BIDIRECTIONAL MODULATION OF GABA-GATED CHLORIDE CHANNELS BY DIVALENT-CATIONS - INHIBITION BY CA2+ AND ENHANCEMENT BY MG2+

The effects of the divalent cations Ca2+, Sr2+, Ba2+, Mg2+, Mn2+, and Cd2+ were studied on gamma-aminobutyric acid(A) (GABA(A)) responses in rat cerebral cortical synaptoneurosomes. The divalent cations produced bidirectional modulation of muscimol-induced Cl-36(-) 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 GABA(A) responses. In contrast to other studies, we demonstrate that Zn2+ inhibits GABA(A) responses in an adult neuronal preparation. Zn2+ produced a concentration-dependent inhibition (limited to 40%) of muscimol responses with an EC(50) of 60 mu M. The inhibition of muscimol-induced Cl-36(-) 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 GABA(A) receptors sensitive to Zn2+ may reflect a specific subunit composition in cerebral cortex as previously observed for recombinant GABA(A) receptors in several expression systems. The modulation of GABA(A) receptor 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.