Possible homologies between photorefractoriness in sheep and birds: the effect of thyroidectomy on the length of the ewe's breeding season

Hypophysial angiogenesis decodes annual time and underlies physiological adaptation to seasonal changes in the environment

Adaptation to annual changes in the environment is controlled by hypophysial hormones. In temperate zones, photoperiod is the primary external cue that regulates annual biological cycles and is translated by the pattern of melatonin secretion acting primarily in the hypophysial pars tuberalis. Angiogenic mechanisms within this tissue contribute to decode the melatonin signal through alternative splicing of the vascular endothelial growth factor A (VEGF-A) gene in both the pars tuberalis and the capillary loops of the infundibulum. The resulting melatonin-evoked differential productions of VEGF-A isoforms will induce seasonal remodeling of the vascular connection between the hypothalamus and hypophysis, and act as paracrine messengers in the pars distalis to generate the required seasonal endocrine response. Specifically, the long melatonin signal in winter upregulates antiangiogenic VEGF-A isoforms, which will reduce the number of vascular loops and the density of VEGF receptors in endocrine and folliculo-stellate (FS) cells, inhibit prolactin secretion, and stimulate FSH. In contrast, the short melatonin signal in summer upregulates proangiogenic VEGF-A isoforms that will increase the number of vascular loops and the density of VEGF receptors in endocrine and FS cells, stimulate prolactin secretion, and suppress FSH. A similar system has been identified in long day seasonal breeders, revealing that this is a conserved mechanism of adaptation across species. Thus, an angiogenesis-based, intrahypophysial system for annual time measurement controls local microvascular plasticity and conveys the photoperiodic signal readout from the melatonin sensitive pars tuberalis to the endocrine cells of the pars distalis to regulate seasonal adaptation to the environment.

Seasonal variations on semen quality attributes in turkey and egg type chicken male breeders

A biological experiment was carried out in twenty-four adult healthy breeder males each in turkey (Beltsville small white) and egg type chicken (White Leghorn Babcock) in order to assess the seasonal influence on semen production and quality. The birds were maintained in individual cages under uniform husbandry conditions throughout the year. The birds were fed with breeder ration and water ad libitum was offered with a constant photoperiod of 14 h/day. Physical and biochemical characteristics of semen, serum hormones (testosterone and thyroxine), and antioxidant activity (catalase and lipid peroxidation) were evaluated throughout the year (January-December). Based on the THI calculations, the observations were classified under three different seasons, namely, winter (November-February), spring (March, April, and October), and summer (May-September). Semen physical parameters, sperm concentration, motility, live sperm percentage, and sperm plasma membrane integrity were superior during the winter season. In seminal plasma, biochemical parameters (phosphorus, ALT, ALP, AST, and uric acid) had a significant (P < 0.05) difference between seasons. There was a significant difference (P < 0.05) among serum hormones (testosterone and thyroxine) that were higher during the winter season. Significant variation was observed in catalase and lipid peroxidation antioxidant enzyme activities (seminal and blood plasma) in winter than in the other two seasons. Both the turkey and egg type chicken breeders exhibited superior seminal characteristics, sex hormone profile, and antioxidant enzyme activity during winter seasons.

GnRH and the photoperiodic control of seasonal reproduction: Delegating the task to kisspeptin and RFRP-3

Synchronization of mammalian breeding activity to the annual change of photoperiod and environmental conditions is of the utmost importance for individual survival and species perpetuation. Subsequent to the early 1960s, when the central role of melatonin in this adaptive process was demonstrated, our comprehension of the mechanisms through which light regulates gonadal activity has increased considerably. The current model for the photoperiodic neuroendocrine system points to pivotal roles for the melatonin-sensitive pars tuberalis (PT) and its seasonally-regulated production of thyroid-stimulating hormone (TSH), as well as for TSH-sensitive hypothalamic tanycytes, radial glia-like cells located in the basal part of the third ventricle. Tanycytes respond to TSH through increased expression of thyroid hormone (TH) deiodinase 2 (Dio2), which leads to heightened production of intrahypothalamic triiodothyronine (T3) during longer days of spring and summer. There is strong evidence that this local, long-day driven, increase in T3 links melatonin input at the PT to gonadotropin-releasing hormone (GnRH) output, to align breeding with the seasons. The mechanism(s) through which T3 impinges upon GnRH remain(s) unclear. However, two distinct neuronal populations of the medio-basal hypothalamus, which express the (Arg)(Phe)-amide peptides kisspeptin and RFamide-related peptide-3, appear to be well-positioned to relay this seasonal T3 message towards GnRH neurons. Here, we summarize our current understanding of the cellular, molecular and neuroendocrine players, which keep track of photoperiod and ultimately govern GnRH output and seasonal breeding.

Transcriptome Analysis Revealed Long Non-Coding RNAs Associated with mRNAs in Sheep Thyroid Gland under Different Photoperiods

The thyroid gland is a vital endocrine organ involved in the reproduction of animals via the regulation of hormone synthesis and secretion. LncRNAs have been proven to play important roles in reproductive regulation; however, the associated mechanism in the thyroid gland has not been clarified. In this study, we investigated to identify photoperiod-induced lncRNAs and mRNAs in the thyroid gland in Sunite ewes by comparing the expression profiles of short photoperiod (SP) and long photoperiods (LP). A total of 41,088 lncRNAs were identified in the thyroid gland through RNA-Seq. Functional analysis of differentially expressed lncRNAs using the R package revealed that reproductive hormone- and photoperiod response-related pathways, including the prolactin signaling, cAMP signaling, and circadian rhythm pathways, were significantly enriched. An mRNA-lncRNA interaction analysis suggested that the lncRNA LOC1056153S88 trans targets ARG2 and CCNB3, and the lncRNA LOC105607004 trans targets DMXL2, both of these might be involved in seasonal sheep breeding reproduction. Together, these results will provide resources for further studies on seasonal reproduction in sheep.

Expression of Aquaglyceroporins in Spermatozoa from Wild Ruminants Is Influenced by Photoperiod and Thyroxine Concentrations

This work identified the presence of AQPs in frozen-thawed sperm of wild ruminants and assessed the influence of the interaction between photoperiod and thyroxine on AQP expression, and on testosterone secretion. Thyroxine and melatonin were administered to ibexes. In a second experiment, performed in mouflons, circulating thyroxine was reduced via treatment with propylthiouracil (PTU), and an artificial long day (LD) photoperiod established. In the ibexes, the melatonin treatment increased the blood plasma testosterone concentration, reduced the cryoresistance ratio (CR) for sperm viability and the presence of an intact acrosome, and increased the percentage of sperm with AQP7 in the acrosome and of AQP3 and AQP10 in the midpiece. In the mouflons, neither the PTU treatment, the LD, nor the combination of both affected the CR of any sperm variable. The percentage of sperm with AQP3 increased in the post-acrosome region but decreased in the tail in the LD+PTU group. The percentage of sperm with AQP10 in the principal piece and endpiece was lower in the PTU+LD group than in the control and LD groups. The influence of photoperiod/melatonin on AQP expression might be indirectly exerted through changes in the testosterone concentration, and thus ultimately affect sperm cryoresistance.

Seasonal differences in hypothalamic thyroid-stimulating hormone β, gonadotropin-releasing hormone-I and deiodinase expression between migrant and resident subspecies of white-crowned sparrow (Zonotrichia leucophrys)

Across taxa, the seasonal transition between non-breeding and breeding states is controlled by localised thyroid hormone signalling in the deep brain via reciprocal switching of deiodinase enzyme expression from type 3 (DIO3) to type 2 (DIO2). This reciprocal switch is considered to be mediated by increasing thyroid-stimulating hormone β (TSHβ) release from the pars tuberalis, which occurs in response to a change in photoperiod. Although well characterised in a handful of model organisms in controlled laboratory settings, this pathway remains largely unexplored in free-living animals under natural environmental conditions. In this comparative gene expression study, we investigated hypothalamic thyroid hormone signalling in two seasonally breeding subspecies of white-crowned sparrow (Zonotrichia leucophrys), across the entirety of their annual cycles. The migratory Gambel's (Z. l. gambelii) and resident Nuttall's (Z. l. nuttalii) subspecies differ with respect to timing of reproduction, as well as life history stage and migratory strategies. Although DIO3 mRNA expression was elevated and DIO2 mRNA expression was reduced in the wintering period in both subspecies, DIO2 peaked in both subspecies prior to the onset of reproduction. However, there was differential timing between subspecies in peak DIO2 expression. Intriguingly, seasonal modulation of TSHβ mRNA was only observed in migrants, where expression was elevated at the start of breeding, consistent with observations from other highly photoperiodic species. There was no correlation between TSHβ, DIO2 and gonadotropin-releasing hormone-I mRNA or reproductive metrics in residents. Based on these observed differences, we discuss potential implications for our understanding of how changes in medial basal hypothalamic gene expression mediates initiation of seasonal reproduction.

Thyroid hormone regulation of adult neural stem cell fate: A comparative analysis between rodents and primates

Thyroid hormone (TH) signaling, a highly conserved pathway across vertebrates, is crucial for brain development and function throughout life. In the adult mammalian brain, including that of humans, multipotent neural stem cells (NSCs) proliferate and generate neuronal and glial progenitors. The role of TH has been intensively investigated in the two main neurogenic niches of the adult mouse brain, the subventricular and the subgranular zone. A key finding is that T3, the biologically active form of THs, promotes NSC commitment toward a neuronal fate. In this review, we first discuss the roles of THs in the regulation of adult rodent neurogenesis, as well as how it relates to functional behavior, notably olfaction and cognition. Most research uncovering these roles of TH in adult neurogenesis was conducted in rodents, whose genetic background, brain structure and rate of neurogenesis are considerably different from that of humans. To bridge the phylogenetic gap, we also explore the similarities and divergences of TH-dependent adult neurogenesis in non-human primate models. Lastly, we examine how photoperiodic length changes TH homeostasis, and how that might affect adult neurogenesis in seasonal species to increase fitness. Several aspects by which TH acts on adult NSCs seem to be conserved among mammals, while we only start to uncover the molecular pathways, as well as how other in- and extrinsic factors are intertwined. A multispecies approach delivering more insights in the matter will pave the way for novel NSC-based therapies to combat neurological disorders.

Thyroid hormone and hypothalamic stem cells in seasonal functions

Seasonal rhythms are a pervasive feature of most living organisms, which underlie yearly timeliness in breeding, migration, hibernation or weight gain and loss. To achieve this, organisms have developed inner timing devices (circannual clocks) that endow them with the ability to predict then anticipate changes to come, usually using daylength as the proximate cue. In Vertebrates, daylength interpretation involves photoperiodic control of TSH production by the pars tuberalis (PT) of the pituitary, which governs a seasonal switch in thyroid hormone (TH) availability in the neighboring hypothalamus. Tanycytes, specialized glial cells lining the third ventricle (3V), are responsible for this TH output through the opposite, PT-TSH-driven, seasonal control of deiodinases 2/3 (Dio 2/3). Tanycytes comprise a photoperiod-sensitive stem cell niche and TH is known to play major roles in cell proliferation and differentiation, which suggests that seasonal control of tanycyte proliferation may be involved in the photoperiodic synchronization of seasonal rhythms. Here we review our current knowledge of the molecular and neuroendocrine pathway linking photoperiodic information to seasonal changes in physiological functions and discuss the potential implication of tanycytes, TH and cell proliferation in seasonal timing.

Molecular economy of nature with two thyrotropins from different parts of the pituitary: pars tuberalis thyroid-stimulating hormone and pars distalis thyroid-stimulating hormone

Thyrotropin (TSH) is classically known to be regulated by negative feedback from thyroid hormones and stimulated by thyrotropin-releasing hormone (TRH) from the hypothalamus. At the end of the 1990s, studies showed that thyrotroph cells from the pars tuberalis (PT) did not have TRH receptors and their TSH regulation was independent from TRH stimulation. Instead, PT-thyrotroph cells were shown to have melatonin-1 (MT-1) receptors and melatonin secretion from the pineal gland stimulates TSH-β subunit formation in PT. Electron microscopy examinations also revealed some important differences between PT and pars distalis (PD) thyrotrophs. PT-TSH also have low bioactivity in the peripheral circulation. Studies showed that they have different glycosylations and PT-TSH forms macro-TSH complexes in the periphery and has a longer half-life. Photoperiodism affects LH levels in animals via decreased melatonin causing increased TSH-β subunit expression and induction of deiodinase-2 (DIO-2) in the brain. Mammals need a light stimulus carried into the suprachiasmatic nucleus (which is a circadian clock) and then transferred to the pineal gland to synthesize melatonin, but birds have deep brain receptors and they are stimulated directly by light stimuli to have increased PT-TSH, without the need for melatonin. Photoperiodic regulations via TSH and DIO 2/3 also have a role in appetite, seasonal immune regulation, food intake and nest-making behaviour in animals. Since humans have no clear seasonal breeding period, such studies as recent ''domestication locus'' studies in poultry are interesting. PT-TSH that works like a neurotransmitter in the brain may become an important target for future studies about humans.

Discontinuity in the molecular neuroendocrine response to increasing daylengths in Ile-de-France ewes: Is transient Dio2 induction a key feature of circannual timing?

In mammals, melatonin is responsible for the synchronisation of seasonal cycles to the solar year. Melatonin is secreted by the pineal gland with a profile reflecting the duration of the night and acts via the pituitary pars tuberalis (PT), which in turn modulates hypothalamic thyroid hormone status via seasonal changes in the production of locally-acting thyrotrophin. Recently, we demonstrated that, in the Soay sheep, photoperiodic induction of Tshb expression and consequent downstream hypothalamic changes occur over a narrow range of photoperiods between 12 and 14 hours in duration. In the present study, we aimed to extend our molecular characterisation of this pathway, based on transcriptomic analysis of photoperiodic changes in the pituitary and hypothalamus of ovariectomised, oestradiol-implanted Ile-de-France ewes. We demonstrate that photoperiodic treatments applied before the winter solstice elicit two distinctive modes of accelerated reproductive switch off compared to ewes held on a simulated natural photoperiod, with shut-down occurring markedly faster on photoperiods of 13 hours or more than on photoperiods of 12 hours and less. This pattern of response was reflected in gene expression profiles of photoperiodically sensitive markers, both in the PT (Tshb, Fam150b, Vmo1, Ezh2 and Suv39H2) and in tanycytes (Tmem252 and Dct). Unexpectedly, the expression of Dio2 in tanycytes did not show any noticeable increase in expression with lengthening photoperiods. Finally, the expression of Kiss1, the key activator of gonadotrophin-releasing hormone release, was proportionately decreased by lengthening photoperiods, in a pattern that correlated strongly with gonadotrophin suppression. These data show that stepwise increases in photoperiod lead to graded molecular responses at the level of the PT, a progressive suppression of Kiss1 in the hypothalamic arcuate nucleus and luteinising hormone/follicle-stimulating hormone release by the pituitary, despite apparently unchanged Dio2 expression in tanycytes. We hypothesise that this apparent discontinuity in the seasonal neuroendocrine response illustrates the transient nature of the thyroid hormone-mediated response to long days in the control of circannual timing.

Maternal photoperiodic programming enlightens the internal regulation of thyroid-hormone deiodinases in tanycytes

Seasonal rhythms in physiology are widespread among mammals living in temperate zones. These rhythms rely on the external photoperiodic signal being entrained to the seasons, although they persist under constant conditions, revealing their endogenous origin. Internal long-term timing (circannual cycles) can be revealed in the laboratory as photoperiodic history-dependent responses, comprising the ability to respond differently to similar photoperiodic cues based on prior photoperiodic experience. In juveniles, history-dependence relies on the photoperiod transmitted by the mother to the fetus in utero, a phenomenon known as "maternal photoperiodic programming" (MPP). The response to photoperiod in mammals involves the nocturnal pineal hormone melatonin, which regulates a neuroendocrine network including thyrotrophin in the pars tuberalis and deiodinases in tanycytes, resulting in changes in thyroid hormone in the mediobasal hypothalamus. This review addresses MPP and discusses the latest findings on its impact on the thyrotrophin/deiodinase network. Finally, commonalities between MPP and other instances of endogenous seasonal timing are considered, and a unifying scheme is suggested in which timing arises from a long-term communication between the pars tuberalis and the hypothalamus and resultant spontaneous changes in local thyroid hormone status, independently of the pineal melatonin signal.

Cloning and expression characterization in hypothalamic Dio2/3 under a natural photoperiod in the domesticated Brandt's vole (Lasiopodomys brandtii)

The Dio2/3 gene is related to the photoperiodic response in mammals and plays an important role in the development of gonadal organs and seasonal breeding. Our previous studies have reported synchronous variations in the gonadal mass and photoperiodical transition around the summer solstice in a wild Brandt's vole population, a species with striking seasonal breeding. To investigate the role of the Dio2/3 gene in the control of seasonal breeding in this species, we cloned and characterized its expression levels by high-throughput Real-Time PCR during the period around the summer solstice. We selected a domesticated strain to ensure similar development of samples. The synchronous variation pattern between the Dio2/3 expression levels and gonadal mass around the summer solstice supports the prediction that the Dio2/3 gene plays an important role in the seasonal transition in this species. We suggest that the observed photoperiod response may be triggered by differences in the day length rather than the absolute daylength in this species. However, the similar Dio2/3 gene expression patterns but inconsistent gonadal mass patterns between the domesticated strain and the wild strain in the samples collected on Sep 8th, an absolute nonbreeding stage in the wild, lead us to speculate that the core function of the Dio2/3 gene should be restricted in response to the photoperiod rather than factors directly regulating gonadal development, and this laboratory strain could be used as an animal model to test the mechanism of environmental adaptation.

The impact of thyroid hormone in seasonal breeding has a restricted transcriptional signature

Thyroid hormone (TH) directs seasonal breeding through reciprocal regulation of TH deiodinase (Dio2/Dio3) gene expression in tanycytes in the ependymal zone of the medio-basal hypothalamus (MBH). Thyrotropin secretion by the pars tuberalis (PT) is a major photoperiod-dependent upstream regulator of Dio2/Dio3 gene expression. Long days enhance thyrotropin production, which increases Dio2 expression and suppresses Dio3 expression, thereby heightening TH signaling in the MBH. Short days appear to exert the converse effect. Here, we combined endocrine profiling and transcriptomics to understand how photoperiod and TH control the ovine reproductive status through effects on hypothalamic function. Almost 3000 genes showed altered hypothalamic expression between the breeding- and non-breeding seasons, showing gene ontology enrichment for cell signaling, epigenetics and neural plasticity. In contrast, acute switching from a short (SP) to a long photoperiod (LP) affected the expression of a much smaller core of 134 LP-responsive genes, including a canonical group previously linked to photoperiodic synchronization. Reproductive switch-off at the end of the winter breeding season was completely blocked by thyroidectomy (THX), despite a very modest effect on the hypothalamic transcriptome. Only 49 genes displayed altered expression between intact and THX ewes, including less than 10% of the LP-induced gene set. Neuroanatomical mapping showed that many LP-induced genes were expressed in the PT, independently of the TH status. In contrast, TH-sensitive seasonal genes were principally expressed in the ependymal zone. These data highlight the distinctions between seasonal remodeling effects, which appear to be largely independent of TH, and TH-dependent localised effects which are permissive for transition to the non-breeding state.

The pars tuberalis: The site of the circannual clock in mammals?

Accurate timing and physiological adaptation to anticipate seasonal changes are an essential requirement for an organism's survival. In contrast to all other environmental cues, photoperiod offers a highly predictive signal that can be reliably used to activate a seasonal adaptive programme at the correct time of year. Coupled to photoperiod sensing, it is apparent that many organisms have evolved innate long-term timekeeping systems, allowing reliable anticipation of forthcoming environmental changes. The fundamental biological processes giving rise to innate long-term timing, with which the photoperiod-sensing pathway engages, are not known for any organism. There is growing evidence that the pars tuberalis (PT) of the pituitary, which acts as a primary transducer of photoperiodic input, may be the site of the innate long-term timer or "circannual clock". Current research has led to the proposition that the PT-specific thyrotroph may act as a seasonal calendar cell, driving both hypothalamic and pituitary endocrine circuits. Based on this research we propose that the mechanistic basis for the circannual rhythm appears to be deeply conserved, driven by a binary switching cell based accumulator, analogous to that proposed for development. We review the apparent conservation of function and pathways to suggest that these broad principles may apply across the vertebrate lineage and even share characteristics with processes driving seasonal adaptation in plants.

The hypothalamic-pituitary-thyroid axis and biological rhythms: The discovery of TSH's unexpected role using animal models

Thyroid hormones (TH) are important for development, growth, and metabolism. It is also clear that the synthesis and secretion of TH are regulated by the hypothalamic-pituitary-thyroid (HPT) axis. Animal models have helped advance our understanding of the roles and regulatory mechanisms of TH. The animals' bodies develop through coordinated timing of cell division and differentiation. Studies of frog metamorphosis led to the discovery of TH and their role in development. However, to adapt to rhythmic environmental changes, animals also developed various endocrine rhythms. Studies of rodents clarified the neural and molecular mechanisms underlying the circadian regulation of the HPT axis. Moreover, birds have a sophisticated seasonal adaptation mechanism, and recent studies of quail revealed unexpected roles for thyroid-stimulating hormone (TSH) and TH in the seasonal regulation of reproduction. Interestingly, this mechanism is conserved in mammals. Thus, we review how animal studies have shaped our general understanding of the HPT axis in relation to biological rhythms.

60 YEARS OF NEUROENDOCRINOLOGY: Regulation of mammalian neuroendocrine physiology and rhythms by melatonin

The isolation of melatonin was first reported in 1958. Since the demonstration that pineal melatonin synthesis reflects both daily and seasonal time, melatonin has become a key element of chronobiology research. In mammals, pineal melatonin is essential for transducing day-length information into seasonal physiological responses. Due to its lipophilic nature, melatonin is able to cross the placenta and is believed to regulate multiple aspects of perinatal physiology. The endogenous daily melatonin rhythm is also likely to play a role in the maintenance of synchrony between circadian clocks throughout the adult body. Pharmacological doses of melatonin are effective in resetting circadian rhythms if taken at an appropriate time of day, and can acutely regulate factors such as body temperature and alertness, especially when taken during the day. Despite the extensive literature on melatonin physiology, some key questions remain unanswered. In particular, the amplitude of melatonin rhythms has been recently associated with diseases such as type 2 diabetes mellitus but understanding of the physiological significance of melatonin rhythm amplitude remains poorly understood.

"Seasonal changes in the neuroendocrine system": some reflections

This perspective considers first the general issue of seasonality and how it is shaped ecologically. It asks what is the relative importance of "strategic" (photoperiod-dependent) versus "tactical" (supplemental) cues in seasonality and what neural circuits are involved? It then considers recent developments as reflected in the Special Issue. What don't we understand about the photoperiodic clock and also the long-term timing mechanisms underlying refractoriness? Are these latter related to the endogenous annual rhythms? Can we finally identify the opsins involved in photodetection? What is the present position with regard to melatonin as "the" annual calendar? An exciting development has been the recognition of the involvement of thyroid hormones in seasonality but how does the Dio/TSH/thyroid hormone pathway integrate with downstream components of the photoperiodic response system? Finally, there are the seasonal changes within the central nervous system itself--perhaps the most exciting aspect of all.

Seasonal regulation of structural plasticity and neurogenesis in the adult mammalian brain: focus on the sheep hypothalamus

To cope with variations in the environment, most mammalian species exhibit seasonal cycles in physiology and behaviour. Seasonal plasticity during the lifetime contributes to seasonal physiology. Over the years, our ideas regarding adult brain plasticity and, more specifically, hypothalamic plasticity have greatly evolved. Along with the two main neurogenic regions, namely the hippocampal subgranular and lateral ventricle subventricular zones, the hypothalamus, which is the central homeostatic regulator of numerous physiological functions that comprise sexual behaviours, feeding and metabolism, also hosts neurogenic niches. Both endogenous and exogenous factors, including the photoperiod, modulate the hypothalamic neurogenic capacities. The present review describes the effects of season on adult morphological plasticity and neurogenesis in seasonal species, for which the photoperiod is a master environmental cue for the successful programming of seasonal functions. In addition, the potential functional significance of adult neurogenesis in the mediation of the seasonal control of reproduction and feeding is discussed.

Photoperiodic time measurement and seasonal immunological plasticity

Seasonal variations in immunity are common in nature, and changes in day length are sufficient to trigger enhancement and suppression of immune function in many vertebrates. Drawing primarily on data from Siberian hamsters, this review describes formal and physiological aspects of the neuroendocrine regulation of seasonal changes in mammalian immunity. Photoperiod regulates immunity in a trait-specific manner, and seasonal changes in gonadal hormone secretion and thyroid hormone signaling all participate in seasonal immunomodulation. Photoperiod-driven changes in the hamster reproductive and immune systems are associated with changes in iodothyronine deiodinase-mediated thyroid hormone signaling, but photoperiod exerts opposite effects on select aspects of the epigenetic regulation of reproductive neuroendocrine and lymphoid tissues. Photoperiodic changes in immunocompetence may explain a proportion of the annual variance in disease incidence and severity in nature, and provide a useful framework to help understand brain-immune interactions.

Clocks for all seasons: unwinding the roles and mechanisms of circadian and interval timers in the hypothalamus and pituitary

Adaptation to the environment is essential for survival, in all wild animal species seasonal variation in temperature and food availability needs to be anticipated. This has led to the evolution of deep-rooted physiological cycles, driven by internal clocks, which can track seasonal time with remarkable precision. Evidence has now accumulated that a seasonal change in thyroid hormone (TH) availability within the brain is a crucial element. This is mediated by local control of TH-metabolising enzymes within specialised ependymal cells lining the third ventricle of the hypothalamus. Within these cells, deiodinase type 2 enzyme is activated in response to summer day lengths, converting metabolically inactive thyroxine (T4) to tri-iodothyronine (T3). The availability of TH in the hypothalamus appears to be an important factor in driving the physiological changes that occur with season. Remarkably, in both birds and mammals, the pars tuberalis (PT) of the pituitary gland plays an essential role. A specialised endocrine thyrotroph cell (TSH-expressing) is regulated by the changing day-length signal, leading to activation of TSH by long days. This acts on adjacent TSH-receptors expressed in the hypothalamic ependymal cells, causing local regulation of deiodinase enzymes and conversion of TH to the metabolically active T3. In mammals, the PT is regulated by the nocturnal melatonin signal. Summer-like melatonin signals activate a PT-expressed clock-regulated transcription regulator (EYA3), which in turn drives the expression of the TSHβ sub-unit, leading to a sustained increase in TSH expression. In this manner, a local pituitary timer, driven by melatonin, initiates a cascade of molecular events, led by EYA3, which translates to seasonal changes of neuroendocrine activity in the hypothalamus. There are remarkable parallels between this PT circuit and the photoperiodic timing system used in plants, and while plants use different molecular signals (constans vs EYA3) it appears that widely divergent organisms probably obey a common set of design principles.

Universality and diversity in the signal transduction pathway that regulates seasonal reproduction in vertebrates

Most vertebrates living outside the tropical zone show robust physiological responses in response to seasonal changes in photoperiod, such as seasonal reproduction, molt, and migration. The highly sophisticated photoperiodic mechanism in Japanese quail has been used to uncover the mechanism of seasonal reproduction. Molecular analysis of quail mediobasal hypothalamus (MBH) revealed that local thyroid hormone activation within the MBH plays a critical role in the photoperiodic response of gonads. This activation is accomplished by two gene switches: thyroid hormone-activating (DIO2) and thyroid hormone-inactivating enzymes (DIO3). Functional genomics studies have shown that long-day induced thyroid-stimulating hormone (TSH) in the pars tuberalis (PT) of the pituitary gland regulates DIO2/3 switching. In birds, light information received directly by deep brain photoreceptors regulates PT TSH. Recent studies demonstrated that Opsin 5-positive cerebrospinal fluid (CSF)-contacting neurons are deep brain photoreceptors that regulate avian seasonal reproduction. Although the involvement of TSH and DIO2/3 in seasonal reproduction has been confirmed in various mammals, the light input pathway that regulates PT TSH in mammals differs from that of birds. In mammals, the eye is the only photoreceptor organ and light information received by the eye is transmitted to the pineal gland through the circadian pacemaker, the suprachiasmatic nucleus. Nocturnal melatonin secretion from the pineal gland indicates the length of night and regulates the PT TSH. In fish, the regulatory machinery for seasonal reproduction, from light input to neuroendocrine output, has been recently demonstrated in the coronet cells of the saccus vasculosus (SV). The SV is unique to fish and coronet cells are CSF-contacting neurons. Here, we discuss the universality and diversity of signal transduction pathways that regulate vertebrate seasonal reproduction.

Seasonal time measurement during reproduction

Most species living outside the tropical zone undergo physiological adaptations to seasonal environmental changes and changing day length (photoperiod); this phenomenon is called photoperiodism. It is well known that the circadian clock is involved in the regulation of photoperiodism such as seasonal reproduction, but the mechanism underlying circadian clock regulation of photoperiodism remains unclear. Recent molecular analysis have revealed that, in mammals and birds, the pars tuberalis (PT) of the pituitary gland acts as the relay point from light receptors, which receive information about the photoperiod, to the endocrine responses. Long-day (LD)-induced thyroid-stimulating hormone (TSH) in the PT acts as a master regulator of seasonal reproduction in the ependymal cells (ECs) within the mediobasal hypothalamus (MBH) and activates thyroid hormone (TH) by inducing the expression of type 2 deiodinase in both LD and short-day (SD) breeding animals. Furthermore, the circadian clock has been found to be localized in the PT and ECs as well as in the circadian pacemaker(s). This review purposes to summarize the current knowledge concerning the involvement of the neuroendocrine system and circadian clock in seasonal reproduction.

Regulation of seasonal reproduction by hypothalamic activation of thyroid hormone

Organisms living outside the tropics measure the changes in the length of the day to adapt to seasonal changes in the environment. Animals that breed during spring and summer are called long-day breeders, while those that breed during fall are called short-day breeders. Although the influence of thyroid hormone in the regulation of seasonal reproduction has been known for several decades, its precise mechanism remained unknown. Recent studies revealed that the activation of thyroid hormone within the mediobasal hypothalamus plays a key role in this phenomenon. This localized activation of the thyroid hormone is controlled by thyrotropin (thyroid-stimulating hormone) secreted from the pars tuberalis of the pituitary gland. Although seasonal reproduction is a rate-limiting factor in animal production, genes involved in photoperiodic signal transduction pathway could emerge as potential targets to facilitate domestication.

Thyroid hormone and seasonal rhythmicity

Living organisms show seasonality in a wide array of functions such as reproduction, fattening, hibernation, and migration. At temperate latitudes, changes in photoperiod maintain the alignment of annual rhythms with predictable changes in the environment. The appropriate physiological response to changing photoperiod in mammals requires retinal detection of light and pineal secretion of melatonin, but extraretinal detection of light occurs in birds. A common mechanism across all vertebrates is that these photoperiod-regulated systems alter hypothalamic thyroid hormone (TH) conversion. Here, we review the evidence that a circadian clock within the pars tuberalis of the adenohypophysis links photoperiod decoding to local changes of TH signaling within the medio-basal hypothalamus (MBH) through a conserved thyrotropin/deiodinase axis. We also focus on recent findings which indicate that, beyond the photoperiodic control of its conversion, TH might also be involved in longer-term timing processes of seasonal programs. Finally, we examine the potential implication of kisspeptin and RFRP3, two RF-amide peptides expressed within the MBH, in seasonal rhythmicity.

The role of thyroid hormones in regulation of chicken ovarian steroidogenesis

In all vertebrates, including birds, the normal development of the ovary and ovarian follicles is under the regulatory influence of hormones produced by the reproductive axis. In recent years, it has become clear that in birds an adequate level of thyroid hormones (THs), i.e. thyroxine (T4) and triiodothyronine (T3), in blood circulation is of primary importance for normal female reproductive functions. In avian species, characterized by seasonal reproduction, THs are involved in the photoperiodic regulation of reproduction acting at the mediobasal hypothalamus. In domestic fowl, where the seasonality of reproduction has been eliminated, the role of THs in ovarian function is not fully elucidated. Recent studies have revealed that ovarian follicles of the laying hen express mRNAs of TH nuclear receptors (TRα and TRβ0) as well as integrin (αVβ3) plasma membrane receptors, indicating genomic and nongenomic action of THs in the chicken ovary. In vivo experiments carried out on laying hens have showed that the bolus injection of T3 decreases levels of luteinizing hormone (LH) and estradiol (E2) in blood, and a hyperthyroid state evoked by administration of T3 for few days diminishes LH, E2 and progesterone (P4) levels, reduces the weight of the ovary, induces atresia of preovulatory follicles and eventually causes stoppage of egg laying. In vitro studies have demonstrated that T3 decreases E2 secretion from white nonhierarchical follicles and the theca layer of yellow preovulatory follicles, while on the other hand, it elevates P4 production from the granulosa layer of these follicles. These effects have been associated with steroidogenic enzyme expression and cyclic AMP synthesis. This review summarizes the current knowledge concerning the role of THs in regulation of steroidogenesis in chicken ovarian follicles.

Thyroid hormone and seasonal regulation of reproduction

Organisms living outside the tropics use changes in photoperiod to adapt to seasonal changes in the environment. Several models have contributed to an understanding of this mechanism at the molecular and endocrine levels. Subtropical birds are excellent models for the study of these mechanisms because of their rapid and dramatic response to changes in photoperiod. Studies of birds have demonstrated that light is perceived by a deep brain photoreceptor and long day-induced thyrotropin (TSH) from the pars tuberalis (PT) of the pituitary gland causes local thyroid hormone activation within the mediobasal hypothalamus (MBH). The locally generated bioactive thyroid hormone, T₃, regulates seasonal gonadotropin-releasing hormone (GnRH) secretion, and hence gonadotropin secretion. In mammals, the eyes are the only photoreceptor involved in photoperiodic time perception and nocturnal melatonin secretion provides an endocrine signal of photoperiod to the PT to regulate TSH. Here, I review the current understanding of the hypothalamic mechanisms controlling seasonal reproduction in mammals and birds.

Circadian clocks and the measurement of daylength in seasonal reproduction

Temperate zone organisms measure changes in daylength to adapt to seasonal changes in their environment. Recent studies have revealed that the long day (LD)-induced thyrotropin (TSH) in the pars tuberalis (PT) of the pituitary gland act as a master factor regulating seasonal reproduction on the the ependymal cells (ECs) within the mediobasal hypothalamus (MBH) to induce expression of type 2 deiodinase (Dio2), a thyroid hormone (TH)-activating enzyme in both LD and short day (SD) breeders. Locally activated TH in the MBH is believed to trigger GnRH secretion from the hypothalamus in LD breeders, while it terminates reproductive activity in SD breeders. Circadian clock is involved in seasonal time measurement and clock genes are expressed in the PT and ECs. Although circadian and melatonin-dependent control of TSH appears to link the circadian clock and the photoperiodic response in mammals, how this circadian clock measure daylength remains to be clarified.

Melatonin-dependent timing of seasonal reproduction by the pars tuberalis: pivotal roles for long daylengths and thyroid hormones

Most mammals living at temperate latitudes exhibit marked seasonal variations in reproduction. In long-lived species, it is assumed that timely physiological alternations between a breeding season and a period of sexual rest depend upon the ability of day length (photoperiod) to synchronise an endogenous timing mechanism called the circannual clock. The sheep has been extensively used to characterise the time-measurement mechanisms of seasonal reproduction. Melatonin, secreted only during the night, acts as the endocrine transducer of the photoperiodic message. The present review is concerned with the endocrine mechanisms of seasonal reproduction in sheep and the evidence that long day length and thyroid hormones are mandatory to their proper timing. Recent evidence for a circadian-based molecular mechanism within the pars tuberalis of the pituitary, which ties the short duration melatonin signal reflecting long day length to the hypothalamic increase of triiodothyronine (T3) through a thyroid-stimulating hormone/deiodinase2 paracrine mechanism is presented and evaluated in this context. A parallel is also drawn with the golden hamster, a long-day breeder, aiming to demonstrate that features of seasonality appear to be phylogenetically conserved. Finally, potential mechanisms of T3 action within the hypothalamus/median eminence in relationship to seasonal timing are examined.

The hypophysial pars tuberalis transduces photoperiodic signals via multiple pathways and messenger molecules

Located between the median eminence, the portal vessels, and the pars distalis (PD) of the hypophysis, the hypophysial pars tuberalis (PT) is an important center for transmission of photoperiodic information to neuroendocrine circuits involved in the control of reproduction, metabolism and behavior. Despite enormous and long lasting efforts, output pathways and messenger molecules from the PT have been unraveled only recently. Most interestingly, the PT sends its signals in two directions: via a "retrograde" pathway to the hypothalamus and via an "anterograde" pathway to the PD. TSH has been identified as a messenger of the "retrograde" pathway. As discovered in Japanese quail, TSH triggers molecular cascades mediating thyroid hormone conversion in the mediobasal hypothalamus (MBH) to activate the gonadal axis. These molecular mechanisms are conserved in photoperiodic mammals, and even in non-photoperiodic laboratory mice. The search for molecules of the "anterograde" pathway was for a long time focused on PT-specific neuropeptides, the so-called "tuberalins". The discovery of a PT-intrinsic endocannabinoid system in hamsters which is regulated by the photoperiod provides strong experimental evidence that the PT also synthesizes lipidergic messengers. To date, 2-arachidonoylglycerol (2-AG) appears as the most important lipidergic messenger from the PT. The primary target of 2-AG, the cannabinoid receptor 1 (CB1) is expressed in the hamster PD. A PT-intrinsic endocannabinoid system also exists in man and CB1 receptors are demonstrated in ACTH-producing cells and folliculo-stellate cells of the human PD. These data lend support to the hypothesis that endocannabinoids function as messengers of the anterograde pathway.

Thyroid hormones in the cerebrospinal fluid of the third ventricle of adult female sheep during different periods of reproductive activity

Thyroid hormones (THs) are obligatory for transition from breeding season to anestrus in sheep. In this process, THs act during a very limited time of the year and primarily within the brain. In ewes chronically equipped for sampling cerebrospinal fluid (CSF) from the third ventricle, we have characterized the concentrations of total and free thyroxine (T4), triiodothyronine (T3), and total reverse T3 (rT3) in the CSF during breeding season, anestrus and during a critical period required for transition to anestrus (December-March). The total T4, T3, rT3 and free T3 average concentrations (+/- SEM) in CSF were 1.5 +/- 0.07 ng/ml, 14.5 +/- 1.2 pg/ml, 43 +/- 7.4 pg/ml, and 0.6 +/- 0.05 pg/ml, respectively, and all were significantly lower (p < 0.001) than in blood plasma except free T4 (12.6 +/- 1.1 pg/ml), which was similar to that in plasma. There was a seasonal trend (p < 0.05) in the concentration of total T3 (highest in December) and free T4 (highest in November) in the CSF that does not follow that in blood plasma. During the period of transition to anestrus the CSF total T3/TT4 molar ratio and free T3/T4 ratio were significantly lower (p < 0.05 and p < 0.01, respectively) than in blood plasma, while the total rT3/T4 ratio was significantly higher (p < 0.01) at the end of this period (March). Additionally, the CSF total rT3 concentrations were also significantly correlated with the CSF total T4 levels (r = 0.57; p < 0.05). In conclusion, the CSF in sheep may serve as a considerable source of thyroid hormones for neuroendocrine events. The lack of significant changes in THs concentrations in the CSF during the period of transition to anestrus indicate that neither seasonal changes of THs circulating in the blood plasma nor THs circulating in the CSF actively drive the transition to anestrus.

Neuroendocrine mechanism of seasonal reproduction in birds and mammals

In temperate zones, animals use changes in day length as a calendar to time their breeding season. However, the photoreceptive and neuroendocrine mechanisms of seasonal reproduction are considered to differ markedly between birds and mammals. This can be understood from the fact that the eye is the only photoreceptive organ, and melatonin mediates the photoperiodic information in mammals, whereas in birds, photoperiodic information is directly received by the deep brain photoreceptors and melatonin is not involved in seasonal reproduction. Recent molecular and functional genomics analysis uncovered the gene cascade regulating seasonal reproduction in birds and mammals. Long day-induced thyroid stimulating hormone in the pars tuberalis of the pituitary gland regulates thyroid hormone catabolism within the mediobasal hypothalamus. Further, this local thyroid hormone catabolism appears to regulate seasonal gonadotropin-releasing hormone secretion. These findings suggest that although the light input pathway is different between birds and mammals (i.e. light or melatonin), the core mechanisms are conserved in these vertebrates.

Identification of the photoperiodic signaling pathway regulating seasonal reproduction using the functional genomics approach

Animals measure photoperiod (daylength) and adapt to seasonal changes in the environment by altering their physiology and behavior accordingly. Although this photoperiodic response has long been of interest, the underlying mechanism has only recently begun to be uncovered at the molecular level. Japanese quail provide an excellent model to study the molecular mechanism underlying the vertebrate photoperiodic response. The recent sequencing of the chicken genome allowed a system-level analysis of photoperiodic time measurement in quail, and this approach uncovered the key event in the photoperiodic signaling cascade that regulates seasonal reproduction. Long photoperiod-induced expression of thyrotropin in the pars tuberalis of the pituitary gland was found to trigger local thyroid hormone catabolism in the mediobasal hypothalamus, which increases the activity of the reproductive neuroendocrine system resulting in gonadal development. Since thyrotropin was only known to stimulate the thyroid gland, a traditional hypothesis-driven approach would not have been expected to predict this discovery. Thus, a functional genomics approach, which is a discovery-driven approach, provides new insights in the field of endocrinology.

Dopamine-melatonin neurons in the avian hypothalamus and their role as photoperiodic clocks

A timing mechanism in the brain governs reproduction in seasonally breeding temperate zone birds by triggering gonad development in response to long days in the spring. The neural mechanism(s) responsible for the timing and induction of reproductive activity by this clock are unknown. Utilizing in situ hybridization, immunocytochemistry and reverse transcriptase-polymerase chain reaction techniques, a group of dopamine (DA) neurons in the premammillary nucleus (PMM) of the caudal turkey hypothalamus that synthesize and colocalize both DA and melatonin (MEL) were identified. In addition, these neurons are found to express clock genes and the circadian photoreceptor melanopsin. DA-MEL neurons reach threshold activation (c-fos expression) when a light pulse is given during the photosensitive phase. This is associated with increases in the number of gonadotropin releasing hormone-I (GnRH-I) neurones activated, as well as an up-regulation of GnRH-I mRNA expression. The expression of tyrosine hydroxylase (TH; the rate limiting enzyme in DA biosynthesis) and tryptophan hydroxylase 1, (TPH1; the first enzyme in MEL biosynthesis) and consequently DAergic-MELergic activities are associated with the daily light-dark cycle. TPH1 mRNA expression shows low levels during the light phase and high levels during the dark phase of the light/dark illumination cycle and is 180 degrees out of phase with the rhythm of TH mRNA expression. Hypothalamic DA-MEL neurons may constitute a critical cellular process involved in the generation and expression of seasonal reproductive rhythms and suggests a previously undescribed mechanism(s) by which light signals gain access to neural targets.

New insights into ancient seasonal life timers

Organisms must adapt to seasonal changes in the environment and time their physiology accordingly. In vertebrates, the annual change in photoperiod is often critical for entraining the neuroendocrine pathways, which drive seasonal metabolic and reproductive cycles. These cycles depend on thyroid hormone (TH), reflecting its ancestral role in metabolic control. Recent studies reveal that--in mammals and birds--TH effects are mediated by the hypothalamus. Photoperiodic manipulations alter hypothalamic TH availability by regulating the expression of TH deiodinases (DIO). In non-mammalian vertebrates, light acts through extraretinal, 'deep brain' photoreceptors, and the eyes are not involved in seasonal photoperiodic responses. In mammals, extraretinal photoreceptors have been lost, and the nocturnal melatonin signal generated from the pineal gland has been co-opted to provide the photoperiodic message. Pineal function is phased to the light-dark cycle by retinal input, and photoperiodic changes in melatonin secretion control neuroendocrine pathway function. New evidence indicates that these comparatively divergent photosensensory mechanisms re-converge in the pars tuberalis of the pituitary, lying beneath the hypothalamus. In all vertebrates studied, the pars tuberalis secretes thyrotrophin in a light- or melatonin-sensitive manner, to act on neighbouring hypothalamic DIO expressing cells. Hence, an ancient and fundamentally conserved brain thyroid signalling system governs seasonal biology in vertebrates.

Thyroid hormones and seasonal reproductive neuroendocrine interactions

Many animals that breed seasonally measure the day length (photoperiod) and use these measurements as predictive information to prepare themselves for annual breeding. For several decades, thyroid hormones have been known to be involved in this biological process; however, their precise roles remain unknown. Recent molecular analyses have revealed that local thyroid hormone activation in the hypothalamus plays a critical role in the regulation of the neuroendocrine axis involved in seasonal reproduction in both birds and mammals. Furthermore, functional genomics analyses have revealed a novel function of the hormone thyrotropin. This hormone plays a key role in signaling day-length changes to the brain and thus triggers seasonal breeding. This review aims to summarize the currently available knowledge on the interactions between elements of the thyroid hormone axis and the neuroendocrine system involved in seasonal reproduction.

Photoperiodic signalling through the melatonin receptor turns full circle

Photoperiod exerts profound influence on the physiology of mammals through the action of melatonin on the neuroendocrine system. Over the last 20 years, studies have moved away from a melatonin receptor-focused approach to understanding how photoperiod regulates neuroendocrine activity through studies of downstream effects on gene expression. This paper reviews the recent progress made in our understanding of the effects of photoperiod on gene expression in the hypothalamus, and considers how this new information can be reconciled with the species-specific location of melatonin receptors.

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

The molecular mechanism underlying photoperiodism is not well understood in any organism. Long-day-induced conversion of prohormone T(4) to bioactive T(3) within the mediobasal hypothalamus (MBH) is critical for the photoperiodic regulation of reproduction. However, because thyroidectomy does not completely block the photoperiodic response in some species, the existence of a thyroid hormone-independent regulatory mechanism appears certain. To identify this novel mechanism, differential subtractive hybridization analysis was performed using MBH of quail kept under short-day and long-day conditions. This analysis identified a gene encoding TGFalpha. Expression of TGFalpha mRNA was induced in the median eminence by the stimulus of long days, and this induction was observed at dusk on the first long day. This rapid induction of TGFalpha mRNA was similar to induction of the thyroid hormone-activating enzyme gene [Dio2 (type 2 iodothyronine deiodinase)], which is the earliest event yet determined in the photo-induction process. Expression analysis of epidermal growth factor receptors revealed strong expression of erbB4 and weak expression of erbB1 and erbB2 in the median eminence. Intracerebroventricular infusion of physiological dose of TGFalpha induced LH secretion and testicular growth under short-day conditions. Finally, we demonstrate that T(3) implantation and TGFalpha infusion into the MBH, either of which causes testicular growth, do not affect the expression of TGFalpha and Dio2, respectively. Thus, long-day-induced activation of the TGFalpha signaling pathway appears to mediate a thyroid hormone-independent pathway for the photoperiodic regulation of reproduction.

Hypothalamic expression of thyroid hormone-activating and -inactivating enzyme genes in relation to photorefractoriness in birds and mammals

Photorefractoriness is the insensitivity of gonadal development to the stimulatory effects of long photoperiods in birds and to the inhibitory effects of short photoperiods in small mammals. Its molecular mechanism remains unknown. Recently, it has been shown that reciprocal expression of thyroid hormone-activating enzyme [type 2 deiodinase (Dio2)] and -inactivating enzyme [type 3 deiodinase (Dio3)] genes in the mediobasal hypothalamus is critical for photoperiodically induced gonadal growth. Since thyroid hormones are required not only for photoinduction, but also for the induction of photorefractoriness, we examined the expression of these genes in relation to photorefractoriness in birds and mammals. Transfer of birds to long photoperiods induced strong expression of Dio2. This was maintained in tree sparrow when they later became photorefractory, but decreased somewhat in quail. In hamsters, transfer to long photoperiods also induced strong expression of Dio2. High values were not maintained under long photoperiods, and, indeed, expression decreased at the same rate as in animals transferred to short photoperiods. There was no renewed expression of Dio2 associated with testicular growth as animals became refractory to short photoperiods. Expression of Dio3 was high under short photoperiods and low under long photoperiods in all the animals examined, except for the short photoperiod-refractory hamsters. Our present study revealed complex regulation of deiodinase genes in the photoinduction and photorefractory processes in birds and mammals. These gene changes may be involved in the regulation of photorefractoriness, as well as photoinduction.

Melatonin regulates type 2 deiodinase gene expression in the Syrian hamster

In seasonal species, photoperiod organizes various physiological processes, including reproduction. Recent data indicate that the expression of type 2 iodothyronine deiodinase (Dio2) is modulated by photoperiod in the mediobasal hypothalamus of some seasonal species. Dio2 is believed to control the local synthesis of bioactive T(3) to regulate gonadal response. Here we used in situ hybridization to study Dio2 expression in the hypothalamus of a photoperiodic rodent, the Syrian hamster. Dio2 was highly expressed in reproductively active hamsters in long day, whereas it was dramatically reduced in sexually inhibited hamsters maintained in short day. This contrasted with the laboratory rat, a nonphotoperiodic species, in which no evidence for Dio2 photoperiodic modulation was seen. We also demonstrate that photoperiodic variations of Dio2 expression in hamsters are independent from secondary changes in gonadal steroids. Studies in pinealectomized hamsters showed that the photoperiodic variation of Dio2 expression is melatonin dependent, and injections of long day hamsters with melatonin for only 7 d were sufficient to inhibit Dio2 expression to that of short day levels. Finally, because in some seasonal species thyroid hormones are involved in photorefractoriness, we examined Dio2 expression in short day-refractory hamsters and found that Dio2 mRNA levels remained low despite full reproductive recrudescence. Altogether, these results demonstrate that in the Syrian hamster Dio2 is photoperiodically modulated via a melatonin-dependent process. Furthermore, refractoriness to photoperiod in hamsters appears to occur independently of Dio2. These results raise new perspectives for understanding how thyroid hormones are involved in the control of photoperiodic neuroendocrine processes.

Potential role of thyroid hormones and prolactin in the programming of photorefractoriness in turkey hens

The domestic turkey hen is a seasonal breeder, requiring a period of short days to establish photosensitivity and a long day length to initiate egg production. The reproductive season is then limited by the onset of photorefractoriness (PR), which causes a decline, and then termination, of egg laying. In passerine birds, PR is programmed early in the reproductive season by the presence of thyroid hormones and a long photoperiod. High circulating prolactin (PRL) is thought to hasten the onset of PR. In a prior study, we reported that hens destined to have PR exhibited lower levels of thyroxine (T4) and PRL at certain points (weeks) following photostimulation than did hens destined to remain photosensitive (PS), a result opposite to what might be expected. The present study was conducted to further explore the possible relationship between circulating hormone levels and subsequent PR in the commercial turkey hen at times (days) closer to photostimulation than our previous study. Plasma levels of triiodothyronine (T3), T4, and PRL were compared in 2 subpopulations of hens identified retrospectively after 50 wk of egg production: A group of 17 hens that exhibited PR (mean onset = 27 wk of photostimulation) and a group of "good" layers that remained PS (mean production = 210 eggs/50 wk). Results showed no differences between groups in plasma T3 or T4 levels or in the T3:T4 ratio at -6, 0, 1, 3, and 7 d from photostimulation. Plasma PRL levels were significantly higher at 8 and 9 wk after photostimulation in hens that remained PS vs. those that became PR. We conclude that thyroid hormone levels around the time of photostimulation either are not actively related to programming of subsequent PR in turkeys or programming for PR in the turkey hen occurs later in the reproductive cycle than in passerine birds. We further conclude that hens that exhibit PR tend to have lower circulating PRL levels early in the reproductive season than hens that remain PS and lay at a relatively high rate.

Molecular mechanism of photoperiodic time measurement in the brain of Japanese quail

In most organisms living in temperate zones, reproduction is under photoperiodic control. Although photoperiodic time measurement has been studied in organisms ranging from plants to vertebrates, the underlying molecular mechanism is not well understood. The Japanese quail (Coturnix japonica) represents an excellent model to study this problem because of the rapid and dramatic photoperiodic response of its hypothalamic-pituitary-gonadal axis. Recent investigations of Japanese quail show that long-day-induced type 2 deiodinase (Dio2) expression in the mediobasal hypothalamus (MBH) plays an important role in the photoperiodic gonadal regulation by catalyzing the conversion of the prohormone thyroxine (T(4)) to bioactive 3,5,3'-triiodothyronine (T3). The T3 content in the MBH is approximately 10-fold higher under long than short days and conditions, and the intracerebroventricular infusion of T3 under short days and conditions mimics the photoperiodic gonadal response. While Dio2 generates active T3 from T4 by outer ring deiodination, type 3 deiodinase (Dio3) catalyzes the conversion of both T3 and T4 into inactive forms by inner ring deiodination. In contrast to Dio2 expression, Dio3 expression in the MBH is suppressed under the long-day condition. Photoperiodic changes in the expression of both genes during the photoinduction process occur before the changes in the level of luteinizing hormone (LH) secretion, suggesting that the reciprocal changes in Dio2 and Dio3 expression act as gene switches of the photoperiodic molecular cascade to trigger induction of LH secretion.