Temperature affects liver and muscle metabolism in photostimulated migratory redheaded buntings (Emberiza bruniceps)

Effect of Photoperiod and Illuminance on Daily Activity Patterns, Physiology, and NPY Peptide Expression in Migratory Redheaded Buntings (Emberiza bruniceps)

INTRODUCTION

Light is the primary source of energy and regulates seasonal changes in physiology and behavior. The role of photoperiod has been much investigated in several bird species, but the role of illumination in seasonal adaptations of passerine finches is less understood. We, therefore, investigated the effects of photoperiod and illuminance on migratory physiology in a Palearctic-Indian migratory finch, redheaded bunting (Emberiza bruncieps).

METHODS

Photosensitive buntings maintained under short days (8L:16D) were divided into three groups receiving 5, 25, and 100 lux of white daytime illuminance, respectively. Thereafter, using photoperiodic manipulation three life history states, i.e., nonmigratory (NM), premigratory (PM), and migratory (MIG) states were induced in the buntings. The birds in the MIG state were consecutively perfused after seven nights of Zugunruhe (nighttime migratory restlessness) for neuropeptide Y (NPY)-immunohistochemistry, which is involved in a wide range of functions including energy homeostasis, vision, and fat deposition in birds.

RESULTS

We found differential effects of illuminance on locomotor activity and physiology. Photostimulated birds showed intense nighttime activity in the MIG state. We observed premigratory hyperphagia in the birds, with increased food intake in the 100 lux group, which was reflected in the body mass gain in the MIG state. NPY expression on the periphery of the nucleus rotundus suggests its potential role in visual acuity, where the NPY-cell count significantly decreased under 25 lux illumination.

CONCLUSION

We demonstrate that migrating birds may also experience physiological effects from changes in daytime illumination. We observed illuminance-dependent variations in the quantity of food consumed by the birds. It indicates that the illuminance may also impact the encephalic centers that control food intake.

Understanding the role of temperature in seasonal timing: Effects on behavioural, physiological and molecular phenotypes

Organisms adapt to daily and seasonal environmental changes to maximise their metabolic and reproductive fitness. For seasonally breeding animals, photoperiod is considered the most robust cue to drive these changes. It, however, does not explain the interannual variations in different seasonal phenotypes. Several studies have repeatedly shown the influence of ambient temperature on the timing of different seasonal physiologies including the timing of migration, reproduction and its associated behaviours, etc. In the present review, we have discussed the effects of changes in ambient temperature on different seasonal events in endotherms with a focus on migratory birds as they have evolved to draw benefits from distinct but largely predictable seasonal patterns of natural resources. We have further discussed the physiological and molecular mechanisms by which temperature affects seasonal timings. The primary brain area involved in detecting temperature changes is the hypothalamic preoptic area. This area receives thermal inputs via sensory neurons in the peripheral ganglia that measure changes in thermoregulatory tissues such as the skin and spinal cord. For the input signals, several thermal sensory TRP (transient receptor potential ion channels) channels have been identified across different classes of vertebrates. These channels are activated at specific thermal ranges. Once perceived, this information should activate an effector function. However, the link between temperature sensation and the effector pathways is not properly understood yet. Here, we have summarised the available information that may help us understand how temperature information is translated into seasonal timing.

Photoperiodic modulation of melatonin receptor and immune genes in migratory redheaded bunting

The transcriptional regulation of innate immune function across annual life history states (LHS) remains obscure in avian migrants. We, therefore, investigated this in a migratory passerine songbird, redheaded bunting (Emberiza bruniceps), which exhibits long-distance vernal migration from India to Central Asia. We exposed the birds (N = 10) to differential photoperiodic conditions to induce a non-migratory (NM), pre-migratory (PM), migratory (MIG), and refractory (REF) state, and performed gene expression assays of melatonin receptors (MEL1A and MEL1B), and innate immunity-linked genes (IL1B, IL6, TLR4, and NFKB) in spleen and blood. We found a significant reduction in splenic mass and volume, and a parallel increase in fat accumulation, and testicular growth in birds under migratory state. The gene expression assay revealed an upregulation of MEL1A and MEL1B mRNA levels in both the tissues in MIG. Additionally, we found a nocturnal increase of splenic IL1B expression, and IL1B, IL6, and TLR4 expression in the blood. The mRNA expression of melatonin receptors and proinflammatory cytokine showed a positive correlation. These results suggest that melatonin relays the photoperiodic signal to peripheral immune organs, which shows LHS-dependent changes in mRNA expression of immune genes.

Daytime light spectrum affects photoperiodic induction of vernal response in obligate spring migrants

It is not well understood how the spectral composition (wavelength) of daylight that varies considerably during the day and seasons affects photoperiodic responses in a seasonal species. Here, we investigated the molecular underpinnings of wavelength-dependent photoperiodic induction in migratory redheaded buntings transferred to 13 h long days in neutral (white), 460 nm (blue), 500 nm (green) or 620 nm (red) wavelength that were compared with one another, and to short day controls for indices of the migratory (body fattening and weight gain, and Zugunruhe) and reproductive (testicular maturation) responses. Buntings showed wavelength-dependent photoperiodic response, with delayed Zugunruhe and slower testis maturation under 620 nm red light. Post-mortem comparison of gene expressions further revealed wavelength-dependence of the photoperiodic molecular response. Whereas there were higher retinal expressions of opn2 (rhodopsin) and opn5 (neuropsin) genes in red daylight, and of rhodopsin-like opsin (rh2) gene in green daylight, the hypothalamic opn2 mRNA levels were higher in blue daylight. Similarly, we found in birds under blue daylight an increased hypothalamic expression of genes involved in the photoperiodic induction (thyroid stimulating hormone subunit beta, tshb; eye absent 3, eya3; deiodinase type 2, dio2) and associated neural responses such as the calcium signaling (ATPase sarcoplasmic/endoplasmic reticulum Ca2+ transporting 2, atp2a2), dopamine biosynthesis (tyrosine hydroxylase, th) and neurogenesis (brain-derived neurotrophic factor, bdnf). These results demonstrate transcriptional changes in parallel to responses associated with migration and reproduction in buntings, and suggest a role of daylight spectrum in photoperiodic induction of the vernal response in obligate spring avian migrants.

Molecular changes associated with migratory departure from wintering areas in obligate songbird migrants

Day length regulates the development of spring migratory and subsequent reproductive phenotypes in avian migrants. This study used molecular approaches, and compared mRNA and proteome-wide expression in captive redheaded buntings that were photostimulated under long-day (LD) conditions for 4 days (early stimulated, LD-eS) or for ∼3 weeks until each bird had shown 4 successive nights of Zugunruhe (stimulated, LD-S); controls were maintained under short days. After ∼3 weeks of LD, photostimulated indices of the migratory preparedness (fattening, weight gain and Zugunruhe) were paralleled with upregulated expression of acc, dgat2 and apoa1 genes in the liver, and of cd36, fabp3 and cpt1 genes in the flight muscle, suggesting enhanced fatty acid (FA) synthesis and transport in the LD-S state. Concurrently, elevated expression of genes involved in the calcium ion signalling and transport (camk1 and atp2a2; camk2a in LD-eS), cellular stress (hspa8 and sod1, not nos2) and metabolic pathways (apoa1 and sirt1), but not of genes associated with migratory behaviour (adcyap1 and vps13a), were found in the mediobasal hypothalamus (MBH). Further, MBH-specific quantitative proteomics revealed that out of 503 annotated proteins, 28 were differentially expressed (LD-eS versus LD-S: 21 up-regulated and 7 down-regulated) and they enriched five physiological pathways that are associated with FA transport and metabolism. These first comprehensive results on gene and protein expression suggest that changes in molecular correlates of FA transport and metabolism may aid the decision for migratory departure from wintering areas in obligate songbird migrants.

Ambient temperature affects multiple drivers of physiology and behaviour: adaptation for timely departure of obligate spring migrants

We investigated the role of ambient temperature in departure from wintering areas of migratory black-headed buntings in spring. Birds transferred at 22 and 35°C to long days were compared with one another and with controls held on short days for indices of readiness to migrate (Zugunruhe, fattening, mass gain), levels of testosterone and gonadal recrudescence. Temperature affected the development of migratory behaviour and physiology: buntings under long days at 35°C, compared with those at 22°C, showed altered migratory behaviour (daily activity and Zugunruhe onset), and enhanced muscle growth and plasma testosterone levels, but showed no effect on testis growth. Temperature was perceived at both peripheral and central levels, and affected multiple molecular drivers culminating into the migratory phenotype. This was evidenced by post-mortem comparison of the expression of 13 genes with known functions in the skin (temperature-sensitive TRP channels: trpv4 and trpm8), hypothalamus and/or midbrain (migration-linked genes: th, ddc, adcyap1 and vps13a) and flight muscles (muscle growth associated genes: ar, srd5a3, pvalb, mtor, myod, mstn and hif1a). In photostimulated birds, the expression of trpv4 in skin, th in the hypothalamus and midbrain, and srd5a3, ar, pvalb and mtor in flight muscle, in parallel with testosterone levels, was greater at 35°C than at 22°C. These results demonstrate the role of ambient temperature in development of the spring migration phenotype, and suggest that transcriptional responsiveness to temperature is a component of the overall adaptive strategy in latitudinal songbird migrants for timely departure from wintering areas in spring.

Metabolic plasticity mediates differential responses to spring and autumn migrations: Evidence from gene expression patterns in migratory buntings

NEW FINDINGS

What is the central question of this study? What are the molecular underpinnings of seasonal metabolic plasticity during spring and autumn migrations in songbirds? What is the main finding and its importance? We report differences in mRNA levels of genes involved in the regulation of glucose and fat metabolism between photoinduced non-migratory and migratory states and between the spring and autumn migratory states. Higher expression of genes associated with fat mobilization and energy generation in the spring than in the autumn migration suggests differential activation of the metabolic pathways or alteration in the efficiency of existing functional machinery during annual journeys between nearly fixed destinations.

ABSTRACT

The molecular underpinnings of metabolic plasticity underlying differential responses to spring and autumn migrations are not well understood. We investigated this by examining the differences in mRNA levels of metabolic genes in the liver, muscle and adipose tissues of night-migratory red-headed buntings between photostimulated non-migratory and migratory states and between spring and autumn migratory states. Buntings accumulated more subcutaneous fat and hepatic lipid, had higher body mass, larger adipose cells and higher circulating triglyceride and free fatty acid levels and exhibited more intense Zugunruhe in the spring migratory state than in the autumn migratory state. More importantly, we found differences in the hepatic expression of pdc and pdk genes, indicating a differential acetyl-CoA requirement, and of the mdh and ogdh genes, suggesting differential oxidative phosphorylation between the non-migratory and migratory states and between the spring and autumn migratory states. Differences in fasn, bmal1 and glut1 mRNA levels were consistent with this and suggested seasonal differences in lipogenesis and/or glucose uptake. Likewise, differences in mRNA levels of genes coding for lipases (atgl and lpl) suggested that adipose triglycerides and free fatty acids serve largely as the metabolic substrate. Furthermore, changes in mRNA levels of genes coding for the fatty acid binding protein (fabp3) and fatty acid translocases (cd36) were consistent with differential fat fuel supply (via circulating free fatty acids) to aerobically exercising flight muscles between the spring and autumn migrations. These results show seasonal adaptation of genetic pathway(s) underlying seasonal metabolic plasticity that seems to mediate differential responses to spring and autumn migrations in latitudinal migratory songbirds.