Multiple binding sites for nicotine receptor antagonists in inhibiting [3H](-)-nicotine binding in rat cortex

The displacement of [3H](-)-nicotine from its binding site in rat cerebral cortex by a number of antagonists was investigated. [3H](-)-Nicotine appeared to bind to a single site with a dissociation constant (KD) of 5.5 nM; pancuronium, gallamine and trimetaphan displaced [3H](-)-nicotine with inhibition constants (KI) of 57, 99 and 621 microM, respectively, whereas mecamylamine only displaced 50% of nicotine binding in concentration > 1 mM. For hexamethonium and (+)-tubocurarine the displacement of [3H](-)-nicotine binding appeared to involve two sites; the higher affinity site comprising 30% of the total binding for hexamethonium but 76% of the sites for (+)-tubocurarine. In the presence of mecamylamine (100 microM) the displacement of [3H](-)-nicotine binding by (+)-tubocurarine appeared to involve only a single site with an affinity similar to that for the high affinity site in the absence of mecamylamine whereas that for hexamethonium still involved two sites. It is suggested that (+)-tubocurarine may act at both the binding site for [3H](-)-nicotine per se and also at an allosteric site. The failure of mecamylamine to influence the binding of hexamethonium suggests that there may be more than one allosteric site or that hexamethonium may distinguish between subtypes of nicotine receptors in the cortex.