The influence of serotonin afferents on
tyrosine hydroxylase expression in differentiating
neurons of the rat arcuate nucleus was studied
in vivo and in vitro. In the in vivo study, pchlorophenylalanine
inhibited serotonin synthesis
in fetal brain from the 11th
to the 20th
embryonic
day. We then used semiquantitative immunocytochemistry
to evaluate tyrosine hydroxylase
levels in neurons of the arcuate nucleus in
fetuses at the 21st
embryonic day or in offspring
at the 35th
postnatal day. Serotonin depltion
significantly decreased the tyrosine hydroxylase
content in neurons of males and females at the
21st
embryonic day and in males at the 35th
postnatal day. For the in vitro study, embryonic
neurons of the arcuate nucleus were cocultured
with embryonic neurons of the raphe
nucleus, the main source of serotonin innervation
of the brain, including the arcuate nucleus.
Co-culture of the neurons resulted in a genderspecific
increase of the tyrosine hydroxylase
level in the neurons of the arcuate nucleus. In
turn, the neurons of the raphe nucleus showed
increased levels .of serotonin in both males and
females, with no sexual dimorphism. Thus, our
results suggest a stimulatory, long-lasting effect
of serotonin afferents on tyrosine hydroxylase
expression in the differentiating neurons of the
rat arcuate nucleus during prenatal ontogenesis.
