CYTOPLASMIC OXYTOCIN AND VASOPRESSIN GENE TRANSCRIPTS DECLINE POSTPARTUM IN THE HYPOTHALAMUS OF THE LACTATING RAT

Oxytocin (OT) and vasopressin (AVP) gene expression are enhanced in th e rat hypothalamus in late gestation and during the second and third w eeks of lactation. We report that during the first 3 postpartum days, OT and AVP cytoplasmic mRNAs in the supraoptic and paraventricular nuc lei of lactating rats decreased dramatically, reaching less than one f ifth of peak gestational levels by day 2 postpartum. Differences in th e temporal pattern of OT and AVP expression were observed in the supra optic and paraventricular nuclei from days 4-10 of lactation. We also compared OT and AVP cytoplasmic mRNAs isolated from the hypothalamus o f day 3 lactating rats to cohorts that had litters removed at the time of parturition. Lactating rats had significantly lower OT and AVP cyt oplasmic mRNA levels than their nonlactating cohorts. We further compa red OT and AVP cytoplasmic mRNAs in the hypothalamus of day 12 lactati ng rats that had been ovariectomized or sham ovariectomized on day 3 o f lactation. Ovariectomized day 12 lactating animals had significantly lower OT and AVP cytoplasmic mRNA levels than their intact cohorts. T hese data refute the hypothesis that lactation is characterized by per sistently elevated hypothalamic cytoplasmic OT and AVP mRNAs produced as a result of continuous stimulation by suckling and suggest that ova rian steroids may exert a modulatory effect on hypothalamic OT and AVP expression during early lactation.