Possible interaction of [3H]glutamate binding sites with anion channels in rat neural tissues

Abstract

The effect of various ions on [(3)H]l-glutamic acid (Glu) binding was examined using crude synaptic membrane preparations from the rat brain. In vitro addition of sodium acetate (1-100 mM) exhibited a significant enhancement of the binding in a concentration dependent manner. Ammonium chloride (20 mM) prevented the potentiation by sodium acetate at 2 degrees C, whereas sodium acetate exerted an inhibitory action on the ammonium chloride-induced augmentation of the binding at 30 degrees C. Ammonium chloride (1-100 mM) itself elicited a temperature dependent stimulation of the binding, which was invariably attenuated by an antagonist for the anion channels such as picrotoxinin (10(?3) M) as well as by inhibitors of anion transport including ethacrynic acid (10(?3) M) and 4,4?-diisothiocyanatostilbene-2,2?-disulfonic acid (10(?4)?10(?3) M), respectively. The later two inhibitors also caused a significant additional raise of the sodium acetate-induced enhancement of the binding. A significant augmentation of the binding resulted from the addition (20 mM) of various anions known to penetrate the anion channels such as bromide, iodide, nitrate, bicarbonate and thiocyanate in a permeability related manner, while that of non-permeable anions including fluoride, sulfate, acetate, formate, phosphate, oxalate, lactate, succinate and tartarate had no such a profound effect on the binding. Addition of d-aspartic acid resulted in the complete abolition of the Na(+)-dependent binding while sparing the Cl(?)-dependent binding. Scatchard analysis revealed that Cl(?) ions induced a two-fold increase in the number of the binding sites without affecting their affinity, whereas Na(+) ions reduced the affinity with a concomitant increase of the number of the binding sites. Addition of quisqualic acid (10(?5)?10(?3) M) inhibited the Cl(?)-dependent binding of [(3)H]Glu to a significantly greater extent than the inhibition on Na(+)-dependent binding. N- Methyl- d -aspartic acid and kainic acid exerted no preventive action on the basal, Cl(?)-dependent and Na(+)-dependent binding. respectively. The highest basal binding activity was found in the retina among various central structures examined. A significant basal binding activity of [(3)H]Glu was also detected in the pituitary and adrenal but not in the kidney. Chloride ions exhibited a significant facilitation of [(3)H]Glu binding to central regions without altering that to peripheral tissues such as pituitary and adrenal. In contrast, Na(+) ions induced significant attenuation of the binding to the pituitary, adrenal and retina despite the occurrence of augmentation of the binding to other central structures. These results suggest the Glu binding sites may be linked to the anion channels in the rat central nervous system and that this linkage may be absent from the pituitary, adrenal and retina.