NADPH-diaphorase expression in neurons and glia of the normal adult rat retina.

In the rat retina, nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) staining has been described previously in a population of amacrine cells, most of which were located in the inner nuclear layer. In the present study, a number of parameters such as the nature of the fixative, the time of fixation and photointensification were optimised to obtain the strongest possible reaction for this enzyme. As a result, a very different staining pattern emerged: with short paraformaldehyde fixation, numerous neurons (identified as a combination of ganglion cells and amacrines) were labelled in the ganglion cell layer, NADPH-d-positive amacrine cells (described previously) were seen in the inner nuclear layer and Müller cells were labelled strongly, particularly in the inner retina. Glutaraldehyde fixation of the same duration resulted in the preferential staining of Müller cells while neurons appeared less reactive. Therefore, fixation conditions are a determining factor in the cellular localisation of NADPH-d in the rat retina. By taking fixation into account, future studies should gain more rigorous insights into the possible functions of this enzyme in the vertebrate retina.