CARRIER-MEDIATED GABA RELEASE ACTIVATES GABA RECEPTORS ON HIPPOCAMPAL-NEURONS

gamma-Aminobutyric acid (GABA) transporters are electrogenic and sodiu m-dependent and can operate in reverse when cells are depolarized or w hen there is reversal of the inward sodium gradient. However, the func tional relevance of this phenomenon is unclear. We have examined wheth er depolarization induced by a physiologically relevant increase in ex tracellular [K+] leads to sufficient amounts of carrier-mediated GABA release to activate GABAA receptors on neurons. Patch-clamp recordings were made from rat hippocampal neurons in culture with solutions desi gned to isolate chloride currents in the recorded neuron. Pressure mic roejection was used to increase extracellular [K+] from 3 to 12 mM. Af ter blockade of vesicular GABA release by removal of extracellular cal cium, this stimulus induced a large conductance increase in hippocampa l neurons [18.9 +/- 6.8 (SD) nS; n = 16]. This was blocked by the GABA , receptor antagonists picrotoxin and bicuculline and had a reversal p otential that followed the Nernst potential for chloride, indicating t hat it was mediated by GABA, receptor activation. Similar responses oc curred after block of vesicular neurotransmitter release by tetanus to xin. GABA, receptors also were activated when an increase in extracell ular [K+] (from 3 to 13 mM) was combined with a reduction in extracell ular [Na+] or when cells were exposed to a decrease in extracellular [ Na+] alone. These results indicate that depolarization and/or reversal of the Na+ gradient activated GABA receptors via release of GABA from neighboring cells. We found that the GABA transporter antagonists 1-( 4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF 89976A; 20-100 mu M) and 1-(2-{[(diphenylmethylene)amino] xy}ethyl)-1, 2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride (NO-711; 10 mu M) both decreased the responses, indicating that the release of GAB A resulted from reversal of the GABA transporter. We propose that carr ier-mediated GABA release occurs in vivo during high-frequency neurona l firing and seizures, and dynamically modulates inhibitory tone.