THE EFFECTS OF AMPHETAMINE, APOMORPHINE, SKF-38393, QUINPIROLE AND BROMOCRIPTINE ON RESPONDING FOR CONDITIONED REWARD IN RATS

The present study investigated the effects of the dopamine (DA) D1 selective agonist, SKF 38393, and the D2 selective agonists, quinpirole and bromocriptine, on responding for conditioned reward. The nonselective DA agonist apomorphine and the indirect agonist amphetamine, were also evaluated. Male rats (n = 302) were tested in a procedure consisting of three distinct phases. During the pre-exposure phase the rats were exposed to an operant chamber containing two levers; one lever produced a lights-off stimulus (3 s) and the other a tone stimulus (3 s). This was followed by 4 conditioning sessions during which the levers were removed and rats received pairings of the lights-off stimulus (80 per day) and food, presented according to a variable time 45 s schedule. Two test sessions followed during which the levers were present and the number of responses made on each lever was calculated as a ratio of the number of responses made during pre-exposure. Drugs were administered prior to each test session. A saline group showed a higher ratio of responding for the lights-off stimulus than the tone stimulus, indicating that the lights-off stimulus had become a conditioned reward. Amphetamine (0.01-2.0 mg/kg) and to a lesser extent, quinpirole (0.01-5.0 mg/kg) and bromocriptine (0.05-10.0 mg/kg) dose-dependently increased responding and specifically enhanced responding on the lever producing the conditioned reward. Apomorphine (0.1-5.0 mg/kg) increased responding on both levers at higher doses but the conditioned reward effect was lost. SKF 38393 (0.1-10.0 mg/kg) appeared to impair the acquisition of responding for conditioned reward. The results were interpreted as indicating that responding for conditioned reward may be dependent on a D1 receptor-mediated reward signal.