Effect of extracellular pH on GABA-activated current in rat recombinant receptors and thin hypothalamic slices

We studied the effects of extracellular pH (pH(o)) on gamma-aminobutyric ac id (GABA)-mediated Cl- current in rat hypothalamic neurons and recombinant type-A GABA (GABA,) receptors stably expressed in human embryonic kidney ce lls (HEK 293), using whole cell and outside-out patch-clamp recordings. In alpha 3 beta 2 gamma 2s receptors, acidic pH decreased, whereas alkaline pH increased the response to GABA in a reversible and concentration-dependent manner. Acidification shifted the GABA concentration-response curve to the right, significantly increasing the EC,, for GABA without appreciably chan ging the slope or maximal current induced by GABA. We obtained similar effe cts of pH in alpha 1 beta 2 gamma 2 receptors and in GABA-activated current s recorded from thin hypothalamic brain slices. In outside-out patches reco rded from alpha 3 beta 2 gamma 2 recombinant receptors, membrane patches we re exposed to 5 mu M GABA at control (7.3), acidic (6.4), or alkaline (8.4) pH. GABA activated main and subconductance states of 24 and 16 pS, respect ively, in alpha 3 beta 2 gamma 2 receptors. Alkaline pH, increased channel opening frequency and decreased the duration of the long closed state, resu lting in an increase in open probability (from 0.0801 +/- 0.015 in pH 7.3 t o 0.138 +/- 0.02 in pH 8.4). Exposure of the channels to acidic pH(o) had t he opposite effects on open probability (decreased to 0.006 +/- 0.0001). Ta ken together, our results indicate that the function of GABA, receptors is modulated by extracellular pH. The proton effect is similar in recombinant and native receptors and is dependent on GABA concentration. In addition, t he effect appears to be independent of the cw-subunit isoform, and is due t o the ability of H+ to alter the frequency of channel opening. Our findings indicate that GABAergic signaling in the CNS may be significantly altered during conditions that increase or decrease pH.