ZINC DOWN-MODULATES THE GABA(C) RECEPTOR CURRENT IN CONE HORIZONTAL CELLS ACUTELY ISOLATED FROM THE CATFISH RETINA

1. We studied the effect of zinc on the gamma-aminobutyric acid-C (GABA(C)) receptor in acutely isolated catfish cone horizontal cells using the whole cell patch-clamp technique. 2. GABA activates the GABA(C) receptor with a half-activation concentration (EC(50)) of 2.99 mu M. The Hill coefficient is 1.32. Desensitization of the receptor is evident when GABA concentration is >3 mu M. 3. Zinc downmodulates the GABA(C) receptor current, elicited by 30 mu M GABA, with a half-inhibition concentration (IC50) of 8.20 mu M. 4. The inhibition of zinc is both competitive and noncompetitive. In the presence of 10 mu M zinc, the maximum GABA response was reduced to similar to 60 percent of control and the EC(50) increased to 17.32 mu M, whereas the Hill coefficient (1.39) was not significantly altered. 5. The steady-state block by zinc is virtually voltage independent. 6. These results suggest that the GABA(C) receptor of horizontal cells can be modulated by endogenous zinc found in photoreceptors.