Photoperiodic regulation of type 2 deiodinase gene in Djungarian hamster: possible homologies between avian and mammalian photoperiodic regulation of reproduction

Hypothalamic ventricular ependymal thyroid hormone deiodinases are an important element of circannual timing in the Siberian hamster (Phodopus sungorus)

Exposure to short days (SD) induces profound changes in the physiology and behaviour of Siberian hamsters, including gonadal regression and up to 30% loss in body weight. In a continuous SD environment after approximately 20 weeks, Siberian hamsters spontaneously revert to a long day (LD) phenotype, a phenomenon referred to as the photorefractory response. Previously we have identified a number of genes that are regulated by short photoperiod in the neuropil and ventricular ependymal (VE) cells of the hypothalamus, although their importance and contribution to photoperiod induced physiology is unclear. In this refractory model we hypothesised that the return to LD physiology involves reversal of SD expression levels of key hypothalamic genes to their LD values and thereby implicate genes required for LD physiology. Male Siberian hamsters were kept in either LD or SD for up to 39 weeks during which time SD hamster body weight decreased before increasing, after more than 20 weeks, back to LD values. Brain tissue was collected between 14 and 39 weeks for in situ hybridization to determine hypothalamic gene expression. In VE cells lining the third ventricle, expression of nestin, vimentin, Crbp1 and Gpr50 were down-regulated at 18 weeks in SD photoperiod, but expression was not restored to the LD level in photorefractory hamsters. Dio2, Mct8 and Tsh-r expression were altered by SD photoperiod and were fully restored, or even exceeded values found in LD hamsters in the refractory state. In hypothalamic nuclei, expression of Srif and Mc3r mRNAs was altered at 18 weeks in SD, but were similar to LD expression values in photorefractory hamsters. We conclude that in refractory hamsters not all VE cell functions are required to establish LD physiology. However, thyroid hormone signalling from ependymal cells and reversal of neuronal gene expression appear to be essential for the SD refractory response.

Circannual variation in thyroid hormone deiodinases in a short-day breeder

At temperate latitudes, many mammals and birds show internally timed, long-term changes in seasonal physiology, synchronised to the seasons by changing day length (photoperiod). Photoperiodic control of thyroid hormone levels in the hypothalamus dictates the timing. This is effected through reciprocal regulation of thyroid hormone deiodinase gene expression. The local synthesis of type 2 deiodinase (Dio2) promotes triiodothyronine (T3) production and summer biology, whereas type 3 deiodinase (Dio3) promotes T3 degradation and winter biology. In the present study, we investigated the extent to which the hypothalamic expression of Dio2 and Dio3 is circannually regulated in the Soay sheep, a short-day breeding mammal. Male sheep were exposed to a long photoperiod (LP; 16 : 24 h light/dark cycle) or a short photoperiod (SP; 8 : 24 h light/dark cycle), for up to 28 weeks to establish four different endocrine states: (i) LP animals in a spring/summer-like state of reproductive arrest; (ii) LP refractory (LPR) animals showing spontaneous reproductive reactivation; (iii) SP animals showing autumn/winter-like reproductive activation; and (iv) SP refractory (SPR) animals showing spontaneous reproductive arrest. A complex pattern of hypothalamic Dio2 and Dio3 expression was observed, revealing distinctive photoperiod-driven and internally timed effects for both genes. The patterns of expression differed both spatially and temporally, with phases of peak Dio2 expression in the median eminence and tuberoinfundibular sulcus, as well as in the paraventricular zone (PVZ) (maximal under LP), whereas Dio3 expression was always confined to the PVZ (maximal under SP). These effects likely reflect the distinct roles of these enzymes in the localised control of hypothalamic T3 levels. The spontaneous decline in Dio2 and spontaneous increase in Dio3 in LPR animals occurred with a corresponding decline in thyroid-stimulating hormone β expression in the neighbouring pars tuberalis (PT), although this relationship did not hold for the corresponding Dio2 increase/Dio3 decrease seen in SPR animals. We conclude that internally timed and spatially regulated changes in Dio2 and Dio3 expression may drive the cycling between breeding and nonbreeding states in long-lived seasonal species, and may be either PT-dependent or PT-independent at different phases of the circannual cycle.

Circannual transitions in gene expression: lessons from seasonal adaptations

Circannual timing is important for the coordination of seasonal activities, particularly promoting the survival of individuals in adverse conditions through adaptive physiological and behavioral changes. This includes optimizing the survival of offspring by coordinating reproductive efforts at appropriate times. Thus, timing is very important for overall fitness. In this chapter, we provide several examples of circannually timed events, including mammalian hibernation, discussing the physiological changes that accompany these events, and some of the known genes and pathways underlying these changes. We then describe five candidate systems that are potentially involved in circannual timing. Finally, we discuss several recent advances in molecular biology and animal husbandry that have made the use of nonmodel organisms for research more feasible, which will hopefully promote and encourage further advancement in the knowledge of circannual timing.

Analysis on DNA sequence of TSHB gene and its association with reproductive seasonality in goats

Thyroid stimulating hormone beta chain (TSHB) is mainly expressed in pituitary and its expression is closely related to photoperiodic control of seasonal reproduction in animals. In the present study, ten primer pairs have been used to clone the DNA sequence and to detect genetic mutations of goat TSHB gene. Two DNA fragments of goat TSHB gene were obtained, which were 2,614 and 1,031 bp in length, respectively. They comprised about 2.5 kb 5' regulatory region, all of the three exons and two introns. Goat TSHB gene has a coding region of 417 bp, encoding 138 amino acids which was predicted to be a secretory protein with a signal peptide of 16 amino acids. The sequence of TSHB gene is highly conserved among mammals. In addition, five mutations (C53A, 3 bp Indel at the 287-289 locus, 34 bp Indel at the 584-617 locus, A1819C and E2_72TA) were found in goat TSHB gene and they were shown to be in strong linkage disequilibrium. Interestingly, the genotype distributions for both single locus and haplotype have shown to be significant different between seasonal and nonseasonal goat breeds. And haplotype H2 and diplotype H2/H4 may be related to year-round estrus. We preliminarily presumed that the five closely linked mutations of goat TSHB gene may be part of the causal sources for the diversities of reproductive seasonality in goats. Our study may provide a possible efficient genetic way to decrease seasonality in goats.

A vitamin for the brain

In the central nervous system (CNS) the function of retinoic acid, the active metabolite of vitamin A, is best understood from its action in guiding embryonic development; as development comes to completion, retinoic acid signaling declines. However, it is increasingly recognized that this signaling mechanism does not disappear in the adult brain but becomes more regionally focused and takes on new roles. These functions are often tied to processes of neural plasticity whether in the hippocampus, through homeostatic neural plasticity, the olfactory bulb or the hypothalamus. The role of retinoic acid in the control of plastic processes has led to suggestions of its involvement in neural disorders, both degenerative and psychiatric. This review presents a snapshot of developments in these areas over recent years.

Dynamic mRNA distribution pattern of thyroid hormone transporters and deiodinases during early embryonic chicken brain development

Maternal thyroid hormones (THs) are important in early brain development long before the onset of embryonic TH secretion, but information about the regulation of TH availability in the brain at these early stages is still limited. We therefore investigated in detail the mRNA distribution pattern of the TH activating type 2 and inactivating type 3 deiodinases (D2 and D3) and the TH transporters, organic anion transporting polypeptide 1c1 (Oatp1c1) and monocarboxylate transporter 8 (Mct8), in chicken embryonic brain as well as in retina and inner ear from day 3 to day 10 of development. Oatp1c1, Mct8 and D3 are expressed in the choroid plexus and its precursors allowing selective uptake of THs at the blood-cerebrospinal fluid-barrier with subsequent inactivation of excess hormone. In contrast, the developing blood-brain-barrier does not express Oatp1c1 or Mct8 but appears to be a site for TH activation by D2. Expression of D3 in several sensory brain centers may serve as protection against premature TH action. Expression of D2 and Mct8 but not D3 in the developing pituitary gland allows accumulation of active THs even at early stages. Mct8 is widely expressed in gray matter throughout the brain. This is the first comprehensive study on the dynamic distribution pattern of TH-transporters and deiodinases at stages of embryonic brain development when only maternal THs are available. It provides the essential background for further research aimed at understanding early developmental processes depending on maternal THs.

Hypothalamic gene expression rapidly changes in response to photoperiod in juvenile Siberian hamsters (Phodopus sungorus)

Siberian hamsters are seasonal mammals that survive a winter climate by making adaptations in physiology and behaviour. This includes gonadal atrophy, reduced food intake and body weight. The underlying central mechanisms responsible for the physiological adaptations are not fully established but involve reducing hypothalamic tri-iodthyronine (T3) levels. Juvenile Siberian hamsters born or raised in short days (SD) respond in a similar manner, although with an inhibition of gonadal development and growth instead of reversing an established long day (LD) phenotype. Using juvenile male hamsters, the present study aimed to investigate whether the central mechanisms are similar before the establishment of the mature LD phenotype. By in situ hybridisation, we examined the response of genes involved in thyroid hormone (Dio2 and Dio3, which determine hypothalamic T3 levels) and glucose/glutamate metabolism in the ependymal layer, histamine H3 receptor and VGF as representatives of the highly responsive dorsomedial posterior arcuate nucleus (dmpARC), and somatostatin, a hypothalamic neuropeptide involved in regulating the growth axis. Differential gene expression of type 2 and type 3 deiodinase in the ependymal layer, histamine H3 receptor in the dmpARC and somatostatin in the ARC was established by the eighth day in SD. These changes are followed by alterations in glucose metabolism related genes in the ependymal layer by day 16 and increased secretogranin expression in the dmpARC by day 32. In conclusion, our data demonstrate similar but rapid and highly responsive changes in gene expression in the brain of juvenile Siberian hamsters in response to a switch from LD to SD. The data also provide a temporal definition of gene expression changes relative to physiological adaptations of body weight and testicular development and highlight the likely importance of thyroid hormone availability as an early event in the adaptation of physiology to a winter climate in juvenile Siberian hamsters.

Glutamine and glutamic acid enhance thyroid-stimulating hormone β subunit mRNA expression in the rat pars tuberalis

Thyroid-stimulating hormone (TSH)-producing cells of the pars tuberalis (PT) display distinct characteristics that differ from those of the pars distalis (PD). The mRNA expression of TSHβ and αGSU in PT has a circadian rhythm and is inhibited by melatonin via melatonin receptor type 1; however, the detailed regulatory mechanism for TSHβ expression in the PT remains unclear. To identify the factors that affect PT, a microarray analysis was performed on laser-captured PT tissue to screen for genes coding for receptors that are abundantly expressed in the PT. In the PT, we found high expression of the KA2, which is an ionotropic glutamic acid receptor (iGluR). In addition, the amino acid transporter A2 (ATA2), also known as the glutamine transporter, and glutaminase (GLS), as well as GLS2, were highly expressed in the PT compared to the PD. We examined the effects of glutamine and glutamic acid on TSHβ expression and αGSU expression in PT slice cultures. l-Glutamine and l-glutamic acid significantly stimulated TSHβ expression in PT slices after 2- and 4-h treatments, and the effect of l-glutamic acid was stronger than that of l-glutamine. In contrast, treatment with glutamine and glutamic acid did not affect αGSU expression in the PT or the expression of TSHβ or αGSU in the PD. These results strongly suggest that glutamine is taken up by PT cells through ATA2 and that glutamic acid locally converted from glutamine by Gls induces TSHβ expression via the KA2 in an autocrine and/or paracrine manner in the PT.

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.

Effects of manipulating hypothalamic triiodothyronine concentrations on seasonal body weight and torpor cycles in Siberian hamsters

Siberian hamsters display photoperiodically regulated annual cycles in body weight, appetite, and reproduction. Previous studies have revealed a profound up-regulation of type 3 deiodinase (DIO3) mRNA in the ventral ependyma of the hypothalamus associated with hypophagia and weight loss in short-day photoperiods. DIO3 reduces the local availability of T(3), so the aim of this study was to test the hypothesis that decreased hypothalamic T(3) availability underlies the short-day-induced catabolic state. The experimental approach was to determine whether a local increase in T(3) in the hypothalamus of hamsters exposed to short days could reverse the behavioral and physiological changes induced by this photoperiod. In study 1, microimplants releasing T(3) were placed bilaterally into the hypothalamus. This treatment rapidly induced a long-day phenotype including increased appetite and body weight within 3 wk of treatment and increased fat mass and testis size by the end of the 10-wk study period. In study 2, hypothalamic T(3) implants were placed into hamsters carrying abdominal radiotelemetry implants. Again body weight increased significantly, and the occurrence of winter torpor bouts was dramatically decreased to less than one bout per week, whereas sham-implanted hamsters entered torpor up to six times a week. Our findings demonstrate that increased central T(3) induces a long-day metabolic phenotype, but in neither study was the molt cycle affected, so we infer that we had not disrupted the initial detection of photoperiod. We conclude that hypothalamic thyroid hormone availability plays a key role in seasonal regulation of appetite, body weight, and torpor.

Melatonin controls photoperiodic changes in tanycyte vimentin and neural cell adhesion molecule expression in the Djungarian hamster (Phodopus sungorus)

The Djungarian hamster displays photoperiodic variations in gonadal size synchronized to the seasons by the nightly secretion of the pineal hormone melatonin. In short photoperiod (SP), the gonads regress in size, and circulating sex steroids levels decline. Thus, the brain is subject to seasonal variations of both melatonin and sex steroids. Tanycytes are specialized glial cells located in the ependymal lining of the third ventricle. They send processes either to the meninges or to blood vessels of the medio-basal hypothalamus. Furthermore, they are known to locally modulate GnRH release in the median eminence and to display seasonal structural changes. Seasonal changes in tanycyte morphology might be mediated either through melatonin or sex steroids. Therefore, we analyzed the effects of photoperiod, melatonin, and sex steroids 1) on tanycyte vimentin expression by immunohistochemistry and 2) on the expression of the neural cell adhesion molecule (NCAM) and polysialic acid as markers of brain plasticity. Vimentin immunostaining was reduced in tanycyte cell bodies and processes in SP. Similarly, tanycytes and their processes contained lower amounts of NCAM in SP. These changes induced by SP exposure could not be restored to long photoperiod (LP) levels by testosterone supplementation. Likewise, castration in LP did not affect tanycyte vimentin or NCAM expression. By contrast, late afternoon melatonin injections mimicking a SP-like melatonin peak in LP hamsters reduced vimentin and NCAM expression. Thus, the seasonal changes in vimentin and NCAM expression in tanycytes are regulated by melatonin independently of seasonal sex steroid changes.

Thyroid hormone and the neuroglia: both source and target

Thyroid hormone plays a crucial role in the development and function of the nervous system. In order to bind to its nuclear receptor and regulate gene transcription thyroxine needs to be activated in the brain. This activation occurs via conversion of thyroxine to T3, which is catalyzed by the type 2 iodothyronine deiodinase (D2) in glial cells, in astrocytes, and tanycytes in the mediobasal hypothalamus. We discuss how thyroid hormone affects glial cell function followed by an overview on the fine-tuned regulation of T3 generation by D2 in different glial subtypes. Recent evidence on the direct paracrine impact of glial D2 on neuronal gene expression underlines the importance of glial-neuronal interaction in thyroid hormone regulation as a major regulatory pathway in the brain in health and disease.

Influence of photoperiod on hormones, behavior, and immune function

Photoperiodism is the ability of plants and animals to measure environmental day length to ascertain time of year. Central to the evolution of photoperiodism in animals is the adaptive distribution of energetically challenging activities across the year to optimize reproductive fitness while balancing the energetic tradeoffs necessary for seasonally-appropriate survival strategies. The ability to accurately predict future events requires endogenous mechanisms to permit physiological anticipation of annual conditions. Day length provides a virtually noise free environmental signal to monitor and accurately predict time of the year. In mammals, melatonin provides the hormonal signal transducing day length. Duration of pineal melatonin is inversely related to day length and its secretion drives enduring changes in many physiological systems, including the HPA, HPG, and brain-gut axes, the autonomic nervous system, and the immune system. Thus, melatonin is the fulcrum mediating redistribution of energetic investment among physiological processes to maximize fitness and survival.

The role of hypothalamic tri-iodothyronine availability in seasonal regulation of energy balance and body weight

Seasonal cycles of body weight provide a natural model system to understand the central control of energy balance. Studies of such cycles in Siberian hamsters suggest that a change in the hypothalamic availability of thyroid hormone is the key determinant of annual weight regulation. Uptake of thyroid hormone into the hypothalamus from the peripheral circulation occurs largely through a specific monocarboxylate transporter expressed by tanycyte cells lining the third ventricle. Tanycytes are the principal brain cell type expressing type II and type III deiodinases, so they control the local concentrations of T4, T3, and inactive metabolites. Type III deiodinase mRNA in tanycytes is photoperiodically upregulated in short photoperiod. This would be expected to reduce the availability of T3 in the hypothalamus by promoting the production of inactive metabolites such as rT3. Experimental microimplantation of T3 directly into the hypothalamus during short-days promotes a long-day phenotype by increasing food intake and body weight without affecting the peripheral thyroid axis. Thus, thyroid hormone exerts anabolic actions within the brain that play a key role in the seasonal regulation of body weight. Understanding the precise actions of thyroid hormone in the brain may identify novel targets for long-term pharmacological manipulation of body weight.

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.

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.

Photoperiod and reproductive condition are associated with changes in RFamide-related peptide (RFRP) expression in Syrian hamsters (Mesocricetus auratus)

To conserve scarce energetic resources during winter, seasonal breeders inhibit reproduction and other nonessential behavioral and physiological processes. Reproductive cessation is initiated in response to declining day lengths, a stimulus represented centrally as a long-duration melatonin signal. The melatonin signal is not decoded by the reproductive axis directly, but by an unidentified neurochemical system upstream of gonadotropin-releasing hormone (GnRH). The dorsomedial nucleus of the hypothalamus (DMH) has been implicated in seasonal changes in reproductive function in Syrian hamsters (Mesocricetus auratus), although the specific-cell phenotype decoding photoperiodic information remains unknown. RFamide-related peptide (RFRP; the mammalian homolog of the gonadotropin-inhibitory hormone (GnIH) gene identified in birds) has emerged as a potent inhibitory regulator of the reproductive axis and, significantly, its expression is localized to cell bodies of the DMH in rodents. In the present study, the authors explored the relationship between RFRP expression, photoperiod exposure, and reproductive condition/hormonal status. In male hamsters that respond to short days with reproductive inhibition, RFRP-ir and mRNA expression are markedly reduced relative to long-day animals. Replacement of testosterone in short-day animals did not affect this response, suggesting that alterations in RFRP expression are not a result of changing sex steroid concentrations. A subset of the hamster population that ignores day length cues and remains reproductively competent in short days (nonresponders) exhibits RFRP-ir expression comparable to long-day hamsters. Analysis of cell body and fiber density suggests a potential interplay between peptide production and release rate in differentially regulating the reproductive axis during early and late stages of reproductive regression. Together, the present findings indicate that photoperiod-induced suppression of reproduction is associated with changes in RFRP and mRNA expression, providing opportunity for further exploration on the role that RFRP plays in this process.

Seasonal breeding as a neuroendocrine model for puberty in sheep

Puberty is defined as the awakening of the hypothalamic-pituitary gonadal axis. Sheep are seasonal breeders, experiencing an annual period of reproductive quiescence and renaissance that can be utilized as a model for the onset of puberty. Kisspeptin and gonadotropin-inhibitory hormone appear to be important for the seasonal shift in reproductive activity and the former is mandatory for puberty. The non-breeding season is characterized by an increase in the negative feedback effect of estrogen on GnRH and gonadotropin secretion, as is the case in the pre-pubertal period. This effect of estrogen may be transmitted by kisspeptin cells. Additionally, dopaminergic A14/A15 neurons facilitate the seasonal change in estrogen negative feedback. Integrated function of these three groups of neurons appears to modulate the annual shift in photoperiod to a physiological change in fertility. This review compares and contrasts seasonal cycles of reproduction with the mechanisms that relate to the onset of puberty.

Photoperiodic regulation of puberty in seasonal species

Puberty occurs seasonally in the majority of mammals native to temperate or arctic latitudes, and in species with sufficiently long life spans puberty can be considered to reoccur on an annual basis. The precise timing of puberty and the annual reoccurrence of fertility reflects an interaction of changes in ambient daylength (photoperiod) and endogenous long-term timing processes, which in some species constitute circannual clocks. Recent studies reveal an unexpected common signalling pathway for photoperiodic information in mammals and birds: changes in secretory activity of the pars tuberalis in the pituitary stalk signal to the tanycyte cells in the ependyma lining the third ventricle. The target genes in the tanycytes encode the deiodinase enzymes that regulate the availability of thyroid hormone in the hypothalamus. Central availability of thyroid hormone appears to be the key determinant of seasonal reproductive transitions. Given the necessity of thyroid hormone for the initial development of the central nervous system, it is hypothesized that at puberty and seasonal reoccurrences of fertility it is the changing local levels of thyroid hormone that orchestrate hypothalamic plasticity, ultimately impinging upon the secretion of GnRH.

Endocrine mechanisms of seasonal adaptation in small mammals: from early results to present understanding

Seasonal adaptation is widespread among mammals of temperate and polar latitudes. The changes in physiology, morphology and behaviour are controlled by the photoneuroendocrine system that, as a first step, translates day lengths into a hormonal signal (melatonin). Decoding of the humoral melatonin signal, i.e. responses on the cellular level to slight alterations in signal duration, represents the prerequisite for appropriate timing of winter acclimatization in photoperiodic animals. Corresponding to the diversity of affected traits, several hormone systems are involved in the regulation downstream of the neural integration of photoperiodic time measurement. Results from recent studies provide new insights into seasonal control of reproduction and energy balance. Most intriguingly, the availability of thyroid hormone within hypothalamic key regions, which is a crucial determinant of seasonal transitions, appears to be regulated by hormone secretion from the pars tuberalis of the pituitary gland. This proposed neuroendocrine pathway contradicts the common view of the pituitary as a gland that acts downstream of the hypothalamus. In the present overview of (neuro)endocrine mechanisms underlying seasonal acclimatization, we are focusing on the dwarf hamster Phodopus sungorus (long-day breeder) that is known for large amplitudes in seasonal changes. However, important findings in other mammalian species such as Syrian hamsters and sheep (short-day breeder) are considered as well.

Relationship between polysialylated neural cell adhesion molecule and beta-endorphin- or gonadotropin releasing hormone-containing neurons during activation of the gonadotrope axis in short daylength in the ewe

Morphological plasticity has been demonstrated between breeding and anestrous seasons in the ewe hypothalamus, particularly for the gonadotropin-releasing hormone (GnRH) system. We sought to determine the impact of a photoperiodic transition, from long days (LD, 16 h light/24 h) to short days (SD; 8 h light/24 h), on the association between a marker of cerebral plasticity, the polysialylated form of neural cell adhesion molecule (PSA-NCAM), and two diencephalic populations: the GnRH and beta-endorphin (beta-END) neurons, the latter being potent inhibitors of GnRH neuronal activity. We also estimated the number of contacts on GnRH neurons after the passage to SD, using synaptophysin as a marker for synaptic buttons. Those parameters were evaluated in ovariectomized estradiol-replaced ewes using double immunocytochemistry and confocal microscopy at different times after the transition to SD: day 0 (D0), D30, D45, D60 and D112. Luteinizing hormone (LH) secretion was recorded throughout the experiment. High LH levels were observed only at D112. Significantly more PSA-NCAM was found in the GnRH neuron perimeters in the D112 group than in the other groups. This increase was not associated with any change in the number of synaptophysin-immunoreactive contacts on GnRH neurons. The beta-END peri-neuronal space was affected negatively by the transition to SD: the percentage of PSA-NCAM on beta-END neurons decreased between D45 and D112 in the posterior two thirds of the arcuate nucleus (ARC). These results suggest that photoperiod may reorganize cell interactions in different hypothalamic areas, ultimately reactivating GnRH neurons, in our model of ovariectomized-estradiol replaced ewes.

Independent changes of thyroid hormones in blood plasma and cerebrospinal fluid after melatonin treatment in ewes

The authors measured the effects of exogenous melatonin treatment on the concentrations of total (T) and free (f) fractions of thyroxine (T4) and triiodothyronine (T3) in cerebrospinal fluid (CSF) and blood plasma as well as the expression of their binding/transporter protein, transthyretin (TTR), in the choroid plexus of ewes from May to August. Melatonin implantation in May and July mainly prevented the decrease in plasma for fT3 and TT3 exhibited in untreated group, and induced a limited decrease in TT4 in June. By contrast, melatonin implantation prevented the decrease in CSF fT3 observed in the untreated group. No effect of melatonin was found on the expression of TTR mRNA in the choroid plexus There were a correlations between blood fT4 and CSF TT4 concentrations in both control and melatonin treated group (r(2)-0.4; P < 0.01 vs. r(2)-0.14; P < 0.05), as well as between blood fT3 and CSF TT3 concentrations but only in the melatonin-treated group (r(2)-0.26; P < 0.02). We conclude that T3, the active form of the hormone within the brain, is regulated by melatonin independently of the peripheral changes within the blood. The lack of correlation between plasma fT3 and CSF TT3 in the control group suggests that an increase in local T3 conversion could contribute as an additional source of T3 in the CSF during the period of increasing day length. These data seem to confirm a local nature for recently discovered connections between the pineal melatonin signal and thyroid-dependent seasonal biology in mammals.

Seasonal regulation of reproduction: altered role of melatonin under naturalistic conditions in hamsters

The seasonal reproductive cycle of photoperiodic rodents is conceptualized as a series of discrete melatonin-dependent neuroendocrine transitions. Least understood is the springtime restoration of responsiveness to winter-like melatonin signals (breaking of refractoriness) that enables animals to once again respond appropriately to winter photoperiods the following year. This has been posited to require many weeks of long days based on studies employing static photoperiods instead of the annual pattern of continually changing photoperiods under which these mechanisms evolved. Maintaining Siberian hamsters under simulated natural photoperiods, we demonstrate that winter refractoriness is broken within six weeks after the spring equinox. We then test whether a history of natural photoperiod exposure can eliminate the requirement for long-day melatonin signalling. Hamsters pinealectomized at the spring equinox and challenged 10 weeks later with winter melatonin infusions exhibited gonadal regression, indicating that refractoriness was broken. A photostimulatory effect on body weight is first observed in the last four weeks of winter. Thus, the seasonal transition to the summer photosensitive phenotype is triggered prior to the equinox without exposure to long days and is thereafter melatonin-independent. Distinctions between photoperiodic and circannual seasonal organization erode with the incorporation in the laboratory of ecologically relevant day length conditions.

MT1 melatonin receptors mediate somatic, behavioral, and reproductive neuroendocrine responses to photoperiod and melatonin in Siberian hamsters (Phodopus sungorus)

Environmental day length drives nocturnal pineal melatonin secretion, which in turn generates or entrains seasonal cycles of physiology, reproduction, and behavior. In mammals, melatonin (MEL) binds to a number of receptor subtypes including high-affinity (MT1 and MT2) and low-affinity (MT3, nuclear orphan receptors) binding sites, which are distributed throughout the central nervous system and periphery. The MEL receptors that mediate photoperiodic reproductive and behavioral responses to MEL have not been identified in a reproductively photoperiodic species. Here I tested the hypothesis that MT1 receptors are necessary and sufficient to engage photoperiodic responses by challenging male Siberian hamsters (Phodopus sungorus), a species that does not express functional MT2 receptors, with ramelteon (RAM), a specific MT1/MT2 receptor agonist. In hamsters housed in a long-day photoperiod, late-afternoon RAM treatment inhibited gonadotropin secretion, induced gonadal regression, and suppressed food intake and body mass, mimicking effects of MEL. In addition, chronic (24 h/d) RAM infusions were sufficient to obscure endogenous MEL signaling, and these treatments attenuated gonadal regression in short days. Together, the outcomes indicate that signaling at the MT1 receptor is sufficient and necessary to mediate the effects of photoperiod-driven changes in MEL on behavior and reproductive function in a reproductively photoperiodic mammal.

Current knowledge on the photoneuroendocrine regulation of reproduction in temperate fish species

Seasonality is an important adaptive trait in temperate fish species as it entrains or regulates most physiological events such as reproductive cycle, growth profile, locomotor activity and key life-stage transitions. Photoperiod is undoubtedly one of the most predictable environmental signals that can be used by most living organisms including fishes in temperate areas. This said, however, understanding of how such a simple signal can dictate the time of gonadal recruitment and spawning, for example, is a complex task. Over the past few decades, many scientists attempted to unravel the roots of photoperiodic signalling in teleosts by investigating the role of melatonin in reproduction, but without great success. In fact, the hormone melatonin is recognized as the biological time-keeping hormone in fishes mainly due to the fact that it reflects the seasonal variation in daylength across the whole animal kingdom rather than the existence of direct evidences of its role in the entrainment of reproduction in fishes. Recently, however, some new studies clearly suggested that melatonin interacts with the reproductive cascade at a number of key steps such as through the dopaminergic system in the brain or the synchronization of the final oocyte maturation in the gonad. Interestingly, in the past few years, additional pathways have become apparent in the search for a fish photoneuroendocrine system including the clock-gene network and kisspeptin signalling and although research on these topics are still in their infancy, it is moving at great pace. This review thus aims to bring together the current knowledge on the photic control of reproduction mainly focusing on seasonal temperate fish species and shape the current working hypotheses supported by recent findings obtained in teleosts or based on knowledge gathered in mammalian and avian species. Four of the main potential regulatory systems (light perception, melatonin, clock genes and kisspeptin) in fish reproduction are reviewed.

Photoperiodic control of TSH-beta expression in the mammalian pars tuberalis has different impacts on the induction and suppression of the hypothalamo-hypopysial gonadal axis

Seasonal reproduction depends on photoperiod-regulated activation or suppression of the gonadal axis. Recent studies in quail have identified long-day induced TSH-beta expression in the pars tuberalis (PT) as a rapid trigger of gonadal activation. Thyroid-stimulating hormone (TSH) induces type 2 deiodinase (Dio2) in the ependymal cell layer (EC) of the infundibular recess to stimulate the gonadal axis. A similar mechanism is proposed in sheep and mice, but the experimental data on the temporal patterns of induction and suppression of TSH-beta and Dio2 expression are incomplete. In the present study, we examined the expression of TSH-beta and Dio2 in hamsters transferred from short- to long-day conditions for 9 days, and demonstrate the induction of TSH-beta and Dio2 on day 8 after transition. These data demonstrate the close relationship between TSH-beta and Dio2 expression in the inductive pathway. The temporal expression of TSH-beta and Dio2 in the suppressive pathway was also examined by s.c. melatonin injection, which mimics the transition from long to short days. Importantly, Dio2 expression in the EC is suppressed on day 1 after the onset of injection, whereas TSH-beta expression in the PT was not suppressed until day 10. These data suggest that regulated transcription of TSH-beta is involved in the induction of the gonadal axis in mammals, whereas the suppression of this axis is mediated by different mechanisms.

Photoperiodic regulation of retinoic acid signaling in the hypothalamus

Both retinoic acid (RA) and thyroid hormone (TH) regulate transcription via specific nuclear receptors. TH regulates hypothalamic homeostasis and active T3 is generated by deiodinase enzymes in tanycytes surrounding the third ventricle. However, RA has not been previously considered in such a role. Data presented here highlights novel parallels between the TH and RA synthetic pathways in the hypothalamus implying that RA also acts to regulate hypothalamic gene expression and function. Key elements of the RA cellular signaling pathway were shown to be regulated in the rodent hypothalamus. Retinoid synthetic enzymes and the retinol transport protein Stra6 were located in the cells lining the third ventricle allowing synthesis of RA from retinol present in the CNS to act via RA receptors and retinoid X receptors in the hypothalamus. Photoperiod manipulation was shown to alter the expression of synthetic enzymes and receptors with lengthening of photoperiod leading to enhanced RA signaling. In vitro RA can regulate the hypothalamic neuroendocrine peptide adrenocorticotrophic hormone. This work presents the new concept of controlled RA synthesis by hypothalamic tanycytes giving rise to possible involvement of this system in endocrine, and possibly vitamin A, homeostasis.

Impact of melatonin and molecular clockwork components on the expression of thyrotropin beta-chain (Tshb) and the Tsh receptor in the mouse pars tuberalis

Photoperiodic regulation of reproduction in birds and mammals involves thyrotropin beta-chain (TSHb), which is secreted from the pars tuberalis (PT) and controls the expression of deiodinase type 2 and 3 in the ependymal cell layer of the infundibular recess (EC) via TSH receptors (TSHRs). To analyze the impact of melatonin and the molecular clockwork on the expression of Tshb and Tshr, we investigated melatonin-proficient C3H wild-type (WT), melatonin receptor 1-deficient (MT1-/-) or clockprotein PERIOD1-deficient (mPER1-/-) mice. Expression of Tshb and TSHb immunoreactivity in PT were low during day and high during the night in WT, high during the day and low during the night in mPER1-deficient, and equally high during the day and night in MT1-deficient mice. Melatonin injections into WT acutely suppressed Tshb expression. Transcription assays showed that the 5' upstream region of the Tshb gene could be controlled by clockproteins. Tshr levels in PT were low during the day and high during the night in WT and mPER1-deficient mice and equally low in MT1-deficient mice. Tshr expression in the EC did not show a day/night variation. Melatonin injections into WT acutely induced Tshr expression in PT but not in EC. TSH stimulation of hypothalamic slice cultures of WT induced phosphorylated cAMP response element-binding protein in PT and EC and deiodinase type 2 in the EC. Our data suggest that Tshb expression in PT is controlled by melatonin and the molecular clockwork and that melatonin activates Tshr expression in PT but not in EC. They also confirm the functional importance of TSHR in the PT and EC.

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.

KiSS‐1: A Likely Candidate for the Photoperiodic Control of Reproduction in Seasonal Breeders

In seasonal species, photoperiod exerts tight regulation of reproduction to ensure that birth occurs at the most favorable time of yr. A distinct photoneuroendocrine circuit composed of the retina, suprachiasmatic nucleus (SCN) of the hypothalamus, and pineal gland transduces daylength into a rhythmic secretion of melatonin. The duration of the night-time rise of this hormone conveys daylength information to the organism. Melatonin is known to mediate the control of seasonal reproduction, but how it modulates sexual activity is far from understood. Recent data indicate that the product of the KiSS-1 gene is a potent stimulator of the hypothalamic-pituitary-gonadal axis and may play, together with its receptor GPR54, a central role in the neuroendocrine regulation of gonadotropin secretion. This article briefly reviews these findings and presents arguments that KiSS-1 could take part in the seasonal control of reproduction.

Melatonin transmits photoperiodic signals through the MT1 melatonin receptor

Melatonin transmits photoperiodic signals that regulate reproduction. Two melatonin receptors (MT1 and MT2) have been cloned in mammals and additional melatonin binding sites suggested, but the receptor that mediates the effects of melatonin on the photoperiodic gonadal response has not yet been identified. We therefore investigated in mice whether and how targeted disruption of MT1, MT2, or both receptor types affects the expression level of two key genes for the photoperiodic gonadal regulation, type 2 and 3 deiodinase (Dio2 and Dio3, respectively). These are expressed in the ependymal cell layer lining the infundibular recess of the third ventricle and regulated by thyrotropin produced in the pars tuberalis. In wild-type C3H mice, Dio2 expression was constantly low, and no photoperiodic changes were observed, whereas Dio3 expression was upregulated under short-day conditions. In C3H with targeted disruption of MT1 and MT1/MT2, Dio2 expression was constitutively upregulated, Dio3 expression was constitutively downregulated, and the photoperiodic effect on Dio3 expression was abolished. Under short-day conditions, C3H with targeted disruption of MT2 displayed similar expression levels of Dio2 and Dio3 as wild-type animals, but they responded to long-day condition with a stronger suppression of Dio3 than wild-type mice. Melatonin injections into wild-type C57BL mice suppressed Dio2 expression and induced Dio3 expression under long-day conditions. These effects were abolished in C57BL mice with targeted disruption of MT1. All data suggest that the melatonin signal that transmits photoperiodic information to the hypothalamo-hypophysial axis acts on the MT1 receptor.

Kisspeptin and the seasonal control of reproduction in hamsters

Reproduction is a complex and energy demanding function. When internal and external conditions might impair reproductive success (negative energy balance, stress, harsh season) reproductive activity has to be repressed. Recent evidence suggests that these inhibitory mechanisms operate on Kiss1-expressing neurons, which were recently shown to be implicated in the regulation of GnRH release. Hamsters are seasonal rodents which are sexually active in long photoperiod and quiescent in short photoperiod. The photoperiodic information is transmitted to the reproductive system by melatonin, a pineal hormone whose secretion is adjusted to night length. The photoperiodic variation in circulating melatonin has been shown to synchronize reproductive activity with seasons, but the mechanisms involved in this effect of melatonin were so far unknown. Recently we have observed that Kiss1 mRNA level in the arcuate nucleus of the Syrian hamster is lower in short photoperiod, when animals are sexually quiescent. Notably, intracerebroventricular infusion of Kiss1 gene product, kisspeptin, in hamsters kept in short photoperiod is able to override the inhibitory photoperiod and to reactivate sexual activity. The inhibition of Kiss1 expression in short photoperiod is driven by melatonin because pinealectomy prevents decrease in Kiss1 mRNA level in short photoperiod and melatonin injection in long photoperiod down regulates Kiss1 expression. Whether melatonin acts directly on arcuate Kiss1 expressing neurons or mediates its action via interneurons is the subject of the current investigations.

Cellular and molecular basis of deiodinase-regulated thyroid hormone signaling

The iodothyronine deiodinases initiate or terminate thyroid hormone action and therefore are critical for the biological effects mediated by thyroid hormone. Over the years, research has focused on their role in preserving serum levels of the biologically active molecule T(3) during iodine deficiency. More recently, a fascinating new role of these enzymes has been unveiled. The activating deiodinase (D2) and the inactivating deiodinase (D3) can locally increase or decrease thyroid hormone signaling in a tissue- and temporal-specific fashion, independent of changes in thyroid hormone serum concentrations. This mechanism is particularly relevant because deiodinase expression can be modulated by a wide variety of endogenous signaling molecules such as sonic hedgehog, nuclear factor-kappaB, growth factors, bile acids, hypoxia-inducible factor-1alpha, as well as a growing number of xenobiotic substances. In light of these findings, it seems clear that deiodinases play a much broader role than once thought, with great ramifications for the control of thyroid hormone signaling during vertebrate development and metamorphosis, as well as injury response, tissue repair, hypothalamic function, and energy homeostasis in adults.

Rhythmic expression of clock genes in the ependymal cell layer of the third ventricle of rodents is independent of melatonin signaling

Reproductive physiology is regulated by the photoperiod in many mammals. Decoding of the photoperiod involves circadian clock mechanisms, although the molecular basis remains unclear. Recent studies have shown that the ependymal cell layer lining the infundibular recess of the third ventricle (EC) is a key structure for the photoperiodic gonadal response. The EC exhibits daylength-dependent changes in the expression of photoperiodic output genes, including the type 2 deiodinase gene (Dio2 ). Here we investigated whether clock genes (Per1 and Bmal1) and the albumin D-binding protein gene (Dbp) are expressed in the EC of Syrian hamsters, and whether their expression differs under long-day and short-day conditions. Expression of all three genes followed a diurnal rhythm; expression of Per1 and Dbp in the EC peaked around lights-off, and expression of Bmal1 peaked in the early light phase. The amplitude of Per1 and Dbp expression was higher in hamsters kept under long-day conditions than in those kept under short-day conditions. Notably, the expression of these genes was not modified by exogenous melatonin within 25 h after injection, whereas Dio2 expression was inhibited 19 h after injection. Targeted melatonin receptor (MT1, MT2, and both MT1 and MT2) disruption in melatonin-proficient C3H mice did not affect the rhythmic expression of Per1 in the EC. These data show the existence of a molecular clock in the rodent EC. In the hamster, this clock responds to long-term changes in the photoperiod, but is independent of acute melatonin signals. In mice, the EC clock is not affected by deletion of melatonin receptors.

Involvement of thyrotropin in photoperiodic signal transduction in mice

Local thyroid hormone catabolism within the mediobasal hypothalamus (MBH) by thyroid hormone-activating (DIO2) and -inactivating (DIO3) enzymes regulates seasonal reproduction in birds and mammals. Recent functional genomics analysis in birds has shown that long days induce thyroid-stimulating hormone production in the pars tuberalis (PT) of the pituitary gland, which triggers DIO2 expression in the ependymal cells (EC) of the MBH. In mammals, nocturnal melatonin secretion provides an endocrine signal of the photoperiod to the PT that contains melatonin receptors in high density, but the interface between the melatonin signal perceived in the PT and the thyroid hormone levels in the MBH remains unclear. Here we provide evidence in mice that TSH participates in this photoperiodic signal transduction. Although most mouse strains are considered to be nonseasonal, a robust photoperiodic response comprising induced expression of TSHB (TSH beta subunit), CGA (TSH alpha subunit), and DIO2, and reduced expression of DIO3, was observed in melatonin-proficient CBA/N mice. These responses could not be elicited in melatonin-deficient C57BL/6J, but treatment of C57BL/6J mice with exogenous melatonin elicited similar effects on the expression of the above-mentioned genes as observed in CBA/N after transfer to short-day conditions. The EC was found to express TSH receptor (TSHR), and ICV injection of TSH induced DIO2 expression. Finally, we show that melatonin administration did not affect the expression of TSHB, DIO2, and DIO3 in TSHR-null mice. Taken together, our findings suggest that melatonin-dependent regulation of thyroid hormone levels in the MBH appears to involve TSH in mammals.

Ancestral TSH mechanism signals summer in a photoperiodic mammal

In mammals, day-length-sensitive (photoperiodic) seasonal breeding cycles depend on the pineal hormone melatonin, which modulates secretion of reproductive hormones by the anterior pituitary gland [1]. It is thought that melatonin acts in the hypothalamus to control reproduction through the release of neurosecretory signals into the pituitary portal blood supply, where they act on pituitary endocrine cells [2]. Contrastingly, we show here that during the reproductive response of Soay sheep exposed to summer day lengths, the reverse applies: Melatonin acts directly on anterior-pituitary cells, and these then relay the photoperiodic message back into the hypothalamus to control neuroendocrine output. The switch to long days causes melatonin-responsive cells in the pars tuberalis (PT) of the anterior pituitary to increase production of thyrotrophin (TSH). This acts locally on TSH-receptor-expressing cells in the adjacent mediobasal hypothalamus, leading to increased expression of type II thyroid hormone deiodinase (DIO2). DIO2 initiates the summer response by increasing hypothalamic tri-iodothyronine (T3) levels. These data and recent findings in quail [3] indicate that the TSH-expressing cells of the PT play an ancestral role in seasonal reproductive control in vertebrates. In mammals this provides the missing link between the pineal melatonin signal and thyroid-dependent seasonal biology.

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.

Comparative endocrinology, environment and global change

All organisms respond to environmental cues that allow them to organize the timing and duration of life history stages that make up their life cycles. Superimposed on this predictable life cycle are unpredictable events that have the potential to be stressful. Environmental and social stresses have deleterious effects on life history stages such as migration, reproductive function and molt in vertebrates. Global climate change, human disturbance and endocrine disruption from pollutants are increasingly likely to pose additional stresses that could have a major impact on organisms. Such impacts have great relevance to conservation as well as basic biology. Although some populations of vertebrates temporarily resist environmental and social stresses, and breed successfully, many show varying decrees of failure sometimes resulting in marked population decline. Alternatively, many aspects of global change may not be overtly stressful but timing of life history events becomes out of step with phenology because pertinent environmental signals normally used have been changed. There is much we do not know about how organisms respond to their natural environment, particularly how salient signals are perceived and then transduced into neuroendocrine and endocrine secretions. Comparative endocrinology has a key role to play in resolving mechanisms underlying responses to the environment. In the face of increasing human disturbance and global climate change there is an urgent need for more integration of ecological, evolutionary and mechanistic studies on stress biology of organisms in their natural world.

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.

Sexually dimorphic gene expression in the brains of mature zebrafish

The molecular signalling pathways mediating sexual dimorphism have principally been investigated in the gonads, and to a lesser extent in other organs. The brain plays a central role in coordinating sexual function, including the regulation of reproductive development, maturation and sexual behaviour in both sexes. In this study, we investigated sex-related differences in gene expression in the brains of breeding zebrafish (Danio rerio) to establish a greater understanding of the sex-specific physiology of the brain in lower vertebrates. The brain transcriptomic profiles of males and females were interrogated to identify the genes showing sexually dimorphic gene expression. 42 genes were differentially expressed between the sexes, from which 18 genes were over-expressed in males and 24 genes were over-expressed in females. In males, these included deiodinase, iodothyronine, type II and ribosomal protein S8, and in females, superoxide dismutase [Cu-Zn], sprouty-4, frizzled 10 and testis enhanced gene transcript. Estrogen responsive elements were found in the regulatory regions for 3 genes over-expressed in males and 7 genes over-expressed in females. We have demonstrated the existence of dimorphic patterns of gene expression in the brain of a sexually mature, non-mammalian, vertebrate model, with implications for studies into reproduction and chemical disruption of brain function.

Hypothalamic thyroid hormone in energy balance regulation

Thyroid hormone has been known for decades as a hormone with profound effects on energy expenditure and ability to control weight. The regulation of energy expenditure by thyroid hormone primarily occurs via regulation of the activity, or expression, of uncoupling proteins in peripheral tissues. However, mechanistically this requires a signal from the brain to change circulating levels of thyroxine and thyroid hormone or increased sympathetic drive to peripheral tissues to alter local thyroid hormone levels via increased expression of type 2 deiodinase. However, little consideration has been given to the potential role and involvement of thyroid hormones action in the brain in the regulation of energy balance. Recent evidence implicates thyroid hormone as a shortterm signal of energy deficit imposed by starvation. Furthermore, thyroid hormone action within the hypothalamus is involved in adjusting long-term energy expenditure in seasonal animals which endure food shortages in winter. Evidence from several studies suggests that regulation of type 2 and type 3 deiodinase enzymes in tanycytes of the third ventricle are gatekeepers of thyroid hormone levels in the hypothalamus. This paper reviews some of the evidence for the role of deiodinase enzymes and the actions of thyroid hormone in the hypothalamus in the regulation of energy balance.

Temporal dynamics of type 2 deiodinase expression after melatonin injections in Syrian hamsters

In many species living in temperate zones, reproduction is controlled by the photoperiod. Recent findings have clarified that type 2 iodothyronine deiodinase (Dio2) plays a significant role in the photoperiodic response of gonads in the mediobasal hypothalamus, converting the prohormone T(4) into bioactive T(3). In mammals, Dio2 expression is suppressed by long-term melatonin injections, although the signal transduction pathways that link the melatonin signal to Dio2 expression are unknown. As a first step to approach the problem, we have here investigated the temporal dynamics of the melatonin effect on Dio2 expression using male Syrian hamsters. Dio2 mRNA levels were found to show diurnal rhythms under long-day conditions in an area adjacent to the tuberoinfundibular sulcus and in the ependymal cell layer lining the ventrobasal walls of the third ventricle. Daily sc melatonin injections given in the late afternoon under long-day condition suppressed the Dio2 mRNA levels already at the first day after the onset of the treatment in the ependymal cell layer lining the ventrobasal walls of the third ventricle, and 1 d later in an area adjacent to the tuberoinfundibular sulcus. These suppressive effects were sustained for at least 2 d after a single injection. Furthermore, we examined the temporal changes of the Dio2 expression after the onset of the treatment, showing that the suppression did not occur until midday of the next day. These data suggest that melatonin is involved in the signal transduction mechanisms controlling the photoperiodic response of gonads by acting on Dio2 expression rather rapidly through indirect pathways.

Hypothalamic thyroid hormone catabolism acts as a gatekeeper for the seasonal control of body weight and reproduction

Seasonal adaptations in physiology exhibited by many animals involve an interface between biological timing and specific neuroendocrine systems, but the molecular basis of this interface is unknown. In this study of Siberian hamsters, we show that the availability of thyroid hormone within the hypothalamus is a key determinant of seasonal transitions. The expression of the gene encoding type III deiodinase (Dio3) and Dio3 activity in vivo (catabolism of T(4) and T(3)) is dynamically and temporally regulated by photoperiod, consistent with the loss of hypothalamic T(3) concentrations under short photoperiods. Chronic replacement of T(3) in the hypothalamus of male hamsters exposed to short photoperiods, thus bypassing synthetic or catabolic deiodinase enzymes located in cells of the ependyma of the third ventricle, prevented the onset of short-day physiology: hamsters maintained a long-day body weight phenotype and failed to undergo testicular and epididymal regression. However, pelage moult to a winter coat was not affected. Type II deiodinase gene expression was not regulated by photoperiod in these hamsters. Collectively, these data point to a pivotal role for hypothalamic DIO3 and T(3) catabolism in seasonal cycles of body weight and reproduction in mammals.