Potentiation of γ‐Aminobutyric Acid‐Mediated Chloride Flux by Pentobarbital and Diazepam but Not Ethanol

Abstract

The influx of 36Cl- into cerebral cortical and cerebellar microsacs from ICR mice and Sprague-Dawley rats was studied in incubations lasting 3 s, 500 ms, or 21 ms. In the 3-s assay, 10-40 mM ethanol did not affect either basal or gamma-aminobutyric acid (GABA)-mediated Cl- flux, at any GABA concentration tested. Only at a concentration of 600 mM did ethanol potentiate Cl- flux in both mouse and rat preparations. Ethanol (20 mM) also did not affect the significant potentiation of GABA-mediated flux produced by 50 microM pentobarbital or 2 microM diazepam in ICR mouse microsacs. In 21- and 500-ms incubations (quench-flow method), 50 microM pentobarbital significantly potentiated GABA-mediated Cl- flux in rat cortical microsacs, but 10-50 mM ethanol did not. These studies suggest that some as yet unrecognized factor is essential for ethanol enhancement of GABA-mediated Cl- flux, as reported by others in brain homogenates and in tissue culture.

Direct Evidence for a Cause‐Effect Link Between Ethanol Potentiation of GABAA Receptor Function and Intoxication From Hyperbaric Studies in C57, LS, and SS Mice

“…These findings replicate and extend initial findings in LS mouse brain microsacs (Davies and Alkana, 1998) and demonstrate that the antagonistic effects of 12 ATA heliox on ethanol potentiation of GABA-activated Cl Ϫ uptake are not due to alterations in Cl Ϫ basal levels. Further, basal levels of Cl Ϫ (5-8 nmol/mg protein) in these studies are well within the range reported with mouse microsacs Alkana, 1998, 2001;Harris and Allan, 1985;Mihic et al, 1992).…”

Effects of Ethanol on GABAA Receptors in GABAergic and Glutamatergic Presynaptic Nerve Terminals

“…There are a number of conflicting reports that EtOH, at intoxicating concentrations (1-100 mM), induces a potentiation (Reynolds and Prasad, 1991) or inhibition (Siggins et al, 1987;Nie et al, 2000) of extrasynaptic GABA responses, whereas in other reports EtOH has no effect on GABA responses, even at the higher range of concentrations (Mihic et al, 1992). In the accumbens dorsomedial cells, GABA A receptors containing ␦ subunits have a high sensitivity to EtOH, with their responses being enhanced at concentrations of 3 to 30 mM (Nie et al, 2011).…”

Low dose acute alcohol effects on GABAA receptor subtypes

“…It is perhaps not surprising that the results of such Cl − flux measurements vary, depending on the brain regions they are derived from, the age of the animals used, the exact protocol used to isolate the membrane fractions, the content of synaptoneurosomes, and the amount of concentrations of GABA or GABA agonists like muscimol added to measure EtOH stimulation. Difficulties in the reproducibility of low dose ethanol enhancement in these Cl − flux assays in different laboratories (Uusi-Oukari & Korpi, 1989;Mihic et al, 1992) might have raised skepticism concerning the validity of these data to explain low dose alcohol effects on GABA A Rs. In addition, synaptic physiologists might have considered the conditions under which 36 Cl − flux assays are performed non-physiologically, but they do make sense when considering that continuously active, highly GABA-sensitive GABA A Rs (like those containing the δ subunit) were measured under these experimental conditions.…”