The tuberoinfundibular system of the rat as demonstrated by immunohistochemical localization of retrogradely transported wheat germ agglutinin (WGA) from the median eminence.

The origin of neuronal perikarya which project to the external zone of the median eminence (the tuberoinfundibular neuronal system) was determined in the rat after injection or diffusion of wheat germ agglutinin (WGA) into the median eminence. The retrogradely transported lectin was detected in neurons using an immunohistochemical method based on the peroxidase-antiperoxidase technique. Immunoreactive cell bodies were found both in hypothalamic and extrahypothalamic regions. Within the hypothalamus, the majority of peroxidase-positive cells were present in the dorsomedial and basolateral portions of the arcuate nucleus, regions of the periventricular nucleus, and the preoptic region, particularly at the level of the organum vasculosum of the lamina terminalis (OVLT). Within the extrahypothalamic regions, WGA-positive perikarya were found in the diagonal band of Broca, the region of the medical septum and the brainstem. Only rare cells were labeled in the ventromedial nucleus of the hypothalamus and no cells were labeled in any region of the amygdala. These data demonstrate that neurons with afferent projections to the median eminence are more widely distributed in the rat brain than previously recognized and therefore, that the concept of the tuberoinfundibular neuronal system must be expanded.