CRF-containing neuron systems in the rat hypothalamus: retrograde tracing and immunohistochemical studies

By employing a combination of the immunohistochemistry for rat corticotropin-releasing factor (rCRF) and retrograde tracing with biotinylated wheat germ agglutinin (b-WGA) injected into the posterior pituitary (group 1) or into the middle portion of the median eminence (group 2), functionally different populations of CRF neurons were identified in the rat hypothalamus. In the group 1 animals, WGA-labeling was seen not only in the posterior lobe but also in the intermediate lobe, in which CRF fibers exist. In these animals, WGA-labeling occurred for almost all large neurons in the anterior commissural, paraventricular (PV), and supraoptic (SO) nuclei, some of the neurons showing a slight immunoreactivity for anti-rCRF. Conversely, CRF positive neurons appeared in large numbers, some being labeled with WGA, in the caudal periventricular region (CPR), and in the dorsomedial to lateral hypothalamic area (DLH), especially in the latter. In the group 2 animals, WGA was disparsed throughout the subependymal, internal, and external layers of the medial portion of the median eminence, and was taken up by many small cells in the PV, almost half of the cells being immunoreactive for CRF. Slight WGA-labeling further occurred in some large neurons of the PV and SO, and in some cells of the DLH. It is concluded that hypophysiotropic CRF existing in the external layer of the median eminence originates from small neurons located in the PV, whereas CRF distributed in the posterior and intermediate pituitary originates from the magnocellular PV and SO, and from some neurons in the DLH and CPR.