NITRIC-OXIDE SYNTHASE IN THE HYPOTHALAMOPITUITARY PATHWAYS

Several histochemical and physiological studies in the literature sugg est that nitric oxide (NO) is involved in the posterior pituitary regu lation. This study was set out to demonstrate the distribution of the pituitary projecting nitric oxide containing hypothalamic pathways. Th e fluorescent retrograde tracer fluorogold (FG) was injected into the pituitary gland in order to reveal the hypothalamic nuclei projecting to the pituitary gland. Nitric oxide synthase (NOS) was subsequently v isualized from the same sections by NADPH-diaphorase histochemistry. M ore than 70% of the FG labelled neurons in the preoptic area, paravent ricular, supraoptic, retrochiasmatic, anterior commissural and circula r nuclei as well as in many other small neurosecretory centers in the hypothalamus contained also NADPH-diaphorase activity (NADPH-DA). In a ddition, some of the neurons in the periventricular nucleus as well as occasional neurons in the lateral hypothalamus projecting to the pitu itary gland contained NADPH-DA. The present results give the first sys tematic neuroanatomical description of the hypothalamic NO synthase co ntaining neurons projecting into the pituitary gland. They indicate th at the pituitary receives a widely distributed NO innervation, which o riginates mostly in the magnocellular neurosecretory hypothalamo-pitui tary system. In addition, the finding that some presumably nonneurosec retory pituitary-projecting neurons contain NO as well suggests that N O might be a more widely used regulator of the posterior pituitary sec retion than previously expected.