Ethanol dissociates thiopental and GABA-receptor interaction

Ethanol enhances synaptically evoked GABAA receptor-mediated responses in cerebral cortical neurons in rat brain slices

Previous intracellular electrophysiological studies on rat hippocampal brain slices have shown very little effect of acute ethanol application on synaptically evoked GABAA receptor-mediated responses recorded in CA1 pyramidal neurons. The present study was designed to compare the effects of ethanol on pyramidal neurons in the hippocampus and cerebral cortex. Using conventional intracellular microelectrodes (60-80 M omega) to impale cortical neurons in brain slices, 80 mM ethanol application did not affect the membrane input impedance nor evoked EPSPs, but significantly affected the resting membrane potential (usually a 2-5 mV hyperpolarization). When stimulus-evoked GABAA-mediated IPSCs were studied using whole-cell recordings from cortical neurons voltage-clamped at depolarizing potentials, monophasic IPSCs were evoked that were blocked by bicuculline, increased by pentobarbital, and enhanced by ethanol superfusion in a dose dependent manner over the range of 20-160 mM. Hippocampal IPSCs recorded under identical conditions were not enhanced by ethanol. Parallel studies of GABA-stimulated 36Cl- flux measurements in microsacs prepared from hippocampal, cerebral cortical and cerebellar tissue demonstrated that ethanol significantly enhanced (30-50%) 36Cl- flux in microsacs derived from the cerebral cortex and cerebellum, but not in microsacs prepared from the hippocampus. These results demonstrate that there are clear brain region-dependent differences in the way that GABAA receptor function is altered by acute ethanol, and that these differences are apparent not only as an enhancement of responses to exogenous GABA, but also as a facilitation of the responses to endogenous GABA released from inhibitory nerve terminals during synaptic activation.

Brain region-dependent sensitivity of GABAA receptor-mediated responses to modulation by ethanol

Simultaneous extracellular and intracellular electrophysiological recordings were made from the CA1 region of rat hippocampal brain slices during superfusion with ethanol. Ethanol (80 mM) had a biphasic effect on the extracellularly recorded population spike, with an initial increase followed by a significant reduction (38%) in this response, which was maximal 10 to 15 min after the start of ethanol application. Concurrent intracellular recordings in the CA1 showed a small (0.7 mV) hyperpolarization of the resting membrane potential, with no significant change in the input impedance, EPSP, GABAA and GABAB IPSPs, or after hyperpolarization (AHP) following depolarizing current injection. Ethanol reduced the amplitude and duration of depolarizing responses to brief, localized pressure-ejection of N-methyl-D-aspartate (NMDA) onto pyramidal neuron dendrites, but did not affect the GABAA receptor-mediated depolarizing responses to the dendritic application of GABA. In parallel studies, the effect of ethanol on GABA-stimulated 36Cl- flux was measured in microsac preparations from rat hippocampus, cerebellum, and cerebral cortex. Ethanol application caused substantial enhancement of the chloride uptake from cerebellar and cerebral cortical microsacs, but had no effect on 36Cl- influx in hippocampal microsacs. These results suggest that there are important brain region-dependent differences in the sensitivity of the GABAA receptor/chloride channel to modulation by ethanol.

Effect of ethanol on hippocampal GABA receptors in the rat brain

The effect of ethanol on GABA receptors was studied in hippocampal slices and membrane preparations of the rat brain. In slice preparation, ethanol enhanced the GABA inhibition of the population spikes evoked in CA 1 pyramidal cells by Schaffer collateral stimulation. The effect of ethanol was dose-dependent being observed with minimal concentration of 70 mM. In 3H-GABA binding experiments, ethanol enhanced the binding to fresh membranes with no change in the affinity. Maximal stimulation was observed at ethanol concentration of 70 mM. In hippocampal slices or membrane preparations obtained from ethanol- or barbital-administered rats chronically, ethanol failed to enhance GABA function. Thus, the present study demonstrated the involvement of GABA in central effect of ethanol. Also suggested in GABA was the involvement in the development of cross-tolerance between ethanol and barbiturates.