Role of HCO3- ions in depolarizing GABAA receptor-mediated responses in pyramidal cells of rat hippocampus.

1. Activation of GABAA receptors can produce both hyperpolarizing and depolarizing responses in CA1 pyramidal cells. The hyperpolarizing response is mediated by a Cl- conductance, but the ionic basis of the depolarizing response is not clear. We compared the GABAA receptor-mediated depolarizations induced by synaptically released gamma-aminobutyric acid [GABA; depolarizing inhibitory postsynaptic potentials (dIPSPs)] with those produced by exogenous GABA (depolarizing GABA responses). Short trains of high-frequency (200 Hz) stimuli were used to generate dIPSPs. We found that dIPSPs generated by trains of stimuli and depolarizing responses to exogenous GABA were accompanied by a conductance increase and had a similar reversal potential, indicating a similar ionic basis for both responses. 2. We wished to determine whether an HCO3- current contributed to the GABAA-mediated depolarizations. We found that dIPSPs and depolarizing GABA responses were sensitive to perfusion with HCO3(-)-free medium. Interpretation of these data was complicated by the mixed nature of the responses: dIPSPs were invariably accompanied by conventional, Cl(-)-mediated fast hyperpolarizing IPSPs (fIPSPs), and response to exogenous GABA usually consisted of biphasic hyperpolarizing and depolarizing responses. However, it was sometimes possible to elicit responses to GABA that appeared purely depolarizing (monophasic depolarizing GABA responses). 3. We analyzed monophasic depolarizing GABA responses and found no change in reversal potential when slices were perfused with HCO(3-)-free medium. We also made whole-cell recordings from CA1 pyramidal cells, attempting to reduce [HCO3-]i, and compared the reversal potential for monophasic depolarizing GABA responses with similar responses recorded with fine intracellular microelectrodes. We found no difference in reversal potential. We also examined effects of the carbonic anhydrase inhibitor acetazolamide (ACTZ) on depolarizing GABA responses. ACTZ reduced these responses but did not change their reversal potential. 4. Effects of HCO(3-)-free medium were not specific to GABAA receptor-mediated responses. GABAB receptor-mediated slow IPSPs (sIPSPs) were also reduced, as were excitatory postsynaptic potentials (EPSPs). Analyses of field potentials and spontaneous fIPSPs suggested a decrease in presynaptic excitability during perfusion with HCO(3-)-free medium. In addition, pyramidal cells showed decreased input resistance when perfused with HCO(3-)-free medium. 5. The sensitivity of GABAA receptor-mediated depolarizations to HCO(3-)-free medium can be explained by a decrease in presynaptic excitability and an increased resting conductance in postsynaptic neurons.(ABSTRACT TRUNCATED AT 400 WORDS)