Sodium channel blockers partially inhibit nicotine-stimulated 86Rb+ efflux from mouse brain synaptosomes

The possible contribution of non-nicotinic cation channels to nicotine-stimulated 86Rb+ efflux was investigated. Dendrotoxin, charybdotoxin and apamin did not affect nicotine-stimulated 86Rb+ efflux. Although 4-aminopyridine and tetraethyl-ammonium completely inhibited nicotine-stimulated 86Rb+ efflux, the concentrations required to achieve this inhibition differed markedly from those required to inhibit K(+)-stimulated 86Rb+ efflux. Cs+ reduced basal 86Rb+ efflux, but did not affect nicotine-stimulated efflux. Tetrodotoxin completely inhibited veratridine-stimulated 86Rb+ efflux (IC50 = 3.4 nM) and partially inhibited (about 42%) nicotine-stimulated efflux (IC50 = 1.3 nM). Saxitoxin also completely inhibited veratridine-stimulated efflux (IC50 = 19.5 nM) and partially inhibited (about 47%) nicotine-stimulated efflux (IC50 = 5.4 nM). Tetrodotoxin had no significant effect on the EC50 for nicotine, but decreased the efflux rate and reduced modestly the rate of desensitization measured with prolonged exposure to nicotine. Tetrodotoxin (50 nM) also inhibited maximal nicotine-stimulated 86Rb+ efflux to approximately the same extent in each of ten brain regions examined. Nicotine-stimulated 86Rb+ efflux measured in the presence or absence of 100 nM tetrodotoxin is correlated with the density of [3H]nicotine, but not [3H]saxitoxin, binding sites in 12 regions, suggesting that the density of nicotinic receptors, not the number of Na+ channels, is the rate-limiting step for nicotine-stimulated 86Rb+ efflux. Therefore, activation of Na+ channels secondary to the stimulation of nicotinic receptors seems to contribute to the nicotine-stimulated 86Rb+ efflux observed in mouse brain synaptosomes. In contrast, no evidence for K+ channel modulation of nicotine-stimulated 86Rb+ efflux from thalamic synaptosomes was obtained.