SOLUBLE GUANYLATE-CYCLASE AND NITRIC-OXIDE SYNTHASE IN SYNAPTOSOMAL FRACTIONS OF BOVINE RETINA

Cyclic GMP has been shown in recent years to directly activate ion cha nnels in bipolar and ganglion cells, and to indirectly regulate coupli ng between horizontal cells, and between bipolar and amacrine cells. I n all of these cases, the effects of cyclic GMP are mimicked by nitric oxide. An increase in calcium concentration stimulates the production of nitric oxide by neuronal and endothelial forms of nitric oxide syn thase, which in turn activates soluble guanylate cyclases, enhancing t he synthesis of cyclic GMP. Though some effects of nitric oxide do not involve cyclic GMP, the nitric oxide-cyclic GMP cascade is well recog nized as a signaling mechanism in brain and other tissues. The widespr ead occurrence of nitric oxide/cyclic GMP-regulated ion channel activi ty in retinal neurons raises the possibility that nitric-oxide-sensiti ve soluble guanylate cyclases play an important role in cell-cell comm unication, and possibly, synaptic transmission. Immunohistochemical st udies have indicated the presence of soluble guanylate cyclase in reti nal synaptic layers, but such studies are not suitable for determinati on of the density or quantitative subcellular distribution of the enzy me. Microanalytical methods involving microdissection of frozen retina also showed the presence of cyclase activity in retinal plexiform lay ers but these methods did not permit distinction between nitric oxide sensitive and insensitive cyclases. In this study, we fractionated ret inal homogenate into the cytosolic and synaptosomal fractions and inve stigated the specific activity and distribution of soluble guanylate c yclase and nitric oxide synthase. The results show that both enzymes a re present in the synaptosomal fractions derived from inner and outer plexiform layers. The synaptosomal fraction derived from inner retina was highly enriched in cyclase activity. Nitric oxide synthase activit y was also higher in the inner than outer retinal synaptosomal fractio n. The results suggest that the nitric oxide-cyclic GMP system is oper ational in both synaptic layers of retina and that it may play a more significant role in the inner retina.