HALOTHANES EFFECTS ON GABA-GATED CHLORIDE FLUX IN MICE SELECTIVELY BRED FOR SENSITIVITY OR RESISTANCE TO DIAZEPAM

The DS (diazepam-sensitive) and DR (diazepam-resistant) lines of mice, selected on the basis of their ataxic response to diazepam, also dive rge in the physiologic response of their brain gamma-aminobutyric acid (A) (GABA(A)) receptors to benzodiazepines, as indicated by augmentati on of GABA-mediated chloride flux. Cross-sensitivity and -resistance t o other sedatives known to interact with the GABA(A)-receptor have als o been demonstrated in DS and DR mice. Based on the finding that these mice also show cross-sensitivity and -resistance to obtundation by ha lothane, we predicted that their GABA(A)-receptors would also exhibit a differential response to halothane as assayed by an in vitro (Cl)-C- 36- influx assay using purified brain microvesicles. Consistent with t his prediction, therapeutic concentrations of halothane enhanced 1 mum ol/l GABA-gated flux with significantly greater potency in DS than in DR mice (halothane EC50 336+/-64 mumol/l (S.E.M.) vs. 605+/-110 mumol/ l, respectively, P = 0.03), but there was no difference in maximal flu x enhancement between the two lines (DS 4.7+/-0.4 nmol - mg-1. 3 s-1, vs. DR 4.7+/-0.5 nmol - mg-1.3 s-1). Halothane (500 mumol/1) also shif ted the entire GABA concentration-flux relationship significantly to t he left, decreasing the EC50 for GABA in both the DS and DR lines. Imp ortantly, the shift in the GABA concentration-flux response in the pre sence of halothane was more pronounced in the DS mice (GABA EC50 1.8+/ -0.4 mumol/l vs. 14.7+/-0.9 /mumol/l without halothane) than in the DR mice (GABA EC50 4.7+/-0.6 mumol/l vs. 14.7+/-0.9 mumol/l without halo thane). This effect of halothane was highly significant, both when com pared to control, and between the selected lines (P < 0.001). The find ings that halothane enhances GABA-gated flux and enhances GABA's chann el gating potency support the hypothesis that differential enhancement of agonist-stimulated chloride permeability at GABA(A) receptors coul d be a mechanism underlying the differential obtunding potency of halo thane in DS and DR mice. However, at high GABA concentrations halothan e decreased maximal chloride flux, more in DS than in DR mice (P < 0.0 01), which is not consistent with such a mechanism.