Dopamine receptors in the central nervous system

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.