Zinc enhances Ionic currents induced in skate Muller (glial) cells by the Inhibitory neuretransmitter GABA

摘要

We describe here a novel effect of zinc on GABA receptors of glial cells in the skate retina. The GABA-induced currents of skate Muller cells, the radial glia of the retina, are mediated by activation of GABA_A receptors (GABA_ARs). In other parts ofthe nervous system, GABA_A-mediated currents are inhibited by zinc.. However, in isolated, voltage-clamped Muller cells, coapplication of zinc (10 μM) and GABA (1μM) resulted in an enhancement of the GABA_AR current. Surprisingly, zinc alone induced acurrent similar in many respects to that elicited by GABA, i..e. the reversal potential was the same as for the GABA-induced current, the current was blocked by bicuculline and picrotoxin, and the current-voltage relation obtained in the presence of 10 μM zinc was virtually identical to that obtained with 1 μM GABA, Both bicuculline and picrotoxin suppressed a current that was present with cells bathed only in Ringer, suggesting that some of the GABA channels were spontaneously open in the absence ofexternally applied GABA, This possibility was supported by cell-attached patch recordings. Under conditions in which potassium and calcium currents were suppressed, spontaneous channel activity was observed Moreover, the frequency of these channel openings was greater when zinc was included in the pipette solution, and reduced when bicuculline was added, These findings suggest that zinc acts directly to enhance the GABA_A receptor activity of the Muller cells, and raise the possibility that the subunitcomposition of the GABA_ARs of skate Mulier cells differs from that of GABA_ARs identified previously in other neuronal and glial preparations.