Estrogen attenuates the drinking response induced by activation of angiotensinergic pathways from the lateral hypothalamic area to the subfornical organ in female rats

Changes in taste palatability across the estrous cycle are modulated by hypothalamic estradiol signaling

Food intake varies across the stages of a rat's estrous cycle. It is reasonable to hypothesize that this cyclic fluctuation in consumption reflects an impact of hormones on taste palatability/preference, but evidence for this hypothesis has been mixed, and critical within-subject experiments in which rats sample multiple tastes during each of the four main estrous phases (metestrus, diestrus, proestrus, and estrus) have been scarce. Here, we assayed licking for pleasant (sucrose, NaCl, saccharin) and aversive (quinine-HCl, citric acid) tastes each day for 5-10 days while tracking rats' estrous cycles through vaginal cytology. Initial analyses confirmed the previously-described increased consumption of pleasant stimuli 24-48 hours following the time of high estradiol. A closer look, however, revealed this effect to reflect a general magnification of palatability-higher than normal preferences for pleasant tastes and lower than normal preferences for aversive tastes-during metestrus. We hypothesized that this phenomenon might be related to estradiol processing in the lateral hypothalamus (LH), and tested this hypothesis by inhibiting LH estrogen receptor activity with ICI 182,780 during tasting. Control infusions replicated the metestrus magnification of palatability pattern; ICI infusions blocked this effect as predicted, but failed to render preferences "cycle free," instead delaying the palatability magnification until diestrus. Clearly, estrous phase mediates details of taste palatability in a manner involving hypothalamic actions of estradiol; further work will be needed to explain the lack of a flat response across the cycle with hypothalamic estradiol binding inhibited, a result which perhaps suggests dynamic interplay between brain regions or hormones.

Significance Statement

Consummatory behaviors are impacted by many variables, including naturally circulating hormones. While it is clear that consumption is particularly high during the stages following the high-estradiol stage of the rodent's estrous (and human menstrual) cycle, it is as of yet unclear whether this phenomenon reflects cycle stage-specific palatability (i.e., whether pleasant tastes are particularly delicious, and aversive tastes particularly disgusting, at particular phases). Here we show that palatability is indeed modulated by estrous phase, and that this effect is governed, at least in part, by the action of estradiol within the lateral hypothalamus. These findings shed light on the mechanisms underlying the adverse impact on human welfare due to irregularities observed across the otherwise cyclic menstrual process.

Permeability of the windows of the brain: feasibility of dynamic contrast-enhanced MRI of the circumventricular organs

BACKGROUND

Circumventricular organs (CVOs) are small structures without a blood-brain barrier surrounding the brain ventricles that serve homeostasic functions and facilitate communication between the blood, cerebrospinal fluid and brain. Secretory CVOs release peptides and sensory CVOs regulate signal transmission. However, pathogens may enter the brain through the CVOs and trigger neuroinflammation and neurodegeneration. We investigated the feasibility of dynamic contrast-enhanced (DCE) MRI to assess the CVO permeability characteristics in vivo, and expected significant contrast uptake in these regions, due to blood-brain barrier absence.

METHODS

Twenty healthy, middle-aged to older males underwent brain DCE MRI. Pharmacokinetic modeling was applied to contrast concentration time-courses of CVOs, and in reference to white and gray matter. We investigated whether a significant and positive transfer from blood to brain could be measured in the CVOs, and whether this differed between secretory and sensory CVOs or from normal-appearing brain matter.

RESULTS

In both the secretory and sensory CVOs, the transfer constants were significantly positive, and all secretory CVOs had significantly higher transfer than each sensory CVO. The transfer constants in both the secretory and sensory CVOs were higher than in the white and gray matter.

CONCLUSIONS

Current measurements confirm the often-held assumption of highly permeable CVOs, of which the secretory types have the strongest blood-to-brain transfer. The current study suggests that DCE MRI could be a promising technique to further assess the function of the CVOs and how pathogens can potentially enter the brain via these structures.

TRIAL REGISTRATION

Netherlands Trial Register number: NL6358, date of registration: 2017-03-24.

Oestradiol effects on neuroendocrine responses induced by water deprivation in rats

Water deprivation (WD) induces changes in plasma volume and osmolality, which in turn activate several responses, including thirst, the activation of the renin-angiotensin system (RAS) and vasopressin (AVP) and oxytocin (OT) secretion. These systems seem to be influenced by oestradiol, as evidenced by the expression of its receptor in brain areas that control fluid balance. Thus, we investigated the effects of oestradiol treatment on behavioural and neuroendocrine changes of ovariectomized rats in response to WD. We observed that in response to WD, oestradiol treatment attenuated water intake, plasma osmolality and haematocrit but did not change urinary volume or osmolality. Moreover, oestradiol potentiated WD-induced AVP secretion, but did not alter the plasma OT or angiotensin II (Ang II) concentrations. Immunohistochemical data showed that oestradiol potentiated vasopressinergic neuronal activation in the lateral magnocellular PVN (PaLM) and supraoptic (SON) nuclei but did not induce further changes in Fos expression in the median preoptic nucleus (MnPO) or subfornical organ (SFO) or in oxytocinergic neuronal activation in the SON and PVN of WD rats. Regarding mRNA expression, oestradiol increased OT mRNA expression in the SON and PVN under basal conditions and after WD, but did not induce additional changes in the mRNA expression for AVP in the SON or PVN. It also did not affect the mRNA expression of RAS components in the PVN. In conclusion, our results show that oestradiol acts mainly on the vasopressinergic system in response to WD, potentiating vasopressinergic neuronal activation and AVP secretion without altering AVP mRNA expression.

Assessment of brain AT1-receptor on the nocturnal basal and angiotensin-induced thirst and sodium appetite in ovariectomised rats

Objective. Considering the controversial data regarding the role of the brain renin-angiotensin system (RAS) on the thirst and sodium appetite in ovariectomised rats, we aimed to evaluate the role of the brain angiotensin II (Ang II) AT1-receptor on the nocturnal fluids intake.

Materials and methods. Groups of Wistar female rats were ovariectomised and chronically given oestrogen or vehicle to evaluate its influence on effects induced by i.c.v. injection of losartan,Ang I and Ang II.

Results. The i.c.v. losartan decreased basal water intake in the ovariectomised group.Ang II but not Ang I-induced nocturnal dipsogenic and natriorexigenic responses in ovariectomised rats. In oestrogen-treated rats, both peptides increased fluids intake. Previously, i.c.v. losartan abolished these effects in all groups. Oestrogen replacement decreased the nocturnal fluids intake, attenuated the losartan and Ang II effects, and highlighted the Ang I response.

Conclusions. The present study has shown for the first time the involvement of AT1-receptor in regulating nocturnal basal water and salt intake in ovariectomised rats. In addition, our data have revealed an unexpected increased brain Ang I-mediated fluid intake in oestrogen-treated ovariectomised compared to ovariectomised rats, which was blocked by previous i.c.v. losartan. Our data have therefore shown that oestrogen influences homeostatic behaviours dependent on brain RAS.

Divergent effects of ERα and ERβ on fluid intake by female rats are not dependent on concomitant changes in AT1R expression or body weight

Estradiol (E2) decreases both water and saline intakes by female rats. The ERα and ERβ subtypes are expressed in areas of the brain that control fluid intake; however, the role that these receptors play in E2's antidipsogenic and antinatriorexigenic effects have not been examined. Accordingly, we tested the hypothesis that activation of ERα and ERβ decreases water and saline intakes by female rats. We found a divergence in E2's inhibitory effect on intake: activation of ERα decreased water intake, whereas activation of ERβ decreased saline intake. E2 decreases expression of the angiotensin II type 1 receptor (AT1R), a receptor with known relevance to water and salt intakes, in multiple areas of the brain where ERα and ERβ are differentially expressed. Therefore, we tested for agonist-induced changes in AT1R mRNA expression by RT-PCR and protein expression by analyzing receptor binding to test the hypothesis that the divergent effects of these ER subtypes are mediated by region-specific changes in AT1R expression. Although we found no changes in AT1R mRNA or binding in areas of the brain known to control fluid intake associated with agonist treatment, the experimental results replicate and extend previous findings that body weight changes mediate alterations in AT1R expression in distinct brain regions. Together, the results reveal selective effects of ER subtypes on ingestive behaviors, advancing our understanding of E2's inhibitory role in the controls of fluid intake by female rats.

Neurohumoral Integration of Cardiovascular Function by the Lamina Terminalis

The mechanisms involved in cardiovascular regulation, such as vascular tone, fluid volume and blood osmolarity, are quite often mediated by signals circulating in the periphery, such as angiotensin II and sodium concentration. Research has identified areas within the lamina terminalis (LT), specifically the sensory circumventricular organs (CVOs), the subfornical organ and the organum vasculosum of the lamina terminalis, as playing crucial roles detecting and integrating information derived from these circulating signals. The median preoptic nucleus (MnPO) is a third integrative structure within the LT that influences cardiovascular homeostasis, although to date, its role is not as clearly elucidated. More recent studies have demonstrated that the CVOs are not only essential in the detection of traditional cardiovascular signals but also signals primarily considered to be important in the regulation of metabolic, reproductive and inflammatory processes that have now also been implicated in cardiovascular regulation. In this review, we highlight the critical roles played by the LT in the detection and integration of circulating signals that provide critical feedback control information contributing to cardiovascular regulation.

Control of fluid intake by estrogens in the female rat: role of the hypothalamus

Body fluid homeostasis is maintained by a complex network of central and peripheral systems that regulate blood pressure, fluid and electrolyte excretion, and fluid intake. The behavioral components, which include well regulated water and saline intake, are influenced by a number of hormones and neuropeptides. Since the early 1970s, it has been known that the ovarian estrogens play an important role in regulating fluid intake in females by decreasing water and saline intake under a variety of hypovolemic conditions. Behavioral, electrophysiological, gene and protein expression studies have identified nuclei in the hypothalamus, along with nearby forebrain structures such as the subfornical organ (SFO), as sites of action involved in mediating these effects of estrogens and, importantly, all of these brain areas are rich with estrogen receptors (ERs). This review will discuss the multiple ER subtypes, found both in the cell nucleus and associated with the plasma membrane, that provide diversity in the mechanism through which estrogens can induce behavioral changes in fluid intake. We then focus on the relevant brain structures, hypothesized circuits, and various peptides, such as angiotensin, oxytocin, and vasopressin, implicated in the anti-dipsogenic and anti-natriorexigenic actions of the estrogens.

Tolerance for chronic heat exposure is greater in female than male mice

PURPOSE

Chronic heat exposure in mice has cellular and physiological effects that improve thermal tolerance [1], but also modifies innate immune responses with potential adverse consequences [2]. While male and female mice are known to respond differently to acute exposure to severe hyperthermia, sex-based differences in responses to chronic moderate heat exposure have not been reported. The major objective of this study was to compare the tolerance of male and female mice for chronic heat exposure.

MATERIALS AND METHODS

We used a mouse model of 5-day moderate heat exposure (ambient temperature ∼37°C) to compare the physiological and cellular heat shock response in male and female mice. Core temperature, heart rate, and activity were monitored telemetrically and heat shock protein levels were measured in brain and lung by western blotting.

RESULTS

Adult CD-1 female mice maintained a 1.2°C lower core temperature (38.31 ± 0.64 versus 39.51 ± 0.72°C; p = 0.002), experienced less weight loss (1.54 ± 0.45 versus 4.54 ± 1.97 g; p = 0.0007), and had improved survival (16/16 survived versus 13/21, p < 0.006) than male mice of the same age. After 5 days of moderate heat exposure Hsp72 levels in brain and lung increased 2.1-fold (p = 0.007) and 5-fold (p = 0.048) in male mice compared with 1.3- (p = 0.054) and 1.5-fold (p = 0.134) in female mice.

CONCLUSIONS

This study reveals previously unknown and potentially important differences between male and female mice in physiological and cellular responses to chronic heat exposure, which had consequences for survival. Future studies may identify biomarkers of differential heat tolerance and treatments to improve heat tolerance in humans.

Neuroendocrine regulation of fluid and electrolyte balance by ovarian steroids: contributions from central oestrogen receptors

Like other hormonally mediated mechanisms, maintenance of body fluid osmolality requires integrated responses from multiple signals at various tissue locales, a large number of which are open to modulation by circulating endocrine factors including the ovarian steroid, oestrogens (E(2)). However, the precise mechanism and the site of action of E(2) in regulating fluid osmolality are not properly understood. More importantly, the biological significance of this action is not clear and the physiological circumstances in which this modulation is engaged remain incomplete. The demonstration of oestrogen receptors (ER) in neural tissues that bear no direct relation to reproduction led us to examine and characterise the expression of ER in brain nuclei that are critical for the maintenance of fluid osmolality. In the rat, ERbeta is prominently expressed in the vasopressin magnocellular neuroendocrine cells of the hypothalamus, whereas ERalpha is localised extensively in the sensory circumventricular organ neurones in the basal forebrain. These nuclei are the primary brain sites that are engaged in defense of fluid perturbation, thus providing a neuroendocrine basis for oestrogenic influence on body fluid regulation. Plasticity in receptor expression that accompanies fluid disturbances at these central loci suggests the functional importance of the receptors and implicates E(2) as one of the fluid regulating hormones in water homeostasis.

Estrogen influences stimulated water intake by ovariectomized female rats

To further elucidate the influence of estrogen on water consumption, we examined water intake by adult female rats stimulated by water deprivation, injection of hypertonic saline or injection of isoproterenol (ISOP), a beta-adrenergic agonist that activates the renin-angiotensin system (RAS). Rats were ovariectomized (OVX) then injected with estradiol benzoate (EB; 10 microg/0.1 ml oil) or the oil vehicle (OIL; 0.1 ml) for 2 consecutive days. Twenty-four hours after the second injection, rats were deprived of food and water. On the following day, rats were given water and intake was measured after 2 h. EB significantly decreased water intake compared with that by OIL-treated rats following water deprivation. Two additional groups of adult female rats were OVX and treated with EB or OIL. Forty-eight hours after EB or OIL treatment, rats were injected with hypertonic saline (1 ml of 2 M NaCl) or ISOP (30 microg/kg in 0.15 M saline) and water intake was measured after 2 h. EB significantly attenuated water intake following ISOP but not after hypertonic saline. Finally, we examined plasma sodium concentration (pNa) after hypertonic saline and plasma renin activity (PRA) after ISOP in EB- and OIL-treated rats and found no differences in pNa or PRA. These results suggest that the stimuli for water intake after hypertonic saline and ISOP were comparable in EB- and OIL-treated rats. Taken together, these results raise the possibility that EB attenuation of stimulated water intake is specific to water intake elicited by activation of the RAS.

Angiotensin II reduces serotonin release in the rat subfornical organ area

In the present study we used intracerebral microdialysis techniques to examine whether angiotensin II (ANG II) modulates the release of serotonin (5-hydroxytryptamine, 5-HT) in the subfornical organ (SFO) in freely moving rats. Extracellular concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the region of the SFO were significantly decreased by microinjection of ANG II (10 pmol, 50 nl), but not by vehicle, into the dialysis site. No significant changes in the 5-HT and 5-HIAA levels caused by ANG II were observed in the sites away from the SFO. Water ingestion significantly enhanced the amount of the decrease in the 5-HT and 5-HIAA concentrations in the SFO area elicited by the ANG II injection. These results show that ANG II may reduce the release of 5-HT in the SFO area, and imply that the 5-HT receptor mechanism in the SFO area may participate in the elicitation of the drinking behavior to ANG II.

Drinking attenuates the noradrenaline release in the lateral hypothalamic area induced by angiotensin II activation of the subfornical organ in rats

Experiments were conducted to investigate the role of noradrenergic systems in the lateral hypothalamus area (LHA) in the water intake caused by injection of angiotensin II (ANG II) into the subfornical organ (SFO) in rats. Intracerebral microdialysis techniques were utilized to quantify the extracellular content of noradrenaline (NA) in the LHA. Microinjection of ANG II into the SFO significantly increased NA release in the LHA when water was not available for drinking. The increase in the release of NA in the LHA was significantly attenuated by water intake. In urethane-anesthetized rats, injections of ANG II into the SFO significantly enhanced the release of NA in the LHA that accompanied an elevation in mean arterial pressure (MAP). Intravenous administration of the alpha-agonist metaraminol, on the other hand, significantly decreased the NA release in the LHA that accompanied an increase in MAP, suggesting that the enhanced NA release in the LHA caused by ANG II into the SFO may be not mediated by increasing in arterial pressure. These results show the involvement of the noradrenergic systems in the LHA in the dipsogenic response induced by angiotensinergic activation of the SFO.

Reduced dipsogenic response induced by angiotensin II activation of subfornical organ projections to the median preoptic nucleus in estrogen-treated rats

The present study was carried out to investigate whether estrogen modulates the drinking response induced by activation of angiotensinergic neural pathways from the subfornical organ (SFO) to the median preoptic nucleus (MnPO). Microinjection of angiotensin II (ANG II, 10(-10) M, 0.2 microl) into the SFO elicited drinking in ovariectomized (OVX) female rats that were treated with either propylene glycol (PG) vehicle or estrogen benzoate (EB). The amount of water intake induced by the ANG II injection was significantly greater in the PG-treated than in the EB-treated animals. In both groups of female rats, previous injections of saralasin (Sar, 10(-10) M, 0.2 microl), a specific ANG II antagonist, into the MnPO resulted in the significant attenuation of the drinking response to ANG II, showing that the ANG II-induced drinking response may be mediated in part by the angiotensinergic SFO projections to the MnPO. Injections of ANG II (10(-10) M, 0.2 microl) into the MnPO caused drinking in both groups, while no significant difference was found between the groups in the amount of water intake. These results suggest that increases in the circulating level of estrogen may attenuate the drinking response induced by ANG II activation of the SFO projections to the MnPO.

Estrogen decreases the responsiveness of subfornical organ neurons to angiotensinergic neural inputs from the lateral hypothalamic area in the female rat

Twenty-eight subfornical organ (SFO) neurons in ovariectomized (OVX) female rats that were treated with propylene glycol (PG) vehicle and 26 SFO neurons in OVX female rats that were treated with estrogen benzoate (EB) were antidromically activated by electrical stimulation of the hypothalamic paraventricular nucleus (PVN) under urethane anesthesia. No significant differences were observed between the PG-treated and EB-treated OVX animals in the latency, conduction velocity, or threshold of antidromic activation. The mean spontaneous discharge rate was significantly lower in the EB-treated than in the PG-treated OVX animals. In both groups, the activity of the majority (86% in the PG-treated animals and 88% in the EB-treated animals) of identified SFO neurons were activated by microiontophoretic application of angiotensin II (ANG II). Electrical stimulation of the lateral hypothalamic area (LHA) increased the excitability of these ANG II-sensitive SFO neurons (58% in the PG-treated animals and 52% in the EB-treated animals). The excitatory response to either ANG II or LHA stimulation was blocked by microiontophoretic application of the ANG II antagonist saralasin (Sar), suggesting that the excitatory response to LHA stimulation may be mediated by angiotensinergic LHA projections to the SFO. The magnitude of excitatory response to either ANG II or the LHA stimulation was much greater in the PG-treated than in the EB-treated animals. These results suggest that estrogen decreases the responsiveness of SFO neurons projecting to the PVN to angiotensinergic inputs from the LHA.