Centrally mediated effects of neurohypophyseal hormones

Intracerebroventricular administration of oxytocin to virgin female rats that had been ovariectomized and primed with estrogen 48 hours previously induced a rapid onset of full maternal behavior. The maternal behavior persisted and its incidence was dose-related. Tocinoic acid, the ring structure of oxytocin, also rapidly induced the onset of persistent, full maternal behavior. Arginine vasopressin induced persistent maternal behavior, but this behavior had a later onset. Prostaglandin F2 alpha induced strong partial maternal behavior, which showed early onset but did not persist. Many other peptides, ovarian steroids, and prostaglandin E2 were no more effective than saline. These findings suggest that the release of oxytocin and prostaglandin F2 alpha during labor may promote maternal behavior in rats.

Experiments using two retrogradely transported fluorescent dyes (bisbenzimide-true blue, and Evans blue-granular blue) were performed in order to determine whether the same or different populations of neurons of the paraventricular nucleus of the hypothalamus (PVH) project to the pituitary gland, dorsal vagal complex, and spinal cord in the rat. The results suggest that cells projecting to the pituitary gland are concentrated in the magnocellular core of the nucleus, while the descending connections arise primarily from the surrounding parvocellular division. The occurrence of neurons double-labeled with both dyes further indicate that at lease 10-15% of the labeled cells in the parvocellular division send divergent axon collaterals to the dorsal vagal complex and to the spinal cord. Cell counts suggest that at least 1,500 cells in the PVH project to the medulla and/or spinal cord. These results, combined with a cytoarchitectonic analysis, show that the PVH consists of eight distinct subdivisions, three magnocellular and five parvocellular. The lateral hypothalamic area and zona incerta also contain a large number of cells projecting to the dorsomedial medulla and spinal cord; approximately 15% of such cells are the double-labeled following injections of separate tracers into these two regions of the same animal.

We have summarized here recent evidence that clarifies the cellular organization and connections of the paraventricular nucleus of the hypothalamus (PVH) in the rat. The nucleus consists of a magnocellular division, with three distinct parts, and a parvocellular division with five distinct parts. Most neurons in the magnocellular division contain either oxytocin or vasopressin, and project to the posterior lobe of the pituitary gland. Separate cell populations centered in the parvocellular division give rise to projections to the median eminence, or to the brain stem and spinal cord including the intermediolateral column; some cells project both to the dorsal vagal complex and to the spinal cord. Cells with long descending projections may contain either oxytocin, vasopressin, somatostatin, or dopamine, although the biochemical specificity of most such neurons has not been determined. Noradrenergic fibers are found preferentially within those parts of the magnocellular division that are predominantly vasopressinergic. The parvocellular division is innervated by adrenergic as well as noradrenergic fibers from the brain stem, and by fibers from the dorsal vagal complex and the parabrachial nucleus. The bed nucleus of the stria terminalis and adjacent parts of the hypothalamus also innervate the PVH. The evidence indicates that subpopulations of neurons in the PVH are directly related to autonomic and neuroendocrine effector mechanisms, and suggests that the nucleus plays an important role in the regulation of visceral responses in the periphery and in the CNS itself.