Ontogenesis of peptide-histidine-isoleucine (PHI)-containing neurons in the suprachiasmatic nucleus (SCN) of the rat.

The morphologic and ontogenetic changes in PHI (peptide-histidine-isoleucine)-containing neurons in the suprachiasmatic nucleus (SCN) of the rat were studied using immunohistochemistry. PHI immunoreactivity first appeared in neural perikarya and processes as early as day 18 of gestation, long before they became evident (day 10 postnatally) in other brain structures. PHI neurons in the SCN were initially small and exhibited faint staining. They gradually increased in number during fetal life, and were found throughout this nucleus by day 21. After birth, they rapidly increased in number and staining intensity. In the 20-day-old rat. PHI-containing perikarya were concentrated in the ventral and medial aspects of the SCN while fibers were located primarily within the dorsal and caudal portions, as in the adult rat. These findings suggest that PHI exerts a physiological role in the developing SCN.