Zn²⁺ modulates light responses of color-opponent bipolar and amacrine cells in the carp retina

The effects of Zn²⁺ on color-opponent bipolar cells (BCs) and amacrine cells (ACs) were studied in the isolated superfused carp retina using intracellular recording techniques. Bath-applied Zn²⁺ (25μM) depolarized R⁺G^--type BCs and suppressed both depolarizing responses of these cells to red (680nm) flashes and hyperpolarizing ones to green (500nm) flashes. Following Zn²⁺ application, G⁺R^--type BCs were hyperpolarized, which was accompanied by a potentiation of their depolarizing responses to green flashes and a suppression of hyperpolarizing ones to red flashes. Similar Zn²⁺ effects were observed in R⁺G^-- and G⁺R^--type ACs. The Zn²⁺ effects persisted in the presence of picrotoxin and strychnine, suggesting that modulation by Zn²⁺ of GABA and glycine receptors was unlikely involved. Using whole-cell recording techniques, it was found Ca²⁺ currents in cone terminals were dose-dependently suppressed by Zn²⁺, suggesting that Zn²⁺ may reduce glutamate release from cone photoreceptors. Furthermore, lowering extracellular Ca²⁺, a procedure that increases glutamate release from photoreceptors, exerted actions on R⁺G^-- and G⁺R^--type BCs, almost opposite to the Zn²⁺ effects on these two types of BCs. It is therefore postulated that the Zn²⁺ effects reported in the present work may reflect a consequence of the changes in input resistances of color-opponent BCs and driving forces for their light responses resulted from the reduced glutamate release by Zn²⁺.