Melatonin effects on prolactin secretion in pituitary-grafted female rats.

Melatonin influences prolactin (PRL) secretion through unknown mechanisms. This work was undertaken to study the effects of melatonin administration of PRL secretion in pituitary-grafted female rats. Melatonin administration resulted in a marked and rapid decrease of previously high plasma PRL levels in pituitary-grafted rats. Luteinizing hormone releasing hormone (LHRH) administration resulted in a marked decrease of plasma PRL levels in both sham-operated and grafted animals given saline whereas a statistically significant increase was observed in grafted, melatonin-treated rats. Estradiol benzoate administration caused an increase in plasma PRL levels of greater magnitude in the afternoon than in the morning in sham-operated animals given saline whereas in grafted animals no response to estradiol benzoate was detected. Melatonin increased the plasma PRL response to estradiol benzoate in sham-operated rats as compared to vehicle-treated animals. A normalization of PRL response to estradiol benzoate was shown in melatonin-treated, pituitary grafted rats. While the suppressive effects of melatonin in plasma PRL levels of pituitary-grafted animals is substantial the mechanism of this inhibition remains unknown.