Re-evaluating NADPH-diaphorase histochemistry as an indicator of nitric oxide synthase: an examination of the olfactory system of coho salmon (Oncorhynchus kisutch).

The NADPH-diaphorase (NADPH-d) histochemical technique is commonly used to localize the nitric oxide (NO)-producing enzyme NO synthase (NOS) in neural tissues. In the olfactory tissues of vertebrates, however, NADPH-d staining can be present without the detection of NOS by other methods. We used pharmacological controls to determine whether NADPH-d staining was indicative of NOS in olfactory tissues from coho salmon (Oncorhynchus kisutch). We also compared NADPH-d staining with immunoreactivity to NOS. NADPH-d staining localized to the olfactory epithelium and to the glomerular layer of the olfactory bulb. Two NOS inhibitors, L-N-nitroarginine and L-N-methyl-L-arginine, failed to block this staining, but the less specific flavin-binder, diphenylene iodonium chloride, decreased or eliminated NADPH-d staining in all olfactory tissues. Immunoreactivity to NOS was present in short axon cells of the bulb, but was absent from the epithelium. Our findings show that NADPH-d staining was not representative of NOS in the olfactory tissues of salmon.