Vagus nerve mediates the increase in estrogen receptors in the hypothalamic paraventricular nucleus and nucleus of the solitary tract during fasting in ovariectomized rats

Suprachiasmatic nucleus and vagus nerve trigger preovulatory LH and ovulation

In brief

In the proestrus day, the neural and endocrine signals modulate ovarian function. This study shows vagus nerve plays a role in the multisynaptic pathways of communication between the suprachiasmatic nucleus and the ovaries where such neural information determines ovulation.

Abstract

The suprachiasmatic nucleus (SCN) regulates the activity of several peripheral organs through a parasympathetic-sympathetic pathway. Previously, we demonstrated that atropine (ATR) microinjection in the right SCN of rats during proestrus blocks ovulation. In the present study, we analysed whether the vagus nerve is one of the neural pathways by which the SCN regulates ovulation. For this, CIIZ-V strain cyclic rats on the day of proestrus were microinjected with a saline solution (vehicle) or ATR in the right or left SCN, which was followed by ventral laparotomy or ipsilateral vagotomy to the microinjection side. Some animal groups were sacrificed (i) on the same day of the surgery to measure oestradiol, progesterone and luteinizing hormone (LH) levels or (ii) at 24 h after surgery to evaluate ovulation. The left vagotomy in rats microinjected with ATR in the left SCN did not modify ovulation. In rats with ATR microinjection in the right SCN, the right vagotomy increased the levels of steroids and LH on the proestrus and ovulatory response. The present results suggest that the right vagus nerve plays a role in the multisynaptic pathways of communication between the SCN and the ovaries and indicate that such neural information participates in the regulation of the oestradiol and progesterone surge, which triggers the preovulatory peak of LH and determines ovulation.

Hypothalamic Estrogen Signaling and Adipose Tissue Metabolism in Energy Homeostasis

Obesity has become a global epidemic, and it is a major risk factor for other metabolic disorders such as type 2 diabetes and cardiometabolic disease. Accumulating evidence indicates that there is sex-specific metabolic protection and disease susceptibility. For instance, in both clinical and experimental studies, males are more likely to develop obesity, insulin resistance, and diabetes. In line with this, males tend to have more visceral white adipose tissue (WAT) and less brown adipose tissue (BAT) thermogenic activity, both leading to an increased incidence of metabolic disorders. This female-specific fat distribution is partially mediated by sex hormone estrogens. Specifically, hypothalamic estrogen signaling plays a vital role in regulating WAT distribution, WAT beiging, and BAT thermogenesis. These regulatory effects on adipose tissue metabolism are primarily mediated by the activation of estrogen receptor alpha (ERα) in neurons, which interacts with hormones and adipokines such as leptin, ghrelin, and insulin. This review discusses the contribution of adipose tissue dysfunction to obesity and the role of hypothalamic estrogen signaling in preventing metabolic diseases with a particular focus on the VMH, the central regulator of energy expenditure and glucose homeostasis.

Sex and metabolic state interact to influence expression of passive avoidance memory in rats: Potential contribution of A2 noradrenergic neurons

Competing motivational drives coordinate behaviors essential for survival. For example, interoceptive feedback from the body during a state of negative energy balance serves to suppress anxiety-like behaviors and promote exploratory behaviors in rats. Results from past research suggest that this shift in motivated behavior is linked to reduced activation of specific neural populations within the caudal nucleus of the solitary tract (cNTS). However, the potential impact of metabolic state and the potential role of cNTS neurons on conditioned avoidance behaviors has not been examined. The present study investigated these questions in male and female rats, using a task in which rats learn to avoid a context (i.e., a darkened chamber) after it is paired with a single mild footshock. When rats later were tested for passive avoidance of the shock-paired chamber, male rats tested in an overnight food-deprived state and female rats (regardless of feeding status) displayed significantly less avoidance compared to male rats that were fed ad libitum prior to testing. Based on prior evidence that prolactin-releasing peptide (PrRP)-positive noradrenergic neurons and glucagon-like peptide 1 (GLP1)-positive neurons within the cNTS are particularly sensitive to metabolic state, we examined whether these neural populations are activated in conditioned rats after re-exposure to the shock-paired chamber, and whether neural activation is modulated by metabolic state. Compared to the control condition, chamber re-exposure activated PrRP+ noradrenergic neurons and also activated neurons within the anterior ventrolateral bed nucleus of the stria terminalis (vlBNST), which receives dense input from PrRP+ terminals, in both male and female rats when fed ad libitum. In parallel with sex differences in passive avoidance behavior, PrRP+ neurons were less activated in female vs. male rats after chamber exposure. GLP1+ neurons were not activated in either sex. In both sexes, overnight food deprivation before chamber re-exposure reduced activation of PrRP+ neurons, and also reduced vlBNST activation. Our results support the view that PrRP+ noradrenergic neurons and their inputs to the vlBNST contribute to the expression of passive avoidance memory, and that this contribution is modulated by metabolic state.

Phoenixin influences the excitability of nucleus of the solitary tract neurones, effects which are modified by environmental and glucocorticoid stress

Phoenixin (PNX) is a neuropeptide shown to play roles in the control of reproduction. The nucleus of the solitary tract (NTS), a critical autonomic integrating centre in the hindbrain, is one of many areas with dense expression of PNX. Using coronal NTS slices obtained from male Sprague-Dawley rats, the present study characterised the effects of PNX on both spike frequency and membrane potential of NTS neurones. Extracellular recordings demonstrated that bath-applied 10 nmol L^-1 PNX increased the firing frequency in 32% of NTS neurones, effects which were confirmed with patch-clamp recordings showing that 50% of NTS neurones tested depolarised in response to application of the peptide. Surprisingly, the responsiveness to PNX in NTS neurones then declined suddenly to 9% (P < 0.001). This effect was subsequently attributed to stress associated with construction in our animal care facility because PNX responsiveness was again observed in slices from rats delivered and maintained in a construction-free facility. We then examined whether this loss of PNX responsiveness could be replicated in rats placed on a chronic stress regimen involving ongoing corticosterone (CORT) treatment in the construction-free facility. Slices from animals treated in this way showed a similar lack of neuronal responsiveness to PNX (9.1 ± 3.9%) within 2 weeks of CORT treatment. These effects were specific to PNX responsiveness because CORT treatment had no effect on the responsiveness of NTS neurones to angiotensin II. These results are the first to implicate PNX with respect to directly controlling the excitability of NTS neurones and also provide intriguing data showing the plasticity of these effects associated with environmental and glucocorticoid stress levels of the animal.

Higher Circulating Cortisol in the Follicular vs. Luteal Phase of the Menstrual Cycle: A Meta-Analysis

Although results of animal research show that interactions between stress and sex hormones are implicated in the development of affective disorders in women, translation of these findings to patients has been scarce. As a basic step toward advancing this field of research, we analyzed findings of studies which reported circulating cortisol levels in healthy women in the follicular vs. luteal phase of the menstrual cycle. We deemed this analysis critical not only to advance our understanding of basic physiology, but also as an important contrast to the findings of future studies evaluating stress and sex hormones in women with affective disorders. We hypothesized that cortisol levels would be lower in the follicular phase based on the proposition that changes in levels of potent GABAergic neurosteroids, including allopregnanolone, during the menstrual cycle dynamically change in the opposite direction relative to cortisol levels. Implementing strict inclusion criteria, we compiled results of high-quality studies involving 778 study participants to derive a standardized mean difference between circulating cortisol levels in the follicular vs. luteal phase of the menstrual cycle. In line with our hypothesis, our meta-analysis found that women in the follicular phase had higher cortisol levels than women in the luteal phase, with an overall Hedges' g of 0.13 (p < 0.01) for the random effects model. No significant between-study difference was detected, with the level of heterogeneity in the small range. Furthermore, there was no evidence of publication bias. As cortisol regulation is a delicate process, we review some of the basic mechanisms by which progesterone, its potent metabolites, and estradiol regulate cortisol output and circulation to contribute to the net effect of higher cortisol in the follicular phase.

Regional differences in estradiol effects on numbers of HSD2-containing neurons in the nucleus of the solitary tract of rats

Estrogens affect body fluid balance, including sodium ingestion. Recent findings of a population of neurons in the hindbrain nucleus of the solitary tract (NTS) of rats that are activated during sodium need suggest a possible central substrate for this effect of estrogens. We used immunohistochemistry to label neurons in the NTS that express 11-β-hydroxysteroid dehydrogenase type 2 (HSD2), an enzyme that promotes aldosterone binding, in male rats, and in ovariectomized (OVX) rats given estradiol benzoate (EB) or oil vehicle (OIL). During baseline conditions, the number of HSD2 immunoreactive neurons in the NTS immediately rostral to the area postrema was greater in EB-treated OVX rats compared to those in OIL-treated OVX and male rats. A small number of HSD2 immunoreactive neurons was also labeled for dopamine-β-hydroxylase (DBH), an enzyme involved in norepinephrine biosynthesis. Double-labeled neurons in the NTS were located primarily in the more lateral portion of the HSD2 population, at the level of the area postrema in all three groups, with no sex or estrogen-mediated differences in the number of double-labeled neurons. These results suggest that two subpopulations of HSD2 neurons are present in the NTS. One subpopulation, which does not colocalize with DBH and is increased during conditions of elevated estradiol, may contribute to the effects of estrogens on sodium ingestion. The role of the other, smaller subpopulation, which colocalizes with DBH and is not affected by estradiol, remains to be determined, but one possibility is that these latter neurons are part of a larger network of catecholaminergic input to neuroendocrine neurons in the hypothalamus.

Vagal complex monocarboxylate transporter-2 expression during hypoglycemia

Astrocytic provision of lactate provision to neurons may be a critical indicator of substrate fuel availability in metabolic sensing sites in the brain, including the hindbrain dorsal vagal complex. We examined the hypothesis that vagal complex monocarboxylate transporter protein levels are gender dependent and estrogen dependent, and that estrogen influences adaptation of these protein responses during repeated insulin-induced hypoglycemia. Western blot analyses showed that male and estrogen-treated ovariectomized female rats exhibit opposite changes in monocarboxylate transporter-2 levels after one insulin injection, as well as divergent patterns of adaptation to this metabolic challenge. The data suggest that sex differences in hypoglycemic patterns in vagal complex lactate transport may underlie disparate signaling of cellular energy imbalance.

Inhibition of luteinizing hormone secretion by localized administration of estrogen, but not dihydrotestosterone, is enhanced in the ventromedial hypothalamus during feed restriction in the young wether

The ability of steroids to inhibit LH secretion is enhanced during undernutrition. To identify potential hypothalamic sites at which this enhancement may occur, we examined LH secretion in feed-restricted or fed young wethers treated with locally administered metabolites of testosterone. In experiment 1, microimplants containing crystalline estradiol-17beta (E) or cholesterol were administered via chronic guide tubes directed to the preoptic area (POA) or ventromedial hypothalamus (VMH) in fed or feed-restricted wethers. E treatment in the VMH decreased LH pulse frequency, pulse amplitude, and mean LH concentration in feed-restricted, but not fed, wethers. E may act in the POA to suppress LH under feed restriction, but definite conclusions cannot be drawn because of steroid-independent effects of feed restriction on LH pulse frequency. In experiment 2, the effect of dihydrotestosterone (DHT) in the VMH was determined. DHT administration to the VMH did not alter LH secretion in either feed-restricted or fed wethers. Thus the VMH is one site wherein E negative feedback is enhanced during feed restriction in the wether. In contrast, we found no evidence for enhanced responsiveness to androgen negative feedback within the VMH of feed-restricted wethers. We suggest that increased sensitivity within the VMH to E, but not to DHT, is important for suppressing LH secretion in undernourished male sheep.

Involvement of brainstem catecholaminergic inputs to the hypothalamic paraventricular nucleus in estrogen receptor alpha expression in this nucleus during different stress conditions in female rats

In the present study, we determined the involvement of brainstem catecholaminergic inputs to the paraventricular nucleus (PVN) on estrogen receptor alpha (ERalpha) expression in this nucleus during conditions of 48-h fasting, 2-deoxy-d-glucose (2DG)-induced acute glucoprivation and 1-h immobilization, in ovariectomized rats. Our approach was to examine the effect of lesioning catecholaminergic inputs to the PVN using DSAP [saporin-conjugated anti-DBH (dopamine-beta-hydroxylase)]. Bilateral injection of DSAP into the PVN, 2 wk before stress, prevented fasting-, glucoprivation-, and immobilization-induced increase in ERalpha-immunopositive cells in the PVN. The DBH-immunoreactive (ir) terminals in the PVN were severely depleted by DSAP injection in all experimental groups. Among the brainstem noradreneregic cell groups examined, DBH-ir cell bodies were significantly reduced in the A2 region of all experimental groups treated with DSAP compared with the saporin- and vehicle-injected controls. PVN DSAP injection caused a small, but not significant, decrease in A1 DBH-ir cell bodies in fasted and immobilized rats, and a significant, but slight, reduction in A1 DBH-ir cell bodies of iv 2DG- injected rats compared with PVN vehicle-injected or PVN saporin-injected controls. The A6 DBH-ir cell bodies in all experimental groups treated with DSAP, saporin, or vehicle did not show any significant difference. These results suggest that the brainstem catecholaminergic inputs to the PVN, especially from the A2 cell group, may play a major role in mediating the induction of ERalpha expression in the PVN by metabolic stressors such as fasting, acute glucoprivation, and less specific stressors, such as immobilization, in female rats.

Energy balance and reproduction

The physiological mechanisms that control energy balance are reciprocally linked to those that control reproduction, and together, these mechanisms optimize reproductive success under fluctuating metabolic conditions. Thus, it is difficult to understand the physiology of energy balance without understanding its link to reproductive success. The metabolic sensory stimuli, hormonal mediators and modulators, and central neuropeptides that control reproduction also influence energy balance. In general, those that increase ingestive behavior inhibit reproductive processes, with a few exceptions. Reproductive processes, including the hypothalamic-pituitary-gonadal (HPG) system and the mechanisms that control sex behavior are most proximally sensitive to the availability of oxidizable metabolic fuels. The role of hormones, such as insulin and leptin, are not understood, but there are two possible ways they might control food intake and reproduction. They either mediate the effects of energy metabolism on reproduction or they modulate the availability of metabolic fuels in the brain or periphery. This review examines the neural pathways from fuel detectors to the central effector system emphasizing the following points: first, metabolic stimuli can directly influence the effector systems independently from the hormones that bind to these central effector systems. For example, in some cases, excess energy storage in adipose tissue causes deficits in the pool of oxidizable fuels available for the reproductive system. Thus, in such cases, reproduction is inhibited despite a high body fat content and high plasma concentrations of hormones that are thought to stimulate reproductive processes. The deficit in fuels creates a primary sensory stimulus that is inhibitory to the reproductive system, despite high concentrations of hormones, such as insulin and leptin. Second, hormones might influence the central effector systems [including gonadotropin-releasing hormone (GnRH) secretion and sex behavior] indirectly by modulating the metabolic stimulus. Third, the critical neural circuitry involves extrahypothalamic sites, such as the caudal brain stem, and projections from the brain stem to the forebrain. Catecholamines, neuropeptide Y (NPY) and corticotropin-releasing hormone (CRH) are probably involved. Fourth, the metabolic stimuli and chemical messengers affect the motivation to engage in ingestive and sex behaviors instead of, or in addition to, affecting the ability to perform these behaviors. Finally, it is important to study these metabolic events and chemical messengers in a wider variety of species under natural or seminatural circumstances.

A rat model for the energetic regulation of gonadotropin secretion: role of the glucose-sensing mechanism in the brain

Energy availability has been considered to regulate gonadal activity by modulating the release of gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) at various reproductive phases, such as lactation and puberty in domestic as well as wild animals. Experimental models with rats and sheep have demonstrated that fasting or glucoprivation suppresses pulsatile LH release. From those experiments, the information on energy deficiency is considered to be detected by specific central sensors and conveyed to the hypothalamus to regulate LH release as well as food intake. Noradrenergic neurons, originating in the medulla oblongata and projecting to the hypothalamic paraventricular nucleus (PVN), is reported to be one of the pathways mediating the response of LH release to energy deficiency. The other component is considered to be an energy-sensing mechanism in the brain. Glucose or other oxidizable fuels may function as a metabolic signal to regulate LH release. Previous studies suggest the presence of a glucose-sensing mechanism in the rat hindbrain. From our previous results in the rat, the ependymocytes lining the wall of the cerebroventricle could possibly serve as a glucose sensor to regulate GnRH/LH release. Greater understanding of the nature of the energy-sensing mechanism in the brain will contribute to the nutritional manipulation of reproductive performance in domestic animals in various conditions.

Acute fasting decreases sexual receptivity and neural estrogen receptor-alpha in female rats

Acute food deprivation or chronic food restriction suppresses reproduction in female mammals. Although a link between undernutrition and ovarian function is well established in rats, a similar link with reproductive behavior in this species is yet to be described. Therefore, we compared the display of estrous behaviors induced by exogenous steroid hormone treatment in ovariectomized fed and fasted rats. In addition, estrogen receptor-alpha immunoreactivity (ERIR) was measured in fed and fasted animals to determine whether changes in behavior were associated with changes in the number of detectable ERIR-containing cells in several brain regions. Fasting for 74 h decreased lordosis quotients (LQ) and lordosis ratings (LR) in ovariectomized, steroid-primed rats. The number of detectable ERIR cells decreased after a 74-h fast in the mid-region of the arcuate (ARC), paraventricular (PVN) and ventromedial nuclei of the hypothalamus (VMH) and the ventral bed nucleus of the stria terminalis (BST) but did not change in a number of other areas examined. Taken together, these data demonstrate that, similar to the effect on the reproductive-endocrine axis, food deprivation for 74 h suppresses steroid-induced display of lordosis in adult, female rats. Furthermore, this suppression in sexual receptivity is associated with a decrease in ERIR in a number of areas, including the VMH, a region of the hypothalamus known to be critical for the display of reproductive behaviors in female rats.

Non-metabolic and metabolic factors causing lactational anestrus: rat models uncovering the neuroendocrine mechanism underlying the suckling-induced changes in the mother

Follicular development and ovulation are strongly inhibited during lactation. Administration of a high dose of estrogen induces luteinizing hormone (LH) surges in ovariectomized lactating rats, suggesting that brain mechanisms regulating cyclic LH release remain intact in lactating mothers. On the other hand, tonic LH release is profoundly suppressed in lactating rats. This suggests that lactational anestrus is mainly due to suppression of the mechanism regulating pulsatile gonadotropin-releasing hormone secretion in the hypothalamus, which is responsible for follicular development and steroid production. Both metabolic and non-metabolic factors are involved in suppressing pulsatile LH secretion throughout lactation in rats. During the first half of lactation, pulsatile LH secretion is strongly suppressed, even if milk production is attenuated by pharmacological blockade of prolactin secretion in ovariectomized lactating rats. Pulsatile LH release quickly recovers by removing pups or blocking neuronal input by hypothalamic deafferentation during the period. These data suggest that the suckling stimulus itself is responsible for suppression of LH release during the first half of lactation. During the second half of lactation, negative energy balance, which is caused by the milk production, appears to play a dominant role in suppressing LH secretion. Blockade of milk production by inhibiting prolactin release causes a gradual increase in LH release even if the vigorous suckling stimulus by foster pups remains. In conclusion, the suckling stimulus itself predominantly suppresses LH pulses during the first half of lactation and metabolic factors take over the role of the suckling stimulus during the second half of lactation.

Leptin and metabolic control of reproduction

Leptin treatment prevents the effects of fasting on reproductive processes in a variety of species. The mechanisms that underlie these effects have not been elucidated. Progress in this area of research might be facilitated by viewing reproductive processes in relation to mechanisms that maintain fuel homeostasis. Reproduction, food intake, and fuel partitioning can be viewed as homeostatic responses controlled by a sensory system that monitors metabolic signals. These signals are generated by changes in intracellular metabolic fuel availability and oxidation rather than by changes in the amount of body fat or by changes in any aspect of body composition. Leptin might be viewed as either a mediator or as a modulator of the intracellular metabolic signal. Consistent with its purported action as a mediator of the metabolic signal, leptin synthesis and secretion are influenced acutely by changes in metabolic fuel availability, and these changes might lead to changes in reproductive function. The effects of leptin treatment on reproduction are blocked by treatments that inhibit intracellular fuel oxidation. Metabolic signals that inhibit reproduction in leptin-treated animals might act via neural pathways that are independent of leptin's action. Alternatively, both leptin and metabolic inhibitors might interact at the level of intracellular fuel oxidation. In keeping with the possibility that leptin modulates the metabolic signal, leptin treatment increases fuel availability, uptake, and oxidation in particular tissues. Leptin might affect reproduction indirectly by altering fuel oxidation or other peripheral processes such as gastric emptying. Reproductive processes are among the most energetically expensive in the female repertoire. Because leptin increases energy expenditure while simultaneously inhibiting energy intake, it may have limited use as a long-term treatment for infertility.

Muscarinic blockade inhibits gastric emptying of mixed-nutrient meal: effects of weight and gender

We compared the vagal contribution to gastric emptying in lean and obese subjects by monitoring gastric emptying of a meal during muscarinic blockade. Lean (n = 6) and obese subjects (n = 6) underwent two treatments: 1) saline infusion and 2) atropine infusion [0.4 mg/m2 bolus, 0.4 mg. (m2)-1. h-1] for 2 h, initiated 30 min before ingestion of a 600-kcal breakfast (64% carbohydrate, 23% fat, 13% protein) composed of orange juice (labeled with Indium-111), egg sandwich (labeled with Technetium-99m), cereal, milk, and banana. Anterior and posterior images were taken every 90 s for 90 min using a dual-headed camera. Atropine significantly delayed emptying of both solid (P < 0.007) and liquid (P < 0.002). Obese subjects exhibited a greater delay in liquid emptying during muscarinic blockade compared with lean subjects (P < 0.02). Female subjects exhibited a slower rate of gastric emptying and were less sensitive to atropine. These data suggest that obese subjects exhibit altered gastric cholinergic activity compared with lean subjects and that gender differences in gastric emptying rate may be due to differences in autonomic tone.

Effects of cold exposure on estrous behavior and neural estrogen receptor in Syrian hamsters

Two experiments examined the effects of 72-h exposure to reduced environmental temperature (5 degrees C) on steroid-induced estrous behavior and neural estrogen-receptor immunoreactivity (ERIR) in ovariectomized Syrian hamsters. Cold exposure significantly inhibited sexual receptivity induced by sequential injections of estradiol benzoate (2.5 microg) and progesterone (500 microg), but only if the animals were not permitted to overeat (limited to 110% of ad lib intake at 22 degrees C). The suppression of sexual receptivity was accompanied by decreases in the number of detectable ERIR cells in the ventromedial hypothalamus (VMH) and by increases in the number of ERIR cells in the medial preoptic area (mPOA). The cold-induced decreases in estrous behavior and in VMH ERIR cells were prevented by lesions of the area postrema (AP), but AP lesions did not prevent the increases in mPOA ERIR cells. Thus, cold exposure mimics the effects of treatment with metabolic inhibitors, experimental diabetes, food deprivation, and insulin-induced fattening on these endpoints. These findings are consistent with the hypothesis that dwelling in the cold affects reproduction indirectly via its actions on metabolic fuel availability, rather than by acting directly on neuroendocrine processes.

AP lesions block suppression of estrous behavior, but not estrous cyclicity, in food-deprived Syrian hamsters

Food deprivation inhibits ovulatory cycles and estrous behavior in Syrian hamsters. Lesions of the area postrema (AP) prevented the suppression of estrous behavior in food-deprived hamsters, but they did not prevent the suppression of estrous cyclicity or the increase in running-wheel activity caused by food deprivation. Food deprivation or treatment with pharmacological inhibitors of glycolysis and fatty acid oxidation decreased estrogen-receptor immunoreactivity (ERIR) in the ventromedial hypothalamus (VMH), increased ERIR in the arcuate nucleus (Arc) and the posterior parvicellular paraventricular nucleus (PaPo), but had no effect on ERIR in the posterodorsal medial amygdala or the anterior parvicellular paraventricular nucleus. Lesions of the AP prevented the food deprivation-induced decrease in VMH ERIR and the increase in Arc ERIR, but they did not prevent the increase in ERIR in the PaPo. Thus, whatever physiological cues are produced by food deprivation, an intact AP is required for their transmission to the neural circuits controlling estrous behavior, VMH ERIR, and Arc ERIR. The AP is not essential for transmission of this information to the neural circuits controlling estrous cyclicity, running-wheel activity, or PaPo ERIR.