Effects of selective dopamine receptor agonists in rats trained to discriminate apomorphine from saline

Rats (N = 12) were trained to discriminate apomorphine (0.25 mg/kg, IP) from saline in a two-lever, food-reinforced (FR 30) drug discrimination paradigm. When the discrimination was acquired, various doses of apomorphine as well as several other dopamine receptor agonists were injected before test sessions. Apomorphine (0.03-0.25 mg/kg, IP) produced a dose-related increase in the percent of responses that occurred on the drug lever during test sessions. The selective DA2 receptor agonist piribedil (0.25-8.0 mg/kg, IP) produced a dose-related increase in drug lever responding that was similar to that seen with apomorphine. On the other hand, administration of the selective DA1 receptor agonist SKF 38393 (1.0-32 mg/kg, IP) resulted in principally saline lever responding, even at doses that substantially reduced the rate of responding. Administration of dopamine (1.0-8.0 mg/kg, IP), which does not readily cross the blood-brain barrier, also resulted in principally saline lever responding. These results suggest that the discriminative stimulus properties of apomorphine are based on its action at a receptor that is similar to the DA2 receptor that has been characterized in the periphery and that this receptor is centrally located.