Cyclic estradiol treatment phasically potentiates endogenous cholecystokinin's satiating action in ovariectomized rats.

The influence of ovarian cycling and of exogenous estradiol on the cholecystokinin (CCK) satiety-signalling system was investigated in intact and ovariectomized Long-Evans rats, respectively. Intraperitoneal injection of 1 mg/kg devazepide, the most potent and selective CCK(A) receptor antagonist, increased test meal size during estrus, but not during diestrus, confirming the influence of hypothalamic-pituitary-gonadal function on CCK satiety in intact rats. Devazepide was then tested in ovariectomized rats that received chronic cyclic estradiol (2 microg estradiol benzoate on Tuesday and Wednesday each week) or oil treatment. Devazepide did not increase meal size in estradiol-treated rats on Tuesday, prior to estradiol treatment, compared to oil-treated rats, but did selectively increase meal size on Friday, late in the estradiol replacement cycle, compared to Tuesday, early in the cycle. These results suggest that a phasic potentiation of the endogenous CCK satiety-signalling system is part of the mechanism for the decrease in meal size in female rats during estrus.     

Ovarian hormones and food intake in female guinea pigs and rhesus monkeys

Food intake was measured during ovarian cycles of female guinea pigs and rhesus monkeys. In both species, the period of lowest food intake occurred around the expected time of ovulation. Feeding levels were lowest in the guinea pig during the time of estrus, estrus being estimated on the basis of vaginal membrane changes. Minimum food intake in the rhesus occurred prior to the midpoint of the menstrual cycle, on Day 13, approximately 15–16 days before the next menstruation. In ovariectomized females of both species, either multiple or single estradiol injections significantly lowered food intake. Single progesterone injections to such females were found to have no significant effect on levels of feeding.     

Modulation of appetite by gonadal steroid hormones

Several sex differences in eating, their control by gonadal steroid hormones and their peripheral and central mediating mechanisms are reviewed. Adult female rats and mice as well as women eat less during the peri-ovulatory phase of the ovarian cycle (estrus in rats and mice) than other phases, an effect under the control of cyclic changes in estradiol secretion. Women also appear to eat more sweets during the luteal phase of the cycle than other phases, possibly due to simultaneous increases in estradiol and progesterone. In rats and mice, gonadectomy reveals further sex differences: orchiectomy decreases food intake by decreasing meal frequency and ovariectomy increases food intake by increasing meal size. These changes are reversed by testosterone and estradiol treatment, respectively. A variety of peripheral feedback controls of eating, including ghrelin, cholecystokinin (CCK), glucagon, hepatic fatty acid oxidation, insulin and leptin, has been shown to be estradiol-sensitive under at least some conditions and may mediate the estrogenic inhibition of eating. Of these, most progress has been made in the case of CCK. Neurons expressing estrogen receptor-alpha in the nucleus tractus solitarius of the brainstem appear to increase their sensitivity to CCK-induced vagal afferent input so as to lead to an increase in the satiating potency of CCK, and consequently decreased food intake, during the peri-ovulatory period in rats. Central serotonergic mechanisms also appear to be part of the effect of estradiol on eating. The physiological roles of other peripheral feedback controls of eating and their central mediators remain to be established.     

Estradiol increases glucagon's satiating potency in ovariectomized rats

Estradiol decreases meal size, food intake, and body weight in female rats. To investigate whether these effects of estradiol involve a change in the sensitivity of the signaling pathway through which pancreatic glucagon released during meals contributes to meal termination (satiation), glucagon or glucagon antibodies were infused via the hepatic portal vein in ovariectomized rats that were chronically treated with estradiol benzoate (2 microg/day sc) or vehicle alone (100 microl sesame oil). Infusions began at 1 h after dark onset, as rats were refed after 7 h of food deprivation. Glucagon (3 microg/min for 30 min) decreased feeding during the initial 45 min of food access in both groups of rats, but the inhibition was significantly greater in estradiol- than in oil-treated rats. Similarly, antagonism of endogenous glucagon by infusion of glucagon antibodies (a dose neutralizing 3 ng of glucagon in vitro during the first 3 min of refeeding) increased feeding significantly more in estradiol- than in oil-treated rats. These data indicate that an increase in the activity of the endogenous glucagon satiation-signaling pathway may be part of the mechanism for estradiol's inhibitory effect on feeding.     

Estradiol, CCK and satiation.

Estradiol has long been known to inhibit feeding in animals, but the mechanism(s) mediating its effects have not been clear. Demonstrations that estradiol's feeding effects are expressed as decreases in meal size coupled with the emerging consensus that cholecystokinin (CCK) released from the small intestines during meals is a physiological negative-feedback signal controlling meal size (i.e. satiation) suggested a new approach to the problem of the mechanisms of estradiol's inhibitory effect on feeding. Progress on this approach is reviewed here. Experimental manipulations of exogenous and endogenous CCK and estradiol have produced converging evidence that estradiol cyclically increases the activity of the CCK satiation-signaling pathway so that meal size and food intake decrease during the ovulatory or estrus phase of the ovarian cycle. This is a striking example of the modulation of the operation of a control of meal size by the physiological context in which the meal occurs. Estradiol also produces a tonic decrease in meal size, but this apparently does not involve the CCK satiation-signaling pathway. Where and how estradiol acts to increase the potency of the CCK satiating-signaling pathway are not known. Several possible sites are suggested by the observations that estradiol treatment increases feeding- and CCK-induced expression of c-Fos in ovariectomized animals in brain areas including the nucleus tractus solitarius, paraventricular nucleus of the hypothalamus, and central nucleus of the amygdala. Tests with null mutation mice indicate that estrogen receptor-alpha is necessary for estradiol's feeding effects. Finally, the possibilities that estradiol exerts important influences on normal or disordered eating in women are discussed. It is concluded that estradiol exerts a biologically significant action on CCK satiation in animals. Further research to determine whether this action of estradiol has a role in the pathogenesis, course, or treatment of disordered eating in women is indicated.     

The daidzein- and estradiol- induced anorectic action in CCK or leptin receptor deficiency rats

We investigated the effect of daidzein feeding and estradiol treatment on food intake in cholecystokinin-1 receptor (CCK1R) deficiency, leptin receptor (ObRb) deficiency rats and their wild-type rats. These rats underwent an ovariectomy or a sham operation. For the 5 week experiment, each rat was divided in three groups: control, daidzein (150 mg/kg diet), and estradiol (4.2 μg/rat/day) groups. In both CCK1R+ and CCK1R? rats, daidzein feeding and estradiol treatment significantly decreased food intake. Daidzein feeding significantly reduced food intake in ovariectomized ObRb? rats, although not in ObRb+ rats. Estradiol treatment significantly lowered food intake in ovariectomized ObRb+ and ObRb? rats. In the ovariectomized rats, estradiol treatment significantly increases uterine weight, while daidzein feeding did not change it, suggesting that daidzein might have no or weak estrogenic effect in our experiment. These results suggest that CCK1R and ObRb signalings were not essential for the daidzein- and estradiol-induced anorectic action.     

Acute activation of ER alpha decreases food intake, meal size, and body weight in ovariectomized rats

Estradiol exerts many of its actions by coupling with two nuclear estrogen receptor (ER) proteins, ER alpha, and ER beta. While the acute, anorexigenic effect of estradiol appears to involve such a mechanism, the relative contributions of ERalpha and ERbeta are equivocal. To address this problem, food intake was monitored in ovariectomized (OVX) rats following acute administration of a selective ER alpha agonist (4,4',4'-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol, PPT; dose range = 0-200 microg), a selective ER beta agonist (2,3-bis(4-hydroxyphenyl)-propionitrile, DPN; dose range = 0-600 microg), and a physiological (4 microg) dose of estradiol benzoate (EB). While PPT-treated rats displayed dose-dependent decreases in daily food intake and body weight, neither of these measures was influenced by any dose of DPN. In addition, DPN failed to modulate the anorexigenic effect of PPT when the two ER agonists were coadministered. Meal pattern analysis revealed that the anorexigenic effect of 75 microg PPT (a dose of PPT that produced a similar decrease in daily food intake as 4 microg EB) was mediated by a decrease in meal size, not meal number. Thus, PPT, like EB and endogenous estradiol, decreases food intake by selectively affecting the controls of meal size. The finding that acute administration of 75 microg PPT failed to induce a conditioned taste aversion suggests that the anorexigenic effect of this dose of PPT is not secondary to malaise. Taken together, our findings demonstrate that selective activation of ER alpha decreases food intake, body weight, and meal size in the ovariectomized rat.     

Endogenous cholecystokinin's satiating action increases during estrus in female rats.

Food intake and meal size are reduced in female Long-Evans rats during estrus. To investigate the contribution of the satiating action of endogenous cholecystokinin (CCK) to this, rats were injected with 1 mg/kg of the potent, selective CCK(A) receptor antagonist, devazepide, during diestrus, when meal size is maximal, and during estrus, when it is minimal. Devazepide increased spontaneous food intake and meal size during estrus, but not during diestrus. Meal frequency was not affected by devazepide treatment. These results indicate that the potency of the CCK satiety-signaling system increases during estrus.     

Progesterone stimulates running wheel activity in adrenalectomized-ovariectomized rats.

Daily treatment with progesterone (5 mg) increased running wheel activity, food intake, and body weight of adrenalectomized-ovariectomized rats. These effects of progesterone are quite similar to those of various corticosteroid treatments in adrenalectomized rats reported previously. In addition, the activity-stimulating action of progesterone is just the opposite of its effect in intact and estradiol-primed ovariectomized rats. These observations are consistent with the hypothesis that the principal role of progesterone in the regulation of body weight is to antagonize the actions of estradiol and that the actions of excessive doses of progesterone in adrenalectomized-ovariectomized rats are simply a by-product of its corticosteroidlike, health-promoting properties.     

Reduced feeding response to neuropeptide Y in senescent Fischer 344 rats

The anorexia of aging syndrome in humans is characterized by spontaneous body weight loss reflecting diminished food intake. We reported previously that old rats undergoing a similar phenomenon of progressive weight loss (i.e., senescent rats) also display altered feeding behavior, including reduced meal size and duration. Here, we tested the hypothesis that blunted responsiveness to neuropeptide Y (NPY), a feeding stimulant, occurs concurrently with senescence-associated anorexia/hypophagia. Young (8 mo old, n = 9) and old (24-30 mo old, n = 11) male Fischer 344 rats received intracerebroventricular NPY or artificial cerbrospinal fluid injections. In response to a maximum effective NPY dose (10 microg), the net increase in size of the first meal after injection was similar in old weight-stable (presenescent) and young rats (10.85 +/- 1.73 and 12.63 +/- 2.52 g/kg body wt (0.67), respectively). In contrast, senescent rats that had spontaneously lost approximately 10% of body weight had significantly lower net increases at their first post-NPY meal (1.33 +/- 0.33 g/kg body wt (0.67)) than before they began losing weight. Thus altered feeding responses to NPY occur in aging rats concomitantly with spontaneous decrements in food intake and body weight near the end of life.     

Estradiol treatment increases CCK-induced c-Fos expression in the brains of ovariectomized rats

The ovarian hormone estradiol reduces meal size and food intake in female rats, at least in part by increasing the satiating potency of CCK. Here we used c-Fos immunohistochemistry to determine whether estradiol increases CCK-induced neuronal activation in several brain regions implicated in the control of feeding. Because the adiposity signals leptin and insulin appear to control feeding in part by increasing the satiating potency of CCK, we also examined whether increased adiposity after ovariectomy influences estradiol's effects on CCK-induced c-Fos expression. Ovariectomized rats were injected subcutaneously with 10 microg 17beta-estradiol benzoate (estradiol) or vehicle once each on Monday and Tuesday for 1 wk (experiment 1) or for 5 wk (experiment 2). Two days after the final injection of estradiol or vehicle, rats were injected intraperitoneally with 4 microg/kg CCK in 1 ml/kg 0.9 M NaCl or with vehicle alone. Rats were perfused 60 min later, and brain tissue was collected and processed for c-Fos immunoreactivity. CCK induced c-Fos expression in the nucleus of the solitary tract (NTS), area postrema (AP), paraventricular nucleus of the hypothalamus (PVN), and central nucleus of the amygdala (CeA) in vehicle- and estradiol-treated ovariectomized rats. Estradiol treatment further increased this response in the caudal, subpostremal, and intermediate NTS, the PVN, and the CeA, but not in the rostral NTS or AP. This action of estradiol was very similar in rats tested before (experiment 1) and after (experiment 2) significant body weight gain, suggesting that adiposity does not modulate CCK-induced c-Fos expression or interact with estradiol's ability to modulate CCK-induced c-Fos expression. These findings suggest that estradiol inhibits meal size and food intake by increasing the central processing of the vagal CCK satiation signal.     

Hyperphagia and obesity in OLETF rats lacking CCK-1 receptors

The brain-gut peptide cholecystokinin (CCK) inhibits food intake following peripheral or site directed central administration. Peripheral exogenous CCK inhibits food intake by reducing the size and duration of a meal. Antagonist studies have demonstrated that the actions of the exogenous peptide mimic those of endogenous CCK. Antagonist administration results in increased meal size and meal duration. The feeding inhibitory actions of CCK are mediated through interactions with CCK-1 receptors. The recent identification of the Otsuka-Long-Evans-Tokushima Fatty (OLETF) rat as a spontaneous CCK-1 receptor knockout model has allowed a more comprehensive evaluation of the feeding actions of CCK. OLETF rats become obese and develop non-insulin dependent diabetes mellitus (NIDDM). Consistent with the absence of CCK-1 receptors, OLETF rats do not respond to exogenous CCK. OLETF rats are hyperphagic and their increased food intake is characterized by a large increase in meal size with a decrease in meal frequency that is not sufficient to compensate for the meal size increase. Deficits in meal size control are evident in OLETF rats as young as 2 days of age. OLETF obesity is secondary to the increased food intake. Pair feeding to amounts consumed by intact control rats normalizes body weight, body fat and elevated insulin and glucose levels. Hypothalamic arcuate nucleus peptide mRNA expression in OLETF rats is appropriate to their obesity and is normalized by pair feeding. In contrast, pair fed and young pre-obese OLETF rats have greatly elevated dorsomedial hypothalamic (DMH) neuropeptide Y (NPY) mRNA expression. Elevated DMH NPY in OLETF rats appears to be a consequence of the absence of CCK-1 receptors. In intact rats NPY and CCK-1 receptors colocalize to neurons within the compact subregion of the DMH and local CCK administration reduces food intake and decreases DMH NPY mRNA expression. We have proposed that the absence of DMH CCK-1 receptors significantly contributes to the OLETF's inability to compensate for their meal size control deficit leading to their overall hyperphagia. Access to a running wheel and the resulting exercise normalizes food intake and body weight in OLETF rats. When given access to running wheels for 6 weeks shortly after weaning, OLETF rats do not gain weight to the same degree as sedentary OLETF rats and do not develop NIDDM. Exercise also prevents elevated levels of DMH NPY mRNA expression, suggesting that exercise exerts an alternative, non-CCK mediated, control on DMH NPY. The OLETF rat is a valuable model for characterizing actions of CCK in energy balance and has provided novel insights into interactions between exercise and food intake.     

Effects of gonadal hormones on eating and body weight in mongolian gerbils (Meriones unguiculatus)

The effects of gonadal steroids on food intake and body weight were studied in Mongolian gerbils. Orchiectomy of adult male gerbils caused significant increases in body weight but had no detectable effect on food intake. Treatment with testosterone propionate or 5α-dihydrotestosterone propionate (100 μg/day) had no effect on food intake or body weight of orchiectomized males, but withdrawal of exogenous androgen treatment had the same effect as orchiectomy, increased body weight with no increase in food intake. Treatment with estradiol benzoate (EB; 2 μg/day) increased food intake and body weight of ovariectomized gerbils, but progesterone (1 mg/day) had no effect on these measures when given by itself. However, when progesterone was given concurrently with EB it synergized with the estrogen and further increased eating and body weight. These results are contrasted with previous work in other mammalian species.     

Effects of ovarian hormones on eating behaviors, body weight, and glucoregulation in rhesus monkeys

The influences of ovarian hormones on food intake, taste preferences, and glucoregulation were examined in intact and ovariectomized rhesus monkeys. Intake of intact monkeys was lowest in the preovulatory stage of the cycle, when estrogen levels are elevated, and exogenous estradiol transiently suppressed food intake of ovariectomized monkeys in a dose-related manner, confirming previous observations. Progesterone treatment did not affect food intake when given alone, but it did attenuate the effect of estradiol when both hormones were given concurrently. Preferences for sweet solutions were not detectably influenced by chronic treatment with estradiol or progesterone, and compensatory responses to ingestion of sugar were unaltered by hormone treatment. Glucose tolerance tests did not reveal significant influences of ovarian hormones on glycemia, but insulin levels were elevated during periods of progesterone treatment. These results do not support the suggestion that fluctuations in caloric intake during the menstrual cycle are secondary to changes in taste preference or glucoregulation. However, possible changes in sweet preference and glucoregulation shortly after initiation of estrogen treatment, i.e., during the time of suppressed food intake, remain to be examined.     

Role played by the ovary and adrenals in early receptivity during 4-day cycle in the female rat

The mechanisms involved in the control of precocious sexual receptivity were studied in 4-day cyclic female Wistar rats injected with 10 μg estradiol benzoate (EB) and caged with a male during the night from diestrus II to proestrus. Early mating frequencies were compared in intact females, in animals ovariectomized on the morning of diestrus I, in adrenalectomized and in adrenalectomized-ovariectomized females. No change in early sexual receptivity occurred either in ovariectomized, or in adrenalectomized animals. On the contrary, a significant decrease of precocious mating frequencies was noted in adrenalectomized-ovariectomized females. The role played by the ovary in the control of precocious receptivity was supposed to be due to the secretion of progesterone which has been evidenced on the late afternoon of diestrus II in estrogen treated females.Concerning the mechanisms by which the adrenals may compensate for the ovaries in the control of early sexual receptivity in estrogen-primed females it was observed that notwithstanding an inhibitory action exerted by EB on the adrenal progesterone secretion, a low rate of progesterone was maintained in the peripheral plasma which was compatible with early mating in ovariectomized animals.     

Estradiol treatment increases feeding-induced c-Fos expression in the brains of ovariectomized rats

The steroid hormone estradiol decreases meal size by increasing the potency of negative-feedback signals involved in meal termination. We used c-Fos immunohistochemistry, a marker of neuronal activation, to investigate the hypothesis that estradiol modulates the processing of feeding-induced negative-feedback signals within the nucleus of the solitary tract (NTS), the first central relay of the neuronal network controlling food intake, and within other brain regions related to the control of food intake. Chow-fed, ovariectomized rats were injected subcutaneously with 10 microg 17-beta estradiol benzoate or sesame oil vehicle on 2 consecutive days. Forty-eight hours after the second injections, 0, 5, or 10 ml of a familiar sweet milk diet were presented for 20 min at dark onset. Rats were perfused 100 min later, and brain tissue was collected and processed for c-Fos-like immunoreactivity. Feeding increased the number of c-Fos-positive cells in the NTS, the paraventricular nucleus of the hypothalamus (PVN), and the central nucleus of the amygdala (CeA) in oil-treated rats. Estradiol treatment further increased this response in the caudal, subpostremal, and intermediate NTS, which process negative-feedback satiation signals, but not in the rostral NTS, which processes positive-feedback gustatory signals controlling meal size. Estradiol treatment also increased feeding-induced c-Fos in the PVN and CeA. These results indicate that modest amounts of food increase neuronal activity within brain regions implicated in the control of meal size in ovariectomized rats and that estradiol treatment selectively increases this activation. They also suggest that estradiol decreases meal size by increasing feeding-related neuronal activity in multiple regions of the distributed neural network controlling meal size.     

Characterization of the estrous cycle in Octodon degus

We characterized the reproductive cycle of Octodon degus to determine whether reproductive maturation is spontaneous in juveniles and if ovarian cyclicity and luteal function are spontaneous in adults. Laboratory-reared prepubertal and adult females were monitored for vaginal patency and increased wheel-running. Sexual receptivity was assessed by pairing adult females with a male 1) continuously, 2) at the time of vaginal patency, or 3) following estradiol treatment. Blood samples were assayed for estradiol and progesterone concentrations on Days 1, 4, 8, and 16 relative to vaginal opening. Ovarian tissues were collected 6 and 16 days after behavioral estrus and 6 days after copulation for histology. In juveniles, the onset of cyclic vaginal patency and increased wheel-running activity was spontaneous, occurred in the absence of proximal male cues, and appeared at regular intervals (17.5 ± 1.4 days). In adults, vaginal patency and increased wheel-running occurred cyclically (21.2 ± 0.6 days) in the absence of proximal male cues, and these traits predicted the time of sexual receptivity. Corpora lutea develop spontaneously and are maintained for 12-14 days. The ovaries had well-developed corpora lutea 6 days after mating and 6 days after estrus without mating. Progesterone concentrations were highest in the second half of the cycle when corpora lutea were present and estradiol concentrations peaked on the day of estrus. Thus, female degus appear to exhibit a spontaneous reproductive cycle consistent with other Hystricognathi rodents. Octodon degus is a novel model with which to examine the mechanisms underlying different reproductive cycles.     

Changes in the levels of protein and steroid hormones in the plasma and steroid hormone receptors in the uterus of normal cycling guinea pigs

This study deals with the estrous cycle of guinea pigs in relation to sexual behavior, uterine weight, levels of gonadotropins, steroid hormones, and steroid hormone receptors in the uterus. The guinea pigs in this study showed cyclic changes in various reproductive functions broadly similar to other laboratory species studied. The increase in the uterine weight coincided with high concentration of steroid hormones (estradiol and progesterone) secreted during proestrus and estrus. The elevated levels of steroid hormone receptor concentrations in the uterus during these periods also confirm the role of these hormones. The rise in progesterone level from day 14 of the cycle was associated with lordosis and its related behavior. It was noted that the "estrus behavior" is the most accurate external marker for ovulation and sexual receptivity to males. It was also observed that there is an association between follicle-stimulating hormone and luteinizing hormone during the preovulatory period that was not demonstrated in previous studies.     

Effect of serum estradiol and leptin levels on thyroid function, food intake and body weight gain in female Wistar rats

We evaluated the interplay among estrogen, leptin and thyroid function in the regulation of body mass in female rats. Adult female rats were divided into four groups: control (C, sham-operated), ovariectomized (OVX), ovariectomized treated with estradiol benzoate (Eb) 0.7 or 14 μg/100 g bw per day, during 21 days. OVX led to an increase in body mass, food intake and food efficiency (change in body mass as function of the amount of food ingested) which were normalized by the lower Eb dose, and decreased significantly when the higher dose was given. Serum leptin levels were increased more than two-fold in all ovariectomized groups. Serum T4 levels of the Eb treated OVX were significantly lower than in the controls. Serum T3 and TSH were unaffected by OVX or by Eb treatment. Uterine type 2 iodothyronine deiodinase (D2) activity changed in parallel with serum estradiol: decreased after OVX, returned to control levels after the lower E2 treatment, and increased significantly after the high Eb dosage. The hypothalamic D2 activity was reduced around 30% in all castrated groups, treated or not with estrogen, whereas in the brown adipose tissue the enzyme was not changed. Interestingly, although estrogen-treated OVX rats had lower body weight, serum leptin was high, suggesting that estrogen increases leptin secretion. Our results show that estradiol is necessary for the hypothalamic action of leptin, since the increase in leptin levels observed in all ovariectomized rats was associated with a decrease in food intake and food efficiency only in the rats treated with estrogen.     

Body development and puberty in spontaneously hypertensive rats

The body growth and the onset of puberty in spontaneously hypertensive rats (SHR) and in normotensive controls (WKY) have been studied. In female rats the onset of puberty was determined by both the age and the body weight at which the vaginal opening and first estrus appeared, as well as the ability of estradiol and progesterone to induce pituitary LH release. For this purpose females were injected with estradiol benzoate (0.1 mg/kg) and progesterone (1 mg/rat). Control animals received only oil vehicle. In male rats, puberty was assessed by studying the age and body weight at the time of balano-preputial separation. In another experiment, SH and WKY rats were decapitated on day 30 to determine FSH, LH, PRL, GH and testosterone plasma levels in males and FSH and LH in females. The results obtained show: a) A greater body weight, at all the ages studied (every 4 days between days 28 and 92) in SHR animals. b) A delay in vaginal opening and first estrus presentation in SHR females. c) Absence of spontaneous LH peaks in WKY females. d) Advancement in balano-preputial separation in SHR males and e) Higher plasma FSH levels in SHR males than in WKY males, without differences in other hormones.