Do allosteric coupling pathways for the benzodiazepine, barbiturate and neuroactive steroid recognition sites on the GABA(A) receptor represent initial sites of action for ethanol

Past results indicate that exposure to low level hyperbaric helium-oxygen gas mixtures (12 ATA heliox--pressure) represents a direct, mechanistic ethanol antagonist that can be used as a tool to identify ethanol's initial site(s) of behavioral action. These results suggest that allosteric coupling pathways, which transduce binding events on ligand-gated ion channels, may represent initial sites of action for ethanol. The present dissertation used pressure to test the hypothesis that one or more of the allosteric coupling pathways for the benzodiazepine, barbiturate and neuroactive steroid recognition sites on the GABA$sb{rm A}$ receptor, represent initial sites of action for ethanol. This was accomplished by using pressure, in conjunction with behavioral and biochemical approaches, to test predictions based on this hypothesis. (1) Pressure should selectively antagonize the behavioral and biochemical effects of ethanol and other drugs whose effects on the GABA$sb{rm A}$ receptor are mediated through allosteric modulation. (2) Pressure should not antagonize the behavioral and biochemical effects of drugs that bind to and directly alter GABA$sb{rm A}$ receptor function. Exposure to 12 ATA heliox antagonized the behavioral effects of the allosteric modulators ethanol, diazepam and pentobarbital but not those of 4, 5, 6, 7-tetrahydroisoxazolo-pyridin-3-ol-(GABA prodrug) or picrotoxin in C57 mice. Further, pressure antagonized the anticonvulsant effects of ethanol, diazepam and pentobarbital in LS mice. Pressure also antagonized flunitrazepam and pentobarbital potentiation of GABA-activated $sp{36}$Cl$sp-$ uptake in LS mouse brain membrane preparations. In contrast, pressure did not alter GABA stimulation or picrotoxin blockade in this system, nor did pressure alter the behavioral or biochemical effects of the neuroactive steroid 3$alpha$-hydroxy-5$beta$-pregnan-20-one. Collectively, these results fit the predictions and support the hypothesis that allosteric coupling pathways represent initial sites of ethanol action. The selectivity of pressure antagonism amongst allosteric modulators suggest that ethanol may act on a subgroup of coupling pathways and that pressure can be used to identify coupling pathways that have similar physicochemical characteristics to ethanol's site(s) of action and which, therefore, are likely targets for ethanol.