Differentiation of tyrosine hydroxylase‐synthesizing and/or aromatic L ‐amino acid decarboxylase‐synthesizing neurons in the rat mediobasal hypothalamus: Quantitative double‐immunofluorescence study

In this double‐immunofluorescence study, we first quantified the neurons of the arcuate nucleus as immunoreactive (+) for tyrosine hydroxylase (TH) and/or aromatic L ‐amino acid decarboxylase (AADC) in rats at embryonic day 21 (E21), at postnatal day 9 (P9), and in adulthood by using conventional fluorescent or confocal microscopy. On E21, monoenzymatic (TH + AADC immunonegative (−) and TH − AADC + ) neurons and bienzymatic (TH + AADC + ) neurons accounted for 99% and 1%, respectively, of the whole neuron population expressing enzymes of dopamine synthesis. Further development was characterized by the dramatic increase in TH + AADC − dorsomedial and TH + AADC + dorsomedial populations from E21 to P9 as well as by the increase in the TH + AADC + dorsomedial population (in females) and a drop in the TH + AADC − ventrolateral and TH + AADC − dorsomedial (in males) populations from P9 to adulthood. In contrast to TH + AADC − (in males) and TH + AADC + neurons, the TH − AADC + neurons did not change in number from E21 to adulthood. Thus, in rat fetuses, the neurons synthesizing TH and/or AADC were mainly monoenzymatic, whereas during postnatal life the fraction of bienzymatic neurons increased by up to 60%. J. Comp. Neurol. 446:114–122, 2002. © 2002 Wiley‐Liss, Inc.