A comparison of the effects of the D1 receptor antagonists SCH 23390 and SCH 39166 on suppression of feeding behavior by the D1 agonist SKF38393.

The hypophagic effect of the D1 receptor agonist SKF 38393 is not dose-dependently antagonized by the D1 antagonist SCH 23390. Moreover, the receptor specificity of this interaction remains in question, since SCH 23390 has significant activity at both 5-HT2 and 5-HT1C receptors, and SKF 38393 also interacts with 5-HT1C receptors. To determine the relative significance of these actions, a comparison was made between the anorectic effects in rats of SCH 23390 (0.1-1.0 mg/kg) and the benzonaphthazepine SCH 39166 (0.1-3.0 mg/kg), a D1 antagonist with negligible affinity for 5-HT sites. Both compounds inhibited food-intake dose-dependently, with SCH 23390 being approximately twice as potent as SCH 39166. Behaviorally inactive and active doses of both antagonists were tested in combination with the D1 agonist SKF 38393 (10-56 mg/kg). Neither antagonist was able to produce more than a marginal attenuation of the agonist-induced hypophagia. This demonstrates that previous failures to reverse the behavioral actions of SKF 38393 by SCH 23390 were not due to specific actions of this particular antagonist. Finally, like SCH 23390, SCH 39166 (0.3 mg/kg) was able to attenuate fully the anorectic effects of the D1 agonist SKF 82958 (1.0 and 3.0 mg/kg), demonstrating that neither compound is intrinsically unable to block D1 receptor-mediated hypophagia. The results demonstrate the generality of the D1 antagonist-mediated effect on feeding and call into question the use of SKF 38393 as a D1 agonist in studies of feeding, and perhaps in other contexts as well.