The hypothalamus-pituitary-adrenal (HPA) axis is the major neuroendocrine stress response system. Corticotropin-releasing hormone (CRH) neurons in the parvocellular paraventricular nucleus (pPVN) play a key role in coordinating responses of this system to stressors. The cytokine interleukin-1␤ (IL-1␤), mimicking infection, robustly activates these CRH neurons via a noradrenergic input arising from the nucleus tractus solitarii (NTS). In late pregnancy, HPA axis responses to stressors, including IL-1␤, are attenuated by a central opioid mechanism that auto-inhibits noradrenaline release in the PVN. Here we show that the neuroactive progesterone metabolite allopregnanolone induces these changes in HPA responsiveness to IL-1␤ in pregnancy. In late pregnancy, inhibition of 5␣-reductase (an allopregnanolone-synthesizing enzyme) with finasteride restored HPA axis responses (rapidly increased pPVN CRH mRNA expression, ACTH, and corticosterone secretion) to IL-1␤. Conversely, allopregnanolone reduced HPA responses in virgin rats. In late pregnancy, activity of the allopregnanolone-synthesizing enzymes (5␣-reductase and 3␣-hydroxysteroid dehydrogenase) was increased in the hypothalamus as was mRNA expression in the NTS and PVN. Naloxone, an opioid antagonist, restores HPA axis responses to IL-1␤ in pregnancy but had no additional effect after finasteride, indicating a causal connection between allopregnanolone and the endogenous opioid mechanism. Indeed, allopregnanolone induced opioid inhibition over HPA responses to IL-1␤ in virgin rats. Furthermore, in virgin rats, allopregnanolone treatment increased, whereas in pregnant rats finasteride decreased proenkephalin-A mRNA expression in the NTS. Thus, in pregnancy, allopregnanolone induces opioid inhibition over HPA axis responses to immune challenge. This novel opioid-mediated mechanism of allopregnanolone action may alter regulation of other brain systems in pregnancy.

In cells expressing Ca 2؉ -mobilizing receptors, InsP 3 -induced Ca 2؉ release from intracellular stores is commonly associated with extracellular Ca 2؉ influx. Operation of these two Ca 2؉ signaling pathways mediates thyrotropin-releasing hormone (TRH) and angiotensin II (AII)-induced prolactin secretion from rat pituitary lactotrophs. After an initial hyperpolarization induced by Ca 2؉ mobilization from the endoplasmic reticulum (ER), these agonists generated an increase in the steadystate firing of action potentials, further facilitating extracellular Ca 2؉ influx and prolactin release. Like TRH and AII, endothelin-1 (ET-1) also induced a rapid release of Ca 2؉ from the ER and a concomitant spike prolactin secretion during the first 3-5 min of stimulation. However, unlike TRH and AII actions, Ca Typically, in anterior pituitary cells operated by calciummobilizing agonists, release of Ca 2ϩ from intracellular Ca 2ϩ stores is associated with extracellular Ca 2ϩ influx. The coordinate actions of these two pathways provide long-lasting Ca 2ϩ signals and secretion during sustained agonist stimulation (1).Endothelins (ETs) 1 are common calcium-mobilizing agonists for secretory pituitary cells and operate through ET A receptors (2, 3) coupled to phospholipase C-(4) but not phospholipase D-dependent signaling pathways (5). In lactotrophs, the ET-1-induced release of Ca 2ϩ from intracellular pools is associated with a rapid and transient increase in prolactin secretion (6, 7) followed by a prolonged inhibition to below basal levels (8). This bidirectional effect of a calcium-mobilizing agonist on secretion is uncommon among cells expressing phospholipase C-coupled receptors and is unique for cells expressing ET A receptors. The mechanism underlying the paradoxical action of ETs on hormone secretion is still unknown. It is unlikely that a rapid desensitization of ET A receptors could explain the inhibition of secretion observed in lactotrophs. Therefore, as the sustained phase in hormone secretion is affected in ET-1-stimulated lactotrophs, we addressed the alternate hypothesis that Ca 2ϩ influx is uncoupled from Ca 2ϩ mobilization in ET-1-stimulated lactotrophs, leading to depletion of the ER calcium pool and inhibition of hormone secretion.To test this hypothesis, rat pituitary lactotrophs were employed. These cells exhibit spontaneous, extracellular Ca 2ϩ -dependent action potential (AP) activity (9), which is tightly coupled to basal prolactin secretion (10). In addition to ET A receptors, lactotrophs express thyrotropin-releasing hormone (TRH) receptors (11), the calcium-mobilizing actions of which have been well characterized (12). Angiotensin II (AII) also stimulates prolactin release, presumably through activation of AT 1B receptors (13). Two calcium entry pathways are proposed to operate in lactotrophs: voltage-gated (VGCC) and store-operated (SOCC) calcium channels (12). Our results indicate that Ca 2ϩ influx through VGCC rather than through SOCC is essential for the sustained secretagogue actions of TRH and ...

influx. This action occurred through the G z signaling pathway; the adenylyl cyclase-signaling cascade could mediate sustained inhibition of secretion, whereas rapid inhibition also occurred at elevated cAMP levels regardless of the status of phospholipase C, tyrosine kinases, and protein kinase C. In a nanomolar concentration range, endothelin also inhibited voltagegated Ca 2؉ influx through the G i/o signaling pathway. Thus, the coupling of seven-transmembrane domain endothelin receptors to G z proteins provided a pathway that effectively blocked hormone secretion distal to Ca 2؉ entry, whereas the cross-coupling to G i/o proteins reinforced such inhibition by simultaneously reducing the pacemaking activity.

The changes in corticotrophin-releasing hormone (CRH), ACTH and dehydroepiandrosterone (DHEA) in maternal and fetal plasma were estimated in women undergoing spontaneous and oxytocininduced labour to correlate hormone changes with the mode of parturition. Blood was sampled from a maternal peripheral vein 2 days before labour, during the second stage of labour and on the second postnatal day, and also from umbilical vessels just after delivery. Hormone concentrations were measured by RIA and ELSA methods. The maternal plasma CRH concentration before labour was significantly higher in the group of women delivered spontaneously and declined during the labour through to the second postnatal day. Measured in umbilical vessels, CRH as well as ACTH concentrations were higher in the umbilical vein than artery. The mean maternal plasma ACTH was similar in both groups before delivery, then increased significantly in both groups during the labour, decreasing on the second day after delivery. There were no changes in DHEA concentrations among the groups and at all time points of collection. No correlations between CRH and ACTH or DHEA were observed. Our results suggest that the maternal pituitary can respond to stress factors during delivery but peripheral CRH, probably mainly of placental origin, is not a major modulator of pituitary action.