The Pattern of Prolactin Secretion during Pseudopregnancy in the Rat: A Daily Nocturnal Surge12

Abstract

The patterns of prolactin secretion during the estrous cycle and pseudopregnancy were determined in rats bearing chronic aortic catheters from which blood samples were withdrawn at 2-hr intervals throughout both reproductive states. Prolactin concentrations were determined by radioimmunoassay. Except for the spontaneous proestrus surges observed at the beginning and end of the blood collection, plasma prolactin concentrations during the estrous cycle were low, never rising above 10 ng/ml. In pseudopregnant animals, prolactin concentrations were highly erratic but significantly elevated during the 24 hr period following cervical stimulation. Beginning on day 1 and at approximately 24-hr intervals thereafter through day 11, a surge of prolactin secretion occurred, beginning at 0100 hr, peaking at 0300–0500 hr and terminating by 0900–1100 hr. Maximal mean prolactin values during the surges ranged from 40–110 ng/ml. These daily nocturnal surges of pseudopregnancy alternate daily with a period from 1100 hr to 2300 hr in which the plasma prolactin concentrations resemble those observed during the estrous cycle. The failure of prolactin levels to be continuously elevated during pseudopregnancy suggests that information derived from cervical stimulation is stored in the brain and is “remembered” each night, resulting in a surge of prolactin secretion. (Endocrinology90: 1292, 1972)