Neurochemical regulation of hypothalamic oxytocin messenger ribonucleic acid levels during early lactation in rats.

The objective of the present studies was to investigate whether the stimulatory influence of the suckling offspring on the level of hypothalamic oxytocin (OT) messenger RNA (mRNA) in early lactation in rats is mediated by activation of the central noradrenergic and/or oxytocinergic systems, both of which have been strongly implicated in suckling-induced OT release. Experiments tested whether the effect of litter separation immediately postpartum to reduce hypothalamic OT mRNA could be mimicked by pharmacological disruption of either noradrenergic or oxytocinergic signals. Bilateral microinjections of the catecholamine neurotoxin 6-hydroxydopamine into either the supraoptic nucleus (SON) or paraventricular nucleus/anterior commissural nucleus regions significantly reduced the concentrations of norepinephrine in these areas, but did not alter the relative levels of OT mRNA in these regions, suggesting that the stimulatory inputs provided by the suckling offspring are not transmitted through the noradrenergic system. However, 24-h infusion of the OT antagonist desGLY-NH2, d(CH2)5 [Tyr (Me)2 Thr4]OVT into the third ventricle of rats maintaining their litters immediately postpartum reduced the level of OT mRNA in the SON. Conversely, chronic infusion of OT into the third ventricle of rats separated from their litters immediately postpartum attenuated the decline in OT mRNA in the SON. OT mRNA levels in the paraventricular nucleus/anterior commissural nucleus region did not change in response to litter separation, or infusion of OT or OT antagonist, implying differential regulation of OT mRNA expression in the magnocellular nuclei. The present results suggest that in addition to promoting OT secretion, OT released centrally by suckling may facilitate OT gene expression, at least during the early postpartum period when OT mRNA is subject to some degree of afferent regulation.

Afferent stimulation regulates oxytocin messenger ribonucleic Acid during early lactation in rats.

The physiological factors that regulate the levels of oxytocin (OT) mRNA in the rat hypothalamo-neurohypophyseal system during lactation are unknown. The major objective of the present studies was to test whether afferent stimuli provided by the offspring influence the level of OT mRNA in the magnocellular nuclei of the hypothalamus, i.e. the paraventricular nucleus (PVN), supraoptic nucleus (SON) and anterior commissural nucleus (ACN), during lactation. In addition, these studies provide a detailed description of the changes that occur in OT mRNA levels in these nuclei during pregnancy and lactation. Levels of OT mRNA were determined in the total RNA extracted from pooled PVN/SON/ACN, which were microdissected from pregnant and lactating Holtzman rats, by slot-blot hybridization. RNA blots were hybridized sequentially with a [(32) P]5'end-labeled, 25-base oligonucleotide probe complementary to bases 912-936 of the OT gene and with a random primer-labeled cDNA complementary to α-tubulin mRNA, which was used for normalization. Autoradiographs were quantitated by scanning laser densitometry. Compared to the levels on day 1 of pregnancy (the day of mated estrus) the relative levels of OT mRNA were decreased on pregnancy days 7 and 14, and then returned to the mated estrus value on days 16 to 20. The level of OT mRNA again declined prior to parturition on day 23, although it was restored to the estrus value the day after parturition and generally remained at this level throughout lactation. To assess the influence of stimuli provided by the offspring for the regulation of OT mRNA levels during lactation, lactating females were separated from their litters immediately after parturition or on lactation day 8. As compared to nonseparated, time-matched controls, removal of the litters immediately after parturition resulted in a statistically significant reduction of approximately 25% in the levels of OT mRNA 24 and 48 h later, which returned to control levels by 72 h. In contrast, removal of the litters on lactation day 8 did not significantly alter the level of OT mRNA in the PVN/SON/ACN over the next 3 days. These findings suggest that the level of OT mRNA in hypothalamic magnocellular nuclei is influenced to some extent by afferent stimuli provided by the offspring during an early period of lactation but, thereafter, becomes unresponsive to removal of this influence. Afferent suckling stimuli may be one component of a multifactorial regulation responsible for the maintenance of OT mRNA expression during lactation.