Ethanol, at physiologically relevant concentrations, significantly enhanced high-affinity neuronal nicotinic acetylcholine receptor (NnAChR) currents insensitive to alpha-bungarotoxin (alpha-BuTX-ICs) in cultured rat cortical neurons in a fast and reversible manner, as determined by standard whole-cell patch-clamp recording techniques. The enhancement was (mean +/- S.D.) 7.7 +/- 5% to 192 +/- 52% upon coapplication of 3 to 300 mM ethanol with 1 to 3 microM ACh. No plateau for this ethanol-induced enhancement of alpha-BuTX-ICs was reached. The maximal alpha-BuTX-IC evoked by very high concentrations of ACh also was increased upon coapplication of ethanol. In contrast, ethanol weakly inhibited low-affinity NnAChR currents sensitive to alpha-BuTX (alpha-BuTX-SCs) (5 +/- 4% to 29 +/- 6% inhibition by 10 to 300 mM ethanol at 300 to 1000 microM ACh). This neuronal preparation also enabled comparison of ethanol action on NnAChRs with its action on N-methyl-D-aspartate receptor currents and gamma-aminobutyric acid receptor currents within the same neurons. Ethanol (100 mM) was more potent at enhancing NnAChR alpha-BuTX-ICs (61 +/- 9% enhancement) than it was at enhancing gamma-aminobutyric acid receptor current (3 +/- 3% enhancement-not statistically significant) or at inhibiting N-methyl-D-aspartate receptor currents (approximately 35 +/- 7% inhibition). Thus, NnAChRs, particularly those insensitive to alpha-BuTX, may be sensitive conduits through which ethanol can mediate some of its actions in the brain.

General anesthetics are thought to act on both excitatory and inhibitory neuronal pathways at both post-and presynaptic sites. However, the literature in these regards is somewhat controversial. The aim of the present study was to reassess the relative importance of the various anesthetic actions using a common preparation. Rat cortical neurons in primary culture were used to record spontaneous miniature postsynaptic currents by the whole-cell patch-clamp technique. Halothane at clinically relevant concentrations prolonged the decay phase of spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded in the presence of tetrodotoxin and at higher concentrations decreased the frequency of mIPSCs. The mIPSC amplitudes underwent little change. Spontaneous action potential-dependent IPSCs recorded in the absence of tetrodotoxin were similarly affected by halothane. Halothane also decreased the frequency of spontaneous miniature non-N-methyl-D-aspartate (NMDA) excitatory postsynaptic currents (mEPSCs) as well as spontaneous action potential-dependent NMDA EPSCs and non-NMDA EPSCs without affecting their decay phase. The halothane effect on mIPSC and mEPSC frequency was dependent on the external calcium concentration. In contrast to halothane, the only effect of propofol was the prolongation of the decay phase of mIPSCs and IPSCs. The prolongation of mIPSCs and IPSCs by halothane and propofol coupled with the ineffectiveness on mEPSCs and EPSCs suggests a selective postsynaptic modulation of GABA A receptors. The additional calcium-dependent inhibition of mIPSC and mEPSC frequency by halothane (but not propofol) suggests a more general mechanism by this anesthetic on presynaptic transmitter release.

The effects of ethanol on the GABA A receptors, which are regarded as one of the most important target sites of ethanol, are very controversial, ranging from potentiation to no effect. The ␦ subunit-containing GABA A receptors expressed in Xenopus oocytes were recently reported to be potently augmented by ethanol. We performed patch-clamp experiments using the cerebellar granule cells and mammalian cells expressing recombinant GABA A receptors. In granule cells, the sensitivity to GABA increased from 7 to 11 days in vitro. Furosemide, an antagonist of ␣6-containing GABA A receptors, inhibited GABAinduced currents more potently at 11 to 14 days than at 7 days. Ethanol at 30 mM had either no effect or an inhibitory effect on currents induced by low concentrations of GABA in granule cells. On ␣4␤2␦, ␣6␤2␦, or ␣6␤3␦GABA A receptors expressed in Chinese hamster ovary cells, ethanol at 10, 30, and 100 mM had either no effect or an inhibitory effect on GABA currents. Ethanol inhibition of GABA A receptor was observed in all of the subunit combinations examined. In contrast, the perforated patch-clamp method to record the GABA currents revealed ethanol effects on the ␣6␤2␦ subunits ranging from slight potentiation to slight inhibition. Ethanol seems to exert a dual action on the GABA A receptors and the potentiating action may depend on intracellular milieu. Thus, the differences between the GABA A receptors expressed in mammalian host cells and those in Xenopus oocytes in the response to ethanol might be due to changes in intracellular components under patch-clamp conditions.

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