Effect of light stimulation on the cytochemical localization of cytochrome C oxidase in photoreceptor cells of cichlid fish.

Cytochrome oxidase activity was evaluated cytochemically in rod photoreceptor cells in the retina of the cichlid fish Oreochromis mossambicus after different stimulation protocols. The enzyme activity was assessed semiquantitatively by estimating the volume ratio of mitochondria classified according to the intensity of enzyme reactivity. Dark adaptation for 5 hr induced an increase of cytochrome oxidase activity both in vivo and in vitro, i.e., in isolated retinas. Short-term illumination (1 hr) of isolated retinas adapted previously in vivo to darkness caused a significant decrease of enzyme activity, whereas short-term darkening after in vivo light adaptation had no effect. Chemical stimulation for 15 min with increased K+ concentration (20 mM) reduced the enzyme activity, i.e., chemical depolarization did not have the same effect as depolarization induced by darkening. Significant changes in cytochrome oxidase activity were apparent within 1 hr of stimulation, so that this method for analysis of neuronal activity can be applied even in short-term experiments.