GABA(A)-RESPONSES OF CA3 NEURONS - CONTRIBUTION OF BICARBONATE AND OF CL--EXTRUSION MECHANISMS

POSTSYNAPTIC GABA(A)-responses of cortical neurones may be composed of an early 'somatic' hyperpolarization (h-GABA(A)) and a 'dendritic' depolarization (d-GABA(A)). In order to study underlying anion-fluxes the gradient of bicarbonate across the membrane of CA3-neurones in hippocampal slices (guinea-pig) was reduced by several manoeuvres (NH4-prepulse-technique, CO2/bicarbonate-withdrawal-technique, acetazolamide, DIDS). Each manoeuvre resulted in an attenuation of d-GABA(A). The inhibition of a Cl-/bicarbonate-exchanger with DIDS could also weaken h-GABA(A) which was selectively impaired by ethacrynic acid-known to block an ATP-driven somatic Cl--extrusion mechanism. In conclusion, bicarbonate-fluxes contribute to d-GABA(A) rather than to h-GABA(A) which is mainly driven by Cl--fluxes. An uneven distribution of Cl--extrusion mechanisms along the adult CA3-neurones is likely to build up the anion-gradients necessary to generate biphasic GABA(A)- responses.