MODULATION OF EXTRACELLULAR PH BY GLUTAMATE AND GABA IN RAT HIPPOCAMPAL SLICES

1. Alkaline extracellular pH transients evoked by afferent stimulation, and local pressure ejection of glutamate and gamma-aminobutyric acid (GABA), were studied in the CA1 region of rat hippocampal slices. Amino acid-evoked responses were obtained by use of a dual micromanipulator, with the tip of a double-barreled pH-sensitive microelectrode positioned 50-mu-m from a pressure ejection pipette. 2. At 31-degrees-C, in Ringer solutions buffered with 26 mM HCO3- and 5% CO2, mean extracellular pH in submerged 300-mu-m slices was 7.15 +/- 0.12 (n = 27 slices), at a tissue depth of approximately 150-mu-m. In Ringer buffered with 35 mM HCO3- and 5% CO2, extracellular pH was 7.29 +/- 0.10 (n = 19 slices). 3. Repetitive stimulation of the Schaffer collaterals caused an extracellular alkaline shift in stratum oriens, pyramidale, and radiatum, averaging 0.05 +/- 0.03 pH units among all regions (n = 138), with a maximum response of 0.16 pH units. Alkaline transients of similar appearance were obtained by local ejection of glutamate (0.01-0.12 pH units, n = 110) and GABA (0.01-0.18 pH units, n = 137). Control ejection of these amino acids into dilute agar caused only small acid shifts. 4. Superfusion of 100-mu-M picrotoxin abolished the GABA-evoked alkaline shift but failed to inhibit the Schaffer collateral- and glutamate-evoked alkalinizations. 5. Superfusion of 10(-5)-10(-3) M acetazolamide acidified the baseline by 0.05-0.10 pH units and amplified the Schaffer collateral- and glutamate-evoked alkaline shifts. At concentrations of less-than-or-equal-to 10-mu-M, acetazolamide variably affected baseline pH, but still enhanced the alkaline transients. The maximum observed enhancement was ninefold. Approximately half-maximal amplification occurred at 500 nM. Saturation of this effect occurred above 10-mu-M. 6. Acetazolamide inhibited the GABA-evoked alkaline shift at concentrations > 50 nM. Increasing concentrations caused the response to become diphasic, with a reduced alkaline transient and a late acid shift. At 5-mu-M acetazolamide, pressure ejection of GABA elicited a rapid acid shift that was partially blocked by picrotoxin. 7. Benzolamide, a less membrane-permeant inhibitor of carbonic anhydrase, had similar effects on the Schaffer collateral-, glutamate-, and GABA-evoked alkaline shifts, but was approximately 10 times more potent than acetazolamide. Use of tetramethylammonium as an extracellular indicator suggested that the latency of the benzolamide effect was a few tens of seconds at most. 8. These results indicate that glutamate- and Schaffer collateral-evoked alkaline shifts arise through a similar bicarbonate-independent mechanism. In contrast, the GABA-evoked alkalinization is consistent with a bicarbonate efflux across GABA-A receptor-gated anion channels. The GABA response does not make a significant contribution to the Schaffer collateral-evoked alkaline shift. The potency and speed of benzolamide suggest that the acid-base dynamics of the brain cell microenvironment are governed by extracellular carbonic anhydrase.