Multiple receptors for brain dopamine.

This study was done to obtain direct in vitro evidence for the possible existence of more than one type of dopaminergic binding site in homogenates of the caudate nucleus from calf brain. Five radioligands for dopaminergic sites were tested. The inhibitions of two agonist radioligands ([3H]dopamine, [3H]apomorphine) by haloperidol, chlorpromazine, or piflutixol were biphasic. The inhibitions of [3H]haloperidol, [3H]spiroperidol, and [3H]dihydroergocryptine binding by dopamine and (-)-norepinephrine were also biphasic. Thus, both 3H-labeled agonists and 3H-labeled antagonists were possibly binding to two high-affinity sites.