COEXISTENCE OF OXYTOCIN AND NADPH-DIAPHORASE IN MAGNOCELLULAR NEURONSOF THE PARAVENTRICULAR AND THE SUPRAOPTIC NUCLEI OF THE RAT HYPOTHALAMUS

Nitric oxide (NO), which was firstly identified as an endothelium-deri ved relaxing factor, has recently been demonstrated to be a neurotrans mitter in the central and peripheral nervous systems. In the hypothala mmus, abundant nitric oxide synthase (NOS) immunoreactivity and its hi stochemical marker, NADPH-diaphorase activity, have been demonstrated in the hypothalamo-neurohypophyseal system. In the present study, we e xamined whether NOS is coexpressed with posterior pituitary hormones i n the rat hypothalamus by combination of oxytocin and vasopressin immu nofluorescence and NADPH-diaphorase histochemistry. Most oxytocin-immu noreactive neurons in the paraventricular and supraoptic nuclei expres sed NADPH-diaphorase activity, but virtually no vasopressin-immunoreac tive neurons contained NADPH-diaphorase activity. This suggests that o xytocin neurons are the main source of NO production in the hypothalam ic-pituitary system.