MECHANISM OF MODULATION OF [H-3] RACLOPRIDE BINDING TO DOPAMINERGIC RECEPTORS IN RAT STRIATAL MEMBRANES BY SODIUM-IONS

The mechanism of modulation of [H-3]raclopride binding to dopaminergic receptors in rat brain striatal membranes by sodium ions was studied by means of equilibrium and kinetic measurements. Among different mono - and divalent cations studied, only sodium and lithium ions significa ntly enhanced [H-3]raclopride binding to rat striatal membranes, but t he effect of lithium was considerably smaller if compared with that of sodium. The equilibrium binding studies revealed that the increase in Na+ concentration from 0.5 to 150 mM increased both the radioligand a ffinity and the number of binding sites. The meaning of these changes was established by kinetic studies, which yielded hyperbolic plots of [H-3]raclopride binding rate constants over the radioligand concentrat ion. These plots correspond to the two-step ligand binding reaction me chanism, involving fast binding equilibrium followed by a slow isomeri zation of the receptor-antagonist complex. Sodium ions did not influen ce the antagonist affinity for the receptor sites in the first step of the binding process, nor the rate of isomerization of the receptor-li gand complex, but slowed down the rate of deisomerization. This led to a change in the value of the receptor-ligand dissociation constant K- d determined under equilibrium conditions. The same change in deisomer ization rate was also sufficient to alter the receptor density (B-max) , measured by the conventional ligand binding procedure. (C) 1997 Else vier Science Ltd.