ABSTRACT
Dopamine receptors in the central nervous system can be studied by measuring the specific binding of [3H]dopamine, [3H]haloperidol, d-[3H]LSD, [3H]dihydroergocryptine or [3H]apomorphine. The receptors are stereoselectively blocked by +)-butaclamol, a neuroleptic. All neuroleptics inhibit the specific binding of [3H]haloperidol in relation to their clinical potencies. The radioligand that desorbs most slowly from the receptor is [3H]apomorphine, thus making it a reliable ligand for dopamine receptors. Dopamine agonists that compete for [3H]apomorphine binding do so at concentrations that correlate with their potency in stimulating striatal adenylate cyclase. Structure-activity analysis, using [3H]apomorphine, confirms that the active dopamine-mimetic conformation is the beta rotamer of dopamine. Prolonged exposure in vitro of caudate homogenate to high concentrations of dopamine leads to increased binding of [3H]apomorphine or [3H]haloperidol, suggesting receptor "sensitization." Chronic haloperidol treatment of rats leads to an increased number of dopamine/neuroleptic receptors in the striatum, but a decrease in the pituitary.
Subject(s)
Central Nervous System , Receptors, Dopamine , Adenylyl Cyclases , Animals , Apomorphine/pharmacology , Butaclamol/pharmacology , Cattle , Dihydroergotoxine/pharmacology , Haloperidol/pharmacology , Lysergic Acid Diethylamide/pharmacology , Radioligand Assay , Receptors, Adrenergic/metabolismABSTRACT
Because dihydroergocryptine (DHE) and closely related ergots are dopamine-mimetic agonists, we tested [3H]DHE as a possible ligand for [3H]dopamine receptors in the calf caudate. In order to avoid [3H]DHE from tagging alpha-adrenergic receptors, an excess of 500 nM phentolamine was used to block these sites, permitting the dopamine receptors to be measured separately. Specific binding of [3H]DHE was defined as total binding minus that occurring in the presence of 1 muM (+)-butaclamol. Excess phentolamine reduced the specific binding of [3H]DHE from 328 down to 138 fmol/mg, the difference presumably representing alpha-receptors. The KD for [3H]DHE was 0.55 nM (with or without phentolamine), and this high affinity site was blocked (in the presence of phentolamine) by 250nM apomorphine, 650 nM dopamine, and 1200 mM (minus)-norepinephrine, indicating that[3H]DHE was binding to dopamine receptors. All neuroleptics blocked specific [3H]DHE binding in direct relation to the clinical potency of the neuroleptic. The displacement of specific [3H]DHE binding by dopamine or by norepinephrine (in the presence of phentolamine) revealed two subsets of dopamine receptors.
