CHANGES IN EXTRACELLULAR K+ EVOKED BY GABA, THIP AND BACLOFEN IN THE GUINEA-PIG HIPPOCAMPAL SLICE

Changes in [K+]0 evoked by the inhibitory amino acid transmitter, GABA (gamma-aminobutyric acid) and its agonists were recorded with ion-selective microelectrodes in the CA1 stratum pyramidale of guinea-pig hippocampal slices. Bath applications of GABA (0.1-10 mM) produced dose-dependent increases in [K+]0 (EC50 = 4 mM, R(max) = 1.6 mM), with a peak and decline during exposure, followed by undershoot during recovery. In contrast the selective GABA(A) agonist, THIP (4,5,6,7-tetrahydroisoxazolo-(5,4-c)-pyridin-3-ol) (0.01-1 mM) showed almost-equal-to ten-fold greater potency and evoked only increases in [K+]0 (EC50 = 0.5 mM, R(max) = 2 mM). Reduction of temperature from 34-degrees to 22-degrees-C caused a more than two-fold augmentation of the K0+ accumulation evoked by GABA, but no change in that due to THIP. The GABA(A) antagonist, BMI (bicuculline methiodide) (100-mu-M) completely blocked responses to THIP and partially antagonized those to GABA. Responses to GABA were synergistically enhanced by pentobarbital (100-mu-M). Only small, delayed and inconsistent changes could be evoked by relatively high concentrations of the GABA(B) agonist, DL-baclofen (0.01-1 mM). The K+ changes evoked by GABA appear to be mediated by the activation of GABA(A) receptors with low affinity and to be related to their depolarizing action. Although the response includes an electrogenic component which suggests the involvement of Na-dependent transmitter uptake/transport, the increase in K0+ probably reflects an outward counter/co-transport of K+ with Cl/HCO3 anion shifts and/or activation of a voltage-dependent K+ conductance.