EFFERENT PROJECTIONS FROM THE PERIVENTRICULAR AND MEDIAL PARVICELLULAR SUBNUCLEI OF THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS TO CIRCUMVENTRICULAR ORGANS OF THE RAT - A PHASEOLUS-VULGARIS-LEUKOAGGLUTININ (PHA-L) TRACING STUDY

The heterogeneous hypothalamic paraventricular nucleus (PVN) is intimately involved in the regulation of several homeostatic functions. These regulations might, at least partly, be mediated via neuronal projections from the PVN to circumventricular organs outside the blood-brain barrier. To study the efferent projections of the medial and periventricular parvicellular subnuclei of the PVN with particular emphasis on the projections to the circumventricular organs, anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) was applied. Three major efferent pathways and one minor one coursed from the medial and periventricular parvicellular subnuclei to the circumventricular organs. The major fiber projections included a rostral, a lateral, and a dorsocaudal projection tract, whereas the minor projection coursed ventrally. Fibers of the rostral projection were followed to the preoptic area and along the fornix to the subfornical organ. Single fibers originating from this projection coursed further rostrally to the organum vasculosum laminae terminalis. The lateral projection equivalent to the hypothalamo-pituitary tract passed through the lateral hypothalamic area to the median eminence, and nerve terminals were observed throughout the rostrocaudal extent of this structure. A few fibers of this bundle continued into the infundibular stalk and some terminated in the posterior pituitary lobe. Few fibers of the lateral projection descended to caudal pontine levels, where they reached descending fibers of the dorsocaudal projection. The dorsocaudal projection was essentially restricted to midline structures. Along the midline, fibers were followed from the hypothalamus either dorsally through the thalamus to the dorsal part of the third ventricle or caudally alongside the ventricular wall to the mesencephalic periaqueductal grey. The density of fibers decreased along the caudal direction of the neuraxis. The dorsal part of this projection gave rise to terminals in the deep pineal gland and pineal stalk, whereas the caudal part of this projection sent terminating fibers into the area postrema. The minor ventrally directed projection could be followed through the periventricular region to the rostral part of the median eminence. The number of terminals in the circumventricular organs varied. Within the median eminence, a high density of afferents was observed in the entire rostrocaudal extent of the external zone, whereas a low density of fibers was seen in the internal zone. A medium density of afferents was observed in the organum vasculosum laminae terminalis, whereas a relative low density of nerve terminals was observed in the posterior pituitary, the deep pineal gland, the subfornical organ, and the area postrema. The morphology of the PHA-L immunoreactive nerve fibers and terminals in these organs suggests that specific PVN subnuclei, in addition to a neurosecretory release to the blood stream, may have an influence on the parenchymal cells in some circumventricular organs. In conclusion, since neurons of the medial and periventricular parvicellular subnuclei of the PVN project to several circumventricular organs, they may be regarded as superior integrators of neuroendocrine outputs.