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Interaction of H + and Zn 2+ on recombinant and native rat neuronal GABA A receptors
Author(s) -
Krishek Belinda J.,
Moss Stephen J.,
Smart Trevor G.
Publication year - 1998
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1998.639bs.x
Subject(s) - gabaa receptor , receptor , antagonism , ic50 , chemistry , xenopus , cerebellum , recombinant dna , antagonist , biophysics , biochemistry , stereochemistry , biology , endocrinology , in vitro , gene
1 The interaction of Zn 2+ and H + ions with GABA A receptors was examined using Xenopus laevis oocytes expressing recombinant GABA A receptors composed of subunits selected from α1, β1, γ2S and δ types, and by using cultured rat cerebellar granule neurones. 2 The potency of Zn 2+ as a non‐competitive antagonist of GABA‐activated responses on α1β1 receptors was reduced by lowering the external pH from 7.4 to 5.4, increasing the Zn 2+ IC 50 value from 1.2 to 58.3 μM. Zinc‐induced inhibition was largely unaffected by alkaline pH up to pH 9.4. 3 For α1β1δ subunits, concentration‐response curves for GABA were displaced laterally by Zn 2+ in accordance with a novel mixed/competitive‐type inhibition. The Zn 2+ IC 50 at pH 7.4 was 16.3 μM. Acidification of Ringer solution resulted in a reduced antagonism by Zn 2+ (IC 50 , 49.0 μM) without affecting the type of inhibition. At pH 9.4, Zn 2+ inhibition remained unaffected. 4 The addition of the γ2S subunit to the α1β1δ construct caused a marked reduction in the potency of Zn 2+ (IC 50 , 615 μM), comparable to that observed with α1β1γ2S receptors (IC 50 639 μM). GABA concentration‐response curves were depressed in a mixed/non‐competitive fashion. 5 In cultured cerebellar granule neurones, Zn 2+ inhibited responses to GABA in a concentration‐dependent manner. Lowering external pH from 7.4 to 6.4 increased the IC 50 from 139 to 253 μM. 6 The type of inhibition exhibited by Zn 2+ on cerebellar granule neurones, previously grown in high K + ‐containing culture media, was complex, with the GABA concentration‐response curves shifting laterally with reduced slopes and similar maxima. The Zn 2+ ‐induced shift in the GABA EC 50 values was reduced by lowering the external pH from 7.4 to 6.4. 7 The interaction of H + and Zn 2+ ions on GABA A receptors suggests that they share either a common regulatory pathway or coincident binding sites on the receptor protein. The apparent competitive mode of block induced by Zn 2+ on α1β1δ receptors is shared by GABA A receptors on cerebellar granule neurones, which are known to express δ‐subunit‐containing receptors. This novel mechanism is masked when a γ2 subunit is incorporated into the receptor complex, revealing further diversity in the response of native GABA A receptors to endogenous cations.