The nonapeptide oxytocin (OT) is important for milk ejection during lactation, uterine contractility at parturition, and the onset of maternal behavior. Sequential exposure to estradiol (E2) and progesterone (P) followed by P withdrawal increases OT mRNA in the paraventricular nucleus (PVN), and to a lesser degree the supraoptic nucleus (SON), of the rat 48 hours after the P is removed. Although increases in PVN OT mRNA are not accompanied by changes in posterior pituitary OT peptide content, the PVN contains OT neurons that project to both the posterior pituitary (magnocellular group) and extra pituitary sites (parvocellular groups). Steroid-induced increases in OT mRNA occur in both the magnocellular and the parvocellular regions of the PVN. The latter are believed to contribute to CNS release of OT which may be important for certain behaviors including the onset of maternal behavior. The same steroid sequence that increases PVN OT mRNA also induces maternal behavior in virgin ovariectomized rats. Exposure of animals to E2 and P for 2 weeks resulted in the shortest latency to the onset of maternal behavior in ovariectomized rats, whereas exposure for 6 days was associated with a longer latency. In this study we questioned if the duration of E2 and P exposure prior to P withdrawal is an important regulator of PVN OT mRNA levels. We compared OT mRNA levels in the PVN of virgin ovariectomized rats administered no steroid or sequential E2 and P for 2 weeks versus 6 days. On day 1 animals received steroid-filled or empty capsules followed by P-filled or empty capsules on day 3. In one steroid-treated group, E2 and P were continued for 6 days and in the other group for 14 days prior to P removal. Animals were sacrificed 48 hours after P removal. Levels of OT mRNA were compared among 6 day and 2 week steroid-treated animals and sham-treated animals. The relative abundance of OT mRNA was significantly increased, P < 0.05, in animals receiving the 2-week, but not the 6-day, steroid treatment compared to sham-treated animals. Pituitary OT peptide content was not significantly different among the three groups. We conclude that the duration of steroid exposure may be an important regulator of the level of OT mRNA in the PVN of the rat.
We recently reported that sequential administration of estrogen and progesterone and subsequent withdrawal of progesterone increased the level of hypothalamic oxytocin (OT) messenger RNA (mRNA) in the female rat. Both estrogen priming and progesterone withdrawal are critical components of this regimen. Rats experience this ovarian steroid pattern during certain lactational events such as on days 10-12 of lactation or with interruption of nursing for 48-72 h. In the present study, we used Northern blot and in situ hybridization to determine the association between the steroid exposure and the level of OT mRNA during these lactational conditions. Week 1 lactating rats that had pups removed for 24 and 48 h, but not 12 h, had significantly higher serum estradiol concentrations than animals continuously nursing their pups on a comparable day of lactation (F4, 15 = 6; P < 0.005). Serum progesterone levels declined significantly during the 48 h after litter removal (F4, 15 = 130.5; P < 0.0001). Significant increases in OT mRNA levels were found at 48 h, but not 12 or 24 h, after litter removal (F4, 15 = 4.3; P < 0.02). Implantation of progesterone-filled capsules to compensate for the spontaneous decline in progesterone that occurs with interruption of nursing attenuated this increase in OT mRNA levels. OT mRNA was significantly higher in the hypothalamic paraventricular nuclei (P < 0.0002) and supraoptic nuclei (P < 0.002) of nursing interrupted animals receiving empty vs. progesterone-filled implants. Implantation of day 12 lactating rats with progesterone capsules for 5 days before being killed blunted the increase in OT mRNA that normally occurs on this day of lactation. The data highlight the pivotal role of estrogen priming and progesterone withdrawal in the increased expression of the OT gene during select lactational events.
We recently reported that sequential administration of estrogen and progesterone and subsequent withdrawal of progesterone increased the level of hypothalamic oxytocin (OT) messenger RNA (mRNA) in the female rat. Both estrogen priming and progesterone withdrawal are critical components of this regimen. Rats experience this ovarian steroid pattern during certain lactational events such as on days 10-12 of lactation or with interruption of nursing for 48-72 h. In the present study, we used Northern blot and in situ hybridization to determine the association between the steroid exposure and the level of OT mRNA during these lactational conditions. Week 1 lactating rats that had pups removed for 24 and 48 h, but not 12 h, had significantly higher serum estradiol concentrations than animals continuously nursing their pups on a comparable day of lactation (F4, 15 = 6; P < 0.005). Serum progesterone levels declined significantly during the 48 h after litter removal (F4, 15 = 130.5; P < 0.0001). Significant increases in OT mRNA levels were found at 48 h, but not 12 or 24 h, after litter removal (F4, 15 = 4.3; P < 0.02). Implantation of progesterone-filled capsules to compensate for the spontaneous decline in progesterone that occurs with interruption of nursing attenuated this increase in OT mRNA levels. OT mRNA was significantly higher in the hypothalamic paraventricular nuclei (P < 0.0002) and supraoptic nuclei (P < 0.002) of nursing interrupted animals receiving empty vs. progesterone-filled implants. Implantation of day 12 lactating rats with progesterone capsules for 5 days before being killed blunted the increase in OT mRNA that normally occurs on this day of lactation. The data highlight the pivotal role of estrogen priming and progesterone withdrawal in the increased expression of the OT gene during select lactational events.
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