Neuroendocrine regulation of salt and water metabolism

Animal models for diabetes insipidus

The hormone arginine vasopressin (AVP) is a nonapeptide synthesized by hypothalamic magnocellular nuclei and secreted from the posterior pituitary into the bloodstream. It binds to AVP receptor 2 in the kidney to promote the insertion of aquaporin channels (AQP2) and antidiuretic responses. AVP secretion deficits produce central diabetes insipidus (CDI), while renal insensitivity to the antidiuretic effect of AVP causes nephrogenic diabetes insipidus (NDI). Hereditary and acquired forms of CDI and NDI generate hypotonic polyuria, polydipsia, hyperosmolality, and hypernatremia. The AVP mutant (Brattleboro) rat is the principal animal model of hereditary CDI, while neurohypophysectomy, pituitary stalk compression, hypophysectomy, and mediobasal hypothalamic lesions produce acquired CDI. In animals, hereditary NDI is mainly caused by mutations in AVP2R or AQP2 genes, while acquired NDI is most frequently induced by lithium. We report here on the determinants of the intake and excretion of water and mineral salts and on the different types of DI in humans. We then describe the hydromineral characteristics of these animal models and the responses observed after administration of hypertonic NaCl or when they are fed with low-sodium diets. Finally, we report on the effects of drugs such as AVP analogues and/or oxytocin, another neuropeptide that increases sodium excretion in animal models and humans with CDI, and sildenafil, a compound that increases the expression and function of AQP2 channels in animal models and humans with NDI.

Effects of oxytocin administration on the hydromineral balance of median eminence-lesioned rats

In the clinical setting, acute injuries in hypothalamic mediobasal regions, along with polydipsia and polyuria, have been observed in patients with cerebral salt wasting (CSW). CSW is also characterised by hypovolaemia and hyponatraemia as a result of an early increase in natriuretic peptide activity. Salt and additional amounts of fluid are the main treatment for this disorder. Similarly, experimental lesions to these brain regions, which include the median eminence (ME), produce a well-documented neurological model of polydipsia and polyuria in rats, which is preceded by an early sodium excretion of unknown cause. In the present study, oxytocin (OT) was used to increase the renal sodium loss and prolong the hydroelectrolyte abnormalities of ME-lesioned animals during the first few hours post-surgery. The objective was to determine whether OT-treated ME-lesioned animals increase their sodium appetite and water intake to restore the volume and composition of extracellular body fluid. Electrolytic lesion of the ME increased water intake, urinary volume and sodium excretion of food-deprived rats and also decreased urine osmolality and estimated plasma sodium concentration. OT administration at 8 hours post-surgery reduced water intake, urine output and plasma sodium concentration and also increased urine osmolality and urine sodium excretion between 8 and 24 hours post-lesion. From 24 to 30 hours, more water and hypertonic NaCl was consumed by OT-treated ME-lesioned rats than by physiological saline-treated-ME-lesioned animals. Food availability from 30 to 48 hours reduced the intake of hypertonic saline solution by ME/OT animals, which increased their water and food intake during this period. OT administration therefore appears to enhance the natriuretic effect of ME lesion, producing hydroelectrolyte changes that reduce the water intake of food-deprived animals. Conversely, the presence of hypertonic NaCl increases the fluid intake of these animals, possibly as a result of the plasma sodium depletion and hypovolaemic states previously generated. Finally, the subsequent increase in food intake by ME/OT animals reduces their need for hypertonic NaCl but not water, possibly in response to osmotic thirst. These results are discussed in relation to a possible transient activation of the ME with the consequent secretion of natriuretic peptides stored in terminal swellings, which would be augmented by OT administration. Electrolytic lesion of the ME may therefore represent a useful neurobiological model of CSW.

Long-term effects of intracerebroventricular insulin microinjection on renal sodium handling and arterial blood pressure in rats

The role of the central nervous system (CNS) in the control of hydrosaline homeostasis has been strikingly demonstrated by several studies. Our laboratory recently showed that centrally administered insulin produced a dose-related increase in the urinary output of sodium, which was abolished by bilateral renal denervation, nitric oxide synthase inhibition and cerebroventricular streptozotocin administration in rats. Recent studies have shown that hyperinsulinemia induces subtle derangements of intracellular insulin-insulin receptor trafficking and insulin metabolism, which are associated with an impairment of insulin signaling. The long-term effect of high insulin levels on the periventricular region could alter insulin signaling, which in turn, may modify the central natriuretic and cardiovascular effects of this peptide. In order to evaluate this hypothesis, we investigated the effects of 7-day i.c.v. insulin administration on tubular handling and blood pressure in conscious, unrestrained rats and their controls, randomly assigned to one of two separate groups: (a) i.c.v. 0.15M NaCl-injected (n=7) and (b) i.c.v. 126.0 ng insulin-injected rats (n=7). In the current study, there were no significant differences between the blood pressure, daily tap water intake and serum sodium, potassium, lithium and creatinine levels in control i.c.v. 0.15M NaCl-injected rats, compared with the insulin-treated group. Conversely, there was a significant decrease in the daily solid rat chow intake (Co: 16.4+/-3.5 g vs. Ins: 10.3+/-2.6g, P=0.003) in 7-day long-term insulin-treated rats, compared with the control group. We confirmed that centrally administered insulin produced a substantial increase in the urinary output of Na+, Li+ and K+, and that the response was significantly enhanced in long-term i.c.v. insulin pre-treated animals, when compared with controls (fractional sodium excretion (FE(Na)) from basal: 0.047+/-0.18% to Ins-treated: 0.111+/-0.035%, P=0.001). Additionally, we demonstrated that insulin-induced natriuresis occurred by increasing fractional proximal (FEP(Na)) from basal (16.8+/-2.6% to Ins-treated: 26.7+/-2.8%, P=0.001) and post-proximal sodium excretion (FEPP(Na)) from basal (0.37+/-0.03% to Ins-treated: 0.42+/-0.05%, P=0.043), despite a decreased Na(+) filtered load and rat food intake. The current data suggest that centrally injected insulin maintain its CNS ability to amplify neuronal hypotensive and natriuretic pathways that counteract the known peripheral antinatriuretic effects of insulin.

Role of the serotoninergic system in the sodium appetite control

The present article reviews the role of the serotoninergic system in the regulation of the sodium appetite. Data from the peripheral and icv administration of serotoninergic (5-HTergic) agents showed the participation of 5-HT2/3 receptors in the modulation of sodium appetite. These observations were extended with the studies carried out after brain serotonin depletion, lesions of DRN and during blockade of 5-HT2A/2C receptors in lateral parabrachial nucleus (LPBN). Brain serotonin depletion and lesions of DRN increased the sodium appetite response, in basal conditions, after sodium depletion and hypovolemia or after beta-adrenergic stimulation as well. These observations raised the hypothesis that the suppression of ascending pathways from the DRN, possibly, 5-HTergic fibers, modifies the angiotensinergic or sodium sensing mechanisms of the subfornical organ involved in the control of the sodium appetite. 5-HTergic blockade in LPBN induced to similar results, particularly those regarded to the natriorexigenic response evoked by volume depletion or increase of the hypertonic saline ingestion induced by brain angiotensinergic stimulation. In conclusion, many evidences lead to acceptation of an integrated participation resulting of an interaction, between DRN and LPBN, for the sodium appetite control.

Prolactin/Leptin interactions in the control of food intake in rats

Recent evidence suggests that the peptide hormone prolactin (PRL) modulates energy balance through a number of mechanisms, including acting in the brain to increase food intake. In the current studies, we first demonstrated that chronic infusions of PRL into the lateral ventricles increased food intake in cycling rats without disrupting estrous cyclicity. In subsequent experiments the hypothesis that at least part of PRL's ability to increase food intake resulted from PRL-induced leptin resistance was tested. Female rats given chronic infusions of PRL (5 microg/h) into the cerebral ventricles for 10 d did not show a reduction in food intake or body weight after a central injection of 4 microg murine leptin, whereas the expected reduction in both of these parameters was seen in vehicle-infused rats. Leptin injections were without effect on these parameters, whether they were administered to free feeding PRL-infused rats or after 24-h food deprivation. This lack of a behavioral response to leptin was accompanied by an attenuation in Fos induction and phosphorylation of signal transducer and activator of transcription 3 after leptin administration in PRL-infused rats in both the ventromedial hypothalamus and paraventricular hypothalamic nucleus.

Prolactin and the hyperphagia of lactation

The nutritional needs of developing young place a large energetic demand on lactating females. In this paper some of the mechanisms through which lactating rats meet these demands are described. Emphasis is placed on the mechanisms that drive increased food intake in suckled rats that are independent of milk delivery and negative energy balance. Evidence is presented to suggest that prolactin (PRL), released from the pituitary in response to suckling stimulation, acts centrally to stimulate food intake during lactation. Brain areas in which PRL may act to stimulate food intake as well as its interactions with other hormones and neuropeptides involved in energy balance are discussed.

Altered renal sodium handling in spontaneously hypertensive rats (SHR) after hypertonic saline intracerebroventricular injection: role of renal nerves

The mechanism by which blood pressure rises in the SHR strain remains to be elucidated. Also, there is a surprising lack of experimental data on the natriuretic mechanisms induced by intracerebroventricular (ICV) injection of hyperosmotic saline (HoS) in SHR. In normotensive animals ICV injection of HoS causes coordinated responses including natriuresis and inhibition of renal sympathetic nerve activity. In the present study, we hypothesized that presumable blunting of the sympathoinhibitory response to centrally injected HoS may contribute to a lack of suppression of efferent renal nerve outflow in SHR. To test this hypothesis, the present study evaluates the influence of renal denervation after central HoS injection at increasing concentration on urinary sodium handling in SHR compared with age-matched normotensive WKy rats. The study confirmed previous data showing pronounced natriuretic response to centrally HoS stimuli but also demonstrated that the creatinine clearance (C(Cr)) and fractional sodium excretion responses diminished as graded NaCl concentrations were increased in WKy rats but not in SHR. In SHR, increased FE(Na) obtained by central administration of 0.90 M NaCl was produced by increases in proximal (FEP(Na)) and post-proximal fractional urinary sodium rejection without changes in C(Cr), indicating a direct tubular effect. Renal denervation caused significant antinatriuresis by decreased C(Cr) and increased FEP(Na) reabsorption in WKy but not in SHR. This study suggests that natriuresis observed only after higher centrally HoS stimuli with a rightward shift of dose-response curve provides evidence of a down-regulation of target organ responsiveness of periventricular areas of genetic hypertensive rats.

Early altered renal sodium handling determined by lithium clearance in spontaneously hypertensive rats (SHR): Role of renal nerves

The mechanism by which blood pressure rises in the SHR strain remains to be elucidated. Since the long-term changes in renal sodium tubule handling associated with genetic hypertension have not been examined in detail, we hypothesized that SHR hypertension development may result from sustained renal sympathetic nerve overactivity and consequently decreased urinary sodium excretion. To test this hypothesis, we assessed renal sodium handling and cumulative sodium balance for 10 consecutive weeks in unanesthetized renal-denervated SHR, performed prior to the start of the entire 10-week metabolic studies, and their age-matched normotensive and hypertensive controls. The present investigation shows that SHR excreted less sodium than Wistar-Kyoto (WKy) rats during the initial 3-week observation period (p <0.05). This tendency was reversed when SHR were 10-wk old. Fractional urinary sodium excretion (FENa+) was significantly lower in 3 and 6-wk-old SHR when compared with the WKy age-matched group, as follows: SHR3-wk-old: 0.33 +/- 0.09% and WKy3-wk-old: 0.75 +/- 0.1% (P <0.05); SHR(6-wk-old): 0.52 +/- 0.12% and WKy6-wk-old: 0.83 +/- 0.11%. The decreased FENa+ in young SHR was accompanied by a significant increase in proximal sodium reabsorption (FEPNa+) compared with the normotensive age-matched control group (P <0.01). This increase occurred despite unchanged creatinine clearance (CCr) and fractional post-proximal sodium excretion (FEPPNa+)in all groups studied. The decreased urinary sodium excretion response in SHR up to the age of 6 weeks was significantly eradicated by bilateral renal denervation of SHR3-wk-old: 0.33 +/- 0.09% and SHR6-wk-old: 0.52 +/- 0.12% to DxSHR 3-wk-old: 1.02 +/- 0.2% and DxSHR 6-wk-old: 0.94 +/- 0.2% (P <0.01), in renal denervated rats. The current data suggest that neural pathways may play an instrumental role on renal sodium reabsorption as result of sustained sympathetic nervous system overexcitability.

Acute intracerebroventricular insulin microinjection after nitric oxide synthase inhibition of renal sodium handling in rats

The role of the central nervous system (CNS) in the control of hydrosaline homeostasis has been strikingly demonstrated by several studies. Recent and growing evidence suggests that insulin or a nonapeptide-derived from the C-terminus of the insulin beta-chain may influence many brain functions. However, there is little information on the insulin-activated neural pathways regulating urinary sodium excretion. Also, we examined the influence of nitric oxide synthase activity by chronic oral administration of N(omega)-nitro-l-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthesis, after previous i.c.v. administration of insulin to unanesthetized, unrestrained rats that were randomly assigned to one of seven separated groups: (a) i.c.v. 0.15 M NaCl-injected (n = 11) and i.c.v. 126 ng (n = 11) insulin-injected rats; (b) i.c.v. insulin-injected in systemic L-NAME-treated (n = 10) and vehicle-treated insulin-injected rats (n = 10); and (c) subcutaneously (SC) insulin-injected rats (n = 5). We showed that centrally administered insulin produced increase in the urinary output of sodium (from 0.15 M NaCl: 855.6 +/- 85.1 Delta%.min(-1) to 126 ng insulin: 2055 +/- 310.6 Delta%.min(-1)) and potassium (126 ng: from 0.15 M NaCl: 460.4 +/- 100 Delta%.min(-1) to 126 ng insulin: 669 +/- 60.8 Delta%.min(-1)). The urinary sodium excretion response to i.c.v. 126 ng insulin microinjection was significantly abolished by previous systemic treatment of animals with 15 mg/kg/day L-NAME (from vehicle + 126 ng insulin: 1935 +/- 258.3 Delta%. min(-1) to L-NAME + 126 ng insulin: 582.3 +/- 69.6 Delta%. min(-1)). In addition, we showed that insulin-induced natriuresis occurred by increasing post-proximal tubule sodium rejection (FEPP(Na)), despite an unchanged glomerular filtration rate (C(Cr)). The current data suggests the novel concept that CNS NO-dependent neural pathways may play an instrumental role on efferent insulin-sensitive nerve activity from periventricular region. Speculatively, it seems interesting to suggest that perhaps one of the efferent signals triggered by insulin in the CNS may be nitrergic in nature, and that defects in this efferent signal could result in insulin central resistance, inability of renal tubules to handle the hydro electrolyte balance and hypertension.

Effects of intracerebroventricular insulin microinjection on renal sodium handling in kidney-denervated rats

The role of the central nervous system (CNS) in the control of hydrosaline homeostasis has been strikingly demonstrated by several studies. Growing evidence suggests that insulin may exert an influence in the modulation of many brain functions. However, there are no available data examining the CNS effect of insulin injection on renal sodium handling. Also, to examine the influence of renal nerve activity during i.c.v. administration of insulin, unanesthetized, unrestrained rats were randomly assigned to one of nine separated groups: (a) sham-operated i.c.v. 0.15 M NaCl-injected (Co, pooled data, n = 37) and sham-operated i.c.v. 0.42 ng. microl(-1) (n = 12), 4.2 ng.microl(-1) (n = 10) and 42.0 ng.microl(-1) (n = 11) insulin-injected rats (In); (b) renal-denervated i.c.v. 0.15 M NaCl (Co(Dx), n = 5), and insulin-injected rats (In(Dx), n = 5); and (c) subcutaneously insulin-injected rats (SC, n = 5). We showed that centrally administered insulin produced dose-related increased urinary output of sodium [Co: 855 +/- 85 Delta% min, 0.42 ng.microl(-1) In: 1189 +/- 308 Delta% min, 4.2 ng.microl(-1) In: 1461 +/- 594 Delta% min (p = 0.048), and 42.0 ng.microl(-1) In: 2055 +/- 411 Delta% min (p = 0.0001)], and dose-independently increased potassium excretion [Co: 460 +/- 28 Delta% min, 0.42 ng.microl(-1) In: 649 +/- 100 Delta% min (p = 0.016), 4.2 ng.microl(-1) In: 671 +/- 175 Delta% min (p = 0.003), and 42.0 ng.microl(-1) In: 669 +/- 70 Delta% min (p = 0.002)] compared to control. The urinary sodium excretion response to i.c.v. 42 ng.microl(-1) insulin injections were abolished by bilateral renal denervation. In addition, we showed that insulin-induced natriuresis occurred by increasing postproximal tubule sodium rejection (FEPP(Na)), and changed glomerular filtration rate (C(Cr)) at 42.0 ng.microl(-1) (p = 0.023) i.c.v. insulin microinjection but not at smaller insulin dose. The current data suggests that a blunted efferent insulin-sensitive nerve activity from periventricular region may contribute to the inability of renal tubules to handle the hydroelectrolyte balance.

Polyethylene glycol-induced hypovolemia affects the expression of MCH mRNA, but not dynorphin or secretogranin II mRNAs, in the rat lateral hypothalamus

Two prominent neuron populations of the rat lateral hypothalamus express genes encoding respectively the prepromelanin-concentrating hormone (MCH) or dynorphin (DYN) and secretogranin II (SGII). Their roles remain hypothetical in mammals. In the present study, we examined the changes in MCH, DYN and SGII gene expression in dehydrated rats compared to controls. Dehydration was obtained by subcutaneous injection of polyethylene glycol (PEG) resulting in a large reduction of the extracellular fluid volume. Using competitive semi-quantitative RT-PCR and in situ hybridization methods, PEG-injected animals showed a significant increase of MCH mRNA level but no variation of DYN and SGII mRNA levels. These results confirm previous observations suggesting that intra- and extracellular dehydration challenges affect different regulation circuits; they indicate that both neuron populations could be involved in the maintenance of body fluid homeostasis, directly, or indirectly, as integrators of various information leading to goal-oriented behaviour.

Interaction between NO and oxytocin: influence on LHRH release

Nitric oxide synthase (NOS)-containing neurons have been localized in various parts of the CNS. These neurons occur in the hypothalamus, mostly in the paraventricular and supraoptic nuclei and their axons project to the neural lobe of the pituitary gland. We have found that nitric oxide (NO) controls luteinizing hormone-releasing hormone (LHRH) release from the hypothalamus acting as a signal transducer in norepinephrine (NE)-induced LHRH release. LHRH not only releases LH from the pituitary but also induces sexual behavior. On the other hand, it is known that oxytocin also stimulates mating behavior and there is some evidence that oxytocin can increase NE release. Therefore, it occurred to us that oxytocin may also stimulate LHRH release via NE and NO. To test this hypothesis, we incubated medial basal hypothalamic (MBH) explants from adult male rats in vitro. Following a preincubation period of 30 min, MBH fragments were incubated in Krebs-Ringer bicarbonate buffer in the presence of various concentrations of oxytocin. Oxytocin released LHRH at concentrations ranging from 0.1 nM to 1 microM with a maximal stimulatory effect (P < 0.001) at 0.1 microM, but with no stimulatory effect at 10 microM. That these effects were mediated by NO was shown by the fact that incubation of the tissues with NG-monomethyl-L-arginine (NMMA), a competitive inhibitor of NOS, blocked the stimulatory effects. Furthermore, the release of LHRH by oxytocin was also blocked by prazocin, an alpha 1-adrenergic receptor antagonist, indicating that NE mediated this effect. Oxytocin at the same concentrations also increased the activity of NOS (P < 0.01) as measured by the conversion of [14C]arginine to citrulline, which is produced in equimolar amounts with NO by the action of NOS. The release of LHRH induced by oxytocin was also accompanied by a significant (P < 0.02) increase in the release of prostaglandin E2 (PGE2), a mediator of LHRH release that is released by NO. On the other hand, incubation of neural lobes with various concentrations of sodium nitroprusside (NP) (300 or 600 microM), a releaser of NO, revealed that NO acts to suppress (P < 0.01) the release of oxytocin. Therefore, our results indicate that oxytocin releases LHRH by stimulating NOS via NE, resulting in an increased release of NO, which increases PGE2 release that in turn induces LHRH release. Furthermore, the released NO can act back on oxytocinergic terminals to suppress the release of oxytocin in an ultrashort-loop negative feedback.