Lipopolysaccharide endotoxin potentiates the effect of osmotic stimulation on vasopressin synthesis and secretion in the rat hypothalamus

The Interaction of Vasopressin with Hormones of the Hypothalamo-Pituitary-Adrenal Axis: The Significance for Therapeutic Strategies in Cardiovascular and Metabolic Diseases

A large body of evidence indicates that vasopressin (AVP) and steroid hormones are frequently secreted together and closely cooperate in the regulation of blood pressure, metabolism, water-electrolyte balance, and behavior, thereby securing survival and the comfort of life. Vasopressin cooperates with hormones of the hypothalamo-pituitary-adrenal axis (HPA) at several levels through regulation of the release of corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and multiple steroid hormones, as well as through interactions with steroids in the target organs. These interactions are facilitated by positive and negative feedback between specific components of the HPA. Altogether, AVP and the HPA cooperate closely as a coordinated functional AVP-HPA system. It has been shown that cooperation between AVP and steroid hormones may be affected by cellular stress combined with hypoxia, and by metabolic, cardiovascular, and respiratory disorders; neurogenic stress; and inflammation. Growing evidence indicates that central and peripheral interactions between AVP and steroid hormones are reprogrammed in cardiovascular and metabolic diseases and that these rearrangements exert either beneficial or harmful effects. The present review highlights specific mechanisms of the interactions between AVP and steroids at cellular and systemic levels and analyses the consequences of the inappropriate cooperation of various components of the AVP-HPA system for the pathogenesis of cardiovascular and metabolic diseases.

Knockdown of endoplasmic reticulum chaperone BiP leads to the death of parvocellular AVP/CRH neurons in mice

Arginine vasopressin (AVP) is expressed in both magnocellular (magnAVP) and parvocellular AVP (parvAVP) neurons of the paraventricular nucleus, and AVP colocalizes with corticotropin-releasing hormone (CRH) only in the parvocellular neurons. The immunoglobulin heavy chain binding protein (BiP) is a major endoplasmic reticulum (ER) chaperone which regulates the unfolded protein response under ER stress. We previously demonstrated that knockdown of BiP in magnAVP neurons exacerbated ER stress, which resulted in the autophagy-associated cell death of magnAVP neurons. Using the same approach, in the present study we examined the role of BiP in mouse parvAVP/CRH neurons. Our data demonstrate that BiP is expressed in mouse parvAVP/CRH neurons under nonstress conditions and is upregulated in proportion to the increase in CRH expression after adrenalectomy. For BiP knockdown in parvAVP/CRH neurons, we utilized a viral approach in combination with shRNA interference. Knockdown of BiP expression induced ER stress in parvAVP/CRH neurons, as reflected by the expression of C/EBP homologous protein. Furthermore, BiP knockdown led to the loss of parvAVP/CRH neurons after 4 weeks. In summary, our results demonstrate that BiP plays a pivotal role in parvAVP/CRH neurons, which function as neuroendocrine cells producing a large number of secretory proteins.

Inflammatory Stress Induced by Intraperitoneal Injection of LPS Increases Phoenixin Expression and Activity in Distinct Rat Brain Nuclei

Due to phoenixin’s role in restraint stress and glucocorticoid stress, as well as its recently shown effects on the inflammasome, we aimed to investigate the effects of lipopolysaccharide (LPS)-induced inflammatory stress on the activity of brain nuclei-expressing phoenixin. Male Sprague Dawley rats (n = 6/group) were intraperitoneally injected with either LPS or control (saline). Brains were processed for c-Fos and phoenixin immunohistochemistry and the resulting slides were evaluated using ImageJ software. c-Fos was counted and phoenixin was evaluated using densitometry. LPS stress significantly increased c-Fos expression in the central amygdaloid nucleus (CeM, 7.2-fold), supraoptic nucleus (SON, 34.8 ± 17.3 vs. 0.0 ± 0.0), arcuate nucleus (Arc, 4.9-fold), raphe pallidus (RPa, 5.1-fold), bed nucleus of the stria terminalis (BSt, 5.9-fold), dorsal motor nucleus of the vagus nerve (DMN, 89-fold), and medial part of the nucleus of the solitary tract (mNTS, 121-fold) compared to the control-injected group (p < 0.05). Phoenixin expression also significantly increased in the CeM (1.2-fold), SON (1.5-fold), RPa (1.3-fold), DMN (1.3-fold), and mNTS (1.9-fold, p < 0.05), leading to a positive correlation between c-Fos and phoenixin in the RPa, BSt, and mNTS (p < 0.05). In conclusion, LPS stress induces a significant increase in activity in phoenixin immunoreactive brain nuclei that is distinctively different from restraint stress.

The multiple faces of the oxytocin and vasopressin systems in the brain

Classically, hypothalamic neuroendocrine cells that synthesise oxytocin and vasopressin were categorised in two major cell types: the magnocellular and parvocellular neurones. It was assumed that magnocellular neurones project exclusively to the pituitary gland where they release oxytocin and vasopressin into the systemic circulation. The parvocellular neurones, on the other hand, project within the brain to regulate discrete brain circuitries and behaviours. Within the last few years, it has become evident that the classical view of these projections is outdated. It is now clear that oxytocin and vasopressin in the brain are released extrasynaptically from dendrites and from varicosities in distant axons. The peptides act principally to modulate information transfer through conventional synapses (such as glutamate synapses) by actions at respective receptors that may be preferentially localised to synaptic regions (on either side of the synapse) to alter the 'gain' of conventional synapses.

Epithelial transport during septic acute kidney injury

A goal for scientists studying septic acute kidney injury (AKI) should be to formulate a conceptual model of disease that is able to coherently reconcile the molecular and inflammatory consequences of sepsis with impaired epithelial tubular function, diminished glomerular filtration rate (GFR) and ultimately kidney failure. Recent evidence has shed light on how sepsis modulates the tubular regulation of ion, glucose, urea and water transport and acid-base homeostasis in the kidney. The present review summarizes recent discoveries on changes in epithelial transport under septic and endotoxemic conditions as well as the mechanisms that link inflammation with impaired tubular membrane transport. This paper also proposes that the tubular dysfunction that is mediated by inflammation in sepsis ultimately leads to increased sodium and chloride delivery to the distal tubule and macula densa, contributing to tubuloglomerular feedback and impaired GFR. We feel that this conceptual model resolves many of the physiologic and clinical paradoxes that septic AKI presents to practicing researchers and clinicians.

Altered oxytocinergic hypothalamus systems in sepsis

Sepsis is known to affect neuroendocrine circuits: injections of lipopolysaccaride are potent stimulators of oxytocin secretion from the posterior lobe, acute sepsis leads to uterus contractions and spontaneous abort. Here, we report changes in expression and distribution of hypothalamic oxytocin in rats that had been subjected to caecal ligation and puncture which led to acute sepsis. Septic animals showed loss of oxytocin immunostaining in perikarya of the supraoptic and paraventricular nuclei and an increase of oxytocin positive fibres, suggesting a shift of oxytocin pools into the axonal compartment. Immunostaining of the posterior lobe revealed reduction of oxytocin in septic rats. Magnocellular neurons in supraoptic- and to a lesser extent in paraventricular nuclei showed nuclear immunoreactivity for the protooncogene c-Fos, indicating stimulation of transcriptional activity upon sepsis. Contrary to magnocellular oxytocin immunoreactivity, we observed increased oxytocin immunoreactivity in cell bodies and processes of periventricular nucleus and in perivascular neurons. Oxytocin neurons in other regions of the hypothalamus and the preoptic region did not appear to be affected by acute sepsis. Our findings suggest a differential activation of neurohypophyseal and cerebrospinal fluid contacting oxytocin systems while centrally projecting oxytocin neurons may not be affected. Systemic oxytocin levels may serve as additional diagnostic marker for sepsis.

Changes in pericytic expression of NG2 and PDGFRB and vascular permeability in the sensory circumventricular organs of adult mouse by osmotic stimulation

The blood-brain barrier (BBB) is a barrier that prevents free access of blood-derived substances to the brain through the tight junctions and maintains a specialized brain environment. Circumventricular organs (CVOs) lack the typical BBB. The fenestrated vasculature of the sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO) and area postrema (AP), allows parenchyma cells to sense a variety of blood-derived information, including osmotic ones. In the present study, we utilized immunohistochemistry to examine changes in the expression of NG2 and platelet-derived growth factor receptor beta (PDGFRB) in the OVLT, SFO and AP of adult mice during chronic osmotic stimulation. The expression of NG2 and PDGFRB was remarkably prominent in pericytes, although these angiogenesis-associated proteins are highly expressed at pericytes of developing immature vasculature. The chronic salt loading prominently increased the expression of NG2 in the OVLT and SFO and that of PDGFRB in the OVLT, SFO and AP. The vascular permeability of low-molecular-mass tracer fluorescein isothiocyanate was increased significantly by chronic salt loading in the OVLT and SFO but not AP. In conclusion, the present study demonstrates changes in pericyte expression of NG2 and PDGFRB and vascular permeability in the sensory CVOs by chronic osmotic stimulation, indicating active participation of the vascular system in osmotic homeostasis.

Hyponatremia and inflammation: the emerging role of interleukin-6 in osmoregulation

Although hyponatremia is a recognized complication of several inflammatory diseases, its pathophysiology in this setting has remained elusive until recently. A growing body of evidence now points to an important role for interleukin-6 in the non-osmotic release of vasopressin. Here, we review this evidence by exploring the immuno-neuroendocrine pathways connecting interleukin-6 with vasopressin. The importance of these connections extends to several clinical scenarios of hyponatremia and inflammation, including hospital-acquired hyponatremia, postoperative hyponatremia, exercise-associated hyponatremia, and hyponatremia in the elderly. Besides insights in pathophysiology, the recognition of the propensity for antidiuresis during inflammation is also important with regard to monitoring patients and selecting the appropriate intravenous fluid regimen, for which recommendations are provided.

Osmoregulation of vasopressin secretion is altered in the postacute phase of septic shock

OBJECTIVE

To determine whether septic shock patients have an abnormal reponse to increasing osmolarity.

DESIGN

Prospective interventional study.

SETTING

Intensive care unit at Raymond Poincaré and Etampes Hospitals.

PATIENTS

Normonatremic patients at > 72 hrs from septic shock onset.

INTERVENTION

Osmotic challenge consisting of infusing 500 mL of hypertonic saline solution (with cumulative amount of sodium not exceeding 24 g) over 120 mins.

MEASUREMENTS AND MAIN RESULTS

Plasma arginine vasopressin levels were measured 15 mins before the test and then four times every 30 mins. A slope of the relation between arginine vasopressin and plasma sodium levels of < 0.5 pg/mEq defined nonresponders. Among the 33 included patients, 17 (52%) were nonresponders. During osmotic challenge, variations throughout the test in plasma sodium levels, blood pressure, and central venous pressure were comparable between the two groups. Arginine vasopressin increased from 4.8 pg/mL [3.3-6.4 pg/mL] to 14.4 pg/mL [11.2-23.3 pg/mL] in responders but only from 2.8 pg/mL [2.3-4.0 pg/mL] to 4.0 pg/mL [3.1-5.3 pg/mL] in nonresponders (p < .0001). Responders had a higher plasma arginine vasopressin levels at baseline and a more severe hematosis alteration. Nonresponders had more frequently bacteremia and liver dysfunction, been referred from the ward and undergone surgery. Critical illness severity, hemodynamic alteration, hydroelectrolytic disturbances, treatment, and outcome did not differ between the two groups.

CONCLUSION

Osmoregulation is dramatically altered in half of patients with prolonged septic shock.

Central NOS inhibition differentially affects vasopressin gene expression in hypothalamic nuclei in septic rats

Our aim was to investigate the effect of central NOS inhibition on hypothalamic arginine vasopressin (AVP) gene expression, hormone release and on the cardiovascular response during experimental sepsis. Male Wistar rats were intracerebroventricularly injected with the non-selective NO synthase (NOS) inhibitor (L-NAME) or aminoguanidine, a selective inhibitor of the inducible isoform (iNOS). After 30 min, sepsis was induced by cecal ligation and puncture (CLP) causing an increase in heart rate (HR), as well as a reduction in median arterial pressure (MAP) and AVP expression ratio (AVP(R)), mainly in the supraoptic nucleus. AVP plasma levels (AVPp) increased in the early but not in the late phase of sepsis. L-NAME pretreatment increased MAP but did not change HR. It also resulted in an increase in AVPp at all time points, except 24h, when it returned to basal levels. AVP(R), however remained reduced in both nuclei. Aminoguanidine pretreatment resulted in increased MAP in the early phase and higher AVP(R) in the supraoptic, but not in the paraventricular nucleus, while AVPp remained elevated at all time points. We suggest that increased central NO production, mainly inducible NOS-derived, reduces AVP gene expression differentially in supraoptic and paraventricular nuclei, and that this may contribute to low AVP plasma levels and hypotension in the late phase of sepsis.

Vasopressin synthesis by the magnocellular neurons is different in the supraoptic nucleus and in the paraventricular nucleus in human and experimental septic shock

Impaired arginine vasopressin (AVP) synthesis and release by the neurohypophyseal system, which includes the neurohypophysis and magnocellular neurons of the paraventricular and supraoptic nuclei, have been postulated in septic shock, but changes in this system have never been assessed in human septic shock, and only partially experimentally. We investigated AVP synthesis and release by the neurohypophyseal system in 9 patients who died from septic shock and 10 controls, and in 20 rats with fecal peritonitis-induced sepsis and 8 sham-operation controls. Ten rats died spontaneously from septic shock, and the others were sacrificed. In patients with septic shock, as in rats that died spontaneously following sepsis induction, AVP immunohistochemical expression was decreased in the neurohypophysis and supraoptic magnocellular neurons, whereas it was increased in the paraventricular magnocellular neurons. No significant change was observed in AVP messenger RiboNucleic Acid (mRNA) expression assessed by in situ hybridization in either paraventricular or supraoptic magnocellular cells. This study shows that both in human and experimental septic shock, AVP posttranscriptional synthesis and transport are differently modified in the magnocellular neurons of the supraoptic and paraventricular nuclei. This may account for the inappropriate AVP release in septic shock and suggests that distinct pathogenic mechanisms operate in these nuclei.

VASOPRESSIN MEDIATES THE PRESSOR EFFECT OF HYPERTONIC SALINE SOLUTION IN ENDOTOXIC SHOCK

The administration of lipopolysaccharide (LPS) to experimental animals results in a septic shock-like syndrome characterized by hypotension, and the hemodynamic management includes the restoration of adequate tissue perfusion by administration of resuscitation fluids to achieve an effective circulating volume. In the present study, we sought to investigate the effects of hypertonic saline solution administration on vasopressin secretion and mean arterial pressure in endotoxic shock. The pressor response to isotonic saline solution (0.9% sodium chloride) or hypertonic saline (7.5% sodium chloride, 4 mL/kg i.v.) was evaluated 4 h after LPS (1.5 mg/kg) administration. At this moment, plasma vasopressin did not differ from control; however, the blood pressure was lower in the LPS-treated group. The hypertonic saline administration was followed by an immediate recovery of blood pressure and also by an increase in plasma vasopressin levels compared with isotonic saline solution. The vasopressin V1 receptor antagonist (10 microg/kg, i.v., 5 min before infusion) blocked the pressor response to hypertonic saline solution. These data suggest that the recovery of blood pressure after hypertonic saline solution administration during endotoxic shock is mediated by vasopressin secretion.

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.

[Brain injury during severe sepsis]

Sepsis-associated encephalopathy is a global cerebral dysfunction induced by the systemic response to inflammation and infection, without a liver or renal injury. Alteration of consciousness, from confusion to coma, is the main clinical symptom. This encephalopathy is associated with an increase in mortality due to sepsis. Its physiopathology is unknown. There is frequently an increased permeability of the blood-brain barrier, which might explain a role of endotoxins on cerebral metabolism. Changes in neurotransmitter release or concentrations (norepinephrine, serotonin, dopamine, GABA) have been reported. There is not any specific treatment of septic encephalopathy. In most cases, this syndrome is rapidly reversible after the treatment of sepsis.

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.

Antidiuretic hormone-V2-receptor-aquaporin-2 system in rat kidneys during acute inflammation

Expression of antidiuretic hormone V(2)-receptor, water channel protein aquaporin-2, and cytokines interleukin-1b and interleukin-6 was studied in the kidneys of rats with acute inflammation produced by intraperitoneal injection of lipopolysaccharide in a dose of 250 microg/100 g. Reduced expression of aquaporin-2 and V(2)-receptor led to impairment of concentration capacity in the kidneys and decrease in urine osmolarity.

Acute endotoxemia in rats induces down-regulation of V2 vasopressin receptors and aquaporin-2 content in the kidney medulla

BACKGROUND

Endotoxemia can lead to fluid metabolism alterations despite unchanged or elevated plasma vasopressin (VP) levels, suggesting a refractoriness of the kidney to the effect of the peptide. To test this hypothesis, we examined the effect of lipopolysaccharide (LPS) injection on the expression of V2 receptors and aquaporin-2 in the kidney.

METHODS

Plasma VP and urine osmolality, and binding of [3H]VP to kidney membranes, Western blot, and immunohistochemical analysis of aquaporin-2, in situ hybridization for V2 VP receptors and cytokines mRNAs were measured in the kidney 3 to 24 hours after LPS injection, 250 microg/100 g, intraperitoneally.

RESULTS

LPS injection caused prolonged decreases in urine osmolality (up to 24 hours) without significant changes in plasma levels of sodium or VP. This was associated with marked decreases in V2 VP receptor mRNA and VP receptor number in the kidney, which were evident for up to 12 hours after LPS injection. Aquaporin-2 in kidney inner medulla was also reduced by about 50%. LPS induced interleukin (IL)-1beta in the kidney medulla by 3 hours, reached maximum at 6 hours, and started to decline by 12 hours, while it increased IL-6 mRNA significantly only at 3 hours. Interleukin mRNA expression was absent in kidneys of control rats. In vitro incubation of kidney medulla slices with IL-1beta reduced VP binding.

CONCLUSION

The inflammatory response to acute endotoxemia down regulates V2 VP receptors and aquaporin-2 of the kidney inner medulla resulting in prolonged impairment of the renal capacity to concentrate urine.