Regulation Of Sodium Appetite Research Articles

Role of Dorsal Raphe Serotonergic Neurons in the Regulation of Sodium Appetite

An animal with sodium deficiency develops sodium appetite which drives it to consume more sodium. A lot of studies have been performed to understand how sodium depletion may lead to the development of sodium appetite, but there are still unanswered questions. In particular, while the relationship between serotonin receptors and sodium appetite has been identified in previous studies, it is still unclear how serotonergic neurons control sodium appetite in response to sodium depletion. In this study, we focused on serotonergic neurons in the dorsal raphe nucleus (DRN) which releases more than 50% of serotonin in the brain. We found that DRN serotonergic neurons are activated in response to sodium depletion. In addition, we found evidence that angiotensin II is a key molecule herein to evoke sodium appetite in sodium depletion. Our results provide refined insight how sodium appetite develops in sodium depletion. This work was supported by grants from Samsung Science & Technology Foundation (SSTF-BA1901-11) to Jong-Woo Sohn. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Angiotensin II facilitates GABAergic neurotransmission at postsynaptic sites in rat amygdala neurons

The central nucleus of the amygdala (CeA) is critical in the regulation of sodium appetite. Angiotensin II (Ang II) is important in the generation of sodium appetite and may function as a neurotransmitter or modulator to affect the synaptic transmission and the excitability of neurons. However, the role of Ang II in the CeA remains unclear. In this study, we determined the effects of Ang II on the excitatory and inhibitory synaptic inputs to the CeA neurons in brain slices with whole-cell patch-clamp recordings. Ang II (0.5–5 μM) significantly potentiated the amplitude of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) in a concentration-dependent manner. Ang II (2 μM) significantly increased the amplitude of miniature GABAergic inhibitory postsynaptic currents (mIPSCs) without affecting the frequency. This effect was blocked by Ang II type 1 (AT1) receptor antagonist, losartan. One mM guanosine 5′-O-(-2-thiodiphosphate) (GDP-β-s) in the pipette internal solution eliminated the facilitatory effect of Ang II on GABAergic synaptic transmission. In contrast, Ang II had no effect on the spontaneous glutamatergic excitatory postsynaptic currents (EPSCs) and did not alter the frequency and amplitude of miniature EPSCs at concentrations that facilitated IPSCs. Furthermore, Ang II decreased the firing activity of CeA neurons, and this effect was abolished by losartan and GDP-β-s. In addition, Ang II failed to inhibit CeA neurons in the presence of bicuculline. These data provide substantial new evidence that Ang II inhibits the CeA neurons by facilitation of GABAergic synaptic input efficacy through activation of postsynaptic AT1 receptors.

Abstract P284: Neuron Specific (Pro)renin Receptor Knockout Reduces Sodium Appetite and Attenuates the Development of DOCA-salt Induced Hypertension

The (pro) renin receptor (PRR) is a key component of the renin-angiotensin system that is highly expressed in the brain. We previously showed that the neuronal PRR deletion attenuates deoxycorticosterone acetate (DOCA)-salt induced hypertension. However, the mechanism underlying remains unclear. To test our hypothesis that PRR is involved in the regulation of sodium appetite during DOCA-salt hypertension, we used a neuron-specific PRR knockout (PRRKO) mouse model generated using the Cre-LoxP system. The PRRKO and their wildtype controls (WT) were implanted with 50 mg DOCA pellet with free access to regular water and 0.9 % saline as the drinking solution. Blood pressure (BP) was monitored by telemetry system in conscious free moving mice. The fluid intake and urine output were monitored along 21 days of DOCA-Salt treatment. The BP is significantly lower in PRRKO compared with WT mice following 21 days of DOCA-salt treatment (112 ± 2 vs. 134 ± 7 mmHg, P= 0.0186). Interestingly, we found that saline intake (27.8 ± 1.8 vs. 15.9 ± 1.2 ml/day, P=0.0007) and total fluid intake (31.1 ± 1.9 vs. 21.1 ± 1.4 ml/day, P=0.003) were higher; while the regular water intake was lower (3.4 ± 0.6 vs. 5.2 ± 0.3 ml/day, P=0.03) in WT compared to PRRKO mice. PRR deletion in the neurons reduced sodium appetite presented as the ratio of saline intake over total fluid intake (0.75 ± 0.016 vs. 0.89 ± 0.019, P=0.0005), as well as total sodium intake (2.45 ± 0.19 vs. 4.28 ± 0.28 mmol/day, P=0.0007) compared with WT mice at the end of the protocol. In addition, the urinary sodium excretion was lower (13.3±1.17 vs. 20±1.17 mmol/day, P<0.0001), but not potassium excretion (0.64 ± 0.028 vs. 0.56 ± 0.05 mmol/day, P=0.1291) in PRRKO compared with WT mice; however, there is no difference in urine sodium and potassium concentrations. Furthermore, plasma vasopressin level (19.0 ± 2.7 vs. 33.6 ± 2.7 pg/ml, P=0.0037) is lower in the PRRKO compared with WT mice at the end of DOCA-salt treatment. In summary, PRR deletion in the neurons reduced sodium appetite, circulating vasopressin level, and attenuated the development of DOCA-salt induced hypertension. Taken together, the present findings suggest that PRR regulates the BP and plays a key role in salt-sensitive hypertension, at least in part, by modulating sodium appetite.

Aldosterone-Sensing Neurons in the NTS Exhibit State-Dependent Pacemaker Activity and Drive Sodium Appetite via Synergy with Angiotensin II Signaling

Sodium deficiency increases angiotensin II (ATII) and aldosterone, which synergistically stimulate sodium retention and consumption. Recently, ATII-responsive neurons in the subfornical organ (SFO) and aldosterone-sensitive neurons in the nucleus of the solitary tract (NTSHSD2 neurons) were shown to drive sodium appetite. Here we investigate the basis for NTSHSD2 neuron activation, identify the circuit by which NTSHSD2 neurons drive appetite, and uncover an interaction between the NTSHSD2 circuit and ATII signaling. NTSHSD2 neurons respond to sodium deficiency with spontaneous pacemaker-like activity-the consequence of "cardiac" HCN and Nav1.5 channels. Remarkably, NTSHSD2 neurons are necessary for sodium appetite, and with concurrent ATII signaling their activity is sufficient to produce rapid consumption. Importantly, NTSHSD2 neurons stimulate appetite via projections to the vlBNST, which is also the effector site for ATII-responsive SFO neurons. The interaction between angiotensin signaling and NTSHSD2 neurons provides a neuronal context for the long-standing "synergy hypothesis" of sodium appetite regulation.

Area postrema projects to FoxP2 neurons of the pre-locus coeruleus and parabrachial nuclei: Brainstem sites implicated in sodium appetite regulation

The area postrema (AP) is a circumventricular organ located in the dorsal midline of the medulla. It functions as a chemosensor for blood-borne peptides and solutes, and converts this information into neural signals that are transmitted to the nucleus tractus solitarius (NTS) and parabrachial nucleus (PB). One of its NTS targets in the rat is the aldosterone-sensitive neurons which contain the enzyme 11 β-hydroxysteroid dehydrogenase type 2 (HSD2). The HSD2 neurons are part of a central network involved in sodium appetite regulation, and they innervate numerous brain sites including the pre-locus coeruleus (pre-LC) and PB external lateral-inner (PBel-inner) cell groups of the dorsolateral pons. Both pontine cell groups express the transcription factor FoxP2 and become c-Fos activated following sodium depletion. Because the AP is a component in this network, we wanted to determine whether it also projects to the same sites as the HSD2 neurons. By using a combination of anterograde axonal and retrograde cell body tract-tracing techniques in individual rats, we show that the AP projects to FoxP2 immunoreactive neurons in the pre-LC and PBel-inner. Thus, the AP sends a direct projection to both the first-order medullary (HSD2 neurons of the NTS) and the second-order dorsolateral pontine neurons (pre-LC and PB-el inner neurons). All three sites transmit information related to systemic sodium depletion to forebrain sites and are part of the central neural circuitry that regulates the complex behavior of sodium appetite.

Activation of lateral parabrachial afferent pathways and endocrine responses during sodium appetite regulation

Modulation of salt appetite involves interactions between the circumventricular organs (CVOs) receptive areas and inhibitory hindbrain serotonergic circuits. Recent studies provide support to the idea that the serotonin action in the lateral parabrachial nucleus (LPBN) plays an important inhibitory role in the modulation of sodium appetite. The aim of the present work was to identify the specific groups of neurons projecting to the LPBN that are activated in the course of sodium appetite regulation, and to analyze the associated endocrine response, specifically oxytocin (OT) and atrial natriuretic peptide (ANP) plasma release, since both hormones have been implicated in the regulatory response to fluid reestablishment. For this purpose we combined the detection of a retrograde transported dye, Fluorogold (FG) injected into the LPBN with the analysis of the Fos immunocytochemistry brain pattern after sodium intake induced by sodium depletion. We analyzed the Fos-FG immunoreactivity after sodium ingestion induced by peritoneal dialysis (PD). We also determined OT and ANP plasma concentration by radioimmunoassay (RIE) before and after sodium intake stimulated by PD. The present study identifies specific groups of neurons along the paraventricular nucleus, central extended amygdala, insular cortex, dorsal raphe nucleus, nucleus of the solitary tract and the CVOs that are activated during the modulation of sodium appetite and have direct connections with the LPBN. It also shows that OT and ANP are released during the course of sodium satiety and fluid reestablishment. The result of this brain network activity may enable appropriate responses that re-establish the body fluid balance after induced sodium consumption.

Central regulation of sodium appetite

Sodium appetite, the behavioural drive to ingest salt, is stimulated by prolonged physiological sodium deficiency in many animal species. The same neural mechanisms that are responsible for sodium appetite in laboratory animals may influence human behaviour as well, with particular relevance to the dietary salt intake of patients with diseases such as heart failure, renal failure, liver failure and salt-sensitive hypertension. Since the original experimental work of Curt Richter in the 1930s, much has been learned about the regulation of salt-ingestive behaviour. Here, we review data from physiology, pharmacology, neuroanatomy and neurobehavioural investigations into the stimulatory and inhibitory signals that regulate sodium appetite. A rudimentary framework is proposed for the brain circuits that integrate peripheral information representing the need for sodium with neural signals for the gustatory detection of salt in order to drive a motivated ingestive response. Based on this model, areas of remaining uncertainty are highlighted where future information would allow a more detailed understanding of the neural circuitry responsible for sodium appetite.

Sodium deprivation and salt intake activate separate neuronal subpopulations in the nucleus of the solitary tract and the parabrachial complex

Salt intake is an established response to sodium deficiency, but the brain circuits that regulate this behavior remain poorly understood. We studied the activation of neurons in the nucleus of the solitary tract (NTS) and their efferent target nuclei in the pontine parabrachial complex (PB) in rats during sodium deprivation and after salt intake. After 8-day dietary sodium deprivation, immunoreactivity for c-Fos (a neuronal activity marker) increased markedly within the aldosterone-sensitive neurons of the NTS, which express the enzyme 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2). In the PB, c-Fos labeling increased specifically within two sites that relay signals from the HSD2 neurons to the forebrain--the pre-locus coeruleus and the innermost region of the external lateral parabrachial nucleus. Then, 1-2 hours after sodium-deprived rats ingested salt (a hypertonic 3% solution of NaCl), c-Fos immunoreactivity within the HSD2 neurons was virtually eliminated, despite a large increase in c-Fos activation in the surrounding NTS (including the A2 noradrenergic neurons) and area postrema. Also after salt intake, c-Fos activation increased within pontine nuclei that relay gustatory (caudal medial PB) and viscerosensory (rostral lateral PB) information from the NTS to the forebrain. Thus, sodium deficiency and salt intake stimulate separate subpopulations of neurons in the NTS, which then transmit this information to the forebrain via largely separate relay nuclei in the PB complex. These findings offer new perspectives on the roles of sensory information from the brainstem in the regulation of sodium appetite.

Functional brainstem pathways linked to sodium deprivation and salt intake

To reveal brainstem pathways activated by sodium deprivation and salt ingestion, nuclear c-Fos immunoreactivity (a marker for neuronal activity) was examined in neurons within the nucleus of the solitary tract (NTS) and in their efferent target nuclei within the pontine parabrachial complex (PB) in rats. After 8d dietary sodium deprivation, the aldosterone-sensitive neurons in the NTS, which express 11-β-hydroxysteroid dehydrogenase type 2 (HSD2), exhibited a marked increase in c-Fos immunoreactivity. In the PB, c-Fos labeling increased specifically within the two nuclei that relay information from the HSD2 neurons to the forebrain – the pre-locus coeruleus and the inner subdivision of the external lateral nucleus. In separate groups of rats that ingested salt (3% NaCl solution) for 1 or 2h following 8d sodium-deprivation, c-Fos immunoreactivity within the HSD2 neurons was virtually eliminated, whereas a prominent increase in c-Fos labeling was found in the surrounding medial NTS (including A2 noradrenergic neurons) and area postrema. Salt ingestion also increased c-Fos labeling in PB nuclei that relay gustatory (caudal medial PB) and viscerosensory (rostral lateral PB) information from the NTS to the forebrain. Thus, sodium deficiency and salt ingestion stimulate separate subpopulations of neurons within the NTS, which transmit this information to the forebrain via largely separate nuclei in the PB. The existence of parallel, ascending signals for sodium need and salt intake offers a new perspective on the potential functions of brainstem sensory pathways in the regulation of sodium appetite.

Local inputs to aldosterone-sensitive neurons of the nucleus tractus solitarius

Aldosterone-sensitive neurons in the nucleus tractus solitarius (NTS) become activated during sodium depletion and could be key neural elements regulating sodium intake. The afferent inputs to these neurons have not yet been defined, but one source may be neurons in the area postrema, a neighboring circumventricular organ that innervates the NTS and exerts a powerful inhibitory influence on sodium appetite [Contreras RJ, Stetson PW (1981) Changes in salt intake after lesions of the area postrema and the nucleus of the solitary tract in rats. Brain Res 211:355–366]. After an anterograde axonal tracer was injected into the area postrema in rats, sections through the NTS were immunolabeled for the enzyme 11-β-hydroxysteroid dehydrogenase type 2 (HSD2), a marker for aldosterone-sensitive neurons, and examined by confocal microscopy. We found that some of the aldosterone-sensitive neurons received close appositions from processes originating in the area postrema, suggesting that input to the HSD2 neurons could be involved in the inhibition of sodium appetite by this site. Axonal varicosities originating from the area postrema also made close appositions with other neurons in the medial NTS, including the neurotensin-immunoreactive neurons in the dorsomedial NTS. Besides these projections, a dense field of neurotensinergic axon terminals overlapped the distribution of the HSD2 neurons. Neurotensin-immunoreactive axon terminals were identified in close apposition to the dendrites and cell bodies of some HSD2 neurons, as well as unlabeled neurons lying in the same zone within the medial NTS. A local microcircuit involving the area postrema, HSD2 neurons, and neurotensinergic neurons may play a major role in the regulation of sodium appetite.

Reduced sodium appetite and increased oxytocin gene expression in mutant mice lacking β-endorphin

Central opioid and oxytocinergic systems have been involved in the regulatory control of sodium appetite. In addition, previous studies support the existence of a functional interaction between opioid peptides and oxytocinergic pathways, and suggest that β-endorphin neurons would modulate the activity of central oxytocinergic pathways, its pituitary secretion and sodium appetite. To investigate the role of this opioid peptide in the control of oxytocin (OT) synthesis and sodium appetite regulation we used mice with gene dosage-dependent variations in brain β-endorphin content, expressing either 100%, 50%, or 0% of normal β-endorphin content. Our results show that β-endorphin knockout (KO) and heterozygous (HT) mutant mice consume approximately a 50% less 2% NaCl solution compared with wild type mice (WT), after furosemide and low sodium diet treatment. These data suggest that β-endorphin may facilitate induced sodium appetite, giving new evidence about the role of β-endorphin on sodium appetite behavior. Our data also indicate that OT mRNA levels evaluated by in situ hybridization significantly increased within the hypothalamic paraventricular nucleus of WT animals after induced sodium ingestion, giving support to former evidence indicating an inhibitory role for central OT in the control of sodium appetite. Moreover, β-endorphin mutated mice have similar higher levels of OT mRNA expression after the different conditions analyzed: basal, control or experimental, compared with WT mice. Both control HT and KO mice showed higher OT mRNA expression levels than control WT group and these levels did not change after induced sodium intake. Taken together, our data suggest that the reduced sodium ingestion observed in β-endorphin deficient mice could be due to a higher expression of the OT gene. This conclusion would support the hypothesis that OT inhibits sodium intake and provides new evidence about β-endorphin modulation of OT synthesis and sodium appetite.

Body sodium and volume homeostasis.

maintenance of a “milieu interieur,” which allows our cells to function away from the ancient sea, is one of the most important functions of the body. Maintenance of constant extracellular osmolarity and sodium concentration depends on a precise balance between intake and excretion of sodium and

The amygdala: Site of genomic and nongenomic arousal of aldosterone-induced sodium intake

BACKGROUND.: Mineralocorticoids act on the brain to influence sodium intake, and they do so via intracellular type I receptors and possibly also via a direct membrane action, as they do in the kidney. One brain area implicated by lesion studies investigating the regulation of sodium appetite aroused by adrenal steroids is the amygdala. To examine the mechanism by which mineralocorticoids act in the amygdala to arouse salt intake via a genomic and or membrane mode of action, rats were bilaterally fitted with cannulae directed to terminate in the amygdala. The genomic action of mineralocorticoids in arousing sodium intake was investigated by the administration of antisense oligodeoxynucleotides (ASDNs) against the mineralocorticoid receptor, and its effects on deoxycorticosterone (DOCA)-induced sodium intake over the course of several days was examined. The nongenomic action of mineralocorticoids on sodium intake was investigated by implantation into the amygdala of DOCA, aldosterone (ALDO), or their A-ring-reduced tetrahydro derivatives, 15 minutes prior to access to saline. Sodium intake was monitored immediately thereafter. Treatment of rats in the amygdala with ASDN against the mineralocorticoid receptor inhibited DOCA-induced sodium intake, whereas ASDN against the glucocorticoid receptor or sense/scrambled sequences had no effect. DOCA and ALDO increased saline intake within 15 minutes after steroid application. Similarly, the application of A-ring-reduced 3beta,5beta tetrahydroaldosterone and 5 alpha-tetrahydrodeoxycorticosterone produced the same increases in sodium intake. Together, the data imply that adrenal steroids, in addition to acting through classic cytosolic receptors, may also act on membrane receptor systems, producing rapid changes in behavior.

Adrenal steroid regulation of central angiotensin II receptor subtypes and oxytocin receptors in rat brain

The neuropeptides angiotensin II (AngII) and oxytocin (OT) play important but opposing roles in the regulation of sodium appetite in the rat, AngII as a stimulatory peptide and OT as an inhibitory peptide. Adrenal steroids increase the density of AngII receptors in brain following in vivo administration, although the neuroanatomical and subtype specificity have not been thoroughly examined. Furthermore, previous studies demonstrate that adrenalectomy (ADX) leads to a reduction in OT receptors, although regions associated with sodium appetite remain to be examined. In the present study, quantitative receptor autoradiography was used to locate regions where perturbations in circulating adrenal steroids affect the density of oxytocin receptors and the angiotensin receptor subtypes AT1 and AT2. The results show that ADX results in a small, but significant decrease in AT1 expression in the paraventricular nucleus of the hypothalamus, subfornical organ, and the area postrema. That this effect is reversed by either aldosterone or low-dose corticosterone replacement suggests that occupancy of the mineralocorticoid receptor is responsible for the steroid effect. No changes were observed in AT2 or OT receptors in nuclei associated with sodium appetite, indicating that perturbations in adrenal steroids did not affect these receptors in brain regions implicated in the control of salt appetite.

The effects of lesions of the bed nucleus of the stria terminalis on sodium appetite

Lesions of the bed nucleus of the stria termina1is result in decrements in sodium ingestion. Specifically, sodium ingestion that is aroused by mineralocorticoids, in addition to sodium depletion, was reduced in rats with large lesions of the bed nucleus of the stria terminalis. Need-free salt ingestion was also reduced. These results suggest that the bed nucleus of the stria termina1is may be involved in the regulation of sodium appetite.

Mineralocorticoid regulation of salt intake is preserved in hippocampectomized rats.

Considering the high concentration of binding sites for [3H]-aldosterone in the hippocampus, a relationship between these sites and the regulation of sodium appetite by aldosterone was investigated. After surgical hippocampectomy, rats showed an approximately 80% depletion of [3H]-aldosterone binding sites. In spite of this reduction, hippocampectomized rats developed a sodium appetite after adrenalectomy; this sodium appetite was suppressed by continuous administration of aldosterone, a response similar to the pattern found in normal rats. These results suggest that the hippocampus is not the main target of the effect of the hormone on salt intake.