Makoto Kaneda

In the retina, zinc is believed to be a modulator of synaptic transmission and a constituent of metalloenzymes. To determine whether the intracellular localization of zinc correlates with function, we examined the localization of endogenous zinc in the rat retina using the silver amplification method. By light microscopy, reaction products were detected in the pigment epithelial cells (PE), the inner segment of photoreceptors (IS), the outer nuclear layer (ONL) and the inner nuclear layer (INL), the outer plexiform layer (OPL) and the inner plexiform layer (IPL), and the ganglion cell layer (GC). The heaviest accumulation of precipitate was observed in PE and IS, whereas only a little precipitate was found in GC. When the intracellular zinc was chelated with diethyldithiocarbamate, a small amount of precipitate was observed only in ONL. By electron microscopy, zinc was associated with three compartments. In OPL and IPL, zinc was associated with neural processes, while in PE, IS, INL, and GC it was associated with the Golgi apparatus. In ONL, zinc was associated with the nucleus. Zinc in the neural processes is believed to act as a modulator of synaptic transmission, and zinc associated with the Golgi apparatus is assumed to catalyze metalloenzyme reactions.

The localization of endogenous Zn²⁺ in the mouse retina was examined histochemically and the inhibitory action of Zn²⁺ on GABA-induced responses was studied in bipolar cells isolated from the mouse retina. Accumulation of endogenous Zn²⁺ was detected in photoreceptors, bipolar, and/or amacrine cells by either the bromopyridylazo-diethylaminophenol method or the dithizone method. Under whole-cell recording conditions, GABA induced a Cl⁻ current in isolated bipolar cells. The current consisted of two components. The first component was inhibited completely by application of 100 μM bicuculline, suggesting that this is a GABA_A-receptor mediated current. The second component was inhibited completely by 100 μM 3-aminopropyl-(methyl)-phosphinic acid, suggesting that this is a GABA_C-receptor mediated current. GABA_C receptors were present at a higher density on the axon terminal than on dendrites. Zn²⁺ inhibited both GABA_A and GABA_C receptors. GABA_C receptors were more susceptible to Zn²⁺; the IC₅₀ for the GABA_A receptor was 67.4 μM and that for the GABA_C receptor was 1.9 μM. These results suggest that Zn²⁺ modulates the inhibitory interaction between amacrine and bipolar cells, particularly that mediated by the GABA_C receptor.