Pharmacological characterization of nicotinic receptor-stimulated GABA release from mouse brain synaptosomes

Several recent electrophysiological studies have demonstrated that nicotinic agonists stimulate the release of gamma-aminobutyric acid (GABA) from rodent brain tissue. Our studies used a neurochemical approach to characterize nicotinic receptor-stimulated [H-3]-GABA release from mouse brain synaptosomes, Nicotine increased [H-3]-GABA release from synaptosomes preloaded with [H-3]-GABA in a concentration-dependent manner. This release appeared rapidly, was Ca++ dependent, and was partially (about 50%) blocked by 100 nM tetrodotoxin and totally blocked by mecamylamine and dihydro-beta-erythroidine. alpha-Bungarotoxin had no effect. Twelve nicotinic agonists were compared for their effects on [H-3]-GABA release. The agonists differed in potency (EC50) and efficacy (E-max). The EC50 and E-max values were significantly correlated (r = 0.95, P < .001 for EC50; r = 0.93, P < .01 for E-max) to values obtained for these same agonists when Rb-86(+) efflux was determined. A significant correlation (r = 0.84, P < .01) was found when the EC50 values for agonist-stimulated [H-3]-GABA release and IC50 values for agonist inhibition of [H-3]-L-nicotine binding were compared. Differences in [H-3]-GABA release were detected in 12 brain regions and maximal release was significantly correlated with [H-3]-nicotine binding. The pharmacological and regional comparisons suggest that the nAChR that stimulates [H-3]-GABA release is the one that binds [H-3]-nicotine with high affinity (alpha 4 beta 2), Unequivocal evidence that the receptor that modulates nicotine-stimulated [H-3]-GABA release contains a beta 2 subunit was obtained in a study using wild-type, heterozygous and homozygous beta 2 null mutant mice. [H-3]-GABA release and [H-3]-nicotine binding decreased along with the number of copies of the null mutant gene.