Distribution and co-localization of nitric oxide synthase and argininosuccinate synthetase in the cat hypothalamus.

Argininosuccinate synthetase (ASS) and nitric oxide synthase (NOS) comprise part of the cyclic metabolic pathway to produce nitric oxide (NO). ASS is one of the arginine synthesis enzymes which synthesizes argininosuccinate from aspartate and citrulline, and NOS forms NO and citrulline from arginine. This study examines the localization of ASS and NOS in the cat hypothalamus using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and immunohistochemistry against ASS and NOS. NADPH-d positive and/or ASS-immunoreactive neurons were localized in the following areas: the anterior hypothalamic area, the anterior hypothalamic nucleus, the supraoptic nucleus, the suprachiasmatic nucleus, the periventricular complex, the paraventricular nucleus, the parvocellular nucleus, the lateral hypothalamic area, the dorsomedial hypothalamic nucleus, the dorsal hypothalamic area, the posterior hypothalamic area, and the supramammillary nucleus. NOS and ASS double-labeled neurons were found in the anterior hypothalamic area, the supraoptic nucleus, the central part of the paraventricular nucleus of the hypothalamus, the lateral hypothalamic area, ventral part of the parvocellular hypothalamic nucleus, the posterior hypothalamic area, and the supramammillary nucleus. Double-labeled neurons in the hypothalamus comprised 20.7-32.0% of ASS-immunoreactive neurons and 10.2-26.3% of NOS-immunoreactive neurons. The results suggest the existence of the 'NO cycle' in situ and the physiological importance of NO and argininosuccinate in several regions of the cat hypothalamus.