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

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 wildtype and ␣4-, ␣5-, ␣7-, ␤2-, and ␤4-null mutant mice to identify nAChR subtypes involved in acetylcholine (ACh)-evoked GABA release. ]GABA release revealed biphasic concentration-response relationships in the four brain regions examined from all wild-type animals and in ␣5 null mutant mice. Moreover, a selective reduction in the maximum response of the high-affinity component was apparent in ␣5-null mutant mice. The results demonstrate that ␣4␤2-type nAChRs are critical for ACh-stimulated [ 3 H]GABA release from all four brain regions examined. In addition, the results suggest that ␣5-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.The nicotinic acetylcholine receptor (nAChR) arguably represents one of the most evolutionarily conserved and well characterized neurotransmitter receptors, and the majority of high-resolution structural data have been accumulated from studies of the torpedo-and muscle-type nAChR (Millar and Gotti, 2009). It is generally believed that the structures of neuronal nAChRs are very similar to the muscle-type nAChR subunits because the amino acid sequences of neuronal nAChR subunits are similar to the muscle-type subunits (Lindstrom et al., 1998). Assuming that the neuronal nAChRs are pentameric assemblies that resemble the peripheral-type nAChR, the fact that mammalian neurons express mRNAs for nine nAChR genes (designated ␣2-␣7 and ␤2-␤4) suggests that many different nAChR subtypes might be expressed in the central nervous system.The importance of identifying the sites of expression and subunit compositions of those nAChRs that are actually expressed in brain is demonstrated by observations that the biophysical and pharmacological properties of nAChRs are affected when different subunit combinations are examined in heterologous expression systems (Zwart and Vijverberg, 1998;Moroni et al., 2006;Kuryatov et al., 2008). Of particular importance is the observation that ␣4␣5␤2 nAChRs are more cation-permeable than other high-affinity heteromeric nAChRs, and the calcium-permeability of these ␣4␣5␤2 nAChRs is exceeded only by ␣7-type nAChRs and N-methyl-D-aspartate glutamate receptors (Kuryatov et al., 2008). Indeed, previous studies performed in our laboratory (Brown et al., 2007) showed that deletion of the ␣5 subunit decreased maximal agonist-evoked 86 Rbϩ efflux in several brain re-

α6β2* and α4β2* Nicotinic Acetylcholine Receptors As Drug Targets for Parkinson's Disease

“…An almost complete dopaminergic lesion leads to only a 30 to 50% decline in striatal ␣4␤2* nAChRs; this would suggest that 50 to 70% of the ␣4␤2* receptors are present on dopaminergic terminals (Kulak et al, 2002a;Zoli et al, 2002;Champtiaux et al, 2003;Quik et al, 2003). Studies using synaptosomal preparations, which primarily contain nerve terminals, suggest that stri- Zoli et al, 2002;Quik et al, 2005 atal presynaptic ␣4␤2* nAChRs consist of both the ␣4␤2 and the ␣4␣5␤2 subtypes and are present on dopaminergic as well as other neurons (McClure-Begley et al, 2009;Grady et al, 2010b). Further evidence for this stems from studies showing that the ␣5 subunit declines by ϳ90% with nigrostriatal damage whereas the ␣4 subunit is decreased by only ϳ50% .…”

“…There is also evidence for functional ␣4␤2* (including ␣4␣5␤2) nAChRs on GABAergic terminals from striatum (Grilli et al, 2009;McClure-Begley et al, 2009). These could arise from GABA interneurons, axon collaterals from the medium spiny projection neurons or collateral projections from the GABAergic neurons in the globus pallidus (Bolam et al, 2000).…”

Prenatal Nicotine Exposure Increases GABA Signaling and Mucin Expression in Airway Epithelium

“…Certainly in the CNS, cholinergic signaling commonly modifies GABAergic signaling (17)(18)(19), and a similar interaction may occur in bronchial epithelial cells. Therefore, the present study sought to determine if the nicotinic cholinergic activation of GABAergic signaling occurs in BECs, and thus may underlie some of the connections between smoking and lung disease.…”

Nicotinic acetylcholine receptors and the ascending dopamine pathways

“…In addition to GABAergic projection neurons, the striatum also contains of a small population of GABAergic interneurons [75]. The nicotinic facilitation of [ 3 H]GABA release from mouse striatal synaptosomes has implicated α4β2* and α4α5β2* nAChRs on GABAergic terminals, from comparison of nAChR subunit null mutant mice [76]. It is unclear if these receptors indirectly modulate dopamine release, although the influence of dopamine on local GABA release is well documented [77,78].…”