Gamma-aminobutyric acid and benzodiazepine receptors in cultured cerebellar granule cells: effects of taurine and its lipophilic derivatives

Effects of ammonia in vitro on endogenous taurine efflux and cell volume in rat cerebrocortical minislices: influence of inhibitors of volume-sensitive amino acid transport

Rat cerebrocortical minislices were incubated with physiological saline in the absence or presence of 5 mM ammonium acetate ("ammonia") and/or inhibitors of osmosensitive amino acid transport: 50 microM niflumic acid and 100 microM N-ethyl-maleimide for 60 min, with medium changes after 20 min and 40 min. The efflux of endogenous taurine, glutamate and glutamine was assayed by high-performance liquid chromatography, and steady-state cell volumes were monitored in the slices with the [14C]inulin method. In the absence of ammonia, niflumic acid abolished taurine efflux but did not affect glutamate or glutamine efflux at all time-points, and increased cell volume at 20 min and 60 min. N-Ethyl-maleimide increased taurine, glutamine and glutamate efflux at 20 min and 40 min, inhibited taurine and glutamine efflux at 60 min, and increased cell volume at 20 min. Ammonia strongly stimulated taurine (by 380% at 20 min), and only moderately glutamate (30% at 20 min) or glutamine efflux (76% at 20 min). Ammonia increased cell volume above the control level at all time-points. Niflumic acid inhibited, but did not abolish ammonia-dependent taurine and glutamine efflux, and did not change glutamate efflux. The effects of ammonia + niflumic acid on cell volume did not differ from the effects of each compound separately. N-Ethyl-maleimide inhibited ammonia-dependent efflux of all three amino acids except for stimulation of glutamate efflux at 20 min. N-Ethyl-maleimide + ammonia decreased the cell volumes more than did each compound separately. It is concluded that although ammonia-induced taurine efflux is accompanied by an increase in cell volume, the underlying mechanism is not simply a cell volume regulatory response normally observed in hypoosmotic stress. Increased efflux of taurine, which is an inhibitory amino acid and a cell membrane protectant, may serve to counteract the deleterious effects of increased excitatory transmission accompanying acute hyperammonemic insult.

Interactions of taurine with GABAB binding sites in mouse brain

The interactions of taurine with GABAB receptors were studied in membranes from the brain of the mouse by measuring the binding of [3H]baclofen and that of [3H]GABA in the presence of isoguvacine. Taurine displaced ligand binding to GABAB receptors concentration-dependently with an IC50 in the micromolar range. The effects of baclofen on the release of taurine and GABA from slices of cerebral cortex of the mouse were assessed using a superfusion system. Potassium-stimulated release of both [3H]taurine and [3H]GABA was unaffected by baclofen but potentiated by delta-aminovalerate. The enhancement of release of [3H]taurine by delta-aminovalerate was partially antagonized by baclofen, suggesting that baclofen-sensitive receptors could modify the release.