Tyrosine administration decreases serum prolactin levels in chronically reserpinized rats

Abstract

The injection of tyrosine, 200 mg/kg, decreased serum prolactin levels and elevated hypothalamic (and striatal) concentrations of two dopamine metabolites, dihydroxyphenylacetic acid and homovanillic acid, in chronically reserpinized rats. Tyrosine administration had none of these effects in otherwise untreated rats, and did not block the increase in serum prolactin that occurred 4 hours after a single injection of reserpine. As anticipated, the injection of dopa decreased serum prolactin in all rats. Valine, another large neutral amino acid, did not modify serum prolactin in chronically reserpinized animals. Since prolactin secretion is normally inhibited by dopamine released from the hypothalamus, reserpine treatment probably elevates serum prolactin by depleting the hypothalamus of dopamine. Our data suggest that tyrosine injection suppresses serum prolactin levels in chronically reserpinized rats by enhancing the synthesis and release of hypothalamic dopamine. Thus, administration of tyrosine, dopamine's dietary precursor, can alter physiologic functions that depend on dopamine.