Oxytocin and vasopressin gene expression in the hypothalamo-neurohypophyseal system of the rat during the estrous cycle, pregnancy, and lactation.

To study the state of hypothalamic expression of the oxytocin (OT) and vasopressin (VP) genes in different conditions of the female rat, OT and VP mRNA levels were determined during the estrous cycle, pregnancy, and lactation. OT and VP mRNA levels of the supraoptic nucleus were quantified by Northern blot and dot-blot analysis. OT and VP contents of the pituitary gland were determined by RIA. During the estrous cycle, OT mRNA levels of the supraoptic nucleus were significantly increased 1.5- to 2-fold at estrous relative to the other periods of the cycle. No significant cyclic variation was observed in VP mRNA levels. The OT content of the pituitary gland was significantly decreased at estrus and also the VP contents showed a cyclic variation with the lowest levels occurring at estrus and metestrus. At the last day of gestation a 3- and 2-fold increase of OT mRNA and VP mRNA, respectively, was measured, but no changes were observed at gestational day 12, 15, and 18. In rats lactating for 15 days, OT and VP mRNA levels were approximately 3-fold higher than during the estrous cycle. Concomitantly the OT and VP contents of the pituitary gland were decreased by approximately 50%. It is concluded that both OT and VP gene expression are stimulated just before birth and during lactation indicating that both genes are sensitive to common factors. A dissociation of the regulation of the OT and VP genes occurs in the estrous cycle.