Chronic Naltrexone Therapy Is Associated with Improved Cardiac Function in Volume Overloaded Rats.

PURPOSE: Myocardial opioid receptors were demonstrated in animals and humans and seem to colocalize with membranous and sarcolemmal calcium channels of the excitation-contraction coupling in the left ventricle (LV). Therefore, this study investigated whether blockade of the cardiac opioid system by naltrexone would affect cardiac function and neurohumoral parameters in Wistar rats with volume overload-induced heart failure. METHODS: Volume overload in Wistar rats was induced by an aortocaval fistula (ACF). Left ventricular cardiac opioid receptors were identified by immunohistochemistry and their messenger ribonucleic acid (mRNA) as well as their endogenous ligand mRNA quantified by real-time polymerase chain reaction (RT-PCR). Following continuous delivery of either the opioid receptor antagonist naltrexone or vehicle via minipumps (n = 5 rats each), hemodynamic and humoral parameters were assessed 28 days after ACF induction. Sham-operated animals served as controls. RESULTS: In ACF rats mu-, delta-, and kappa-opioid receptors colocalized with voltage-gated L-type Ca2+ channels in left ventricular cardiomyocytes. Chronic naltrexone treatment of ACF rats reduced central venous pressure (CVP) and left ventricular end-diastolic pressure (LVEDP), and improved systolic and diastolic left ventricular functions. Concomitantly, rat brain natriuretic peptide (rBNP-45) and angiotensin-2 plasma concentrations which were elevated during ACF were significantly diminished following naltrexone treatment. In parallel, chronic naltrexone significantly reduced mu-, delta-, and kappa-opioid receptor mRNA, while it increased the endogenous opioid peptide mRNA compared to controls. CONCLUSION: Opioid receptor blockade by naltrexone leads to improved LV function and decreases in rBNP-45 and angiotensin-2 plasma levels. In parallel, naltrexone resulted in opioid receptor mRNA downregulation and an elevated intrinsic tone of endogenous opioid peptides possibly reflecting a potentially cardiodepressant effect of the cardiac opioid system during volume overload.

Overhydration, underhydration, and total body sodium: A tricky "ménage a trois" in dialysis patients.

Overhydration is a frequent complication in dialysis patients. It has been linked with hypertension, left ventricular hypertrophy, arterial stiffness, atherosclerosis uremic cardiomyopathy, and all-cause mortality or cardiovascular morbidity. In addition, predialysis underhydration is also associated with increased risk of death in ESRD patients. In this context, the optimal evaluation of hydration status is a must. However, this mission is not easy or accurate. In the last 10 years, several new methods have been tested in dialysis patients, particularly bioimpedance and lung ultrasonography. The precise clinical value of these techniques in the daily care of hemodialysis patients is not obvious yet. Sodium is also an important piece of this puzzle. Salt intake and/or removal of sodium during dialysis are essential determinants of optimal hydration status. Recent studies have revealed that salt and water homeostasis is also dependent of tissue sodium storage-increased in hemodialysis patients. However, the significance of increased sodium tissue storage as a cardiovascular risk factor and the relationship between tissue sodium content and hard CV endpoint have not yet been elucidated yet.

Locomotion in young rats with induced brain cellular edema - effects of recombinant human erythropoietin.

OBJECTIVES: Effect of recombinant human erythropoietin (rhEPO) on spontaneous motor activity was tested in young rats after intraperitoneal (i.p.) administration of rhEPO, followed by induction of cellular brain edema (CE). Induced changes in the spontaneous horizontal locomotor activity was studied by open field test (OFT). METHODS: CE was induced by water intoxication (WI) using standard method of fractional hyperhydration accompanied with desmopressin administration. Using the accepted method of OFT average time spent in locomotion (s) was determined. 48 young rats at the age of 25, and 35 days were divided into three groups - controls, rats after WI (OFT followed after 44 hours), and rats administered with rhEPO prior to application WI (OFT after 48 hours). RESULTS: In 35-day-old rats rhEPO administration increased the spontaneous locomotor activity, previously decreased by cellular edema. In 25-day-old rats, rhEPO administration prior to the induced CE, decreased spontaneous locomotor activity. CONCLUSION: Presented results demonstrate the neuroprotective capacity of rhEPO, manifested by elimination of the suppressive influence of CE on the locomotion in 35-day-old rats. In 25-day-old rats the neuroprotective effect was not present. These results confirmed that the 10 day interval in the development may represent a different stage of brain maturation in the relation to the neuroprotective effect of rhEPO.

Cellular brain edema induced by water intoxication in rat experimental model.

OBJECTIVES: This paper presents our own rat model of the cellular brain edema, induced by water intoxication (WI). The basic principle of the model is an osmotic imbalance in the cell membrane followed by an intracellular flow of sodium and simultaneous accumulation of water leading to the subsequent increase of BBB permeability. METHODS: The usefulness of the model was tested in precisely specified conditions whose results were clearly expressed. The procedure determined both how WI induces cellular edema as well as the disturbances caused by cellular edema. RESULTS: The evidence of existing cellular edema with increased BBB permeability was proved by intracellular accumulation of intravital dye with a large molecular size; increased brain-water content was confirmed by using the dry/wet weight method and by the decrease in CT density; the elevated intracranial pressure (ICP) due to the expanding volume was determined by continuous monitoring the ICP; the structural lesions were proved by identification of the myelin disintegration; and the impaired nervous functions was demonstrated by the of open field test method. CONCLUSION: Our experimental model can help the future studies of pathophysiology of cellular brain edema and is suitable for testing neuroprotective agents.

Study of locomotion, rearing and grooming activity after single and/or concomitant lesions of central and peripheral nervous system in rats.

OBJECTIVE: Locomotion, rearing and grooming represent different forms of behaviour and motor activity in rats. In this study, changes in these activities were analysed in relation to impaired function of the nervous system by single and/or concomitant lesions representing an experimental model of the dual diagnosis. METHODS: 32 rats were divided into 4 groups of 8 rats: intact rats, rats with single lesion of peripheral nervous system (PNS) - Marcaine neuropathy, rats with single CNS lesion - cellular brain edema induced by water intoxication, and the concomitant lesions (combination of CNS and PNS lesion in one rat). Water intoxication was performed in a standard way by fractionated hyperhydration. The average time spent by locomotion, rearing and grooming was registered and analyzed using an open field test. RESULTS: All activities of the rats after water intoxication became inhibited due to the generally suppressive effect of brain edema. Lesion of PNS reduced activity in locomotion only, because for rearing and grooming activities, the function of the forelimb is not dominant. Combination of lesions (dual diagnosis) reduced locomotion and rearing activity more than single lesions, and enhances the stressogenic effect, which was manifested by a long periods of grooming. CONCLUSION: Results of our study confirmed the physiological and pathophysiological differences in the movement stereotype between locomotion, rearing and grooming caused by the characteristics and algorithms of the movements, which are inborn to rats - the dominant role of the forelimbs in locomotion, the dominant exploratory activity in rearing, and the precise syntactic movement pattern in grooming.

An experimental model of the "dual diagnosis": Effect of cytotoxic brain edema plus peripheral neuropathy on the spontaneous locomotor activity of rats.

OBJECTIVE: The aim of the study was to find how a simultaneous impairment of the CNS (cellular brain edema induced by water intoxication) and PNS (blockade of the right forelimb brachial plexus by local anesthewtic - Marcaine) affects spontaneous locomotor activity of adult rats. METHODS: Rats were divided into groups of animals without water intoxication (without WI) - A,B,C, and those that were water intoxicated (induction of brain edema - after WI) - D,E,F. Both groups were further divided into intact ones (A,D), animals with PNS lesion (Marcaine) (B,E) and sham-operated animals (C,F). Locomotor activity (LA) of the rats was tested by the open field test. RESULTS: LA of rats with both CNS and PNS impairment (WI + Marcaine) was significantly suppressed compared to the activity of control rats. Comparison of LA of rats with a single lesion - PNS impairment only (Marcaine only), CNS lesion only (WI) to those animals with both lesions (WI + Marcaine) revealed even larger decrease of LA of rats with combined lesions, which represents a model of the dual diagnosis. Also the pattern of behaviour of rats in both sham operated groups was different, which apparently depended on water intoxication. CONCLUSION: The presented results show that the LA of rats with combined lesions is significantly lower compared to the activity of rats with a single lesion in the CNS or PNS. Results also indicate that the already induced endoneurial edema prevents subsequent accumulation of water applied to the intimate vicinity of the peripheral nervous structures.

Neuronal excitability after water intoxication in young rats.

OBJECTIVES: Our previous experiments with animal models revealed that water intoxication induces brain oedema and opens plasma membranes. Present study is aimed to determine whether the standard method of hyperhydration can influence cerebral microenvironment also in young rats. Neuronal functions were tested by standard electrical cortical stimulation. METHODS: Hyperhydration was induced by administration of distilled water (DW) intraperitoneally. Three groups of young rats were used: 12, 25, and 35-day-old. Cortical excitability was tested 19 to 20 hours after DW administration by electrical stimulation of the sensorimotor cortex with intensity necessary to elicit cortical afterdischarges (AD). Water content in the brain was estimated by dry/wet ratio and value of natremia by standard biochemical examination. Control animals of the same age groups were tested in the same way, only they did not receive DW. RESULTS: Brain water content in hyperhydrated animals was smaller than in controls in all studied age groups. Natremia was the same (normal) in both the hyperhydrated and control animals aged 25 days. Excitability of cortical neurons in young hyperhydrated animals was significantly inhibited in comparison to the same age groups of controls. CONCLUSION: Hyperhydration induced in young rats (12, 25, 35-day-old) had different effects than in adults. Absence of hyponatremia, lower water content in the brain and significant inhibition of cortical excitability can be explained on the basis of ontogenetically dependent aquaporine expression (AQP 4) and different activity of ionic membrane transporters.

Assessment of brain compliance using ICP waveform analysis in water intoxication rat model.

Intracranial pressure (ICP) monitoring has been used widely for patients with intracranial hypertension. However, the data of mean ICP do not reflect various brain conditions correctly. Therefore, we performed ICP -waveform analysis to assess brain compliance. Data for ICP -waveform analysis were obtained by stereotactic intraventricle puncture. ICP waveform is expressed as a three-phase wave. Analyzed differential waveforms in a water intoxication model and continuous infusion models were evaluated respectively. In the water intoxication models, the second wave (P2) known to reflect compliance is elevated. ICP waveform analysis will be valuable for the assessment of the pathological condition of the brain.

Risk factors for rhabdomyolysis in self-induced water intoxication (SIWI) patients.

Self-induced water intoxication (SIWI) patients present with various neurological and non-neurological symptoms. However, it is reported that non-neurological manifestations such as rhabdomyolysis are comparatively rare. The mechanism underlying rhabdomyolysis remains controversial. To investigate this further, we evaluated 22 SIWI patients for rhabdomyolysis. We reviewed the records of 22 patients with SIWI and evaluated their clinical characteristics. These patients were divided into the following two groups: Group A with rhabdomyolysis and Group B without it. We compared these groups to study the risk factors underlying the occurrence of rhabdomyolysis. Furthermore, we compared the complications and the duration of hospitalization between the two groups. The maximum serum sodium correction speed per hour, the increase in the serum sodium level in the initial 24 h, and the duration of hospitalization for group A were faster, higher, and longer, respectively, when compared with those in group B. Only group A patients showed complications. The rapid correction of hyponatremia may possibly trigger rhabdomyolysis in SIWI patients.

Two unusual pediatric cases of dilutional hyponatremia.

Dilutional hyponatremia, although not uncommon, is an underestimated problem in the pediatric population. In most cases, it results from excessive hydration or water retention, also described as the so-called water intoxication. One of the most known causes is the use of desmopressin in enuretic children. This drug enhances the free water reabsorption in the renal collecting ducts. The addition of the anticholinergic agent oxybutynin aggravated the condition by causing a dry mouth with excessive thirst and water intake in our first case. Dietary water overconsumption, either voluntary or involuntary, is a phenomenon seen in formula-fed babies. But in our second case, a game involving forced ingestion of large amounts of water had serious consequences including hyponatremia-related coma. An effort should therefore be made to inform caretakers about the risks of these games. These cases, provoked by rather unusual and peculiar causes, illustrate again that electrolytes and especially serum [Na(+)] are key points to be determined in a child with diminished consciousness. Moreover, an accurate history including the intake of medication and dietary information should be made.

[Oxytocin and syndrome of inappropriate secretion of antidiuretic neonatal hormone. Case report of early severe hyponatremia and literature review]. / Oxitocina y sindrome de secreción inapropiada de hormona antidiurética neonatal. Reporte de un caso de hiponatremia severa temprana y revisión de la bibliografia.

This is a clinical case presentation of a full term newborn infant who suffered severe hyponatremia and early seizures, associated with maternal fluid overload with electrolyte free solutions and high doses of oxytocin for labor augmentation. Although this condition has been recognized since the 1960's with isolated reports, this particular case has features that needs further investigation, not only for the unsually severe hyponatremia, but most importantly we think, for the prominent signs of fluid retention, the infant had, that suggest excessive antidiuretic activity probably due to oxytocin. These findings are consistent with syndrome of inappropriate secretion of antidiuretic hormone. Although until now there is no proof that oxytocin by itself produces this syndrome. We think the association is possible in certain clinical circumstances, such as those found in this case. We also, briefly discussed the pathophysiology of perinatal hyponatremia, the neonatal treatment of this condition and the current guidelines for the women in labor. Hyponatremia should not be considered a benign condition, since in the neonate, it may affect brain function.

A new experimental mouse model of water intoxication with sustained increased intracranial pressure and mild hyponatremia without side effects of antidiuretics.

The most used experimental mouse model of hyponatremia and elevated intracranial pressure (ICP) is intraperitoneal injection of water in combination with antidiuretics. This model of water intoxication (WI) results in extreme pathological changes and death within 1 h. To improve preclinical studies of the pathophysiology of elevated ICP, we characterized diuresis, cardiovascular parameters, blood ionogram and effects of antidiuretics in this model. We subsequently developed a new mouse model with mild hyponatremia and sustained increased ICP. To investigate the classical protocol (severe WI), C57BL/6mice were anesthetized and received an intraperitoneal injection of 20% body weight of MilliQ water with or without 0.4 µg·kg-1 desmopressin acetate (dDAVP). Corresponding Sham groups were also studied. In the new WI protocol (mild WI), 10% body weight of a solution containing 6.5 mM NaHCO3, 1.125 mM KCl and 29.75 mM NaCl was intraperitoneally injected. By severe WI, ICP and mean arterial pressure increased until brain stem herniation occurred (23 ± 3 min after injection). The cardiovascular effects were accelerated by dDAVP. Severe WI induced a halt to urine production irrespective of the use of dDAVP. Following the new mild WI protocol, ICP also increased but was sustained at a pathologically high level without inducing herniation. Mean arterial pressure and urine production were not affected during mild WI. In conclusion, the new mild WI protocol is a superior experimental model to study the pathophysiological effects of elevated ICP induced by water intoxication.

Proper Hydration During Ultra-endurance Activities.

The health and performance of ultra-endurance athletes is dependent on avoidance of performance limiting hypohydration while also avoiding the potentially fatal consequences of exercise-associated hyponatremia due to overhydration. In this work, key factors related to maintaining proper hydration during ultra-endurance activities are discussed. In general, proper hydration need not be complicated and has been well demonstrated to be achieved by simply drinking to thirst and consuming a typical race diet during ultra-endurance events without need for supplemental sodium. As body mass is lost from oxidation of stored fuel, and water supporting the intravascular volume is generated from endogenous fuel oxidation and released with glycogen oxidation, the commonly promoted hydration guidelines of avoiding body mass losses of >2% can result in overhydration during ultra-endurance activities. Thus, some body mass loss should occur during prolonged exercise, and appropriate hydration can be maintained by drinking to the dictates of thirst.

Forelimb Movement Disorder in Rat Experimental Model.

Study of motor activity is an important part of the experimental models of neural disorders of rats. It is used to study effects of the CNS impairment, however studies on the peripheral nervous system lesions are much less frequent. The aim of the study was to extend the spectrum of experimental models of anterior limb movement disorders in rats by blockade of the right anterior limb brachial plexus with the local anesthetic Marcaine (Ma), or with aqua for injection administered into the same location (Aq) (with control intact group C). Two other groups with anterior limb movement disorders underwent induction of cellular brain edema by water intoxication (MaWI and AqWI). Results showed a lower spontaneous motor activity of animals in all experimental groups versus controls, and lower spontaneous motor activity of animals in the MaWI group compared to other experimental groups in all categories. There was no difference in spontaneous activity between the groups Ma, Aq and AqWI. Our study indicates that alterations of spontaneous motor activity may result from the impaired forelimb motor activity induced by the anesthetic effect of Marcaine, by the volumetric effect of water, as a result of induced brain edema, or due to combination of these individual effects.

Of Mice and Men-The Physiology, Psychology, and Pathology of Overhydration.

The detrimental effects of dehydration, to both mental and physical health, are well-described. The potential adverse consequences of overhydration, however, are less understood. The difficulty for most humans to routinely ingest ≥2 liters (L)-or "eight glasses"-of water per day highlights the likely presence of an inhibitory neural circuit which limits the deleterious consequences of overdrinking in mammals but can be consciously overridden in humans. This review summarizes the existing data obtained from both animal (mostly rodent) and human studies regarding the physiology, psychology, and pathology of overhydration. The physiology section will highlight the molecular strength and significance of aquaporin-2 (AQP2) water channel downregulation, in response to chronic anti-diuretic hormone suppression. Absence of the anti-diuretic hormone, arginine vasopressin (AVP), facilitates copious free water urinary excretion (polyuria) in equal volumes to polydipsia to maintain plasma tonicity within normal physiological limits. The psychology section will highlight reasons why humans and rodents may volitionally overdrink, likely in response to anxiety or social isolation whereas polydipsia triggers mesolimbic reward pathways. Lastly, the potential acute (water intoxication) and chronic (urinary bladder distension, ureter dilation and hydronephrosis) pathologies associated with overhydration will be examined largely from the perspective of human case reports and early animal trials.

Cytotoxic brain edema induced by water intoxication and vasogenic brain edema induced by osmotic BBB disruption lead to distinct pattern of ICP elevation during telemetric monitoring in freely moving rats.

OBJECTIVES: A novel method of long-term telemetric monitoring of mean arterial pressure (MAP) and intracranial pressure (ICP) for the determination of current cerebral perfusion pressure (CPP) and the time course of ICP in freely moving rats under physiological conditions and with increased ICP due to the induced cerebral edema were studied. METHODS: The brain edema, that caused volume enlargement and ICP elevation was achieved in entirely experimental conditions without any parallel pathological process. Vasogenic/extracellular edema was induced by osmotic blood-brain barrier disruption (BBBd) and for induction of cytotoxic/intracellular edema the water intoxication model (WI) was used. RESULTS: The results showed significantly elevated values of ICP both in conditions of osmotic blood-brain barrier disruption (BBBd model) and cytotoxic/intracellular edema (WI model) compared to intact rats. The average values of ICP were significantly higher in WI model compared to osmotic BBBd model. Distinct pattern of elevated ICP, related to the selected way of experimental brain edema induction, was found. In the experimental model of osmotic BBB disruption, the elevation of ICP started earlier but was of very short duration. In WI model the elevation of ICP was present during the whole period of monitoring. CONCLUSION: Our results indicate that purely experimental models of brain edema (WI, BBBd) without any parallel pathological process can compromise the basic brain homeostatic activity.

Oculohypotensive effects of foeniculum vulgare in experimental models of glaucoma.

PURPOSE: Evaluation of oculohypotensive activity of single drop application of aqueous extract of Foeniculum vulgare in experimental models of glaucoma. METHODS: The evaluation of oculohypotensive activity of Foeniculum vulgare was done in rabbits with normal intraocular pressure (IOP) and with experimentally elevated IOP. The experimental increase in IOP was achieved using water loading and steroid induced glaucoma models. RESULTS: The aqueous seed extract of Foeniculum vulgare exhibited 17.49, 21.16 and 22.03% reduction of intraocular pressure (IOP) in normotensive rabbits at 0.3%, 0.6% and 1.2% (w/v) concentrations respectively. The 0.6% concentration was further evaluated in acute and chronic models of glaucoma. A maximum mean difference of 31.20% was observed between vehicle treated and extract treated eyes in water loading model while a maximum mean IOP lowering of 31.29% was observed in steroid induced model of glaucoma. CONCLUSIONS: The aqueous extract of Foeniculum vulgare possesses significant oculohypotensive activity, which was found to be comparable to that of timolol. Further investigations into the mechanism of action, possible toxicity and human clinical trials are warranted before the Foeniculum vulgare finds place in the arsenal of antiglaucoma drugs prescribed by physicians.

Three distinct roles of aquaporin-4 in brain function revealed by knockout mice.

Aquaporin-4 (AQP4) is expressed in astrocytes throughout the central nervous system, particularly at the blood-brain and brain-cerebrospinal fluid barriers. Phenotype analysis of transgenic mice lacking AQP4 has provided compelling evidence for involvement of AQP4 in cerebral water balance, astrocyte migration, and neural signal transduction. AQP4-null mice have reduced brain swelling and improved neurological outcome in models of (cellular) cytotoxic cerebral edema including water intoxication, focal cerebral ischemia, and bacterial meningitis. However, brain swelling and clinical outcome are worse in AQP4-null mice in models of vasogenic (fluid leak) edema including cortical freeze-injury, brain tumor, brain abscess and hydrocephalus, probably due to impaired AQP4-dependent brain water clearance. AQP4 deficiency or knock-down slows astrocyte migration in response to a chemotactic stimulus in vitro, and AQP4 deletion impairs glial scar progression following injury in vivo. AQP4-null mice also manifest reduced sound- and light-evoked potentials, and increased threshold and prolonged duration of induced seizures. Impaired K+ reuptake by astrocytes in AQP4 deficiency may account for the neural signal transduction phenotype. Based on these findings, we propose modulation of AQP4 expression or function as a novel therapeutic strategy for a variety of cerebral disorders including stroke, tumor, infection, hydrocephalus, epilepsy, and traumatic brain injury.

Role of plasma vasopressin in impaired water excretion of glucocorticoid deficiency.

In the present study, the effect of selective glucocorticoid deficiency on renal water excretion was investigated in conscious, trained, adrenalectomized dogs. The animals were studied before and after a water load while on replacement therapy of desoxycorticosterone acetate, 5 mg/day, and dexamethasone, 0.8 mg/day (group I), and while off dexamethasone for 5-9 days (group II). Before the water load the weight, inulin space, cardiac output, blood pressure, glomerular filtration rate, renal blood flow, plasma osmolality, and plasma antidiuretic hormone measured by radioimmunoassay were similar in both groups I and II. However, after a 40 ml/kg water load a marked impairment in renal water excretion in the glucocorticoid deficient dogs became apparent. Maximal free water clearance was -0.046+/-0.16 vs. 6.51+/-0.72 ml/min (P < 0.001) and minimal urinary osmolality was 425+/-56 vs. 82+/-3.5 mosmol/kg H(2)O (P < 0.001) in group II as compared to group I. Plasma antidiuretic hormone was maximally suppressed during the water load in group I to 0.34+/-0.08 pg/ml but remained elevated at 9.18+/-1.79 pg/ml (P < 0.005) in group II. This nonsuppressibility of plasma antidiuretic hormone during water loading in group II was associated with a significant tachycardia of 145+/-6 vs. 87+/-6 beats/min (P < 0.001) in group I and a significantly lower stroke volume of 27+/-0 vs. 59+/-0.5 ml/beat (P < 0.001). In conclusion, our results implicate a persistent secretion of antidiuretic hormone as an important factor in the impaired water excretion of glucocorticoid deficiency. A deleterious effect of glucocorticoid deficiency on cardiac function was observed and this hemodynamic alteration could be involved in initiating a nonosmolar, baroreceptor-mediated release of vasopressin.

Neuroendocrine responses to a cold pressor stimulus in polydipsic hyponatremic and in matched schizophrenic patients.

Schizophrenia, many believe, reflects an enhanced vulnerability to psychological stress. Controlled exposure to stressors, however, has produced inconclusive results, particularly with regards to neurohormones. Some of the variability may be attributable to the nature and psychological significance of the stimulus and failure to control physiologic confounds. In addition, it is possible that the heterogeneity of schizophrenia is an important factor. In a carefully designed study and in a controlled setting, we measured the neuroendocrine response of eight polydipsic hyponatremic (PHS), seven polydipsic normonatremic (PNS), and nine nonpolydipsic normonatremic (NNS) (ie normal water balance) schizophrenic in-patients as well as 12 healthy controls (HC) to two different stressors: one of which appears to influence neuroendocrine secretion through its psychological (cold pressor) and the other (upright posture) through its systemic actions. Subjects in the three psychiatric groups were stabilized and acclimated to the research setting, and all received saline to normalize plasma osmolality. Following the cold pressor, plasma adrenocorticotropin and cortisol levels showed a more prolonged rise in PHS patients relative to PNS patients. NNS patients, in contrast, exhibited blunted responses relative to both of the polydipsic groups and the HC. Peak vasopressin responses were also greater in PHS and blunted in NNS patients. Responses to the postural stimulus were similar across patient groups. These findings provide a mechanism for life threatening water intoxication in schizophrenia; help to reconcile conflicting findings of stress responsiveness in schizophrenia; and potentially identify a discrete patient subset with enhanced vulnerability to psychological stress.