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

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Abstract
Because it was known that [3H]dihydroergocryptine can label .alpha.-adrenergic receptors as well as dopamine receptors, this study was done to establish the conditions under which [3H]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. In the presence of spiperone, [3H]dihydroergocryptine exhibited saturable binding with a Kd of 0.73 nM and a total number of sites of 150 fmol/mg of protein. The catechol neurotransmitters competed for [3H]-dihydroergocryptine binding in the potencies order epinephrine > (-)-norepinephrine > dopamine, indicating that [3H]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. Chlorpromazine was more potent than haloperidol in competing for [3H]dihydroergocryptine, also in accord with the properties of .alpha. receptors. These results with [3H]dihydroergocryptine as an .alpha.-adrenergic receptor ligand correlate well with those published for [3H]WB-4101 [2-[.gamma.-(2,6-dimethoxyphenoxyethyl)] aminomethyl-1,4-benzodioxane].