[Expression of nitric oxide synthase in enkephalin and dynorphin systems of the rat hypothalamus]

Nitric oxide (NO), a simple gas with free radical chemical properties, is synthesized by nitric oxide synthase (NOS) from arginine in neurons and acts as a neurotransmitter in the central and peripheral nervous systems. The immunohistochemical demonstration of NOS-immunoreactivity and its histochemical marker, NADPH-diaphorase activity in many neurons of the hypothalamus, suggest that NO plays a role in controlling the production and/or release of hypothalamic neuroendocrine peptides. In the present study, the expression of NOS in the enkephalin and dynorphin systems of the rat hypothalamus was examined by the combined method of the NADPH-diaphorase histochemistry and the immunocytochemistry of methionine enkephalin (M-Enk) or dynorphin B (Dyn-B). About 6 to 9% of M-Enk immunoreactive neurons in the paraventricular, arcuate and ventromedial nuclei expressed NADPH-diaphorase activity. Dyn-B immunoreactive neurons, however, showed NADPH-diaphorase activity in high ratio (37%-84%) in the supraoptic nucleus and the parvocellular and magnocellular paraventricular nucleus. These results revealed that a part of the enkephalin and dynorphin neurons in the rat hypothalamus have the ability to produce NO. The high ratio of expression of NO in magnocellular neurosecretory dynorphin containing neurons suggested that NO participates in controlling posterior pituitary hormone secretion together with dynorphin.