S. Vanhatalo et S. Soinila, DOPAMINERGIC PERIVENTRICULO-HYPOPHYSEAL NERVES SHOW TRYPTOPHAN-HYDROXYLASE IMMUNOREACTIVITY BUT LACK SEROTONIN SYNTHESIS, Brain research, 687(1-2), 1995, pp. 205-210
Hypothalamic dopaminergic periventricular and arcuate nuclei are known
to project to the pituitary gland and contain serotonin in their term
inals. In order to elucidate the potential of these neurons to synthes
ize serotonin, we studied immunohistochemically the possible tryptopha
n hydroxylase content of periventriculo-hypophyseal neurons, identifie
d by retrograde tracing from the pituitary gland. These neurons were f
ound to contain tryptophan hydroxylase-immunoreactivity (TpOH-IR), whi
ch was enhanced after colchicine treatment. All of the TpOH-IR neurons
contained tyrosine hydroxylase-immunoreactivity as well. However, non
e of them were immunoreactive for serotonin in either intact animals o
r in animals pretreated with serotonin precursor L-tryptophan and MAO
inhibitor pargyline. Thus, neurons of the dopaminergic periventriculo-
hypophyseal pathway express tryptophan hydroxylase, but are unable to
synthesize serotonin. These findings (i) raise the possibility that, i
n these nerves, serotonin might serve a function other than regular sy
naptic transmission, and (ii) suggest that expression of an enzyme syn
thesizing certain transmitter does not necessarily confirm the corresp
onding transmitter phenotype of that neuron.