Administration of long-term estradiol and progesterone followed by progesterone withdrawal does not alter the plasma oxytocin secretory response to cholecystokinin or the pituitary oxytocin content in ovariectomized rats

Oestradiol effects on neuroendocrine responses induced by water deprivation in rats

Water deprivation (WD) induces changes in plasma volume and osmolality, which in turn activate several responses, including thirst, the activation of the renin-angiotensin system (RAS) and vasopressin (AVP) and oxytocin (OT) secretion. These systems seem to be influenced by oestradiol, as evidenced by the expression of its receptor in brain areas that control fluid balance. Thus, we investigated the effects of oestradiol treatment on behavioural and neuroendocrine changes of ovariectomized rats in response to WD. We observed that in response to WD, oestradiol treatment attenuated water intake, plasma osmolality and haematocrit but did not change urinary volume or osmolality. Moreover, oestradiol potentiated WD-induced AVP secretion, but did not alter the plasma OT or angiotensin II (Ang II) concentrations. Immunohistochemical data showed that oestradiol potentiated vasopressinergic neuronal activation in the lateral magnocellular PVN (PaLM) and supraoptic (SON) nuclei but did not induce further changes in Fos expression in the median preoptic nucleus (MnPO) or subfornical organ (SFO) or in oxytocinergic neuronal activation in the SON and PVN of WD rats. Regarding mRNA expression, oestradiol increased OT mRNA expression in the SON and PVN under basal conditions and after WD, but did not induce additional changes in the mRNA expression for AVP in the SON or PVN. It also did not affect the mRNA expression of RAS components in the PVN. In conclusion, our results show that oestradiol acts mainly on the vasopressinergic system in response to WD, potentiating vasopressinergic neuronal activation and AVP secretion without altering AVP mRNA expression.

Neurochemical bases of plasticity in the magnocellular oxytocin system during gestation

The central and systemic release of oxytocin (OT) has been well documented during parturition and lactation. In preparation for the demands of these events, the magnocellular hypothalamic neurons of the central OT system undergo a variety of biochemical, molecular, electrophysiological, and anatomical adaptations during gestation. However, the mechanisms responsible for these changes have not been well established. A number of neurochemical mediators have been implicated in contributing to the plasticity in the OT magnocellular system during gestation, including ovarian hormones, as well as central neurotransmitters, such as glutamate, gamma-amino butyric acid (GABA), and central neurosteroids, e.g., allopregnanolone. In addition, several lines of evidence suggest that central OT release and subsequent OT receptor stimulation may contribute to adaptations of the OT system during gestation, and may be necessary for its subsequent functioning during lactation. Here, we review evidence for involvement of the neurochemical systems implicated in contributing to adaptations that occur in the OT system during the course of gestation.

Oxytocinergic activity is linked to lower blood pressure and vascular resistance during stress in postmenopausal women on estrogen replacement

Estrogen administration results in increased release of the oxytocin (OT) prohormone reflected by increases in oxytocin intermediate peptide (OT Int) in both animal models and humans, and sequential treatment of ovariectomized rats with estrogen/progesterone then progesterone withdrawal leads to increased hypothalamic OT mRNA. Blood pressure (BP) reductions have been related to increased exogenous and endogenous OT in rats and to higher endogenous OT activity in premenopausal women, but not previously in postmenopausal women. Thus, we used plasma obtained at rest and during a speech stressor from 54 postmenopausal women who participated in a 6-month randomized trial of oral conjugated estrogens vs. placebo to examine effects of estrogen replacement therapy (ERT) on plasma OT and OT Int levels and their relationships to changes in BP during the trial. ERT alone and with progesterone (but not placebo) led to significant increases in plasma levels of OT Int, but no change in plasma OT levels. Women showing greater increases in OT Int during treatment showed greater decreases in BP and total vascular resistance during a series of behavioral stressors compared to women with moderate or no increases in OT Int, even after controlling for effects related to treatment condition or to changes in plasma estradiol. The findings suggest that enhanced oxytocinergic activity may contribute to BP decreases associated with ERT in more responsive postmenopausal women.

Differential sensitivity of intranuclear and systemic oxytocin release to central noradrenergic receptor stimulation during mid- and late gestation in rats

A number of changes occur in the oxytocin (OT) system during gestation, such as increases in hypothalamic OT mRNA, increased neural lobe and systemic OT, and morphological and electrophysiological changes in OT-containing magnocellular neurons, suggestive of altered neuronal sensitivity, which may be mediated by ovarian steroids. Because central norepinephrine (NE) and histamine (HA) are potent stimulators of OT release during parturition and lactation, the present study investigated the effects of central noradrenergic and histaminergic receptor activation on systemic (NE, HA) and intranuclear (NE) OT release in pregnant rats and in ovariectomized rats treated with ovarian steroids. Plasma OT levels in late gestation were significantly higher compared with all other groups, and neither adrenergic nor histaminergic receptor blockade decreased these elevated levels. Furthermore, the alpha-adrenergic agonist phenylephrine, but not histamine, stimulated systemic OT release to a significantly greater extent in late gestation than in midpregnant, ovariectomized, or steroid-treated females. Although basal extracellular OT levels in the paraventricular nucleus, as measured with microdialysis, were unchanged during pregnancy or steroid treatment, noradrenergic receptor stimulation of intranuclear OT release was significantly elevated in midgestation females compared with all other groups. These studies indicate that sensitivity of intranuclear and systemic OT release to noradrenergic receptor activation differentially varies during the course of gestation.

Effects of gonadal hormone replacement on measures of basal forebrain cholinergic function

The effects of different hormone replacement regimens on basal forebrain cholinergic function were examined by measuring changes in choline acetyltransferase activity and high affinity choline uptake in adult, ovariectomized, rats. Increases in choline acetyltransferase activity were detected in the frontal cortex (20. 1%) and olfactory bulbs (30.4%) following two weeks, but not four weeks, of repeated treatment with estrogen plus progesterone. Increases in high affinity choline uptake were detected in the frontal cortex (39.5-55.1%), hippocampus (34.9-48.9%), and olfactory bulbs (29.9%) after two weeks, but not four weeks, of either continuous estrogen administration, repeated progesterone administration, or repeated treatment with estrogen plus progesterone. Repeated administration of estradiol (2-25 microg/250 g body weight) for two or four weeks, and continuous estrogen administration for four weeks and six months, produced no significant changes in choline acetyltransferase activity or high affinity choline uptake in the hippocampus, frontal cortex or olfactory bulbs. Continuous estrogen administration for 13 months produced a significant decrease in high affinity choline uptake across all regions with the largest effect (-28.1%) detected in the hippocampus. The findings demonstrate that short-term treatment with estrogen and/or progesterone can significantly enhance cholinergic function within specific targets of the basal forebrain cholinergic projections. Most important is the fact that the effects varied considerably according to the manner and regimen of hormone replacement and did not persist with prolonged treatment. These findings could have important implications for the effective use of hormone replacement strategies in the prevention and treatment of Alzheimer's disease and age-related cognitive decline in women.

The duration of estradiol and progesterone exposure prior to progesterone withdrawal regulates oxytocin mRNA levels in the paraventricular nucleus of the rat

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.