Circadian clock genes and photoperiodism: Comprehensive analysis of clock gene expression in the mediobasal hypothalamus, the suprachiasmatic nucleus, and the pineal gland of Japanese Quail under various light schedules

One Health: Circadian Medicine Benefits Both Non-human Animals and Humans Alike

Circadian biology's impact on human physical health and its role in disease development and progression is widely recognized. The forefront of circadian rhythm research now focuses on translational applications to clinical medicine, aiming to enhance disease diagnosis, prognosis, and treatment responses. However, the field of circadian medicine has predominantly concentrated on human healthcare, neglecting its potential for transformative applications in veterinary medicine, thereby overlooking opportunities to improve non-human animal health and welfare. This review consists of three main sections. The first section focuses on the translational potential of circadian medicine into current industry practices of agricultural animals, with a particular emphasis on horses, broiler chickens, and laying hens. The second section delves into the potential applications of circadian medicine in small animal veterinary care, primarily focusing on our companion animals, namely dogs and cats. The final section explores emerging frontiers in circadian medicine, encompassing aquaculture, veterinary hospital care, and non-human animal welfare and concludes with the integration of One Health principles. In summary, circadian medicine represents a highly promising field of medicine that holds the potential to significantly enhance the clinical care and overall health of all animals, extending its impact beyond human healthcare.

Avian migration clocks in a changing world

Avian long-distance migration requires refined programming to orchestrate the birds' movements on annual temporal and continental spatial scales. Programming is particularly important as long-distance movements typically anticipate future environmental conditions. Hence, migration has long been of particular interest in chronobiology. Captivity studies using a proxy, the shift to nocturnality during migration seasons (i.e., migratory restlessness), have revealed circannual and circadian regulation, as well as an innate sense of direction. Thanks to rapid development of tracking technology, detailed information from free-flying birds, including annual-cycle data and actograms, now allows relating this mechanistic background to behaviour in the wild. Likewise, genomic approaches begin to unravel the many physiological pathways that contribute to migration. Despite these advances, it is still unclear how migration programmes are integrated with specific environmental conditions experienced during the journey. Such knowledge is imminently important as temporal environments undergo rapid anthropogenic modification. Migratory birds as a group are not dealing well with the changes, yet some species show remarkable adjustments at behavioural and genetic levels. Integrated research programmes and interdisciplinary collaborations are needed to understand the range of responses of migratory birds to environmental change, and more broadly, the functioning of timing programmes under natural conditions.

Cloning, tissue distribution, and effects of different circadian rhythms on the mRNA expression levels of circadian clock genes Per1a and Per1b in Phoxinus lagowskii

This study describes the cloning and characterization of Period 1a and Period 1b genes and the analysis of their mRNA and protein expression in Amur minnow (Phoxinus lagowskii) after exposure to different light cycles. The full-length P. lagowskii Per1a and Per1b genes encode proteins consisting of 1393 and 1409 amino acids, and share high homology with the per1 genes of other freshwater fish species. The Per1a and Per1b genes were widely expressed within the brain, eye, and peripheral tissues. The acrophase of the Per1a gene in the pituitary gland occurred during the dark phase at ZT15 (zeitgeber time 15, 12 L: 12 D) and ZT18 (8 L, 16 D), whereas the acrophase of the Per1b gene in the pituitary gland was observed during the light phase. Our study suggests that the expression of Per1a and Per1b in P. lagowskii varied depending on differences in circadian rhythm patterns. The results of our dual-luciferase reporter assays demonstrated that the P. lagowskii Per1b gene enhances the activation of NF-κB. This study is the first to examine the circadian clock gene Per1a and Per1b in the high-latitude fish P. lagowskii, offering valuable insights into the effects of different light periods on this fish species.

Circadian gene variation in relation to breeding season and latitude in allochronic populations of two pelagic seabird species complexes

Annual cues in the environment result in physiological changes that allow organisms to time reproduction during periods of optimal resource availability. Understanding how circadian rhythm genes sense these environmental cues and stimulate the appropriate physiological changes in response is important for determining the adaptability of species, especially in the advent of changing climate. A first step involves characterizing the environmental correlates of natural variation in these genes. Band-rumped and Leach's storm-petrels (Hydrobates spp.) are pelagic seabirds that breed across a wide range of latitudes. Importantly, some populations have undergone allochronic divergence, in which sympatric populations use the same breeding sites at different times of year. We investigated the relationship between variation in key functional regions of four genes that play an integral role in the cellular clock mechanism-Clock, Bmal1, Cry2 and Per2-with both breeding season and absolute latitude in these two species complexes. We discovered that allele frequencies in two genes, Clock and Bmal1, differed between seasonal populations in one archipelago, and also correlated with absolute latitude of breeding colonies. These results indicate that variation in these circadian rhythm genes may be involved in allochronic speciation, as well as adaptation to photoperiod at breeding locations.

Investing in a nest egg: intraspecific variation in the timing of egg laying across a latitudinal gradient

Avian reproductive strategies vary widely, and many studies of life-history variation have focused on the incubation and hatching stages of nesting. Birds make proximate decisions regarding reproductive investment during the laying stage, and these decisions likely constrain and tradeoff with other traits and subsequent behaviors. However, we know relatively little about egg-laying stage behaviors given the difficulty of locating and monitoring nest sites from the onset of laying. We used non-invasive continuous video recording to quantify variation in the egg-laying behaviors of burrowing owls (Athene cunicularia) along a 1400-km latitudinal gradient in western North America. Burrowing owls laid eggs disproportionately in the morning hours, and that tendency was strongest among first eggs in a clutch. However, selection appeared to act more strongly on laying intervals (the time between laying of consecutive eggs) than on the diel time of laying, and laying intervals varied widely among and within clutches. Laying intervals declined seasonally and with increasing clutch size but increased with increasing burrow temperature and as a function of laying stage nest attentiveness, which may be a strategy to preserve egg viability. Laying interval was positively correlated with the duration of hatching intervals, suggesting that laying interval duration is one mechanism (along with timing of incubation onset) that generates variation in hatching asynchrony. Our results lend support to two general hypotheses to explain laying schedules; selection favors laying eggs in the morning, but other selective pressures may override that pattern. These conclusions indicate that allocation decisions during laying are an important part of avian life-history strategies which are subject to energetic constraints and tradeoffs with other traits.

Targeted differential photostimulation alters reproductive activities of domestic birds

Modern poultry production systems use environmentally controlled houses providing only artificial illumination. The role of light in reproduction of poultry depends on light quality (photoperiod, intensity/brightness, and spectrum), which enables us to provide custom-made illumination, targeted for the elevation of reproductive activities. Artificial targeted illumination significantly affects poultry reproduction. This phenomenon is based on the mechanism of light absorption in birds, which consists of two main components: the eye (retinal photoreceptors) and brain extraretinal photoreceptors. Several experiments on turkey hens and broiler breeder males and females have shown that photostimulation of brain extraretinal photoreceptors, while maintaining retinal photoreceptors under non-photostimulatory conditions, elevates reproductive activity by increasing egg production of hens and semen quality of roosters. In addition, we found acceleration in all gonadal axis parameters, leading to the acceleration in the production rate. Furthermore, we studied the role of retinal activation in gonadal axis suppuration and identified the role of serotonin in this phenomenon. As for today, several broiler breeder farms use targeted illumination based on our studies with excellent results.

Photoperiodic regulation of avian physiology: From external coincidence to seasonal reproduction

Seasonal cycles of environmental cues generate variation in the timing of life-history transition events across taxa. It is through the entrainment of internal, endogenous rhythms of organisms to these external, exogenous rhythms in environment, such as cycling temperature and daylight, by which organisms can regulate and time life history transitions. Here, we review the current understanding of how photoperiod both stimulates and terminates seasonal reproduction in birds. The review describes the role of external coincidence timing, the process by which photoperiod is proposed to stimulate reproductive development. Then, the molecular basis of light detection and the photoperiodic regulation of neuroendocrine timing of seasonal reproduction in birds is presented. Current data indicates that vertebrate ancient opsin is the predominant photoreceptor for light detection by the hypothalamus, compared to neuropsin and rhodopsin. The review then connects light detection to well-characterized hypothalamic and pituitary gland molecules involved in the photoperiodic regulation of reproduction. In birds, Gonadotropin-releasing hormone synthesis and release are controlled by photoperiodic cues via thyrotropin-stimulating hormone-β (TSHβ) independent and dependent pathways, respectively. The review then highlights the role of D-box and E-box binding motifs in the promoter regions of photoperiodic genes, in particular Eyes-absent 3, as the key link between circadian clock function and photoperiodic time measurement. Based on the available evidence, the review proposes that at least two molecular programs form the basis for external coincidence timing in birds: photoperiodic responsiveness by TSHβ pathways and endogenous internal timing by gonadotropin synthesis.

Molecular expression of clock genes in central and peripheral tissues of white-rumped munia (Lonchura striata)

To synchronize with the fluctuating environment, organisms have evolved an endogenous time tracking mechanism referred to as the biological clock(s). This clock machinery has been identified in almost all cells of vertebrates and categorized as central and peripheral clocks. In birds, three independent circadian clocks have been identified in the hypothalamus, the pineal and the retina which interact and generate circadian time at a functional level. However, there is a limited knowledge of molecular clockwork and integration between central and peripheral clocks in birds. Therefore, we studied the daily expression of clock genes (Bmal1, Clock, Per2, Cry1, Npas2, Rev-Erbα, E4bp4, Pparα, Hlf and Tef) in three central circadian clocks (hypothalamus, pineal and retina), other brain areas (cerebellum, optic tectum and telencephalon) and in the peripheral tissues (liver, intestine, muscle and blood) of white-rumped munia. Adult birds were exposed to equinox photoperiod (12 L:12D) for 2 weeks and were then sampled (N = 5 per time point) at six-time points (ZT1, ZT5, ZT9, ZT13, ZT17 and ZT21). Daily expressions of clock genes were studied using qPCR. We observed daily variations and tissue-specific expression patterns for clock genes. These results are consistent with the autoregulatory circadian feedback loop proposed for the mammalian system and thus suggest a conserved tissue-level circadian time generation in white-rumped munia.

More Light Please: Daphnia Benefit From Light Pollution by Increased Tolerance Toward Cyanobacterial Chymotrypsin Inhibitors

Cryptochromes are evolutionary ancient blue-light photoreceptors that are part of the circadian clock in the nervous system of many organisms. Cryptochromes transfer information of the predominant light regime to the clock which results in the fast adjustment to photoperiod. Therefore, the clock is sensitive to light changes and can be affected by anthropogenic Artificial Light At Night (ALAN). This in turn has consequences for clock associated behavioral processes, e.g., diel vertical migration (DVM) of zooplankton. In freshwater ecosystems, the zooplankton genus Daphnia performs DVM in order to escape optically hunting predators and to avoid UV light. Concomitantly, Daphnia experience circadian changes in food-supply during DVM. Daphnia play the keystone role in the carbon-transfer to the next trophic level. Therefore, the whole ecosystem is affected during the occurrence of cyanobacteria blooms as cyanobacteria reduce food quality due to their production of digestive inhibitors (e.g., protease inhibitors). In other organisms, digestion is linked to the circadian clock. If this is also the case for Daphnia, the expression of protease genes should show a rhythmic expression following circadian expression of clock genes (e.g., cryptochrome 2). We tested this hypothesis and demonstrated that gene expression of the clock and of proteases was affected by ALAN. Contrary to our expectations, the activity of one type of proteases (chymotrypsins) was increased by ALAN. This indicates that higher protease activity might improve the diet utilization. Therefore, we treated D. magna with a chymotrypsin-inhibitor producing cyanobacterium and found that ALAN actually led to an increase in Daphnia’s growth rate in comparison to growth on the same cyanobacterium in control light conditions. We conclude that this increased tolerance to protease inhibitors putatively enables Daphnia populations to better control cyanobacterial blooms that produce chymotrypsin inhibitors in the Anthropocene, which is defined by light pollution and by an increase of cyanobacterial blooms due to eutrophication.

Impact of light stimulation during incubation on hatching traits and post-hatch performance of commercial broilers

Light in terms of photo- and scoto-periods is the key ambient factor affecting the physiology of birds through establishing normal biological clock and circadian rhythms. In natural incubation light significantly influences embryonic development, however, at commercial setups eggs are incubated under a dark environment. Presently not a single commercial poultry hatchery is using light during incubation; hence, comprehensive studies are needed to address the industry for considering light as a potential embryonic growth stimulant. In the present study, white Light-emitting diodes (LEDs; 5000 K) were installed in the incubator and 250 lx light intensity was provided for 0, 12, and 24 h per day during the whole incubation period. A total of 900 broiler hatching eggs (Hubbard classic; from 58 weeks old parents) were randomly allocated to 3 treatment groups, having 5 replicates of 60 eggs each, a tray was considered as replicate during incubation and these eggs were incubated under standard incubation protocols. After hatching, a total of 300 chicks were picked and divided into 3 described treatments (0, 12, and 24 h of photo-stimulation to eggs during incubation) having 5 replicates of 20 birds each. The results indicated that incubation of eggs under 12 and 24 h of lighting significantly improved (P ≤ 0.05) hatch window, hatchability % (0.0002), a hatch of fertile % (0.001), and carcass yield % (0.0454). Embryonic mortality, dead germs, and dead in shell embryos were lower in eggs incubated under 12 h light. Significantly better FCR (0.0006), stress susceptibilities such as H/L ratio (0.0227), and physical asymmetry (0.0065) were observed among the birds incubated under 12 h light (P ≤ 0.05). In conclusion, an appropriate light stimuli (12 h) may help to improve hatching traits and post-hatch performance of commercial broiler.

Moonlight is a key entrainer of lunar clock in the brain of the tropical grouper with full moon preference

Background

Many animals in coral reefs exhibit lunar cycles in their reproduction, showing synchronous gametogenesis and spawning at a particular moon phase. How these lunar reproductive cycles are endogenously regulated remains unknown, although changes in moonlight between the new moon and full moon are likely involved in this rhythmic event.

Results

This study evaluated the possible role of cryptochrome (cry1, cry2, cry3) in phase shifting and setting in the honeycomb grouper Epinephelus merra, which is a typical lunar spawner with full moon preference. qPCR analysis revealed that when fish were reared under alternating light-dark conditions, the transcript levels of cry1 and cry2, but not of cry3, in the diencephalon and pituitary gland showed daily variations. Weekly collection at midnight showed increases in the transcript levels of cry1 and cry2 in the diencephalon, but not the pituitary gland, from the first quarter moon through the last quarter moon. In comparison to the new moon, these transcript levels were significantly lower at all other sampling times. The artificial full moon conditions for 1 month resulted in increased cry transcript levels in both tissues at 2 (cry1) or 2 and 4 (cry2) weeks after the initiation of full moon conditions.

Conclusions

These results indicate the importance of transient changes in “brightness at night” in the response to moonlight for the phase shift and of “darkness at night” during the new moon for the phase set to the determined moon phase. We concluded that the moon phase-dependent oscillation of clock genes plays a role in lunar cycle-dependent behaviors in fish.

Developmental effects of constant light on circadian behaviour and gene expressions in zebra finches: Insights into mechanisms of metabolic adaptation to aperiodic environment in diurnal animals

A most crucial feature of biological adaptation is the maintenance of a close temporal relationship of behaviour and physiology with prevailing 24-h light-dark environment, which is rapidly changing with increasing nighttime illumination. This study investigated developmental effects of the loss of night on circadian behaviour, metabolism and gene expressions in diurnal zebra finches born and raised under LL, with controls on 12L:12D. Birds under LD were entrained, and showed normal body mass and a significant 24-h rhythm in both activity-rest pattern and mRNA expression of candidate genes that we measured. But, under LL, birds gained weight and accumulated lipid in the liver. Intriguingly, at the end of the experiment, the majority (4/5th) of birds under LL were rhythmic in activity despite arrhythmic expression in the hypothalamus of c-Fos (neuronal activity), Rhodopsin and Mel1-a genes (light perception), and clock genes (Bmal1, Per2 and Rev-erb β). In peripheral tissues, LL induced variable clock gene expressions. Whereas 24-h mRNA rhythm was abolished for Bmal1 in both liver and gut, it persisted for Per2 and Rev-erb β in liver, and for Per2 in gut. Further, we found under LL, the loss of 24-h rhythm in hepatic expression of Fasn and Cd36/Fat (biosynthesis and its uptake), and gut expression of Sglt1, Glut5, Cd36 and Pept1 (nutrient absorption) genes. As compared to LD, baseline mRNA levels of Fasn and Cd36 genes were attenuated under LL. Among major transporter genes, Sglt1 (glucose) and Cd36 (fat) genes were arrhythmic, while Glut5 (glucose) and Pept1 (protein) genes were rhythmic but with phase differences under LL, compared to LD. These results demonstrate dissociation of circadian behaviour from clock gene rhythms, and provide molecular insights into possible mechanisms at different levels (behaviour and physiology) that diurnal animals might employ in order to adapt to an emerging overly illuminated-night urban environment.

Photoperiodic induction without light-mediated circadian entrainment in a High Arctic resident bird

Organisms use changes in photoperiod to anticipate and exploit favourable conditions in a seasonal environment. While species living at temperate latitudes receive day length information as a year-round input, species living in the Arctic may spend as much as two-thirds of the year without experiencing dawn or dusk. This suggests that specialised mechanisms may be required to maintain seasonal synchrony in polar regions. Svalbard ptarmigan (Lagopus muta hyperborea) are resident at 74-81°N latitude. They spend winter in constant darkness (DD) and summer in constant light (LL); extreme photoperiodic conditions under which they do not display overt circadian rhythms. Here, we explored how Arctic adaptation in circadian biology affects photoperiodic time measurement in captive Svalbard ptarmigan. For this purpose, DD-adapted birds, showing no circadian behaviour, either remained in prolonged DD, were transferred into a simulated natural photoperiod (SNP) or were transferred directly into LL. Birds transferred from DD to LL exhibited a strong photoperiodic response in terms of activation of the hypothalamic thyrotropin-mediated photoperiodic response pathway. This was assayed through expression of the Eya3, Tshβ and deiodinase genes, as well as gonadal development. While transfer to SNP established synchronous diurnal activity patterns, activity in birds transferred from DD to LL showed no evidence of circadian rhythmicity. These data show that the Svalbard ptarmigan does not require circadian entrainment to develop a photoperiodic response involving conserved molecular elements found in temperate species. Further studies are required to define how exactly Arctic adaptation modifies seasonal timer mechanisms.

Circadian rhythm in photoperiodic expressions of GnRH-I and GnIH regulating seasonal reproduction in the Eurasian tree sparrow, Passer montanus

The present study investigates the involvement of circadian rhythm in photoperiodic expressions of GnRH-I and GnIH in the hypothalamus controlling seasonal reproduction in the Eurasian tree sparrow (Passer montanus). Groups of photosensitive birds were exposed for four weeks to resonance light dark cycles comprising of a light phase of 6 h (L) combined with dark phase of different durations (D) such that the period of LD cycles varied by 12 h increments viz. 12- (6 L/6D), 24- (6 L/18D), 36- (6 L/30D), 48- (6 L/42D), 60- (6 L/54D) and 72- (6 L/66D)h. In addition, a control group (C) was maintained under long day length (14 L/10D). Observations, recorded at the beginning and end of experiment, revealed significant testicular growth with corresponding increase in the hypothalamic expression of GnRH-I peptide but low levels of GnIH mRNA and peptide in the birds exposed to resonance cycles of 12, 36 and 60 h which were read as long days. On the other hand, birds experiencing resonance cycles of 24, 48 and 72 h read them as short days wherein they maintained their quiescent gonads and low levels of GnRH-I peptide but exhibited significant increase in GnIH mRNA and peptide expressions. Thus, sparrows responded to resonance light dark cycles differently despite the fact that each of them contained only 6 h of light. These findings suggest that an endogenous circadian rhythm is involved in photoperiodic expressions of above molecules and indicate a shift in their expressions depending upon whether the light falls in the photoinducible or non-photoinducible phase of an endogenous circadian rhythm.

Time course of photo-induced Egr-1 expression in the hypothalamus of a seasonally breeding songbird

Many seasonally-breeding species use daylength to time reproduction. Light-induced release of progonadal hormones involves a complex cascade of responses both inside and outside the brain. In this study, we used induction of early growth response 1 (Egr-1), the protein product of an immediate early gene, to evaluate the time course of such responses in male white-throated sparrows (Zonotrichia albicollis) exposed to a single long day. Induction of Egr-1 in the pars tuberalis began ∼11 h after dawn. This response was followed ∼6 h later by dramatic induction in the tuberal hypothalamus, including in the ependymal cells lining the third ventricle. At approximately the same time, Egr-1 was induced in dopaminergic and vasoactive intestinal peptide neurons in the tuberal hypothalamus and in dopaminergic neurons of the premammillary nucleus. We noted no induction in gonadotropin-releasing hormone (GnRH) neurons until 2 h after dawn the following morning. Overall, our results indicate that Egr-1 responses in GnRH neurons occur rather late during photostimulation, compared with responses in other cell populations, and that such induction may reflect new synthesis related to GnRH depletion rather than stimulation by light cues.

Should I Lay or Should I Grow: Photoperiodic Versus Metabolic Cues in Chickens

While photoperiod has been generally accepted as the primary if not the exclusive cue to stimulate reproduction in photoperiodic breeders such as the laying hen, current knowledge suggests that metabolism, and/or body composition can also play an influential role to control the hypothalamic-pituitary gonadal (HPG)-axis. This review thus intends to first describe how photoperiodic and metabolic cues can impact the HPG axis, then explore and propose potential common pathways and mechanisms through which both cues could be integrated. Photostimulation refers to a perceived increase in day-length resulting in the stimulation of the HPG. While photoreceptors are present in the retina of the eye and the pineal gland, it is the deep brain photoreceptors (DBPs) located in the hypothalamus that have been identified as the potential mediators of photostimulation, including melanopsin (OPN4), neuropsin (OPN5), and vertebrate-ancient opsin (VA-Opsin). Here, we present the current state of knowledge surrounding these DBPs, along with their individual and relative importance and, their possible downstream mechanisms of action to initiate the activation of the HPG axis. On the metabolic side, specific attention is placed on the hypothalamic integration of appetite control with the stimulatory (Gonadotropin Releasing Hormone; GnRH) and inhibitory (Gonadotropin Inhibitory Hormone; GnIH) neuropeptides involved in the control of the HPG axis. Specifically, the impact of orexigenic peptides agouti-related peptide (AgRP), and neuropeptide Y (NPY), as well as the anorexigenic peptides pro-opiomelanocortin (POMC), and cocaine-and amphetamine regulated transcript (CART) is reviewed. Furthermore, beyond hypothalamic control, several metabolic factors involved in the control of body weight and composition are also presented as possible modulators of reproduction at all three levels of the HPG axis. These include peroxisome proliferator-activated receptor gamma (PPAR-γ) for its impact in liver metabolism during the switch from growth to reproduction, adiponectin as a potential modulator of ovarian development and follicular maturation, as well as growth hormone (GH), and leptin (LEP).

Effect of monochromatic light on the temporal expression of N-acetyltransferase in chick pineal gland

The avian pineal gland is an independent molecular oscillator that receives external light information that regulates the synthesis and secretion of melatonin. Arylalkylamine N-acetyltransferase plays an important role in the pineal gland by controlling the rhythmic production of melatonin. Previous study showed that monochromatic light influences the secretion of melatonin, which is regulated by the molecular circadian clock genes in chick pineal gland. This study was designed to investigate the effect of monochromatic light on the circadian rhythm of levels of cAanat, clock protein (CLOCK and BMAL1), cCreb, and opsins (cOpnp, Pinopsin; cOpn4-1, Melanopsin-1; cOpn4-2, Melanopsin-2) in chick pineal gland. A total of 240 post-hatching day (P) 0 broiler chickens were reared under white (WL), red (RL), green (GL), and blue light (BL) with light (L)-dark (D) cycle of 12L:12D for 14 d. The results show significant circadian rhythms in the expression of cAanat, CLOCK, BMAL1, cCreb, cOpnp, cOpn4-1, and cOpn4-2, but not for cOpnp under RL. Compared with WL, GL increased the level of cAanat mRNA, while RL decreased it. Meanwhile, CLOCK and BMAL1 proteins were expressed at high levels in GL. Furthermore, the peak of the 24 h pattern of cOpnp mRNA in GL was earlier than that of in WL, RL, and BL. These results demonstrated that monochromatic light affects the daily expression of cAanat in the chick pineal gland via the biological clock. GL activates the transcription of cAanat, while RL suppresses the transcription of cAanat. Meanwhile, GL appears to induce the peak of cOpnp mRNA in advance to affect the transmission of light. Thus, monochromatic light regulates cAanat in the chick pineal gland by affecting the levels of clock regulators via entraining the expression of pineal gland opsins.

The quail genome: insights into social behaviour, seasonal biology and infectious disease response

BACKGROUND

The Japanese quail (Coturnix japonica) is a popular domestic poultry species and an increasingly significant model species in avian developmental, behavioural and disease research.

RESULTS

We have produced a high-quality quail genome sequence, spanning 0.93 Gb assigned to 33 chromosomes. In terms of contiguity, assembly statistics, gene content and chromosomal organisation, the quail genome shows high similarity to the chicken genome. We demonstrate the utility of this genome through three diverse applications. First, we identify selection signatures and candidate genes associated with social behaviour in the quail genome, an important agricultural and domestication trait. Second, we investigate the effects and interaction of photoperiod and temperature on the transcriptome of the quail medial basal hypothalamus, revealing key mechanisms of photoperiodism. Finally, we investigate the response of quail to H5N1 influenza infection. In quail lung, many critical immune genes and pathways were downregulated after H5N1 infection, and this may be key to the susceptibility of quail to H5N1.

CONCLUSIONS

We have produced a high-quality genome of the quail which will facilitate further studies into diverse research questions using the quail as a model avian species.

Light-exposure at night impairs mouse ovary development via cell apoptosis and DNA damage

The alternation of light and dark rhythm causes a series of physiological, biochemical and metabolic changes in animals, which also alters the growth and development of animals, and feeding, migration, reproduction and other behavioral activities. In recent years, many studies have reported the effects of long-term (more than 6 weeks) illumination on ovarian growth and development. In the present study, we observed the damage, repair and apoptosis of ovarian DNA in a short period of illumination. The results showed that, in short time (less than 2 weeks) illumination conditions, the 24-h light treatment caused the reduction of total ovarian follicle number and down-regulation of circadian clock related genes. Furthermore, the changed levels of serum sex hormones were also detected after 24-h light exposure, of which the concentrations of LH (luteinizing hormone), FSH (follicle-stimulating hormone) and E2 (estradiol) were increased, but the concentration of PROG (progesterone) was decreased. Moreover, 24-h light exposure increased the expression of DNA damage and repair related genes, the number of TUNEL and RAD51 positive cells. These results indicated that 24-h light exposure for 4, 8 and 12 days increased DNA damage and cell apoptosis, thereby affecting the development of ovary.

Photoperiodic Regulation of Reproduction in Vertebrates

Organisms use changes in photoperiod for seasonal reproduction to maximize the survival of their offspring. Birds have sophisticated seasonal mechanisms and are therefore excellent models for studying these phenomena. Birds perceive light via deep-brain photoreceptors and long day–induced thyroid-stimulating hormone (TSH, thyrotropin) in the pars tuberalis of the pituitary gland (PT), which cause local thyroid hormone activation within the mediobasal hypothalamus. The local bioactive thyroid hormone controls seasonal gonadotropin-releasing hormone secretion and subsequent gonadotropin secretion. In mammals, the eyes are believed to be the only photoreceptor organ, and nocturnal melatonin secretion triggers an endocrine signal that communicates information about the photoperiod to the PT to regulate TSH. In contrast, in Salmonidae fish the input pathway to the neuroendocrine output pathway appears to be localized in the saccus vasculosus. Thus, comparative analysis is an effective way to uncover the universality and diversity of fundamental traits in various organisms.

Effect of pinealectomy on the circadian clock of the chick retina under different monochromatic lights

The avian circadian rhythm pacemaker is composed of the retina, pineal gland and suprachiasmatic nucleus. As an intact input-pacemaker-output system, each of these structures is linked within a neuroendocrine loop to influence downstream processes and peripheral oscillations. While our previous study found that monochromatic light affected the circadian rhythms of clock genes in the chick retina, the effect of the pineal gland on the response of the retinal circadian clock under monochromatic light still remains unclear. In this study, a total of 144 chicks, including sham-operated and pinealectomized groups, were exposed to white, red, green or blue light. After 2 weeks of light illumination, the circadian expression of six core clock genes (cClock, cBmal1, cCry1, cCry2, cPer2 and cPer3), melanopsin (cOpn4-1, cOpn4-2), Arylalkylamine N-acetyltransferase (cAanat) and melatonin was examined in the retina. The cBmal1, cCry1, cPer2, cPer3, cOpn4-1, cOpn4-2 and cAanat genes as well as melatonin had circadian rhythmic expression in both the sham-operated and pinealectomized groups under different monochromatic lights, while cClock and cCry2 had arrhythmic 24 h profiles in all of the light-treated groups. After pinealectomy, the rhythmicity of the clock genes, melanopsins, cAanat and melatonin in the chick retina did not change, especially the mesors, amplitudes and phases of cBmal1, cOpn4-1, cOpn4-2, cAanat and melatonin. Compared to the white light group, however, green light increased the mRNA expression of the positive-regulating clock genes cBmal1, cAanat, cOpn4-1 and cOpn4-2 as well as the melatonin content in pinealectomized chicks, whereas red light decreased their expression. These results suggest that the chick retina is a relatively independent circadian oscillator from the pineal gland, whose circadian rhythmicity (including photoreception, molecular clock and melatonin output) is not altered after pinealectomization. Moreover, green light increases ocular cAanat expression and melatonin synthesis by accelerating the expression of melanopsin and positive-regulating clock genes cBmal1 and cClock.

The underlying mechanisms of vertebrate seasonal reproduction

Animals make use of changes in photoperiod to adapt their physiology to the forthcoming breeding season. Comparative studies have contributed to our understanding of the mechanisms of seasonal reproduction in vertebrates. Birds are excellent models for studying these phenomena because of their rapid and dramatic responses to changes in photoperiod. Deep brain photoreceptors in birds perceive and transmit light information to the pars tuberalis (PT) in the pituitary gland, where the thyroid-stimulating hormone (TSH) is produced. This PT-TSH locally increases the level of the bioactive thyroid hormone T₃ via the induction of type 2 deiodinase production in the mediobasal hypothalamus, and an increased T₃ level, in turn, controls seasonal gonadotropin-releasing hormone secretion. In mammals, the eyes are the only photoreceptive structure, and nocturnal melatonin secretion encodes day-length information and regulates the PT-TSH signaling cascade. In Salmonidae, the saccus vasculosus plays a pivotal role as a photoperiodic sensor. Together, these studies have uncovered the universality and diversity of fundamental traits in vertebrates.

Effect of Monochromatic Light on Circadian Rhythm of Clock Genes in Chick Pinealocytes

The avian circadian system is a complex of mutually coupled pacemakers residing in pineal gland, retina and suprachiasmatic nucleus. In this study, the self-regulation mechanism of pineal circadian rhythm was investigated by culturing chick primary pinealocytes exposed to red light (RL), green light (GL), blue light (BL), white light (WL) and constant darkness (DD), respectively. All illuminations were set up with a photoperiod of 12 light: 12 dark. The 24-h expression profiles of seven core clock genes (cBmal1/2, cClock, cCry1/2 and cPer2/3), cAanat and melatonin showed significant circadian oscillation in all groups, except for the loss of cCry1 rhythm in BL. Compared to WL, GL increased the amplitudes and mesors of positive elements (cClock and cBmal1/2) and reduced those of negative elements (cCry1/2 and cPer2/3), in contrast to RL. The temporal patterns of cAanatmRNA and melatonin secretion have always been consistent with the positive genes. Besides, GL advanced the acrophases of the positive elements, cAanat and melatonin, but RL and BL showed the opposite effect. Thereby, GL could promote the secretion of melatonin by enhancing the expressions of positive clock genes and repressing the expressions of negative clock genes.

Seasonal Reproduction in Vertebrates: Melatonin Synthesis, Binding, and Functionality Using Tinbergen's Four Questions

One of the many functions of melatonin in vertebrates is seasonal reproductive timing. Longer nights in winter correspond to an extended duration of melatonin secretion. The purpose of this review is to discuss melatonin synthesis, receptor subtypes, and function in the context of seasonality across vertebrates. We conclude with Tinbergen's Four Questions to create a comparative framework for future melatonin research in the context of seasonal reproduction.

Daily, circadian and seasonal changes of rhodopsin-like encephalic photoreceptor and its involvement in mediating photoperiodic responses of Gambel's white-crowned Sparrow, Zonotrichia leucophrys gambelii

Extra-retinal, non-pineal, encephalic photoreceptors (EP) play important roles in mediating development of the reproductive system by the annual change in day length (photoperiodic gonadal response - PGR) in birds. However, the distribution of rhodopsin-like EPs and their functional daily, circadian and seasonal changes are still unclear in the avian brain. This study identifies two novel groups of rhodopsin-immunoreactive cells in the nucleus paraventricularis magnocellularis (PVN) of the hypothalamus and in the medial basal hypothalamus (MBH) in a seasonally breeding species, Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii). In the PVN, rhodopsin-ir cell number showed both daily and circadian changes with more labeled cells apparent in the night phase in photosensitive birds, while only circadian changes were observed involving fewer labeled cells in the night phase in photorefractory birds. Single long day photo-stimulation significantly decreased the rhodopsin-ir cell number only in photosensitive birds, coincident with a rise in plasma levels of luteinizing hormone (LH). In the MBH, rhodopsin-ir cell number did not show daily, circadian or single long day induced changes in either photoperiodic states. But, overall these rhodopsin expressing neurons significantly increased from photosensitive to photorefractory states. In the median eminence (ME), more intense rhodopsin-ir was detected in photorefractory birds compared to photosensitive birds. For expression of GnRH and vasoactive intestinal polypeptide (VIP), seasonal differences were found with opposite relationships, consistent with previous studies. Our results suggest different roles of the two groups of rhodopsin-like EPs in the regulation of PGR in white-crowned sparrows.

Signaling pathways to and from the hypophysial pars tuberalis, an important center for the control of seasonal rhythms

Seasonal (circannual) rhythms play an important role for the control of body functions (reproduction, metabolism, immune responses) in nearly all living organisms. Also humans are affected by the seasons with regard to immune responses and mental functions, the seasonal affective disorder being one of the most prominent examples. The hypophysial pars tuberalis (PT), an important interface between the hypophysial pars distalis and neuroendocrine centers in the brain, plays an essential role in the regulation of seasonal functions and may even be the seat of the circannual clock. Photoperiodic signals provide a major input to the PT. While the perception of these signals involves extraocular photoreceptors in non-mammalian species (birds, fish), mammals perceive photoperiodic signals exclusively in the retina. A multisynaptic pathway connects the retina with the pineal organ where photoperiodic signals are translated into the neurohormone melatonin that is rhythmically produced night by night and encodes the length of the night. Melatonin controls the functional activity of the mammalian PT by acting upon MT1 melatonin receptors. The PT sends its output signals via retrograde and anterograde pathways. The retrograde pathway targetting the hypothalamus employs TSH as messenger and controls a local hypothalamic T3 system. As discovered in Japanese quail, TSH triggers molecular cascades mediating thyroid hormone conversion in the ependymal cell layer of the infundibular recess of the third ventricle. The local accumulation of T₃ in the mediobasal hypothalamus (MBH) appears to activate the gonadal axis by affecting the neuro-glial interaction between GnRH terminals and tanycytes in the median eminence. This retrograde pathway is conserved in photoperiodic mammals (sheep and hamsters), and even in non-photoperiodic laboratory mice provided that they are capable to synthesize melatonin. The anterograde pathway is implicated in the control of prolactin secretion, targets cells in the PD and supposedly employs small molecules as signal substances collectively denominated as "tuberalins". Several "tuberalin" candidates have been proposed, such as tachykinins, the secretory protein TAFA and endocannabinoids (EC). The PT-intrinsic EC system was first demonstrated in Syrian hamsters and shown to respond to photoperiodic changes. Subsequently, the EC system was also demonstrated in the PT of mice, rats and humans. To date, 2-arachidonoylglycerol (2-AG) appears as the most important endocannabinoid from the PT. Likely targets for the EC are folliculo-stellate cells that contain the CB1 receptor and appear to contact lactotroph cells. The CB1 receptor was also found on corticotroph cells which appear as a further target of the EC. Recently, the CB1 receptor was also localized to CRF-containing nerve fibers running in the outer zone of the median eminence. This finding suggests that the EC system of the PT contributes not only to the anterograde, but also to the retrograde pathway. Taken together, the results support the concept that the PT transmits its signals via a "cocktail" of messenger molecules which operate also in other brain areas and systems rather than through PT-specific "tuberalins". Furthermore, they may attribute a novel function to the PT, namely the modulation of the stress response and immune functions.

Temporal expression of clock genes in central and peripheral tissues of spotted munia under varying light conditions: Evidence for circadian regulation of daily physiology in a non-photoperiodic circannual songbird species

We investigated if the duration and/or frequency of the light period affect 24-h rhythm of circadian clock genes in central and peripheral tissues of a non-photoperiodic songbird, the spotted munia (Lonchura punctulata), in which a circannual rhythm regulates the reproductive cycle. We monitored activity-rest pattern and measured 24-h mRNA oscillation of core clock (Bmal1, Clock, Per2, Cry1 and Cry2) and clock-controlled (E4bp4, Rorα and Rev-erbα) genes in the hypothalamus, retina, liver and gut of spotted munia subjected to an aberrant light-dark (LD) cycle (3.5L:3.5D; T7, T = period length of LD cycle) and continuous light (LL, 24L:0D), with controls on 24-h LD cycle (T24, 12L:12D). Munia exhibited rhythmic activity-rest pattern with period matched to T7 or T24 under an LD cycle and were arrhythmic with a scattered activity pattern and higher activity duration under LL. At the transcriptional level, both clock and clock-controlled genes showed a significant 24-h rhythm in all four tissues (except Clock in the liver) under 12L:12D, suggesting a conserved tissue-level circadian time generation in spotted munia. An exposure to 3.5L:3.5D or LL induced arrhythmicity in transcriptional oscillation of all eight genes in the hypothalamus (except Rev-erbα) and liver (except Bmal1 and Rev-erbα under T7 and Cry1 under LL). In the retina, however, all genes showed arrhythmic 24-h mRNA expression under LL, but not under T7 (except in E4bp4 and Rorα). Interestingly, unlike in the liver, Bmal1, Per2, Cry1, Rorα and Rev-erbα mRNA expressions were rhythmic in the gut under both T7 (except Rorα) and LL conditions. These results showed variable relationship of internal circadian clocks with the external light environment and suggested a weak coupling of circadian clocks between the central (hypothalamus and retina) and peripheral (liver and gut) tissues. We suggest tissue-level circadian clock regulation of daily physiology and behavior in the spotted munia.

Mapping the brain of the chicken (Gallus gallus), with emphasis on the septal-hypothalamic region

There has been remarkable progress in discoveries made in the avian brain, particularly over the past two decades. This review first highlights some of the discoveries made in the forebrain and credits the Avian Brain Nomenclature Forum, responsible for changing many of the terms found in the cerebrum and for stimulating collaborative research thereafter. The Forum facilitated communication among comparative neurobiologists by eliminating confusing and inaccurate names. The result over the past 15yearshas been a standardized use of avian forebrain terms. Nonetheless, additional changes are needed. The goal of the paper is to encourage a continuing effort to unify the nomenclature throughout the entire avian brain. To emphasize the need for consensus for a single name for each neural structure, I have selected specific structures in the septum and hypothalamus that our laboratory has been investigating, to demonstrate a lack of uniformity in names applied to conservative brain regions compared to the forebrain. The specific areas reviewed include the distributions of gonadotropin-releasing hormone neurons and their terminal fields in circumventricular organs, deep-brain photoreceptors, gonadotropin inhibitory neurons and a complex structure and function of the nucleus of the hippocampal commissure.

Temporal Expression of c-fos and Genes Coding for Neuropeptides and Enzymes of Amino Acid and Amine Neurotransmitter Biosynthesis in Retina, Pineal and Hypothalamus of a Migratory Songbird: Evidence for Circadian Rhythm-Dependent Seasonal Responses

This study investigated whether, in photoperiodic songbirds, the circadian pacemaker system (CPS) connects to the seasonal photoperiodic responses, by changes at transcriptional level in the level and 24-h rhythm of its constituent neurotransmitters. We used black-headed buntings (Emberiza melanocephala), which exhibit distinct seasonal states in captivity under appropriate photoperiods and hence served as a useful model system. Under short days, buntings remain in the photosensitive state (Pse) (winter phenotype: non-migratory, non-breeding). Under long days, however, buntings undergo through early-photostimulated (spring phenotype: pre-migratory, pre-breeding), late photostimulated (summer phenotype: migratory, breeding) and photorefractory (autumn phenotype: post-breeding) states. During all four seasonal states, we measured in the retina, pineal and hypothalamus, which together form avian CPS, 4-hourly mRNA expression of c-fos (a neuronal-activity marker) and of genes coding for neuropeptides (vasoactive intestinal peptide, vip; somatostatin, sst; neuropeptide Y, npy) and for intermediary enzymes of amino acid (glutamate: glutaminase, gls and glutamic-oxaloacetic transaminase 2, got2; GABA: glutamic acid decarboxylase, gad65) and amine (dopamine: tyrosine hydroxylase, th) neurotransmitters biosynthetic pathway. There was a significant alteration in level and 24-h pattern of mRNA expression, albeit with seasonal differences in presence, waveform parameters and phase relationship of 24-h rhythm, of different genes. Particularly, mRNA expression of all candidate genes (except hypothalamic vip, pineal gls and retinal th) was arrhythmic in late photostimulated state. These results underscore that circadian rhythm of peptide, amino acid and amine neurotransmitter biosynthesis in CPS plays a critical role in the photoperiodic regulation of seasonal states in birds.

Circannual testis and moult cycles persist under photoperiods that disrupt circadian activity and clock gene cycles in spotted munia

We investigated whether circannual rhythms underlying annual testis maturation and moult cycles are independent of duration and frequency of the light period and circadian clock control in non-photoperiodic spotted munia. Birds were subjected to an aberrant light-dark (LD) cycle (3.5 h L:3.5 h D; T7, where T is the period length of the LD cycle) and continuous light (LL, 24 h L:0 h D), with controls on 12 h L:12 h D (T24, 24 h LD cycle). We measured the behavioural activity pattern of the birds and 24 h mRNA oscillations of circadian clock genes (bmal1, clock, per2, cry1, cry2) in the hypothalamus, the putative site of seasonal timing. Diurnal munia were rhythmic in behaviour with the period of the activity-rest cycle matched to T7 and T24, and became behaviourally arrhythmic with activity scattered throughout 24 h under LL. Similarly, exposure to 3.5 h L:3.5 h D and LL caused arrhythmicity in 24 h clock gene expression, suggesting disruption of internal circadian timing at the transcriptional level; a significant rhythm was found under 12 h L:12 h D. During an exposure of 80 weeks, munia showed two to three cycles of testis maturation and wing primaries moult under all photoperiods, although with a longer period under 12L:12D. Thus, the frequency of light period under 3.5 h L:3.5 h D or LL disrupted circadian clock gene cycles, but did not affect the generation of circannual testis and moult cycles. We conclude that the prevailing light environment and hypothalamic circadian gene cycles do not exert direct control on the timing of the annual reproductive cycle in spotted munia, suggesting independent generation of the circadian and circannual rhythms in seasonally breeding species.

Effect of monochromatic light on circadian rhythmic expression of clock genes and arylalkylamine N-acetyltransferase in chick retina

Birds have more developed visual function. They not only have the ability to detect light and darkness but also have the color vision. Previous study showed that monochromatic light influenced avian physiological processes, which were controlled by clock genes. Therefore, bird's eye is a good model to studying the impact of color of light on circadian rhythms. Avian retina is one of the most important central oscillations. The study was designed to investigate the effect of color of light on the expression of clock genes and arylalkylamine N-acetyltransferase (Aanat) mRNA expression in chick retina. A total of 240 post-hatching day (P) 0 broiler chickens were exposed to blue (BL), green (GL), red (RL) and white light (WL) from a LED system under a light-dark cycle 12L:12D for 14 d. The results show that the significant daily variations existed in the gene expression of cBmal1, cBmal2, cCry1, cCry2, cPer2 and cPer3, but not for cClock under four light treatments. The genes cBmal1, cCry1, cPer2 and cPer3 presented circadian rhythmic expression under the various monochromatic lights. When compared with WL, GL elevated the expression of positive regulators of cellular clock (cBmal1, cBmal2 and cClock) and cAanat mRNA level, whereas RL increased the mRNA levels of negative regulators of cellular clock (cCry1, cCry2, cPer2 and cPer3) and decreased the cAanat mRNA expression in the retina. These results demonstrated that monochromatic light affect the periodic expression levels of the biological clock mRNA by positive and negative feedback loop interactions, GL activated the transcription of cAanat; while RL suppressed the transcription of cAanat. Thereby, color of light regulates ocular cAanat expression by affecting on expression of cellular clock regulators.

Effect of monochromatic light on circadian rhythmic expression of clock genes in the hypothalamus of chick

To clarify the effect of monochromatic light on circadian clock gene expression in chick hypothalamus, a total 240 newly hatched chickens were reared under blue light (BL), green light (GL), red light (RL) and white light (WL), respectively. On the post-hatched day 14, 24-h profiles of seven core clock genes (cClock, cBmal1, cBmal2, cCry1, cCry2, cPer2 and cPer3) were measured at six time points (CT 0, CT 4, CT 8, CT 12, CT 16, CT 20, circadian time). We found all these clock genes expressed with a significant rhythmicity in different light wavelength groups. Meanwhile, cClock and cBmal1 showed a high level under GL, and followed a corresponding high expression of cCry1. However, RL decreased the expression levels of these genes. Be consistent with the mRNA level, CLOCK and BMAL1 proteins also showed a high level under GL. The CLOCK-like immunoreactive neurons were observed not only in the SCN, but also in the non-SCN brain region such as the nucleus anterior medialis hypothalami, the periventricularis nucleus, the paraventricular nucleus and the median eminence. All these results are consistent with the auto-regulatory circadian feedback loop, and indicate that GL may play an important role on the circadian time generation and development in the chick hypothalamus. Our results also suggest that the circadian clock in the chick hypothalamus such as non-SCN brain region were involved in the regulation of photo information.

Seasonal alterations in the daily rhythms in hypothalamic expression of genes involved in the photoperiodic transduction and neurosteroid-dependent processes in migratory blackheaded buntings

The present study investigated seasonal alterations in the daily rhythms of hypothalamic expression of genes involved in the photoperiodic regulation of annual cycles in birds. We measured the 4-hourly mRNA expression of genes involved in the photoperiodic transduction (OPN5, EYA3, CGA, TSHβ, DIO2, DIO3) and neurosteroid-dependent processes (AR, CYP19, ERα, ERβ) in the hypothalamus of migratory blackheaded buntings photoinduced with photosensitive, photostimulated (early and late stimulated) and photorefractory seasonal states. There were significant differences in daily mRNA profiles between the photoperiodic states. Particularly, increased CGA, TSHβ and DIO2 and decreased DIO3 mRNA levels in the early photostimulated state, compared to the photosensitive state, suggest that thyroid hormones have a role in photostimulation in buntings. Similar differences in the expression of genes coding for the aromatase enzyme (CYP19) and receptors for oestrogen (ERα, ERβ) (but not androgen; AR) indicate that there is seasonal alteration in the neuro-oestrogen-mediated functions. Furthermore, peak expression times of CGA, TSHβ and DIO2 genes at hours 14-15 of the day in the early stimulated state indicated molecular regulation of the daily rhythm of photoinducibility in buntings. Most significantly, however, we found an attenuated daily rhythm in thyroid hormone modulatory genes and a switch of peak expression time from day to night in CYP19 mRNA rhythm in the subsequent late photostimulated state, although testicular maturation still persisted. These alterations in daily rhythms may have signalled the initiation of processes underlying other seasonal phenologies in parallel with the gonadal response, such as a manifestation of the night-time flight in buntings. These results show alterations in daily rhythms underlying the transcriptional regulation of the photoperiod-induced seasonal states in migratory blackheaded buntings.

Effects of continuous white light and 12h white-12h blue light-cycles on the expression of clock genes in diencephalon, liver, and skeletal muscle in chicks

The core circadian clock mechanism relies on a feedback loop comprised of clock genes, such as the brain and muscle Arnt-like 1 (Bmal1), chriptochrome 1 (Cry1), and period 3 (Per3). Exposure to the light-dark cycle synchronizes the master circadian clock in the brain, and which then synchronizes circadian clocks in peripheral tissues. Birds have long been used as a model for the investigation of circadian rhythm in human neurobiology. In the present study, we examined the effects of continuous light and the combination of white and blue light on the expression of clock genes (Bmal1, Cry1, and Per3) in the central and peripheral tissues in chicks. Seventy two day-old male chicks were weighed, allocated to three groups and maintained under three light schedules: 12h white light-12h dark-cycles group (control); 24h white light group (WW group); 12h white light-12h blue light-cycles group (WB group). The mRNA levels of clock genes in the diencephalon were significantly different between the control and WW groups. On the other hand, the alteration in the mRNA levels of clock genes was similar between the control and WB groups. Similar phenomena were observed in the liver and skeletal muscle (biceps femoris). These results suggest that 12h white-12h blue light-cycles did not disrupt the circadian rhythm of clock gene expression in chicks.

Extra-hypothalamic brain clocks in songbirds: Photoperiodic state dependent clock gene oscillations in night-migratory blackheaded buntings, Emberiza melanocephala

The avian circadian pacemaker system is comprised of independent clocks in the retina, pineal and hypothalamus, as shown by daily and circadian oscillations of core clock genes (Per2, Cry1, Bmal1 and Clock) in several birds including migratory blackheaded buntings (Emberiza melanocephala). This study investigated the extra-hypothalamic brain circadian clocks in blackheaded buntings, and measured Per2, Cry1, Cry2, Bmal1 and Clock mRNA expressions at 4h intervals over 24h beginning 1h after light-on in the left and right telencephalon, optic tectum and cerebellum, the brain regions involved in several physiological and cognitive functions. Because of seasonal alterations in the circadian clock dependent brain functions, we measured daily clock gene oscillations in buntings photoperiod-induced with the non-migratory state under short days (SDnM), and the pre-migratory (LDpM), migratory (LDM) and post-migratory (refractory, LDR) states under long days. Daily Per2 oscillations were not altered with changes in the photoperiodic states, except for about 2-3h phase difference in the optic tectum between the SDnM and LDpM states. However, there were about 3-5h differences in the phase and 2 to 4 fold change in the amplitude of daily Bmal1 and Cry1 mRNA oscillations between the photoperiod-induced states. Further, Cry2 and Clock genes lacked a significant oscillation, except in Cb (Cry2) and TeO and Rt (Clock) under LDR state. Overall, these results show the presence of circadian clocks in extra-hypothalamic brain regions of blackheaded buntings, and suggest tissue-dependent alterations in the waveforms of mRNA oscillations with transitions in the photoperiod-induced seasonal states in a long-day species.

Molecular and Neuroendocrine Mechanisms of Avian Seasonal Reproduction

Animals living outside tropical zones experience seasonal changes in the environment and accordingly, adapt their physiology and behavior in reproduction, molting, and migration. Subtropical birds are excellent models for the study of seasonal reproduction because of their rapid and dramatic response to changes in photoperiod. For example, testicular weight typically changes by more than a 100-fold. In birds, the eyes are not necessary for seasonal reproduction, and light is instead perceived by deep brain photoreceptors. Functional genomic analysis has revealed that long day (LD)-induced thyrotropin from the pars tuberalis of the pituitary gland causes local thyroid hormone (TH) activation within the mediobasal hypothalamus. This local bioactive TH, triiodothyronine (T₃), appears to regulate seasonal gonadotropin-releasing hormone (GnRH) secretion through morphological changes in neuro-glial interactions. GnRH, in turn, stimulates gonadotropin secretion and hence, gonadal development under LD conditions. In marked contrast, low temperatures accelerate short day (SD)-induced testicular regression in winter. Interestingly, low temperatures increase circulating levels of T₃ to support adaptive thermogenesis, but this induction of T₃ also triggers the apoptosis of germ cells by activating genes involved in metamorphosis. This apparent contradiction in the role of TH has recently been clarified. Central activation of TH during spring results in testicular growth, while peripheral activation of TH during winter regulates adaptive thermogenesis and testicular regression.

Role of monochromatic light on daily variation of clock gene expression in the pineal gland of chick

The avian pineal gland is a master clock that can receive external photic cues and translate them into output rhythms. To clarify whether a shift in light wavelength can influence the circadian expression in chick pineal gland, a total of 240 Arbor Acre male broilers were exposed to white light (WL), red light (RL), green light (GL) or blue light (BL). After 2weeks light illumination, circadian expressions of seven core clock genes in pineal gland and the level of melatonin in plasma were examined. The results showed after illumination with monochromatic light, 24h profiles of all clock gene mRNAs retained circadian oscillation, except that RL tended to disrupt the rhythm of cCry2. Compared to WL, BL advanced the acrophases of the negative elements (cCry1, cCry2, cPer2 and cPer3) by 0.1-1.5h and delayed those of positive elements (cClock, cBmal1 and cBmal2) by 0.2-0.8h. And, RL advanced all clock genes except cClock and cPer2 by 0.3-2.1h, while GL delayed all clock genes by 0.5-1.5h except cBmal2. Meanwhile, GL increased the amplitude and mesor of positive and reduced both parameters of negative clock genes, but RL showed the opposite pattern. Although the acrophase of plasma melatonin was advanced by both GL and RL, the melatonin level was significantly increased in GL and decreased in RL. This tendency was consistent with the variations in the positive clock gene mRNA levels under monochromatic light and contrasted with those of negative clock genes. Therefore, we speculate that GL may enhance positive clock genes expression, leading to melatonin synthesis, whereas RL may enhance negative genes expression, suppressing melatonin synthesis.

Molecular basis for regulating seasonal reproduction in vertebrates

Animals that inhabit mid- to high-latitude regions exhibit various adaptive behaviors, such as migration, reproduction, molting and hibernation in response to seasonal cues. These adaptive behaviors are tightly regulated by seasonal changes in photoperiod, the relative day length vs night length. Recently, the regulatory pathway of seasonal reproduction has been elucidated using quail. In birds, deep brain photoreceptors receive and transmit light information to the pars tuberalis in the pituitary gland, which induces the secretion of thyroid-stimulating hormone. Thyroid-stimulating hormone locally activates thyroid hormone via induction of type 2 deiodinase in the mediobasal hypothalamus. Thyroid hormone then induces morphological changes in the terminals of neurons that express gonadotropin-releasing hormone and facilitates gonadotropin secretion from the pituitary gland. In mammals, light information is received by photoreceptors in the retina and neurally transmitted to the pineal gland, where it inhibits the synthesis and secretion of melatonin, which is crucial for seasonal reproduction. Importantly, the signaling pathway downstream of light detection and signaling is fully conserved between mammals and birds. In fish, the regulatory components of seasonal reproduction are integrated, from light detection to neuroendocrine output, in a fish-specific organ called the saccus vasculosus. Various physiological processes in humans are also influenced by seasonal environmental changes. The findings discussed herein may provide clues to addressing human diseases, such as seasonal affective disorder.

Annual gonadal cycles in birds: modeling the effects of photoperiod on seasonal changes in GnRH-1 secretion

This paper reviews current knowledge of photoperiod control of GnRH-1 secretion and proposes a model in which two processes act together to regulate GnRH1 secretion. Photo-induction controls GnRH1 secretion and is directly related to prevailing photoperiod. Photo-inhibition, a longer term process, acts through GnRH1 synthesis. It progresses each day during daylight hours, but reverses during darkness. Thus, photo-inhibition gradually increases when photoperiods exceed 12h, and reverses under shorter photoperiods. GnRH1 secretion on any particular day is the net result of these two processes acting in tandem. The only difference between species is their sensitivity to photo-inhibition. This can potentially explain differences in timing and duration of breeding seasons between species, why some species become absolutely photorefractory and others relatively photorefractory, why breeding seasons end at the same time at different latitudes within species, and why experimental protocols sometimes produce results that appear counter to what happens naturally.