Bidirectional Modulation of GABA‐Gated Chloride Channels by Divalent Cations: Inhibition by Ca2+and Enhancement by Mg2+

Abstract:The effects of the divalent cations Ca2+, Sr2+, Ba2+, Mg2+, Mn2+, and Cd2+were studied on γ‐aminobutyric acidA(GABAA) responses in rat cerebral cortical synaptoneurosomes. The divalent cations produced bidirectional modulation of muscimol‐induced36Cl−uptake consistent with their ability to permeate and block Ca2+channels. The order of potency for inhibition of muscimol responses was Ca2+> Sr2+> Ba2+, similar to the order for permeation of Ca2+channels in neurons. The order of potency for enhancement of muscimol responses was Cd2+> Mn2+> Mg2+, similar to the order for blockade of Ca2+channels in neurons. Neither Ca2+nor Mg2+caused accumulation of GABA in the extravesicular space due to increased GABA release or decreased reuptake of GABA by the synaptoneurosomes. The inhibition of muscimol responses by Ca2+was most likely via an intracellular site of action because additional inhibition could be obtained in the presence of the Ca2+ionophore, A23187. This confirms electrophysiologic findings in cultured neurons from several species. In contrast, the effects of Cd2+, Mn2+, and Mg2+may be mediated via blockade of Ca2+channels or by intracellular sites, although the results of these studies do not distinguish between the two loci. The effects of Zn2+were also studied, because this divalent cation is reported to have widely divergent effects on GABAAresponses. In contrast to other studies, we demonstrate that Zn2+inhibits GABAAresponses in an adult neuronal preparation. Zn2+produced a concentration‐dependent inhibition (limited to 40%) of muscimol responses with an EC50of 60 μM. The inhibition of muscimol‐induced38Cl−uptake by Zn2+was noncompetitive. The effect of Zn2+was reduced in the presence of Mg2+in a competitive or allosteric manner. The portion of GABAAreceptors sensitive to Zn2+may reflect a specific subunit composition in cerebral cortex as previously observed for recombinant GABAAreceptors in several expression systems. The modulation of GABAAreceptor function by Ca2+and other divalent cations may play an important role in the development and/or attenuation of neuronal excitability associated with pathologic conditions such as seizure activity and cerebral ischemia.