The Twenty-four Hour Pattern of Prolactin Secretion in Aging Female Rats1

Abstract

The 24 h patterns of prolactin secretion were determined in 20–26-month-old female rats during the constant estrous (CE) and pseudopregnant (PSP) states. Each female was fitted with a chronic intra-atrial cannula and serial blood samples were obtained at 3 h intervals for measurement of plasma prolactin levels. This sampling procedure for a 24 h cycle was repeated 1–3 times for each rat over a 6 day period. The pituitaries were removed at the conclusion of the study for histological examination. Prolactin levels in the CE rats were increased from baseline values to 254 ± 111 (SEM) ng/ml at 1700 h and subsequently fell to 61 ± 11 ng/ml by 2300 h. Most of the CE rats also showed a smaller nocturnal rise in prolactin between 0200–0500 h and the intervals between the diurnal surge and the nocturnal prolactin rise ranged from 9 to 15 h. In contrast to CE rats, the PSP rats exhibited large nocturnal surges of prolactin release, with peak values of 248 ± 44 and 215 ± 27 ng/ml occurring at 0200 and 0500 h. In addition, most of the PSP rats showed a smaller increase in plasma prolactin between 1400–1700 h. Both CE and PSP rats that had evidence of pituitary tumors and/or lesions exhibited static high levels of prolactin but no obvious surges of secretion. These results demonstrate that predominantly diurnal and nocturnal prolactin surges occur, respectively, in aging CE and PSP rats. Since the CE and PSP states are characterized by high levels of estrogen and progestin secretion, respectively, these distinctly different steroid hormonal milieux may be involved in the control of daily prolactin secretion patterns in aging females.