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 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.