raclopride\r\n(0, 0.1, and 0.2 mg/kg) were administered via an intra-peritoneal route in a between-group design. Data\r\nfrom Experiment 1 showed that both SCH23390 and raclopride suppressed expression of the COR and\r\nfood-cup approach, but only the impairment produced by raclopride reached a significant level. In\r\nExperiment 2, with SCH23390 being administered during the acquisition phase, the suppressed COR\r\nwas completely restored in a subsequent (24 h later) drug-free session. In contrast, the suppressed COR\r\nin raclopride-pretreated groups was only partially restored. These findings support the view that the\r\nDA system plays a role in the neural substrates underlying this appetitive conditioning. In addition, D2\r\nreceptors are more likely involved in the modulation of learning process of the COR than D1\r\n receptors.