3H-domperidone binding sites differ in rat striatum and pituitary

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Abstract
3H-Domperidone (3H-DOMP) binding sites were compared in rat striatum and pituitary, regarding the effects of the non-hydrolysable GTP analog, Gpp(NH)p and inhibition by various dopamine (DA) antagonists. Gpp(NH)p (0.1 mM) elicited in both tissues a rightward shift in DA concentration-inhibition curves, but the changes in either IC50 values or pseudo-Hill coefficients were larger in pituitary than in striatum. Computer analysis of the data showed that, in the presence of Gpp(NH)p, the curve obtained in striatum is best explained by the presence of two classes of binding site, a high-affinity site (Ki=95 nM, 27% of total binding) and a low-affinity site (Ki=5, 100 nM, 73% of total binding), whereas in pituitary only a low-affinity site (Ki=5,070 nM) could be detected. In striatum, several discriminant benzamide derivatives (DBD), (−)-sulpiride and recently developped compounds, allowed to distinguish two components among 3H-DOMP binding sites: a high-affinity site representing one-third of total binding and a low-affinity component displaying a 8–17 fold lower affinity. Classical neuroleptics like haloperidol, chlorpromazine and metoclopramide inhibited striatal 3H-DOMP binding in a monophasic manner. In pituitary a single component could be detected for all tested antagonists including the DBD and Ki values for the latters were identical to those found for the low-affinity component in striatum. Gpp(NH)p (0.1 mM) had no effect on the biphasic inhibition of striatal binding by the most discriminant compound RIV 2093, suggesting that the presence of two components for this DBD is not related to the two conformational states of the D-2 receptor. Also the discriminant property of RIV 2093 is not related to its binding to a sodium-dependent site, since the sodium effects (leftward shift of the inhibition curve) are very similar for RIV 2093 and the non discriminant substituted benzamide metoclopramide. It is concluded that, in addition to a fully GTP-sensitive site, analogous to the D-2 receptor detected in pituitary, striatum also contains a distinct class of binding site, not or less affected by GTP, for which DBD display a high affinity.