NITRIC oxide (NO), apparently identical to endothelium-derived relaxing factor in blood vessels1-3, is also formed by cytotoxic macrophages4,5, in adrenal gland6 and in brain tissue7-9, where it mediates the stimulation by glutamate of cyclic GMP formation in the cerebellum10,11. Stimulation of intestinal12 or anococcygeal13-15 nerves liberates NO, and the resultant muscle relaxation is blocked by arginine derivatives that inhibit NO synthesis. It is, however, unclear whether in brain or intestine, NO released following nerve stimulation is formed in neurons, glia, fibroblasts, muscle or blood cells, all of which occur in proximity to neurons and so could account for effects of nerve stimulation on cGMP and muscle tone. We have now localized NO synthase protein immunohisto-chemically in the rat using antisera to the purified enzyme16. We demonstrate NO synthase in the brain to be exclusively associated with discrete neuronal populations. NO synthase is also concentrated in the neural innervation of the posterior pituitary, in autonomic nerve fibres in the retina, in cell bodies and nerve fibres in the myenteric plexus of the intestine, in adrenal medulla, and in vascular endothelial cells. These prominent neural localizations provide the first conclusive evidence for a strong association of NO with neurons.
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