PHARMACOLOGICAL CHARACTERIZATION OF NICOTINIC RECEPTOR-STIMULATED GABA RELEASE FROM MOUSE-BRAIN SYNAPTOSOMES

Several recent electrophysiological studies have demonstrated that nic otinic 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 s ynaptosomes preloaded with [H-3]-GABA in a concentration-dependent man ner. This release appeared rapidly, was Ca++ dependent, and was partia lly (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 efficac y (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 obta ined for these same agonists when Rb-86(+) efflux was determined. A si gnificant correlation (r = 0.84, P < .01) was found when the EC50 valu es for agonist-stimulated [H-3]-GABA release and IC50 values for agoni st inhibition of [H-3]-L-nicotine binding were compared. Differences i n [H-3]-GABA release were detected in 12 brain regions and maximal rel ease was significantly correlated with [H-3]-nicotine binding. The pha rmacological and regional comparisons suggest that the nAChR that stim ulates [H-3]-GABA release is the one that binds [H-3]-nicotine with hi gh affinity (alpha 4 beta 2), Unequivocal evidence that the receptor t hat modulates nicotine-stimulated [H-3]-GABA release contains a beta 2 subunit was obtained in a study using wild-type, heterozygous and hom ozygous 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.