PITUITARY-GLAND RECEIVES BOTH CENTRAL AND PERIPHERAL NEUROPEPTIDE-Y INNERVATION

Neuropeptide Y (NPY)-containing neural projections to the rat pituitar y gland were studied by combining NPY immunohistochemistry with retrog rade tracing with Fluorogold as well as central and peripheral denerva tions. Numerous pituitary-projecting, i.e. Fluorogold-labelled, neuron s in the superior cervical ganglion, as well as in the hypothalamic ma gnocellular nuclei were NPY-immunoreactive (NPY-IR). In contrast, no o ther hypothalamic NPY-IR neurons, e.g, in the arcuate nucleus or the p reoptic area, were observed to be projecting into the pituitary. Withi n the posterior lobe of the pituitary gland two morphologically distin ct NPY-LR fiber populations were discovered, namely thinner parenchyma l terminals, distinct from the neurosecretory terminals, and thicker, perivascular fibers. Neurosecretory nerve terminals, in contrast, were devoid of NPY-IR, being consistent with the previous reports on their sensitivity to osmotic stimulation. On the other hand, the anterior a nd intermediate lobes contained no NPY-IR fibers. Bilateral extirpatio n of the superior cervical ganglion resulted in disappearance of the p erivascular NPY-lR fibers leaving the parenchymal NPY-IR fibers unaffe cted, while transection of the pituitary stalk abolished all of the pa renchymal NPY-IR neurons, leaving the perivascular fibers unaffected. These findings together with the observed colocalization of tyrosine h ydroxylase and NPY in the posterior lobe perivascular fibers indicated that they are sympathetic nerve endings. The thin parenchymal termina ls, instead, are suggested to stem from central sources other than hyp othalamus. Our findings indicate that the pituitary gland receives NPY -containing innervation from at least three distinct sources, and NPY may thus affect pituitary functions in various ways, such as blood flo w and vasopressin release.