N-ETHYLMALEIMIDE BLOCKS DEPOLARIZATION-INDUCED SUPPRESSION OF INHIBITION AND ENHANCES GABA RELEASE IN THE RAT HIPPOCAMPAL SLICE IN-VITRO

Regulation of synaptic, GABAA receptor-mediated inhibition is a proces s of critical importance to normal brain function. Recently, we have d escribed a phenomenon in hippocampus of a transient, yet marked, decre ase in spontaneous, GABA(A) receptor-mediated IPSCs after depolarizati on activated Ca2+ influx into a pyramidal cell. This process, depolari zation-induced suppression of inhibition (DSI), is absent in hippocamp al cells that previously had been exposed to pertussis toxin in vivo, implicating a G-protein in the DSI process. To circumvent the problem that a single cell cannot be studied before and after G-protein block using the pertussis toxin pretreatment method, we have used the sulfhy dryl alkylating agent N-ethylmaleimide (NEM), which blocks pertussis t oxin-sensitive G-proteins, to determine whether acute inhibition of G- proteins can eliminate DSI of spontaneous IPSCs (sIPSCs). In whole-cel l recordings from CAI pyramidal cells that were first determined to ex press DSI, we have found that NEM does block DSI of sIPSCs. We also re port that DSI of monosynaptic, evoked IPSCs is blocked by NEM, suggest ing that a similar mechanism underlies both forms of DSI. It was of in terest that DSI was abolished at a time when NEM had increased, not de creased, GABA transmission. Indeed, NEM greatly increased quantal GABA release by a Ca2+-independent mechanism, an observation with potentia lly important implications for understanding synaptic GABA release.