NITRIC-OXIDE IN THE RETINA - A DOUBLE-FAC E MEDIATOR

The free radical nitric oxide (NO) is synthesized from L-arginine by a n enzyme termed NO synthase (NOS). Three isoforms of NOS, coded by thr ee disctint genes, have been identified, NOS-I and NOS-III, termed con stitutive isoforms and NOS-II, generally termed as inducible isoform. The three isoforms are expressed in the retina and the specific role f or each of them begin to be understood. The small quantities generate by isoforms I and III in a regulated manner, is involved in nuerotrans mission between some amacrine cells and bipolar or ganglion cells, and in the regulation of retinal blood flow. In contrast, NOS-II is expre ssed in retinal pigmented epithelial and Muller glial cells, during so me pathological status or after in vitro stimulation by cytokines. The large amounts of NO, released by NOS-II, can act as a cytostatic and cytotoxic molecule against invading agents but also against healthy ce lls. In this context the enhanced formation of NO could disrupt the fu nctional of NO could disrupt the functional retinal integrity, could l ead to retinal inflammation and then could participate to retinal dege neration.