Cell typing and connections of the rat suprachiasmatic nucleus.

Microiontophoretic delivery of horseradish peroxidase and Lucifer Yellow liposomes into several nuclei (arcuate nucleus, median eminence, ventromedial hypothalamic nucleus, and retrochiasmatic area) of the medial basal hypothalamus revealed an intrinsic pattern of connections to the suprachiasmatic nucleus that was correlated to immunocytochemistry (LH-RH, leuenkephalin, and ACTH) of the fibers and cell types. Bilateral projections of the suprachiasmatic nucleus into the arcuate nucleus were found. The number of connections between the supraoptic nucleus and nuclei of the mediobasal hypothalamus was found to be greater than hitherto reported. A direct projection from the suprachiasmatic nucleus into the median eminence could not be demonstrated with the retrograde horseradish peroxidase and the Lucifer Yellow liposome technique. Intermediobasal hypothalamic connections are not only organized over the third ventricle but also ventrally through the median eminance. ACTH-like fibers penetrate the suprachiasmatic nucleus. These connections originate in the arcuate nucleus of the mediobasal hypothlamus. Leu-enkephalin was demonstrated within the center of the suprachiasmatic nucleus; LH-RH positive cells, however, surround the suprachiasmatic nucleus.