Physiological and genomic insight into neuroendocrine regulation of puberty in gilts

The timing of pubertal attainment in gilts is a critical factor for pork production and is an early indicator of future reproductive potential. Puberty, defined as age at first standing estrus in the presence of a boar, is brought about by an escape from estrogen inhibition of the GnRH pulse generator, which allows for increasing LH pulses leading to the onset of cyclicity. The biological mechanisms that control the timing of these events is related to decreasing inhibitory signals with a concomitant increase in stimulatory signals within the hypothalamus. The roles of gamma-aminobutyric acid, endogenous opioid peptides, and gonadotropin-inhibitory hormone in negatively regulating gonadotropin secretion in gilts is explored. Developmental changes in stimulatory mechanisms of glutamatergic and kisspeptin neurons are important for increased LH pulsatility required for the occurrence of puberty in pigs. Age at first estrus of gilts is metabolically gated, and numerous metabolites, metabolic hormones, and appetite-regulating neurotransmitters have been implicated in the nutritional regulation of gonadotropin secretion. Leptin is an important metabolic signal linking body energy reserves with age at puberty in gilts. Leptin acting through neuropeptide Y and proopiomelanocortin neurons in the hypothalamus has important impacts on the function of the reproductive neurosecretory axis of gilts. Age at puberty in swine is heritable, and genomic analyses reveal it to be a polygenic trait. Genome-wide association studies for pubertal age in gilts have revealed several genomic regions in common with those identified for age at menarche in humans. Candidate genes have been identified that have important functions in growth and adiposity. Numerous genes regulating hypothalamic neuronal function, gonadotropes in the adenohypophysis, and ovarian follicular development have been identified and illustrate the complex maturational changes occurring in the hypothalamic-pituitary-ovarian axis during puberty in gilts.

Nutritional control of puberty in the bovine female: prenatal and early postnatal regulation of the neuroendocrine system

Puberty is a complex biological event that requires maturation of the reproductive neuroendocrine axis and subsequent initiation of high-frequency, episodic release of GnRH and LH. Nutrition is a critical factor affecting the neuroendocrine control of puberty. Although nutrient restriction during juvenile development delays puberty, elevated rates of body weight gain during this period facilitate pubertal maturation by programming hypothalamic centers that underlie the pubertal process. Recent findings suggest that maternal nutrition during gestation can also modulate the development of the fetal neuroendocrine axis, thus influencing puberty and subsequent reproductive function. Among the several metabolic signals, leptin plays a critical role in conveying metabolic information to the brain and, consequently, controlling puberty. The effects of leptin on GnRH secretion are mediated via an upstream neuronal network because GnRH neurons do not express the leptin receptor. Two neuronal populations located in the arcuate nucleus that express the orexigenic peptide neuropeptide Y (NPY), and the anorexigenic peptide alpha melanocyte-stimulating hormone (αMSH), are key components of the neurocircuitry that conveys inhibitory (NPY) and excitatory (αMSH) inputs to GnRH neurons. In addition, neurons in the arcuate nucleus that coexpress kisspeptin, neurokinin B, and dynorphin (termed KNDy neurons) are also involved in the metabolic control of puberty. Our studies in the bovine female demonstrate that increased planes of nutrition during juvenile development lead to organizational and functional changes in hypothalamic pathways comprising NPY, proopiomelanocortin (POMC, the precursor of αMSH), and kisspeptin neurons. Changes include alterations in the abundance of NPY, POMC, and Kiss1 mRNA and in plasticity of the neuronal projections to GnRH neurons. Our studies also indicate that epigenetic mechanisms, such as modifications in the DNA methylation pattern, are involved in this process. Finally, our most recent data demonstrate that maternal nutrition during gestation can also induce morphological and functional changes in the hypothalamic NPY system in the heifer offspring that are likely to persist long after birth. These organizational changes occurring during fetal development have the potential to not only impact puberty but also influence reproductive performance throughout adulthood in the bovine female.

mRNA decapping is an evolutionarily conserved modulator of neuroendocrine signaling that controls development and ageing

Eukaryotic 5'-3' mRNA decay plays important roles during development and in response to stress, regulating gene expression post-transcriptionally. In Caenorhabditis elegans, deficiency of DCAP-1/DCP1, the essential co-factor of the major cytoplasmic mRNA decapping enzyme, impacts normal development, stress survival and ageing. Here, we show that overexpression of dcap-1 in neurons of worms is sufficient to increase lifespan through the function of the insulin/IGF-like signaling and its effector DAF-16/FOXO transcription factor. Neuronal DCAP-1 affects basal levels of INS-7, an ageing-related insulin-like peptide, which acts in the intestine to determine lifespan. Short-lived dcap-1 mutants exhibit a neurosecretion-dependent upregulation of intestinal ins-7 transcription, and diminished nuclear localization of DAF-16/FOXO. Moreover, neuronal overexpression of DCP1 in Drosophila melanogaster confers longevity in adults, while neuronal DCP1 deficiency shortens lifespan and affects wing morphogenesis, cell non-autonomously. Our genetic analysis in two model-organisms suggests a critical and conserved function of DCAP-1/DCP1 in developmental events and lifespan modulation.

Developmental programming of the female neuroendocrine system by steroids

Developmental programming refers to processes that occur during early life that may have long-term consequences, modulating adult health and disease. Complex diseases, such as diabetes, cancer and cardiovascular disease, have a high prevalence in different populations, are multifactorial, and may have a strong environmental component. The environment interacts with organisms, affecting their behaviour, morphology and physiology. This interaction may induce permanent or long-term changes, and organisms may be more susceptible to environmental factors during certain developmental stages, such as the prenatal and early postnatal periods. Several factors have been identified as responsible for inducing the reprogramming of various reproductive and nonreproductive tissues. Among them, both natural and synthetic steroids, such as endocrine disruptors, are known to have either detrimental or positive effects on organisms depending on the dose of exposure, stage of development and biological sexual background. The present review focuses on the action of steroids and endocrine disruptors as agents involved in developmental programming and on their modulation and effects on female neuroendocrine functions.

Epigenetic mechanisms in the placenta related to infant neurodevelopment

As the 'third brain' the placenta links the developing fetal brain and the maternal brain enabling study of epigenetic process in placental genes that affect infant neurodevelopment. We described the characteristics and findings of the 17 studies on epigenetic processes in placental genes and human infant neurobehavior. Studies showed consistent findings in the same cohort of term healthy infants across epigenetic processes (DNA methylation, genome wide, gene and miRNA expression) genomic region (single and multiple genes, imprinted genes and miRNAs) using candidate gene and genome wide approaches and across biobehavioral systems (neurobehavior, cry acoustics and neuroendocrine). Despite limitations, studies support future work on molecular processes in placental genes related to neurodevelopmental trajectories including implications for intervention.

Dynamic modulation of sociality and aggression: an examination of plasticity within endocrine and neuroendocrine systems

Endocrine and neuroendocrine systems are key mediators of behavioural plasticity and allow for the ability to shift social behaviour across dynamic contexts. These systems operate across timescales, modulating both rapid responses to environmental changes and developmental plasticity in behavioural phenotypes. Thus, not only do endocrine systems mediate behavioural plasticity, but also the systems themselves exhibit plasticity in functional capabilities. This flexibility at both the mechanistic and behavioural levels can be crucial for reproduction and survival. Here, we discuss how plasticity in nonapeptide and steroid actions may influence the expression of, and allow rapid shifts between, sociality and aggression-behavioural shifts that can be particularly important for social interactions. Recent findings of overlap in the mechanisms that modulate social and aggressive behaviour suggest the potential for a mechanistic continuum between these behaviours. We briefly discuss the potential for a sociality-aggression continuum and novel techniques that will enable probing of the functional connectivity of social behaviours. From an evolutionary perspective, we suggest that plasticity in endocrine and neuroendocrine mechanisms of behaviour may be important targets of selection, and discuss the conditions under which we would predict selection to have resulted in differences in endocrine plasticity across species that differ in social organization.This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Neuroendocrine disruption of organizational and activational hormone programming in poikilothermic vertebrates

In vertebrates, sexual differentiation of the reproductive system and brain is tightly orchestrated by organizational and activational effects of endogenous hormones. In mammals and birds, the organizational period is typified by a surge of sex hormones during differentiation of specific neural circuits; whereas activational effects are dependent upon later increases in these same hormones at sexual maturation. Depending on the reproductive organ or brain region, initial programming events may be modulated by androgens or require conversion of androgens to estrogens. The prevailing notion based upon findings in mammalian models is that male brain is sculpted to undergo masculinization and defeminization. In absence of these responses, the female brain develops. While timing of organizational and activational events vary across taxa, there are shared features. Further, exposure of different animal models to environmental chemicals such as xenoestrogens such as bisphenol A-BPA and ethinylestradiol-EE2, gestagens, and thyroid hormone disruptors, broadly classified as neuroendocrine disrupting chemicals (NED), during these critical periods may result in similar alterations in brain structure, function, and consequently, behaviors. Organizational effects of neuroendocrine systems in mammals and birds appear to be permanent, whereas teleost fish neuroendocrine systems exhibit plasticity. While there are fewer NED studies in amphibians and reptiles, data suggest that NED disrupt normal organizational-activational effects of endogenous hormones, although it remains to be determined if these disturbances are reversible. The aim of this review is to examine how various environmental chemicals may interrupt normal organizational and activational events in poikilothermic vertebrates. By altering such processes, these chemicals may affect reproductive health of an animal and result in compromised populations and ecosystem-level effects.

Innervation of Pulmonary Neuroendocrine Cells and Neuroepithelial Bodies in Developing Rabbit Lung

We investigated the development of innervation of the pulmonary neuroendocrine cell (PNEC) system composed of single cells and organoid cell clusters, neuroepithelial bodies (NEB) in rabbit fetal and neonatal lungs. To visualize the nerve fibers and their contacts with PNECs/NEBs, we used confocal microscopy and multilabel immunohistochemistry (IHC) with pan-neural marker, synaptic vesicle protein 2 (SV2), and serotonin (5-HT) as markers for PNECs/NEBs, and smooth muscle actin or cytokeratin to identify airway landmarks. The numbers and distribution of PNEC/NEB at different stages of lung development (E16, 18, 21, 26, and P2) and the density of innervation were quantified. First PNECs immunoreactive for 5-HT were identified in primitive airway epithelium at E18 as single cells or as small cell clusters with or without early nerve contacts. At E21 a significant increase in the number of PNECs with formation of early innervated NEB corpuscules was observed. The overall numbers of PNECs/NEBs and the density of mucosal, submucosal, and intercorpuscular innervation increased with progressing gestation and peaked postnataly (P2). At term, the majority of NEBs and single PNECs within airway mucosa possessed neural contacts. Such an extensive and complex innervation of the PNEC system indicates a multifunctional role in developing lung and during neonatal adaptation.

Maternal Vitamin D Deficiency Programs Reproductive Dysfunction in Female Mice Offspring Through Adverse Effects on the Neuroendocrine Axis

Vitamin D (VitD) deficiency affects more than 1 billion people worldwide with a higher prevalence in reproductive-aged women and children. The physiological effects of maternal VitD deficiency on the reproductive health of the offspring has not been studied. To determine whether maternal VitD deficiency affects reproductive physiology in female offspring, we monitored the reproductive physiology of C57BL/6J female offspring exposed to diet-induced maternal VitD deficiency at three specific developmental stages: 1) in utero, 2) preweaning, or 3) in utero and preweaning. We hypothesized that exposure to maternal VitD deficiency disrupts reproductive function in exposed female offspring. To test this hypothesis, we assessed vaginal opening and cytology and ovary and pituitary function as well as gonadotropin and gonadal steroid levels in female offspring. The in utero, preweaning, and in utero and preweaning VitD deficiency did not affect puberty. However, all female mice exposed to maternal VitD deficiency developed prolonged and irregular estrous cycles characterized by oligoovulation and extended periods of diestrus. Despite similar gonadal steroid levels and GnRH neuron density, females exposed to maternal VitD deficiency released less LH on the evening of proestrus. When compared with control female offspring, there was no significant difference in the ability of females exposed to maternal VitD deficiency to respond robustly to exogenous GnRH peptide or controlled ovarian hyperstimulation. These findings suggest that maternal VitD deficiency programs reproductive dysfunction in adult female offspring through adverse effects on hypothalamic function.

Do fatty acids affect fetal programming?

BACKGROUND

In this study discussed the primary and regulatory roles of fatty acids, and investigated the affects of fatty acids on metabolic programming.

METHODS

Review of the literature was carried out on three electronic databases to assess the roles of fatty acids in metabolic programming. All abstracts and full-text articles were examined, and the most relevant articles were selected for screening and inclusion in this review.

RESULTS

The mother's nutritional environment during fetal period has important effects on long term health. Fatty acids play a primary role in growth and development. Alterations in fatty acid intake in the fetal period may increase the risk of obesity and metabolic disorders in later life. Maternal fatty acid intakes during pregnancy and lactation are passed to the fetus and the newborn via the placenta and breast milk, respectively. Imbalances in fatty acid intake during the fetal period change the fatty acid composition of membrane phospholipids, which can cause structural and functional problems in cells. Additionally, the metabolic and neuroendocrine environments of the fetus and the newborn play key roles in the regulation of energy balance.

CONCLUSIONS

Imbalances in fatty acid intake during pregnancy and lactation may result in permanent changes in appetite control, neuroendocrine function and energy metabolism in the fetus, leading to metabolic programming. Further studies are needed to determine the role of fatty acid intake in metabolic programming.

Dopaminergic Neurons Controlling Anterior Pituitary Functions: Anatomy and Ontogenesis in Zebrafish

Dopaminergic (DA) neurons located in the preoptico-hypothalamic region of the brain exert a major neuroendocrine control on reproduction, growth, and homeostasis by regulating the secretion of anterior pituitary (or adenohypophysis) hormones. Here, using a retrograde tract tracing experiment, we identified the neurons playing this role in the zebrafish. The DA cells projecting directly to the anterior pituitary are localized in the most anteroventral part of the preoptic area, and we named them preoptico-hypophyseal DA (POHDA) neurons. During development, these neurons do not appear before 72 hours postfertilization (hpf) and are the last dopaminergic cell group to differentiate. We found that the number of neurons in this cell population continues to increase throughout life proportionally to the growth of the fish. 5-Bromo-2'-deoxyuridine incorporation analysis suggested that this increase is due to continuous neurogenesis and not due to a phenotypic change in already-existing neurons. Finally, expression profiles of several genes (foxg1a, dlx2a, and nr4a2a/b) were different in the POHDA compared with the adjacent suprachiasmatic DA neurons, suggesting that POHDA neurons develop as a distinct DA cell population in the preoptic area. This study offers some insights into the regional identity of the preoptic area and provides the first bases for future functional genetic studies on the development of DA neurons controlling anterior pituitary functions.

[NEUROENDOCRINE REPRODUCTIVE CENTERS OF CORTICOMEDIAL DIVISION OF THE AMYGDALA]

In this review for the first time systematized available in modern literature data, which characterize the structural and functional organization of the reproductive centers of corticomedial division of the Amygdala. Given information about physiological mechanisms of their involvement in the organization of sexual behavior, regulation of secretion and excretion of gonadotropines, influence on the processes of sexual maturation of organisms. Involvement of Amygdala in functional systems of the brain, which determine reproductive functions, predefined its participation in the processes of sexual differentiation of the brain. Important role in the implementation of reproductive functions plays the olfactory stimuli, which through the Amygdala switches to the centers of the pre-optic-hypothalamic region, which controls the secretion of gonadotropins and sexual behavior.

Shaping the Reproductive System: Role of Semaphorins in Gonadotropin-Releasing Hormone Development and Function

The semaphorin proteins, which contribute to the morphogenesis and homeostasis of a wide range of systems, are among the best-studied families of guidance cues. Much recent research has focused on the role of semaphorins in the development and adult activity of hormone systems and, reciprocally, how circulating reproductive hormones regulate their expression and function. Specifically, several reports have focused on the molecular mechanisms underlying the effects of semaphorins on the migration, survival and structural and functional plasticity of neurons that secrete gonadotropin-releasing hormone (GnRH), essential for the acquisition and maintenance of reproductive competence in mammals. Alterations in the development of this neuroendocrine system lead to anomalous or absent GnRH secretion, resulting in heterogeneous reproductive disorders such as congenital hypogonadotropic hypogonadism (CHH) or other conditions characterized by infertility or subfertility. This review summarizes current knowledge of the role of semaphorins and their receptors on the development, differentiation and plasticity of the GnRH system. In addition, the involvement of genetic deficits in semaphorin signaling in some forms of CHH in humans is discussed.

[Plasticity of neuroendocrine and immune systems in early development]

This article provides an analysis of our own and published data on the reciprocal morphogenetic influence of the neiuroendocriie and imnimune systems on their formation and function in mammals. It is substantiated that, in early ontogeny, neurohormones regulate the growth and differentiation of various tissues in the body, including the lymphoid tissue. Thymicpeptides, in turn, affect the development of the hypothalamic-pitiitary-adrenal and gonadal-systems. Various adverse factors and changes in the physiological concentrations of hormones in the critical periods of development of these systems change their functions, and the plasticity of physiological systems in early ontogeny allows the body to adapt to new conditions. Disturbances in the interaction of the neuroendocrineand immune systems in the perinatal period induce apredisposition to various diseases in progeny.

A critical point of male gonad development: neuroendocrine correlates of accelerated testicular growth in rats during early life

Testis growth during early life is important for future male fertility and shows acceleration during the first months of life in humans. This acceleration coincides with the peak in gonadotropic hormones in the blood, while the role of hypothalamic factors remains vague. Using neonatal rats to assess this issue, we found that day 9 of life is likely critical for testis development in rats. Before this day, testicular growth was proportional to body weight gain, but after that the testes showed accelerated growth. Hypothalamic kisspeptin and its receptor mRNA levels begin to elevate 2 days later, at day 11. A significant increase in the mRNA levels for gonadotropin-releasing hormone (GnRH) receptors in the hypothalamus between days 5 and 7 was followed by a 3-fold decrease in GnRH mRNA levels in this brain region during the next 2 days. Starting from day 9, hypothalamic GnRH mRNA levels increased significantly and positively correlated with accelerated testicular growth. Triptorelin, an agonist of GnRH, at a dose that had no effect on testicular growth during “proportional” period, increased testis weights during the period of accelerated growth. The insensitivity of testicular growth to GnRH during “proportional” period was supported by inability of a 2.5-fold siRNA knockdown of GnRH expression in the hypothalamus of the 7-day-old animals to produce any effect on their testis weights. GnRH receptor blockade with cetrorelix was also without effect on testis weights during “proportional” period but the same doses of this GnRH antagonist significantly inhibited “accelerated” testicular growth. GnRH receptor mRNA levels in the pituitary as well as plasma LH concentrations were higher during “accelerated” period of testicular growth than during “proportional” period. In general, our data defined two distinct periods in rat testicular development that are primarily characterized by different responses to GnRH signaling.

Reproductive aging in men

Aging in men is associated with a decrease in serum testosterone levels due to attrition in testicular Leydig cells and slowing of the hypothalamic GnRH pulse generator. The practicing endocrinologist is frequently consulted for consideration of testosterone therapy in older men with late-onset hypogonadism (LOH), a condition that many clinicians fail to distinguish from organic hypogonadism. Recent data using syndromic definition show that only 2% of 40-80-year-old men have LOH. Co-morbidities and obesity strongly contribute to LOH, suggesting that testosterone is a biomarker of health. Hence, prevention and treatment of these co-morbidities might attenuate age-related decline in androgen levels.

Evidence for increased tissue androgen sensitivity in neurturin knockout mice

Neurturin (NTN) is a member of the glial cell line-derived neurotrophic factor (GDNF) family and signals through GDNF family receptor alpha 2 (GFRα2). We hypothesised that epithelial atrophy reported in the reproductive organs of Ntn (Nrtn)- and Gfrα2 (Gfra2)-deficient mice could be due to NTN affecting the hormonal environment. To investigate this, we compared the reproductive organs of Ntn- and Gfrα2-deficient male mice in parallel with an analysis of their circulating reproductive hormone levels. There were no significant structural changes within the organs of the knockout mice; however, serum and intratesticular testosterone and serum LH levels were very low. To reconcile these observations, we tested androgen sensitivity by creating a dihydrotestosterone (DHT) clamp (castration plus DHT implant) to create fixed circulating levels of androgens, allowing the evaluation of androgen-sensitive endpoints. At the same serum DHT levels, serum LH levels were lower and prostate and seminal vesicle weights were higher in the Ntn knockout (NTNKO) mice than in the wild-type mice, suggesting an increased response to androgens in the accessory glands and hypothalamus and pituitary of the NTNKO mice. Testicular and pituitary responsiveness was unaffected in the NTNKO males, as determined by the response to the human chorionic gonadotrophin or GNRH analogue, leuprolide, respectively. In conclusion, our results suggest that NTN inactivation enhances androgen sensitivity in reproductive and neuroendocrine tissues, revealing a novel mechanism to influence reproductive function and the activity of other androgen-dependent tissues.

Maternal mood and neuroendocrine programming: effects of time of exposure and sex

Adverse exposures that influence growth in prenatal and early postnatal periods are considered to influence vulnerability to chronic diseases via their effects on the neuroendocrine system. In humans, the assessment of the underlying mechanisms has been restricted. The present study aimed to investigate the effects of adverse early-life exposures, specifically maternal mood, on hypothlamic-pituitary-adrenal (HPA) axis, sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) responses to an acute physiological stressor. In addition, we conducted a preliminary examination into whether these effects varied by time of exposure and sex. One hundred and thity-nine individuals (mean age 15.12 years) were recruited from the ALSPAC (Avon Longitudinal Study of Parents and Children) birth cohort. Participants underwent the CO(2) stress test and indices of the PNS, SNS and HPA axis were measured. Pre-existing data on demographic and psychosocial factors of the mothers during pregnancy (18 and 32 weeks) and postnatally (8 weeks and 8 months) were extracted, as were participants' clinical and demographic data at birth. Increases in both pre- and postnatal anxiety and depression were associated with greater SNS reactivity to the stressor and slower recovery, as well as blunted HPA axis responses. Programming effects on the SNS appeared to be restricted to male offspring only. No consistent relationships were evident for any of the measures of pre-stress function. We have found preliminary evidence that both pre- and postnatal maternal anxiety and depression have sustained programming effects on the SNS and HPA axis. Effects on the SNS were restricted to male offspring.

Developing brain as an endocrine organ: secretion of dopamine

This study was aimed to test our hypothesis that the developing brain operates as an endocrine organ before the establishment of the blood-brain barrier (BBB), in rats up to the first postnatal week. Dopamine (DA) was selected as a marker of the brain endocrine activity. The hypothesis was supported by the observations in rats of: (i) the physiological concentration of DA in peripheral blood of fetuses and neonates, before the BBB establishment, and its drop by prepubertal period, after the BBB development; (ii) a drop of the DA concentration in the brain for 54% and in blood for 74% on the 3rd postnatal day after the intraventricular administration of 50 μg of α-methyl-p-tyrosine, an inhibitor of DA synthesis, with no changes in the DA metabolism in peripheral DA-producing organs. Thus, the developing brain is a principal source of circulating DA which is capable of providing an endocrine regulation of peripheral organs and the brain.

Epigenetic effects of endocrine-disrupting chemicals on female reproduction: an ovarian perspective

The link between in utero and neonatal exposure to environmental toxicants, such as endocrine-disrupting chemicals (EDCs) and adult female reproductive disorders is well established in both epidemiological and animal studies. Recent studies examining the epigenetic mechanisms involved in mediating the effects of EDCs on female reproduction are gathering momentum. In this review, we describe the developmental processes that are susceptible to EDC exposures in female reproductive system, with a special emphasis on the ovary. We discuss studies with select EDCs that have been shown to have physiological and correlated epigenetic effects in the ovary, neuroendocrine system, and uterus. Importantly, EDCs that can directly target the ovary can alter epigenetic mechanisms in the oocyte, leading to transgenerational epigenetic effects. The potential mechanisms involved in such effects are also discussed.

Neuroendocrine targets of endocrine disruptors

The central neuroendocrine systems are responsible for the control of homeostatic processes in the body, including reproduction, growth, metabolism and energy balance, as well as stress responsiveness. These processes are initiated by signals in the central nervous system, specifically the hypothalamus, and are conveyed first by neural and then by endocrine effectors. The neuroendocrine systems, as the links between the brain and peripheral endocrine organs, play critical roles in the ability of an organism to respond to its environment under normal circumstances. When neuroendocrine homeostasis is disrupted by environmental endocrine-disrupting chemicals, a variety of perturbations can ensue, particularly when endocrine disruption occurs during critical developmental time periods. This article will discuss the evidence for environmental endocrine disruption of neuroendocrine systems and the effects on endocrine and reproductive functions.

[Endocrine functions of the brain in adult and developing mammals]

The main prerequisite for organism's viability is the maintenance of the internal environment despite changes in the external environment, which is provided by the neuroendocrine control system. The key unit in this system is hypothalamus exerting endocrine effects on certain peripheral organs and anterior pituitary. Physiologically active substances of neuronal origin enter blood vessels in the neurohemal parts of hypothalamus where no blood-brain barrier exists. In other parts of the adult brain, the arrival of physiologically active substances is blocked by the blood-brain barrier. According to the generally accepted concept, the neuroendocrine system formation in ontogeny starts with the maturation of peripheral endocrine glands, which initially function autonomously and then are controlled by the anterior pituitary. The brain is engaged in neuroendocrine control after its maturation completes, which results in a closed control system typical of adult mammals. Since neurons start to secrete physiologically active substances soon after their formation and long before interneuronal connections are formed, these cells are thought to have an effect on brain development as inducers. Considering that there is no blood-brain barrier during this period, we proposed the hypothesis that the developing brain functions as a multipotent endocrine organ. This means that tens of physiologically active substances arrive from the brain to the systemic circulation and have an endocrine effect on the whole body development. Dopamine, serotonin, and gonadotropin-releasing hormone were selected as marker physiologically active substances of cerebral origin to test this hypothesis. In adult animals, they act as neurotransmitters or neuromodulators transmitting information from neuron to neuron as well as neurohormones arriving from the hypothalamus with portal blood to the anterior pituitary. Perinatal rats--before the blood-brain barrier is formed--proved to have equally high concentration of dopamine, serotonin, and gonadotropin-releasing hormone in the systemic circulation as in the adult portal system. After the brain-blood barrier is formed, the blood concentration of dopamine and gonadotropin-releasing hormone drops to zero, which indirectly confirms their cerebral origin. Moreover, the decrease in the blood concentration of dopamine, serotonin, and gonadotropin-releasing hormone before the brain-blood barrier formation after the microsurgical disruption of neurons that synthesize them or inhibition of dopamine and serotonin synthesis in the brain directly confirm their cerebral origin. Before the blood-brain barrier formation, dopamine, serotonin, gonadotropin-releasing hormone, and likely many other physiologically active substances of cerebral origin can have endocrine effects on peripheral target organs--anterior pituitary, gonads, kidney, heart, blood vessels, and the proper brain. Although the period of brain functioning as an endocrine organ is not long, it is crucial for the body development since physiologically active substances exert irreversible effects on the targets as morphogenetic factors during this period. Thus, the developing brain from the neuron formation to the establishment of the blood-brain barrier functions as a multipotent endocrine organ participating in endocrine control of the whole body development.

Evaluation of the hypothalamic-pituitary-adrenal axis function in childhood and adolescence

The hypothalamic-pituitary-adrenal (HPA) axis plays an important role in the maintenance of basal and stress-related homeostasis. The hypothalamus controls the secretion of adrenocorticotropic hormone (ACTH) from the anterior pituitary, which in turn stimulates the secretion of glucocorticoids from the adrenal cortex. Glucocorticoids, the final effectors of the HPA axis, regulate a broad spectrum of physiologic functions essential for life and exert their effects through their ubiquitously distributed intracellular receptors. Alterations in the activity of the HPA axis may present with symptoms and signs of glucocorticoid deficiency or excess. Detailed endocrinologic evaluation is of primary importance in determining the diagnosis and/or etiology of the underlying condition. We review the most common endocrinologic investigations used in the evaluation of the HPA axis integrity and function.

Programming neuroendocrine stress axis activity by exposure to glucocorticoids during postembryonic development of the frog, Xenopus laevis

Exposure to elevated glucocorticoids during early mammalian development can have profound, long-term consequences for health and disease. However, it is not known whether such actions occur in nonmammalian species, and if they do, whether the molecular physiological mechanisms are evolutionarily conserved. We investigated the effects of dietary restriction, which elevates endogenous corticosterone (CORT), or exposure to exogenous CORT added to the aquarium water of Xenopus laevis tadpoles on later-life measures of growth, feeding behavior, and neuroendocrine stress axis activity. Dietary restriction of prometamorphic tadpoles reduced body size at metamorphosis, but juvenile frogs increased food intake, showed catch-up growth through 21 d after metamorphosis, and had elevated whole-body CORT content compared with controls. Dietary restriction causes increased CORT in tadpoles, so to mimic this increase, we treated tadpoles with 100 nm CORT or vehicle for 5 or 10 d and then reared juvenile frogs to 2 months after metamorphosis. Treatment with CORT decreased body weight at metamorphosis, but juvenile frogs showed catch-up growth and had elevated basal plasma (CORT). Immunohistochemical analysis showed that CORT exposure as a tadpole led to decreased glucocorticoid receptor immunoreactivity in brain regions involved with stress axis regulation and in the anterior pituitary gland of juvenile frogs. The elevated CORT in juvenile frogs, which could result from decreased negative feedback owing to down-regulation of glucocorticoid receptor, may drive the hyperphagic response. Taken together, our findings suggest that long-term, stable phenotypic changes in response to elevated glucocorticoids early in life are an ancient and conserved feature of the vertebrate lineage.

Foetal Leydig cells and the neuroendocrine system

It has been shown that adult human Leydig cells express a number of neuroendocrine markers, and, therefore, could be considered as a part of the neuroendocrine system in the adult. A limited number of studies have dealt with the dynamics of development of human foetal Leydig cells, whereas studies on their neuroendocrine nature are still extremely rare. Therefore, the aim of our study was to investigate the development of human foetal Leydig cells in different weeks of gestation (wg) and to check if these cells express certain markers characteristic of the diffuse neuroendocrine system (DNS). Qualitative, quantitative histological studies and immunohistochemical analyses of human foetal testicular tissue have been performed. According to our data, Leydig cells formed a dynamic population of cells within the interstitum of testes in the period between 15 and 36 wg. The total number of Leydig cells of human foetal testes changed through different stages of gestation by means of 'pulsatile' dynamics (most likely, by following the variable level of gonadotropins). At early stages of development (15-17 wg) immunohistochemical reactions for the expression of neuron specific enolase (NSE) were positive within sex cords and between them, in the interstitum. Pro-spermatogonia in the sex cords were positive, as well as elongated spindle-shaped cells of the interstitum (very likely, precursors of Leydig cells). During the later stages of development (28-36 wg), excluding the pro-spermatogonia, the interstitial Leydig cells were also positive. The results of the immunohistochemical analyses (the expression of NSE) confirmed the hypothesis that human foetal Leydig cells were of neuroendocrine nature.

Windows of the brain: Towards a developmental biology of circumventricular and other neurohemal organs

We review the anatomical and functional features of circumventricular organs in vertebrates and their homologous neurohemal organs in invertebrates. Focusing on cyclostomes (lamprey) and urochordates (ascidians), we discuss the evolutionary origin of these organs as a function of their cell type specification and morphogenesis.

Estrogenic environmental endocrine-disrupting chemical effects on reproductive neuroendocrine function and dysfunction across the life cycle

Endocrine disrupting chemicals (EDCs) are natural or synthetic compounds that interfere with the normal function of an organism's endocrine system. Many EDCs are resistant to biodegradation, due to their structural stability, and persist in the environment. The focus of this review is on natural and artificial EDCs that act through estrogenic mechanisms to affect reproductive neuroendocrine systems. This endocrine axis comprises the hypothalamic gonadotropin-releasing hormone (GnRH), pituitary gonadotropins, and gonadal steroid hormones, including estrogens. Although it is not surprising that EDCs that mimic or antagonize estrogen receptors may exert actions upon reproductive targets, the mechanisms for these effects are complex and involve all three levels of the hypothalamic-pituitary-gonadal (HPG) system. Nevertheless, considerable evidence links exposure to estrogenic environmental EDCs with neuroendocrine reproductive deficits in wildlife and in humans. The effects of an EDC are variable across the life cycle of an animal, and are particularly potent when exposure occurs during fetal and early postnatal development. As a consequence, abnormal sexual differentiation, disrupted reproductive function, or inappropriate sexual behavior may be detected later in life. This review will cover the effects of two representative classes of estrogenic EDCs, phytoestrogens and polychlorinated biphenyls (PCBs), on neuroendocrine reproductive function, from molecules to behavior, across the vertebrate life cycle. Finally, we identify the gaps of knowledge in this field and suggest future directions for study.

Impact of neonatal hypothyroidism on reproduction in the male hamster, Mesocricetus auratus

Thyroid hormones play an important role in brain development. In the present study, we examined the influence of transient postnatal hypothyroidism on reproductive neuroendocrine and behavioral outcomes in the male Syrian (golden) hamster. Hamster pups were rendered hypothyroid following exposure to the goitrogen, 6-n-propyl-2-thiouracil (PTU), between postnatal (PN) day 0 (birth) and PN25 (weaning). By 15 days after cessation of PTU, exposure (PN40) serum thyroxine levels had returned to control levels. The testes of treated males were approximately 30% heavier than controls and daily sperm production was increased by 73%. Immunocytochemistry for GnRH revealed that the total number of GnRH neurons did not vary between groups; however, a shift in the distribution of GnRH neurons was observed in treated males such that more GnRH immunoreactive neurons were found in the caudal portion of their normal distribution. The shift in GnRH distribution was associated with a significant reduction (40-50%) in pituitary gonadotropin secretion. Behaviorally, treated males took significantly longer to investigate the anogenital region and then mount a receptive female. A corresponding reduction in the total number of anogenital investigations and mounts was observed. This difference between treated males and controls was reduced, but not eliminated, over successive trials and by the third trial the number of intromission was similar between treated and control males. We conclude that the full complement of adult reproductive functions observed in the male golden hamster requires thyroid hormones during the early postnatal period. The severity of the effects induced by early hypothyroidism in this species varies from transient to permanent, depending on the endpoint.

Reproductive ageing in women

The traditional view in respect to female reproduction is that the number of oocytes at birth is fixed and continuously declines towards the point when no more oocytes are available after menopause. In this review we briefly discuss the embryonic development of female germ cells and ovarian follicles. The ontogeny of the hypothalamic-pituitary-gonadal axis is then discussed, with a focus on pubertal transition and normal ovulatory menstrual cycles during female adult life. Biochemical markers of menopausal transition are briefly examined. We also examine the effects of age on female fertility, the contribution of chromosomal abnormalities of the oocyte to the observed decline in female fertility with age and the possible biological basis for the occurrence of such abnormalities. Finally, we consider the effects of maternal age on obstetric complications and perinatal outcome. New data that have the potential to revolutionize our understanding of mammalian oogenesis and follicular formation, and of the female reproductive ageing process, are also briefly considered.

Long-Term Exposure of Female Sheep to Physiologic Concentrations of Estradiol: Effects on the Onset and Maintenance of Reproductive Function, Pregnancy, and Social Development in Female Offspring1

As steroids and steroid-like compounds accumulate in the environment, it has become important to understand how low-dose exposure affects reproductive function. Ovary-intact sheep were used in a multigenerational study, to determine whether chronic exposure to low levels of estrogen disrupts reproductive function and behavior. We assessed parameters of reproductive performance in control and postnatally estradiol-treated females (Generation 1, G1), and their offspring (Generation 2, G2). In the G1 animals, 17beta-estradiol (E) was administered continuously from 4 wk of age at two doses via subcutaneous implants (ultralow E [<1 pg/ml in circulation, n = 8] or low E [1-3 pg/ml, n = 8]). Both doses delayed puberty; low E also produced pronounced prepubertal and seasonal anestrus hypogonadotropism, and delayed the onset of the second breeding season. All G1 animals conceived and produced offspring (G2), the treatment of which resulted from continuous maternal exposure during pregnancy and lactation. Behavioral observations of G2 females revealed that low prenatal E modestly masculinized play behavior and increased the frequency of attempts to displace competitors relative to ultralow E and control animals. The timing and magnitude of the LH surge also differed in prepubertal low prenatal E females relative to the controls, although these differences were not evident when retested at one year of age. These findings support the hypothesis that chronic exposure to physiologic amounts of exogenous estrogens has multigenerational effects on behavior and neuroendocrine function. Despite these disruptive steroid actions, ovarian cyclicity and fertility are not invariably compromised, pointing to an impressive resiliency of the reproductive axis to insult by exogenous estrogenic compounds.

Sex- and age-specific effects of anabolic androgenic steroids on reproductive behaviors and on GABAergic transmission in neuroendocrine control regions

Illicit use of anabolic androgenic steroids (AAS) has become a prevalent health concern not only among male professional athletes, but, disturbingly, among a growing number of women and adolescent girls. Despite the increasing use of AAS among women and adolescents, few studies have focused on the effects of these steroids in females, and female adolescent subjects are particularly underrepresented. Among the hallmarks of AAS abuse are changes in reproductive behaviors. Here, we discuss work from our laboratories on the actions of AAS on the onset of puberty and sexual behaviors in female rodents, AAS interactions and sex- and age-specific effects of these steroids on neural transmission mediated by gamma-aminobutyric acid receptors within forebrain neuroendocrine control regions that may underlie AAS-induced changes in these behaviors.

Specification and development of the pars intercerebralis and pars lateralis, neuroendocrine command centers in the Drosophila brain

The central neuroendocrine system in the Drosophila brain includes two centers, the pars intercerebralis (PI) and pars lateralis (PL). The PI and PL contain neurosecretory cells (NSCs) which project their axons to the ring gland, a complex of peripheral endocrine glands flanking the aorta. We present here a developmental and genetic study of the PI and PL. The PI and PL are derived from adjacent neurectodermal placodes in the dorso-medial head. The placodes invaginate during late embryogenesis and become attached to the brain primordium. The PI placode and its derivatives express the homeobox gene Dchx1 and can be followed until the late pupal stage. NSCs labeled by the expression of Drosophila insulin-like peptide (Dilp), FMRF, and myomodulin form part of the Dchx1 expressing PI domain. NSCs of the PL can be followed throughout development by their expression of the adhesion molecule FasII. Decapentaplegic (Dpp), secreted along the dorsal midline of the early embryo, inhibits the formation of the PI and PL placodes; loss of the signal results in an unpaired, enlarged placodeal ectoderm. The other early activated signaling pathway, EGFR, is positively required for the maintenance of the PI placode. Of the dorso-medially expressed head gap genes, only tailless (tll) is required for the specification of the PI. Absence of the corpora cardiaca, the endocrine gland innervated by neurosecretory cells of the PI and PL, does not affect the formation of the PI/PL, indicating that inductive stimuli from their target tissue are not essential for early PI/PL development.

Induction of a hypothyroid state during juvenile development delays pubertal reactivation of the neuroendocrine system governing luteinising hormone secretion in the male rhesus monkey (Macaca mulatta)

The present study aimed to determine the influence of thyroid status on the timing of the pubertal resurgence in gonadotrophin-releasing hormone pulse generator activity [tracked by circulating luteinising hormone (LH) levels] in male rhesus monkeys. Six juvenile monkeys were orchidectomised and then treated with the antithyroid drug, methimazole, from 15-19 months until 36 months of age, at which time thyroxine (T(4)) replacement was initiated. Four additional agonadal monkeys served as controls. Blood samples were drawn weekly for hormonal assessments. Body weight, crown-rump length and bone age were monitored at regular intervals. By 8 weeks of methimazole treatment, plasma T(4) had fallen sharply, and the decline was associated with a plasma thyroid-stimulating hormone increase. In controls, plasma LH levels remained undetectable until the pubertal rise occurred at 29.3 +/- 0.2 months of age. This developmental event occurred in only half of the methimazole-treated animals before 36 months of age when T(4) replacement was initiated. The hypothyroid state was associated with a profound arrest of growth and bone maturation, but increased body mass indices and plasma leptin levels. T(4) replacement in methimazole-treated monkeys was associated with the pubertal rise in LH in the remaining three animals and accelerated somatic development in all six animals. Although pubertal resurgence in LH secretion occurred at a later chronological age in methimazole-treated animals compared to controls, bone age, crown-rump length and body weight at that time did not differ between groups. There were no long-term differences in plasma prolactin between groups. We conclude that juvenile hypothyroidism in male primates causes a marked delay in the pubertal resurgence of LH secretion, probably occasioned at the hypothalamic level. Whether this effect is meditated by an action of thyroid hormone directly on the hypothalamus or indirectly as a result of the concomitant deficit in somatic development remains to be determined.

The male monkey as a model for the study of the neurobiology of puberty onset in man

This chapter is based on the material that was presented in the Symposium titled "Puberty in mechanistic perspective: animal models" at Sixth International Conference on the Control of the Onset of Puberty held in Evian, May 2005.

A store-operated current (SOC) mediates oxytocin autocontrol in the developing rat hypothalamus

Oxytocin (OT) and vasopressin (VP) autocontrol their secreting neurons in the supraoptic nucleus (SON) by modulating action potential firing through activation of specific metabotropic receptors. However, the mechanisms linking receptor activation to firing remain unknown. In almost all cell types, activation of plasma membrane metabotropic receptors triggers signalling cascades that induce mobilization of calcium from intracellular stores. In turn, emptying the calcium stores may evoke calcium influx through store-operated channels (SOCs), the functions of which remain largely unknown in neurons. In this study, we show that these channels play a key role in the SON, at least in the response to OT. In isolated rat SON neurons, store depletion by thapsigargin induced an influx of calcium, demonstrating the presence of SOCs in these neurons. This calcium influx was specifically inhibited by 0.2 mM 1-(2-trifluoromethylphenyl-)imidazole (TRIM). At 2 mM, this compound affected neither the resting electrophysiological properties nor the voltage-dependant inward currents. In fresh slices, TRIM (2 mM) did not affect the resting potential of SON neurons, action potential characteristics, spontaneous action potential firing or synaptic activity; this compound thus appears to be a specific blocker of SOCs in SON neurons. TRIM (0.2 mM) specifically reduced the increase in action potential firing triggered by OT but did not affect the VP-induced response. These observations demonstrate that store operated channels exist in hypothalamic neurons and specifically mediate the response to OT in the SON.

Sex differences and the development of the rabbit brain: effects of vinclozolin

The preoptic/anterior hypothalamic area (POA/AH) is one of the most sexually dimorphic areas of the vertebrate brain and plays a pivotal role in regulating male sexual behavior. Vinclozolin is a fungicide thought to be an environmental antiandrogen, which disrupts masculine sexual behavior when administered to rabbits during development. In this study, we examined several characteristics of the rabbit POA/AH for sexual dimorphism and endocrine disruption by vinclozolin. Pregnant rabbits were dosed orally with vinclozolin (10 mg/kg body weight) or carrot paste vehicle once daily for 6 wk beginning at midgestation and continuing through nursing until Postpartum Week 4. At 6 wk, offspring were perfused with 4% paraformaldehyde and brains processed for immunocytochemical localization of tyrosine hydroxylase, calbindin, gonadotropin-releasing hormone (GnRH), or Nissl stain. There were significant sex differences in the distribution of calbindin in the POA/AH and the size of cells in the dorsal POA/AH (values greater in females than in males), but not in the number or distribution of tyrosine hydroxylase or GnRH neurons. In both sexes, exposure to vinclozolin significantly increased calbindin expression in the ventral POA/AH and significantly decreased number of GnRH neurons selectively in the region of the organum vasculosum of the lamina terminalis (OVLT) but not more caudally in the POA/AH. This is the first documentation of a sexually dimorphic region in the rabbit brain, and further supports the use of this species as a model for studying the influence of vinclozolin on reproductive development with potential application to human systems.

Activation of gonadotropin-releasing hormone neurons by kisspeptin as a neuroendocrine switch for the onset of puberty

We examined the role of kisspeptin and its receptor, the G-protein-coupled receptor GPR54, in governing the onset of puberty in the mouse. In the adult male and female mouse, kisspeptin (10-100 nM) evoked a remarkably potent, long-lasting depolarization of >90% of gonadotropin-releasing hormone (GnRH)-green fluorescent protein neurons in situ. In contrast, in juvenile [postnatal day 8 (P8) to P19] and prepubertal (P26-P33) male mice, kisspeptin activated only 27 and 44% of GnRH neurons, respectively. This developmental recruitment of GnRH neurons into a kisspeptin-responsive pool was paralleled by an increase in the ability of centrally administered kisspeptin to evoke luteinizing hormone secretion in vivo. To learn more about the mechanisms through which kisspeptin-GPR54 signaling at the GnRH neuron may change over postnatal development, we performed quantitative in situ hybridization for kisspeptin and GPR54 transcripts. Approximately 90% of GnRH neurons were found to express GPR54 mRNA in both juvenile and adult mice, without a detectable difference in the mRNA content between the age groups. In contrast, the expression of KiSS-1 mRNA increased dramatically across the transition from juvenile to adult life in the anteroventral periventricular nucleus (AVPV; p < 0.001). These results demonstrate that kisspeptin exerts a potent depolarizing effect on the excitability of almost all adult GnRH neurons and that the responsiveness of GnRH neurons to kisspeptin increases over postnatal development. Together, these observations suggest that activation of GnRH neurons by kisspeptin at puberty reflects a dual process involving an increase in kisspeptin input from the AVPV and a post-transcriptional change in GPR54 signaling within the GnRH neuron.

[Sex characteristics of neuroendocrine effects of prenatal exposure to exogenous glucocorticoids]

The effects of hydrocortisone acetate treatment of rats during the last gestational week on neurochemical and morphological characteristics of the brain in early postnatal and mature offspring were studied. Disappearance of sexual differences both in aromatase and 5alpha-reductase activities and noradrenaline concentration in the preoptic area in 10-day old rats was found. Meanwhile a sexual dimorphism in serotonin metabolism emerged. In adult offspring, the prenatal exposure to glucocorticoids resulted in disappearance of sexual differences in neurocytes' nuclei volume in medial preoptic and suprachiasmatic nuclei. The adrenocortical reaction to noradrenaline infusion to the 3rd brain ventricle was absent in the experimental males and intensified in females. In males, adrenocortical reaction to restraint decreased while post-stress changes in hypothalamic noradrenaline concentration and hippocampal glutamate decarboxylase activity were not observed. In the similar experiments in females both the augmentation of adrenocortical reaction and inhibition of GABA-ergic system were revealed. The results obtained indicate the modifying effect of prenatal exposure to glucocorticoids on sexual dimorphism of neuroendocrine system.

Increased CRF-like and NPY-like immunoreactivity in adult rats exposed to nicotine during adolescence: relation to anxiety-like and depressive-like behavior

OBJECTIVE

Recently, animal models have been developed that demonstrate that adolescent nicotine exposure produces neurobehavioral changes which persist into adulthood. This study further examined the impact of adolescent nicotine exposure on anxiety-like and depressive-like behavior, as well as on levels of corticotropin-releasing factor (CRF) and neuropeptide Y (NPY) in this model.

METHODS

Male adolescent rats (35-40 days old) were administered nicotine using Nicoderm CQ patches (Smith-Kline Beecham). Behavior in the elevated plus maze (EPM) and forced swim test (FST) was assessed 2-3 weeks after exposure ended. Brain levels of CRF and NPY were then assessed 5-6 weeks after behavioral tests were completed. In addition, blood and brain levels of nicotine resulting from nicotine treatment were examined.

RESULTS

After 5 days of exposure to 5 mg/kg/day nicotine, blood levels of nicotine averaged 66+/-5 ng/ml and brain nicotine levels averaged 52+/-4 ng/g. Rats exposed to nicotine displayed an anxiety-like profile in the EPM (i.e., decreased time spent in the open arms) and an antidepressant-like profile in the FST (i.e., less time spent immobile). Rats exposed to nicotine also had increased hypothalamic and frontal cortical CRF, increased hypothalamic and hippocampal NPY, and a decreased ratio of NPY to CRF in the amygdala.

CONCLUSIONS

This study demonstrates that adolescent nicotine exposure produces lasting increases in anxiety-like behavior and may reduce depressive-like behavior. These behavioral changes also occurred in concert with alterations in CRF and NPY systems. Thus, lasting neurobehavioral changes associated with adolescent nicotine exposure may be related to allostatic changes in stress peptide systems.

Relationship between serum concentrations of leptin, soluble leptin receptor, testosterone and IGF-I, and growth during the first year of postnatal life in the male rhesus monkey, Macaca mulatta

OBJECTIVES

Subnormal leptin levels in low birth weight infants may allow for catch-up growth during infancy. Scant data are available that relate growth with circulating leptin during normal infancy in primates. The current study objective was to examine the association between serum leptin, its soluble receptor (sOB-R), testosterone and IGF-I concentrations, and body weight during infancy in male rhesus monkeys.

DESIGN

Hormone levels were assessed longitudinally in animals (n = 7) from birth until 1 year of age.

RESULTS

Body weight increased during the first 6 months of life and was strongly correlated with rising IGF-I levels and, as IGF-I plateaued and then declined during the second half of the year, body weight gain decelerated. In contrast, leptin levels declined gradually with age during the first year of life in conjunction with increasing body weight. There was no association between body weight gain and serum leptin levels or between serum testosterone and leptin values. Since sOB-R levels also declined with leptin values, it does not appear that levels of bioavailable leptin changed during infancy.

CONCLUSIONS

The data do not support the contention that leptin regulates growth during infancy, but the close association between IGF-I levels and body weight suggested that this hormone may regulate growth in infant male monkeys. The failure to observe an association between serum testosterone and leptin concentrations suggested that leptin is not involved in the activation of the hypothalamic-pituitary -testicular axis during this developmental period.

Ancient origins of human developmental plasticity

Animals have the ability to alter development, physiology, growth, and behavior in response to different environmental conditions. These responses represent critical assessments of both external and internal factors. For example, the timing of metamorphosis, hatching, or birth depends on the trade-offs between growth opportunity and mortality risk in the developmental habitat. Physiological sensors compute these trade-offs as a function of energy balance and environmental stress, and effectors initiate physiological, developmental, and behavioral responses to these determinations. The neuroendocrine stress axis provides a means for animals to integrate information from multiple sources and to respond accordingly. Considerable evidence now supports the view that the secretion of hormones critical to development (corticosteroid and thyroid hormones) is controlled by a common neuroendocrine stress pathway involving corticotropin-releasing factor (CRF) and related peptides. CRF produced in the hypothalamus stimulates the biosynthesis and secretion of both thyroid and corticosteroid hormones, leading to accelerated tadpole metamorphosis. Similarly, in mammals CRF of fetal and placental origin has been shown to influence the timing of birth. Studies in several experimental animal models and in humans show that early life experience can have long-term phenotypic consequences. Furthermore, there is evidence that phenotypic expression is strongly influenced by the actions of stress hormones produced during development. The integrated neuroendocrine response to stress, and its role in timing critical life history transitions and establishing long-term phenotypic expression, arose early in the evolution of vertebrates.

Life history and the early origins of health differentials

Current epidemiologic models concerning the fetal origins of later health risk are evaluated from the perspectives of evolutionary and developmental biology. Claims of adaptive value for and biological status of fetal programming are critically examined. Life history theory is applied to identify key trade-offs in adaptive strategies that constrain developmental design to use information from the environment to guide ontogeny and establish cost-benefit trade-offs that weigh early survival advantage against remote or unlikely future costs. Expectable environments of evolutionary adaptedness, particularly of gestation, are characterized and their impact on human adaptive design discussed. The roles of neuroendocrine mechanisms in scaffolding life course development, negotiating ongoing cost-benefit trade-offs, and mediating their long-term impacts on function and health are reviewed in detail. Overviews of gestational biology and the postnatal physiologic, cognitive-affective, and behavioral effects of gestational stress identify a shared central role for the hypothalamic-pituitary-adrenal (HPA) axis. Rather than merely mediating stress responses, the axis emerges an agent of resource allocation that draws a common thread among conditions of gestation, postnatal environments, and functional and health-related outcomes. The preponderance of evolutionary and developmental analysis identifies environments as agents on both sides of the health risk equation, by influencing vulnerabilities and capacities established in early and later life course development, and determining exposures and demands encountered over the life course.

Regulators of neurite outgrowth: role of cell adhesion molecules

Neuronal differentiation is a fundamental event in the development of the nervous system as well as in the regeneration of damaged nervous tissue. The initiation and guidance of a neurite are accomplished by positive (permissive or attractive), negative (inhibitory or repulsive), or guiding (affecting the advance of the growth cone) signals from the extracellular space. The signals may arise from either the extracellular matrix (ECM) or the surface of other cells, or be diffusible secreted factors. Based on this classification, we briefly describe selected positive, negative, and guiding signaling cues focusing on the role of cell adhesion molecules (CAMs). CAMs not only regulate cell-cell and cell-ECM adhesion "mechanically," they also trigger intracellular signaling cascades launching neurite outgrowth. Here, we describe the structure, function, and signaling of three key CAMs found in the nervous system: N-cadherin and two Ig-CAMs, L1 and the neural cell adhesion molecule NCAM.

Neuroendocrine hypothesis of aging: the role of corticoadrenal steroids

The age-related modifications of neuroendocrine secretions normally capable of integrative activity in the whole body might partly promote and amplify many aging phenomena. The quantitative and qualitative changes of hypothalamus-pituitary-adrenal axis are a significant example. The dampening of cortisol and dehydroepiandrosterone (DHEA) circadian fluctuation and the progressive decrease of DHEA/cortisol ratio are at the basis of multiple clinical implications: the shift from anabolic to catabolic status, the activation of atherosclerosis progression, the deterioration of immune competence, the impairment of cognitive and affective performances and the glico- and lipometabolic disorders. The hypothesis of a DHEA supplementation strategy comes out from these premises.

Fetal origins of developmental plasticity: Animal models of induced life history variation

The interaction of the genetic program with the environment shapes the development of an individual. Accumulating data from animal models indicate that prenatal and early-postnatal events (collectively called "early-life events") can initiate long-term changes in the expression of the genetic program which persist, or may only become apparent, much later in the individual's life. Researchers working with humans or animal models of human diseases often view the effects of early-life events through the lens of pathology, with a focus on whether the events increase the risk for a particular disease. Alternatively, comparative biologists often view the effects of early-life events through the lens of evolution and adaptation by natural selection; they investigate the processes by which environmental conditions present early in life may prompt the adoption of different developmental pathways leading to alternative life histories. Examples of both approaches are presented in this article. This article reviews the concepts of phenotypic plasticity, natural selection, and evidence from animal models that early-life events can program the activity of the neuroendocrine system, at times altering life history patterns in an adaptive manner. Data from seasonally breeding rodents are used to illustrate the use of maternally derived information to alter the life history of young. In several species, the maternal system transfers photoperiodic information to the young in utero. This maternally derived information alters the response of young to photoperiods encountered later and life, producing seasonally distinct life histories.

Divergent stress responses and coping styles in psychogenetically selected Roman high-(RHA) and low-(RLA) avoidance rats: behavioural, neuroendocrine and developmental aspects

The Swiss sublines of Roman high-(RHA/Verh) and low-(RLA/Verh) avoidance rats have been genetically selected for good vs. poor performance in two-way active avoidance since 1972. RLA/Verh rats show increased stress responses (e.g. freezing behaviour, ACTH, corticosterone and prolactin secretion) and adopt a more passive (or reactive) coping style when confronted with a novel environment. In the open field, elevated plus-maze, black/white box test, and in a new light/dark open field test, RLA/Verh rats appear to be more anxious than their RHA/Verh counterparts. Anxiety may result from their particular psychophysiological profile, i.e. increased emotionality combined with a passive coping style. In contrast, RHA/Verh rats are less responsive to stress, they show little anxiety in novel situations and tend to be impulsive and novelty (sensation) seekers. Some behavioural differences are already noticeable shortly after birth, but the full pattern appears to stabilize only after puberty. Gene-environment interactions are critical in establishing this pattern. The data reviewed indicate that the differences between RHA/Verh and RLA/Verh rats probably result from a complex interaction among divergent anxiety/emotionality characteristics, differences in locomotor activity and novelty/reward seeking, as well as active vs. passive coping styles. It is proposed further that these divergent personality types are to be found not only in other selective breeding programs but in the form of individual differences in most populations of rats used for this type of research.

Neurosecretory identity conferred by the apterous gene: lateral horn leucokinin neurons in Drosophila

The LIM-HD protein Apterous has been shown to regulate expression of the FMRFamide neuropeptide in Drosophila neurons (Benveniste et al. [1998] Development 125:4757-4765). To test whether Apterous has a broader role in controlling neurosecretory identity, we analyzed the expression of several neuropeptides in apterous (ap) mutants. We show that Apterous is necessary for expression of the Leucokinin neuropeptide in a pair of brain neurons located in the lateral horn region of the protocerebrum (LHLK neurons). ap null mutants are depleted of Leucokinin in these cells, whereas hypomorphic mutants show reduced Leucokinin expression. Other Leucokinin-containing neurons are not affected by mutations in ap gene. Co-expression of apterous and Leucokinin is observed exclusively in the LHLK neurons, from larval stages to adulthood. Rescue assays performed in null ap mutants, by expressing Apterous protein under apGAL4 and elavGAL4 drivers, demonstrate the recovery of Leucokinin in the LHLK neurons. These results reinforce the emerging role of the LIM-HD proteins in determining neuronal identity. They also clarify the neuroendocrine phenotype of apterous mutants.

The role of insulin-like growth factor-I in neuroendocrine function and the consequent effects on sexual maturation: inferences from animal models

It is known that growth hormone (GH) plays an important role in growth and development.Additionally, emerging evidence suggest that it also influences hypothalamic-pituitary-gonadal function. We have found that GH from different species has different effects in mice. In rodents, human GH (hGH) binds to both GH and prolactin (PRL) receptors; it has both somatotrophic and lactotrophic effects. Since PRL has a profound effect on neuroendocrine function, the results obtained from hGH treatment or from transgenic animals expressing the hGH gene reflect PRL-like effects of this hormone. However, bovine GH (bGH) is purely somatogenic and therefore the effects of bGH represent the function of the natural GH produced in rodents. Furthermore, our studies in mice and rats have shown that not all effects of GH are stimulatory and the duration of exposure of the hypothalamo-hypophyseal-gonadal system to GH might influence the secretions of gonadotropins and gonadal steroids. In humans, excess productions of GH in acromegaly and GH resistance in Laron syndrome adversely affect reproduction. Similarly, it has been demonstrated that in transgenic mice expressing various GH genes, in insulin-like growth factor-I (IGF-I) gene-knockout mice, in GH receptor gene-disrupted (GHR-KO) mice, and in Ames dwarf mice the onset of puberty and/or fertility is altered. Therefore, excess or subnormal secretion of GH can affect reproduction. We have shown that the hypothalamic-pituitary functions are affected in transgenic mice expressing the GH genes, Ames dwarf mice and in GH receptor gene knockout mice. The majority of the GH effects are mediated via IGF-I and the aforementioned effects may be due to the GH-induced IGF-I secretion or due to the absence of this peptide production. It is important to realize that the syntheses and actions of IGF binding proteins are controlled by IGF-I. Furthermore, some IGF binding proteins can inhibit IGF-I action. Therefore, the concentrations of IGF binding proteins and the ratio of these binding proteins and IGF-I within the body might play a pivotal role in modulating IGF-I effects on the neuroendocrine-gonadal system.