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

Comparing the ontogeny, neurobiology, and function of social play in hamsters and rats

Syrian hamsters show complex social play behavior and provide a valuable animal model for delineating the neurobiological mechanisms and functions of social play. In this review, we compare social play behavior of hamsters and rats and underlying neurobiological mechanisms. Juvenile rats play by competing for opportunities to pin one another and attack their partner's neck. A broad set of cortical, limbic, and striatal regions regulate the display of social play in rats. In hamsters, social play is characterized by attacks to the head in early puberty, which gradually transitions to the flanks in late puberty. The transition from juvenile social play to adult hamster aggression corresponds with engagement of neural ensembles controlling aggression. Play deprivation in rats and hamsters alters dendritic morphology in mPFC neurons and impairs flexible, context-dependent behavior in adulthood, which suggests these animals may have converged on a similar function for social play. Overall, dissecting the neurobiology of social play in hamsters and rats can provide a valuable comparative approach for evaluating the function of social play.

Melanocortin receptor 3 and 4 mRNA expression in the adult female Syrian hamster brain

Melanocortin 3 receptors (MC3R) and melanocortin 4 receptors (MC4R) are vital in regulating a variety of functions across many species. For example, the dysregulation of these receptors results in obesity and dysfunction in sexual behaviors. Only a handful of studies have mapped the expression of MC3R and MC4R mRNA across the central nervous system, with the primary focus on mice and rats. Because Syrian hamsters are valuable models for functions regulated by melanocortin receptors, our current study maps the distribution of MC3R and MC4R mRNA in the Syrian hamster telencephalon, diencephalon, and midbrain using RNAscope. We found that the expression of MC3R mRNA was lowest in the telencephalon and greatest in the diencephalon, whereas the expression of MC4R mRNA was greatest in the midbrain. A comparison of these findings to previous studies found that MC3R and MC4R expression is similar in some brain regions across species and divergent in others. In addition, our study identifies novel brain regions for the expression of MC3Rs and MC4Rs, and identifies cells that co-express bothMC3 and MC4 receptors within certain brain regions.

The Importance of Leptin to Reproduction

A healthy nutritional state is required for all aspects of reproduction and is signaled by the adipokine leptin. Leptin acts in a relatively narrow concentration range: too much or too little will compromise fertility. The leptin signal timing is important to prepubertal development in both sexes. In the brain, leptin acts on ventral premammillary neurons which signal kisspeptin (Kiss1) neurons to stimulate gonadotropin releasing hormone (GnRH) neurons. Suppression of Kiss1 neurons occurs when agouti-related peptide neurons are activated by reduced leptin, because leptin normally suppresses these orexigenic neurons. In the pituitary, leptin stimulates production of GnRH receptors (GnRHRs) and follicle-stimulating hormone at midcycle, by activating pathways that derepress actions of the messenger ribonucleic acid translational regulatory protein Musashi. In females, rising estrogen stimulates a rise in serum leptin, which peaks at midcycle, synchronizing with nocturnal luteinizing hormone pulses. The normal range of serum leptin levels (10-20 ng/mL) along with gonadotropins and growth factors promote ovarian granulosa and theca cell functions and oocyte maturation. In males, the prepubertal rise in leptin promotes testicular development. However, a decline in leptin levels in prepubertal boys reflects inhibition of leptin secretion by rising androgens. In adult males, leptin levels are 10% to 50% of those in females, and high leptin inhibits testicular function. The obesity epidemic has elucidated leptin resistance pathways, with too much leptin in either sex leading to infertility. Under conditions of balanced nutrition, however, the secretion of leptin is timed and regulated within a narrow level range that optimizes its trophic effects.

Stress responses of testicular development, inflammatory and apoptotic activities in male zebrafish (Danio rerio) under starvation

Food deprivation is a severe stress across multiple fields and challenged to organismal development and immune system. Here, adult male zebrafish were used to investigate the starvation stress on organismal development, spermatogenesis, testicular inflammation and apoptosis. Results showed that the biological indexes, blood parameters, and RNA/DNA ratio in testis dramatically decreased after 1-3 weeks of starvation. The testicular architecture was impaired and the spermatogenesis was retarded with increased proportions of spermatogonia and spermatocytes, and decreased proportion of spermatozoa in the starved fish. The mRNA expressions of amh and sycp3 were downregulated, the retinoic acid content increased at later stage of starvation through the transcriptional regulation of aldh1a2 and cyp26a1. Besides, the immune response was elevated with upregulated mRNA and protein expressions of TNF-α, IL-6, and IL-1β, which indicated the inflammation of opportunistic risk in testis. The apoptotic activity was stimulated, accompanied by differentially upregulated expressions of baxa, casp9, casp3, casp2, and decreased ratio of Bcl-2/Bax in the attenuate testis. Taken together, our findings revealed that the stress responses of testicular development, inflammatory and apoptotic activities in male zebrafish under starvation and pointed out the susceptibility of fish gonad to food fluctuation.

Metabolic hormones and the regulation of spermatogenesis in fishes

Metabolic hormones play essential regulatory roles in many biological processes, including morphogenesis, growth, and reproduction through the maintenance of energy balance. Various metabolic hormones originally discovered in mammals, including ghrelin, leptin, and nesfatin-1 have been identified and characterized in fish. However, physiological roles of these metabolic hormones in regulating reproduction are largely unknown in fishes, especially in males. While the information available is restricted, this review attempts to summarize the main findings on the roles of metabolic peptides on the reproductive system in male fishes with an emphasis on testicular development and spermatogenesis. Specifically, the primary goal is to review the physiological interactions between hormones that regulate reproduction and hormones that regulate metabolism as a critical determinant of testicular function. A brief introduction to the localization of metabolic hormones in fish testis is also provided. Besides, the consequences of fasting and food deprivation on testicular development and sperm quality will be discussed with a focus on interactions between metabolic and reproductive hormones.

Association of Gnrhr mRNA With the Stem Cell Determinant Musashi: A Mechanism for Leptin-Mediated Modulation of GnRHR Expression

The cyclic expression of pituitary gonadotropin-releasing hormone receptors (GnRHRs) may be an important checkpoint for leptin regulatory signals. Gonadotrope Lepr-null mice have reduced GnRHR levels, suggesting these receptors may be leptin targets. To determine if leptin stimulated GnRHR directly, primary pituitary cultures or pieces were exposed to 1 to 100 nM leptin. Leptin increased GnRHR protein levels and the percentages of gonadotropes that bound biotinylated analogs of gonadotropin-releasing hormone (bio-GnRH) but had no effect on Gnrhr messenger RNA (mRNA). An in silico analysis revealed three consensus Musashi (MSI) binding elements (MBEs) for this translational control protein in the 3' untranslated region (UTR) of Gnrhr mRNA. Several experiments determined that these Gnrhr mRNA MBE were active: (1) RNA electrophoretic mobility shift assay analyses showed that MSI1 specifically bound Gnrhr mRNA 3'-UTR; (2) RNA immunoprecipitation of pituitary fractions with MSI1 antibody pulled down a complex enriched in endogenous MSI protein and endogenous Gnrhr mRNA; and (3) fluorescence reporter assays showed that MSI1 repressed translation of the reporter coupled to the Gnrhr 3'-UTR. In vitro, leptin stimulation of pituitary pieces reduced Msi1 mRNA in female pituitaries, and leptin stimulation of pituitary cultures reduced MSI1 proteins selectively in gonadotropes identified by binding to bio-GnRH. These findings show that leptin's direct stimulatory actions on gonadotrope GnRHR correlate with a direct inhibition of expression of the posttranscriptional regulator MSI1. We also show MSI1 interaction with the 3'-UTR of Gnrhr mRNA. These findings now open the door to future studies of leptin-modulated posttranscriptional pathways.

Food restriction-induced changes in motivation differ with stages of the estrous cycle and are closely linked to RFamide-related peptide-3 but not kisspeptin in Syrian hamsters

We tested the hypothesis that the effects of food restriction on behavioral motivation are mediated by one or both of the RFamide peptides, RFamide-related peptide-3 (RFRP-3) and kisspeptin (Kp) in female Syrian hamsters (Mesocricetus auratus). Female hamsters fed ad libitum and given a choice between food and adult male hamsters are highly motivated to visit males instead of food on all four days of the estrous cycle, but after 8days of mild food restriction (75% of ad libitum intake) they shift their preference toward food every day of the estrous cycle until the day of estrus, when they shift their preference back toward the males. In support of a role for RFRP-3 in these behavioral changes, the preference for food and the activation of RFRP-3-immunoreactive (Ir) cells in the dorsomedial hypothalamus (DMH) showed the same estrous cycle pattern in food-restricted females, but no association was observed between behavior and the activation of Kp cells in the hypothalamic arcuate nucleus or preoptic area. Next, we tested the hypothesis that food-restriction-induced activation of RFRP-3-Ir cells is modulated by high levels of ovarian steroids at the time of estrus. In support of this idea, on nonestrous days, mild food restriction increased activation of RFRP-3-Ir cells, but failed to do so on the day of estrus even though this level of food restriction did not significantly decrease circulating concentrations of estradiol or progesterone. Furthermore, in ovariectomized females, food-restriction-induced increases in activation of RFRP-3-Ir cells were blocked by systemic treatment with progesterone alone, estradiol plus progesterone, but not estradiol alone. Central infusion with RFRP-3 in ad libitum-fed females significantly decreased sexual motivation and produced significant increases in 90-minute food hoarding, in support of the hypothesis that elevated central levels of RFRP-3 are sufficient to create the shift in behavioral motivation in females fed ad libitum. Together, these results are consistent with the hypothesis that high levels of ingestive motivation are promoted during the nonfertile phase of the estrous cycle by elevated activation of RFRP-3-Ir cells, and RFRP-3-Ir cellular activation is modulated by ovarian steroids around the time of estrus, thereby diverting attention away from food and increasing sexual motivation.

Sex differences in the gut microbiome-brain axis across the lifespan

In recent years, the bidirectional communication between the gut microbiome and the brain has emerged as a factor that influences immunity, metabolism, neurodevelopment and behaviour. Cross-talk between the gut and brain begins early in life immediately following the transition from a sterile in utero environment to one that is exposed to a changing and complex microbial milieu over a lifetime. Once established, communication between the gut and brain integrates information from the autonomic and enteric nervous systems, neuroendocrine and neuroimmune signals, and peripheral immune and metabolic signals. Importantly, the composition and functional potential of the gut microbiome undergoes many transitions that parallel dynamic periods of brain development and maturation for which distinct sex differences have been identified. Here, we discuss the sexually dimorphic development, maturation and maintenance of the gut microbiome-brain axis, and the sex differences therein important in disease risk and resilience throughout the lifespan.

Selective deletion of leptin receptors in gonadotropes reveals activin and GnRH-binding sites as leptin targets in support of fertility

The adipokine, leptin (LEP), is a hormonal gateway, signaling energy stores to appetite-regulatory neurons, permitting reproduction when stores are sufficient. Dual-labeling for LEP receptors (LEPRs) and gonadotropins or GH revealed a 2-fold increase in LEPR during proestrus, some of which was seen in LH gonadotropes. We therefore investigated LEPR functions in gonadotropes with Cre-LoxP technology, deleting the signaling domain of the LEPR (Lepr-exon 17) with Cre-recombinase driven by the rat LH-β promoter (Lhβ-cre). Selectivity of the deletion was validated by organ genotyping and lack of LEPR and responses to LEP by mutant gonadotropes. The mutation had no impact on growth, body weight, the timing of puberty, or pregnancy. Mutant females took 36% longer to produce their first litter and had 50% fewer pups/litter. When the broad impact of the loss of gonadotrope LEPR on all pituitary hormones was studied, mutant diestrous females had reduced serum levels of LH (40%), FSH (70%), and GH (54%) and mRNA levels of Fshβ (59%) and inhibin/activin β A and β B (25%). Mutant males had reduced serum levels of GH (74%), TSH (31%), and prolactin (69%) and mRNA levels of Gh (31%), Ghrhr (30%), Fshβ (22%), and glycoprotein α-subunit (Cga) (22%). Serum levels of LEP and ACTH and mRNA levels of Gnrhr were unchanged. However, binding to GnRH receptors was reduced in LEPR-null LH or FSH gonadotropes by 82% or 89%, respectively, in females (P < .0001) and 27% or 53%, respectively, in males (P < .03). This correlated with reductions in GnRH receptor protein immunolabeling, suggesting that LEP's actions may be posttranscriptional. Collectively, these studies highlight the importance of LEP to gonadotropes with GnRH-binding sites and activin as potential targets. LEP may modulate population growth, adjusting the number of offspring to the availability of food supplies.

Central pathways integrating metabolism and reproduction in teleosts

Energy balance plays an important role in the control of reproduction. However, the cellular and molecular mechanisms connecting the two systems are not well understood especially in teleosts. The hypothalamus plays a crucial role in the regulation of both energy balance and reproduction, and contains a number of neuropeptides, including gonadotropin-releasing hormone (GnRH), orexin, neuropeptide-Y, ghrelin, pituitary adenylate cyclase-activating polypeptide, α-melanocyte stimulating hormone, melanin-concentrating hormone, cholecystokinin, 26RFamide, nesfatin, kisspeptin, and gonadotropin-inhibitory hormone. These neuropeptides are involved in the control of energy balance and reproduction either directly or indirectly. On the other hand, synthesis and release of these hypothalamic neuropeptides are regulated by metabolic signals from the gut and the adipose tissue. Furthermore, neurons producing these neuropeptides interact with each other, providing neuronal basis of the link between energy balance and reproduction. This review summarizes the advances made in our understanding of the physiological roles of the hypothalamic neuropeptides in energy balance and reproduction in teleosts, and discusses how they interact with GnRH, kisspeptin, and pituitary gonadotropins to control reproduction in teleosts.

Neuroendocrine regulation of appetitive ingestive behavior

Food availability in nature is often irregular, and famine is commonplace. Increased motivation to engage in ingestive behaviors increases the chance of survival, providing additional potential opportunities for reproduction. Because of the advantages conferred by entraining ingestive behavior to environmental conditions, neuroendocrine mechanisms regulating the motivation to acquire and ingest food have evolved to be responsive to exogenous (i.e., food stored for future consumption) and endogenous (i.e., body fat stores) fuel availability. Motivated behaviors like eating occur in two phases. The appetitive phase brings animals into contact with food (e.g., foraging, food hoarding), and the more reflexive consummatory phase results in ingestion (e.g., chewing, swallowing). Quantifiable appetitive behaviors are part of the natural ingestive behavioral repertoire of species such as hamsters and humans. This review summarizes current knowledge about neuroendocrine regulators of ingestive behavior, with an emphasis appetitive behavior. We will discuss hormonal regulators of appetitive ingestive behaviors, including the orexigenic hormone ghrelin, which potently stimulates foraging and food hoarding in Siberian hamsters. This section includes a discussion of the hormone leptin, its relation to endogenous fat stores, and its role in food deprivation-induced increases in appetitive ingestive behaviors. Next, we discuss how hormonal regulators interact with neurotransmitters involved in the regulation of ingestive behaviors, such as neuropeptide Y (NPY), agouti-related protein (AgRP) and α-melanocyte stimulating hormone (α-MSH), to regulate ingestive behavior. Finally, we discuss the potential impact that perinatal nutrient availability can have on the neuroendocrine regulation of ingestive behavior. Understanding the hormonal mechanisms that connect metabolic fuel availability to central appetite regulatory circuits should provide a better understanding of the neuroendocrine regulation of the motivation to engage in ingestive behavior.

Prenatal and postnatal energetic conditions and sex steroids levels across the first year of life

OBJECTIVES

Human biologists have documented variability in reproductive maturation, fertility, and cancer risk related to developmental conditions. Yet no previous studies have directly examined the impact of prenatal and postnatal energetic environments on sex steroids in infancy, a critical period for hypothalamic-pituitary-gonadal axis development. Thus, we examined the impact of maternal characteristics, birth size, and feeding practices on fecal sex steroid production in a longitudinal sample of 31 American infants followed from 2 weeks to 12 months of age.

METHODS

Maternal characteristics and birth size were collected at study enrollment, infant diet was assessed through weekly 24-h food diaries, and anthropometrics were measured weekly. Fecal estradiol and testosterone levels were assessed weekly using validated microassay RIA techniques. Mixed models were used to test for associations between maternal and birth characteristics, feeding practices, and sex steroids across the first year of life. Formal mediation analysis examined whether the relationship between infant feeding and hormone levels was mediated by infant size.

RESULTS

Maternal and birth characteristics had persistent effects on fecal sex steroid levels, with taller maternal height and larger birth size associated with lower estradiol levels in girls and higher testosterone levels in boys. Infant diet was also associated with sex steroid levels independently of infant size. Formula feeding was associated with higher estradiol levels in boys and girls and with higher testosterone in girls.

CONCLUSION

These results suggest that markers of early energy availability influence sex hormone levels with potential long-term consequences for reproductive development and function.

Neural and hormonal control of food hoarding

Many animals hoard food, including humans, but despite its pervasiveness, little is known about the physiological mechanisms underlying this appetitive behavior. We summarize studies of food hoarding in humans and rodents with an emphasis on mechanistic laboratory studies of species where this behavior importantly impacts their energy balance (hamsters), but include laboratory rat studies although their wild counterparts do not hoard food. The photoperiod and cold can affect food hoarding, but food availability is the most significant environmental factor affecting food hoarding. Food-deprived/restricted hamsters and humans exhibit large increases in food hoarding compared with their fed counterparts, both doing so without overeating. Some of the peripheral and central peptides involved in food intake also affect food hoarding, although many have not been tested. Ad libitum-fed hamsters given systemic injections of ghrelin, the peripheral orexigenic hormone that increases with fasting, mimics food deprivation-induced increases in food hoarding. Neuropeptide Y or agouti-related protein, brain peptides stimulated by ghrelin, given centrally to ad libitum-fed hamsters, duplicates the early and prolonged postfood deprivation increases in food hoarding, whereas central melanocortin receptor agonism tends to inhibit food deprivation and ghrelin stimulation of hoarding. Central or peripheral leptin injection or peripheral cholecystokinin-33, known satiety peptides, inhibit food hoarding. Food hoarding markedly increases with pregnancy and lactation. Because fasted and/or obese humans hoard more food in general, and more high-density/high-fat foods specifically, than nonfasted and/or nonobese humans, understanding the mechanisms underlying food hoarding could provide another target for behavioral/pharmacological approaches to curb obesity.

The somatotrope as a metabolic sensor: deletion of leptin receptors causes obesity

Leptin, the product of the Lep gene, reports levels of adiposity to the hypothalamus and other regulatory cells, including pituitary somatotropes, which secrete GH. Leptin deficiency is associated with a decline in somatotrope numbers and function, suggesting that leptin may be important in their maintenance. This hypothesis was tested in a new animal model in which exon 17 of the leptin receptor (Lepr) protein was selectively deleted in somatotropes by Cre-loxP technology. Organ genotyping confirmed the recombination of the floxed LepR allele only in the pituitary. Deletion mutant mice showed a 72% reduction in pituitary cells bearing leptin receptor (LEPR)-b, a 43% reduction in LEPR proteins and a 60% reduction in percentages of immunopositive GH cells, which correlated with reduced serum GH. In mutants, LEPR expression by other pituitary cells was like that of normal animals. Leptin stimulated phosphorylated Signal transducer and activator of transcription 3 expression in somatotropes from normal animals but not from mutants. Pituitary weights, cell numbers, IGF-I, and the timing of puberty were not different from control values. Growth curves were normal during the first 3 months. Deletion mutant mice became approximately 30-46% heavier than controls with age, which was attributed to an increase in fat mass. Serum leptin levels were either normal in younger animals or reflected the level of obesity in older animals. The specific ablation of the Lepr exon 17 gene in somatotropes resulted in GH deficiency with a consequential reduction in lipolytic activity normally maintained by GH and increased adiposity.

Sexual behavior in lactating rats: role of estrogen-induced progesterone receptors

Lactation is associated with suppression of reproductive function, the duration of which depends on the number of young suckled and food availability. Although previous studies have documented increasing responsivity to the positive feedback effects of estrogen on luteinizing hormone (LH) secretion with time postpartum, changes in the ability of estrogen to stimulate sexual behavior across these time points and the influence of food restriction on response to estrogen have not been investigated. Thus, we compared the ability of exogenous estrogen administration to stimulate proceptive and receptive behavior in ad libitum fed and food restricted rats on Days 15 and 20 postpartum. Because the ability of estrogen to induce sexual behavior depends on activation of both estrogen receptors and estrogen-induced progesterone receptors, a second study compared estrogen and progesterone-ir within the VMH and MPOA in similar groups. Finally, we investigated the role of the high levels of progesterone typical of lactation in the suppression of estrogen-induced sexual behavior by transient blockade of the progesterone receptor using RU486. As expected there was an increase across time in the ability of estrogen to stimulate sexual behavior that correlated with an increased ability of estrogen to induce progesterone receptors in the MPOA that was most evident in ad libitum fed rats. RU486 administration concomitant with estrogen administration increased solicitation behavior and was most effective in ad libitum fed rats suggesting an inhibitory role of progesterone on estrogen-induced sexual proceptivity in lactating rats.

Interactions between gonadotropin-releasing hormone (GnRH) and orexin in the regulation of feeding and reproduction in goldfish (Carassius auratus)

Links between energy homeostasis and reproduction have been demonstrated in vertebrates. As a general rule, abundant food resources favor reproduction whereas low food availability induces an inhibition of reproductive processes. In both mammals and fish, gonadotropin-releasing hormone (GnRH) and orexin (OX) are hypothalamic neuropeptides that play critical roles in the regulation of sexual behavior and appetite, respectively. In order to assess possible interactions between orexin and GnRH in the control of feeding and reproduction in goldfish, we examined the effects of chicken GnRH (cGnRH-II) intracerebroventricular (ICV) injection on feeding behavior and OX brain mRNA expression as well as the effects of orexin ICV injections on spawning behavior and cGnRH-II brain mRNA expression. Treatment with cGnRH-II at doses that stimulate spawning (0.5 ng/g or 1 ng/g) resulted in a decrease in both food intake and hypothalamic orexin mRNA expression. Treatment with orexin A at doses that stimulate feeding (10 ng/g) induced an inhibition of spawning behavior and a decrease in cGnRH-II expression in the hypothalamus and optic tectum-thalamus. Our results suggest that the anorexigenic actions of cGnRH-II in goldfish might be in part mediated by OX and that orexin inhibits reproductive behavior in part via the inhibition of the GnRH system. Our data suggest the existence of a coordinated control of feeding and reproduction by the orexin and GnRH systems in goldfish.

Influence of ovarian hormones on development of ingestive responding to alterations in fatty acid oxidation in female rats

Adult male rats have been demonstrated to increase food intake in response to administration of drugs that interfere with oxidation of fatty acids (e.g. methyl palmoxirate and mercaptoacetate [MA]), effects that are larger in animals maintained on a high-fat diet. In contrast, while administration of MA has been reported to stimulate food intake in pre-pubertal female rats, food intake is not stimulated by MA in adult female rats. Instead, administration of MA to adult females results in changes in reproductive behavior and physiology. The present experiments were designed to examine the effects of administration of MA on food intake in adult female rats. The results demonstrated that, as previously reported, food intake was stimulated by MA in adult male rats on low-fat and high-fat diets, but food intake was not stimulated by MA in gonadally-intact adult female rats on either low-fat or high-fat diet. Further, MA did not stimulate food intake in female rats ovariectomized as adults. However, when females were ovariectomized prior to the onset of puberty (postnatal day 25-28), food intake was stimulated by administration of MA in adulthood. Finally, cyclic injections of 17-beta-estradiol benzoate given to females ovariectomized prior to the onset of puberty abolished the stimulatory effects of MA on food intake in adult females. Taken together, the data suggest that exposure to estrogens during the time of puberty in female rats can persistently alter adult ingestive responding to signals related to changes in energy utilization.

Fasting and glucose effects on pituitary leptin expression: is leptin a local signal for nutrient status?

Leptin, a potent anorexigenic hormone, is found in the anterior pituitary (AP). The aim of this study was to determine whether and how pituitary leptin-bearing cells are regulated by nutritional status. Male rats showed 64% reductions in pituitary leptin mRNA 24 hr after fasting, accompanied by significant (30-50%) reductions in growth hormone (GH), prolactin, and luteinizing hormone (LH), and 70-80% reductions in target cells for gonadotropin-releasing hormone or growth hormone-releasing hormone. There was a 2-fold increase in corticotropes. Subsets (22%) of pituitary cells coexpressed leptin and GH, and <5% coexpressed leptin and LH, prolactin, thyroid-stimulating hormone, or adrenocorticotropic hormone. Fasting resulted in significant (55-75%) losses in cells with leptin proteins or mRNA, and GH or LH. To determine whether restoration of serum glucose could rescue leptin, LH, and GH, additional fasted rats were given 10% glucose water for 24 hr. Restoring serum glucose in fasted rats resulted in pituitary cell populations with normal levels of leptin and GH and LH cells. Similarly, LH and GH cells were restored in vitro after populations from fasted rats were treated for as little as 1 hr in 10-100 pg/ml leptin. These correlative changes in pituitary leptin, LH, and GH, coupled with leptin's rapid restoration of GH and LH in vitro, suggest that pituitary leptin may signal nutritional changes. Collectively, the findings suggest that pituitary leptin expression could be coupled to glucose sensors like glucokinase to facilitate rapid responses by the neuroendocrine system to nutritional cues.

Immunohistochemical Evidence for the Presence of Glucokinase in the Gonadotropes and Thyrotropes of the Anterior Pituitary Gland of Rat and Monkey

A recent report provides new evidence for the presence of glucokinase (GK) in the anterior pituitary. In the present study, immunohistochemistry was used to identify the cells containing GK in the pituitary of rats and monkeys. In rats, GK was detected as a generalized cytoplasmic staining in a discrete population of cells in the anterior pituitary. In colocalization experiments, the majority of cells expressing follicle-stimulating hormone (FSH) or luteinizing hormone (LH) also contained GK. In addition to the gonadotropes, GK was observed in a subpopulation of corticotropes and thyrotropes. GK was not detected in cells expressing growth hormone or prolactin. In monkeys, GK was also observed in a discrete population of cells. Intracellular distribution differed from the rat in that GK in most cells was concentrated in a perinuclear location that appeared to be associated with the Golgi apparatus. However, similar to rats, colocalization experiments showed that the majority of cells expressing FSH or LH also contained GK. In addition to the gonadotropes, GK was observed in a subpopulation of corticotropes and thyrotropes. In the monkey, only a few cells had generalized cytoplasmic staining for GK. These experiments provide further evidence for the presence of GK in the anterior pituitary. Although some corticotropes and thyrotropes contained GK, the predominant cell type expressing GK was gonadotropes. In view of the generally accepted role of GK as a glucose sensor in a variety of cells including the insulin-producing pancreatic β-cells as the prototypical example, it is hypothesized that hormone synthesis and/or release in pituitary cells containing GK may be directly influenced by blood glucose.

Anterior pituitary leptin expression changes in different reproductive states: in vitro stimulation by gonadotropin-releasing hormone

This study was designed to learn more about the changes in expression of rat anterior pituitary (AP) leptin during the estrous cycle. QRT-PCR assays of cycling rat AP leptin mRNA showed 2-fold increases from metestrus to diestrus followed by an 86% decrease on the morning of proestrus. Percentages of leptin cells increased in proestrus and pregnancy to 55-60% of AP cells. Dual labeling for leptin proteins and growth hormone (GH) or gonadotropins showed that the rise in leptin protein-bearing cells from diestrus to proestrus was mainly in GH cells. Only 10-20% of leptin cells in male or cycling female rats coexpress gonadotropins. In contrast, 50-73% of leptin cells from pregnant or lactating females coexpress gonadotropins and only 19% coexpress GH, indicating plasticity in the distribution of leptin. Leptin cells expressed GnRH receptors, and estrogen and GnRH together increased the coexpression of leptin mRNA and gonadotropins. GnRH increased cellular leptin proteins three to four times and mRNA 9.8 times in proestrous rats and stimulated leptin secretion in cultures from diestrous, proestrous, and pregnant rats. These regulatory influences, and the high expression of AP leptin during proestrus and pregnancy, suggest a supportive role for leptin during key events involved with reproduction.

Repeated quinpirole treatments produce neurochemical sensitization and associated behavioral changes in female hamsters

RATIONALE

Repeated stimulation of dopaminergic pathways with dopamine receptor agonists can produce both neurochemical and behavioral sensitization.

OBJECTIVES

The present study was designed to examine whether repeated treatment with the D2-like dopamine receptor agonist, quinpirole, would produce neurochemical sensitization of D1 dopamine receptor-mediated processes and associated behavioral changes in female hamsters in a manner analogous to that previously used to sensitize heterologous dopamine signaling pathways in derived cell lines.

MATERIALS AND METHODS

Female hamsters received two injections of quinpirole (1.5 mg/kg) or saline each week for 7 weeks, during which time pouching behavior and body weight were monitored. Over the next 2 weeks, hamsters were tested for differences in prepulse inhibition of the acoustic startle response (PPI) and sexual behavior. Adenylate cyclase activation assays were then performed on dissected tissue from the nucleus accumbens and caudate-putamen.

RESULTS

Repeated treatment with quinpirole increased pouching behavior and body weight and disrupted PPI. No changes in sexual activity in response to repeated quinpirole were found. Prior quinpirole treatment enhanced D1 dopamine receptor-stimulated adenylate cyclase activity in the caudate-putamen that was blocked by co-incubation with the D1 dopamine antagonist, SCH23390.

CONCLUSIONS

These results show that repeated activation of D2-like receptors in vivo can produce changes in feeding behavior and sensory processing that is associated with sensitization of D1 dopamine receptor-mediated signaling in the caudate-putamen.

The melanocortin circuit in obese and lean strains of chicks

Agonists of membranal melanocortin 3 and 4 receptors (MC3/4Rs) are known to take part in the complex control mechanism of energy balance. In this study, we compared the physiological response to an exogenous MC3/4R agonist and the hypothalamic expression of proopic melanocortin (POMC) gene, encoding few MC3/4R ligands, between broiler and layer chicken strains. These strains, representing the two most prominent commercial strains of chickens grown for meat (broilers) and egg production (layers), differ in their food intake, fat accumulation, and reproductive performance and, therefore, form a good model of obese and lean phenotypes, respectively. A single i.v. injection of the synthetic peptide melanotan-II (MT-II; 1 mg/kg body weight) into the wing vein of feed-restricted birds led to attenuation of food intake upon exposure to feeding ad libitum in both broiler and layer chickens. A study of the POMC mRNA encoding the two prominent natural MC3/4R agonists, alpha-MSH and ACTH, also revealed a general similarity between the strains. Under feeding conditions ad libitum, POMC mRNA levels were highly similar in chicks of both strains and this level was significantly reduced upon feed restriction. However, POMC mRNA down-regulation upon feed restriction was more pronounced in layers than in broilers. These results suggest: (i) a role for MC3/4R agonists in the control of appetite; (ii) that the physiological differences between broilers and layers are not related to unresponsiveness of broiler chickens to the satiety signal of MC3/4R ligands. Therefore, these findings suggest that artificial activation of this circuit in broiler chicks could help to accommodate with their agricultural shortcomings of overeating, fattening, and impaired reproduction.

Evidence that the type-2 gonadotrophin-releasing hormone (GnRH) receptor mediates the behavioural effects of GnRH-II on feeding and reproduction in musk shrews

Gonadotrophin-releasing hormone (GnRH) is a regulatory neuropeptide of which there are multiple structural variants. In mammals, a hypothalamic form (GnRH-I) controls gonadotrophin secretion whereas a midbrain form (GnRH-II) appears to have a neuromodulatory role affecting feeding and reproduction. In female musk shrews and mice, central administration of GnRH-II reinstates mating behaviour previously inhibited by food restriction. In addition, GnRH-II treatment also decreases short-term food intake in musk shrews. GnRH-II can bind two different mammalian GnRH receptors (type-1 and type-2), and thus it is unclear which receptor subtype mediates the behavioural effects of this peptide. Adult female musk shrews implanted with i.c.v. cannula were food restricted or fed ad lib and then tested for sexual behaviour or food intake. One hour before testing, animals were pretreated with vehicle or Antide, a potent type-1 GnRH receptor antagonist (at a dose that blocks GnRH-I or -II mediated ovulation). Twenty minutes before testing, females were infused a second time with either GnRH-II or vehicle. Additional females were tested after an infusion of 135-18, a type-1 receptor antagonist that displays agonist actions at the primate type-2 receptor. GnRH-II treatment increased sexual behaviour in underfed female shrews; pretreatment with Antide did not block this action, suggesting that the effects of GnRH-II are not mediated via the type-1 receptor. Similarly, the inhibitory effects of GnRH-II on short-term food intake were not prevented by pretreatment with Antide. The behavioural effects of the type-2 receptor agonist 135-18 were similar to those seen in GnRH-II-treated females, with 135-18 promoting sexual behaviour and decreasing food intake. Collectively, these results indicate that GnRH-II does not act via the type-1 GnRH receptor to regulate mammalian behaviour but likely activates the type-2 GnRH receptor.

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.

Metabolic setpoint control mechanisms in different physiological systems at rest and during exercise

Using a number of different homeostatic control mechanisms in the brain and peripheral physiological systems, metabolic activity is continuously regulated at rest and during exercise to prevent catastrophic system failure. Essential for the function of these regulatory processes are baseline "setpoint" levels of metabolic function, which can be used to calculate the level of response required for the maintenance of system homeostasis after system perturbation, and to which the perturbed metabolic activity levels are returned to at the end of the regulatory process. How these setpoint levels of all the different metabolic variables in the different peripheral physiological systems are created and maintained, and why they are similar in different individuals, has not been well explained. In this article, putative system regulators of metabolic setpoint levels are described. These include that: (i) innate setpoint values are stored in a certain region of the central nervous system, such as the hypothalamus; (ii) setpoint values are created and maintained as a response to continuous external perturbations, such as gravity or "zeitgebers", (iii) setpoint values are created and maintained by complex system dynamical activity in the different peripheral systems, where setpoint levels are regulated by the ongoing feedback control activity between different peripheral variables; (iv) human anatomical and biomechanical constraints contribute to the creation and maintenance of metabolic setpoints values; or (v) a combination of all these four different mechanisms occurs. Exercise training and disease processes can affect these metabolic setpoint values, but the setpoint values are returned to pre-training or pre-disease levels if the training stimulus is removed or if the disease process is cured. Further work is required to determine what the ultimate system regulator of metabolic setpoint values is, why some setpoint values are more stringently protected by homeostatic regulatory mechanisms than others, and the role of conscious decision making processes in determining the regulation of metabolic setpoint values.

Characterization and species differences in gastric ghrelin cells from mice, rats and hamsters

Ghrelin is a newly identified gastric peptide hormone that has various important functions, including growth-hormone release and appetite stimulation. Ghrelin-immunoreactive cells (ghrelin cells) are characterized by X-type endocrine cells in the rat stomach. In the present study, we analysed ghrelin cells in fundi of stomach from ICR mice and Syrian hamsters immunohistochemically, immunoelectron microscopically and morphometrically, and compared the results with those from Wistar rats. Immunohistochemistry revealed that ghrelin cells were sparsely distributed in the proper gastric glands in all species. The number of ghrelin cells per unit area in hamsters was significantly lower than that in rats. Immunoelectron microscopy detected ghrelin immunolabelling in granules in the X-type endocrine cells. However, the diameter of granules in the hamsters was significantly smaller than that in the mice and rats. Gastric ghrelin contents were determined by radioimmunoassay, and levels in the hamsters were significantly lower than those in mice and rats. The results from mice were identical to those from rats. In conclusion, gastric ghrelin cells in mice and hamsters are characterized by X-type endocrine cells, as has been observed in rats. However, the data indicated that gastric ghrelin production was lower in hamster than in mouse or rat.

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.

The evolutionarily conserved gonadotropin-releasing hormone II modifies food intake

GnRH is an evolutionarily conserved peptide of which there are multiple structural variants. One form, GnRH II, is the most widespread in vertebrates, but its primary function remains unclear. In female musk shrews, administration of GnRH II, but not GnRH I, reinstates mating behavior previously inhibited by food restriction. Because this finding suggests that the function of GnRH II may be linked to energetic status, we tested whether GnRH II directly affects food intake. Adult female musk shrews were maintained on ad libitum feeding or food restricted for 48 h, after which they were infused centrally with GnRH I (1 microg), GnRH II (1 microg), or saline. Food intake was recorded 90 min, and 3, 6, 24, and 48 h after infusion. GnRH II administration, but not saline or GnRH I, reduced 24-h food intake in ad libitum animals. Short-term food intake (90 min and 3 h) of both ad libitum and underfed shrews receiving GnRH II was also reduced by as much as 33%, relative to the food intake of saline-infused controls. GnRH I infusion did not affect short-term food intake differently than saline infusion in shrews fed ad libitum. In underfed females, GnRH I had an effect on short-term food intake that was intermediate to saline and GnRH II. We conclude that, in addition to its permissive role in regulating reproduction, GnRH II may also modulate food intake in mammals. Because GnRH II is present in primate brain, it may also serve a similar function in humans.