Agonist/antagonist analgesics and nigrostriatal dopamine metabolism in the rat: evidence for receptor dualism.

Agonist/antagonist (Ag/Ant) analgesics possess bell-shaped dose-response curves with regard to nigrostriatal dopamine (DA) metabolism in the rat. Using local drug injections as well as parenteral drug, after acute hemisection, we have determined that this phenomenon is not the result of autoinhibition. Our data clearly indicate that the receptor population antagonizing the agonist actions of Ag/Ant is not localized within the striatum or substantia nigra. These results therefore present evidence for receptor dualism with Ag/Ant analgesics.