Pressure-sensitive and -insensitive coupling in gamma-aminobutyric acid(a)receptors

Rationale: Previous behavioral and biochemical studies suggest that alloste ric coupling processes initiated by benzodiazepines, barbiturates and neuro active steroids can be sub-categorized on the basis of their sensitivities to antagonism by increased atmospheric pressure. However, biochemical evide nce supporting this hypothesis was limited to single concentration studies in long sleep (LS) mice. Objective: The present paper addresses these issue s by extending biochemical investigation of pressure effects on allosteric modulators across a range of concentrations that allosterically enhance gam ma -aminobutyric acid (GABA)A receptor function and alter behavior using tw o mouse genotypes. In addition, the effects of pressure on ligand binding w ere explored to further investigate the mechanism of pressure antagonism of allosteric modulation. Methods: The effects of 12 times normal atmospheric pressure (ATA) of helium-oxygen gas (heliox) on allosteric modulation of G ABAA receptor function and [H-3]flunitrazepam binding was tested in LS and C57BL mouse brain membranes (microsacs) using chloride flux and high-affini ty binding assays. Results: In both genotypes, exposure to 12 ATA heliox an tagonized the allosteric enhancement of GABAA receptor function by flunitra zepam (0.1-10 muM) and pentobarbital (0.1-50 muM) but did not affect allost eric modulation by 3 alpha -hydroxy-5 beta -pregnan-20-one (0.1-1 muM). Pre ssure did not affect benzodiazepine receptor affinity (Kd) or the number of benzodiazepine receptors (B-max)Conclusions: The results: (1) confirm that there are differences in sensitivity to pressure antagonism of allosteric coupling among GABAA allosteric modulators; (2) demonstrate that these diff erences are not concentration or genotype dependent; (3) add evidence that pressure antagonizes allosteric modulation by uncoupling the receptor and ( 4) support the hypothesis that allosteric modulation of receptor function c an be sub-categorized on the basis of sensitivity to pressure antagonism.