Prolactin secretory surge during estrus coincides with increased dopamine activity in the hypothalamus and preoptic area and is not altered by ovariectomy on proestrus

Abstract

Prolactin (PRL) secretory surges have been reported on the afternoons of both proestrus and estrous in cycling rats. As neuroendocrine regulation of estrous PRL surge is poorly understood, the present study aimed to investigate the involvement of hypothalamic dopamine and serotonin as well as of plasma ovarian steroids in this hormonal surge generation. For that, we determined the concentrations of dopamine, serotonin and their respective metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindole-3-acetic acid (5-HIAA) in the mediobasal hypothalamus (MBH) and medial preoptic area (MPOA) throughout the day of estrus and correlated them with plasma PRL levels. In a second study, we evaluated the effect of ovariectomy on the morning of proestrus on PRL surges of both proestrus and estrus. Dopamine turnover, as determined by DOPAC/dopamine ratio, increased in both the MBH and MPOA coinciding with the afternoon PRL surge on estrus. In contrast, both the concentration and turnover (5-HIAA/serotonin) of serotonin within these areas were unaltered during estrus. In addition, ovariectomy reduced plasma estradiol and progesterone levels but did not alter the PRL surges on proestrus and estrus. Considering that dopamine is the main inhibitor of PRL release and that PRL auto-regulates its secretion through a short-loop feedback mechanism, our present results suggest that PRL may suppress its own secretion during the estrus surge through the activation of the dopaminergic neurons in the MBH and MPOA. In addition, the PRL surge on estrus seems do not depend on either the activity of hypothalamic serotonin or the increased secretion of ovarian steroids on proestrus.