Nitric oxide, interleukin and prostaglandin interactions affecting the magnocellular system

Approaches Mediating Oxytocin Regulation of the Immune System

The hypothalamic neuroendocrine system is mainly composed of the neural structures regulating hormone secretion from the pituitary gland and has been considered as the higher regulatory center of the immune system. Recently, the hypothalamo-neurohypophysial system (HNS) emerged as an important component of neuroendocrine–immune network, wherein the oxytocin (OT)-secreting system (OSS) plays an essential role. The OSS, consisting of OT neurons in the supraoptic nucleus, paraventricular nucleus, their several accessory nuclei and associated structures, can integrate neural, endocrine, metabolic, and immune information and plays a pivotal role in the development and functions of the immune system. The OSS can promote the development of thymus and bone marrow, perform immune surveillance, strengthen immune defense, and maintain immune homeostasis. Correspondingly, OT can inhibit inflammation, exert antibiotic-like effect, promote wound healing and regeneration, and suppress stress-associated immune disorders. In this process, the OSS can release OT to act on immune system directly by activating OT receptors or through modulating activities of other hypothalamic–pituitary–immune axes and autonomic nervous system indirectly. However, our understandings of the role of the OSS in neuroendocrine regulation of immune system are largely incomplete, particularly its relationship with other hypothalamic–pituitary–immune axes and the vasopressin-secreting system that coexists with the OSS in the HNS. In addition, it remains unclear about the relationship between the OSS and peripherally produced OT in immune regulation, particularly intrathymic OT that is known to elicit central immunological self-tolerance of T-cells to hypophysial hormones. In this work, we provide a brief review of current knowledge of the features of OSS regulation of the immune system and of potential approaches that mediate OSS coordination of the activities of entire neuroendocrine–immune network.

Prostaglandins and n-3 polyunsaturated fatty acids in the regulation of the hypothalamic-pituitary axis

The hypothalamic-pituitary (H-P) axis integrates complex physiological and environmental signals and responds to these cues by modulating the synthesis and secretion of multiple pituitary hormones to regulate peripheral tissues. Prostaglandins are a component of this regulatory system, affecting multiple hormone synthesis and secretion pathways in the H-P axis. The implications of these actions are that physiological processes or disease states that alter prostaglandin levels in the hypothalamus or pituitary can impinge on H-P axis function. Considering the role of prostaglandins in mediating inflammation, the potential for neuroinflammation to affect H-P axis function in this manner may be significant. In addition, the mitigating effects of n-3 polyunsaturated fatty acids (n-3 PUFA) on the inflammation-associated synthesis of prostaglandins and their role as substrates for pro-resolving lipid mediators may also include effects in the H-P axis. One context in which neuroinflammation may play a role is in the etiology of diet-induced obesity, which also correlates with altered pituitary hormone levels. This review will survey evidence for the actions of prostaglandins and other lipid mediators in the H-P axis, and will address the potential for obesity-associated inflammation and n-3 PUFA to impinge on these mechanisms.

COX-2 disruption leads to increased central vasopressin stores and impaired urine concentrating ability in mice

It was hypothesized that cyclooxygenase-2 (COX-2) activity promotes urine concentrating ability through stimulation of vasopressin (AVP) release after water deprivation (WD). COX-2-deficient (COX-2(-/-), C57BL/6) and wild-type (WT) mice were water deprived for 24 h, and water balance, central AVP mRNA and peptide level, AVP plasma concentration, and AVP-regulated renal transport protein abundances were measured. In male COX-2(-/-), basal urine output and water intake were elevated while urine osmolality was decreased compared with WT. Water deprivation resulted in lower urine osmolality, higher plasma osmolality in COX-2(-/-) mice irrespective of gender. Hypothalamic AVP mRNA level increased and was unchanged between COX-2(-/-) and WT after WD. AVP peptide content was higher in COX-2(-/-) compared with WT. At baseline, plasma AVP concentration was elevated in conscious chronically catheterized COX-2(-/-) mice, but after WD plasma AVP was unchanged between COX-2(-/-) and WT mice (43 ± 11 vs. 70 ± 16 pg/ml). Renal V2 receptor abundance was downregulated in COX-2(-/-) mice. Medullary interstitial osmolality increased and did not differ between COX-2(-/-) and WT after WD. Aquaporin-2 (AQP2; cortex-outer medulla), AQP3 (all regions), and UT-A1 (inner medulla) protein abundances were elevated in COX-2(-/-) at baseline and further increased after WD. COX-2(-/-) mice had elevated plasma urea and creatinine and accumulation of small subcapsular glomeruli. In conclusion, hypothalamic COX-2 activity is not necessary for enhanced AVP expression and secretion in response to water deprivation. Renal medullary COX-2 activity negatively regulates AQP2 and -3. The urine concentrating defect in COX-2(-/-) is likely caused by developmental glomerular injury and not dysregulation of AVP or collecting duct aquaporins.

Preoptic nitric oxide attenuates endotoxic fever in guinea pigs by inhibiting the POA release of norepinephrine

Lipopolysaccharide (LPS) administration induces hypothalamic nitric oxide (NO); NO is antipyretic in the preoptic area (POA), but its mechanism of action is uncertain. LPS also stimulates the release of preoptic norepinephrine (NE), which mediates fever onset. Because NE upregulates NO synthases and NO induces cyclooxygenase (COX)-2-dependent PGE2, we investigated whether NO mediates the production of this central fever mediator. Conscious guinea pigs with intra-POA microdialysis probes received LPS intravenously (2 μg/kg) and, thereafter, an NO donor (SIN-1) or scavenger (carboxy-PTIO) intra-POA (20 μg/μl each, 2 μl/min, 6 h). Core temperature (Tc) was monitored constantly; dialysate NE and PGE2 were analyzed in 30-min collections. To verify the reported involvement of α2-adrenoceptors (AR) in PGE2 production, clonidine (α2-AR agonist, 2 μg/μl) was microdialyzed with and without SIN-1 or carboxy-PTIO. To assess the possible involvement of oxidative NE and/or NO products in the demonstrated initially COX-2-independent POA PGE2 increase, (+)-catechin (an antioxidant, 3 μg/μl) was microdialyzed, and POA PGE2, and Tc were determined. SIN-1 and carboxy-PTIO reduced and enhanced, respectively, the rises in NE, PGE2, and Tc produced by intravenous LPS. Similarly, they prevented and increased, respectively, the delayed elevations of PGE2 and Tc induced by intra-POA clonidine. (+)-Catechin prevented the LPS-induced elevation of PGE2, but not of Tc. We conclude that the antipyretic activity of NO derives from its inhibitory modulation of the LPS-induced release of POA NE. These data also implicate free radicals in POA PGE2 production and raise questions about its role as a central LPS fever mediator.

Suppression of endotoxin-induced fever in near-term pregnant rats is mediated by brain nitric oxide

Over the last three decades, experiments in several mammalian species have shown that the febrile response to bacterial endotoxin is attenuated late in pregnancy. More recent evidence has established that the expression of nitric oxide synthase (NOS) enzymes is increased in the brain late in pregnancy. The current study investigated the possible role of brain nitric oxide in mediating the phenomenon of fever suppression. Core body temperature (Tb) of near-term pregnant rats ( day 19 and 20) was measured following inhibition of brain NOS and intraperitoneal injection of LPS (50 μg/kg); they were compared with both day 15 pregnant and virgin animals. Intracerebroventricular injection with an inhibitor of NOS, NG-monomethyl-l-arginine citrate (l-NMMA; 280 μg), in near-term pregnant rats restored the febrile response to LPS. As expected, near-term dams that received intracerebroventricular vehicle + IP LPS did not increase Tb, in contrast to the 1.0 ± 0.2°C rise in Tb in dams treated with ICV l-NMMA + IP LPS ( P < 0.01). In virgin females and day 15 pregnant controls receiving this treatment, the increases in Tb were 1.5 ± 0.3°C and 1.6 ± 0.4°C, respectively. Thus, blockade of brain NOS restored the febrile response to LPS in near-term dams; at 5 h postinjection, Tb was 60–70% of that observed in virgins and day 15 pregnant animals. Intracerebroventricular l-NMMA alone did not induce a significant change in Tb in any group. These results suggest that the mechanism underlying the suppression of the febrile response in near-term pregnancy is mediated by nitric oxide signaling in the brain.

The influence of interleukin-1beta on oxytocin signalling in primary cells of human decidua

OBJECTIVE

Oxytocin (OT) and its corresponding receptor (OTR), synthesized within the pregnant uterus, play a key role in the process of (preterm) labor as part of a paracrine system that regulates symmetrical contractility. In the setting of intrauterine infection, a major cause of preterm labour and birth, decidua serves as a major source of cytokine production. The present study evaluates the time-dependent effect [0-24 h] of the inflammatory cytokine Interleukin-1beta (IL-1beta) treatment on OT signalling and OT stimulated prostaglandin release in primary cultures of human decidua.

STUDY DESIGN

Primary cultures of human decidua (n=6) were treated with IL-1beta [5 ng/ml] for 0-24h and or indomethacin [100 microM]--an inhibitor of the prostaglandin synthesis--for 0-24 h or for 24 h. OT peptide expression, OTR binding, Inositol triphosphate production (IP(3)), and arachidonic acid (AA) as well as prostaglandin (PGE(2)) release were measured.

RESULTS

IL-1beta transiently reduced cytoplasmic OT peptide at 4-6 h of IL-1beta incubation, while its secretion into the media was increased after 6 h of stimulation. The later was completely blocked by indomethacin. A decrease in OTR mRNA expression and a loss of OTR binding were detected after 8 h and 16 h of IL-1beta treatment, respectively. IL-1beta also decreased IP(3) production and AA release, but significantly enhanced OT mediated PGE(2) production. This effect was completely suppressed by the cyclooxygenase-2 (COX-2) inhibitor NS-398.

CONCLUSION

Our data suggest, that IL-1beta indirectly increases OT secretion in primary cultures of human decidua in a time dependent fashion through the production of prostaglandins through COX-2 and that this increase in OT peptide may secondarily down-regulate the OTR and its signalling cascade. These findings might explain the poor effectiveness of oxytocin receptor antagonists as tocolytic agents in the setting of intrauterine infection.

Response of interleukin-1beta in the magnocellular system to salt-loading

Drinking 2% NaCl decreases interleukin (IL)-1beta in the neural lobe and enhances IL-1 Type 1 receptor expression in magnocellular neurones and pituicytes. To quantify cytokine depletion from the neural lobe during progressive salt loading and determine whether the changes are reversible and correspond with stores of vasopressin (VP) or oxytocin (OT), rats were given water on day 0 and then 2% NaCl to drink for 2, 5, 8 or 5 days followed by 5 days of water (rehydration). Control rats drinking only water were pair-fed amounts eaten by 5-day salt-loaded animals. Animals were decapitated on day 8, the neural lobe frozen and plasma hormones analysed by radioimmunoassay (OT, VP) or enzyme-linked immunosorbent assay (IL-1beta). IL-1beta, VP and OT in homogenates of the neural lobe were quantified by immunocapillary electrophoresis with laser-induced fluorescence detection. Differences were determined by ANOVA, Tukey's t-test, Dunnett's procedure, Fisher's least significant difference and linear regression analysis. In response to salt-loading, rats lost body weight similar to pair-fed controls, drank progressively more 2% NaCl and excreted greater urine volumes. Plasma VP increased at days 2 and 8 of salt-loading, whereas osmolality, OT and cytokine were enhanced after 8 days with IL-1beta remaining elevated after rehydration. In the neural lobe, all three peptides decreased progressively with increasing duration of salt-loading (IL-1beta, r2 = 0.98; OT, r2 = 0.94; VP, r2 = 0.93), beginning on day 2 (IL-1beta; VP) or 5 (OT), with only VP replenished by rehydration. IL-1beta declined more closely (P < 0.0001; ANOVA interaction analysis) with OT (r2 = 0.96) than VP (r2 = 0.86), indicative of corelease from the neural lobe during chronic dehydration. Local effects of IL-1beta on magnocellular terminals, pituicytes and microglia in the neural lobe with activation of forebrain osmoregulatory structures by circulating cytokine may sustain neurosecretion of OT and VP during prolonged salt-loading.

IL-1β directly excites isolated rat supraoptic neurons via upregulation of the osmosensory cation current

Previous studies have shown that IL-1β can excite the magnocellular neurosecretory cells (MNCs) of the hypothalamus. However, it is not known whether IL-1β can have direct IL-1 receptor type 1 (IL-1R1)-mediated effects on MNCs, and little is known about the cellular mechanisms by which IL-1β influences electrical activity in these cells. Here, we used patch-clamp recordings to examine the effects of IL-1β on acutely isolated rat MNCs. We found that IL-1β directly excites MNCs in a dose-dependent manner and that this response can be blocked by an inhibitor of the IL-1R1. Voltage-clamp analysis of the current evoked by IL-1β revealed a linear current-voltage relationship between −90 and −20 mV, and a reversal potential near −35 mV. This value was not affected by reducing the concentration of chloride ions in the external solution, indicating the involvement of a nonselective cation conductance. The effects of IL-1β were inhibited by Na-salicylate, an inhibitor of cyclooxygenase. Moreover. the effects of IL-1β were mimicked and occluded by PGE2, and were inhibited by AH-23848, an antagonist of the PGE2 type 4 (i.e., EP4) receptor. The current evoked by IL-1β was also abolished by 100 μM gadolinium (Gd3+), but was significantly larger when examined in cells preshrunk by negative pressure applied via the recording pipette. IL-1β alone did not cause changes in cell volume nor in the mechanosensitivity of MNCs. We conclude that IL-1β directly excites MNCs via an IL-1R1-mediated induction of PGE2 synthesis and EP4 receptor-dependent autocrine upregulation of the nonselective cation conductance that underlies osmoreception.

Interleukin-1beta depolarizes magnocellular neurons in the paraventricular nucleus of the hypothalamus through prostaglandin-mediated activation of a non selective cationic conductance

Interleukin-1beta (IL-1beta) is involved in hypothalamic regulation of the neuroimmune response by influencing the synthesis and secretion of corticotropin releasing hormone (CRH), vasopressin (VP) and other stress-related mediators. VP secretion from magnocellular (MNC) neurons of the paraventricular nucleus (PVN) of the hypothalamus at the posterior pituitary and/or median eminence contributes to increasing adrenocorticotropin hormone (ACTH) output and ultimately glucocorticoid release, which then contributes to the stress response. In this study, using whole-cell patch clamp recordings from neurons in a slice preparation of the rat PVN, we show that MNC neurons are also influenced by IL-1beta. In response to 1 nM IL-1beta, 62% of MNC neurons tested depolarized (mean depolarization=10.9+/-1.4 mV); effects which were maintained in the presence of a sodium channel blocker, tetrodotoxin (TTX). The effects of IL-1beta on MNC neurons were blocked in the presence of a specific cyclooxygenase (COX)-2 inhibitor, NS-398, indicating a dependence on prostaglandins (PG) in mediating these effects. In response to direct application of 1 muM PGE2, 57% of MNC neurons depolarized, exhibiting a membrane potential change similar to that induced by IL-1beta (mean depolarization=7.8+/-1.1 mV). Voltage clamp experiments examining the effects of PGE2 on the currents evoked by slow voltage ramps revealed activation of a conductance characteristic of a non-selective cationic conductance (NSCC) (voltage-independent, with a reversal potential of -41.8+/-7.6 mV), suggesting that this prostanoid directly modifies cationic currents in MNC neurons. These data provide evidence that IL-1beta depolarizes MNC neurons in the PVN as a result of prostaglandin-mediated activation of a NSCC.

Histamine and prostaglandin interaction in regulation of oxytocin and vasopressin secretion

Prostaglandins and histamine in the hypothalamus are involved in the regulation of oxytocin and vasopressin secretion, and appear to be involved in the mediation of pituitary hormone responses to immunochallenges. Therefore, we investigated in conscious male rats: (i) whether blockade of H1 or H2 receptors affected the oxytocin and vasopressin responses to prostaglandins and (ii) whether blockade of prostaglandin synthesis affected the oxytocin and vasopressin responses to histamine or to Escherichia coli lipopolysaccharide (LPS), in order to determine any interaction between prostaglandins and histamine in the hypothalamus. Oxytocin secretion was dose-dependently stimulated by intracerebroventricular infusion of 1 or 5 microg of PGE1, PGE2 or PGF2alpha, with PGE2 being the most potent of the compounds used. Prior central infusion of the H1 receptor antagonist mepyramine or the H2 receptor antagonist cimetidine significantly inhibited the oxytocin response to all three prostaglandins by approximately 50%. Vasopressin secretion was increased by PGE1 but not by PGE2 or PGF2alpha. The stimulatory effect of PGE1 was almost annihilated by prior administration of mepyramine or cimetidine. Central infusion of histamine or immunochallenge with LPS administered intraperitoneally increased oxytocin and vasopressin secretion four- and two-fold, respectively. Pretreatment with systemic injection of the prostaglandin synthesis inhibitor indomethacin dose-dependently reduced the oxytocin response and prevented the vasopressin response to histamine or LPS. We conclude that histamine and PGE1, PGE2 or PGF2alpha interact in the regulation of oxytocin secretion, whereas histamine and only PGE1 interact in the regulation of vasopressin secretion. Furthermore, histamine as well as LPS may affect oxytocin and vasopressin neurones via activation of prostaglandins, probably in the hypothalamic supraoptic nucleus.

The magnocellular oxytocin system, the fount of maternity: adaptations in pregnancy

Oxytocin secretion from the posterior pituitary gland is increased during parturition, stimulated by the uterine contractions that forcefully expel the fetuses. Since oxytocin stimulates further contractions of the uterus, which is exquisitely sensitive to oxytocin at the end of pregnancy, a positive feedback loop is activated. The neural pathway that drives oxytocin neurons via a brainstem relay has been partially characterised, and involves A2 noradrenergic cells in the brainstem. Until close to term the responsiveness of oxytocin neurons is restrained by neuroactive steroid metabolites of progesterone that potentiate GABA inhibitory mechanisms. As parturition approaches, and this inhibition fades as progesterone secretion collapses, a central opioid inhibitory mechanism is activated that restrains the excitation of oxytocin cells by brainstem inputs. This opioid restraint is the predominant damper of oxytocin cells before and during parturition, limiting stimulation by extraneous stimuli, and perhaps facilitating optimal spacing of births and economical use of the store of oxytocin accumulated during pregnancy. During parturition, oxytocin cells increase their basal activity, and hence oxytocin secretion increases. In addition, the oxytocin cells discharge a burst of action potentials as each fetus passes through the birth canal. Each burst causes the secretion of a pulse of oxytocin, which sharply increases uterine tone; these bursts depend upon auto-stimulation by oxytocin released from the dendrites of the magnocellular neurons in the supraoptic and paraventricular nuclei. With the exception of the opioid mechanism that emerges to restrain oxytocin cell responsiveness, the behavior of oxytocin cells and their inputs in pregnancy and parturition is explicable from the effects of hormones of pregnancy (relaxin, estrogen, progesterone) on pre-existing mechanisms, leading through relative quiescence at term inter alia to net increase in oxytocin storage, and reduced auto-inhibition by nitric oxide generation. Cyto-architectonic changes in parturition, involving evident retraction of glial processes between oxytocin cells so they get closer together, are probably a response to oxytocin neuron activation rather than being essential for their patterns of firing in parturition.