NITRIC-OXIDE SYNTHASE-CONTAINING MAGNOCELLULAR NEURONS OF THE RAT HYPOTHALAMUS SYNTHESIZE OXYTOCIN AND VASOPRESSIN AND EXPRESS FOS FOLLOWING STRESS STIMULI

We investigated the chemical and anatomical features of nitric oxide s ynthase (NOS)-containing neurons in the paraventricular and supraoptic nuclei in the rat hypothalamus using combinations of enzyme histochem istry, in situ hybridization and immuno-histochemistry. Neurons expres sing NOS mRNA completely overlapped with NADPH-diaphorase-positive neu rons. Topographical distribution of NOS was segregated from that of CR F-containing parvicellular neurons in the posterior paraventricular nu cleus but overlapped with that of magnocellular neurons. In the parave ntricular nucleus, 70% of oxytocin neurons contained NOS, which corres ponded to one half of NOS neurons. About one third of vasopressin-immu noreactive neurons were NADPH-diaphorase-positive and the same proport ion of NADPH-diaphorase-positive neurons were vasopressin-immunoreacti ve. In the supraoptic nucleus, 50% of oxytocin neurons were NADPH-diap horase-positive, which corresponded to 40% of NOS neurons. About 25% o f vasopressin neurons were NADPH-diaphorase-positive, and 30% of NADPH -diaphorase-positive neurons were vasopressin-immunoreactive. When NAD PH-diaphorase histochemistry was performed first, subsequent immunosta ining was markedly perturbed. Using fluoro-gold as a retrograde tracer , 4% of NADPH-diaphorase-positive neurons were shown to contribute to the descending projection to the spinal cord. About 40%-50% of NADPH-d iaphorase-positive neurons exhibited Fos immunoreactivity after inject ion of lipopolysaccharide or hypertonic saline, while only 10%-15% of these neurons expressed Fos in response to immobilization or pain. End ogenous NO may be involved in the regulation of magnocellular function s, especially when the internal environment is disturbed.