DIFFERENTIATION OF SHORT-WAVELENGTH-SENSITIVE CONES BY NADPH DIAPHORASE HISTOCHEMISTRY

NADPH diaphorase (NADPH dehydrogenase; EC 1.6.99.1) histochemistry lab els neurons that synthesize the neurotransmitter nitric oxide (NO). In retina, it has been demonstrated that NO can affect the metabolism of cGMP in rod photoreceptors. To investigate potential involvement of N O in cone photoreceptor activity, we utilized NADPH diaphorase histoch emistry to study the cone-dominated retina of the tree shrew (Tupaia b elangeri). Unexpectedly, our results revealed different NADPH diaphora se activity in the cellular subcompartments of the spectral classes of cone photoreceptors. Although all cones showed intense labeling of in ner segment ellipsoids, the short-wavelength-sensitive (SWS or ''blue- sensitive'') cones and the rods displayed intense staining of the myoi d inner segment subcompartment as well. Furthermore, only SWS cones an d rods displayed surface labeling of their nuclei. These findings indi cate a manner in which SWS cones differ biochemically from other cone types and in which they are more similar to rods. Such differences may underlie some of the unusual functional properties of the SWS cone sy stem, which have been attributed to postreceptoral processes.