DISTRIBUTION AND COLOCALIZATION OF NITRIC-OXIDE SYNTHASE AND ARGININOSUCCINATE SYNTHETASE IN THE CAT HYPOTHALAMUS

Argininosuccinate synthetase (ASS) and nitric oxide synthase (NOS) com prise part of the cyclic metabolic pathway to produce nitric oxide (NO ), ASS is one of the arginine synthesis enzymes which synthesizes argi ninosuccinate from aspartate and citrulline, and NOS forms NO and citr ulline from arginine, This study examines the localization of ASS and NOS in the cat hypothalamus using nicotinamide adenine dinucleotide ph osphate-diaphorase (NADPH-d) histochemistry and immunohistochemistry a gainst 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 supra chiasmatic nucleus, the periventricular complex, the paraventricular n ucleus, the parvocellular nucleus, the lateral hypothalamic area, the dorsomedial hypothalamic nucleus, the dorsal hypothalamic area, the po sterior hypothalamic area, and the supramammillary nucleus, NOS and AS S double-labeled neurons were found in the anterior hypothalamic area, the supraoptic nucleus, the central part of the paraventricular nucle us of the hypothalamus, the lateral hypothalamic area, ventral part of the parvocellular hypothalamic nucleus, the posterior hypothalamic ar ea, and the supramammillary nucleus, Double-labeled neurons in the hyp othalamus 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.