p-Tyramine, a Natural Amine, Inhibits Prolactin Releasein Vivo*

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Abstract
P-Tyramine, an endogenous amine with sympathomimetic action, is found in the mammalian hypothalamus. When injected i.p. p-tyramine reduced serum PRL [prolactin] without alterating LH [luteinizing hormone] and TSH serum titers in adult orchidectomized rats and rats subjected to ether or immobilization stress. The hyperprolactinemia achieved by this last procedure was inhibited by both tyramine and dopamine; tyramine produced the same effect as dopamine at a dose 5 times greater. When PRL levels were increased by pretreatment with .alpha.-methyl-p-tyrosine or halperidol, once again both tyramine and dopamine lowered PRL titers. The hypoprolactinemic effect of p-tyramine was also observed in median eminence-lesioned animals, suggesting a pituitary site of action. Apparently, low amounts of tyramine, a naturally occurring amine, can inhibit in vivo increase in PRL levels achieved physiologically or pharmacologically.