Involvement of transforming growth factor alpha in the photoperiodic regulation of reproduction in birds

Photoperiodic physiology of summer breeding birds and a search for the role of eye

Photoperiod regulation of gonadal cycles is well studied and documented in both birds and mammals. Change in photoperiod is considered as the most effective and important cue to time the initiation of the annual physiological cycles in birds. Approaching of long days (as observed in summer months), signal long-day breeding birds to initiation reproduction and other related functions. Birds and other non-mammalian vertebrates use the extraocular photoreceptors which may be present in the mediobasal hypothalamus (MBH) or associated regions to measure the photoperiodic time and so are different from mammals where only the eyes are lone photoreceptive organs. The downstream signaling involves thyroid responsive genes playing a crucial role in mediating photoperiodic signals in both birds and mammals. Role of eyes in the avian seasonal cycle has been a questionable issue with evidences both favoring and negating any role. We propose that morphological as well as physiological data argue that retinal photoreceptors can participate in gonadal cycle, at least in the quail and duck. The present review details the studies of photoneuroendocrine control of gonadal axis in birds and review evidences to decipher the role eyes in photoperiodic mediated physiologies in birds.

The polygamous GnRH neuron: Astrocytic and tanycytic communication with a neuroendocrine neuronal population

To ensure the survival of the species, hypothalamic neuroendocrine circuits controlling fertility, which converge onto neurons producing gonadotropin-releasing hormone (GnRH), must respond to fluctuating physiological conditions by undergoing rapid and reversible structural and functional changes. However, GnRH neurons do not act alone, but through reciprocal interactions with multiple hypothalamic cell populations, including several glial and endothelial cell types. For instance, it has long been known that in the hypothalamic median eminence, where GnRH axons terminate and release their neurohormone into the pituitary portal blood circulation, morphological plasticity displayed by distal processes of tanycytes modifies their relationship with adjacent neurons as well as the spatial properties of the neurohemal junction. These alterations not only regulate the capacity of GnRH neurons to release their neurohormone, but also the activation of discrete non-neuronal pathways that mediate feedback by peripheral hormones onto the hypothalamus. Additionally, a recent breakthrough has demonstrated that GnRH neurons themselves orchestrate the establishment of their neuroendocrine circuitry during postnatal development by recruiting an entourage of newborn astrocytes that escort them into adulthood and, via signalling through gliotransmitters such as prostaglandin E2, modulate their activity and GnRH release. Intriguingly, several environmental and behavioural toxins perturb these neuron-glia interactions and consequently, reproductive maturation and fertility. Deciphering the communication between GnRH neurons and other neural cell types constituting hypothalamic neuroendocrine circuits is thus critical both to understanding physiological processes such as puberty, oestrous cyclicity and aging, and to developing novel therapeutic strategies for dysfunctions of these processes, including the effects of endocrine disruptors.

The Versatile Tanycyte: A Hypothalamic Integrator of Reproduction and Energy Metabolism

The fertility and survival of an individual rely on the ability of the periphery to promptly, effectively, and reproducibly communicate with brain neural networks that control reproduction, food intake, and energy homeostasis. Tanycytes, a specialized glial cell type lining the wall of the third ventricle in the median eminence of the hypothalamus, appear to act as the linchpin of these processes by dynamically controlling the secretion of neuropeptides into the portal vasculature by hypothalamic neurons and regulating blood-brain and blood-cerebrospinal fluid exchanges, both processes that depend on the ability of these cells to adapt their morphology to the physiological state of the individual. In addition to their barrier properties, tanycytes possess the ability to sense blood glucose levels, and play a fundamental and active role in shuttling circulating metabolic signals to hypothalamic neurons that control food intake. Moreover, accumulating data suggest that, in keeping with their putative descent from radial glial cells, tanycytes are endowed with neural stem cell properties and may respond to dietary or reproductive cues by modulating hypothalamic neurogenesis. Tanycytes could thus constitute the missing link in the loop connecting behavior, hormonal changes, signal transduction, central neuronal activation and, finally, behavior again. In this article, we will examine these recent advances in the understanding of tanycytic plasticity and function in the hypothalamus and the underlying molecular mechanisms. We will also discuss the putative involvement and therapeutic potential of hypothalamic tanycytes in metabolic and fertility disorders.

The special relationship: glia-neuron interactions in the neuroendocrine hypothalamus

Natural fluctuations in physiological conditions require adaptive responses involving rapid and reversible structural and functional changes in the hypothalamic neuroendocrine circuits that control homeostasis. Here, we discuss the data that implicate hypothalamic glia in the control of hypothalamic neuroendocrine circuits, specifically neuron-glia interactions in the regulation of neurosecretion as well as neuronal excitability. Mechanistically, the morphological plasticity displayed by distal processes of astrocytes, pituicytes and tanycytes modifies the geometry and diffusion properties of the extracellular space. These changes alter the relationship between glial cells of the hypothalamus and adjacent neuronal elements, especially at specialized intersections such as synapses and neurohaemal junctions. The structural alterations in turn lead to functional plasticity that alters the release and spread of neurotransmitters, neuromodulators and gliotransmitters, as well as the activity of discrete glial signalling pathways that mediate feedback by peripheral signals to the hypothalamus. An understanding of the contributions of these and other non-neuronal cell types to hypothalamic neuroendocrine function is thus critical both to understand physiological processes such as puberty, the maintenance of bodily homeostasis and ageing and to develop novel therapeutic strategies for dysfunctions of these processes, such as infertility and metabolic disorders.

Advances in Understanding of Structural Reorganization in the Hypothalamic Neurosecretory System

The hypothalamic neurosecretory system synthesizes neuropeptides in hypothalamic nuclei and releases them from axonal terminals into the circulation in the neurohypophysis (NH) and median eminence (ME). This system plays a crucial role in regulating body fluid homeostasis and social behaviors as well as reproduction, growth, metabolism, and stress responses, and activity-dependent structural reorganization has been reported. Current knowledge on dynamic structural reorganization in the NH and ME, in which the axonal terminals of neurosecretory neurons directly contact the basement membrane (BM) of a fenestrated vasculature, is discussed herein. Glial cells, pituicytes in the NH and tanycytes in the ME, engulf axonal terminals and interpose their cellular processes between axonal terminals and the BM when hormonal demands are low. Increasing demands for neurosecretion result in the retraction of the cellular processes of glial cells from axonal terminals and the BM, permitting increased neurovascular contact. The shape conversion of pituicytes and tanycytes is mediated by neurotransmitters and sex steroid hormones, respectively. The NH and ME have a rough vascular BM profile of wide perivascular spaces and specialized extension structures called "perivascular protrusions." Perivascular protrusions, the insides of which are occupied by the cellular processes of vascular mural cells pericytes, contribute to increasing neurovascular contact and, thus, the efficient diffusion of hypothalamic neuropeptides. A chronic physiological stimulation has been shown to increase perivascular protrusions via the shape conversion of pericytes and the profile of the vascular surface. Continuous angiogenesis occurs in the NH and ME of healthy normal adult rodents depending on the signaling of vascular endothelial growth factor (VEGF). The inhibition of VEGF signaling suppresses the proliferation of endothelial cells (ECs) and promotes their apoptosis, which results in decreases in the population of ECs and axonal terminals. Pituicytes and tanycytes are continuously replaced by the proliferation and differentiation of stem/progenitor cells, which may be regulated by matching those of ECs and axonal terminals. In conclusion, structural reorganization in the NH and ME is caused by the activity-dependent shape conversion of glial cells and vascular mural cells as well as the proliferation of endothelial and glial cells by angiogenesis and gliogenesis, respectively.

Gonadotrophin-releasing hormone nerve terminals, tanycytes and neurohaemal junction remodelling in the adult median eminence: functional consequences for reproduction and dynamic role of vascular endothelial cells

Although coordinated actions of several areas within the hypothalamus are involved in the secretion of gonadotrophin-releasing hormone (GnRH), the median eminence of the hypothalamus, where the nerve terminals are located, plays a particularly critical role in the release of GnRH. In adult females, prior to the preovulatory surge of GnRH, the retraction of specialised ependymoglial cells lining the floor of the third ventricle named tanycytes allows for the juxtaposition of GnRH nerve terminals with the adjacent pericapillary space of the pituitary portal vasculature, thus forming direct neurohaemal junctions. These morphological changes occur within a few hours and are reversible. Such remodelling may promote physiological conditions to enhance the central release of GnRH and potentiate oestrogen-activated GnRH release. This plasticity involves dynamic cell interactions that bring into play tanycytes, astrocytes, vascular endothelial cells and GnRH neurones themselves. The underlying signalling pathways responsible for these structural changes are comprised of highly diffusible gaseous molecules, such as endothelial nitric oxide, and paracrine communication processes involving receptors of the erbB tyrosine kinase family, transforming growth factor beta 1 and eicosanoids, such as prostaglandin E(2). Some of these molecules, as a result of their ability to diffuse within the median eminence, may also serve as synchronizing cues allowing for the occurrence of functionally meaningful episodes of GnRH secretion by coordinating GnRH release from the GnRH neuroendocrine terminals.

Function-related structural plasticity of the GnRH system: a role for neuronal-glial-endothelial interactions

As the final common pathway for the central control of gonadotropin secretion, GnRH neurons are subjected to numerous regulatory homeostatic and external factors to achieve levels of fertility appropriate to the organism. The GnRH system thus provides an excellent model in which to investigate the complex relationships between neurosecretion, morphological plasticity and the expression of a physiological function. Throughout the reproductive cycle beginning from postnatal sexual development and the onset of puberty to reproductive senescence, and even within the ovarian cycle itself, all levels of the GnRH system undergo morphological plasticity. This structural plasticity within the GnRH system appears crucial to the timely control of reproductive competence within the individual, and as such must have coordinated actions of multiple signals secreted from glial cells, endothelial cells, and GnRH neurons. Thus, the GnRH system must be viewed as a complete neuro-glial-vascular unit that works in concert to maintain the reproductive axis.

Red jungle fowl (Gallus gallus) as a model for studying the molecular mechanism of seasonal reproduction

Photoperiodism is an adaptation mechanism that enables animals to predict seasonal changes in the environment. Japanese quail is the best model organism for studying photoperiodism. Although the recent availability of chicken genome sequences has permitted the expansion from single gene to genome-wide transcriptional analysis in this organism, the photoperiodic response of the domestic chicken is less robust than that of the quail. Therefore, in the present study, we examined the photoperiodic response of the red jungle fowl (Gallus gallus), a predecessor of the domestic chicken, to test whether this animal could be developed as an ideal model for studying the molecular mechanisms of seasonal reproduction. When red jungle fowls were transferred from short-day- to long-day conditions, gonadal development and an increase in plasma LH concentration were observed. Furthermore, rapid induction of thyrotropin beta subunit, a master regulator of photoperiodism, was observed at 16 h after dawn on the first long day. In addition, the long-day condition induced the expression of type 2 deiodinase, the key output gene of photoperiodism. These results were consistent with the results obtained in quail and suggest that the red jungle fowl could be an ideal model animal for the genome-wide transcriptional analysis of photoperiodism.

Photostimulated expression of type 2 iodothyronine deiodinase mRNA is greatly attenuated in the rostral tuberal hypothalamus of the photorefractory turkey hen

For many temperate-zone avian species, termination of breeding occurs when individuals no longer respond to previously stimulatory day lengths, a condition called photorefractoriness. Long day lengths induce significantly greater expression of c-fos and fos-related antigens (FRAs) in the tuberal hypothalamus of the photosensitive hen than that of the photorefractory hen. The tuber is also a site of photoinducible glial expression of type 2 iodothyronine deiodinase (Dio2), which converts thyroxine into its active form, triiodothyronine (T3). T3 induces withdrawal of glial processes from gonadotrophin-releasing hormone (GnRH) I nerve terminals, which is believed to permit the efficient release of GnRH I into the associated portal vasculature. Using a riboprobe, we tested whether long days induce Dio2 mRNA expression in the turkey tuber and, if so, whether this expression is reduced in photorefractory hens. Long days significantly induced rostral and caudal tuberal hypothalamic Dio2 expression in photosensitive hens. Photorefractory hens had reduced expression of Dio2 with most subjects expressing no detectable mRNA in the rostral tuber and variably attenuated amounts throughout the medial and caudal tuber. We also performed double-label immunohistochemistry to identify co-localisation between FRAs and glial fibrillary acidic protein, a glial marker. FRAs were present in the nuclei of a few astrocytes in the median eminence and infundibular nucleus of the tuber. The temporal and spatial coincidence between FRA and Dio2 expression, their mutual association with glia, and the attenuation of their response during photorefractoriness suggests that the two events are linked and that photorefractoriness involves a reduced capacity for photo-inducible gene expression within glia of the tuberal hypothalamus.

Control of the annual cycle in birds: endocrine constraints and plasticity in response to ecological variability

This paper reviews information from ecological and physiological studies to assess how extrinsic factors can modulate intrinsic physiological processes. The annual cycle of birds is made up of a sequence of life-history stages: breeding, moult and migration. Each stage has evolved to occur at the optimum time and to last for the whole duration of time available. Some species have predictable breeding seasons, others are more flexible and some breed opportunistically in response to unpredictable food availability. Photoperiod is the principal environmental cue used to time each stage, allowing birds to adapt their physiology in advance of predictable environmental changes. Physiological (neuroendocrine and endocrine) plasticity allows non-photoperiodic cues to modulate timing to enable individuals to cope with, and benefit from, short-term environmental variability. Although the timing and duration of the period of full gonadal maturation is principally controlled by photoperiod, non-photoperiodic cues, such as temperature, rainfall or food availability, could potentially modulate the exact time of breeding either by fine-tuning the time of egg-laying within the period of full gonadal maturity or, more fundamentally, by modulating gonadal maturation and/or regression. The timing of gonadal regression affects the time of the start of moult, which in turn may affect the duration of the moult. There are many areas of uncertainty. Future integrated studies are required to assess the scope for flexibility in life-history strategies as this will have a critical bearing on whether birds can adapt sufficiently rapidly to anthropogenic environmental changes, in particular climate change.

Thyrotrophin in the pars tuberalis triggers photoperiodic response

Molecular mechanisms regulating animal seasonal breeding in response to changing photoperiod are not well understood. Rapid induction of gene expression of thyroid-hormone-activating enzyme (type 2 deiodinase, DIO2) in the mediobasal hypothalamus (MBH) of the Japanese quail (Coturnix japonica) is the earliest event yet recorded in the photoperiodic signal transduction pathway. Here we show cascades of gene expression in the quail MBH associated with the initiation of photoinduced secretion of luteinizing hormone. We identified two waves of gene expression. The first was initiated about 14 h after dawn of the first long day and included increased thyrotrophin (TSH) beta-subunit expression in the pars tuberalis; the second occurred approximately 4 h later and included increased expression of DIO2. Intracerebroventricular (ICV) administration of TSH to short-day quail stimulated gonadal growth and expression of DIO2 which was shown to be mediated through a TSH receptor-cyclic AMP (cAMP) signalling pathway. Increased TSH in the pars tuberalis therefore seems to trigger long-day photoinduced seasonal breeding.

Tracking the seasons: the internal calendars of vertebrates

Animals have evolved many season-specific behavioural and physiological adaptations that allow them to both cope with and exploit the cyclic annual environment. Two classes of endogenous annual timekeeping mechanisms enable animals to track, anticipate and prepare for the seasons: a timer that measures an interval of several months and a clock that oscillates with a period of approximately a year. Here, we discuss the basic properties and biological substrates of these timekeeping mechanisms, as well as their reliance on, and encoding of environmental cues to accurately time seasonal events. While the separate classification of interval timers and circannual clocks has elucidated important differences in their underlying properties, comparative physiological investigations, especially those regarding seasonal prolactin secretions, hint at the possibility of common substrates.