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.