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.