Light and electron microscopic study of tyrosine hydroxylase-immunoreactive neurons within the developing rat arcuate nucleus

The topography, fine structure, and patterns of connections of tyrosine hydroxylase (TH)-immunoreactive tubero-infundibular dopaminergic (TIDA) neurons were examined by light and electron microscopic immunocytochemistry in the arcuate nucleus of 2-, 15- and 30-day-old female Wistar rats. In 2-day-old animals, TH-immunoreactive perikarya were mainly located in the ventrolateral portion of the arcuate nucleus. In 15-day-old rats numerous TH-positive cell bodies were still present ventrolaterally, but a cluster of labeled cells was also apparent in the mediodorsal segment of the nucleus. In the 30-day-old rats, most TH-immunoreactive neurons were concentrated mediodorsally, as seen in the adult. At the ultrastructural level, TH-immunoreactive somata exhibited, in all age groups, a large nucleus surrounded by a thin rim of cytoplasm containing mitochondria, Golgi apparatus, endoplasmic reticulum, multivesicular bodies and lysosomes. These labeled somata were synaptically contacted by unlabeled axon terminals and often laid adjacent to either labeled or unlabeled dendrites. Similarly, in all age groups, labeled dendrites were synaptically contacted by unlabeled axon terminals and were often directly apposed to either labeled or unlabeled perikarya and dendrites, or to tanycytic processes. These results indicate that TIDA neurons establish extensive connections early in development, and that their pattern of intercellular relationships remains qualitatively unchanged from 2 days to adulthood. It is suggested that TIDA neurons may be already functional at birth, and could therefore, influence the maturation of other arcuate neuronal populations.