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

Because it was known that [3 H]dihydroergocryptine can label α-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 α-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 aK 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 α receptors. The α-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 α receptors. These results with [3 H]dihydroergocryptine as an α-adrenergic receptor ligand correlate well with those published by others for [3 H]WB-4101.