Proton modulation of functionally distinct GABAA receptors in acutely isolated pyramidal neurons of rat hippocampus.
We have studied the effect of extracellular pH (pHo) on the GABAA receptor-mediated chloride conductance in acutely isolated pyramidal neurons from area CA1 of the rat hippocampus under whole-cell voltage clamp in bicarbonate-free solutions. The conductance evoked by saturating or near-saturating concentrations (200-1000 microM) of GABA showed a marked sensitivity to variations of pHo around 7.4. A decrease in pHo between 8.4 and 6.4 increased the GABAA receptor-mediated chloride conductance by about two-fold per pH unit. In contrast, when evoked by a low agonist concentration (1-10 microM) the conductance showed an equally marked decrease upon a decrease in pHo. The half-time for desensitization of the conductance induced by 500 microM GABA was around 900 ms at pHo 6.4 and 7.4, but decreased to 650 ms at pHo 8.4. A fall in pHo decreased the amount of desensitization of the conductance evoked by a 5 s application of 5 microM, but not of 500 microM, GABA. The concentration-response relationship of the GABA-induced conductance showed a local plateau between 50 and 100 microM of GABA, which was particularly evident at high pHo. Assuming two receptor populations with a high and a low affinity for GABA, the effect of H+ on the GABAA receptors could be explained as an increase in the EC50 of the high affinity receptor, and an apparently non-competitive potentiation of both the high and the low affinity receptors. The GABAA receptor-mediated conductance was markedly inhibited by 20-50 microM Zn2+. In addition, Zn2+ reverted the down-modulation by H+ observed at low GABA concentrations to up-modulation. Diazepam (1-10 microM) had only a marginal effect on the GABA-gated conductance. Taken together, the results suggest the coexistence in individual hippocampal neurons of two distinct GABAA receptor populations having differential sensitivities to H+. In the light of the inhibitory action of Zn2+ and the virtual absence of an effect of diazepam it is probable that a significant fraction of the GABAA receptors lack the gamma 2 subunit. The observation that an elevated pH has a strong suppressing effect on the conductance evoked by high concentrations of GABA may at least partly explain why an extracellular alkalosis leads to neuronal hyperexcitability.
Pasternack
M
M
Department of Biosciences, University of Helsinki, Finland. mpasternack@katk.helsinki.fi
Smirnov
S
S
Kaila
K
K
D000818:Animals; D018118:Chloride Channels / Q000187:drug effects / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects; D004594:Electrophysiology; D018755:GABA Agonists / Q000494:pharmacology; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D011522:Protons; D017966:Pyramidal Cells / Q000187:drug effects*; D051381:Rats; D017208:Rats, Wistar; D011963:Receptors, GABA-A / Q000187:drug effects*; D015854:Up-Regulation / Q000187:drug effects
D018118:Chloride Channels; D018755:GABA Agonists; D011522:Protons; D011963:Receptors, GABA-A