Effects of central oxytocin receptor blockade on water and saline intake, mean arterial pressure, and c-Fos expression in rats

Deschloroclozapine exhibits an exquisite agonistic effect at lower concentration compared to clozapine-N-oxide in hM3Dq expressing chemogenetically modified rats

Introduction

Within the realm of chemogenetics, a particular form of agonists targeting designer receptors exclusively activated by designer drugs (DREADDs) has emerged. Deschloroclozapine (DCZ), a recently introduced DREADDs agonist, demonstrates remarkable potency in activating targeted neurons at a lower dosage compared to clozapine-N-oxide (CNO).

Methods

We conducted a comparative analysis of the effects of subcutaneously administered CNO (1 mg/kg) and DCZ (0.1 mg/kg) in our transgenic rats expressing hM3Dq and mCherry exclusively in oxytocin (OXT) neurons.

Results and Discussion

Notably, DCZ exhibited a swift and robust elevation of serum OXT, surpassing the effects of CNO, with a significant increase in the area under the curve (AUC) up to 3 hours post-administration. Comprehensive assessment of brain neuronal activity, using Fos as an indicator, revealed comparable effects between CNO and DCZ. Additionally, in a neuropathic pain model, both CNO and DCZ increased the mechanical nociceptive and thermal thresholds; however, the DCZ-treated group exhibited a significantly accelerated onset of the effects, aligning harmoniously with the observed alterations in serum OXT concentration following DCZ administration. These findings emphasize the remarkable efficacy of DCZ in rats, suggesting its equivalent or potentially superior performance to CNO at considerably lower dosages, thus positioning it as a promising contender among DREADDs agonists.

Involvement of oxytocin receptor deficiency in psychiatric disorders and behavioral abnormalities

Oxytocin and its target receptor (oxytocin receptor, OXTR) exert important roles in the regulation of complex social behaviors and cognition. The oxytocin/OXTR system in the brain could activate and transduce several intracellular signaling pathways to affect neuronal functions or responses and then mediate physiological activities. The persistence and outcome of the oxytocin activity in the brain are closely linked to the regulation, state, and expression of OXTR. Increasing evidence has shown that genetic variations, epigenetic modification states, and the expression of OXTR have been implicated in psychiatric disorders characterized by social deficits, especially in autism. Among these variations and modifications, OXTR gene methylation and polymorphism have been found in many patients with psychiatric disorders and have been considered to be associated with those psychiatric disorders, behavioral abnormalities, and individual differences in response to social stimuli or others. Given the significance of these new findings, in this review, we focus on the progress of OXTR's functions, intrinsic mechanisms, and its correlations with psychiatric disorders or deficits in behaviors. We hope that this review can provide a deep insight into the study of OXTR-involved psychiatric disorders.

Exogenous central angiotensin fails to stimulate a sodium appetite in diabetes insipidus Brattleboro rats

We investigated the effect of central administration of angiotensin II (AngII) on a specific salt appetite (SSA) in homozygous diabetes insipidus Brattleboro (DI) rats because this stimulus induces such a response in all other rat strains. DI rats have a deficiency in the synthesis of arginine vasopressin (AVP) and a reduced content of pituitary oxytocin (OT). They are characterized also by polyuria, polydipsia, and they seldom ingest high concentrations of NaCl solutions. We also tested if the appetite can be influenced by neurohypophyseal hormones especially oxytocin (OT) because it inhibits SSA in other animals. DI rats and Long Evans (LE) controls were fed ad libitum and given a choice between water, and either 0.9% or 1.8% NaCl. The data showed a significant increase of daily spontaneous water intake in DI compared with LE rats. Both DI and LE ingested similar small spontaneous volumes of the isotonic NaCl solution, but DI rats drank significantly less hypertonic NaCl than the LE controls. I.c.v infusion of AngII induced significant sodium intake in LE rats, but only raised water intake in DI rats. When combined with i.c.v. Ang II, OVT enhanced salt intake in LE animals while AVP attenuated water intake in both groups of rats and blocked NaCl intake completely in LE rats. In conclusion, DI rats did not demonstrate a SSA in response to central administration of AngII, although the drinking of water was enhanced. In combination with i.c.v. AngII, AVP inhibits water drinking in both DI and LE rats. In the LE controls OT attenuates AngII-induced SSA but has no effect in DI rats.

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.

The osmoresponsiveness of oxytocin and vasopressin neurones: Mechanisms, allostasis and evolution

In the rat supraoptic nucleus, every oxytocin cell projects to the posterior pituitary, and is involved both in reflex milk ejection during lactation and in regulating uterine contractions during parturition. All are also osmosensitive, regulating natriuresis. All are also regulated by signals that control appetite, including the neural and hormonal signals that arise from the gut after food intake and from the sites of energy storage. All are also involved in sexual behaviour, anxiety-related behaviours and social behaviours. The challenge is to understand how a single population of neurones can coherently regulate such a diverse set of functions and adapt to changing physiological states. Their multiple functions arise from complex intrinsic properties that confer sensitivity to a wide range of internal and environmental signals. Many of these properties have a distant evolutionary origin in multifunctional, multisensory neurones of Urbilateria, the hypothesised common ancestor of vertebrates, insects and worms. Their properties allow different patterns of oxytocin release into the circulation from their axon terminals in the posterior pituitary into other brain areas from axonal projections, as well as independent release from their dendrites.

Localisation of 11β-Hydroxysteroid Dehydrogenase Type 2 in Mineralocorticoid Receptor Expressing Magnocellular Neurosecretory Neurones of the Rat Supraoptic and Paraventricular Nuclei

An accumulating body of evidence suggests that the activity of the mineralocorticoid, aldosterone, in the brain via the mineralocorticoid receptor (MR) plays an important role in the regulation of blood pressure. MR was recently found in vasopressin and oxytocin synthesising magnocellular neurosecretory cells (MNCs) in both the paraventricular (PVN) and supraoptic (SON) nuclei in the hypothalamus. Considering the physiological effects of these hormones, MR in these neurones may be an important site mediating the action of aldosterone in blood pressure regulation within the brain. However, aldosterone activation of MR in the hypothalamus remains controversial as a result of the high binding affinity of glucocorticoids to MR at substantially higher concentrations compared to aldosterone. In aldosterone-sensitive epithelia, the enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) prevents glucocorticoids from binding to MR by converting glucocorticoids into inactive metabolites. The present study aimed to determine whether 11β-HSD2, which increases aldosterone selectivity, is expressed in MNCs. Specific 11β-HSD2 immunoreactivity was found in the cytoplasm of the MNCs in both the SON and PVN. In addition, double-fluorescence confocal microscopy demonstrated that MR-immunoreactivity and 11β-HSD2-in situ hybridised products are colocalised in MNCs. Lastly, single-cell reverse transcriptase-polymerase chain reaction detected MR and 11β-HSD2 mRNAs from cDNA libraries derived from single identified MNCs. These findings strongly suggest that MNCs in the SON and PVN are aldosterone-sensitive neurones.

A previous history of repeated amphetamine exposure modifies brain angiotensin II AT1 receptor functionality

Previous results from our laboratory showed that angiotensin II AT1 receptors (AT1-R) are involved in the neuroadaptative changes induced by amphetamine. The aim of the present work was to study functional and neurochemical responses to angiotensin II (ANG II) mediated by AT1-R activation in animals previously exposed to amphetamine. For this purpose male Wistar rats (250-320 g) were treated with amphetamine (2.5mg/kg/day intraperitoneal) or saline for 5 days and implanted with intracerebroventricular (i.c.v.) cannulae. Seven days after the last amphetamine administration the animals received ANG II (400 pmol) i.c.v. One group was tested in a free choice paradigm for sodium (2% NaCl) and water intake and sacrificed for Fos immunoreactivity (Fos-IR) determinations. In a second group of rats, urine and plasma samples were collected for electrolytes and plasma renin activity determination and then they were sacrificed for Fos-IR determination in Oxytocinergic neurons (Fos-OT-IR).

RESULTS

Repeated amphetamine exposure (a) prevented the increase in sodium intake and Fos-IR cells in caudate-putamen and accumbens nucleus induced by ANG II i.c.v. (b) potentiated urinary sodium excretion and Fos-OT-IR in hypothalamus and (c) increased the inhibitory response in plasma renin activity, in response to ANG II i.c.v. Our results indicate a possible functional desensitisation of AT1-R in response to ANG II, induced by repeated amphetamine exposure. This functional AT1-R desensitisation allows to unmask the effects of ANG II i.c.v. mediated by oxytocin. We conclude that the long lasting changes in brain AT1-R functionality should be considered among the psychostimulant-induced neuroadaptations.

Neurohypophysial Hormones Regulate Amphibious Behaviour in the Mudskipper Goby

The neurohypophysial hormones, arginine vasotocin and isotocin, regulate both hydromineral balance and social behaviors in fish. In the amphibious mudskipper, Periophthalmus modestus, we previously found arginine-vasotocin-specific regulation of aggressive behavior, including migration of the submissive subordinate into water. This migration also implies the need for adaptation to dehydration. Here, we examined the effects of arginine vasotocin and isotocin administration on the amphibious behavior of individual mudskippers in vivo. The mudskippers remained in the water for an increased period of time after 1-8 h of intracerebroventricular (ICV) injection with 500 pg/g arginine vasotocin or isotocin. The 'frequency of migration' was decreased after ICV injection of arginine vasotocin or isotocin, reflecting a tendency to remain in the water. ICV injections of isotocin receptor antagonist with arginine vasotocin or isotocin inhibited all of these hormonal effects. In animals kept out of water, mRNA expression of brain arginine vasotocin and isotocin precursors increased 3- and 1.5-fold, respectively. Given the relatively wide distribution of arginine vasotocin fibres throughout the mudskipper brain, induction of arginine vasotocin and isotocin under terrestrial conditions may be involved also in the preference for an aquatic habitat as ligands for brain isotocin receptors.

Effects of hydrogen sulfide (H2S) on water intake and vasopressin and oxytocin secretion induced by fluid deprivation

During dehydration, responses of endocrine and autonomic control systems are triggered by central and peripheral osmoreceptors and peripheral baroreceptors to stimulate thirst and sodium appetite. Specifically, it is already clear that endocrine system acts by secreting vasopressin (AVP), oxytocin (OT) and angiotensin II (ANG II), and that gaseous molecules, such as nitric oxide (NO) and carbon monoxide (CO), play an important role in modulating the neurohypophyseal secretion as well as ANG II production and thirst. More recently, another gas-hydrogen sulfide (H2S)-has been studied as a neuronal modulator, which is involved in hypothalamic control of blood pressure, heart frequency and temperature. In this study, we aimed to investigate whether H2S and its interaction with NO system could participate in the modulatory responses of thirst and hormonal secretion induced by fluid deprivation. For this purpose, Wistar male rats were deprived of water for 12 and 24h, and the activity of sulfide-generating enzymes was measured. Surprisingly, 24-h water deprivation increased the activity of sulfide-generating enzymes in the medial basal hypothalamus (MBH). Furthermore, the icv injection of sodium sulfide (Na2S, 260nmol), a H2S donor, reduced water intake, increased AVP, OT and CORT plasma concentrations and decreased MBH nitrate/nitrite (NOX) content of 24-h water-deprived animals compared to controls. We thus suggest that H2S system has an important role in the modulation of hormonal and behavioral responses induced by 24-h fluid deprivation.

The role of the hypothalamic paraventricular nucleus and the organum vasculosum lateral terminalis in the control of sodium appetite in male rats

Angiotensin II (AngII) and aldosterone cooperate centrally to produce a robust sodium appetite. The intracellular signaling and circuitry that underlie this interaction remain unspecified. Male rats pretreated with both deoxycorticosterone (DOC; a synthetic precursor of aldosterone) and central AngII exhibited a marked sodium intake, as classically described. Disruption of inositol trisphosphate signaling, but not extracellular-regulated receptor kinase 1 and 2 signaling, prevented the cooperativity of DOC and AngII on sodium intake. The pattern of expression of the immediate early gene product cFos was used to identify key brain regions that may underlie this behavior. In the paraventricular nuclei (PVN) of the hypothalamus, DOC pretreatment diminished both AngII-induced cFos induction and neurosecretion of oxytocin, a peptide expressed in the PVN. Conversely, in the organum vasculosum lateral terminalis (OVLT), DOC pretreatment augmented cFos expression. Immunohistochemistry identified a substantial presence of oxytocin fibers in the OVLT. In addition, when action potentials in the PVN were inhibited with intraparenchymal lidocaine, AngII-induced sodium ingestion was exaggerated. Intriguingly, this treatment also increased the number of neurons in the OVLT expressing AngII-induced cFos. Collectively, these results suggest that the behavioral cooperativity between DOC and AngII involves the alleviation of an inhibitory oxytocin signal, possibly relayed directly from the PVN to the OVLT.

Role of α2-adrenoceptors in the lateral parabrachial nucleus in the control of body fluid homeostasis

Central α₂-adrenoceptors and the pontine lateral parabrachial nucleus (LPBN) are involved in the control of sodium and water intake. Bilateral injections of moxonidine (α₂-adrenergic/imidazoline receptor agonist) or noradrenaline into the LPBN strongly increases 0.3 M NaCl intake induced by a combined treatment of furosemide plus captopril. Injection of moxonidine into the LPBN also increases hypertonic NaCl and water intake and reduces oxytocin secretion, urinary sodium, and water excreted by cell-dehydrated rats, causing a positive sodium and water balance, which suggests that moxonidine injected into the LPBN deactivates mechanisms that restrain body fluid volume expansion. Pretreatment with specific α₂-adrenoceptor antagonists injected into the LPBN abolishes the behavioral and renal effects of moxonidine or noradrenaline injected into the same area, suggesting that these effects depend on activation of LPBN α₂-adrenoceptors. In fluid-depleted rats, the palatability of sodium is reduced by ingestion of hypertonic NaCl, limiting intake. However, in rats treated with moxonidine injected into the LPBN, the NaCl palatability remains high, even after ingestion of significant amounts of 0.3 M NaCl. The changes in behavioral and renal responses produced by activation of α₂-adrenoceptors in the LPBN are probably a consequence of reduction of oxytocin secretion and blockade of inhibitory signals that affect sodium palatability. In this review, a model is proposed to show how activation of α₂-adrenoceptors in the LPBN may affect palatability and, consequently, ingestion of sodium as well as renal sodium excretion.

Role of the lateral parabrachial nucleus in the control of sodium appetite

In states of sodium deficiency many animals seek and consume salty solutions to restore body fluid homeostasis. These behaviors reflect the presence of sodium appetite that is a manifestation of a pattern of central nervous system (CNS) activity with facilitatory and inhibitory components that are affected by several neurohumoral factors. The primary focus of this review is on one structure in this central system, the lateral parabrachial nucleus (LPBN). However, before turning to a more detailed discussion of the LPBN, a brief overview of body fluid balance-related body-to-brain signaling and the identification of the primary CNS structures and humoral factors involved in the control of sodium appetite is necessary. Angiotensin II, mineralocorticoids, and extracellular osmotic changes act on forebrain areas to facilitate sodium appetite and thirst. In the hindbrain, the LPBN functions as a key integrative node with an ascending output that exerts inhibitory influences on forebrain regions. A nonspecific or general deactivation of LPBN-associated inhibition by GABA or opioid agonists produces NaCl intake in euhydrated rats without any other treatment. Selective LPBN manipulation of other neurotransmitter systems [e.g., serotonin, cholecystokinin (CCK), corticotrophin-releasing factor (CRF), glutamate, ATP, or norepinephrine] greatly enhances NaCl intake when accompanied by additional treatments that induce either thirst or sodium appetite. The LPBN interacts with key forebrain areas that include the subfornical organ and central amygdala to determine sodium intake. To summarize, a model of LPBN inhibitory actions on forebrain facilitatory components for the control of sodium appetite is presented in this review.

Endogenous oxytocin is necessary for preferential Fos expression to male odors in the bed nucleus of the stria terminalis in female Syrian hamsters

Successful reproduction in mammals depends on proceptive or solicitational behaviors that enhance the probability of encountering potential mates. In female Syrian hamsters, one such behavior is vaginal scent marking. Recent evidence suggests that the neuropeptide oxytocin (OT) may be critical for regulating this behavior. Blockade of OT receptors in the bed nucleus of the stria terminalis (BNST) or the medial preoptic area (MPOA) decreases vaginal marking responses to male odors; lesion data suggest that BNST, rather than MPOA, mediates this effect. However, how OT interacts with sexual odor processing to drive preferential solicitation is not known. To address this issue, intact female Syrian hamsters were exposed to male or female odors and their brains processed for immunohistochemistry for Fos, a marker of recent neuronal activation, and OT. Additional females were injected intracerebroventricularly (ICV) with an oxytocin receptor antagonist (OTA) or vehicle, and then tested for vaginal marking and Fos responses to sexual odors. Colocalization of OT and Fos in the paraventricular nucleus of the hypothalamus was unchanged following exposure to male odors, but decreased following exposure to female odors. Following injections of OTA, Fos expression to male odors was decreased in BNST, but not in MPOA or the medial amygdala (MA). Fos expression in BNST may be functionally relevant for vaginal marking, given that there was a positive correlation between Fos expression and vaginal marking for BNST, but not MPOA or MA. Together, these data suggest that OT facilitation of neuronal activity in BNST underlies the facilitative effects of OT on solicitational responses to male odors.

SGK1-dependent salt appetite in pregnant mice

AIM

Pregnancy is typically paralleled by substantial increase in maternal extracellular fluid volume, requiring net accumulation of water and NaCl. The positive water and salt balance is accomplished at least in part by increased uptake of salt secondary to enhanced salt appetite. Little is known about the underlying cellular mechanisms. Stimulation of salt appetite by mineralocorticoids, however, is known to be dependent on the serum- and glucocorticoid-inducible kinase SGK1.

METHODS

To test for a role of SGK1 in the stimulation of salt appetite during pregnancy, fluid intake was recorded in pregnant SGK1 knockout mice (sgk1(-/-) ) and their wild type littermates (sgk1(+/+) ). The mice were offered two bottles, one with plain water and the other with isotonic saline.

RESULTS

In early pregnancy, i.e. up to 10 days prior to parturition, the sgk1(+/+) mice displayed a significant preference for saline, whereas the sgk1(-/-) mice preferred water. Accordingly, the water intake was significantly smaller and saline intake was significantly larger in sgk1(+/+) mice than in sgk1(-/-) mice and the preference for water was significantly stronger in sgk1(-/-) mice than in sgk1(+/+) mice. Plasma aldosterone levels were higher in sgk1(-/-) mice than in sgk1(+/+) mice, a difference contrasting the enhanced salt appetite of sgk1(+/+) mice.

CONCLUSIONS

SGK1 participates in the stimulation of salt appetite during pregnancy.

Calcitonin has a vasopressin-like effect on aquaporin-2 trafficking and urinary concentration

The most common cause of hereditary nephrogenic diabetes insipidus is a nonfunctional vasopressin (VP) receptor type 2 (V2R). Calcitonin, another ligand of G-protein-coupled receptors, has a VP-like effect on electrolytes and water reabsorption, suggesting that it may affect AQP2 trafficking. Here, calcitonin increased intracellular cAMP and stimulated the membrane accumulation of AQP2 in LLC-PK1 cells. Pharmacologic inhibition of protein kinase A (PKA) and deficiency of a critical PKA phosphorylation site on AQP2 both prevented calcitonin-induced membrane accumulation of AQP2. Fluorescence assays showed that calcitonin led to a 70% increase in exocytosis and a 20% decrease in endocytosis of AQP2. Immunostaining of rat kidney slices demonstrated that calcitonin induced a significant redistribution of AQP2 to the apical membrane of principal cells in cortical collecting ducts and connecting segments but not in the inner stripe or inner medulla. Calcitonin-treated VP-deficient Brattleboro rats had a reduced urine flow and two-fold higher urine osmolality during the first 12 hours of treatment compared with control groups. Although this VP-like effect of calcitonin diminished over the following 72 hours, the tachyphylaxis was reversible. Taken together, these data show that calcitonin induces cAMP-dependent AQP2 trafficking in cortical collecting and connecting tubules in parallel with an increase in urine concentration. This suggests that calcitonin has a potential therapeutic use in nephrogenic diabetes insipidus.

Neuroendocrine and cardiovascular parameters during simulation of stress-induced rise in circulating oxytocin in the rat

Physiological functions of oxytocin released during stress are not well understood. We have (1) investigated the release of oxytocin during chronic stress using two long-term stress models and (2) simulated stress-induced oxytocin secretion by chronic treatment with oxytocin via osmotic minipumps. Plasma oxytocin levels were significantly elevated in rats subjected to acute immobilization stress for 120 min, to repeated immobilization for 7 days and to combined chronic cold stress exposure for 28 days with 7 days immobilization. To simulate elevation of oxytocin during chronic stress, rats were implanted with osmotic minipumps subcutaneously and treated with oxytocin (3.6 microg/100 g body weight/day) or vehicle for 2 weeks. Chronic subcutaneous oxytocin infusion led to an increase in plasma oxytocin, adrenocorticotropic hormone, corticosterone, adrenal weights and heart/body weight ratio. Oxytocin treatment had no effect on the incorporation of 5-bromo-2-deoxyuridine into DNA in the heart ventricle. Mean arterial pressure response to intravenous phenylephrine was reduced in oxytocin-treated animals. Decrease in adrenal tyrosin hydroxylase mRNA following oxytocin treatment was not statistically significant. Oxytocin treatment failed to modify food intake and slightly increased water consumption. These data provide evidence on increased concentrations of oxytocin during chronic stress. It is possible that the role of oxytocin released during stress is in modulating hypothalamic-pituitary-adrenocortical axis and selected sympathetic functions.

The anorexigenic and hypertensive effects of nesfatin-1 are reversed by pretreatment with an oxytocin receptor antagonist

Nesfatin-1 is an 82-amino acid protein encoded by the nucleobindin2 gene. When injected intracerebroventricularly, nesfatin-1, via a melanocortin 3/4 receptor-dependent mechanism, potently decreased both food and water intakes and elevated mean arterial pressure in a dose-related manner. Because nesfatin-1 colocalized with oxytocin in hypothalamus and because nesfatin-1 had direct depolarizing effects on oxytocin-producing neurons in hypothalamic slice preparations, we hypothesized that the actions of nesfatin-1 required the presence of functional oxytocin receptors. We, therefore, pretreated conscious, unrestrained male rats with the oxytocin receptor antagonist, ornithine vasotocin (OVT), before treatment with nesfatin-1. We found that pretreatment with OVT reversed the effects of nesfatin-1 on both food and water intake and on mean arterial pressure, indicating that the central oxytocin system is a downstream mediator of these actions of nesfatin-1. Additionally, we found that OVT reversed the anorexigenic effect of alpha-melanocyte-stimulating hormone (alpha-MSH), suggesting that the central oxytocin system is downstream of the central melanocortin system. Taken together, these data suggest that nesfatin-1 acts through a serial neuronal circuit, in which nesfatin-1 activates the central melanocortin system, which, in turn, acts through the central oxytocin system, leading to an inhibition of food and water intake and an increase in mean arterial pressure.

Role of serotonergic 5-HT1A and oxytocinergic receptors of the lateral septal area in sodium intake regulation

Several reports have revealed a high density of 5-HT(1A) receptors in the lateral septal area (LSA), as well as a subpopulation of oxytocin (OT) receptors. Increasing evidence shows that 5-HT(1A) and OT neurons inhibit sodium urinary excretion. The aim of this study was to investigate the part played by serotonergic (5-HT(1A)) and oxytocinergic receptors in the LSA in the sodium intake induced in rats by sodium depletion followed by 24h deprivation. Cannulae were implanted bilaterally into the LSA of rats to enable the introduction of receptor ligands into that brain area. Serotonergic injections of 5-HT (10, 20, and 40 microg/0.2 microL) reduced 1.8% NaCl solution intake, but injections (1, 2, and 4 microg/0.2 microL) of 8-OH-DPAT, a 5-HT(1A) agonist, were more effective than 5-HT in reducing 1.8% NaCl intake. Pretreatment of the LSA with the 5-HT(1A) antagonist pMPPF partially reduced the inhibitory effect of 5-HT and totally reversed the effects of 8-OH-DPAT on 1.8% NaCl intake induced by sodium depletion. Previous treatment with the potent oxytocin receptor antagonist d(CH(2))(5)[Tyr(Me)(2)Thr(4), Orn(5), Tyr(NH(2))(9)]-vasotocin also totally blocked the inhibitory effects of 5-HT or 8-OH-DPAT on 1.8% NaCl intake. These results show that 5-HT(1A) serotonergic receptors in the LSA, including some that interact with the oxytocinergic system, modulate sodium intake induced by sodium loss in rats.

Inhibition of brain proinflammatory cytokine synthesis reduces hypothalamic excitation in rats with ischemia-induced heart failure

The expression of proinflammatory cytokines increases in the hypothalamus of rats with heart failure (HF). The pathophysiological significance of this observation is unknown. We hypothesized that hypothalamic proinflammatory cytokines upregulate the activity of central neural systems that contribute to increased sympathetic nerve activity in HF, specifically, the brain renin-angiotensin system (RAS) and the hypothalamic-pituitary-adrenal (HPA) axis. Rats with HF induced by coronary ligation and sham-operated controls (SHAM) were treated for 4 wk with a continuous intracerebroventricular infusion of the cytokine synthesis inhibitor pentoxifylline (PTX, 10 microg/h) or artificial cerebrospinal fluid (VEH). In VEH-treated HF rats, compared with VEH-treated SHAM rats, the hypothalamic expression of proinflammatory cytokines was increased, along with key components of the brain RAS (renin, angiotensin-converting enzyme, angiotensin type 1 receptor) and corticotropin-releasing hormone, the central indicator of HPA axis activation, in the paraventricular nucleus (PVN) of the hypothalamus. The expression of other inflammatory/excitatory mediators (superoxide, prostaglandin E(2)) was also increased, along with evidence of chronic neuronal excitation in PVN. VEH-treated HF rats had higher plasma levels of norepinephrine, ANG II, interleukin (IL)-1beta, and adrenocorticotropic hormone, increased left ventricular end-diastolic pressure, and increased wet lung-to-body weight ratio. With the exception of plasma IL-1beta, an indicator of peripheral proinflammatory cytokine activity, all measures of neurohumoral excitation were significantly lower in HF rats treated with intracerebroventricular PTX. These findings suggest that the increase in brain proinflammatory cytokines observed in rats with ischemia-induced HF is functionally significant, contributing to neurohumoral excitation by activating brain RAS and the HPA axis.

DOCA stimulates salt appetite in Zucker rats: effect of dose, synergistic action with central angiotensin II, and obesity

An enhanced sodium appetite is found in rats by the synergist interaction of peripheral mineralocorticoids, deoxycorticosterone acetate (DOCA), and central angiotensin II (AngII), the synergy theory. We used obese Zucker rats which have a predisposition to develop hypertension under appropriate salt conditions to examine this synergy response between AngII and different low doses of DOCA on 2% NaCl intake. Obese and lean Zucker rats on low sodium food were treated systemically with 0.5, 1 and 2 mg/kg/day of DOCA for 3 days, before receiving i.c.v. AngII (10 pmol) on the fourth day. Food, fluid intakes and urine outputs were measured daily throughout. Plasma aldosterone levels were also analysed. Results showed that AngII alone increased water but not salt intake, whereas all three doses of DOCA by themselves enhanced daily salt intake during the treatment period. The lowest dose of DOCA plus AngII did not stimulate an enhanced sodium consumption. The 1 mg/kg was the threshold dose of DOCA for a synergistic response, and with 2 mg/kg DOCA the obese rats consumed nearly 2-fold more hypertonic NaCl solution than the leans. Moreover, obese baseline plasma levels of aldosterone were more elevated than the lean rats. In conclusion, in adult Zucker rats a threshold level of mineralocorticoid is required for the salt stimulating action of central AngII. In the obese rat the synergistic effect is enhanced with higher doses of mineralocorticoid, suggesting that the plasma level of aldosterone could be a prominent factor, which may predispose the obese to salt-sensitivity and, possibly, subsequently to hypertension under appropriate conditions.

Sertraline, a selective serotonin reuptake inhibitor, affects thirst, salt appetite and plasma levels of oxytocin and vasopressin in rats

We investigated the effects of chronic administration of sertraline (SERT; approximately 20 mg kg(-1) day(-1) in drinking water), a selective serotonin reuptake inhibitor, on water and sodium intake and on plasma levels of oxytocin (OT) and vasopressin (AVP) in basal and stimulated conditions. Basal water intake was reduced in SERT-treated rats. After 24 h of water deprivation, rats treated with SERT for 21 days ingested less water than the control rats (9.7 +/- 0.5 versus 20.0 +/- 0.9 ml, respectively, at 300 min after water presentation, P < 0.0001). Subcutaneous injection of 2 m NaCl or isoproterenol evoked a lower dipsogenic response in rats treated with SERT for 21 days. Fluid and food deprivation also induced a weaker dipsogenic response in SERT-treated rats (1.6 +/- 0.5 versus 10.2 +/- 1.2 ml, at 300 min, P < 0.0001) but had no effect on saline intake. Sodium depletion induced a higher natriorexigenic response in the SERT group (5.6 +/- 1.3 versus 1.2 +/- 0.3 ml, at 300 min, P < 0.0002). Higher urinary density and lower plasma sodium levels were observed after SERT treatment. Sertraline also increased plasma levels of vasopressin and oxytocin (AVP, 2.65 +/- 0.36 versus 1.31 +/- 0.16 pg ml(-1), P < 0.005; OT, 17.16 +/- 1.06 versus 11.3 +/- 1.03 pg ml(-1), P < 0.0009, at the third week post-treatment). These data constitute the first evidence that chronic SERT treatment affects water and sodium intake in rats. These effects seem to be related to the hyponatraemia caused by the higher plasma levels of AVP and OT.

Richter and sodium appetite: from adrenalectomy to molecular biology

Nearly three-quarters of a century ago, Curt Richter removed the adrenal glands from rats and noted that the animal's vitality was dependent on its increased consumption of sodium chloride. In doing so, Richter revealed an innate behavioral mechanism that serves to maintain the hydromineral balance of an animal faced with sodium deficit. This experiment and others like it, led to the development of a field of research devoted to the investigation of salt appetite. The following is a discussion of how Richter's initial observations gave birth to an evolving field that incorporates multiple approaches to examine the drive to consume sodium.

Novel effect of mineralocorticoid receptor antagonism to reduce proinflammatory cytokines and hypothalamic activation in rats with ischemia-induced heart failure

Blocking brain mineralocorticoid receptors (MRs) reduces the high circulating levels of tumor necrosis factor (TNF)-alpha in heart failure (HF) rats. TNF-alpha and other proinflammatory cytokines activate neurons in the paraventricular nucleus (PVN) of hypothalamus, including corticotropin-releasing hormone (CRH) neurons, by inducing cyclooxygenase (COX)-2 activity and synthesis of prostaglandin E2 by perivascular cells of the cerebral vasculature. We tested the hypothesis that systemic treatment with a MR antagonist would reduce hypothalamic COX-2 expression and PVN neuronal activation in HF rats. Rats underwent coronary ligation to induce HF, confirmed by echocardiography, or sham surgery, followed by 6 weeks treatment with eplerenone (30 mg/kg per day, orally) or vehicle (drinking water). Eplerenone-treated HF rats had lower plasma TNF-alpha, interleukin (IL)-1beta and IL-6, less COX-2 staining of small blood vessels penetrating PVN, fewer PVN neurons expressing Fra-like activity (indicating chronic neuronal activation), and fewer PVN neurons staining for TNF-alpha, IL-1beta, and CRH than vehicle-treated HF rats. COX-2 and CRH protein expression in hypothalamus were 1.7- and 1.9-fold higher, respectively, in HF+vehicle versus sham+vehicle rats; these increases were attenuated (26% and 25%, respectively) in HF+eplerenone rats. Eplerenone-treated HF rats had less prostaglandin E2 in cerebrospinal fluid, lower plasma norepinephrine levels, lower left ventricular end-diastolic pressure, and lower right ventricle/body weight and lung/body weight ratios, but no improvement in left ventricular function. Treatment of HF rats with anticytokine agents, etanercept or pentoxifylline, produced very similar results. This study reveals a previously unrecognized effect of MR antagonism to minimize cytokine-induced central neural excitation in rats with HF.

11beta-hydroxysteroid dehydrogenase type 2 activity in hypothalamic paraventricular nucleus modulates sympathetic excitation

Aldosterone stimulates the sympathetic nervous system by binding to a select population of brain mineralocorticoid receptors (MR). These MR have an equal affinity for corticosterone that is present in substantially higher concentrations, but are held in reserve for aldosterone by activity of the enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD-2), which converts corticosterone to an inactive metabolite. Thus, colocalization of MR and 11beta-HSD-2 activity may help identify brain regions that mediate the effects of aldosterone. The present studies tested the hypothesis that 11beta-HSD-2 activity regulates MR-mediated responses in the paraventricular nucleus (PVN) of the hypothalamus, a forebrain region implicated in sympathetic regulation. Real-time-polymerase chain reaction revealed the presence of 11beta-HSD-2 mRNA in PVN. In anesthetized adult male Sprague-Dawley rats, microinjection of the 11beta-HSD-2 inhibitor carbenoxolone (CBX) into PVN increased mean arterial pressure, heart rate, and renal sympathetic nerve activity. Intracerebroventricular injections of CBX excited PVN neurons and increased mean arterial pressure, heart rate, and renal sympathetic nerve activity. The ability of CBX to increase sympathetic activity by inhibiting 11beta-HSD-2, thereby permitting corticosterone to activate MR, was confirmed by the following: Intracerebroventricular glycyrrhizic acid, another 11beta-HSD-2 inhibitor, mimicked the sympathoexcitatory effects of CBX; the sympathoexcitatory effects of CBX were blocked by spironolactone, a MR antagonist. Neither CBX nor glycyrrhizic acid elicited a response in adrenalectomized rats. These findings suggest that MR in PVN contribute to sympathetic regulation and may be activated by aldosterone or corticosterone (or cortisol in humans) depending on the state of 11beta-HSD-2 activity.

SGK1 as a determinant of kidney function and salt intake in response to mineralocorticoid excess

Mineralocorticoids modify salt balance by both stimulating salt intake and inhibiting salt loss. Renal salt retention is accomplished by upregulation of reabsorption, an effect partially mediated by serum- and glucocorticoid-inducible kinase 1 (SGK1). The present study explored the contribution of SGK1 to the regulation of renal function, salt intake, and blood pressure during mineralocorticoid excess. DOCA/1% NaCl treatment increased blood pressure and creatinine clearance to a similar extent in SGK1-deficient sgk1−/−and wild-type sgk1+/+mice but led to more pronounced increase of proteinuria in sgk1+/+mice (by 474 ± 89%) than in sgk1−/−mice (by 154 ± 31%). DOCA/1% NaCl treatment led to significant increase of kidney weight (by 24%) and to hypokalemia (from 3.9 ± 0.1 to 2.7 ± 0.1 mmol/l) only in sgk1+/+mice. The treatment enhanced renal Na+excretion significantly more in sgk1+/+mice (from 3 ± 1 to 134 ± 32 μmol·24 h−1·g body wt−1) than in sgk1−/−mice (from 4 ± 1 to 49 ± 8 μmol·24 h−1·g body wt−1), pointing to SGK1-dependent stimulation of salt intake. With access to two drinking bottles containing 1% NaCl or water, DOCA treatment did not significantly affect water intake in either genotype but increased 1% NaCl intake in sgk1+/+mice (within 9 days from 3.5 ± 0.9 to 16.5 ± 2.4 ml/day) consistent with DOCA-induced salt appetite. This response was significantly attenuated in sgk1−/−mice (from 2.6 ± 0.6 to 5.9 ± 0.9 ml/day). Thus SGK1 contributes to the stimulation of salt intake, kidney growth, proteinuria, and renal K+excretion during mineralocorticoid excess.