Acetylcholine-Stimulated [3H]GABA Release from Mouse Brain Synaptosomes is Modulated by α4β2 and α4α5β2 Nicotinic Receptor Subtypes

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Abstract
Nicotinic acetylcholine receptor (nAChR) agonists stimulate the release of GABA from GABAergic nerve terminals, but the nAChR subtypes that mediate this effect have not been elucidated. The studies reported here used synaptosomes derived from the cortex, hippocampus, striatum, and thalamus of wild-type and α4-, α5-, α7-, β2-, and β4-null mutant mice to identify nAChR subtypes involved in acetylcholine (ACh)-evoked GABA release. Null mutation of genes encoding the α4 or β2 subunits resulted in complete loss of ACh-stimulated [3H]GABA release in all four brain regions. In contrast, α5 gene deletion exerted a small but significant decrease in maximal ACh-evoked [3H]GABA release in hippocampus and striatum, with a more profound effect in cortex. Acetylcholine-stimulated [3H]GABA release from thalamic synaptosomes was not significantly affected by α5 gene deletion. No effect was detected in the four brain regions examined in α7- or β4-null mutant mice. Further analysis of ACh-evoked [3H]GABA release revealed biphasic concentration-response relationships in the four brain regions examined from all wild-type animals and in α5 null mutant mice. Moreover, a selective reduction in the maximum response of the high-affinity component was apparent in α5-null mutant mice. The results demonstrate that α4β2-type nAChRs are critical for ACh-stimulated [3H]GABA release from all four brain regions examined. In addition, the results suggest that α5-containing receptors on GABAergic nerve terminals comprise a fraction of the high ACh-sensitivity component of the concentration-response curve and contribute directly to the ability of nicotinic agonists to evoke GABA release in these regions.

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