BIRTH-DATES OF THE TYROSINE-HYDROXYLASE IMMUNOREACTIVE NEURONS IN THEHYPOTHALAMUS OF MALE AND FEMALE RATS

This study determined the birthdates of the tyrosine hydroxylase(TH) i mmunoreactive (IR) neurons in the zona incerta (ZI), periventricular n ucleus (PeVN) and arcuate nucleus (AN) of male and female rats. 'Long- survival' [H-3]thymidine autoradiography combined with TH immunocytoch emistry, the first enzyme of catecholamine synthesis, was used. In mal es, TH-IR neurons originate in the ZI between embryonic days (E) 12 an d 13, while in the PeVN and AN this process is prolonged until E16. Th e majority of TH-LR neurons became postmitotic at E12 in the ZI, betwe en E12 and E14 in the PeVN and at E15 in the AN. The birthdate of TH-I R neurons was sexually dimorphic with (a) generation of the majority o f TH-IR neurons in the ZI in males preceding that in females, (b) gene ration of TH-IR neurons in the AN of males delayed as compared to fema les, and (c) average daily fractions of the newborn TH-IR neurons in e ach hypothalamic region of females exceeding that seen in males. This sexual dimorphism was observed prior to E16, i.e. before the onset of sex difference in androgen levels, implying a hormone-independent mech anism, determined at the genetic level.