INTERACTION OF H+ AND ZN2+ ON RECOMBINANT AND NATIVE RAT NEURONAL GABA(A) RECEPTORS

1. The interaction of Zn2+ and H+ ions with GABA(A) receptors was exam ined using Xenopus laevis oocytes expressing recombinant GABA(A) recep tors composed of subunits selected from alpha 1, beta 1, gamma 2S and delta types, and by using cultured rat cerebellar granule neurones. 2. The potency of Zn2+ as a non-competitive antagonist of GABA-activated responses on alpha 1 beta 1 receptors was reduced by lowering the ext ernal pH from 7.4 to 5.4, increasing the Zn2+ IC50 value from 1.2 to 5 8.3 mu M. Zinc-induced inhibition was largely unaffected by alkaline p H up to pH 9.4. 3. For alpha 1 beta 1 delta subunits, concentration-re sponse curves for GABA were displaced laterally by Zn2+ in accordance with a novel mixed/competitive-type inhibition. The Zn2+ IC50 at pH 7. 4 was 16.3 mu M. Acidification of Ringer solution resulted in a reduce d antagonism by Zn2+ (IC50, 49.0 mu M) without affecting the type of i nhibition. At pH 9.4, Zn2+ inhibition remained unaffected. 4. The addi tion of the gamma 2S subunit to the alpha 1 beta 1 delta construct cau sed a marked reduction in the potency of Zn2+ (IC50, 615 mu M), compar able to that observed with alpha 1 beta 1 gamma 2S receptors (IC50 639 mu M). GABA concentration-response curves were depressed in a mixed/n on-competitive fashion. 5. In cultured cerebellar granule neurones, Zn 2+ inhibited responses to GABA in a concentration-dependent manner. Lo wering external pH from 7.4 to 6.4 increased the IC50 from 139 to 253 mu M. 6. The type of inhibition exhibited by Zn2+, on cerebellar granu le neurones, previously grown in high K+-containing culture media, was complex, with the GABA concentration-response curves shifting lateral ly with reduced slopes and similar maxima. The Zn2+-induced shift in t he GABA EC50 values was reduced by lowering the external pH from 7.4 t o 6.4. 7. The interaction of H+ and Zn2+ ions on GABA(A) receptors sug gests that they share either a common regulatory pathway or coincident binding sites on the receptor protein. The apparent competitive mode of block induced by Zn2+ on alpha 1 beta 1 delta receptors is shared b y GABA(A) receptors on cerebellar granule neurones, which are known to express delta-subunit-containing receptors. This novel mechanism is m asked when a gamma 2 subunit is incorporated into the receptor complex , revealing further diversity in the response of native GABA(A) recept ors to endogenous cations.