Cyclic GMP systems in the retina

Evidence for the presence of multiple cyclic GMP (cGMP) systems in the vertebrate retina is examined. Levels of this cyclic nucleotide are characteristically high in the photoreceptor cell, whereas in the inner retina they are similar to those found in other areas of the central nervous system. Guanylate cyclase and cGMP phosphodiesterase show a similar uneven distribution, with very high levels of activity in outer segments. At least three forms of guanylate cyclase, one of which may be unique to the photoreceptor cell, have been described. Similarly, differences in the regulation of cGMP phosphodiesterase within localized areas of retina indicate the existence of multiple forms of this enzyme. Factors that influence cGMP levels also have regionally variable effects. Light adaptation leads to reduced cGMP levels in the distal retina, whereas inner retina layers do not show parallel changes. Calcium suppresses the formation of cGMP in photoreceptors, but has no effect or stimulates cGMP formation in the inner retina. Cellular depolarization appears to reduce levels of cGMP in photoreceptors and increases it in the inner retina. Furthermore, endogenous proteins whose phosphorylation is apparently controlled by cGMP seem to be localized to specific retinal cells or subcellular organelles. These data lead to the conclusion that multiple cGMP systems exist in the retina, each with a distinct location and function.