Recent studies imaging nitric oxide (NO) production in the
retina have indicated a much wider distribution of NO production
than would be suggested by previous light-microscopic localizations
of neuronal nitric oxide synthase (nNOS). To help resolve this
discrepancy, the present study analyzed the ultrastructural
localization of nNOS-like immunoreactivity (-LI) in all layers
of the retina. In the ellipsoids of rod photoreceptors and the
accessory elements of double cones, nNOS-LI was associated with
some atypical mitochondria. In the outer plexiform layer, nNOS-LI
was in some postsynaptic horizontal and bipolar cell processes
at photoreceptor ribbon synapses. In some amacrine and ganglion
cell somata, nNOS-LI was diffusely localized in the cytoplasm
and associated with the endoplasmic reticulum. In the inner
plexiform layer, nNOS-LI diffusely filled some amacrine cell
processes, while in other amacrine cells nNOS-LI was selectively
localized at the presynaptic specializations of conventional
synapses. Neuronal NOS-LI was also found at membrane
specializations in bipolar cell terminals that were distinct
from their normal ribbon synapses. Finally, some nNOS-LI was
found in mitochondria in Müller cells. The diverse subcellular
localizations of nNOS-LI indicates that NO may play distinct
functional roles in many retinal cells, which correlates well
with the widespread NO production found in previous NO imaging
studies.