Some in vitro receptor binding properties of [3H]eticlopride, a novel substituted benzamide, selective for dopamine-D2 receptors in the rat brain.

The substituted benzamide compound eticlopride, (S)-(-)-5-chloro-3-ethyl-N-[(1-ethyl-2-pyrrolidinyl) methyl]-6-methoxysalicylamide hydrochloride (FLB 131), has been shown to selectively block dopamine-D2 binding sites in the rat brain. The compound was tritium-labelled to high specific radioactivity and was used for in vitro receptor binding studies. [3H]Eticlopride was found to bind specifically to rat brain homogenates with the highest binding in the striatum and lowest in the hippocampus. The binding was saturable with a high number of binding sites (49.5 pmol/g) and with very high affinity (0.17 nM). As with other benzamides, the binding of [3H]eticlopride was highly sodium-dependent. Lesioning of the striatal neurons with ibotenic acid reduced the binding by 50% while lesioning of the nigrostriatal pathways with 6-hydroxydopamine was without effect on the observed binding. The binding of [3H]eticlopride was inhibited potently by neuroleptic drugs, while compounds known not to interact with the dopamine-D2 binding sites were inactive. It is concluded that this new dopamine-D2 antagonist may be a useful tool for the study of dopamine-D2 binding sites due to its high affinity and good selectivity.