Selective labeling of alpha-adrenergic receptors in caudate nucleus by [3H] dihydroergocryptine in the presence of spiperone-blocked dopamine receptors.

Because it was known that [(3)H]dihydroergocryptine can label alpha-adrenergic receptors as well as dopamine receptors, this study was done to establish the conditions under which [(3)H]dihydroergocryptine would be a reliable ligand for selective labeling of alpha-adrenergic receptors. The calf caudate was chosen because it contains both dopamine and adrenergic receptors, and 5 nM spiperone (spiroperidol) was used to block the neuroleptic/dopamine receptors. Thus, in the presence of spiperone, [(3)H]dihydroergocryptine exhibited saturable binding with a K(d) of 0.73 nM and a total number of sites of 150 fmol/mg of protein. The catechol neurotransmitters competed for [(3)H]dihydroergocryptine binding in the potencies order epinephrine > (-)-norepinephrine > dopamine, indicating that [(3)H]dihydroergocryptine (in the presence of 5 nM spiperone) was revealing alpha receptors. The alpha-adrenergic antagonists also competed for binding in the appropriate order: phentolamine > phenoxybenzamine > dibenamine. Finally, chlorpromazine was more potent than haloperidol in competing for [(3)H]dihydroergocryptine, also in accord with the properties of alpha receptors. These results with [(3)H]dihydroergocryptine as an alpha-adrenergic receptor ligand correlate well with those published by others for [(3)H]WB-4101.