Brain oxytocin receptor antagonism disinhibits sodium appetite in preweanling 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 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.

Angiotensinergic pathway through the median preoptic nucleus in the control of oxytocin secretion and water and sodium intake

Our objective was to study in which situations the median preoptic nucleus (MnPO) interferes with the control of oxytocin secretion and salt intake and the possible mediation of angiotensin II (AII) through their AT1 receptors. Lesion of the MnPO by ibotenic acid in male rats did not change water and NaCl intake in conditions of ad libitum offer, water deprivation or salt load, but it did cause significant decrease of NaCl intake in sodium depleted animals. These animals presented higher water intake or lower NaCl intake after microinjection of AII or losartan into the MnPO, respectively. They decreased plasma oxytocin after microinjection of losartan into the MnPO, but not of AII or isotonic saline. Oxytocin secretion induced by hypertonic saline i.p. was reduced by microinjection of AII, but not losartan into the MnPO. On the other hand, microinjection of losartan in this area, but not AII, reduced plasma oxytocin in animals submitted to the isotonic saline i.p. Thus, it seems that the sodium intake control is performed by MnPO neurons through the stimulatory action of angiotensin II on AT1 receptors under sodium depletion, but not water deprivation or salt overload neither of ad libitum water and salt intake condition. On the other hand, in the high-sodium condition, endogenous angiotensin did not act on MnPO neurons to the control of oxytocin secretion while exogenous angiotensin inhibited oxytocin secretion. These results indicate two possible angiotensinergic neural circuits: one is stimulating and the other is inhibiting oxytocin secretion, depending on sodium balance.

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

Central injection of ANG II has been proposed to have dual effects on salt appetite including a direct stimulatory effect and an indirect inhibitory effect through an activation of central oxytocinergic neurons. The inhibition was demonstrated by pretreating rats with central ornithine vasotocin (OVT; oxytocin antagonist) 30 min before a central ANG II injection. The OVT pretreatment produced a large increase in ANG II-induced saline intake. The present paper reports a failure to replicate that influential experiment. However, we also report for the first time that OVT by itself: 1) provokes drinking of both water and saline solution with a latency almost as short as that produced by ANG II; 2) produces a mild pressor response; and 3) increases c-Fos expression in the organum vasculosum laminae terminalis (OVLT) and the median preoptic nucleus (MnPO). Oxytocin activity may provide an inhibitory control of drinking responses as has been suggested, but the inhibition is tonic and includes both water and saline drinking. Inhibition of this tonic activity may stimulate drinking by increasing neural activity in the OVLT and MnPO.

Salt appetite and the renin-angiotensin system: effect of oxytocin deficiency

To explore the role of oxytocin in the regulation of salt appetite and blood pressure, we conducted studies in oxytocin gene-knockout mice and determined (1) blood pressure and heart rate during day and night periods, (2) salt appetite after iso-osmotic volume depletion, and (3) salt appetite and blood pressure after central injection of angiotensin II. Long-term arterial catheters were inserted, and blood pressure and heart rate were recorded for 24 hours. There was a modest decrease in blood pressure and heart rate in knockout mice. Salt appetite was measured with a 2- bottle choice (water and 2% NaCl), with measurement of licking activity. Mice were injected subcutaneously with 30% polyethylene glycol (0.5 mL), and voluntary intakes were measured for 24 hours. Knockout mice consumed 3 times the amount of NaCl than did controls, 276+/-77 vs 90+/-38 licks/24 h (P<0.05). Water consumption was similar between groups. Angiotensin II (5, 50, and 200 ng/3 microL) injected intracerebroventricularly produced dose-related increases in intake, with no differences between the groups. The 50-ng dose of angiotensin II elicited salt and water intakes of 151+/-43 vs 160+/-33 licks and 250+/-53 vs and 200+/-51 licks, respectively (control vs knockout). The pressor response to angiotensin II was not different between the groups. Results suggest that oxytocin plays a role in the regulation of blood pressure and salt appetite, specifically as mediated by volume receptors, and that the renin-angiotensin system is not involved in these changes.

Enhanced salt intake in oxytocin deficient mice

The maternal roles of oxytocin (OT) are well known, but recent work suggests that OT is also a vital component in fluid balance regulation. To explore the role of OT in salt/volume regulation, we studied NaCl intake in a genetically modified mouse strain lacking OT. Using male control and OT knockout mice (OTKO), we determined the circadian pattern of salt and water intake under need-free conditions. For the study of intake, a two-bottle choice system was used to provide access to water and 2% NaCl with computerized monitoring of licking activity. Salt licking activity (licks/24 h) for controls was 59 +/- 22 vs. 380 +/- 105 in OTKO (P < 0.05). The volume of salt consumed (ml/24 h) was 0.4 +/- 0.1 in controls vs. 1.8 +/- 0.4 in OTKO (P < 0.01). There was no statistical difference in the consumption of water between the groups. However, the initiation of water intake was shifted, with an advancement of almost 3 h in OTKO (P < 0.01). Differences in the timing of salt intake could not be determined due to the low volume of salt consumed by controls. Taken together, these data show that removal of OT amplifies the salt-seeking behavior associated with normal daily fluid fluctuations. The fact that OTKO voluntarily consume a normally aversive salt solution further implies that OT is a powerful regulator of circadian salt appetite.