Thyrotrophin in the pars tuberalis triggers photoperiodic response

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.

Evidence for RGS4 modulation of melatonin and thyrotrophin signalling pathways in the pars tuberalis

In mammals, the pineal hormone melatonin is secreted nocturnally and acts in the pars tuberalis (PT) of the anterior pituitary to control seasonal neuroendocrine function. Melatonin signals through the type 1 Gi-protein coupled melatonin receptor (MT1), inhibiting adenylate cyclase (AC) activity and thereby reducing intracellular concentrations of the second messenger, cAMP. Because melatonin action ceases by the end of the night, this allows a daily rise in cAMP levels, which plays a key part in the photoperiodic response mechanism in the PT. In addition, melatonin receptor desensitisation and sensitisation of AC by melatonin itself appear to fine-tune this process. Opposing the actions of melatonin, thyroid-stimulating hormone (TSH), produced by PT cells, signals through its cognate Gs-protein coupled receptor (TSH-R), leading to increased cAMP production. This effect may contribute to increased TSH production by the PT during spring and summer, and is of considerable interest because TSH plays a pivotal role in seasonal neuroendocrine function. Because cAMP stands at the crossroads between melatonin and TSH signalling pathways, any protein modulating cAMP production has the potential to impact on photoperiodic readout. In the present study, we show that the regulator of G-protein signalling RGS4 is a melatonin-responsive gene, whose expression in the PT increases some 2.5-fold after melatonin treatment. Correspondingly, RGS4 expression is acutely sensitive to changing day length. In sheep acclimated to short days (SP, 8 h light/day), RGS4 expression increases sharply following dark onset, peaking in the middle of the night before declining to basal levels by dawn. Extending the day length to 16 h (LP) by an acute 8-h delay in lights off causes a corresponding delay in the evening rise of RGS4 expression, and the return to basal levels is delayed some 4 h into the next morning. To test the hypothesis that RGS4 expression modulates interactions between melatonin- and TSH-dependent cAMP signalling pathways, we used transient transfections of MT1, TSH-R and RGS4 in COS7 cells along with a cAMP-response element luciferase reporter (CRE-luc). RGS4 attenuated MT1-mediated inhibition of TSH-stimulated CRE-luc activation. We propose that RGS4 contributes to photoperiodic sensitivity in the morning induction of cAMP-dependent gene expression in the PT.

Thyroid hormone signalling genes are regulated by photoperiod in the hypothalamus of F344 rats

Seasonal animals adapt their physiology and behaviour in anticipation of climate change to optimise survival of their offspring. Intra-hypothalamic thyroid hormone signalling plays an important role in seasonal responses in mammals and birds. In the F344 rat, photoperiod stimulates profound changes in food intake, body weight and reproductive status. Previous investigations of the F344 rat have suggested a role for thyroid hormone metabolism, but have only considered Dio2 expression, which was elevated in long day photoperiods. Microarray analysis was used to identify time-dependent changes in photoperiod responsive genes, which may underlie the photoperiod-dependent phenotypes of the juvenile F344 rat. The most significant changes are those related to thyroid hormone metabolism and transport. Using photoperiod manipulations and melatonin injections into long day photoperiod (LD) rats to mimic short day (SD), we show photoinduction and photosuppression gene expression profiles and melatonin responsiveness of genes by in situ hybridization; TSHβ, CGA, Dio2 and Oatp1c1 genes were all elevated in LD whilst in SD, Dio3 and MCT-8 mRNA were increased. NPY was elevated in SD whilst GALP increased in LD. The photoinduction and photosuppression profiles for GALP were compared to that of GHRH with GALP expression following GHRH temporally. We also reveal gene sets involved in photoperiodic responses, including retinoic acid and Wnt/ß-catenin signalling. This study extends our knowledge of hypothalamic regulation by photoperiod, by revealing large temporal changes in expression of thyroid hormone signalling genes following photoperiod switch. Surprisingly, large changes in hypothalamic thyroid hormone levels or TRH expression were not detected. Expression of NPY and GALP, two genes known to regulate GHRH, were also changed by photoperiod. Whether these genes could provide links between thyroid hormone signalling and the regulation of the growth axis remains to be investigated.

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.

Thyroid hormone action in cerebellum and cerebral cortex development

Thyroid hormones (TH, including the prohormone thyroxine (T4) and its active deiodinated derivative 3,3′,5-triiodo-L-thyronine (T3)) are important regulators of vertebrates neurodevelopment. Specific transporters and deiodinases are required to ensure T3 access to the developing brain. T3 activates a number of differentiation processes in neuronal and glial cell types by binding to nuclear receptors, acting directly on transcription. Only few T3 target genes are currently known. Deeper investigations are urgently needed, considering that some chemicals present in food are believed to interfere with T3 signaling with putative neurotoxic consequences.

G protein-coupled receptors: mutations and endocrine diseases

Over the past 20 years, naturally occurring mutations that affect G protein-coupled receptors (GPCRs) have been identified, mainly in patients with endocrine diseases. The study of loss-of-function or gain-of-function mutations has contributed to our understanding of the pathophysiology of several diseases with classic hypophenotypes or hyperphenotypes of the target endocrine organs, respectively. Simultaneously, study of the mutant receptors ex vivo was instrumental in delineating the relationships between the structure and function of these important physiological and pharmacological molecules. Now that access to the crystallographic structure of a few GPCRs is available, the mechanics of these receptors can be studied at the atomic level. Progress in the fields of cell biology, molecular pharmacology and proteomics has also widened our view of GPCR functions. Initially considered simply as guanine nucleotide exchange factors capable of activating G protein-dependent regulatory cascades, GPCRs are now known to display several additional characteristics, each susceptible to alterations by disease-causing mutations. These characteristics include functionally important basal activity of the receptor; differential activation of various G proteins; differential activation of G protein-dependent and independent effects (biased agonism); interaction with proteins that modify receptor function; dimerization-dependent effects; and interaction with allosteric modulators. This Review attempts to illustrate how natural mutations of GPCR could contribute to our understanding of these novel facets of GPCR biology.

The central effects of thyroid hormones on appetite

Obesity is a major public health issue worldwide. Current pharmacological treatments are largely unsuccessful. Determining the complex pathways that regulate food intake may aid the development of new treatments. The hypothalamic-pituitary-thyroid (HPT) axis has well-known effects on energy expenditure, but its role in the regulation of food intake is less well characterised. Evidence suggests that the HPT axis can directly influence food intake. Thyroid dysfunction can have clinically significant consequences on appetite and body weight. Classically, these effects were thought to be mediated by the peripheral effects of thyroid hormone. However, more recently, local regulation of thyroid hormone in the central nervous system (CNS) is thought to play an important role in physiologically regulating appetite. This paper focuses on the role of the HPT and thyroid hormone in appetite and provides evidence for potential new targets for anti-obesity agents.

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.

Extrathyroidal expression of TSH receptor

The TSH receptor expressed on the cell surface of thyroid follicular cells plays a pivotal role in the regulation of thyroid status and growth of the thyroid gland. In recent years it has become evident that the TSH receptor is also expressed widely in a variety of extrathyroidal tissues including: anterior pituitary; hypothalamus; ovary; testis; skin; kidney; immune system; bone marrow and peripheral blood cells; white and brown adipose tissue; orbital preadipocyte fibroblasts and bone. A large body of evidence is emerging to describe the functional roles of the TSH receptor at these various sites but their physiological importance in many cases remains a subject of controversy and much interest. Current understanding of the actions of the TSH receptor in extrathyroidal tissues and their possible physiological implications is discussed.

Historical view of development of comparative endocrinology in Japan

This article describing a brief history of development of comparative endocrinology in Japan is contributed to the journal General and Comparative Endocrinology, in commemoration of the 50th anniversary of its publication. It covers significant works in the field of comparative endocrinology that have been done by Japanese endocrinologists, focusing those achieved during the past 70 years. The contents were arranged according to the taxonomical order of the experimental animals with which individual researchers or research groups have contributed to the acquisition of important knowledge in comparative endocrinology.

Neuronal plasticity and seasonal reproduction in sheep

Seasonal reproduction represents a naturally occurring example of functional plasticity in the adult brain as it reflects changes in neuroendocrine pathways controlling gonadotropin-releasing hormone (GnRH) secretion and, in particular, the responsiveness of GnRH neurons to estradiol negative feedback. Structural plasticity within this neural circuitry may, in part, be responsible for seasonal switches in the negative feedback control of GnRH secretion that underlie annual reproductive transitions. We review evidence for structural changes in the circuitry responsible for seasonal inhibition of GnRH secretion in sheep. These include changes in synaptic inputs onto GnRH neurons, as well as onto dopamine neurons in the A15 cell group, a nucleus that plays a key role in estradiol negative feedback. We also present preliminary data suggesting a role for neurotrophins and neurotrophin receptors as an early mechanistic step in the plasticity that accompanies seasonal reproductive transitions in sheep. Finally, we review recent evidence suggesting that kisspeptin cells of the arcuate nucleus constitute a critical intermediary in the control of seasonal reproduction. Although a majority of the data for a role of neuronal plasticity in seasonal reproduction has come from the sheep model, the players and principles are likely to have relevance for reproduction in a wide variety of vertebrates, including humans, and in both health and disease.

Encoding and decoding photoperiod in the mammalian pars tuberalis

In mammals, the nocturnal melatonin signal is well established as a key hormonal indicator of seasonal changes in day-length, providing the brain with an internal representation of the external photoperiod. The pars tuberalis (PT) of the pituitary gland is the major site of expression of the G-coupled receptor MT1 in the brain and is considered as the main site of integration of the photoperiodic melatonin signal. Recent studies have revealed how the photoperiodic melatonin signal is encoded and conveyed by the PT to the brain and the pituitary, but much remains to be resolved. The development of new animal models and techniques such as cDNA arrays or high throughput sequencing has recently shed the light onto the regulatory networks that might be involved. This review considers the current understanding of the mechanisms driving photoperiodism in the mammalian PT with a particular focus on the seasonal prolactin secretion.

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.

A molecular switch for photoperiod responsiveness in mammals

Seasonal synchronization based on day length (photoperiod) allows organisms to anticipate environmental change. Photoperiodic decoding relies on circadian clocks, but the underlying molecular pathways have remained elusive [1]. In mammals and birds, photoperiodic responses depend crucially on expression of thyrotrophin β subunit RNA (TSHβ) in the pars tuberalis (PT) of the pituitary gland [2-4]. Now, using our well-characterized Soay sheep model [2], we describe a molecular switch governing TSHβ transcription through the circadian clock. Central to this is a conserved D element in the TSHβ promoter, controlled by the circadian transcription factor thyrotroph embryonic factor (Tef). In the PT, long-day exposure rapidly induces expression of the coactivator eyes absent 3 (Eya3), which synergizes with Tef to maximize TSHβ transcription. The pineal hormone melatonin, secreted nocturnally, sets the phase of rhythmic Eya3 expression in the PT to peak 12 hr after nightfall. Additionally, nocturnal melatonin levels directly suppress Eya3 expression. Together, these effects form a switch triggering a strong morning peak of Eya3 expression under long days. Species variability in the TSHβ D element influences sensitivity to TEF, reflecting species variability in photoperiodic responsiveness. Our findings define a molecular pathway linking the circadian clock to the evolution of seasonal timing in mammals.

Vertebrate ancient opsin and melanopsin: divergent irradiance detectors

Both vertebrates and invertebrates respond to light by utilising a wide-ranging array of photosensory systems, with diverse photoreceptor organs expressing a characteristic photopigment, itself consisting of an opsin apoprotein linked to a light-sensitive retinoid chromophore based on vitamin A. In the eye, the pigments expressed in both cone and rod photoreceptors have been studied in great depth and mediate contrast perception, measurement of the spectral composition of environmental light, and thus classical image forming vision. By contrast, the molecular basis for non-visual and extraocular photoreception is far less understood; however, two photopigment genes have become the focus of much study, the vertebrate ancient (va) opsin and melanopsin (opn4). In this review, we discuss the history of discovery for each gene, as well as focusing on the evolution, expression profile, functional role and broader physiological significance of each photopigment. Recently, it has been suggested independently by Arendt et al. and Lamb that an ancestral opsin bifurcated in early metazoans and evolved into two quite different photopigments, one expressed in rhabdomeric photoreceptors and the other in ciliary photoreceptors. This interpretation of the evolution of the metazoan eye has provided a powerful framework for understanding photobiological organization. Their proposal, however, does not encompass all current experimental observations that would be consistent with what we term a central "Evolution of Photosensory Opsins with Common Heredity (EPOCH)" hypothesis to explain the complexity of animal photosensory systems. Clearly, many opsin genes (e.g. va opsin) simply do not fit neatly within this scheme. Thus, the review concludes with a discussion of these anomalies and their context regarding the phylogeny of photoreceptor and photopigment development.

Effect of the photoperiod and administration of melatonin on the pars tuberalis of viscacha (Lagostomus maximus maximus): an ultrastructural study

The pituitary pars tuberalis (PT) is a glandular zone exhibiting well-defined structural characteristics. Morphologically, it is formed by specific secretory cells, folliculostellate cells, and migratory cells coming from the pars distalis. The purpose of this work was to investigate differences in specific cellular characteristics in the PT of viscachas captured in summer (long photoperiod) and winter (short photoperiod), as well as the effects of chronic melatonin administration in viscachas captured in summer and kept under long photoperiod. In summer, the PT-specific cells exhibited cell-like characteristics with an important secretory activity and a moderate amount of glycogen. In winter, the PT-specific granulated cells showed ultrastructural variations with signs of a reduced synthesis activity. Also, PT showed a high amount of glycogen and a great number of cells in degeneration. After melatonin administration, the ultrastructural characteristics were similar to those observed in winter, but the amount of glycogen was higher. These results suggest possible functional implications as a result of morphological differences between long and short photoperiods, and are in agreement with the variations of the pituitary-gonadal axis, probably in response to the natural photoperiod changes through the pineal melatonin. The ultrastructural differences observed in PT, after melatonin administration, were similar to those observed in the short photoperiod, thus supporting the hypothesis that these cytological changes are induced by melatonin.

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.

A mammalian neural tissue opsin (Opsin 5) is a deep brain photoreceptor in birds

It has been known for many decades that nonmammalian vertebrates detect light by deep brain photoreceptors that lie outside the retina and pineal organ to regulate seasonal cycle of reproduction. However, the identity of these photoreceptors has so far remained unclear. Here we report that Opsin 5 is a deep brain photoreceptive molecule in the quail brain. Expression analysis of members of the opsin superfamily identified as Opsin 5 (OPN5; also known as Gpr136, Neuropsin, PGR12, and TMEM13) mRNA in the paraventricular organ (PVO), an area long believed to be capable of phototransduction. Immunohistochemistry identified Opsin 5 in neurons that contact the cerebrospinal fluid in the PVO, as well as fibers extending to the external zone of the median eminence adjacent to the pars tuberalis of the pituitary gland, which translates photoperiodic information into neuroendocrine responses. Heterologous expression of Opsin 5 in Xenopus oocytes resulted in light-dependent activation of membrane currents, the action spectrum of which showed peak sensitivity (lambda(max)) at approximately 420 nm. We also found that short-wavelength light, i.e., between UV-B and blue light, induced photoperiodic responses in eye-patched, pinealectomized quail. Thus, Opsin 5 appears to be one of the deep brain photoreceptive molecules that regulates seasonal reproduction in birds.

Contribution of TNF-alpha and nuclear factor-kappaB signaling to type 2 iodothyronine deiodinase activation in the mediobasal hypothalamus after lipopolysaccharide administration

To determine whether signaling through TNF and/or nuclear factor-kappaB contributes to bacterial lipopolysaccharide (LPS)-induced activation of type 2 iodothyronine deiodinase (D2) in tanycytes lining the floor and infralateral walls of the third ventricle, the effect of a TNF antagonist on D2 gene expression and LPS-induced Ikappa-Balpha expression in tanycytes were studied. Animals treated with soluble, rat, polyethylene glycol-conjugated TNF receptor type 1 (4 mg/kg body weight) before a single ip injection of LPS showed a significant reduction in circulating IL-6 levels but no effect on LPS-induced D2 mRNA in the majority of tanycytes with the exception of a subpopulation of alpha tanycytes in the wall of the third ventricle. LPS induced a rapid increase in Ikappa-Balpha mRNA in the pars tuberalis and a delayed response in alpha tanycytes but absent in all other tanycyte subsets. The LPS-induced increase in Ikappa-Balpha in the pars tuberalis was associated with increased TSHbeta gene expression in this tissue, but cAMP response element-binding protein (CREB) phosphorylation was observed only in a subset of alpha tanycytes. These data suggest that TNF and nuclear factor-kappaB signaling are not the primary, initiating mechanisms mediating the LPS-induced D2 response in tanycytes, but may contribute in part to sustaining the LPS-induced D2 response in a subset of alpha tanycytes. We hypothesize that in addition to TSH, other factors derived from the pars tuberalis may contribute to LPS-induced D2 activation in tanycytes.

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.

Melanopsin expression in dopamine-melatonin neurons of the premammillary nucleus of the hypothalamus and seasonal reproduction in birds

Melanopsin (OPN4) is a photoreceptive molecule regulating circadian systems in mammals. Previous studies from our laboratory have shown that co-localized dopamine-melatonin (DA-MEL) neurons in the hypothalamic premammillary nucleus (PMM) are putatively photosensitive and exhibit circadian rhythms in DAergic and MELergic activities. This study investigates turkey OPN4x (tOPN4x) mRNA distribution in the hypothalamus and brainstem, and characterizes its expression in PMM DA-MEL neurons, using in situ hybridization (ISH), immunocytochemistry (ICC), double-label ISH/ICC, and real time-PCR. The mRNA encoding tOPN4x was found in anatomically discrete areas in or near the hypothalamus and the brainstem, including nucleus preopticus medialis (POM), nucleus septalis lateralis (SL), PMM and the pineal gland. Double ICC, using tyrosine hydroxylase (TH, the rate limiting enzyme in DA synthesis)-and OPN4x antibodies, confirmed the existence of OPN4x protein in DA-MEL neurons. Also, tOPN4x mRNA expression was verified with double ISH/ICC using tOPN4x mRNA and TH immunoreactivity. PMM and pineal gland tOPN4x mRNA expression levels were diurnally high during the night and low during the day. A light pulse provided to short day photosensitive hens during the photosensitive phase at night significantly down-regulated tOPN4x expression. The expression level of tOPN4x mRNA in PMM DA-MEL neurons of photorefractory hens was significantly lower as compared with that of short or long day photosensitive hens. The results implicate tOPN4x in hypothalamic PMM DA-MEL neurons as an important component of the photoreceptive system regulating reproductive activity in temperate zone birds.

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

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

The genetics of the thyroid stimulating hormone receptor: history and relevance

BACKGROUND

The thyroid stimulating hormone receptor (TSHR) is the key regulator of thyrocyte function. The gene for the TSHR on chromosome 14q31 has been implicated as coding for the major autoantigen in the autoimmune hyperthyroidism of Graves' disease (GD) to which T cells and autoantibodies are directed.

SUMMARY

The TSHR is a seven-transmembrane domain receptor that undergoes complex posttranslational processing. In this brief review, we look at the genetics of this important autoantigen and its influence on a variety of tissue functions in addition to its role in the induction of GD.

CONCLUSIONS

There is convincing evidence that the TSH receptor gene confers increased susceptibility for GD, but not Hashimoto's thyroiditis. GD is associated with polymorphisms in the intron 1 gene region. How such noncoding nucleotide changes influence disease susceptibility remains uncertain, but is likely to involve TSHR splicing variants and/or microRNAs arising from this gene region. Whether such influences are confined to the thyroid gland or whether they influence cell function in the many extrathyroidal sites of TSHR expression remains unknown.

Short-days induce weight loss in Siberian hamsters despite overexpression of the agouti-related peptide gene

Many vertebrates express profound annual cycles of body fattening, although it is not clear whether these represent differential activity of the central pathways known to mediate homeostatic control of food intake and energy expenditure, or whether the recent discovery of a major role for pars tuberalis-ependymal signalling points towards novel mechanisms. We examined this in the Siberian hamster (Phodopus sungorus) by using gene transfection to up-regulate a major orexigenic peptide, agouti-related peptide (AgRP), and then determined whether this increased anabolic drive could prevent the short-day induced winter catabolic state. Infusions of a recombinant adeno-associated virus encoding an AgRP construct into the hypothalamus of hamsters in the long-day obese phase of their seasonal cycle produced a 20% gain in body weight over 6 weeks compared to hamsters receiving a control reporter construct, reflecting a significant increase in food intake and a significant decrease in energy expenditure. However, all hamsters showed a significant, prolonged decrease in body weight when exposed to short photoperiods, despite the hamsters expressing the AgRP construct maintaining a higher food intake and lower energy expenditure relative to the control hamsters. Visualisation of the green fluorescent protein reporter and analysis of AgRP-immunoreactivity confirmed widespread expression of the construct in the hypothalamus, which was maintained for the 21-week duration of the study. In conclusion, the over-expression of AgRP in the hypothalamus produced a profoundly obese state but did not block the seasonal catabolic response, suggesting a separation of rheostatic mechanisms in seasonality from those maintaining homeostasis of energy metabolism.

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

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

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.

Weak evidence of bright light effects on human LH and FSH

Background

Most mammals are seasonal breeders whose gonads grow to anticipate reproduction in the spring and summer. As day length increases, secretion increases for two gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH). This response is largely controlled by light. Light effects on gonadotropins are mediated through effects on the suprachiasmatic nucleus and responses of the circadian system. There is some evidence that seasonal breeding in humans is regulated by similar mechanisms, and that light stimulates LH secretion, but primate responses seem complex.

Methods

To gain further information on effects of bright light on LH and FSH secretion in humans, we analyzed urine samples collected in three experiments conducted for other goals. First, volunteers ages 18-30 years and 60-75 commenced an ultra-short 90-min sleep-wake cycle, during which they were exposed to 3000 lux light for 3 hours at balanced times of day, repeated for 3 days. Urine samples were assayed to explore any LH phase response curve. Second, depressed participants 60-79 years of age were treated with bright light or dim placebo light for 28 days, with measurements of urinary LH and FSH before and after treatment. Third, women of ages 20-45 years with premenstrual dysphoric disorder (PMDD) were treated to one 3-hour exposure of morning light, measuring LH and FSH in urine before and after the treatments.

Results

Two of the three studies showed significant increases in LH after light treatment, and FSH also tended to increase, but there were no significant contrasts with parallel placebo treatments and no significant time-of-day treatment effects.

Conclusions

These results gave some support for the hypothesis that bright light may augment LH secretion. Longer-duration studies may be needed to clarify the effects of light on human LH and FSH.

Light, time, and the physiology of biotic response to rapid climate change in animals

Examination of temperate and polar regions of Earth shows that the nonbiological world is exquisitely sensitive to the direct effects of temperature, whereas the biological world is largely organized by light. Herein, we discuss the use of day length by animals at physiological and genetic levels, beginning with a comparative experimental study that shows the preeminent role of light in determining fitness in seasonal environments. Typically, at seasonally appropriate times, light initiates a cascade of physiological events mediating the input and interpretation of day length to the output of specific hormones that ultimately determine whether animals prepare to develop, reproduce, hibernate, enter dormancy, or migrate. The mechanisms that form the basis of seasonal time keeping and their adjustment during climate change are reviewed at the physiological and genetic levels. Future avenues for research are proposed that span basic questions from how animals transition from dependency on tropical cues to temperate cues during range expansions, to more applied questions of species survival and conservation biology during periods of climatic stress.

Identification of Eya3 and TAC1 as long-day signals in the sheep pituitary

Seasonally breeding mammals such as sheep use photoperiod, encoded by the nocturnal secretion of the pineal hormone melatonin, as a critical cue to drive hormone rhythms and synchronize reproduction to the most optimal time of year. Melatonin acts directly on the pars tuberalis (PT) of the pituitary, regulating expression of thyrotropin, which then relays messages back to the hypothalamus to control reproductive circuits. In addition, a second local intrapituitary circuit controls seasonal prolactin (PRL) release via one or more currently uncharacterized low-molecular-weight peptides, termed "tuberalins," of PT origin. Studies in birds have identified the transcription factor Eya3 as the first molecular response activated by long photoperiod (LP). Using arrays and in situ hybridization studies, we demonstrate here that Eya3 is the strongest LP-activated gene in sheep, revealing a common photoperiodic molecular response in birds and mammals. We also demonstrate TAC1 (encoding the tachykinins substance P and neurokinin A) to be strongly activated by LP within the sheep PT. We show that these PRL secretagogues act on primary pituitary cells and thus are candidates for the elusive PT-expressed tuberalin seasonal hormone regulator.

Variation in levels of luteinizing hormone and reproductive photoresponsiveness in a population of white-footed mice (Peromyscus leucopus)

Natural genetic variation in reproduction and life history strategies is a manifestation of variation in underlying regulatory neuronal and endocrine systems. A test of the hypothesis that genetic variation in luteinizing hormone (LH) level could be related to a life history trait, seasonal reproduction, was conducted on artificial selection lines from a wild-source population of white-footed mice (Peromyscus leucopus). Variation exists in the degree of suppression of reproduction by winter short-day photoperiods (SD) in wild-source individuals and in the laboratory population. In this population, most individuals from a photoperiod-responsive (R) artificial selection line are strongly suppressed reproductively in SD, while most individuals from a photoperiod-nonresponsive (NR) artificial selection line are only weakly reproductively suppressed in SD. We assayed levels of LH to test for genetic variation between lines that could contribute to variation in reproductive status in SD. Females from both lines were raised in long-day photoperiods (LD) or SD, ovariectomized under isoflurane anesthesia, and given estradiol implants. Levels of LH were significantly higher in the NR line than in the R line, indicating genetic variation for levels of LH. Levels of LH were higher in LD than in SD, indicating that levels of LH were sensitive to photoperiod treatment even with a controlled level of estradiol negative feedback. The results indicate that there is genetic variation in levels of LH that could be functionally important both in the laboratory in SD and in the wild population in winter. Thus genetic variation in levels of LH is a plausible causal factor determining winter reproductive phenotype in the wild population.

Photoperiodic modulation of clock gene expression in the avian premammillary nucleus

The premammillary nucleus (PMM) has been shown to contain a daily endogenous dual-oscillation in dopamine (DA)/melatonin (MEL) as well as c-fos mRNA expression that is associated with the daily photo-inducible phase of gonad growth in turkeys. In the present study, the expression of clock genes (Bmal1, Clock, Cry1, Cry2, Per2 and Per3) in the PMM was determined under short (8 : 16 h light/dark cycle) and long (16 : 8 h light/dark cycle) photoperiods relative to changes associated with the diurnal rhythm of DA and MEL. Constant darkness (0 : 24 h light/dark cycle) was used to assess the endogenous response of clock genes. In addition, light pulses were given at zeitgeber time (ZT) 8, 14 and 20 to ascertain whether clock gene expression is modulated by light pulse stimulation and therefore has a daily phase-related response. In the PMM, the temporal clock gene expression profiles were similar under short and long photoperiods, except that Per3 gene was phase-delayed by approximately 16 h under long photoperiod. In addition, Cry1 and Per3 genes were light-induced at ZT 14, the photosensitive phase for gonad recrudescence, whereas the Clock gene was repressed. Gene expression in established circadian pacemakers, the visual suprachiasmatic nucleus (vSCN) and the pineal, was also determined. Clock genes in the pineal gland were rhythmic under both photoperiods, and were not altered after light pulses at ZT 14, which suggests that pineal clock genes may not be associated with the photosensitive phase and reproductive activities. In the vSCN, clock gene expression was phase-shifted depending on the photoperiod, with apexes at night under short day length and during the day under long day length. Furthermore, light pulses at ZT 14 induced the Per2 gene, whereas it repressed the Bmal1 gene. Taken together, the changes in clock gene expression observed within the PMM were unique compared to the pineal and vSCN, and were induced by long photoperiod and light during the daily photosensitive phase; stimuli that are also documented to promote reproductive activity. These results show that Cry1 and Per3 are involved in the photic response associated with the PMM neuronal activation and are coincident with an essential circadian mechanism (photosensitive phase) controlling the reproductive neuroendocrine system.

Genome-wide census and expression profiling of chicken neuropeptide and prohormone convertase genes

Neuropeptides regulate cell-cell signaling and influence many biological processes in vertebrates, including development, growth, and reproduction. The complex processing of neuropeptides from prohormone proteins by prohormone convertases, combined with the evolutionary distance between the chicken and mammalian species that have experienced extensive neuropeptide research, has led to the empirical confirmation of only 18 chicken prohormone proteins. To expand our knowledge of the neuropeptide and prohormone convertase gene complement, we performed an exhaustive survey of the chicken genomic, EST, and proteomic databases using a list of 95 neuropeptide and 7 prohormone convertase genes known in other species. Analysis of the EST resources and 22 microarray studies offered a comprehensive portrait of gene expression across multiple conditions. Five neuropeptide genes (apelin, cocaine-and amphetamine-regulated transcript protein, insulin-like 5, neuropeptide S, and neuropeptide B) previously unknown in chicken were identified and 62 genes were confirmed. Although most neuropeptide gene families known in human are present in chicken, there are several gene not present in the chicken. Conversely, several chicken neuropeptide genes are absent from mammalian species, including C-RF amide, c-type natriuretic peptide 1 precursor, and renal natriuretic peptide. The prohormone convertases, with one exception, were found in the chicken genome. Bioinformatic models used to predict prohormone cleavages confirm that the processing of prohormone proteins into neuropeptides is similar between species. Neuropeptide genes are most frequently expressed in the brain and head, followed by the ovary and small intestine. Microarray analyses revealed that the expression of adrenomedullin, chromogranin-A, augurin, neuromedin-U, platelet-derived growth factor A and D, proenkephalin, relaxin-3, prepronociceptin, and insulin-like growth factor I was most susceptible (P-value<0.005) to changes in developmental stage, gender, and genetic line among other conditions studied. Our complete survey and characterization facilitates understanding of neuropeptides genes in the chicken, an animal of importance to biomedical and agricultural research.

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.

Effect of photoperiod on the thyroid-stimulating hormone neuroendocrine system in the European hamster (Cricetus cricetus)

Recent studies have characterised a retrograde mechanism whereby the pineal hormone melatonin acts in the pars tuberalis (PT) of the pituitary gland to control thyroid hormone action in the hypothalamus, leading to changes in seasonal reproductive function. This involves the release of thyroid-stimulating hormone (TSH) from PT that activates type II deiodinase (DIO2) gene expression in hypothalamic ependymal cells, locally generating biologically active T3, and thus triggering a neuroendocrine cascade. In the present study, we investigated whether a similar regulatory mechanism operates in the European hamster. This species utilises both melatonin signalling and a circannual timer to time the seasonal reproductive cycle. We found that expression of betaTSH RNA in the PT was markedly increased under long compared to short photoperiod, whereas TSH receptor expression was localised in the ependymal cells lining the third ventricle, and in the PT, where its expression varied with time and photoperiod. In the ependymal cells at the base of the third ventricle, DIO2 and type III deiodinase (DIO3) expression was reciprocally regulated, with DIO2 activated under long and repressed under short photoperiod, and the reverse case for DIO3. These data are consistent with recent observations in sheep, and suggest that the PT TSH third ventricle-ependymal cell relay plays a conserved role in initiating the photoperiodic response in both long- and short-day breeding mammals.

Seasonal differences of gene expression profiles in song sparrow (Melospiza melodia) hypothalamus in relation to territorial aggression

Background

Male song sparrows (Melospiza melodia) are territorial year-round; however, neuroendocrine responses to simulated territorial intrusion (STI) differ between breeding (spring) and non-breeding seasons (autumn). In spring, exposure to STI leads to increases in luteinizing hormone and testosterone, but not in autumn. These observations suggest that there are fundamental differences in the mechanisms driving neuroendocrine responses to STI between seasons. Microarrays, spotted with EST cDNA clones of zebra finch, were used to explore gene expression profiles in the hypothalamus after territorial aggression in two different seasons.

Methodology/Principal Findings

Free-living territorial male song sparrows were exposed to either conspecific or heterospecific (control) males in an STI in spring and autumn. Behavioral data were recorded, whole hypothalami were collected, and microarray hybridizations were performed. Quantitative PCR was performed for validation. Our results show 262 cDNAs were differentially expressed between spring and autumn in the control birds. There were 173 cDNAs significantly affected by STI in autumn; however, only 67 were significantly affected by STI in spring. There were 88 cDNAs that showed significant interactions in both season and STI.

Conclusions/Significance

Results suggest that STI drives differential genomic responses in the hypothalamus in the spring vs. autumn. The number of cDNAs differentially expressed in relation to season was greater than in relation to social interactions, suggesting major underlying seasonal effects in the hypothalamus which may determine the differential response upon social interaction. Functional pathway analyses implicated genes that regulate thyroid hormone action and neuroplasticity as targets of this neuroendocrine regulation.

Is the gonadotropin releasing hormone system vulnerable to endocrine disruption in birds?

Endocrine disrupting chemicals (EDCs) from a variety of sources occur widely in the environment, but relationships between exposure to EDCs and long term effects on bird populations can be difficult to prove. Embryonic exposure to EDCs may be particularly detrimental, with potential long-term effects on reproduction and ultimately individual fitness. Because many EDCs may have subtle sublethal effects, it is necessary to establish sensitive end points as biomarkers of EDC exposure in birds. Because the effects of EDCs may be both short- and long-term, it is important to determine if embryonic exposure impacts sexual differentiation and development of the reproductive axis in hatchlings and if there are effects on reproductive function in adults. Our studies have focused on the effects of estrogen- and androgen-active EDCs on the hypothalamic gonadotropin releasing hormone-I (GnRH-I) system in an avian model of precocial species, the Japanese quail. Estrogen- or androgen-active EDCs were administered between 0 and embryonic day 4, and hypothalamic GnRH-I was measured in hatchlings and adults. Treatment with vinclozolin and PCB126 depressed the concentration of embryonic GnRH-I peptide while methoxyclor had an inconsistent stimulatory effect. Treatment with atrazine or trenbolone had no significant effects on hypothalamic GnRH-I in adults. Overall these observations support the view that the developing avian GnRH-I neural system may be vulnerable to EDCs with potential to alter lifelong reproductive function.

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.

A riot of rhythms: neuronal and glial circadian oscillators in the mediobasal hypothalamus

Background

In mammals, the synchronized activity of cell autonomous clocks in the suprachiasmatic nuclei (SCN) enables this structure to function as the master circadian clock, coordinating daily rhythms in physiology and behavior. However, the dominance of this clock has been challenged by the observations that metabolic duress can over-ride SCN controlled rhythms, and that clock genes are expressed in many brain areas, including those implicated in the regulation of appetite and feeding. The recent development of mice in which clock gene/protein activity is reported by bioluminescent constructs (luciferase or luc) now enables us to track molecular oscillations in numerous tissues ex vivo. Consequently we determined both clock activities and responsiveness to metabolic perturbations of cells and tissues within the mediobasal hypothalamus (MBH), a site pivotal for optimal internal homeostatic regulation.

Results

Here we demonstrate endogenous circadian rhythms of PER2::LUC expression in discrete subdivisions of the arcuate (Arc) and dorsomedial nuclei (DMH). Rhythms resolved to single cells did not maintain long-term synchrony with one-another, leading to a damping of oscillations at both cell and tissue levels. Complementary electrophysiology recordings revealed rhythms in neuronal activity in the Arc and DMH. Further, PER2::LUC rhythms were detected in the ependymal layer of the third ventricle and in the median eminence/pars tuberalis (ME/PT). A high-fat diet had no effect on the molecular oscillations in the MBH, whereas food deprivation resulted in an altered phase in the ME/PT.

Conclusion

Our results provide the first single cell resolution of endogenous circadian rhythms in clock gene expression in any intact tissue outside the SCN, reveal the cellular basis for tissue level damping in extra-SCN oscillators and demonstrate that an oscillator in the ME/PT is responsive to changes in metabolism.