The distribution of growth‐hormone‐releasing factor (GRF) immunoreactivity in the central nervous system of the rat: An immunohistochemical study using antisera directed against rat hypothalamic GRF

Abstract

Immunohistochemical methods have been used to chart the distribution of rat hypothalamic growth‐hormone‐releasing factor (rhGRF) immunoreactivity in the brains of normal and colchicine‐treated adult albino rats. The results suggest the existence of at least two distinct rhGRF‐containing systems: one responsible for delivery of the peptide to portal vessels in the median eminence, and one whose relationship, if any, to hypophysiotropic function is less direct. A dense plexus of rhGRF‐stained fibers was found throughout the external lamina of the median eminence that is the route by which the peptide is delivered to the anterior pituitary. This projection appears to arise primarily from a group of rhGRF‐immunoreactive neurons centered in the arcuate nucleus. Some 1,000–1,500 rhGRF‐positive neurons were counted on each side of the brain in rats pretreated with colchicine. Colocalization studies, using a sequential double staining technique, indicated that a subset of rhGRF‐immunoreactive neurons in the arcuate region contain neurotensin immunoreactivity. No evidence was obtained for colocalization of rhGRF with either of two pro‐opiomelanocortin‐derived peptides (α‐melanocyte‐stimulating hormone, adrenocorticotropic hormone (1–24)) in individual neurons in the arcuate nucleus. Much smaller groups of neurons were localized in the parvicellular division of the paraventricular nucleus of the hypothalamus and in the dorsomedial nucleus, and it is unclear whether they contribute to the plexus of rhGRF‐stained fibers in the median eminence. The only other region in the rat brain in which rhGRF‐stained cells were found reliably was in the area that roughly encapsulates the caudal aspect of the ventromedial nucleus of the hypothalamus. Because cells in this region are not known to project to the median eminence, they may be assumed to contribute to the extrahypophysiotropic rhGRF‐stained projections outlined below. From the level of the arcuate and ventromedial nuclei, rhGRF‐immunoreactive fibers could be traced along the base of the brain and through the periventricular system to discrete terminal fields limited almost exclusively to the hypothalamus and adjoining parts of the basal telencephalon. All parts of the periventricular region of the hypothalamus receive an input, including the preoptic and anterior parts in which somato‐statin‐containing neurons that project to the median eminence are clustered. Other prominent terminal fields were localized in discrete parts of the dorsomedial, paraventricular, suprachiasmatic, and premammillary nuclei, and in the medial preoptic and lateral hypothalamic areas. Beyond the hypothalamus, sparse projections have been traced to parts of the limbic region of the telencephalon, including the lateral septal nucleus, the bed nucleus of the stria terminalis, and the medial nucleus of the amygdala. No rhGRF‐immunoreactive cells or fibers have been detected in the neocortex, in any structure caudal to the mammillary complex of the hypothalamus, in the spinal cord, or in a number of peripheral tissues. These results suggest that GRF displays the most restricted distribution of the known hypophysiotropic hormones of the hypothalamus. The possibil‐ ity that the central rhGRF system may comprise an integrated unit for the control of growth hormone secretion by the anterior pituitary is discussed.