Caudal brain stem plays a role in metabolic control of estrous cycles in Syrian hamsters

Morphological Analysis of the Hindbrain Glucose Sensor-Hypothalamic Neural Pathway Activated by Hindbrain Glucoprivation

Lowered glucose availability, sensed by the hindbrain, has been suggested to enhance gluconeogenesis and food intake as well as suppress reproductive function. In fact, our previous histological and in vitro studies suggest that hindbrain ependymal cells function as a glucose sensor. The present study aimed to clarify the hindbrain glucose sensor-hypothalamic neural pathway activated in response to hindbrain glucoprivation to mediate counterregulatory physiological responses. Administration of 2-deoxy-D-glucose (2DG), an inhibitor of glucose utilization, into the fourth ventricle (4V) of male rats for 0.5 hour induced messenger RNA (mRNA) expression of c-fos, a marker for cellular activation, in ependymal cells in the 4V, but not in the lateral ventricle, the third ventricle or the central canal without a significant change in blood glucose and testosterone levels. Administration of 2DG into the 4V for 1 hour significantly increased blood glucose levels, food intake, and decreased blood testosterone levels. Simultaneously, the expression of c-Fos protein was detected in the 4V ependymal cells; dopamine β-hydroxylase-immunoreactive cells in the C1, C2, and A6 regions; neuropeptide Y (NPY) mRNA-positive cells in the C2; corticotropin-releasing hormone (CRH) mRNA-positive cells in the hypothalamic paraventricular nucleus (PVN); and NPY mRNA-positive cells in the arcuate nucleus (ARC). Taken together, these results suggest that lowered glucose availability, sensed by 4V ependymal cells, activates hindbrain catecholaminergic and/or NPY neurons followed by CRH neurons in the PVN and NPY neurons in the ARC, thereby leading to counterregulatory responses, such as an enhancement of gluconeogenesis, increased food intake, and suppression of sex steroid secretion.

Review of the neuroanatomic landscape implicated in glucose sensing and regulation of nutrient signaling: immunophenotypic localization of diabetes gene Tcf7l2 in the developing murine brain

Genetic variants in the transcription factor 7-like 2(Tcf7l2) gene have been found to confer a significant risk of type 2 diabetes and attenuated insulin secretion. Based on its genomic wide association Tcf7l2 is considered the single most important predictor of diabetes to date. Previous studies of Tcf7l2 mRNA localization in the adult brain suggest a putative role of Tcf7l2 in the CNS regulation of energy homeostasis. The present study further characterizes the immunophenotypic distribution of peptide expression in the brains of Tcf7l2 progeny during developmental time periods between E12.5 and P1. Tcf7l2(-/-) is lethal beyond P1. Results show that while negligible TCF7L2 expression is found in the developing brains of Tcf7l2(-/-)mice, TCF7L2 protein is relatively widespread and robustly expressed in the brain by E18.5 and exhibits specific expression within neuronal populations and regions of the brain in Tcf7l2(+/-) and Tcf7l2(+/+) progeny. Strong immunophenotypic labeling was found in the diencephalic structure of the thalamus that suggests a role of Tcf7l2 in the development and maintenance of thalamic activity. Strongly expressed TCF7L2 was localized in select hypothalamic and preoptic nuclei indicative of Tcf7l2 function within neurons controlling energy balance. Definitive neuronal staining for TCF7L2 within nuclei of the brain stem and circumventricular organs extends TCF7L2 localization within autonomic neurons and its potential integration with autonomic function. In addition robust TCF7L2 expression was found in the tectal and tegmental structures of the superior and inferior colliculi as well as transient expression in neuroepithelium of the cerebral and hippocampal cortices of E16 and E18.5. Patterns of TCF7L2 peptide localization when compared to the adult protein synthetic chemical/anatomical landscape of glucose sensing exhibit a good correlational fit between its expression and regions, nuclei, and pathways regulating energy homeostasis via integration and response to peripheral endocrine, metabolic and neuronal signaling. TCF was also found co-localized with peptides that regulate energy homeostasis including AgRP, POMC and NPY. TCF7l2, some variants of which have been shown to impair GLP-1-induced insulin secretion, was also found co-localize with GLP-1 in adult TCF wild type progeny. Impaired Tcf7l2-mediated neural regulation may contribute to the risk and/or underlying pathophysiology of type 2 diabetes that has found high expression in genomic studies of Tcf7l2 variants.

Sense and nonsense in metabolic control of reproduction

An exciting synergistic interaction occurs among researchers working at the interface of reproductive biology and energy homeostasis. Reproductive biologists benefit from the theories, experimental designs, and methodologies used by experts on energy homeostasis while they bring context and meaning to the study of energy homeostasis. There is a growing recognition that identification of candidate genes for obesity is little more than meaningless reductionism unless those genes and their expression are placed in a developmental, environmental, and evolutionary context. Reproductive biology provides this context because metabolic energy is the most important factor that controls reproductive success and gonadal hormones affect energy intake, storage, and expenditure. Reproductive hormone secretion changes during development, and reproductive success is key to evolutionary adaptation, the process that most likely molded the mechanisms that control energy balance. It is likely that by viewing energy intake, storage, and expenditure in the context of reproductive success, we will gain insight into human obesity, eating disorders, diabetes, and other pathologies related to fuel homeostasis. This review emphasizes the metabolic hypothesis: a sensory system monitors the availability of oxidizable metabolic fuels and orchestrates behavioral motivation to optimize reproductive success in environments where energy availability fluctuates or is unpredictable.

Central lipoprivation-induced suppression of luteinizing hormone pulses is mediated by paraventricular catecholaminergic inputs in female rats

The present study aims to clarify the role of fatty acids in regulating pulsatile LH secretion in rats. To produce an acute central lipoprivic condition, mercaptoacetate (MA), an inhibitor of fatty acids oxidation, was administered into the fourth cerebroventricle (4V) in ad libitum fed ovariectomized (OVX) rats (0.4, 2, and 10 micromol/rat) with or without an estradiol (E2) implant producing diestrus plasma E2 levels. Pulsatile LH secretion was suppressed by 4V MA administration in a dose-dependent manner in both OVX and OVX plus E2 rats. Mean LH levels and LH pulse frequency and amplitude were significantly reduced by the highest dose of MA in OVX rats, and by the middle and highest dose of MA in E2-treated rats, suggesting that estrogen enhanced LH suppression. Blood glucose levels increased immediately after the highest dose of MA in both groups. Fourth ventricular injection of trimetazidine (2 and 3 micromol/rat), another inhibitor of fatty acids oxidation, also inhibited pulsatile LH release, resulting in significant and dose-dependent suppression of LH pulse frequency and an increase in blood glucose levels in OVX plus E2 rats. In contrast, peripheral injection of the highest 4V dose of MA (10 micromol/rat) did not alter LH release or blood glucose levels. Microdialysis of the hypothalamic paraventricular nucleus (PVN) revealed that norepinephrine release in the region was increased by 4V MA administration. Preinjection of alpha-methyl-p-tyrosine, a catecholamine synthesis inhibitor, into the PVN completely blocked the lipoprivic inhibition of LH and the counter-regulatory increase in blood glucose levels in OVX plus E2 rats. Together, these studies indicate that fatty acid availability may be sensed by a central detector, located in the lower brainstem to maintain reproduction, and that noradrenergic inputs to the PVN mediate this lipoprivic-induced suppression of LH release.

Monosodium glutamate-induced arcuate nucleus damage affects both natural torpor and 2DG-induced torpor-like hypothermia in Siberian hamsters

Siberian hamsters ( Phodopus sungorus) have the ability to express daily torpor and decrease their body temperature to ∼15°C, providing a significant savings in energy expenditure. Daily torpor in hamsters is cued by winterlike photoperiods and occurs coincident with the annual nadirs in body fat reserves and chronic leptin concentrations. To better understand the neural mechanisms underlying torpor, Siberian hamster pups were postnatally treated with saline or MSG to ablate arcuate nucleus neurons that likely possess leptin receptors. Body temperature was studied telemetrically in cold-acclimated (10°C) male and female hamsters moved to a winterlike photoperiod (10:14-h light-dark cycle) ( experiments 1 and 2) or that remained in a summerlike photoperiod (14:10-h light-dark cycle) ( experiment 3). In experiment 1, even though other photoperiodic responses persisted, MSG-induced arcuate nucleus ablations prevented the photoperiod-dependent torpor observed in saline-treated Siberian hamsters. MSG-treated hamsters tended to possess greater fat reserves. To determine whether reductions in body fat would increase frequency of photoperiod-induced torpor after MSG treatment, hamsters underwent 2 wk of food restriction (70% of ad libitum) in experiment 2. Although food restriction did increase the frequency of torpor in both MSG- and saline-treated hamsters, it failed to normalize the proportion of MSG-treated hamsters undergoing photoperiod-dependent torpor. In experiment 3, postnatal MSG treatments reduced the proportion of hamsters entering 2DG-induced torpor-like hypothermia by ∼50% compared with saline-treated hamsters (38 vs. 72%). In those MSG-treated hamsters that did become hypothermic, their minimum temperature during hypothermia was significantly greater than comparable saline-treated hamsters. We conclude that 1) arcuate nucleus mechanisms mediate photoperiod-induced torpor, 2) food-restriction-induced torpor may also be reduced by MSG treatments, and 3) arcuate nucleus neurons make an important, albeit partial, contribution to 2DG-induced torpor-like hypothermia.

Fos-like immunoreactivity in Siberian hamster brain during initiation of torpor-like hypothermia induced by 2DG

Systemic 2-deoxy-d-glucose (2DG) produces pronounced torpor-like hypothermia (not< approximately 15 degrees C) in the Siberian hamster. Siberian hamsters are heterothermic, naturally undergoing photoperiod-dependent torpor during winter-like photoperiods. Fos was used to identify neural structures activated during the initiation of torpor-like hypothermia induced by 2DG treatment. The Fos-like immunoreactivity (Fos-li) in the area postrema and nucleus of the solitary tract that predominantly characterizes other 2DG-induced responses was absent during 2DG-induced torpor in the present experiment. Fos-li was seen in a number of forebrain and hindbrain sites during entry into hypothermia, but the densest Fos-li was found in the parvocellular portion of the paraventricular nucleus. Fos-li in the medial nucleus of the amygdala and the dorsal lateral septum also distinguished 2DG-induced torpor from other 2DG-induced behaviors. The possible involvement of neuropeptide Y pathways during 2DG-induced expression of reversible hypothermia is discussed.

Food deprivation and leptin prioritize ingestive and sex behavior without affecting estrous cycles in Syrian hamsters

Energy consumption is critical for the energetically expensive processes related to reproduction, and thus, mechanisms that increase ingestive behavior are directly linked to reproductive success. Similarly, the mechanisms that inhibit hunger and ingestive behavior might be most adaptive when these mechanisms cause individuals to stop foraging, hoarding and eating in order to find and court potential mates. In the laboratory, ingestive behaviors are typically studied separately from reproductive behaviors even though it is likely that these behaviors evolved under conditions in which both food and mates were available. We examined the choice between paracopulatory and ingestive behaviors in a semi-natural environment in which both food and potential mates were available. Intact female Syrian hamsters showed a high preference for males on days 3 and 4 (day 4 being the day of ovulation and estrous behavior), and a 48-h period of food deprivation significantly decreased preference for sex and increased preference for eating and food hoarding on day 3 in 89% of the hamsters, although none became anestrous. The same period of food deprivation significantly decreased the level of vaginal marking without significant effects on plasma estradiol concentrations. Next, hamsters were either food deprived (FD) or fed ad libitum, and half of each group was treated with vehicle or the adipocyte hormone leptin. The percentage of females with a low preference for sex was significantly greater in the FD compared to the ad libitum-fed groups, and leptin treatment prevented this effect. Metabolic fuels, possibly acting through leptin and other hormones, might influence sensitivity to estradiol or enhance the downstream effects of estradiol, thereby increasing motivation for sex and decreasing the relative motivation to forage, hoard and eat food.

Neuroendocrinology of nutritional infertility

Natural selection has linked the physiological controls of energy balance and fertility such that reproduction is deferred during lean times, particularly in female mammals. In this way, an energetically costly process is confined to periods when sufficient food is available to support pregnancy and lactation. Even in the face of abundance, nutritional infertility ensues if energy intake fails to keep pace with expenditure. A working hypothesis is proposed in which any activity or condition that limits the availability of oxidizable fuels (e.g., undereating, excessive energy expenditure, diabetes mellitus) can inhibit both gonadotropin-releasing hormone (GnRH)/luteinizing hormone secretion and female copulatory behaviors. Decreases in metabolic fuel availability appear to be detected by cells in the caudal hindbrain. Hindbrain neurons producing neuropeptide Y (NPY) and catecholamines (CA) then project to the forebrain where they contact GnRH neurons both directly and also indirectly via corticotropin-releasing hormone (CRH) neurons to inhibit GnRH secretion. In the case of estrous behavior, the best available evidence suggests that the inhibitory NPY/CA system acts primarily via CRH or urocortin projections to various forebrain loci that control sexual receptivity. Disruption of these signaling processes allows normal reproduction to proceed in the face of energetic deficits, indicating that the circuitry responds to energy deficits and that no signal is necessary to indicate that there is an adequate energy supply. While there is a large body of evidence to support this hypothesis, the data do not exclude nutritional inhibition of reproduction by other pathways and processes, and the full story will undoubtedly be more complex than this.

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.

Lessons from experimental disruption of estrous cycles and behaviors

In female mammals reproduction is highly sensitive to the food supply. During lean times, females suspend reproductive attempts in favor of maintaining processes necessary for survival; fertility is restored once the food supply is again abundant. Nearly all aspects of reproduction are affected, including puberty, adult ovulatory cycles, and reproductive behaviors. Work with experimental animals reveals that caloric restriction inhibits release of luteinizing hormone (LH) and female sexual behavior via similar, although separate, processes. The primary metabolic event affecting LH release as well as female sexual behavior is the short-term (minute-to-minute, hour-to-hour) availability of oxidizable metabolic fuels, rather than any aspect of body size or composition (e.g., body fat content). Metabolic fuel availability is detected in the hindbrain and perhaps in peripheral tissues. Metabolic information is then transmitted synaptically from the visceral hindbrain to the forebrain effector circuits. In the forebrain, signaling via corticotropin-releasing hormone receptors appears to be crucial for inhibition of both LH secretion and female sexual behavior.

Neuroendocrine facets of human puberty

The unfolding of pubertal growth and maturation entails multisystem collaboration. Most notably, the outflow of gonadotropins and growth hormone (GH) proceeds both independently and jointly. The current update highlights this unique dependency in the human.

Glucoprivic regulation of estrous cycles in the rat

Previous research has shown that glucoprivation induced by chronic 2-deoxy-D-glucose (2DG) treatment extends estrous cycle length and disrupts reproductive behaviors in female hamsters, similar to food deprivation. Such treatment also suppresses food intake, which is reversed in male rats by reducing brain histamine levels prior to 2DG treatment. We, therefore, determined if 2DG extends estrous cycles in the female rat and if this is due to elevated brain histamine levels. We measured estrous cycle length during 2DG-induced glucoprivation, in the presence and absence of alpha-fluoromethylhistidine (FMH), a treatment that reduces brain histamine levels. Adult female rats were treated for 72 h with either saline (n = 8), 2DG (200 mg/kg S.C. every 6 h; n = 9), or FMH (100 mg/kg i.p. daily) + 2DG (200 mg/kg; n = 7). An additional group was treated with FMH (100 mg/kg i.p.; n = 5) alone. To determine if 2DG extends estrous cycles due to glucoprivation or to decreased caloric intake, a group of rats (n = 7) received a reduced diet equal to the mean daily food intake of rats receiving 2DG alone. 2DG induced more long estrous cycles compared to rats receiving saline, FMH + 2DG, or FMH alone. In rats treated with FMH + 2DG, the percentage of 4-5-day cycles was similar to that of saline-treated rats, and a high percentage of 4-5-day cycles was also observed in rats receiving a reduced diet. These data suggest that 2DG does not suppress estrous cycles through a decrease in total calorie intake, but rather by inducing glucoprivation. In addition, during 2DG-induced glucoprivation, elevated brain histamine levels contribute to the mechanism that suppresses reproductive function.

Metabolic signals, hormones and neuropeptides involved in control of energy balance and reproductive success in hamsters

In the 'postgenome era', most research on the neuroendocrine control of energy homeostasis has focused on hormonal and neuropeptide control of food intake (i.e. the amount of food eaten) in rats and mice. The amount of food consumed is influenced by both the motivation to procure food and the consummatory act of ingestion. In some species, the rate of food intake remains relatively constant, while survival is maintained via changes in food procurement, external storage and internal expenditure. For example, in hamsters, metabolic signals, peripheral hormones and central neuropeptides influence hunger motivation, food hoarding and changes in energy expenditure without necessarily influencing the amount of food ingested. A similar suite of metabolic signals, hormones and neuropeptides is involved in optimizing reproductive success under fluctuating energetic conditions. Reproductive processes are inhibited or delayed when energy expenditure outstrips energy intake and mobilization from storage. Estrous cyclicity in Syrian hamsters is sensitive to the availability of oxidizable glucose, but the presence of central glucose alone is not sufficient for normal estrous cycles. Food deprivation-induced anestrus does not depend upon food deprivation-induced increases in concentrations of adrenal hormones such as glucocorticoids. If hormones such as insulin and leptin play a role, they might do so by modulating the availability of glucose detected at extra-hypothalamic sites, instead of or in addition to direct effects on the mechanisms that control gonadotropin releasing hormone secretion. Despite our ability to measure and manipulate gene transcription, understanding of fuel homeostasis requires examination of indirect effects of hormones and neuropeptides on peripheral metabolism, attention to the motivational as well as consummatory aspects of ingestion, and the study of behaviour in a natural or seminatural context.

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.

Central, but not peripheral, glucose-sensing mechanisms mediate glucoprivic suppression of pulsatile luteinizing hormone secretion in the sheep

Changes in glucose availability are proposed to modulate pulsatile GnRH secretion, and at least two anatomical sites, the liver and hindbrain, may serve as glucose sensors. The present study determined the relative importance of these putative glucose-sensing areas in regulating pulsatile LH secretion in the sheep. Our approach was to administer the antimetabolic glucose analog, 2-deoxy-D-glucose (2DG) into either the hepatic portal vein or the fourth ventricle in gonadectomized females in which LH pulse frequency was high. In the first study, a catheter was placed in the ileocolic vein to determine the effects of local injection of 2DG into the hepatic portal system on the release of LH. After monitoring the pattern of LH secretion for 4 h, 2DG (250 mg/kg) was infused (500 microl/min) into the liver for 2 h. For comparison, animals were also given the same dose of 2DG into a jugular vein for 2 h. Administration of 2DG into either the hepatic portal or jugular vein reduced LH pulse frequency to the same extent. Infusion of the lower dose (50 mg/kg) locally into the hepatic portal vein did not affect plasma LH profiles. Collectively, these results are interpreted to indicate that the liver does not contain special glucose-sensing mechanisms for the glucoprivic suppression of LH pulses. In the second study, 2DG (5 mg/kg) was infused (50 l/min) for 30 min into the fourth ventricle or lateral ventricle. During the subsequent 4-h sampling period, pulsatile LH secretion was significantly suppressed, but there was no significant difference in LH pulse frequency between sites of infusion. Peripheral 2DG concentrations were not detectable after either fourth or lateral ventricle infusions, indicating that the 2DG had acted centrally to suppress LH pulses. Plasma cortisol concentrations increased more in animals infused with 2DG into the fourth ventricle than in those infused into the lateral ventricle, suggesting that 2DG infused into lateral ventricle is transported caudally into the fourth ventricle and acts within the area surrounding the fourth ventricle. Overall, these findings suggest that an important glucose-sensing mechanism is located circumventricularly in the fourth ventricle. Moreover, the liver does not appear to play an important role in detecting glucoprivic action of 2DG to suppress pulsatile LH secretion.

Role of area postrema in control of torpor in Siberian hamsters

Siberian hamsters undergo torpor during the short days of winter and in response to glucoprivation or food restriction. We tested whether the area postrema and the adjacent nucleus of the solitary tract (hereafter the AP), which monitor metabolic fuel availability, also control the onset of torpor. Siberian hamsters that had manifested torpor spontaneously or had entered torpor in response to 2-deoxy-D-glucose (2-DG) treatment were subjected to area postrema ablations (APx). Hamsters continued to display torpor postoperatively; most features of torpor were unaffected by APx. The AP is not necessary for expression of torpor elicited by short day lengths or metabolic challenge. In contrast, decreases in food intake manifested by hamsters treated with 2-DG were counteracted by APx. In Siberian hamsters, the AP appears to mediate effects of 2-DG on food intake but not torpor.

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.

Peripheral or central administration of motilin suppresses LH release in female rats: a novel role for motilin

Motilin is secreted in a clear episodic pattern during fasting or during the interdigestive phase, but feeding promptly stops this secretory pattern, and plasma concentrations of motilin decrease. We have previously determined that fasting markedly suppresses pulsatile luteinizing hormone (LH) secretion in female rats in the presence of oestrogen. In the present study, we wished to learn if motilin may mediate the fasting-induced suppression of LH secretion by determining the effects of motilin administration on LH release and on food intake. Intravenous (i.v.) injection of motilin (37 nmol/rat) suppressed LH release and significantly decreased mean LH concentrations both in ovariectomized (OVX) and oestradiol-implanted ovariectomized (OVX+E2) rats. Food intake was significantly increased by i.v. motilin injection in OVX rats, but not in OVX+E2 rats. It is likely that motilin inhibits LH release via inhibition of the gonadotrophin-releasing hormone (GnRH)-releasing mechanism at the hypothalamic level, because motilin (3.7 nmol/rat) also suppressed LH secretion when centrally administered, and because LH release in i.v. motilin-treated rats increased in response to exogenous GnRH. These results suggest that motilin may be a peripheral signal for the suppression of LH secretion through central sensors.

Central action of insulin regulates pulsatile luteinizing hormone secretion in the diabetic sheep model

This study tested the hypothesis that central mechanisms regulating luteinizing hormone (LH) secretion are responsive to insulin. Our approach was to infuse insulin into the lateral ventricle of six streptozotocin-induced diabetic sheep in an amount that is normally present in the CSF when LH secretion is maintained by peripheral insulin administration. In the first experiment, we monitored cerebrospinal fluid (CSF) insulin concentrations every 3-5 h in four diabetic sheep given insulin by peripheral injection (30 IU). The insulin concentration in the CSF was increased after insulin injection, and there was a positive relationship between CSF and plasma concentrations of insulin (r = 0.80, P < 0.01). In the second experiment, peripheral insulin administration was discontinued, and the sheep received either an intracerebroventricular (i.c.v.) infusion of insulin (12 mU/day in 2.4 ml saline) or saline (2.4 ml/day) for 5 days (n = 6) in a crossover design. The dose of insulin (i.c.v.) was calculated to approximate the increase in CSF insulin concentration found after peripheral insulin treatment. To monitor LH secretory patterns, blood samples were collected by jugular venipuncture at 10-min intervals for 4 h on the day before and 5 days after the start of i.c.v. insulin infusion. To monitor the increase in CSF insulin concentrations, a single CSF sample was collected one and four days after the start of the central infusion. The i.c.v. insulin infusion increased CSF insulin concentrations above those in saline-treated animals (P < 0.05) and maintained them at or above the peak levels achieved after peripheral insulin treatment. Central insulin infusion did not affect peripheral (plasma) insulin or glucose concentrations. LH pulse frequency in insulin-treated animals was greater than that in saline-treated animals (3.5 +/- 0.2 vs. 2.3 +/- 0.3 pulses/4 h, P < 0.01), but it was less than that during peripheral insulin treatment (4.8 +/- 0.2 pulses/4 h, P < 0.01). Our findings suggest that physiologic levels of central insulin supplementation are able to increase pulsatile LH secretion in diabetic sheep with low peripheral insulin. These results are consistent with the notion that central insulin plays a role in regulating pulsatile GnRH secretion.

The area postrema mediates insulin hypoglycaemia-induced suppression of pulsatile LH secretion in the female rat

The caudal brainstem has been implicated in mediating the suppressive effect of glucoprivation on the reproductive neuroendocrine axis, specifically inhibition of pulsatile gonadotrophin-releasing hormone (GnRH)/luteinising hormone (LH) release in the rat. In the present study, removal of the area postrema completely prevented the profound inhibitory effect of insulin-induced hypoglycaemic stress on pulsatile LH release. These results suggest a pivotal role for this brainstem structure in mediating hypoglycaemic stress-induced suppression of the hypothalamic GnRH pulse generator.

The nature of the metabolic signal that triggers onset of puberty in female rats

These experiments examined the effects of different refeeding stimuli on reproductive activity as measured by the onset of first estrus in prepubertal, food-restricted female rats. Such rats were placed on a restricted food intake until day 50 of age to maintain a weight of 80-90 g, and to suppress onset of puberty (normal time of puberty: 37 +/- 1.4 days of age). In Experiment 1, rats were refed at different daily caloric intakes from day 50 through day 62. First estrus was observed in all rats, with highest caloric intake after 5.7 +/- 0.6 days of refeeding. Progressively fewer rats achieved first estrus, and the time to first estrus increased as the caloric intake per day decreased. These results suggest that the highest caloric intake most closely resembles an ad lib diet in such realimented rats. The second experiment determined the duration of an ad lib food stimulus needed to initiate first estrus. Similarly growth-restricted rats were refed (on Day 50 of age) ad lib meals of 67.2 +/- 0.1 kcal, presented for periods of 12, 24, 48, or 72 h. The majority of rats (6 of 7) achieved first estrus when the ad lib meals were presented for 72 h, but progressively fewer rats achieved first estrus when such meals were presented for less time. These data indicate that an extended ad lib food stimulus (72 h) is necessary to cause onset of cycling in the majority of food-restricted, prepubertal female rats.

Localization of glucokinase-like immunoreactivity in the rat lower brain stem: for possible location of brain glucose-sensing mechanisms

Pancreatic glucokinase (GK) is considered an important element of the glucose-sensing unit in pancreatic beta-cells. It is possible that the brain uses similar glucose-sensing units, and we employed GK immunohistochemistry and confocal microscopy to examine the anatomical distribution of GK-like immunoreactivities in the rat brain. We found strong GK-like immunoreactivities in the ependymocytes, endothelial cells, and many serotonergic neurons. In the ependymocytes, the GK-like immunoreactivity was located in the cytoplasmic area, but not in the nucleus. The GK-positive ependymocytes were found to have glucose transporter-2 (GLUT2)-like immunoreactivities on the cilia. In addition, the ependymocytes had GLUT1-like immunoreactivity on the cilia and GLUT4-like immunoreactivity densely in the cytoplasmic area and slightly in the plasma membrane. In serotonergic neurons, GK-like immunoreactivity was found in the cytoplasm and their processes. The present results raise the possibility that these GK-like immunopositive cells comprise a part of a vast glucose-sensing mechanism in the brain.

Energetic and endocrine mediation of natal dispersal behavior in Belding's ground squirrels

Natal dispersal, the permanent departure of an individual from its birth site, is sex biased in most mammals, with males dispersing at higher rates or over greater distances than do female conspecifics. Because dispersal movements may be energetically expensive, their occurrence should theoretically be influenced by energy availability. Moreover, the male bias typical of mammalian dispersal suggests that this behavior might be mediated by gonadal androgens. Using free-living Belding's ground squirrels (Spermophilus beldingi) as subjects, we provisioned juveniles with extra food to evaluate energetic influences on male dispersal behavior. Provisioning increased body mass and body fat of juvenile males and caused them to disperse at younger ages, but did not affect blood glucose levels. Dispersing males were fatter than same-aged males that had not yet dispersed. Moreover, body fat of provisioned and unprovisioned males did not differ when evaluated relative to the week during which they dispersed, suggesting that there may be a fat threshold for dispersal. In a second experiment, we measured plasma concentrations of testosterone (T) in provisioned and unprovisioned, free-living juveniles to evaluate the hypothesis that male dispersal behavior is activated by concurrent high levels of T. We observed no increase in plasma T associated with dispersal by juvenile males, no sex differences in circulating T among juveniles, and no effects of food provisioning on juvenile T levels. In a third experiment with free-living S. beldingi, we concurrently altered early androgen exposure by treating females with T at birth and manipulated energy availability by food provisioning. Perinatal T-treatment increased the likelihood of dispersal among juvenile females. Provisioning increased body mass and body fat of juveniles and caused males and T-treated females to disperse at significantly younger ages than either their unprovisioned counterparts or the few provisioned control females that dispersed. These results suggest that early T exposure in this species determines the probability of dispersal, whereas the amount of energy an individual has stored as fat strongly influences the timing of dispersal. Early T exposure also appears to cause the timing of dispersal to respond to energy availability and body fat in a male-typical way, possibly by organizing masculine mass and fat thresholds for dispersal.

Hypoglycemia and torpor in Siberian hamsters

We tested whether reduced blood glucose concentrations are necessary and sufficient for initiation of torpor in Siberian hamsters. During spontaneous torpor bouts, body temperature (Tb) decreases from the euthermic value of 37 to <31 degrees C. Among hamsters that displayed torpor during maintenance in a short-day length (10 h light/day) at an air temperature of 15 degrees C, blood glucose concentrations decreased significantly by 28% as Tb fell from 37 to <31 degrees C and increased during rewarming so that by the time Tb first was >36 degrees C, glucose concentrations had returned to the value preceding torpor. Hamsters did not display torpor when maintained in a long-day length (16 h light/day) and injected with a range of insulin doses (1-50 U/kg body mass), some of which resulted in sustained, pronounced hypoglycemia. We conclude that changes in blood glucose concentrations may be a consequence rather than a cause of the torpid state and question whether induction of torpor by 2-deoxy-D-glucose is due to its general glucoprivic actions.

Physiological perspectives on leptin as a regulator of reproduction: role in timing puberty

How nutrition regulates reproductive activity remains a major unsolved question of reproductive biology. Reducing the level of nutrition during adulthood can lead to infertility, primarily through reduction of GnRH secretion. Inquiry about such a mechanism has its roots in the search for cues timing the onset of fertility, because the tempo of sexual maturation is much more closely associated with body growth than with chronological age. Growth depends on the quantity and quality of food intake. When food availability is low, small, short-lived species with high metabolism and reduced growth may not even attain puberty before they die. In longer-lived species, puberty is delayed for months or even years until more food becomes available. To appreciate fully how the pubertal progression is timed will require understanding how peripheral signals relating information about energy metabolism are sensed by the brain and how such information is routed through pathways controlling GnRH secretion. Here, we provide some background and physiologic perspective on the question of whether the fat-derived hormone leptin is the unique peripheral signal, is an important signal, is but one of a constellation of signals, or is not a signal timing puberty.

Effect of corticotropin-releasing hormone antagonist on oestrogen-dependent glucoprivic suppression of luteinizing hormone secretion in female rats

Pharmacological reduction of glucose availability with 2-deoxyglucose (2DG) suppresses pulsatile luteinizing hormone (LH) secretion in rats and growth-retarded lambs. Gonadal steroids enhance the glucoprivic suppression of LH secretion in rats. The present study determined if corticotropin-releasing hormone (CRH) plays a role in mediating oestrogen-dependent and -independent glucoprivic suppression of LH secretion. The study was conducted in ovariectomized (OVX) rats some of which received Silastic implants containing oestradiol-17beta (OE2) dissolved in peanut oil at 20 microg/ml to produce a physiological plasma level of OE2 (30 pg/ml). Seven days after ovariectomy, the rats were stereotaxically implanted with a guide cannula into the third cerebral ventricle. Seven days later, blood samples were collected through an indwelling atrial cannula every 6 min for 3 h for LH pulse determination. After the first hour of blood sampling, a CRH antagonist, [D-Phe12, Nle21,38]hCRF-(21-41), or vehicle was injected into the third cerebral ventricle through the implanted cannula before 2DG administration through the indwelling atrial cannula. Pulsatile LH secretion was suppressed by 2DG (200 mg/kg b.w.) in the vehicle-treated rats bearing OE2 implants. The CRH antagonist (5.65 nmol) blocked the suppressive effect of 2DG on pulsatile LH secretion in the OE2-treated OVX animals. On the other hand, in the absence of oestrogen, the effect of a twice greater dose of 2DG (400 mg/kg b.w.) was not blocked by five times greater amount of CRH antagonist (28.3 nmol). These results suggest the mechanisms mediating glucoprivic suppression of LH secretion involve two components: one is oestrogen-dependent and the other oestrogen-independent. CRH may be involved in the oestrogen-dependent component of glucoprivic suppression of LH secretion but not the oestrogen-independent one.

Inhibition of luteinizing hormone secretion and expression of c-fos and corticotrophin-releasing factor genes in the paraventricular nucleus during insulin-induced hypoglycaemia in sheep

Insulin can act within the brain to stimulate ovine luteinizing hormone (LH) secretion, but insulin-induced hypoglycaemia inhibits LH via unknown brain sites, possibly involving corticotrophin-releasing factor (CRF). Castrate male sheep, with (E+) or without (E-) subcutaneous oestradiol implants, were blood sampled every 12 min for 8 h. Insulin (0.25 or 0.5 IU/kg) was injected at 4 h via the carotid artery or jugular vein. All treatments reduced LH output with no differences between dose rate nor route of administration, but sensitivity was greater in E+ than E-sheep. There was no evidence for an effect of insulin on LH 0-1 h postinjection; however, 1-3 h after insulin, when hypoglycaemia was established, LH pulses were inhibited in both E+ and E- sheep (P<0.001). Additional intravenous (i.v.) glucose injections given 1 h (20 mmol) and 2 h (10 mmol) after insulin (0.5 IU/kg) were each followed by an LH pulse within 30 min (75% response in both E+ and E-sheep). In a separate experiment, sheep were killed 2 h after i.v. insulin (0.5 IU/kg) or saline. In-situ hybridization revealed c-fos mRNA in the paraventricular nucleus (PVN), but not in any other hypothalamic nuclei nor in the hindbrain; and this was linked with increased CRF gene expression in the PVN. Similar c-fos and CRF gene expression was seen in insulin-treated sheep given additional i.v. glucose (20 and 10 mmol, respectively, 40 and 20 min ante mortem), but not in saline-treated controls. Therefore, insulin-induced hypoglycaemia inhibited LH secretion, with oestradiol potentiating the effect, and was associated with gonadal steroid-independent c-fos gene expression and increased CRF gene expression in the PVN. The ovine PVN may be involved in mediating insulin-induced hypoglycaemic inhibition of LH by a mechanism which might involve CRF.

Role of endogenous opiates in glucoprivic inhibition of the luteinizing hormone surge and fos expression by preoptic gonadotropin-releasing hormone neurones in ovariectomized steroid-primed female rats

In female mammals, the preovulatory luteinizing hormone (LH) 'surge' elicits ovulation and the subsequent transformation of Graafian follicles into corpora lutea, and is thus a critical component of successful reproduction. In light of evidence that this surge is impaired as a consequence of caloric restriction, the following experiments utilized pharmacological strategies to determine whether glucose substrate homeostasis influences the magnitude and/or duration of this pivotal hormonal event. Groups of oestrogen-and progesterone-primed ovariectomized (OVX) rats were injected intravenously (i.v.) with the glucose antimetabolite, 2-deoxy-D-glucose (2DG: 100 or 400 mg/kg), or the vehicle, saline, prior to onset of the expected LH surge. Other rats were pretreated with 2DG (100 microg/rat) or saline by an intracerebroventricular (i.c.v) route. While glucoprivation did not abolish the afternoon LH surge in these animals, mean plasma LH levels were significantly decreased in groups injected with the higher i.v. dose of 2DG or treated with this drug by an i.c.v route, relative to their vehicle-injected controls. In other studies, i.c.v delivery of the opioid receptor antagonist, naltrexone (NALT), partially reversed the inhibitory effects of 2DG on the gonadal steroid-induced LH surge. Dual-label immunocytochemistry of tissue sections from the preoptic area and anterior hypothalamus of OVX, steroid-primed rats revealed nuclear Fos-immunoreactivity (-ir) in a subpopulation of gonadotropin-releasing hormone-(GnRH-)immunopositive neurones prior to maximal preovulatory LH release. Animals pretreated with 2DG i.c.v showed a significant decrease in mean numbers of GnRH neurones exhibiting Fos-ir, whereas coadministration of 2DG and NALT resulted in numbers of double-labelled neurones that were similar to those detected in the non-drug-treated controls. These studies show that magnitude of the LH surge is decreased by glucose substrate imbalance, and that regulatory effects of this metabolic challenge on the reproductive neuroendocrine axis is correlated with alterations in the transcriptional activation of preoptic GnRH neurones by gonadal steroid positive feedback. The present results also support a role for central opiatergic neurotransmission in glucoprivic regulation of cyclic LH secretion in this animal model.

Seasonal neuroendocrine rhythms in the male Siberian hamster persist after monosodium glutamate-induced lesions of the arcuate nucleus in the neonatal period

The aim of these experiments was to examine the role of the arcuate nucleus in the control of seasonal cycles of body weight, feed intake, moulting and reproduction in the Siberian hamster. The arcuate nucleus has previously been implicated as a central site where systemic feedback signals (e.g. leptin) might act to regulate feed intake and body weight, so it was predicted that hamsters with lesions of this structure would be unable to display the inhibitory effects of short days on these parameters. In the first series of studies, lesions that destroyed approximately 80% of the cells in the arcuate nucleus were produced by treating hamsters neonatally with monosodium glutamate (MSG; 4 mg/g body weight sc), and vehicle- and MSG-treated males were raised from birth in long days (LD) or short days (SD). In hamsters raised in LD, the initial gain in body weight and testicular growth were significantly reduced by MSG treatment, however, growth rate and testis weight were still significantly greater than in vehicle- or MSG-treated hamsters raised in SD. In the second study, hamsters treated neonatally with vehicle or MSG were raised in LD for 8 weeks and, subsequently, approximately half in each group were transferred to SD for 18 weeks. As expected, vehicle-treated hamsters showed a characteristic decline in body weight when exposed to SD, while those remaining in LD continued to increase body weight. Feed intake decreased in parallel with the decline in body weight in SD, a complete moult to the white winter pelage occurred by 16 weeks in SD, and testicular regression occurred. Responses to SD also occurred in the MSG-treated hamsters: body weight decreased in SD but increased in their lesioned litter mates remaining in LD, and feed intake paralleled body weight changes in these groups. The moult to winter pelage was significantly retarded in MSG-treated hamsters transferred to SD. The testes were completely regressed in sham- and MSG-treated hamsters exposed to SD, whereas testes weights in MSG-treated hamsters maintained in LD were intermediate between those in vehicle-treated hamsters in SD and LD. Thus, despite initial effects on growth, the MSG-treated hamsters bearing substantial lesions of the arcuate nucleus were able to show appropriate responses to photoperiod, although not always of the same magnitude as the unlesioned controls. We conclude that feedback mechanisms operating via the arcuate nucleus are not the major regulators of seasonal cycles of body weight, feed intake, pelage and reproduction.

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.

The role of specific macronutrient availability in the effect of food restriction on length of lactational diestrus in rats

In lactating rats, food restriction for the first two weeks postpartum extends the period of lactational diestrus by about 1 week. In these studies we investigated whether this effect results from caloric restriction or the reduced availability of a specific macronutrient. In Experiment 1 lactating rats nursing litters of eight pups were assigned to one of four conditions: 1) ad lib. fed; 2) protein-restricted; 3) carbohydrate-restricted; and 4) fat-restricted. Animals in all the restricted conditions were given access to 50% of ad lib. intake of the appropriate nutrient for Days 1-14 postpartum and ad lib. access to the other two macronutrients. In Experiment 2, ad lib. supplementation from one macronutrient source was provided to lactating rats given restricted access to a composite diet. No differential effect of specific macronutrient deprivation or supplementation on length of lactational diestrus was observed in these studies. Thus, the results of both studies are consistent with the hypothesis that caloric restriction plays a primary role in inducing the prolongation of lactational diestrus in food-restricted rats.

Body fat and time of year interact to mediate dispersal behaviour in ground squirrels

Behaviour is often influenced by energy availability. Hibernation presents an energetic challenge to Belding's ground squirrels, Spermophilus beldingi, which depend entirely on stored energy for survival during their 8-9 month dormant period each year. Dispersal from the natal area may also place energetic demands on S. beldingi. In this study, we assessed the relationship between dispersal behaviour, body mass and body fat of juvenile male S. beldingi over 3 consecutive years, one of which had a substantially delayed juvenile active season due to heavy spring snowfall. We evaluated body fat using the non-destructive method of measuring electrical conductivity of the body. When the active season began late, acquisition of body mass and body fat were accelerated, and dispersal behaviour was inhibited. These results suggest that the ontogeny of mass and fat gain in juvenile male S. beldingi is influenced by a seasonal time-keeping mechanism. Energy allocation was hierarchical in S. beldingi, with pre-hibernation fattening taking precedence over dispersal. Physiological signals reflecting body fat content appear to interact with an endogenous timing mechanism in this species to regulate the dispersal behaviour of juvenile males.\C 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour.

Lifetime reproduction of giant transgenic mice: the energy stress paradigm

Lifetime reproduction of female transgenic rat growth hormone (TRrGH) mice and their normal siblings was evaluated on a high-protein (38%) diet, a standard diet (23% protein), and the standard diet supplemented with sucrose cubes. Compared with those on the standard diet, normal mice fed the high-protein diet showed significant increases in litter size, number of litters, and lifetime fecundity. Number of litters and lifetime fecundity were also enhanced in normal mice fed sucrose. TRrGH mice showed no significant improvements in reproduction on the high-protein diet, but they were significantly smaller. Sucrose dramatically improved reproduction of TRrGH mice, with no reduction in mature mass. The percentage of fertile TRrGH mice increased from 45% on standard chow to 71% with sucrose. The number and size of litters of TRrGH mice also significantly increased with sucrose, mean lifetime fecundity doubling from 9 pups on standard food to 18 pups on sucrose. However, TRrGH mice did not attain the reproductive success of normal mice on any diet. These results suggest that TRrGH mice are energetically stressed by enforced channelling of energy into growth. An immense literature addresses infertility due to energy limitation and stress generally. We synthesize these aspects with growth hormone transgenesis to derive an integrated view of neuroendocrine energy regulation relevant to restoring fertility of transgenic GH animals.

Control of fertility by metabolic cues

In female mammals, reproduction is extremely sensitive to the availability of oxidizable metabolic fuels. When food intake is limited or when an inordinate fraction of the available energy is diverted to other uses such as exercise or fattening, reproductive attempts are suspended in favor of processes necessary for individual survival. Both reproductive physiology and sexual behaviors are influenced by food availability. Nutritional effects on reproductive physiology are mediated by changes in the activity of gonadotropin-releasing hormone (GnRH) neurons in the forebrain, whereas the suppression of sexual behaviors appears to be due, at least in part, to decreases in estrogen receptor in the ventromedial hypothalamus. Work using pharmacological inhibitors of glucose and fatty acid oxidation indicates that reproductive physiology and behavior respond to short-term (minute-to-minute or hour-to-hour) changes in metabolic fuel oxidation, rather than to any aspect of body size or composition (e.g., body fat content or fat-to-lean ratio). These metabolic cues seem to be detected in the viscera (most likely in the liver) and in the caudal hindbrain (probably in the area postrema). This metabolic information is then transmitted to the GnRH-secreting or estradiol-binding effector neurons in the forebrain. There is no evidence to date for direct detection of metabolic cues by these forebrain effector neurons. This metabolic fuels hypothesis is consistent with a large body of evidence and seems to account for the infertility that is seen in a number of situations, including famine, eating disorders, excessive exercise, cold exposure, lactation, some types of obesity, and poorly controlled diabetes mellitus.

Glucoprivic treatments that induce anestrus, but do not affect food intake, increase FOS-like immunoreactivity in the area postrema and nucleus of the solitary tract in Syrian hamsters

Animals make a wide variety of physiological and behavioral adjustments in order to maintain caloric homeostasis. For example, most animals increase food intake when the availability of cellular metabolic fuels is low. The area postrema (AP) and adjacent, reciprocally-innervated nucleus of the solitary tract (NTS) are important brain areas for metabolic control of food intake in rats. However, in Syrian hamsters, food intake is not affected by decreases in metabolic fuel availability such as those that occur with food deprivation or with pharmacological inhibitors of metabolic fuels. Hamsters make other adjustments that conserve energy when the availability of metabolic fuels is low. Estrous cycles are inhibited by treatment with a high dose of 2-deoxy-D-glucose (2DG), a drug that inhibits cellular glucose utilization, but not by treatment with methyl palmoxirate (MP) a drug that inhibits fatty acid utilization. Recent data suggest that the AP/NTS is critical for the effects of glucoprivation on estrous cycles. Lesions of the AP/NTS prevent 2DG-induced anestrus. If the AP/NTS is involved in anestrus induced by glucoprivation, it might be predicted that glucoprivic treatments that induce anestrus would change patterns of neural activation, as measured by FOS-like immunoreactivity (FOS-li), in the AP/NTS. We examined FOS-li in females that were either food deprived or fed ad libitum, and in females treated with 2DG, MP or the appropriate vehicle. FOS-li was increased in the AP/NTS only in hamsters food deprived or treated with 2DG, the two treatments that induce anestrus but have no effect on food intake. These results are consistent with the notion that metabolic control of estrous cycles involves detection of decreases in the availability of metabolic fuels in the AP/NTS.