Prolactin secretion and its dopamine inhibitory control in rat fetuses.

This study has determined in rats the ontogenetic schedule of the onset of pituitary prolactin (PRL) synthesis and release as well as of the establishment of the dopamine (DA) inhibitory control of PRL secretion. RIA recognized PRL traces in the pituitary at the 18th embryonic day (E18), although a clearly detectable amount of this hormone was first measured at E20, suggesting the onset of PRL synthesis. The PRL level in the pituitary increased significantly by E22, in females to a higher extent than in males. Decapitation of fetuses did not cause any change in the PRL plasma level in males showing no PRL release from the pituitary until term. Conversely, there was a slight but significant fall of plasma PRL in decapitated females, suggesting PRL release from the pituitary. An inhibition of DA receptors on lactotropes of fetuses resulted in an increased level of plasma PRL at E20, but not at E18, while the pituitary content of PRL remained unchanged. The same treatment at E22 caused a significant increase of the PRL concentration in plasma and a concomitant fall in the pituitary that could be prevented by preliminary encephalectomy. These data show that the tuberoinfundibular DA system begins to inhibit PRL release from lactotropes between E20 and E22, completely arresting PRL release from the pituitary in males but not in females.