Dopaminergic periventriculo-hypophyseal nerves show tryptophan-hydroxylase immunoreactivity but lack serotonin synthesis.

Hypothalamic dopaminergic periventricular and arcuate nuclei are known to project to the pituitary gland and contain serotonin in their terminals. In order to elucidate the potential of these neurons to synthesize serotonin, we studied immunohistochemically the possible tryptophan hydroxylase content of periventriculo-hypophyseal neurons, identified by retrograde tracing from the pituitary gland. These neurons were found to contain tryptophan hydroxylase-immunoreactivity (TpOH-IR), which was enhanced after colchicine treatment. All of the TpOH-IR neurons contained tyrosine hydroxylase-immunoreactivity as well. However, none of them were immunoreactive for serotonin in either intact animals or 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, in these nerves, serotonin might serve a function other than regular synaptic transmission, and (ii) suggest that expression of an enzyme synthesizing certain transmitter does not necessarily confirm the corresponding transmitter phenotype of that neuron.