Opposing alpha- and beta-adrenergic mechanisms mediate dose-dependent actions of noradrenaline on supraoptic vasopressin neurones in vivo

Catecholamines modulate differentially nonapeptide precursor mRNA expression in the preoptic area and ovary of the catfish Heteropneustes fossilis: An in vitro study

In the catfish Heteropneustes fossilis, three nonapeptide hormone genes were identified in the brain preoptic area (POA) and ovary: a pro-vasotocin (pro-vt) and two isotocin gene paralogs viz., a novel pro-ita and conventional pro-itb. In the present study, the regulatory role of catecholamines [CA: dopamine (DA), noradrenaline (NA), adrenaline (AD)] on the expression of these genes were investigated in vitro. DA (1, 10, and 100 ng/mL) inhibited significantly the mRNA expression in both the POA and ovary. NA upregulated the POA mRNA expression in a biphasic manner, the lower concentrations (1 ng and 10 ng) scaled up and the higher concentration (100 ng) scaled down the expression of pro-vt and pro-itb, while only the 1 ng NA scaled up the pro-ita expression. In the ovary, NA upregulated the mRNA expressions at all concentrations; the pro-vt expression was stimulated only at 10 and 100 ng. AD stimulated pro-vt and pro-ita expression in the POA at all concentrations but the pro-itb expression was inhibited at 1 and 10 ng, and stimulated at 100 ng concentrations. In the ovary, AD elicited varied effects; no significant change in pro-vt, a stimulation of pro-ita, and an inhibition of pro-itb at 1 ng, and stimulation of pro-itb at the 10 and 100 ng. The incubation of the POA and ovary with α-methylparatyrosine (MPT, 250 µg/mL, a tyrosine hydroxylase inhibitor) for 8 h downregulated the mRNA expression in the POA but unaltered the expression in the ovary. Pre-incubation with MPT for 4 h, followed by co-incubation with DA, NA or AD for 4 h elicited varied effects. In the POA, the co-incubations with the CAs rescued the inhibition due to MPT. The MPT + DA and MPT + AD treatments reduced the magnitude of the inhibition of pro-vt and pro-itb by MPT. But the pro-ita expression was modestly stimulated in the MPT + AD group. On the other hand, the MPT + NA treatment rescued the MPT effect and elicited 10-folds increase in the expression levels. In the ovary, the changes were: an inhibition in the MPT + DA group, no significant alteration in the MPT + NA group, and a mild stimulation in the MPT + AD group. The results suggest that CAs modulate brain and ovarian nonapeptide gene expression differentially, which is important in the neuroendocrine/endocrine integration of reproduction in the catfish.

Neuroendocrine Derangements in Early Septic Shock: Pharmacotherapy for Relative Adrenal and Vasopressin Insufficiency

Septic shock is a leading cause of mortality in intensive care units throughout the world. While this disease state represents a highly complex pathophysiology involving numerous organ systems, the early approach to care includes adequate hemodynamic support traditionally achieved via infusions of vasoactive medications after adequate fluid resuscitation. Relative adrenal and vasopressin deficiencies are a common feature of septic shock that contribute to impaired hemodynamics. Hydrocortisone and vasopressin are endocrine system hormone analogues that target the acute neuroendocrine imbalance associated with septic shock. This clinically focused annotated review describes the pathophysiological mechanisms behind their use and explores the potential clinical roles of early administration and synergy when combined.

Structural and functional diversity of nonapeptide hormones from an evolutionary perspective: A review

The article presents an overview of the comparative distribution, structure and functions of the nonapeptide hormones in chordates and non chordates. The review begins with a historical preview of the advent of the concept of neurosecretion and birth of neuroendocrine science, pioneered by the works of E. Scharrer and W. Bargmann. The sections which follow discuss different vertebrate nonapeptides, their distribution, comparison, precursor gene structures and processing, highlighting the major differences in these aspects amidst the conserved features across vertebrates. The vast literature on the anatomical characteristics of the nonapeptide secreting nuclei in the brain and their projections was briefly reviewed in a comparative framework. Recent knowledge on the nonapeptide hormone receptors and their intracellular signaling pathways is discussed and few grey areas which require deeper studies are identified. The sections on the functions and regulation of nonapeptides summarize the huge and ever increasing literature that is available in these areas. The nonapeptides emerge as key homeostatic molecules with complex regulation and several synergistic partners. Lastly, an update of the nonapeptides in non chordates with respect to distribution, site of synthesis, functions and receptors, dealt separately for each phylum, is presented. The non chordate nonapeptides share many similarities with their counterparts in vertebrates, pointing the system to have an ancient origin and to be an important substrate for changes during adaptive evolution. The article concludes projecting the nonapeptides as one of the very first common molecules of the primitive nervous and endocrine systems, which have been retained to maintain homeostatic functions in metazoans; some of which are conserved across the animal kingdom and some are specialized in a group/lineage-specific manner.

Magnocellular Neurons and Posterior Pituitary Function

The posterior pituitary gland secretes oxytocin and vasopressin (the antidiuretic hormone) into the blood system. Oxytocin is required for normal delivery of the young and for delivery of milk to the young during lactation. Vasopressin increases water reabsorption in the kidney to maintain body fluid balance and causes vasoconstriction to increase blood pressure. Oxytocin and vasopressin secretion occurs from the axon terminals of magnocellular neurons whose cell bodies are principally found in the hypothalamic supraoptic nucleus and paraventricular nucleus. The physiological functions of oxytocin and vasopressin depend on their secretion, which is principally determined by the pattern of action potentials initiated at the cell bodies. Appropriate secretion of oxytocin and vasopressin to meet the challenges of changing physiological conditions relies mainly on integration of afferent information on reproductive, osmotic, and cardiovascular status with local regulation of magnocellular neurons by glia as well as intrinsic regulation by the magnocellular neurons themselves. This review focuses on the control of magnocellular neuron activity with a particular emphasis on their regulation by reproductive function, body fluid balance, and cardiovascular status. © 2016 American Physiological Society. Compr Physiol 6:1701-1741, 2016.

Pharmacologic agents for acute hemodynamic instability: recent advances in the management of perioperative shock- a systematic review

Despite the growing body of evidence evaluating the efficacy of vasoactive agents in the management of hemodynamic instability and circulatory shock, it appears no agent is superior. This is becoming increasingly accepted as current guidelines are moving away from detailed algorithms for the management of shock, and instead succinctly state that vasoactive agents should be individualized and guided by invasive hemodynamic monitoring. This extends to the perioperative period, where vasoactive agent selection and use may still be left to the discretion of the treating physician with a goal-directed approach, consisting of close hemodynamic monitoring and administration of the lowest effective dose to achieve the hemodynamic goals. Successful therapy depends on the ability to rapidly diagnose the etiology of circulatory shock and thoroughly understand its pathophysiology as well as the pharmacology of vasoactive agents. This review focuses on the physiology and resuscitation goals in perioperative shock, as well as the pharmacology and recent advances in vasoactive agent use in its management.

Noradrenergic dysregulation in the pathophysiology of PTSD

A central role for noradrenergic dysregulation in the pathophysiology of post-traumatic stress disorder (PTSD) is increasingly suggested by both clinical and basic neuroscience research. Here, we integrate recent findings from clinical and animal research with the earlier literature. We first review the evidence for net upregulation of the noradrenergic system and its responsivity to stress in individuals with PTSD. Next, we trace the evidence that the α₁ noradrenergic receptor antagonist prazosin decreases many of the symptoms of PTSD from initial clinical observations, to case series, to randomized controlled trials. Finally, we review the basic science work that has begun to explain the mechanism for this efficacy, as well as to explore its possible limitations and areas for further advancement. We suggest a view of the noradrenergic system as a central, modifiable link in a network of interconnected stress-response systems, which also includes the amygdala and its modulation by medial prefrontal cortex. Particular attention is paid to the evidence for bidirectional signaling between noradrenaline and corticotropin-releasing factor (CRF) in coordinating these interconnected systems. The multiple different ways in which the sensitivity and reactivity of the noradrenergic system may be altered in PTSD are highlighted, as is the evidence for possible heterogeneity in the pathophysiology of PTSD between different individuals who appear clinically similar. We conclude by noting the importance moving forward of improved measures of noradrenergic functioning in clinical populations, which will allow better recognition of clinical heterogeneity and further assessment of the functional implications of different aspects of noradrenergic dysregulation.

Reactive oxygen species are physiological mediators of the noradrenergic signaling pathway in the mouse supraoptic nucleus

Free radicals are essential for the vasopressin (AVP) response to plasmatic hyperosmolarity. Noradrenergic afferents are the major projections on the supraoptic nucleus (SON) of the hypothalamus and stimulate the expression of AVP via a nitric oxide (NO) pathway. In this study, we investigated the mechanisms linking free radicals and noradrenaline (NA)-induced regulation of AVP. Analysis of Tg8 transgenic mice, invalidated for the monoamine oxidase-A gene and with consequently high levels of brain monoamines and AVP in the SON, showed that free radicals are more abundant in their SON than in that of wild-type mice (WT). Antioxidant superoxide dismutase 1 and 2 and catalase enzyme activities were also higher in these mice than in WT. This may explain the observed absence of cytotoxicity that would otherwise be associated with such high level of free radicals. Treatment of Tg8 mice with α-MPT, a blocking agent for NA synthesis, decreased both the production of free radicals and the AVP levels in the SON. Furthermore, incubation of ex vivo slices including the SON with NA increased the production of free radicals and AVP levels in wild-type mice. When NA was associated with α-lipoic acid, an antioxidant blocking the production of free radicals, AVP remained at its control level, indicating that free radicals are required for the effect of NA on the expression of AVP. In slices incubated with SNP, a producer of NO, free radicals and AVP levels increased. When NA was associated with L-NAME (a NO synthase blocker), the levels of free radicals and AVP were the same as in controls. Thus, the noradrenaline-NO pathway, which stimulates the expression of vasopressin, involves free radicals. This study provides further evidence of the physiological importance of free radicals, which should no longer be considered solely as cytotoxic factors.

Physiological regulation of magnocellular neurosecretory cell activity: integration of intrinsic, local and afferent mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.

Diverse roles of G-protein coupled receptors in the regulation of neurohypophyseal hormone secretion

The magnocellular neurones in the supraoptic nucleus project to the neural lobe and release vasopressin and oxytocin into the peripheral circulation, where they act on the kidney to promote fluid retention or stimulate smooth muscles in the vasculature, uterus and mammary glands to support blood pressure, promote parturition or induce milk let-down, respectively. Hormone release is regulated by complex afferent pathways carrying information about plasma osmolality, blood pressure and volume, cervical stretch, and suckling. These afferent pathways utilise a broad array of neurotransmitters and peptides that activate both ligand-gated ion channels and G-protein coupled receptors (GPCRs). The ligand-gated ion channels induce rapid changes in membrane potential resulting in the generation of action potentials, initiation of exocytosis and the release of hormone into the periphery. By contrast, the GPCRs activate a host of diverse signalling cascades that modulate action potential firing and regulate other cellular functions required to support hormone release (e.g. hormone synthesis, processing, packaging and trafficking). The diversity of these actions is critical for integration of the distinct regulatory signals into a response appropriate for maintaining homeostasis. This review describes several diverse roles of GPCRs in magnocellular neurones, focusing primarily on adrenergic, purinergic and peptidergic (neurokinin and angiotensin) receptors.

Multiple alpha1-adrenergic receptor subtypes support synergistic stimulation of vasopressin and oxytocin release by ATP and phenylephrine

Simultaneous exposure of explants of the hypothalamo-neurohypophyseal system (HNS) to ATP and the α(1)-adrenergic receptor (α(1)-R) agonist, phenylephrine (ATP+PE) induces a synergistic stimulation of vasopressin and oxytocin (VP/OT) release that is sustained for hours. The current studies confirm that the synergism is dependent upon activation of α(1)-R by demonstrating that an α(1)-R antagonist prevents the response. The role of the α(1)A, B, and D-adrenergic receptor subtypes in the synergistic effect of ATP+PE on intracellular calcium ([Ca(2+)](i)) in supraoptic nucleus (SON) neurons and VP/OT release from neural lobe was evaluated. The increase in [Ca(2+)](i) induced by PE in SON predominantly reflects release from intracellular stores and is mediated by activation of the α(1)A adrenergic receptor subtype. The α(1)A subtype is also required for the sustained elevation in [Ca(2+)](i) induced by ATP+PE. In contrast, although synergistic stimulation of VP/OT release was eliminated by removal of PE and was blunted by benoxathian, an α(1)-R antagonist that is not subtype selective, no single α(1)-R subtype selective antagonist prevented sustained stimulation of VP/OT release by ATP+PE. Thus, sustained activation of α(1)-R is essential for the synergistic VP and OT response to ATP+PE, but multiple α(1)-R subtypes can support the response. Redundancy amongst the α(1)-R subunits in supporting this response is consistent with the predicted importance of the response for sustaining the elevated VP release required to prevent cardiovascular collapse during hemorrhage and sepsis.

Decreased catecholamine degradation associates with shock and kidney injury after cardiac surgery

Enzymatic pathways involving catechol-O-methyltransferase (COMT) catabolize circulating catecholamines. A G-to-A polymorphism in the fourth exon of the COMT gene results in a valine-to-methionine amino acid substitution at codon 158, which leads to thermolability and low ("L"), as opposed to high ("H"), enzymatic activity. We enrolled 260 patients postbypass surgery to test the hypothesis that COMT gene variants impair circulating catecholamine metabolism, predisposing to shock and acute kidney injury (AKI) after cardiac surgery. In accordance with the Hardy-Weinberg equilibrium, we identified 64 (24.6%) homozygous (LL), 123 (47.3%) heterozygous (HL), and 73 (28.1%) homozygous (HH) patients. Postoperative catecholamines were higher in homozygous LL patients compared with heterozygous HL and homozygous HH patients (P < 0.01). During their intensive care stay, LL patients had both a significantly greater frequency of vasodilatory shock (LL: 69%, HL: 57%, HH: 47%; P = 0.033) and a significantly longer median duration of shock (LL: 18.5 h, HL: 14.0 h, HH: 11.0 h; P = 0.013). LL patients also had a greater frequency of AKI (LL: 31%, HL: 19.5%, HH: 13.7%; P = 0.038) and their AKI was more severe as defined by a need for renal replacement therapy (LL: 7.8%, HL: 2.4%, HH: 0%; P = 0.026). The LL genotype associated with intensive care and hospital length of stay (P < 0.001 and P = 0.002, respectively), and we observed a trend for higher mortality. Cross-validation analysis revealed a similar graded relationship of adverse outcomes by genotype. In summary, this study identifies COMT LL homozygosity as an independent risk factor for shock, AKI, and hospital stay after cardiac surgery. (ClinicalTrials.gov number, NCT00334009).

The vasopressin and copeptin response to infection, severe sepsis, and septic shock

OBJECTIVE

To compare the course of arginine vasopressin (AVP) and copeptin plasma concentrations between patients with infection, severe sepsis, and septic shock.

DESIGN

Prospective, closed-cohort study.

SETTING

Twelve-bed general and surgical intensive care unit and 33-bed internal medicine ward in a university hospital.

PATIENTS

Ten patients with infection, 22 with severe sepsis, and 28 with septic shock.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Hemodynamic, laboratory and clinical data were recorded daily during the first 7 days after intensive care unit or hospital admission. Parallel thereto, blood was withdrawn to determine plasma AVP (radioimmunoassay) and copeptin (immunoluminometric assay) concentrations. Standard tests, a mixed effects model, and a linear regression analysis were used for statistical analysis. The AVP response was different between the three study groups (p < 0.001) but did not change over time (p = 0.12). Although patients with severe sepsis and septic shock had higher AVP levels than did patients with infection (both p < 0.001), no difference in AVP concentrations was seen between severe sepsis and septic shock patients (p = 0.98). No difference in AVP was observed between survivors and nonsurvivors at day 28 (p = 0.87). In patients with severe sepsis, serum osmolarity (p < 0.001), arterial pH (p = 0.001), lactate (p < 0.001), and Pao2 (p = 0.04) were associated with the course of AVP plasma levels, whereas it was serum osmolarity alone in patients with septic shock (p = 0.03). Plasma AVP concentrations correlated with copeptin (r = .614, p < 0.001), but this correlation was influenced by continuous veno-venous hemofiltration (p = 0.002).

CONCLUSIONS

Severe sepsis induced a stronger AVP response than infection without systemic inflammation. However, the lack of a difference in AVP plasma concentrations between patients with and without shock indicates that the AVP system does not function normally in severe sepsis. Our data support the hypothesis that impaired AVP response is at least partially responsible for the failure to restore vascular tone in septic shock.

Vasopressin in the pediatric cardiac intensive care unit: Myth or reality

Pediatric cardiac surgery is undergoing a metamorphosis, with more and more critical patients being operated in our country today. Although the principles of physiology have not changed, it is imperative that care providers continue to stay abreast with developments and newer drugs that may help modify the outcome. The team dynamics have also become more complex, which necessitates the need for all care providers (surgeons, cardiologists, anesthesiologists, and intensivists) to better understand the interactions and benefits of newer drugs. Vasopressin has been used in our adult patients for more than a decade and recently has found its rightful place in the pediatric armoury. The objective of this article is to review the physiology of vasopressin and the rationale of its use in critically ill children with shock, in context of the available published data.

Mechanisms involved in the pressor response to noradrenaline microinjection into the supraoptic nucleus of unanesthetized rats

We report on the cardiovascular effects of noradrenaline (NA) microinjection into the hypothalamic supraoptic nucleus (SON) as well as the central and peripheral mechanisms involved in their mediation. Microinjections of NA 1, 3, 10, 30 or 45 nmol/100 nL into the SON caused dose-related pressor and bradycardiac response in unanesthetized rats. The response to NA 10 nmol was blocked by SON pretreatment with 15 nmol of the alpha(2)-adrenoceptor antagonist RX821002 and not affected by pretreatment with equimolar dose of the selective alpha(1)-adrenoceptor antagonist WB4101, suggesting that local alpha(2)-adrenoceptors mediate these responses. Pretreatment of the SON with the nonselective beta-adrenoceptor antagonist propranolol 15 nmol did not affect the pressor response to NA microinjection of into the SON. Moreover, the microinjection of the 100 nmol of the selective alpha(1)-adrenoceptor agonist methoxamine (MET) into the SON did not cause cardiovascular response while the microinjection of the selective alpha(2)-adrenoceptor agonists BHT920 (BHT, 100 nmol) or clonidine (CLO, 5 nmol) caused pressor and bradycardiac responses, similar to that observed after the microinjection of NA. The pressor response to NA was potentiated by intravenous pretreatment with the ganglion blocker pentolinium and was blocked by intravenous pretreatment with the V(1)-vasopressin receptor antagonist dTyr(CH2)5(Me)AVP, suggesting an involvement of circulating vasopressin in this response. In conclusion, our results suggest that pressor responses caused by microinjections of NA into the SON involve activation of local alpha(2)-adrenoceptor receptors and are mediated by vasopressin release into circulation.

Arginine vasopressin in the treatment of vasodilatory septic shock

Vasodilatory septic shock is characterized by profound vasodilation of the peripheral circulation, relative refractoriness to catecholamines and a relative deficiency of the posterior pituitary hormone, vasopressin. Arginine vasopressin is effective in restoring vascular tone in vasodilatory septic shock and may be associated with decreased mortality in less severe septic shock as well as improved mortality and decreased renal failure in septic shock patients at risk for renal failure.

The vasopressin and copeptin response in patients with vasodilatory shock after cardiac surgery: a prospective, controlled study

OBJECTIVE

To evaluate arginine vasopressin (AVP) and copeptin plasma concentrations in patients with vasodilatory shock after cardiac surgery.

DESIGN

Prospective, controlled, clinical study.

SETTING

Surgical intensive care unit and cardiac surgery ward in a tertiary university teaching hospital.

PATIENTS AND PARTICIPANTS

Thirty-three critically ill patients with vasodilatory shock after cardiac surgery and ten control patients undergoing uncomplicated aorto-coronary bypass surgery.

MEASUREMENTS AND RESULTS

Hemodynamic, laboratory and clinical data were recorded daily in all patients during the first 7 days after cardiac surgery. At the same time, points blood was withdrawn to determine plasma concentrations of AVP (radioimmunoassay) and copeptin (immunoluminometric assy). Standard tests, a mixed effects model and regression analyses were used for statistical analysis. The course of AVP was significantly different between groups (P < 0.001). While AVP concentrations were lower in the study group on the first postoperative day, they were higher than that in the control group from postoperative day 3 on. There was no difference in the postoperative AVP response between study patients with or without chronic angiotensin-converting enzyme inhibitor therapy. Except during continuous veno-venous hemofiltration, AVP and copeptin correlated significantly with each other (P < 0.001; r = 0.749).

CONCLUSIONS

The AVP response to cardiac surgery is significantly different between patients with vasodilatory shock and patients undergoing uncomplicated aorto-coronary bypass surgery. Although no causative relationship between AVP concentrations and cardiovascular instability can be drawn from these results, our data support the hypothesis that inadequately low AVP plasma levels contribute to the failure to restore vascular tone in vasodilatory shock after cardiac surgery.

Vasopressinergic neurons in rats in ontogenesis: responses to salt loading and their modulation by noradrenergic afferents

Salt loading in adult mammals leads to increased vasopressin secretion by vasopressinergic neurons in the supraoptic nucleus, which is mediated by the actions of a number of hormones and neurotransmitters, including noradrenaline. The present study addressed identification of the stage of ontogenesis at which vasopressinergic neurons start to respond to salt loading and when the noradrenalinergic regulation of this process begins. Studies were performed on rats at embryonic day 21 (E21), postnatal day 3 (P3), and postnatal day 13 (P13) using immunocytochemical and in situ hybridization. Animals were subjected to salt loading, in some cases on the background of the alpha1-adrenoceptor inhibitor prazosin. Salt loading in rats of all age groups induced increases in the synthesis of vasopressin mRNA, probably accompanied by increased synthesis of vasopressin peptide. At E21 and P3, intraneuronal vasopressin levels were increased; there was no change at P13. In salt loading on the background of prazosin administration, vasopressin mRNA and vasopressin contents at E21 showed no change, while at P3 they were increased, which is evidence of the inhibitory effect of noradrenaline on vasopressin expression in the early postnatal period. Thus, vasopressinergic neurons start to respond to salt loading at the end of the prenatal period with increases in vasopressin expression; noradrenergic afferents have inhibitory influences on vasopressin expression in the early postnatal period.

Tachykinin neurokinin 3 receptor signaling in cholecystokinin-elicited release of oxytocin and vasopressin

Neurokinin 3 receptor (NK3R) signaling has an integral role in the stimulated oxytocin (OT) and vasopressin (VP) release in response to hyperosmolarity and hypotension. Peripheral injections of cholecystokinin (CCK) receptor agonists for the CCK-A (sulfated CCK-8) and CCK-B (nonsulfated CCK-8) receptors elicit an OT release in rat. It is unknown whether NK3R contributes to this endocrine response. Freely behaving male rats were administered an intraventricular pretreatment of 250 or 500 pmol of SB-222200, a specific NK3R antagonist, or 0.15 M NaCl before an intraperitoneal or intravenous injection of CCK-8 (nonsulfated or sulfated) or 0.15 M NaCl. Blood samples were taken before intraventricular treatment and 15 min after intraperitoneal or intravenous injection, and plasma samples were assayed for OT and VP concentration. Intraperitoneal injection of both nonsulfated and sulfated CCK-8 significantly increased plasma OT levels and had no effect on plasma VP levels. Intravenous injection of sulfated CCK-8 stimulated an increase in plasma OT levels and did not alter plasma VP levels. However, intravenous injection of nonsulfated CCK-8 stimulated a significant increase in plasma levels of both OT and VP. No other studies have demonstrated CCK-8-stimulated release of VP in rat. NK3R antagonist did not alter baseline levels of either hormone. However, pretreatment of NK3R antagonist significantly blocked the CCK-stimulated release of OT in all CCK treatment groups and blocked VP release in response to intravenous injection of nonsulfated CCK-8. Therefore, central NK3R signaling is required for OT and VP release in response to CCK administration.

Employing dobutamine as a useful agent to reverse the terlipressin-linked impairments in cardiopulmonary hemodynamics and global oxygen transport in healthy and endotoxemic sheep

Although arginine vasopressin and terlipressin have been identified as useful nonadrenergic agents to increase systemic blood pressure in catchecholamine-resistant septic shock, the impairments in cardiac index (CI) and global oxygen transport may limit their clinical applicability. The present study was designed as a prospective controlled laboratory experiment to investigate the effects of dobutamine as an adjunct to terlipressin infusion on cardiopulmonary hemodynamics and global oxygen transport in healthy and endotoxemic sheep. Nine adult ewes were instrumented for chronic study using an established protocol. After a baseline measurement in the healthy state had been performed, 1 mg terlipressin was given as bolus infusion. Thirty minutes later, dobutamine was continuously infused at incremental doses (2 and 5 microg x kg(1) x min(1), each for 1 h). After 24 h of recovery, a hypotensive-hyperdynamic circulation was induced and maintained by a continuous infusion of Salmonella typhosa endotoxin (10 ng x kg(1) x min(1)). After 16 h of endotoxemia, the six surviving sheep received terlipressin and dobutamine according to the same protocol that was used in healthy sheep. Compared with baseline, terlipressin infusion was associated with a significant increase in MAP that, however, occurred at the expense of a compromised CI, oxygen delivery index (DO(2)I), and mixed venous oxygen saturation (SvO(2), each P < 0.05). Dobutamine infusion was followed by a dose-dependent increase in CI, DO(2)I, and SvO(2) in both health and endotoxemia (each P < 0.05). Although the higher dosage of dobutamine exerted favorable effects, such as a decrease in pulmonary vascular resistance index (P < 0.05), the associated onset of tachycardia (P < 0.05) and arterial hypotension (P < 0.05) may limit its therapeutic use under septic conditions. This study provides evidence that dobutamine in a dosage of 2 microg x kg(1) x min(1) is useful to reverse the terlipressin-linked depressions in CI, DO(2)I and SvO(2) in ovine endotoxemia without obvious side effects.

Vasopressin vs. terlipressin in the treatment of cardiovascular failure in sepsis

BACKGROUND

Arginine vasopressin (AVP) and terlipressin (TP) are increasingly used as adjunct vasopressors in the treatment of septic shock. Despite important pharmacological differences between the two drugs (e.g., receptor selectivity, effective half-life) the use of either substance is determined mainly by local availability and institutional inventory. We briefly describe the pathophysiology and pharmacology of septic shock relevant to the treatment with vasopressin analogues. In addition, differences in pharmacokinetics and pharmacodynamics between AVP and TP are discussed.

DISCUSSION

The current literature suggests that neither AVP nor TP should be administered in high doses in patients with septic shock. Furthermore, increasing evidence indicates that early administration of vasopressin analogues may improve outcome as compared to a last-resort treatment. Low-dose infusion of AVP (0.6-2.4 U/h) has been demonstrated to be a safe adjunct in the management of septic shock. The V2 agonistic effects of AVP may exert favorable effects on hepatosplanchnic, renal, pulmonary, and coronary perfusion. However, the higher V1 receptor selectivity of TP may prove more potent in restoring arterial blood pressure and avoiding rebound hypotension, while carrying the risk of sustained global and regional vasoconstriction after bolus injection.

CONCLUSIONS

Evidence from experimental studies and initial clinical reports suggests that continuous low-dose infusion of TP may stabilize hemodynamics in septic shock with reduced side effects. However, randomized, controlled trials are necessary to determine the role of bolus or continuous infusion of TP in the treatment of septic shock before this approach can be recommended for routine clinical use.

Vasopressin for the septic burn patient

BACKGROUND

Exogenous arginine vasopressin (VP) has been increasingly used in the hemodynamic management of critically ill patients with septic shock, but its use in septic burn patients has not been systematically examined.

PURPOSE

To review our experience with the use of VP in septic burn patients.

METHODS

Retrospective review of all patients who received VP at a tertiary care adult regional burn centre. Only patients who strictly met the American College of Chest Physicians/Society of Critical Care Medicine Consensus Criteria for sepsis at the time of VP initiation were analysed.

RESULTS

There were 30 septic burn patients treated on 43 distinct occasions with VP. This group had a mean (+/-S.D.) age of 49+/-19 years, a mean % TBSA burn of 41+/-15% and a 37% incidence of inhalation injury. A significant increase in mean arterial pressure (MAP), a significant decrease in heart rate (HR), and a trend towards increased urine output (UO) occurred following initiation of VP. When VP was added to an existing infusion of norepinephrine (NE), there was a significant NE sparing effect. VP was implicated in the death of one patient who developed diffuse upper gastrointestinal necrosis while on VP. Other complications in patients treated with VP included peripheral ischemia (2), skin graft failure (1) and donor site conversion (1). In all complications, VP had been administered in combination with prolonged NE infusions (mean of 10 microg/min over a mean of 177 h).

CONCLUSION

VP is a useful adjunctive pressor that spares NE requirements in septic burn patients, but its use is not without risks, particularly when VP is combined with sustained moderate to high infusions of NE.

Vasopressin: mechanisms of action on the vasculature in health and in septic shock

BACKGROUND

Vasopressin is essential for cardiovascular homeostasis, acting via the kidney to regulate water resorption, on the vasculature to regulate smooth muscle tone, and as a central neurotransmitter, modulating brainstem autonomic function. Although it is released in response to stress or shock states, a relative deficiency of vasopressin has been found in prolonged vasodilatory shock, such as is seen in severe sepsis. In this circumstance, exogenous vasopressin has marked vasopressor effects, even at doses that would not affect blood pressure in healthy individuals. These two findings provide the rationale for the use of vasopressin in the treatment of septic shock. However, despite considerable research attention, the mechanisms for vasopressin deficiency and hypersensitivity in vasodilatory shock remain unclear.

OBJECTIVE

To summarize vasopressin's synthesis, physiologic roles, and regulation and then review the literature describing its vascular receptors and downstream signaling pathways. A discussion of potential mechanisms underlying vasopressin hypersensitivity in septic shock follows, with reference to relevant clinical, in vivo, and in vitro experimental evidence.

DATA SOURCE

Search of the PubMed database (keywords: vasopressin and receptors and/or sepsis or septic shock) for articles published in English before May 2006 and manual review of article bibliographies.

DATA SYNTHESIS AND CONCLUSIONS

The pathophysiologic mechanism underlying vasopressin hypersensitivity in septic shock is probably multifactorial. It is doubtful that this phenomenon is merely the consequence of replacing a deficiency. Changes in vascular receptors or their signaling and/or interactions between vasopressin, nitric oxide, and adenosine triphosphate-dependent potassium channels are likely to be relevant. Further translational research is required to improve our understanding and direct appropriate educated clinical use of vasopressin.

Noradrenergic regulation of galanin expression in the supraoptic nucleus in the rat hypothalamus. An ex vivo study

Galanin is coexpressed with vasopressin and oxytocin in magnocellular neurons of the rat neuroendocrine hypothalamus. Various physiological stimuli, such as osmotic stimulation or lactation, that affect vasopressin and oxytocin expression and release also modulate galanin expression. Magnocellular neurons are highly innervated by noradrenergic inputs from the brainstem. The noradrenergic system plays a critical excitatory role in the activation of vasopressin-expressing and oxytocin-expressing neurons. Here, we have evaluated the possible regulation of Gal expression by noradrenaline in the magnocellular neurons of supraoptic nucleus in an ex vivo acute model of rat hypothalamic slices. The slices containing the supraoptic nucleus were incubated with 10(-4) M noradrenaline for 1 or 4 hr. The levels of galanin and galanin mRNA were estimated by semiquantitative immunohistochemistry and in situ hybridization, respectively. Our results show that the amount of galanin-immunopositive material in the cell bodies of the magnocellular neurons increased significantly after incubation with noradrenaline compared with control slices at the same time point and that this effect was more pronounced after 4 hr than after 1 hr. In situ hybridization showed that radiolabeling of the supraoptic nucleus with a radioactive galanin probe increased slightly after 1 hr of incubation and increased considerably after 4 hr of incubation with noradrenaline. Our study shows that galanin may be a target in the regulation of the hypothalamic magnocellular-neurohypophysial system by noradrenaline.

Noradrenaline triggers multivesicular release at glutamatergic synapses in the hypothalamus

The origin of large-amplitude miniature EPSCs (mEPSCs) at central synapses remains to be firmly established. Here, we show that at excitatory synapses onto magnocellular neurosecretory cells in the hypothalamus, noradrenaline induces a rapid and robust increase in mEPSC amplitude that requires alpha1-adrenoceptor activation but is impervious to postsynaptic manipulations that block the putative insertion of AMPA receptors. In response to noradrenaline, mEPSCs exhibit a putative multimodal amplitude histogram distribution that is not attributable to random temporal summation, the unveiling of a quiescent synapse, or the release of large vesicles. Large-amplitude mEPSCs are sensitive to a high dose of ryanodine and are associated with an enhanced glutamate cleft concentration. Together, these data are consistent with the hypothesis that large-amplitude mEPSCs result from the synchronous release of multiple vesicles via rapid presynaptic calcium expulsion from intracellular stores.

Effects of combined administration of vasopressin, epinephrine, and norepinephrine during cardiopulmonary resuscitation in pigs*

OBJECTIVE

Synergistic effects of epinephrine and vasopressin may be of benefit during cardiopulmonary resuscitation. However, cerebral perfusion was decreased when epinephrine was combined with vasopressin compared with vasopressin alone. Although a combined infusion of norepinephrine and vasopressin improves hemodynamic variables compared with norepinephrine alone during sepsis, it is unknown whether norepinephrine in addition to vasopressin and epinephrine changes vital organ perfusion during cardiopulmonary resuscitation.

DESIGN

Prospective, randomized animal study.

SETTING

: University hospital research laboratory.

SUBJECTS

Twenty-one domestic pigs.

INTERVENTIONS

After 4 mins of ventricular fibrillation and 3 mins of basic life support, the pigs were randomly assigned to receive either 200 microg/kg epinephrine, 0.4 units/kg vasopressin alone, or 45 microg/kg norepinephrine plus 45 microg/kg epinephrine plus 0.4 units/kg vasopressin before defibrillation.

MEASUREMENTS AND MAIN RESULTS

Organ perfusion was determined by radiolabeled microspheres. Myocardial blood flow (mean +/- sem) before and 90 secs and 5 mins after drug administration was 8 +/- 2, 25 +/- 6, and 7 +/- 1 mL/min/100 g after high-dose epinephrine, 12 +/- 1, 75 +/- 7, and 60 +/- 10 mL/min/100 g after vasopressin, and 9 +/- 2, 95 +/- 26, and 46 +/- 15 mL/min/100 g after vasopressin/epinephrine/norepinephrine, respectively (p < .05 at 90 secs and 5 mins vasopressin vs. epinephrine and vasopressin/epinephrine/norepinephrine vs. epinephrine). At the same time points, cerebral blood flow was 8 +/- 2, 23 +/- 3, and 17 +/- 3 mL/min/100 g after epinephrine, 11 +/- 3, 55 +/- 7, and 52 +/- 7 mL/min/100 g after vasopressin, and 11 +/- 4, 67 +/- 13, and 53 +/- 12 mL/min/100 g after vasopressin/epinephrine/norepinephrine, respectively (p < .05 at 90 secs and 5 mins vasopressin vs. epinephrine and vasopressin/epinephrine/norepinephrine vs. epinephrine). Two of seven animals in the epinephrine group, four of seven animals in the vasopressin/epinephrine/norepinephrine group, and seven of seven animals in the vasopressin group could be successfully resuscitated (p < .05 vasopressin vs. epinephrine).

CONCLUSIONS

Vasopressin with or without epinephrine and norepinephrine resulted in higher myocardial and cerebral perfusion than epinephrine alone, but there was no benefit in adding norepinephrine to vasopressin and epinephrine with regard to cardiac and cerebral blood flow during cardiopulmonary resuscitation.

Einsatz von Vasopressin und Terlipressin bei Sepsis und systemischen Entzündungsreaktionen

Vasopressin and terlipressin are increasingly used as alternative non-adrenergic vasopressors for hemodynamic support of septic patients with arterial hypotension. Despite excellent vasopressive effects, vasopressin analogues may potentially impair macro-hemodynamics, oxygen transport and microvascular blood flow. Due to those unwanted side-effects, vasopressin and terlipressin may potentially compromise organ function and possibly foster the development of multiple organ failure. This review article discusses the results of clinical and experimental studies to judge the effects of vasopressin and terlipressin on microcirculation, oxygen supply, metabolism and organ function in patients with sepsis or systemic inflammatory response syndrome (SIRS). Although vasopressin analogues are emerging as promising alternatives to treat catecholamine-refractory hypotension, there is no evidence that vasopressin receptor agonists improve outcome. To date, vasopressin and terlipressin can, therefore, not be recommended for routine clinical use.

Vasopressin and terlipressin: pharmacology and its clinical relevance

Vasopressin and its analogue, terlipressin, are potent vasopressors that may be useful therapeutic agents in the treatment of cardiac arrest, septic and catecholamine-resistant shock and oesophageal variceal haemorrhage. The aim of this article is to review the physiology and pharmacology of vasopressin and summarise its efficacy and safety in clinical trials and its subsequent therapeutic use. Recent studies indicate that the use of vasopressin during cardiopulmonary resuscitation may improve the survival of patients with asystolic cardiac arrest. Vasopressin deficiency can contribute to refractory shock states associated with sepsis, cardiogenic shock and cardiac arrest. Low doses of vasopressin and terlipressin can restore vasomotor tone in conditions that are resistant to catecholamines, with preservation of renal blood flow and urine output. They are also useful in reducing bleeding and mortality associated with oesophageal variceal haemorrhage. The long-term outcome of the use of these drugs is not known.

Clinical review: Vasopressin and terlipressin in septic shock patients

Vasopressin (antidiuretic hormone) is emerging as a potentially major advance in the treatment of septic shock. Terlipressin (tricyl-lysine-vasopressin) is the synthetic, long-acting analogue of vasopressin, and has comparable pharmacodynamic but different pharmacokinetic properties. Vasopressin mediates vasoconstriction via V₁ receptor activation on vascular smooth muscle. Septic shock first causes a transient early increase in blood vasopressin concentrations; these concentrations subsequently decrease to very low levels as compared with those observed with other causes of hypotension. Infusions of 0.01–0.04 U/min vasopressin in septic shock patients increase plasma vasopressin concentrations. This increase is associated with reduced need for other vasopressors. Vasopressin has been shown to result in greater blood flow diversion from nonvital to vital organ beds compared with adrenaline (epinephrine). Of concern is a constant decrease in cardiac output and oxygen delivery, the consequences of which in terms of development of multiple organ failure are not yet known. Terlipressin (one or two boluses of 1 mg) has similar effects, but this drug has been used in far fewer patients. Large randomized clinical trials should be conducted to establish the utility of these drugs as therapeutic agents in patients with septic shock.

Acute increases in arterial blood pressure do not reduce plasma vasopressin levels stimulated by angiotensin II or hyperosmolality in rats

The present study sought to determine whether an acute increase in arterial blood pressure (ABP) reduces plasma vasopressin (VP) levels stimulated by ANG II or hyperosmolality. During an intravenous infusion of ANG II (100 ng·kg−1·min−1), attenuation of the ANG II-evoked increase in ABP with diazoxide or minoxidil did not further enhance plasma VP levels in rats. When VP secretion was stimulated by an infusion of hypertonic saline, coinfusion of the α-adrenergic agonist phenylephrine (PE) significantly increased ABP but did not reduce plasma VP levels. In fact, plasma VP levels were enhanced. The enhancement of plasma VP levels cannot be explained by a direct stimulatory action of PE, as plasma VP levels of isosmotic rats did not change during a similar infusion of PE. An infusion of endothelin-1 in hyperosmotic rats significantly raised ABP but did not reduce plasma VP levels; rather, VP levels increased as observed with PE. In α-chloralose-anesthetized rats infused with hypertonic saline, inflation of an aortic cuff to increase ABP and stimulate arterial baroreceptors did not reduce plasma VP levels. In each experiment, plasma oxytocin levels paralleled plasma VP levels. Collectively, the present findings suggest that an acute increase in ABP does not inhibit VP secretion.

Arginine vasopressin compromises gut mucosal microcirculation in septic rats

OBJECTIVE

Arginine vasopressin (AVP) is increasingly used in the therapy of septic patients with hypotension. However, its effects on the microvascular networks have not been studied in detail. This study was designed to determine the effects of AVP infusion on the villus microcirculation of the septic rat ileum.

DESIGN

Prospective, placebo-controlled, randomized, single-blinded trial.

SETTING

University research laboratory.

SUBJECTS

Fifteen male Sprague-Dawley rats.

INTERVENTIONS

Twenty-four hours after cecal ligation and perforation to create sepsis (M1), rats (n = 8) received a continuous AVP infusion to increase mean arterial pressure by 20 mm Hg (M2) and 40 mm Hg (M3) from M1. In the control group (n = 7), an equivalent volume of normal saline was infused.

MEASUREMENTS AND MAIN RESULTS

Videomicroscopy was performed on 6-10 villi of ileum mucosa at M1 and was repeated at M2 and M3. Blood was drawn to determine plasma levels of AVP and interleukin-6. At M1, both study groups were hypotensive compared with preseptic data (mean arterial pressure, -25%). The increase in mean arterial pressure was linked to supraphysiologic AVP plasma levels and was accompanied by a decrease in mean mucosal blood flow by 76% at M2 and 81% at M3 (p <.001 vs. control). Red blood cell velocity fell by 45% and 47%, respectively (p <.05 vs. control). Whereas periods of arrested villus blood flow increased from 8.1 +/- 2.6 secs/min to 43.8 +/- 5.2 and 47 +/- 6.2 secs/min at M2 and M3 (p <.001), the diameter of terminal arterioles remained unchanged. In addition, AVP infusion further augmented the sepsis-associated increase in interleukin-6 levels (AVP, 905 +/- 160 vs. control, 638 +/- 55 pg/mL; p =.022).

CONCLUSIONS

This study provides evidence for severe abnormalities in gut mucosal blood flow after AVP infusion in septic rats, accompanied by an augmented inflammatory response to the septic injury. The effects of AVP on microvascular blood flow in this model may be related to AVP activities on larger arterioles (>40 microm), a concomitant reduction in cardiac output, or even both.

[Indications of vasopressin in the management of septic shock]

OBJECTIVE

Vasopressin (antidiuretic hormone) is emerging as a potentially major advancement in the treatment of septic shock. Vasopressin is both a vasopressor and an antidiuretic hormone. It also has haemostatic, gastrointestinal, and thermoregulatory effects. This article reviews the physiology of vasopressin and all the relevant clinical literature on its use in the treatment of septic shock.

DATA SOURCES AND EXTRACTION

Extraction from Pubmed database of French and English articles on the physiology and clinical use of vasopressin. The following key words were selected: vasodilatory shock, vasopressin, septic shock, catecholamines, norepinephrine, renal function, diuresis, mesenteric haemodynamic. The collected articles were reviewed and selected according to their quality and originality.

DATA SYNTHESIS

Vasopressin mediates vasoconstriction via V1-receptor activation on vascular smooth muscle. Septic shock causes first a transient early increase in blood vasopressin concentrations that decreases later to very low concentrations compared to other causes of hypotension. Vasopressin infusion of 0.01-0.04 U min(-1) in septic shock patients increases plasma vasopressin concentrations. This increase is associated with a lesser need for other vasopressors. Vasopressin has been shown to produce greater blood flow diversion from non-vital to vital organ beds than does adrenaline. A large randomized clinical trial should be performed to assess its place as a therapeutic agent of septic shock patient.

Activation by serotonin and noradrenaline of vasopressin and oxytocin expression in the mouse paraventricular and supraoptic nuclei

Noradrenaline and serotonin are known to control arginine-vasopressin (AVP) and oxytocin (OT) secretion in the systemic circulation. The aim of the current study was to investigate whether these monoamines are also able to influence AVP and OT expression in the paraventricular (PVN) and supraoptic nuclei (SON). To test this hypothesis, we used the Tg8 transgenic mice KO for the monoamine oxidase-A gene, which present high levels of noradrenaline and serotonin in the brain. AVP and OT expression were evaluated at peptide and mRNA levels by immunohistochemistry, enzyme immunoassay, and in situ hybridization. Compared with wild type, the amounts of AVP, OT, AVP mRNA, and OT mRNA were increased in the PVN and SON in Tg8 mice. To distinguish the respective contributions of noradrenaline and serotonin to these modifications, we treated Tg8 mice with a synthesis inhibitor of either catecholamines [alpha-methylparatyrosine (alpha-MPT)] or serotonin [parachlorophenylalanine (pCPA)]. Administration of alpha-MPT to Tg8 mice induced a decline in the amounts of AVP, OT, and their mRNA in the PVN and SON. The pCPA treatment in Tg8 mice was also associated with a decrease in OT expression in the PVN and SON and in AVP expression in the PVN, but not in the SON. These results suggest that noradrenaline may activate AVP and OT expression in the PVN and SON. Likewise, serotonin is proposed to stimulate AVP and OT expression in the PVN and only OT expression in the SON.

Neurotransmitter/neuropeptide interactions in the regulation of neurohypophyseal hormone release

Regulation of neurohypophyseal hormone release reflects the convergence of a large number of afferent pathways on the vasopressin (VP)- and oxytocin-producing neurons. These pathways utilize a broad range of neurotransmitters and neuropeptides. In this review, the mechanisms by which this information is coordinated into appropriate physiological responses is discussed with a focus on the responses to agents that are coreleased from A1 catecholamine nerve terminals in the supraoptic nucleus. The A1 pathway transmits hemodynamic information to the vasopressin neurons by releasing several neuroactive agents including ATP, norepinephrine, neuropeptide Y, and substance P. These substances stimulate VP release from explants of the hypothalamo-neurohypophyseal system and certain combinations of these agents elicit potent but selective synergism. Evaluation of the signal cascades elicited by these agents provides insights into mechanisms underlying these synergistic interactions and suggests mechanisms responsible for coordinated responses of the VP neurons to activation of a range of ion-gated ion channel and G-protein-coupled receptors.

Physiology of vasopressin relevant to management of septic shock

Vasopressin is emerging as a rational therapy for the hemodynamic support of septic shock and vasodilatory shock due to systemic inflammatory response syndrome. The goal of this review is to understand the physiology of vasopressin relevant to septic shock in order to maximize its safety and efficacy in clinical trials and in subsequent therapeutic use. Vasopressin is both a vasopressor and an antidiuretic hormone. It also has hemostatic, GI, and thermoregulatory effects, and is an adrenocorticotropic hormone secretagogue. Vasopressin is released from the axonal terminals of magnocellular neurons in the hypothalamus. Vasopressin mediates vasoconstriction via V1-receptor activation on vascular smooth muscle and mediates its antidiuretic effect via V2-receptor activation in the renal collecting duct system. In addition, vasopressin, at low plasma concentrations, mediates vasodilation in coronary, cerebral, and pulmonary arterial circulations. Septic shock causes first a transient early increase in blood vasopressin concentrations that decrease later in septic shock to very low levels compared to other causes of hypotension. Vasopressin infusion of 0.01 to 0.04 U/min in patients with septic shock increases plasma vasopressin levels to those observed in patients with hypotension from other causes, such as cardiogenic shock. Increased vasopressin levels are associated with a lesser need for other vasopressors. Urinary output may increase, and pulmonary vascular resistance may decrease. Infusions of > 0.04 U/min may lead to adverse, likely vasoconstriction-mediated events. Because clinical studies have been relatively small, focused on physiologic end points, and because of potential adverse effects of vasopressin, clinical use of vasopressin should await a randomized controlled trial of its effects on clinical outcomes such as organ failure and mortality.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Purinergic and adrenergic agonists synergize in stimulating vasopressin and oxytocin release

The A1 catecholamine neurons of the caudal ventrolateral medulla transmit hemodynamic information to the vasopressin (VP) neurons in the hypothalamus. These neurons corelease ATP with norepinephrine. Perifused explants of the hypothalamoneurohypophyseal system were used to investigate the role of these substances on VP release. ATP (100 micrometer) increased VP release 1.5-fold (p = 0.027). The response was rapid but unsustained. It was blocked by the P(2) receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS). The alpha(1)-adrenergic agonist phenylephrine (PE; 100 micrometer) also increased VP release by 1.5-fold (p = 0.014). Again, the response was rapid and unsustained. However, simultaneous perifusion of explants with ATP (100 micrometer) and PE (100 micrometer) resulted in a threefold to fourfold increase in VP release, which was sustained for as long as 4 hr. There was a similar synergistic effect of ATP and PE on oxytocin release. Interestingly, the synergistic response was delayed approximately 40 min relative to the response to either agent alone. Several experiments were performed to elucidate the cellular mechanisms of this synergism. The effect was blocked by PPADS, a protein kinase C inhibitor (bisindolylmaleimide I HCl), and actinomycin, an inhibitor of gene transcription. These data suggest that P(2X) receptor activation, PKC-mediated phosphorylation, and gene transcription are required for the synergistic response. The marked synergism of these coreleased agents is probably important to achieve sustained increases in plasma VP in response to prolonged hypotension. These observations may also have broad applications to CNS function, because ATP may be coreleased at noradrenergic synapses throughout the CNS.

Substance P and NPY differentially potentiate ATP and adrenergic stimulated vasopressin and oxytocin release

The supraoptic nuclei are innervated by the A1 neurons of the caudal ventrolateral medulla. Substances colocalized in the A1 terminals include norepinephrine (NE), substance P (SP), ATP, and neuropeptide Y (NPY). ATP, acting at P(2x) receptors, caused rapid and unsustained stimulation of vasopressin (VP) and oxytocin (OT) release from perifused explants of the hypothalamo-neurohypophysial system. SP elicited a concentration-dependent stimulation of VP and OT release that was large and sustained compared with other stimuli. ATP, but not phenylephrine (PE, alpha(1)-adrenergic agonist), augmented the response to SP (1 microM). In contrast, NPY did not alter basal nor ATP-induced VP or OT release, but it did cause sustained potentiation of PE-induced VP and OT release. The Y(1)-agonist, [Leu(31),Pro(34)]-NPY, increased VP and OT release, suggesting that the ineffectiveness of NPY reflects opposing actions at pre- and postsynaptic receptors. However, [Leu(31),Pro(34)]-NPY did not potentiate hormone responses to ATP or PE. The differential responses to these colocalized neurotransmitters and neuropeptides illustrate the range of potential responses that stimulation of this pathway might elicit from supraoptic neurons.