NITRIC-OXIDE SYNTHASE IN MULLER CELLS AND NEURONS OF SALAMANDER AND FISH RETINA

Nitric oxide synthase (NOS) is the biosynthetic enzyme of the signalin g molecule nitric oxide (NO). NO donors have been reported to modulate conductances in cell types throughout the retina, from photoreceptors to ganglion cells. Previously, NOS immunoreactivity has been reported in amacrine cells and cells within the ganglion cell layer. Here, we have examined the cellular localization of NOS in the retinas of salam ander, goldfish, and catfish using both an affinity-purified antiserum to brain NOS and NADPH diaphorase (NADPHd) histochemistry. These mark ers indicate that an NOS-like enzyme is localized not only to presumpt ive amacrine cells but also, depending on the species, to photorecepto r ellipsoids, to somata within the ganglion cell layer, and to horizon tal cells. In addition to these neurons, our results indicate that Mul ler cells, the radial glia of the retina, also contain an NOS-like enz yme. In support of this latter conclusion, cells morphologically simil ar to Muller cells were positive for NADPHd staining in all three spec ies. In salamander, NOS-like immunoreactivity, NADPHd staining, and bi nding of anti-GFAP (a marker for glia) were localized to cells that we re morphologically indistinguishable from Muller cells. In goldfish, r eactivity to both anti-NOS and anti-vimentin (a marker for glia) coloc alized to radial processes extending through the inner retina to the i nner limiting membrane. These observations are the first to indicate t he presence of an NOS-like enzyme in Muller cells and suggest that the se glia could be a ready source of NO for target neurons throughout th e retina.