A Detailed Characterization of Prolactin Secretion Patterns during Daylight in Individual Cycling and Pseudopregnant Rats

Abstract

In order to characterize plasma prolactin (Prl) patterns during the estrous cycle and pseudopregnancy (PSP), I collected sequential blood samples between 8.00 and 19.00 h at 1-hour, 1.5-min or 1-min intervals from conscious, unrestrained female rats. During the afternoon of proestrus and estrus a single Prl surge was observed, while during PSP two daily Prl surges occurred, one nocturnal and one diurnal. During PSP the secretion of Prl occurred occasionally in substantial bursts from baseline levels. This contrasted with the proestrus afternoon surge when plasma Prl levels were constantly elevated due to a more or less continuous release of fluctuating amounts of Prl. The differences in timing and secretion patterns of the three surges suggest a separate neural regulation. The diurnal surge on day 0 of PSP may be composed of two different surges which also may have a separate neural regulation: one peak occurs at 14.00-16.00 h, and is comparable in timing to the Prl surge during the afternoon of estrus; a second surge starts at 17.00 h, and is comparable in time of onset to the diurnal surge on the other days of PSP. Plasma Prl always appeared to increase in an unpredictable manner, discontinuously by means of several bursts, with maximum increments of about 600 ng/ml/min. The shortest half-time values, as calculated from the disappearance of Prl from the circulation, were about 2.2 min. The individual patterns show that Prl release must be the consequence of a very dynamic neural regulatory process.