Abstract

Several recent electrophysiological studies have demonstrated that nicotinic agonists stimulate the release of γ-aminobutyric acid (GABA) from rodent brain tissue. Our studies used a neurochemical approach to characterize nicotinic receptor-stimulated [³H]-GABA release from mouse brain synaptosomes. Nicotine increased [³H]-GABA release from synaptosomes preloaded with [³H]-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-β-erythroidine. α-Bungarotoxin had no effect. Twelve nicotinic agonists were compared for their effects on [³H]-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 ⁸⁶Rb⁺ efflux was determined. A significant correlation (r = 0.84, P < .01) was found when the EC₅₀ values for agonist-stimulated [³H]-GABA release and IC₅₀ values for agonist inhibition of [³H]-l-nicotine binding were compared. Differences in [³H]-GABA release were detected in 12 brain regions and maximal release was significantly correlated with [³H]-nicotine binding. The pharmacological and regional comparisons suggest that the nAChR that stimulates [³H]-GABA release is the one that binds [³H]-nicotine with high affinity (α4β2). Unequivocal evidence that the receptor that modulates nicotine-stimulated [³H]-GABA release contains a β2 subunit was obtained in a study using wild-type, heterozygous and homozygous β2 null mutant mice. [³H]-GABA release and [³H]-nicotine binding decreased along with the number of copies of the null mutant gene.