Sex Hormones, Regulatory Behaviors, and Body Weight

A hamster model for stress-induced weight gain

This review addresses the translational relevance of animal models of stress and their effects on body weight. In humans, stress, whether chronic or acute, has often been associated with increased food intake and weight gain. In view of the current obesity epidemic, this phenomenon is especially relevant. Such observations contrast with reports with commonly used laboratory animals, especially rats and mice. In these species, it is common to find individuals gaining less weight under stress, even with potent social stressors. However, there are laboratory species that present increased appetite and weight gain under stress, such as golden hamsters. Furthermore, these animals also include metabolic and behavioral similarities with humans, including hoarding behavior which is also enhanced under stress. Consequently, we propose that our comparative perspective provides useful insights for future research on the development of obesity in humans as a consequence of chronic stress exposure.

Sex and timing of gonadectomy relative to puberty interact to influence weight, body composition, and feeding behaviors in mice

Gonadal sex steroids are important regulators of energy balance in adult rodents, and gonadectomy (GDX) has opposing effects on weight gain in sexually mature males and females. Puberty is associated with the emergence of sex differences in weight, body composition, and feeding behaviors, yet the role of gonadal hormones at puberty remains unclear. To address this, we performed GDX or sham surgery in male and female C57Bl/6 mice at postnatal day (P)25 (prepubertal) or P60 (postpubertal) timepoints and measured weight and body composition for 35 days, after which ad libitum and operant food intake was measured using Feeding Experimentation Device 3 (FED3s) in the home cage. Consistent with previous studies, postpubertal GDX caused weight gain in females and weight loss in males and increased adiposity in both sexes. However, prepubertal GDX decreased weight gain and altered body composition across the adolescent transition (P25 to P60) in males but had no effect in females. Despite the varied effects on weight, GDX decreased food intake and motivation for food as assessed in operant tasks regardless of sex or timing of surgery relative to puberty. Our findings indicate that GDX interacts with both sex and age at surgery to influence weight, body composition, and feeding behavior.

The Rise and Fall of Physiological Theories of the Control of Human Eating Behavior

Kuhns was the first to suggest that theories in science do not develop in small increments but rather in major leaps to paradigms that examine the same question through very different perspectives. Theories on the mechanism responsible for control of human food intake fall into Kuhn’s description. This article describes how the two major theories of the control of food intake in humans, the Glucostatic Theory, and the Lipostatic Theory, showed initial promise as explanations, but later deteriorated with the slow accumulation experimental data. The locus of theories considered eating behavior as a part of physiological system that regulates the storage of energy on the body. We challenge this fundamental belief with data which suggests that we must be ready to accept a major change in the way we think about eating behavior if we are ever to decrease the prevalence of obesity.

Adiposity, reproductive and metabolic health, and activity levels in zoo Asian elephant (Elephas maximus)

Many captive Asian elephant populations are not self-sustaining, possibly due in part to obesity-related health and reproductive issues. This study investigated relationships between estimated body composition and metabolic function, inflammatory markers, ovarian activity (females only) and physical activity levels in 44 Asian elephants (n=35 females, n=9 males). Deuterium dilution was used to measure total body water from which fat mass (FM) and fat-free mass (FFM) could be derived to estimate body composition. Serum was analyzed for progestagens and estradiol (females only), deuterium, glucose, insulin and amyloid A. Physical activity was assessed by an accelerometer placed on the elephant's front leg for at least 2 days. Relative fat mass (RFM) - the amount of fat relative to body mass - was calculated to take differences in body size between elephants into consideration. Body fat percentage ranged from 2.01% to 24.59%. Male elephants were heavier (P=0.043), with more FFM (P=0.049), but not FM (P>0.999), than females. For all elephants, estimated RFM (r=0.45, P=0.004) was positively correlated with insulin. Distance walked was negatively correlated with age (r=-0.46, P=0.007). When adjusted for FFM and age (P<0.001), non-cycling females had less fat compared with cycling females, such that for every 100 kg increase in FM, the odds of cycling were 3 times higher (P<0.001). More work is needed to determine what an unhealthy amount of fat is for elephants; however, our results suggest higher adiposity may contribute to metabolic perturbations.

Neuroendocrinology of reward in anorexia nervosa and bulimia nervosa: Beyond leptin and ghrelin

The pathophysiology of anorexia nervosa (AN) and bulimia nervosa (BN) are still poorly understood, but psychobiological models have proposed a key role for disturbances in the neuroendocrines that signal hunger and satiety and maintain energy homeostasis. Mounting evidence suggests that many neuroendocrines involved in the regulation of homeostasis and body weight also play integral roles in food reward valuation and learning via their interactions with the mesolimbic dopamine system. Neuroimaging data have associated altered brain reward responses in this system with the dietary restriction and binge eating and purging characteristic of AN and BN. Thus, neuroendocrine dysfunction may contribute to or perpetuate eating disorder symptoms via effects on reward circuitry. This narrative review focuses on reward-related neuroendocrines that are altered in eating disorder populations, including peptide YY, insulin, stress and gonadal hormones, and orexins. We provide an overview of the animal and human literature implicating these neuroendocrines in dopaminergic reward processes and discuss their potential relevance to eating disorder symptomatology and treatment.

Sex Differences in Binge Eating: Gonadal Hormone Effects Across Development

Eating disorders are highly sexually differentiated disorders that exhibit a female predominance in risk. Most theories focus on psychosocial explanations to the exclusion of biological/genetic influences. The purpose of this descriptive review is to evaluate evidence from animal and human studies in support of gonadal hormone effects on sex differences in binge eating. Although research is in its nascent stages, findings suggest that increased prenatal testosterone exposure in males appears to protect against binge eating. Although pubertal testosterone may exert additional protective effects, the prenatal period is likely critical for the decreased risk observed in males. By contrast, studies indicate that, in females, it is the lack of prenatal testosterone coupled with the organizational effects of pubertal ovarian hormones that may lead to increased binge eating. Finally, twin data suggest that changes in genetic risk may underlie these hormone influences on sex differences across development.

Effects of Stimulated Physical Activity on the Circadian Pacemaker of Vertebrates 1

A dogma in the field of circadian rhythms is that in order to keep accurate time, pacemakers that generate such rhythms must be relatively independent of changes in the external and internal environment. While it is true that the period of circadian oscillators is conserved within a narrow range, regardless of alterations in the external and internal envi ronment, numerous perturbations have now been found that can change the period and/or induce a phase shift in circadian pacemakers. Many of these perturbations also alter the overall level of activity and/or metabolic state of the organism. In 1960, Aschoff suggested that alterations in the "level of excitement" may induce changes in circadian clocks. Although little attention has been given to this hypothesis over the past three decades, recent findings support its validity and open new avenues for studying the function and organization of circadian clock systems.

Shaking the myth: Body mass, aggression, steroid hormones, and social dominance in wild house mouse

In social mammals, the position of a male in the group's hierarchy strongly affects his reproductive success. Since a high social rank is often gained through competition with other males, selection should favour bigger males over smaller ones. We may therefore predict faster growth and/or delayed sexual maturity in dominant males. Likewise, dominants should have higher levels of testosterone, hormone important in many aspects of male dominance. Less obvious is the relationship between dominance and levels of corticosterone but generally higher concentrations are expected in subordinate individuals. We studied body growth, sexual maturation and endocrinal changes in males of two house mouse subspecies, raised in fraternal pairs. Since Mus musculus domesticus is the subspecies which dominates mutual encounters with Mus musculus musculus we predicted higher growth rate, delayed puberty and aggression, and higher testosterone and corticosterone levels in domesticus males compared to musculus. In all comparisons, no differences were found between dominant and subordinate musculus brothers. On the other hand, in M. m. domesticus, dominant males revealed a different growth trajectory and lower corticosterone levels than subordinate males but not delayed puberty and higher testosterone concentrations, thus contradicting our predictions. In inter-subspecific comparisons, musculus males matured earlier but became aggressive at the same time as domesticus males. The musculus testosterone ontogeny suggests that social positions in this subspecies remain unfixed for an extended period and that the increasing levels probably reflect prolonged hierarchy contests. It appears that the ontogeny of behaviour and physiological traits diverge cryptically between the two subspecies.

Potential risk of isoflavones: toxicological study of daidzein supplementation in piglets

As a main component of soy isoflavones, daidzein is rich in soy-derived products, which are widely used as feed ingredients in farm animals. However, little research has been conducted on the side effects of dietary daidzein, especially in young animals. In this study, the safety of daidzein was evaluated. Results show that ingesting 400 mg/kg of dietary daidzein for 70 days is associated with a lower average daily weight gain (kilogram) (0.47 ± 0.03 vs 0.54 ± 0.04, P < 0.05) and a higher splenic damage index (1.00 ± 1.10 vs 0.00 ± 0.00, P < 0.05) in young pigs compared with control. Female pigs receiving 200 and 400 mg/kg daidzein showed reduced serum testosterone levels (ng/L) on days 35 and 70 compared with the control group (day 35, 246 ± 74 and 224 ± 20 vs 362 ± 48, P < 0.05; day 70, 252 ± 38 and 219 ± 77 vs 374 ± 38, P < 0.05). Daidzein residue (μg/kg) in pig livers increased (243 ± 80 vs 142 ± 47, P < 0.05, day 70). These results suggest that dietary supplements of 400 mg/kg of daidzein negatively affect the weight gain and splenic morphology of pigs.

Puberty as a critical risk period for eating disorders: a review of human and animal studies

This article is part of a Special Issue "Puberty and Adolescence". Puberty is one of the most frequently discussed risk periods for the development of eating disorders. Prevailing theories propose environmentally mediated sources of risk arising from the psychosocial effects (e.g., increased body dissatisfaction, decreased self-esteem) of pubertal development in girls. However, recent research highlights the potential role of ovarian hormones in phenotypic and genetic risk for eating disorders during puberty. The goal of this paper is to review data from human and animal studies in support of puberty as a critical risk period for eating disorders and evaluate the evidence for hormonal contributions. Data are consistent in suggesting that both pubertal status and pubertal timing significantly impact risk for most eating disorders in girls, such that advanced pubertal development and early pubertal timing are associated with increased rates of eating disorders and their symptoms in both cross-sectional and longitudinal research. Findings in boys have been much less consistent and suggest a smaller role for puberty in risk for eating disorders in boys. Twin and animal studies indicate that at least part of the female-specific risk is due to genetic factors associated with estrogen activation at puberty. In conclusion, data thus far support a role for puberty in risk for eating disorders and highlight the need for additional human and animal studies of hormonal and genetic risk for eating disorders during puberty.

Role of testosterone in mediating prenatal ethanol effects on hypothalamic-pituitary-adrenal activity in male rats

Prenatal ethanol (E) exposure programs the fetal hypothalamic-pituitary-adrenal (HPA) and -gonadal (HPG) axes such that E rats show HPA hyperresponsiveness to stressors and altered HPG and reproductive function in adulthood. Importantly, prenatal ethanol may differentially alter stress responsiveness in adult male and female offspring compared to their control counterparts. To test the hypothesis that alterations in HPA activity in E males are mediated, at least in part, by ethanol-induced changes in the capacity of testosterone to regulate HPA activity, we explored dose-related effects of testosterone on HPA and HPG function in adult male offspring from prenatal E, pair-fed (PF) and ad libitum-fed control (C) dams. Our data suggest that E males show changes in both HPA and HPG regulation, as well as altered sensitivity to the inhibitory effects of testosterone. While gonadectomy (GDX) reduced weight gain in all animals, low testosterone replacement restored body weights in PF and C but not E males. Further, sensitivity of the thymus and adrenal to circulating testosterone was reduced in E rats. In addition, stress-induced corticosterone (CORT) levels were increased in PF and C but not E males following GDX, and while low dose testosterone replacement restored CORT levels for PF and C, high testosterone levels were needed to normalize CORT levels for E males. A negative correlation between pre-stress testosterone and post-stress CORT levels in C but not in E and PF males further supports the finding of reduced sensitivity to testosterone. Importantly, testosterone appeared to have reduced effects on central corticotrophin releasing hormone (CRH) pathways in E, but greater effects on central arginine vasopressin (AVP) pathways in E and/or PF compared to C males. Testosterone also had less of an inhibitory effect on stress-induced luteinizing hormone increases in E than in PF and C males following GDX. In addition, androgen receptor mRNA levels in the medial preoptic nucleus and the principal nucleus of posterior bed nucleus of the stria terminalis were lower in E and PF compared to C males under intact conditions. Together, these data support our previous work suggesting altered sensitivity to testosterone in E males. Furthermore, differential effects of testosterone on the complex balance between central CRH and central AVP pathways may play a role in the HPA alterations observed. That some findings were similar in E and PF males suggest that nutritional effects of diet may have played a role in mediating at least some of the changes seen in E animals.

Validity of test meals for determining binge eating

Assessment of binge eating has been criticized because of serious doubts concerning the accuracy of self-report. This experiment tested the validity of a laboratory test meal as an indicator of binge eating. Eight individuals diagnosed with binge-eating disorder (BED), eight obese non-binge-eaters, and eight normal-weight non-binge-eaters ate a test meal under conditions designed to increase the likelihood of inducing a binge episode. Non-binge-eaters, regardless of weight, felt in control of their eating and ate a relatively small amount of the test meal, while participants with BED ate significantly more food and felt significantly more out of control. Eating behavior during test meals can be a useful indicator of BED diagnostic status and may be a useful method for objectively defining binge eating.

No time to eat: an adaptationist account of periovulatory behavioral changes

A comprehensive review of women's dietary behavior across the menstrual cycle suggests a drop in caloric intake around the time of ovulation; similar patterns occur in many other mammals. The periovulatory nadir is puzzling, as it is not explicable in terms of changes in the energy budget. Existing explanations in the animal literature operate wholly at the proximate level of analysis and hence do not address this puzzle. In this paper, I offer an ultimate explanation for the periovulatory feeding nadir, arguing that the decrease in the set point for satiation during the fertile period of the female cycle is an adaptation produced by natural selection in order to reduce the motivational salience of goals that compete with those directly or indirectly pertaining to mating. In support of this explanation, I adduce evidence of: a) periovulatory reductions in other ingestive behaviors, and b) periovulatory increases in motor activity and the psychological concomitants thereof.

Cyclic estradiol treatment enhances the effects of interleukin-1beta on food intake in female rats

Proinflammatory cytokines elicit behavioral and physiological responses that include decreased food intake, fever, and a general disinterest in usual activities. Ovarian hormones modulate immune system activity and responsiveness to cytokines in female mammals, suggesting that sex differences in immune function may be influenced by gonadal steroids. In this experiment, female adult rats were ovariectomized and given two daily subcutaneous injections of 5.0 or 20.0 microg of estradiol benzoate or the oil vehicle 3 weeks after surgery. Following 2 days of hormone treatment, animals received ip injections of interleukin-1beta (IL-1beta) or saline 1 h before light offset. Food and water intake was measured 2 h after light offset. The results indicate that a cyclic pattern of estradiol treatment enhances the anorectic effect of IL-1beta and suggest that responses to immune system activation are influenced by estradiol.

Changes in progesterone metabolites in the hippocampus can modulate open field and forced swim test behavior of proestrous rats

The purpose of these experiments was to test the hypothesis that attenuating the endogenous increase of the 5alpha-reduced progesterone metabolite 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP) in the hippocampus will alter anxiety and depression behavior of proestrous rats. In Experiment 1, anxiety (open field) and depression (forced swim test) behavior was compared of rats that should have high (proestrous) and low (diestrous and male rats) endogenous hippocampal 3alpha,5alpha-THP. Proestrous rats exhibited more anxiolytic-like (increased central entries in the open field) and anti-depressant-like (less immobility in the forced swim test) behavior than diestrous or male rats. In Experiments 2 and 3, respectively, systemic and intrahippocampal finasteride, a 5alpha-reductase inhibitor which attenuates progesterone's metabolism to 3alpha,5alpha-THP, versus vehicle administration to proestrous rats was compared for effects on open field and forced swim test behavior. Systemic or intrahippocampal finasteride decreased central entries in the open field and increased immobility in the forced swim tests compared to vehicle administration. In Experiment 4, the effects of systemic and intrahippocampal finasteride vs vehicle administration on hippocampal 3alpha,5alpha-THP of proestrous rats was examined. Finasteride, SC or intrahippocampally, reduced 3alpha,5alpha-THP in the hippocampus compared to vehicle administration. Together these data suggest that variations in 3alpha,5alpha-THP levels in the hippocampus may mitigate proestrous changes in anxiety and depressive behavior of cycling rats.

Focus on primary care: evaluation, management, and treatment of obesity in women

Obesity is the number one nutritional disorder in the developed world. It is now well recognized that obesity is a disease that represents a rapidly growing epidemic. It is often a lifelong problem that preferentially affects women. Modern science and medicine are just beginning to unravel the multifactorial pathophysiology of obesity. Current literature emphasizes the metabolic, hormonal, and behavioral interactions as well as genetic predisposition. This understanding has led to new recommendations for the management of the obese patient and to new pharmaceutical approaches for those at high risk. These risks include diabetes, cardiovascular disease, hypertension, gallbladder disease, and osteoarthritis. In addition, there are a number of reproductive health problems unique to obese women. Until recently, however, the evidence for benefit of weight loss was not clear. There are now numerous Category A data indicating significant benefit for weight loss as it affects multiple comorbidities It is, therefore, incumbent upon the providers of women's health care to assess patient risk and to offer resources, guidance, assistance, and reassurance to the overweight and obese patient as she embarks on a program of renewed health through weight loss.

LEARNING OBJECTIVES

After completion of this article, the reader will be able to describe the classification of overweight and obesity, explain the pathophysiology of obesity, outline the evaluation and screening of the obese patient, and list potential therapeutic interventions for weight loss.

Regulation of leptin gene expression and secretion by steroid hormones

Previous work has shown that 17 beta-estradiol is the primary ovarian signal regulating body weight and adiposity, although its mechanisms of action remain unclear. We hypothesized that 17 beta-estradiol could enhance leptin levels as a mechanism of its anorectic effects. Administration of 5 microg 17 beta-estradiol subcutaneously (s.c.) for 2 days significantly elevated leptin mRNA levels in adipose tissue as compared to vehicle controls (P < 0.003). A time-course administration of estrogen showed increased mRNA levels in adipose tissue between 6 and 12 h after estrogen injection as compared to vehicle controls (P < 0.03). Corresponding to the increased leptin mRNA levels at 6 and 12 h, elevated plasma leptin levels were observed at 12 h after estrogen administration as compared to controls (P < 0.05). Administration of progesterone (1 mg/rat) after estradiol injection did not enhance the elevated leptin mRNA levels in adipose tissue. Serum leptin levels from cycling rats did not differ significantly between metestrous and proestrous animals. In conclusion, the present studies demonstrate that 17 beta-estradiol can regulate leptin gene expression and secretion in the female rat, thus providing a better understanding of the possible anorectic effect of estrogens.

Effects of megestrol acetate and testosterone on body composition in castrated male Sprague-Dawley rats

The interrelationships among sex hormones, caloric intake, and intermediary metabolism in health and disease are uncertain. Studies in malnourished patients with AIDS and cancer show that megestrol acetate (MA) therapy increases appetite, body weight, and body fat, while it decreases serum testosterone (T) concentration. In this study, the separate and combined effects of MA and T were investigated in 65 young adult, male, castrated, Sprague-Dawley rats who received subcutaneous implants containing placebo, MA, T, or both MA and T for 11 wk. By hierarchical multiple regression analysis, MA therapy decreased weight gain and food intake (P < 0.01), increased body fat (P = 0.024), decreased body protein (P < 0.001), and decreased the portion of calories accrued as protein rather than fat (P ratio, P < 0.03). T alone decreased fat (P < 0.03), but had no significant effect on food intake, the relative number of consumed calories utilized for growth (food efficiency), body weight, or protein. The interaction of MA and T did not affect food intake or food efficiency, but increased body weight (P < 0.02), protein (P < 0.05) and the P ratio (P < 0.02). The portion of weight gain as fat was reduced from 47.3% with MA alone to 27.4% when MA and T were combined. Thus, megestrol acetate has significant antianabolic effects that are independent of its effects upon food intake. The addition of testosterone to megestrol acetate partially antagonized MA's inhibition of lean mass accretion in these rats.

Sex-specific effects of prolactin on food intake by rats

While prolactin (PRL) has been reported to increase food intake by virgin female rats, its effects on food intake by male rats are relatively unexplored. The present studies examined the possibility that PRL has sex-specific effects on food intake by rats. In the first study, intact female and male rats were given subcutaneous injections of saline vehicle or ovine (o) PRL (1.0 mg/kg) twice daily at 08:00 and 20:00 h for 10 days. Food intake, body weight, and water intake were measured daily. Results indicate that oPRL administration increased food intake by an average of 4.5 g per day in female subjects, but did not significantly alter body weight or water intake. Male rats treated with oPRL did not significantly alter their food intake, even after an additional five days of treatment. In the second study, a wide range of oPRL doses (vehicle, 0.02, 0.2, 2.0, and 20.0 mg/kg/day) were tested in gonadectomized female and male rats. The results indicate that female rats responded to increasingly larger doses of oPRL with greater increases in food intake, with a maximum increase of approximately 6. 1 g per day at a dose of 20.0 mg/kg. In contrast, male rats maintained baseline levels of intake across all oPRL doses tested. These data suggest that PRL has sex-specific effects on food intake.

The estrogenic and antiandrogenic pesticide methoxychlor alters the reproductive tract and behavior without affecting pituitary size or LH and prolactin secretion in male rats

This study was designed to determine if long-term exposure to high doses of methoxychlor (M) would alter pituitary or testicular endocrine functions in either an estrogenic or antiandrogenic manner. Weanling male Long-Evans hooded rats were dosed daily with M (po) at 0, 200, 300, or 400 mg kg-1 day-1 for 10 months. Methoxychlor treatment delayed puberty by as much as 10 weeks and reduced fertility and copulatory plug formation in a dose-related manner at the initial mating. During mating, M-treated males exhibited shorter latencies to mount and ejaculate versus control males, but the number of intromissions prior to ejaculation was unaffected, indicating that M enhanced the arousal level in the males in an estrogen-dependent manner. Most treated males eventually mated but time-to-pregnancy was lengthened. Very low sperm counts were associated with infertility, while prolonged delays in puberty reduced fecundity. Methoxychlor treatment with 200 to 400 mg kg-1 day-1 failed to mimic the chronic effects of a sustained (8 months) low dose of estradiol-17 beta (3-mm silastic implants) on pituitary or testicular hormone levels. Estradiol administration increased pituitary weight 4-fold, serum levels of luteinizing hormone (LH) were reduced by almost 50%, and serum prolactin was increased 40-fold, while M did not affect any of these measures. These data demonstrate that M affects the CNS, epididymal sperm numbers, and the accessory sex glands and delays mating without significantly affecting the secretion of LH, prolactin, or testosterone. These data indicate that M did not alter pituitary endocrine function in either an estrogenic or antiandrogenic manner. To our knowledge, these data provide the first in vivo example of such a pronounced degree of target tissue selectivity to an environmental endocrine-disrupting chemical.

The influence of sex hormones on obesity across the female life span

Women have a higher prevalence of obesity than men in most developed countries. Obesity affects many aspects of women's health by increasing risk for heart disease, diabetes, breast cancer, and infertility. One reason for the gender difference in obesity may be that fluctuations in reproductive hormone concentrations throughout women's lives uniquely predispose them to excess weight gain. Studies in experimental animals and women have shown that hormonal changes across the menstrual cycle affect calorie and macronutrient intake and alter 24-hour energy expenditure. Pregnancy is a significant factor in the development of obesity for many women. Various factors are associated with excess weight retention following pregnancy, including weight gain during pregnancy, ethnicity, dietary patterns, and interval between pregnancies. There is a need to tailor recommendations for energy intake during pregnancy to individual women, and recent evidence also suggests that the timing of weight gain during pregnancy is a critical factor. Menopause is also a high-risk time for weight gain in women. Although the average woman gains 2-5 pounds during menopausal transition, some women are at risk for greater weight gains. There is also a hormonally driven shift in body fat distribution from peripheral to abdominal at menopause, which may increase health risks in older women. Hormone therapies have varying impacts on body weight and fat distribution. In summary, hormonal fluctuations across the female life span may explain the increased risk for obesity in women. Awareness of these factors allows development of targets for prevention and early intervention.

Hormone replacement modifies cholecystokinin-induced changes in sucrose palatability in ovariectomized rats

The taste reactivity test was used to examine the effect of CCK-octapeptide (CCK-8) on the palatability of a sucrose solution in ovariectomized rats either receiving hormonal replacement (estradiol and progesterone; OVX + HRT), or treated with vehicle only (OVX + VEH). Statistical analyses revealed that the OVX + HRT rats treated with CCK-8 exhibited a robust decrease in ingestive responses, and an increase in aversive responses and passive drips to the intraoral sucrose infusions, relative to treatment with the NaCl vehicle. In contrast, a weak effect of CCK-8 on ingestive responses, no significant effect on the frequency of aversive responses, and a reduced effect on passive drips was observed in the OVX + VEH rats. These results show that CCK-8 modifies sucrose palatability, and that this effect is modulated by gonadal hormone levels.

Food cravings, endogenous opioid peptides, and food intake: a review

Extensive research indicates a strong relationship between endogenous opioid peptides (EOPs) and food intake. In the present paper, we propose that food cravings act as an intervening variable in this opioid-ingestion link. Specifically, we argue that altered EOP activity may elicit food cravings which in turn may influence food consumption. Correlational support for this opioidergic theory of food cravings is provided by examining various clinical conditions (e.g. pregnancy, menstruation, bulimia, stress, depression) which are associated with altered EOP levels, intensified food cravings, and increased food intake.

Maternal hormonal manipulations in rats cause obesity and increase medial hypothalamic norepinephrine release in male offspring

In previous work it has been shown that adult male, but not female, offspring of rats that have either been injected with Protamine Zinc Insulin on days 15-20 gestation, or undernourished during the first 2 weeks of gestation, develop significant obesity commencing at about 50 days of age. The present experiment examines the question of whether rats with these two forms of obesity display neurochemical abnormalities in areas of the brain known to influence food intake and body weight. Twenty-one gauge stainless steel guide shafts were surgically implanted using standard stereotaxic procedures. One week later 26 ga microdialysis probes were lowered into the medial hypothalamus. Dialysates collected from male offspring in the two experimental conditions contain significantly higher norepinephrine (NE) levels than did controls. It would appear that in addition to sharing a similar time course of onset and a sex dependent expression of obesity, both of these models are also characterized by elevated medial hypothalamic NE. Since this obesity appears only in males, and at a time when testosterone levels are rapidly rising in males, and since testosterone has been shown to elevate food intake and body weights in rats, we also investigated whether gonadal weights or circulating testosterone levels were differentially elevated by our manipulations.

The aversive and hypophagic effects of estradiol

Estradiol is known to reduce food intake in many species. Recent studies have also shown that estradiol can function as an unconditioned stimulus in taste aversion paradigms, suggesting that it induces nausea and malaise in rats and mice. The experiments reported here compared the hypophagic and aversive effects of estradiol. Using mice as subjects, the first investigation examined the taste aversion properties of the estradiol receptor antagonist MER-25, which is estrogenic with respect to feeding. MER-25 induced a strong taste aversion, contrary to a previous report. Second, progesterone, which counteracts the hypophagic effects of estradiol, did not disrupt the taste aversion induced by estradiol in mice. The third investigation used the Mongolian gerbil, a species in which estradiol increases food intake, in contrast to other species. Despite increasing food intake, estradiol induced a conditioned taste aversion in the gerbil similar to that seen in rats and mice. Taken together, these results indicate that the feeding and aversive effects of estrogen are mediated by different mechanisms.

Effects of systemic and central prolactin injections on food intake, weight gain, and estrous cyclicity in female rats

It has been suggested that prolactin may contribute to the hyperphagia of lactation. Studies examining the effects of the systemic administration of prolactin on food intake, however, have produced varying results. In species other than the rat, central prolactin administration has been found to increase food intake, but it is not known if central prolactin administration increases food intake in rats. In the current study the effects of peripheral and central prolactin administration on food intake, weight gain, and estrous cyclicity in female rats were compared. Prolactin was administered twice daily at 0800 and at 1900 h either subcutaneously at 3 mg/kg or 1 mg/kg b.wt. or by intracerebroventricular (ICV) infusion (2 micrograms/0.5 microliter) for 10 days to female rats. Control animals received similar injections of vehicle. Food intake, body weight, and vaginal smears were taken daily. Results showed that peripheral administration of prolactin increased food intake and weight gain and disrupted vaginal cyclicity. In contrast, ICV administration increased food intake to the same extent as did systemic prolactin administration but had no effect on weight gain or cyclicity. These data suggest that prolactin acts both peripherally and centrally to regulate energy balance in the female rat.

Treatment with alpha-helical-CRF(9-41) prevents the anorectic effect of 17-beta-estradiol

The role of corticotropin-releasing factor (CRF) in the anorexia induced by 17-beta-estradiol (E2) has been assessed in castrated female rats that were trained to eat their daily food ration in three separate meals. Each rat was implanted with a permanent guide cannula that was aimed at the right lateral ventricle of the brain. Seven days after the brain surgery each rat was also subcutaneously implanted with an osmotic minipump containing Buserelin, a potent GnRH agonist that induces reversible castration in rats. Eight rats were used in the study, and each of them underwent four experimental treatments that consisted of a) a subcutaneous (SC) injection of oil combined with an intracerebroventricular (ICV) infusion of saline, b) a SC injection of E2 combined with an ICV infusion of saline c) a SC injection of oil combined with an ICV infusion of alpha-helical CRF(9-41), and d) a SC injection of E2 combined with an ICV injection of alpha-helical CRF(9-41). Subcutaneous injections of E2 or oil were carried out the day before the ICV infusions of alpha-helical CRF(9-41) or saline. Intracerebroventricular infusions were performed 30 min before the meal for which the interaction effect of E2 and alpha-helical CRF(9-41) on food intake was determined. E2 and alpha-helical CRF(9-41) interacted on food intake; E2 brought about a 33% reduction in food intake in rats when infused with saline, whereas it was without effect when infused with alpha-helical-CRF(9-41)-treated rats. The present results provide evidence that CRF is involved in the anorectic effect of E2.

Taste and food preference changes across the course of pregnancy

The present study investigates taste and specific food consumption changes across the course of pregnancy. These variables could potentially play a role in excess pregnancy-associated weight gains. Pregnant and postpartum women were asked to consume a series of everyday foods in the laboratory. Consumption and taste perception of each food were measured. In contrast to the self-report literature on cravings and aversions during pregnancy, which emphasizes changes in the first trimester, this study found that women in the second trimester consumed significantly more sweet food, but not salty or non-sweet/non-salty food, as compared with women at any other point in pregnancy. Subjects were restrained eaters, and so possibly refrained from daily consumption of excess sweet foods. This study suggests that psychological variables may interact with behavioral and physiological variables to control food preferences and eating in pregnancy.

Developmental changes in sugar and starch taste preferences in young rats

Recent studies indicate that neonatal and adult rats are attracted to the taste of sugar as well as to starch-derived polysaccharides (e.g., Polycose). The present study investigated whether the relative preference for sweet and starchy tastes changes during the postweaning to adulthood period in male and female rats. This was accomplished by giving growing rats 24-hr/day one-solution (sucrose or Polycose vs. water) and two-solution (sucrose vs. Polycose) preference tests over a 9-week period (from 28 to 98 days of age). Acceptance (absolute intake) of sucrose and Polycose was also evaluated by comparing solution intakes during one-solution tests with water baseline intakes. Dilute solutions were used to minimize postingestive caloric and/or colligative effects. When tested with 2% saccharide solutions, the rats strongly preferred sucrose to Polycose with the preference increasing in magnitude from weeks 1 to 9 of testing. Sucrose acceptance also increased, relative to that of Polycose, after the third test week. When tested with 0.06 molar solutions, the rats initially consumed more Polycose (week 1) but by the fifth test week their sucrose preference and acceptance exceeded that of Polycose. These results indicate that the preference for sweet taste, relative to that for starchy taste, increases with age. Contrary to previous findings, the sweet taste preference was somewhat stronger in the male rats than in the female rats.

Does dietary hyperphagia contradict the lipostatic theory?

It has frequently been suggested that body weight or fat somehow exerts an inhibitory influence on food intake in a way that acts to maintain a stable body weight or fat. The principal evidence supporting this idea is that animals that have been induced to overeat and become overweight by various means, eat less than control rats when they are permitted to eat freely. If the degree of suppression of appetite by overweight is as large as several experiments suggest, then dietary hyperphagia should be self-limiting. Any overeating induced by dietary treatments should disappear after animals become moderately overweight. Animals fed some kinds of hyperhagia-promoting diets do show this pattern. However, animals fed other kinds of diets do not show this pattern, and with most diets, dietary hyperphagia continues for extended periods. This implies that either 1) overweight does not suppress appetite as much as suggested by various authorities, 2) dietary manipulations can override normal regulatory mechanisms, or 3) certain diets induce irreversible changes in body fat that are not evident from changes in body weight.

Importance of feeding time in pair-fed, ovariectomized rats

The present investigation was designed to determine the effects of feeding time on body weight gain in ovariectomized (OVX) rats pair-fed with sham operated control rats. In pair feeding, a test and a control rat were each fed the same amount of food. In the first experiment, OVX rats and controls were pair-fed at 1900, just before the dark phase of the light-dark cycle. Body weight gain was observed for 36 days. There was no significant difference in weight gain between the two groups. In the second experiment, when rats were pair-fed chow, starting from 1200, there was a significant increase in the body weight of the OVX group compared to the sham operated control group. Observation continued for 56 days. In the third experiment, the OVX group in the pair-fed condition and the sham control group were pair-fed at 1900 for 56 days and feeding was then switched to 0500 (just before the beginning of the light cycle) for 28 days. The body weight gain in the OVX pair-fed group was quite similar to that of the sham control group when feeding started at 1900, but a significant increase in body weight gain of the OVX group was observed after switching the starting time of feeding to 0500. In conclusion, the results suggest that the time of daily feeding may be important for body weight gain during the dynamic phase after ovariectomy.

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.

Food craving, mood and the menstrual cycle

Seventy-six women, with a mean age of 35.7 years, who reported premenstrual craving for sweet foods in a retrospective questionnaire, were assessed prospectively with a pre- and post-menstrual eating questionnaire and daily ratings of craving, mood, irritability and breast tenderness over two menstrual cycles. In 72% of these women a perimenstrual pattern of food craving was confirmed. In 13% this craving was confined to the menstrual phase. There was no consistent association between food craving and mood change, either in timing or severity. Women with more severe mood change did not report more severe craving. There was also no association between food craving and cyclical breast tenderness. Perimenstrual food craving, therefore, appears to be a cyclical phenomenon in its own right, of uncertain aetiology and worthy of further study.

Peripubertal castration of male rats, adult open field ambulation and partner preference behavior

The validity of the hypothesis put forward earlier, that testicular secretions during puberty have an organizing effect on open field ambulation was examined. Male rats were castrated or sham-operated at days 21, 43 or 70. At the age of 17 weeks the males were tested in an automated, octagonal open field (3 consecutive days, 3 min/day) for locomotor activity. Male rats castrated at day 21 or day 43 ambulated more than sham-castrated controls. Males castrated at day 70 did not differ from sham-castrated controls. It thus appears that pubertal testicular secretion(s) organize adult open field locomotor activity in male rats. From 18 weeks of age partner preference behavior was tested in the same open field apparatus with one adjacent cage containing an ovariectomized female and an opposite one containing an ovariectomized female brought into heat. The females in the adjacent cages were separated from the experimental males in the octagonal cage by wire mesh. Peripubertally castrated males did not show a clear-cut partner preference, whereas the intact males preferred the vicinity of the estrous female. There were no differences among the males castrated either before, during or after puberty. Testosterone treatment (crystalline T in silastic capsules) caused peripubertally castrated males to prefer the estrous female. Thus, adult partner preference behavior does not seem to be organized by peripubertal testicular androgens.

Effects of a brain-enhanced chemical delivery system for estradiol on body weight and serum hormones in middle-aged male rats

We have developed a redox-chemical delivery system for brain-enhanced drug delivery of estradiol based on an interconvertible dihydropyridine in equilibrium pyridinium salt carrier. Estradiol, when combined with the carrier, readily crosses the blood-brain barrier and upon oxidation of the carrier is "locked" in the brain. The aim of this study was to evaluate the effects of the estradiol-chemical delivery system (E2-CDS) on body weight change and associated alterations in the secretion of anterior pituitary hormones in middle-aged, male rats. The data revealed that rats receiving E2-CDS exhibited a significant weight loss by 2 days which continued to day 14, the last observation day. A significant weight difference was observed between E2-CDS and DMSO-treated animals. Serum estradiol levels of rats treated with E2-CDS were elevated 100-fold by day 1 and decreased thereafter and serum prolactin concentrations were doubled by 24 hours and continued to increase to the completion of the experiment. Testosterone levels were markedly suppressed by 24 hours while serum levels of LH, TSH, T3, T4 and GH were not significantly altered. These data indicate that the E2-CDS causes a long-term reduction in body weight and testosterone secretion and that these changes are not mediated by alterations in the secretion of anterior pituitary hormones.

Further evidence of noradrenergic regulation of rat hypothalamic estrogen receptor concentration: possible non-functional increase and functional decrease

The regulation of estrogen receptors by the alpha 2-noradrenergic system was studied. A single injection of the alpha 2-noradrenergic antagonist, yohimbine, caused a biphasic effect on the concentration of cytosol estrogen receptors in the mediobasal hypothalamus and anterior pituitary gland. A short-latency increase was seen at 1.5-3 h, followed by a longer-lasting decrease at 8-16 h. Scatchard analysis revealed that the apparent, short-latency increase is in the concentration of binding sites, not in the affinity of the receptor for [3H]estradiol. The increase in the concentration of cytosol estrogen receptors is not blocked by pretreatment with the alpha 2-noradrenergic agonist, clonidine. In addition, no increase is detected in the concentration of cell nuclear estrogen receptors accumulating in response to a saturating dose of estradiol. Therefore, the apparent increase in the concentration of cytosol estrogen receptors may not represent a functional increase in receptors. The decrease in the concentration of estrogen receptors, which occurs 8-16 h after yohimbine treatment, is also seen after injection of the alpha 2-adrenergic antagonist, idazoxan, and is not due to a change in the in vitro rate of association of the receptors with [3H]estradiol. Furthermore, the decrease seems to be a functional decrease in the concentration of receptors capable of cell nuclear accumulation in response to estradiol injection, as indicated by the results of experiments in which the concentration of cell nuclear estrogen receptors was assayed after estradiol injection. These experiments provide further support for the hypothesis that the alpha-noradrenergic system, and perhaps specifically the alpha 2-subtype, is involved in decreasing the concentration of estrogen receptors in parts of the brain and pituitary gland. This interaction provides a mechanism by which the environment could regulate the sensitivity of certain neurons to estradiol. However, the finding that the initial increase in the concentration of cytosol estrogen receptors after yohimbine treatment is not followed by the predicted increase in cell nuclear estrogen receptors after estradiol injection raises questions about the physiological relevance of the apparent increase under some conditions.

Species differences in polysaccharide and sugar taste preferences

This study compared the polysaccharide and sugar taste preferences of humans and four rodent species (laboratory rats, Rattus norvegicus; Golden Syrian hamsters, Mesocricetus auratus; Mongolian gerbils, Meriones unguiculatus; Egyptian spiny mice, Acomys cahirinus). In Experiment 1 human subjects rated the pleasantness, sweetness, and flavor intensity of polysaccharide (Polycose), sucrose, and maltose solutions at concentrations of 0.0125 M to 0.4 M, and 1% to 32% concentrations. At the higher molar concentrations Polycose was rated as less sweet and less pleasant than the sucrose and maltose solutions; there were no differences in the flavor intensity ratings. With the percent concentrations Polycose was rated as less sweet and less flavorable as the sucrose and maltose solutions; there were no reliable differences in the pleasantness ratings. In Experiment 2, the Polycose, sucrose, and maltose preferences of rats, hamsters, gerbils, and spiny mice were compared using 24 hr two-bottle tests (saccharide vs. water) at concentrations of 0.001 M, 0.005 M, 0.01 M, and 0.1 M. In general, the rats displayed stronger preferences for Polycose and maltose than did the other three species. In addition, the gerbils showed a stronger Polycose preference at the 0.1 M concentration than did the hamsters and spiny mice, and the spiny mice display a weaker preference for sucrose than did the other three species. Within species comparisons revealed that all four species displayed preferences for Polycose that were as strong or stronger than their preferences for sucrose and maltose. With only a few exceptions, male and female rodents did not differ in their saccharide preferences. Thus, while rats show the most robust Polycose preference of the four rodent species, all four species were attracted to the taste of polysaccharides. Humans, on the other hand, reported that Polycose solutions were unpleasant. The results suggest that rodents have taste receptors for starch-derived polysaccharides that humans lack.