Evidence for an intracerebral action of phenylalanine in stimulation of prolactin secretion: interaction of large neutral amino acids.

Phenylalanine stimulates PRL secretion when given orally or iv to normal individuals. To differentiate between hypothalamic and pituitary sites of action, we examined the effects of concurrent infusion of valine on the PRL response to iv phenylalanine in eight normal men. Since large neutral amino acids share the same high affinity blood-brain barrier transport system, entry of phenylalanine into the brain will be diminished by the simultaneous presence of high serum concentrations of valine; the pituitary, lying outside the blood-brain barrier, is not subject to these competitive effects. Valine significantly blunted the PRL response to phenylalanine, supporting an effect of phenylalanine within the blood-brain barrier to stimulate PRL release.