Control of gonadotropin feedback: the possible role of estrogen-induced hypothalamic synaptic plasticity

Melanocortin neurons: Multiple routes to regulation of metabolism

The burden of disability, premature death, escalating health care costs and lost economic productivity due to obesity and its associated complications including hypertension, stroke, cardiovascular disease and type 2 diabetes is staggering [1,2]. A better understanding of metabolic homeostatic pathways will provide us with insights into the biological mechanisms of obesity and how to fundamentally address this epidemic [3-6]. In mammals, energy balance is maintained via a homeostatic system involving both peripheral and central melanocortin systems; changes in body weight reflect an unbalance of the energetic state [7-9]. Although the primary cause of obesity is unknown, there is significant effort to understand the role of the central melanocortin pathway in the brain as it has been shown that deficiency of proopiomelanocortin (POMC) [10,11] and melanocortin 4 receptors (MC4R) [12-15] in both rodents and humans results in severe hyperphagia and obesity [16-23]. In this review, we will summarize how the central melanocortin pathway helps regulate body mass and adiposity within a 'healthy' range through the 'nutrient sensing' network [24-28]. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Leptin in anorexia and cachexia syndrome

Leptin is a product of the obese (OB) gene secreted by adipocytes in proportion to fat mass. It decreases food intake and increases energy expenditure by affecting the balance between orexigenic and anorexigenic hypothalamic pathways. Low leptin levels are responsible for the compensatory increase in appetite and body weight and decreased energy expenditure (EE) following caloric deprivation. The anorexia-cachexia syndrome is a complication of many chronic conditions including cancer, chronic obstructive pulmonary disease, congestive heart failure, chronic kidney disease, and aging, where the decrease in body weight and food intake is not followed by a compensatory increase in appetite or decreased EE. Crosstalk between leptin and inflammatory signaling known to be activated in these conditions may be responsible for this paradox. This manuscript will review the evidence and potential mechanisms mediating changes in the leptin pathway in the setting of anorexia and cachexia associated with chronic diseases.

Regulation of the female rat estrous cycle by a neural cell-specific epidermal growth factor-like repeat domain containing protein, NELL2

NELL2, a protein containing epidermal growth factor-like repeat domains, is predominantly expressed in the nervous system. In the mammalian brain, NELL2 expression is mostly neuronal. Previously we found that NELL2 is involved in the onset of female puberty by regulating the release of gonadotropin-releasing hormone (GnRH), and in normal male sexual behavior by controlling the development of the sexually dimorphic nucleus of the preoptic area (POA). In this study we investigated the effect of NELL2 on the female rat estrous cycle. NELL2 expression in the POA was highest during the proestrous phase. NELL2 mRNA levels in the POA were increased by estrogen treatment in ovariectomized female rats. Blocking NELL2 synthesis in the female rat hypothalamus decreased the expression of kisspeptin 1, an important regulator of the GnRH neuronal apparatus, and resulted in disruption of the estrous cycle at the diestrous phase. These results indicate that NELL2 is involved in the maintenance of the normal female reproductive cycle in mammals.

New aspects of melanocortin signaling: a role for PRCP in α-MSH degradation

The role of the central melanocortin system in the regulation of energy metabolism has received much attention during the past decade since gene mutations of key components in melanocortin signaling cause monogenic forms of obesity in animals and humans. In the arcuate nucleus of the hypothalamus the prohormone proopiomelanocortin (POMC) is posttranslationally cleaved to produce α-melanocyte stimulating hormone (α-MSH), a peptide with anorexigenic effects upon activation of the melanocortin receptors (MCRs). α-MSH undergoes extensive post-translational processing and its in vivo activity is short lived due to rapid degradation. The enzymatic process that controls α-MSH inactivation is incompletely understood. Recent evidence suggests that prolyl carboxypeptidase (PRCP) is an enzyme responsible for α-MSH degradation. As for many key melanocortin peptides, gene mutation of PRCP causes a change in the metabolic phenotype of rodents. This review summarizes the current knowledge on the melanocortin system with particular focus on PRCP, a newly discovered component of the melanocortin system.

Estrogens regulate posttranslational modification of neural cell adhesion molecule during the estrogen-induced gonadotropin surge

Estrogen-induced synaptic plasticity (EISP) in the periventricular area (PVA) of the hypothalamus is necessary for the preovulatory gonadotropin surge. Because in situ enzymatic desialization of hypothalamic polysialylated (PSA) neural cell adhesion molecule (NCAM) blocked EISP, we examined the presence and amount of NCAM isotopes, PSA-NCAM, and sialylation enzymes in microdissected mouse hypothalamus tissues from proestrous afternoon [peak of estrogens and nadir of arcuate nucleus (AN) synapses] and metestrous morning (nadir of estrogens and highest AN synapses). Immunohistochemistry confirmed immunoreactive (ir) PSA-NCAM staining in the perineural spaces of the PVA. The extent of staining was cycle dependent, with more dense and complete profiles of individual neurons limned by the ir-PSA-NCAM staining on proestrus and less on metestrus. Western blots showed that high levels of ir-PSA-NCAM on proestrus are accompanied by diminished ir-NCAM-140 and -180 but not ir-NCAM-120 and the reverse on metestrus (P < 0.05). To evaluate the increase of sialylated NCAM at the expense of desialylated protein, expression of the responsible polysialyltransferase enzymes polysialyltransferase (ST8Sia IV) and sialyltransferase (ST8Sia II) mRNA levels were measured using RT-PCR. Both polysialyltransferase and sialyltransferase mRNA are more abundant on proestrus than metestrus (P < 0.05), indicating that these enzymes are regulated by estrogens. These results support estrogen-regulated formation and extrusion of hydrophilic PSA-NCAM into perineural spaces in the PVA as part of the mechanism of EISP.

Sex differences in serotonergic but not gamma-aminobutyric acidergic (GABA) projections to the rat ventromedial nucleus of the hypothalamus

Hormonal conditions that elicit lordosis in female rats are ineffective in males, suggesting that this behavior is actively suppressed in males. Previous studies theorize that serotonergic and gamma-aminobutyric acidergic (GABA) inputs to the ventrolateral division of the ventromedial nucleus of the hypothalamus (VMNvl) may contribute to lordosis inhibition in males. Using triple-label immunofluorescent techniques, the present studies explored potential sex differences in the density of these projections within three hypothalamic sites: the VMNvl, the arcuate nucleus (ARC), and the dorsomedial nucleus of the hypothalamus. Antibodies directed against HuC/D, estrogen receptor (ER)-alpha and either serotonin (5-HT) or the gamma-aminobutyric acid synthetic enzyme glutamic acid decarboxylase-65 were used to compare the densities of glutamic acid decarboxylase (GAD)-65- and 5-HT-containing fibers in each brain area, the percentage of VMNvl HuC/D immunoreactive (ir) neurons that contained ERalpha, and the percentage of HuC/D and ERalpha double-labeled cells receiving apparent contacts from 5-HT fibers between adult, gonadectomized male and female rats. The densities of VMNvl and ARC 5-HT immunolabeled fibers were significantly higher in the males, and the percentage of VMNvl HuC/D-ir neurons containing ERalpha was significantly higher in the females. The percentage of HuC/D-ir cells contacted by 5-HT fibers was significantly higher in the males, compared with the females, but there was no sex difference in the proportion of those cells receiving contacts that were ERalpha-ir. Neonatal administration of estradiol but not genistein masculinized 5-HT content in the adult female VMNvl, but the percentage of HuC/D-ir cells colabeled with ERalpha was not significantly affected by treatment. A similar, but not statistically significant, pattern was observed in the ARC. These findings suggest that the development of serotonergic inputs to the male VMNvl is orchestrated by neonatal estradiol exposure. The hormone-dependent organization of these 5-HT projection patterns may be an important developmental mechanism accounting for sex-specific behaviors in adulthood.

Brain androgen and progesterone metabolizing enzymes: biosynthesis, distribution and function

This review summarizes the biosynthesis, cell type-distribution and function of brain aromatase cytochrome P450 (P450aro) and 5alpha-reductase enzymes. This overview covers the impact of the steroid products of the P450aro and 5alpha-reductase enzymes in establishing sexually dimorphic brain structures, specifically the sexually dimorphic nucleus of the preoptic area (SDN) and the anteroventral periventricular nucleus (AVPV). Additionally, since metabolites of the P450aro and 5alpha-reductase enzymes are known to regulate the calcium-binding protein, calbindin (CALB), CALB is reviewed in relationship to its potential role in determining sexually dimorphic brain structures. Finally, recent reports indicate that phytoestrogens inhibit P450aro and 5alpha-reductase activities in peripheral tissue sites, therefore, the effects of phytoestrogens on brain P450aro and 5alpha-reductase are briefly considered and the impact of consuming a high vs. a low phytoestrogen diet on visual spatial memory in male and female rats is presented.

Estrogen and the aging brain

Evidence is presented indicating a role for estrogen in the function and maintenance of the aging brain. Based on complementary data that estrogen regulates the function of the immune--brain barrier, the hypothesis is presented that estrogen contributes to brain homeostasis via regulation of microglial activation, enabling immune-privileged status in the brain. Diminished estrogen levels during the menopause compromise the immune--brain barrier fostering inflammatory processes in the brain. This has potentially lethal consequences for brain cells, and may contribute to brain pathologies such as Alzheimer's disease.

Prenatal testosterone masculinizes synaptic input to gonadotropin-releasing hormone neurons in sheep

In sheep, the control of tonic and surge GnRH secretion is sexually differentiated by testosterone in utero. However, GnRH neurons are not sexually dimorphic with respect to number, distribution, or gross morphology. Therefore, this study tested the hypothesis that prenatal steroids influence synaptic input to GnRH neurons. We compared the number of synapses on GnRH neurons from male, female, and androgenized female lambs (n = 5 each). Androgenized females were exposed to testosterone during mid-gestation. Yearling lambs were perfused, and GnRH neurons were visualized using the LR-1 antibody. Five to seven GnRH neurons from the rostral preoptic area in each animal were viewed at the ultrastructural level. Afferent synapses and glial ensheathment on each neuron were counted in a single section through the plane of the nucleus. GnRH neurons from females received approximately twice as many contacts (3.6 +/- 0.7 synapses/100 microm plasma membrane) as those from male lambs (1.6 +/- 0.3; p < 0.05), similar to previous reports in rats. In addition, the number of synapses on GnRH neurons from androgenized female lambs (1.5 +/- 0.5) was similar to that from male lambs, suggesting that prenatal steroids give rise to sex differences in synaptic input to GnRH neurons.

Prenatal melatonin exposure influences the maturation of gonadotropin and prolactin estradiol-benzoate feedback system

Gonadotropin and prolactin response to estrogen feedback in female rat offspring of control and melatonin treated (150 microg/100 g BW) mother rats during pregnancy (MEL-offspring) were studied at these periods: infantile, prepubertal and pubertal. In controls negative or absent LH feedback developed after estradiol benzoate (EB) injection up to 30 days of age indicating that the onset of puberty had not occurred. The positive feedback was established from day 33 on. However, in MEL-offspring the first activation of gonadotropin secretion during afternoon, 31 h after EB, was observed at 25 days of age, representing the first neuroendocrine sign of the onset of puberty. This positive response disappeared on day 30 in MEL-offspring. At 33 days of age, the LH positive response to EB was found in both groups, indicating a more advanced sexual development. In controls, this response increased at 35 days of age while in MEL-offspring it was highly depressed. FSH secretion in response to EB showed a negative feedback effect from infantile to the end of prepubertal period in both groups. The positive feedback was observed earlier in MEL-offspring (at 33 days of age) than in controls (at 35 days of age), but at this age it was absent in MEL-offspring. A positive prolactin response to EB at all ages in controls was observed. The typical pulsatility with higher values in the afternoon appeared by the first time at 30 days of age. However, in MEL-offspring no pulsatile response was observed throughout any age. These data suggest that prenatal melatonin administration altered gonadotropin and prolactin response to EB inducing precocious sensitivity during prepubertal period but depressed response during the pubertal period.

Circulating levels of immunoreactive beta-endorphin in polycystic ovary syndrome

The plasma levels of beta-endorphin were studied in 64 women with polycystic ovarian disease (PCOD), from whom was selected a group of 23 women with normal weight and amenorrhea of < 36 days. On day 21, beta-endorphin levels were: mean 64.92 pg/ml; SD 37.32 pg/ml; 95% CI 48.38-81.47 pg/ml. It was also observed that their levels of opioid peptide were reduced, compared with women who had normal ovulatory cycles, both in the follicular phase (mean 70.93 pg/ml; SD 24.59 pg/ml; 95% CI 76.84-99.77 pg/ml) and luteal phase (mean 88.30 pg/ml; SD 31.80 pg/ml; 95% CI 76.84-99.77 pg/ml). The results were statistically significant (p < 0.05) for levels in PCOD patients compared with those of the luteal phase in women with normal ovulatory cycles. The decreased levels of beta-endorphin were negatively related to luteinizing hormone (LH) levels, which might explain the rise of LH levels in women with PCOD who control their weight and at the time of amenorrhea, although it is not clear if central opioid activity is reflected in the peripheral blood.