ABSTRACT
Nitric oxide(NO) has recently been identified as a short-lived intracellular messenger in the central and peripheral nervous systems. Nitric oxide synthase(NOS) is a key enzyme for NO biosynthesis by formation of citrulline from L-arginine. This enzyme reaction requires reduced nicotine adenine dinucleotide phosphate(NADPH) as a co-factor and the electron transferring c-terminal sequence of NOS give rise to its NADPH-diaphorase activity. However, it has yet to prove that NO is a sensory neurotransmitter in olfactory nervous system. The purpose of this study was to localize histochemically the distribution of NADPH-diaphorase in the olfactory system of rats. In the olfactory mucosa, NADPH-diaphorase positive cells were demonstrated in the ciliary layers, Bowmann's glands and olfactory nerve bundles. In main olfactory bulb, a large number of NADPH-diaphorase positive cells were also demonstrated in nerve fibers of olfactory nerve layer, periglomerular cell, superficial short axon cells of external plexiform layer and deep short axon cells between granular and subependymal layers. Furthermore there were NADPH-diaphorase positive cells of plexiform and granular cell layers of the accessory olfactory bulb. Therefore, it is suggested that the presence of NADPH-diaphorase acitivity in nerve fibers of olfactory system may be involved in odor processing.