Specific in vivo binding of 3H-spiperone to individual lobes of the pituitary gland of the rat. Evidence for the labelling of dopamine receptors.

The in vivo binding of 3H-spiperone to individual lobes of the pituitary gland was studied after intravenous injections in unanesthetized male rats. The binding was found to be saturable and reversible. The percentage of total binding of 3H-spiperone that was specific binding was highest in the intermediate (approximately equal to 75%) and lowest in the posterior (approximately equal to 35%) lobes. The regional distribution of 3H-spiperone binding 1 hour after injections was the following: intermediate greater than anterior greater than posterior. Pharmacological analysis of the in vivo 3H-spiperone binding showed that dopamine agonists (e.g. bromocriptine, N-n-propylnorapomorphine) and antagonists could prevent the in vivo binding of 3H-spiperone in all three parts of the gland. The substituted benzamide drugs remoxipride and raclopride blocked the in vivo 3H-spiperone binding in the anterior and intermediate lobes but did not reduce the 3H-spiperone binding in the posterior part, except when given in very high doses. Taken together, the present study has shown that 3H-spiperone can be used in studies of the dopamine receptors in the anterior, intermediate and posterior lobes of the pituitary gland, but the proportion of non-specific binding is higher than in the striatum. The use of in vivo 3H-spiperone binding may thus be a useful method to study the regulation and pharmacology of these receptors in situ.