Lívia da Rocha Natalino Monteiro

In addition to being recognised for involvement in cardiovascular control and hydromineral balance, the renin‐angiotensin system (RAS) has also been associated with the neuroendocrine control of energy balance. One of the main brain sites for angiotensin II (ANG II)/type 1 receptor (AT₁R) signalling is the subfornical organ (SFO), a circumventricular organ related to the control of autonomic functions, motivated behaviours and energy metabolism. Thus, we hypothesised that circulating ANG II may act on the SFO AT₁R receptors to integrate metabolic and hydromineral balance. We evaluated whether food deprivation can modulate systemic RAS activity and Agrt1a brain expression, and if ANG II/AT₁R signalling influences the hypothalamic expression of mRNAs encoding neuropeptides and food and water ingestion in fed and fasted Wistar rats. We found a significant increase in both ANG I and ANG II plasma levels after 24 and 48 h of fasting. Expression of Agrt1a mRNA in the SFO and paraventricular nucleus (PVN) also increased after food deprivation for 48 h. Treatment of fasted rats with low doses of losartan in drinking water attenuated the decrease in glycemia and meal‐associated water intake without changing the expression in PVN or arcuate nucleus of mRNAs encoding selected neuropeptides related to energy homeostasis control. These findings point to a possible role of peripheral ANG II/SFO‐AT₁R signalling in the control of refeeding‐induced thirst. On the other hand, intracerebroventricular losartan treatment decreased food and water intake over dark time in fed but not in fasted rats.

Background/Aims: Furosemide is a loop diuretic widely used in clinical practice for the treatment of oedema and hypertension. The aim of this study was to determine physiological and molecular changes in the hypothalamic-neurohypophysial system as a consequence of furosemide-induced sodium depletion. Methods: Male rats were sodium depleted by acute furosemide injection (10 and 30 mg/kg) followed by access to low sodium diet and distilled water for 24 h. The renal and behavioural consequences were evaluated, while blood and brains were collected to evaluate the neuroendocrine and gene expression responses. Results: Furosemide treatment acutely increases urinary sodium and water excretion. After 24 h, water and food intake were reduced, while plasma angiotensin II and corticosterone were increased. After hypertonic saline presentation, sodium-depleted rats showed higher preference for salt. Interrogation using RNA sequencing revealed the expression of 94 genes significantly altered in the hypothalamic paraventricular nucleus (PVN) of sodium-depleted rats (31 upregulated and 63 downregulated). Out of 9 genes chosen, 5 were validated by quantitative PCR in the PVN (upregulated: Ephx2, Ndnf and Vwf; downregulated: Caprin2 and Opn3). The same genes were also assessed in the supraoptic nucleus (SON, upregulated: Tnnt1, Mis18a, Nr1d1 and Dbp; downregulated: Caprin2 and Opn3). As a result of these plastic transcriptome changes, vasopressin expression was decreased in PVN and SON, whilst vasopressin and oxytocin levels were reduced in plasma. Conclusions: We thus have identified novel genes that might regulate vasopressin gene expression in the hypothalamus controlling the magnocellular neurons secretory response to body sodium depletion and consequently hypotonic stress.

Neuroimmunology and Neuroinflammation is an open access journal, with focuses on neuroimmunology and neuroinflammation research, and coverage extending to other basic and clinical studies related to neuroscience.

New Findings: What is the central question of this study? Can Na⁺ depletion mobilize Na⁺ from the skin reservoir in ovariectomized rats? Does oestrogen replacement change the amount and the dynamics of skin Na⁺ storage? Is the reduced salt appetite after Na⁺ depletion in ovariectomized rats with oestrogen replacement related to changes in the skin Na⁺? What is the main finding and its importance? This work demonstrated that acute body Na⁺ depletion induced by frusemide mobilized the osmotically inactive skin Na⁺ reservoir to become osmotically active. Oestrogen treatment decreased the induced Na⁺ intake in ovariectomized rats but did not modulate the inactive Na⁺ reservoir in control conditions or its mobilization induced by Na⁺ depletion. Oestradiol, which is an important hormone for water and electrolyte balance, also has a role in the inhibition of induced Na⁺ appetite. Sodium can be stored in the skin in osmotically active or inactive forms, and this skin Na⁺ reservoir may be involved in the control of body Na⁺ levels during physiopathological challenges. In this study, we investigated whether the effect of sodium depletion by frusemide can mobilize Na⁺ from the skin reservoir and whether oestradiol replacement changes or mobilizes the Na⁺ reserves in the skin. Ovariectomized Wistar rats were treated with vehicle or oestradiol for 7 days to evaluate the effects of oestrogen on the hydroelectrolyte balance, intake responses and skin Na⁺ and water content in basal conditions. Furthermore, the effects of oestrogen were evaluated after 24 h frusemide‐induced whole‐body Na⁺ depletion. Oestradiol‐replaced rats exhibited reduced water intake without any significant changes in salt intake, Na⁺ excretion or water and Na⁺ skin content in basal conditions. After sodium depletion, both vehicle‐ and oestradiol‐treated rats exhibited an increase in the osmotically active skin Na⁺, which was associated with a decrease of the inactive skin Na⁺ reservoir. Oestrogen decreased the hypertonic saline intake induced by Na⁺ depletion, but it was not associated with any significant changes in the skin Na⁺ reservoir. Thus, sodium depletion is able to change the inactive–active skin Na⁺ reservoir balance. However, the oestrogenic modulation of sodium appetite after Na⁺ depletion is probably not related to the action of this hormone in the skin Na⁺ reservoir balance.

New Findings: What is the central question of this study? The central goal of this study was to understand the effects of central angiotensin‐(1–7) on basal and osmotically stimulated water intake in rats. What is the main finding and its importance? This study demonstrated that central administration of angiotensin‐(1–7) did not induce thirst in basal conditions but increased water intake after osmotic stimulation, such as water deprivation and salt loading. These results indicate a new function for this peptide, which, in turn, allows for future research on the mechanisms through which angiotensin‐(1–7) influences osmotic thirst. Angiotensin‐(1–7) [Ang‐(1–7)] is generated by type 2 angiotensin‐converting enzyme (ACE2) and binds to the MAS receptor. Although it is well known that Ang‐(1–7) functionally antagonizes the effects of the classical renin–angiotensin system in several situations, the role of Ang‐(1–7) in hydromineral homeostasis is not clear. The aim of this study was to assess the role of Ang‐(1–7) on neuroendocrine responses to hyperosmolality in rats. Male Wistar rats were divided into the following three groups: control; 24 h of water deprivation (WD); and 24 h of salt loading (SL; 1.8% NaCl). Intracerebroventricular (i.c.v.) injections of Ang‐(1–7) or vehicle were given to assess water intake and plasma concentration of vasopressin. Additionally, the brains from control and WD groups were collected to evaluate gene expression in the subfornical organ (SFO), paraventricular nucleus (PVN) and supraoptic nucleus (SON). It was found that i.c.v. Ang‐(1–7) did not change water and salt intake in control rats; however, Ang‐(1–7) increased water intake after WD and SL, with no change in salt intake. Plasma vasopressin was not changed by i.c.v. Ang‐(1–7) in control or WD rats. Moreover, WD increased Mas gene expression in the SON and PVN, with no changes in Ace2 mRNA levels. In conclusion, Ang‐(1–7) increases thirst after osmotic stimuli, indicating that a previous sensitization to its action is necessary. This finding is consistent with the increased Mas gene expression in the PVN and SON after water deprivation.

Sodium appetite is regulated by several signalling molecules, among which angiotensinII(Ang II) serves as a key driver of robust salt intake by binding toAng IItype 1 receptors (AT1R) in several regions in the brain. The activation of these receptors recruits the mitogen‐activated protein kinase (MAPK) pathway, which has previously been linked toAng II‐induced increases in sodium appetite. Thus, we addressed the involvement ofMAPKsignalling in the induction of sodium appetite after 4 days of low‐sodium diet consumption. An increase in extracellular signal‐regulated kinase (ERK) phosphorylation in the laminae terminalis and mediobasal hypothalamus was observed after low‐sodium diet consumption. This response was reduced by i.c.v. microinjection of anAT1R antagonist into the laminae terminalis but not the hypothalamus. This result indicates that low‐sodium diet consumption activates theMAPKpathway viaAng II/AT1R signalling on the laminae terminalis. On the other hand, activation of theMAPKpathway in the mediobasal hypothalamus after low‐sodium diet consumption appears to involve another extracellular mediator. We also evaluated whether a low‐sodium diet could increase the sensitivity forAng IIin the brain and activate theMAPKpathway. However, i.c.v. injection ofAng IIincreasedERKphosphorylation on the laminae terminalis and mediobasal hypothalamus; this increase achieved a response magnitude similar to those observed in both the normal and low‐sodium diet groups. These data indicate that low‐sodium diet consumption for 4 days is insufficient to change theERKphosphorylation response toAng IIin the brain. To investigate whether theMAPKpathway is involved in sodium appetite after low‐sodium diet consumption, we performed i.c.v. microinjections of aMAPKpathway inhibitor (PD98059).PD98059 inhibited both saline and water intake after low‐sodium diet consumption. Thus, theMAPKpathway is involved in promoting the sodium appetite after low‐sodium diet consumption.