DEVELOPMENT OF THE TUBEROINFUNDIBULAR SYSTEM IN RATS - BIRTH-DATES OFTHE TYROSINE HYDROXYLASE-IMMUNOPOSITIVE NEURONS

The present study has attempted to determine the birthdates of the tyr osine hydroxylase (TH)-immunopositive neurons in the arcuate nucleus ( AN) of rats, thus evaluating the time of the last mitotic divisions of their cell precursors. For this aim, 'long-survival' [H-3]thymidine a utoradiography was used in combination with immunocytochemistry of TH, the first enzyme of catecholamine synthesis. According to our data, s ome TH-immunopositive neurons in the AN were born as early as the 13th fetal day. At day 14, the number of double-labeled neurons almost tri pled while reaching a maximum at day 15 of the intrauterine developmen t. The fraction of the double-labeled neurons dropped on the following day, and practically disappeared by fetal day 17, showing cessation o f the production of the TH-immunopositive neurons. The birth of the TH -immunopositive neurons is considered as the onset of the development of the tuberoinfundibular dopaminergic system.