Ghrelin affects stopover decisions and food intake in a long-distance migrant

Circulating profile of the appetite-regulating hormone ghrelin during moult-fast and chick provisioning in southern rockhopper penguins (Eudyptes chrysocome chrysocome)

A multitude of animal species undergo prolonged fasting events at regularly occurring life history stages. During such periods of food deprivation, individuals need to suppress their appetite. The satiety signalling gut hormone ghrelin has received much attention in this context in studies looking at mammalian systems. In wild birds, however, knowledge on the ghrelin system and its role during extended fasts is still scarce. In this study, we collected plasma samples for measurements of circulating ghrelin concentrations from adult southern rockhopper penguins (Eudyptes chrysocome chrysocome) during the three to four week-long moult-fast that they repeat annually to replace their feathers. We further sampled chicks before and after feeding bouts and non-moulting adults. Circulating ghrelin levels did not differ significantly between fed and unfed chicks but chicks had significantly lower plasma ghrelin levels compared to adults. Furthermore, penguins in late moult (i.e. individuals at the end of the prolonged fasting bout) had higher ghrelin levels compared to non-moulting adults. Our results show elevated levels of circulating ghrelin during moult and generally lower levels of ghrelin in chicks than in adults regardless of feeding state. Given the scarcity or absence of knowledge on the function of ghrelin in seabirds and in fasting birds in general, our results add greatly to our understanding of the avian ghrelin system.

Effects of ghrelin on food caching behaviour and body composition in black-capped chickadees (Poecile atricapillus)

Several metabolic hormones signal an organism's energy balance to the brain and modulate feeding behaviours accordingly. These metabolic signals may also regulate other behaviour related to energy balance, such as food caching or hoarding. Ghrelin is one such hormone, but it appears to exert different effects on appetite and fat levels in birds and mammals. Ghrelin treatment inhibits food intake and decreases fat stores in some bird species, but these effects may differ between acylated and unacylated (des-acyl) forms of ghrelin. The effect of ghrelin on food caching in birds has been examined in only one study, that found both leptin and unacylated ghrelin reduced food caching and mass gain in coal tits (Periparus ater). We expanded on this to test how both forms of ghrelin affect food caching and body composition in black-capped chickadees (Poecile atricapillus). We injected each bird with acylated ghrelin, unacylated ghrelin, and a saline control and then measured food caching every 20 min for two hours post-injection. We also measured body mass fat levels the day before, and after treatment using quantitative magnetic resonance (QMR). Contrary to prior work, we found no effects of either form of ghrelin on food caching, or body or fat mass. Future work is required to determine if the difference between our results and those of the prior study stems from species differences in response to ghrelin and/or in the motivation to cache food, or ghrelin effects being modulated by energy reserves.

Modular switches shift monarch butterfly migratory flight behavior at their Mexican overwintering sites

Eastern North American migratory monarch butterflies exhibit migratory behavioral states in fall and spring characterized by sun-dependent oriented flight. However, it is unclear how monarchs transition between these behavioral states at their overwintering site. Using a modified Mouritsen-Frost flight simulator, we confirm individual directionality and compass-based orientation (leading to group orientation) in fall migrants, and also uncover sustained flight propensity and direction-based flight reinforcement as distinctly migratory behavioral traits. By testing monarchs at their Mexican overwintering sites, we show that overwintering monarchs show reduced propensity for sustained flight and lose individual directionality, leading to the loss of group-level orientation. Overwintering fliers orient axially in a time-of-day dependent manner, which may indicate local versus long-distance directional heading. These results support a model of migratory flight behavior in which modular, state-dependent switches for flight propensity and orientation control are highly dynamic and are controlled in season- and location-dependent manners.

Toward understanding the endocrine regulation of diverse facultative migration strategies

Migration is an important event in the annual cycle of many animals that facilitates the use of resources that vary across space and time. It can occur with regular and predictable timing, as in obligate migration, or with much greater flexibility, as in facultative migration. Most research aimed at understanding the endocrine mechanisms regulating the transition to a migratory stage has focused on obligate migration, whereas less is known about facultative forms of migration. One challenge for research into the endocrine regulation of facultative migration is that facultative migrations encompass a diverse array of migratory movements. Here, we present a framework to describe and conceptualize variation in facultative migrations that focuses on conditions at departure. Within the context of this framework, we review potential endocrine mechanisms involved in the initiation of facultative migrations in vertebrates. We first focus on glucocorticoids, which have been the subject of most research on the topic. We then examine other potential hormones and neurohormones that have received less attention, but are exciting candidates to consider. We conclude by highlighting areas where future research is particularly needed.

Update on Feeding Regulation by Ghrelin in Birds: Focused on Brain Network

Ghrelin is known to be a feeding stimulatory hormone in mammals, but in birds, in contrast to mammals, the feeding behavior is regulated in inhibitory manners. This is because the neuropeptides associated with the regulation in the brain are different from those in mammals, i.e., it has been shown that, in chickens, a corticotropin-releasing hormone family peptide, urocortin, which is a feeding-inhibitory peptide, is mainly involved in the inhibitory mechanism. However, feeding is also regulated by various neurotransmitters in the brain, and recently, their interaction with the mechanisms underlying feeding inhibition by ghrelin in birds has been intensively studied and clarified. This review summarizes these findings.

Manipulation of photoperiod induces fat storage, but not fat mobilization in the migratory songbird, Dumetella carolinensis (Gray Catbird)

The annual cycle of migratory birds requires significant phenotypic remodeling. We sought to induce the migratory phenotype in Gray Catbirds by exposing them to a short-day light cycle. While adipose storage was stimulated, exceeding that typically seen in wild birds, other aspects of the migratory phenotype were unchanged. Of particular interest, the rate of lipid export from excised adipose tissue was nearly halved. This is in contrast to wild migratory birds in which lipid export rates are increased. These data suggest that exposure to an altered light cycle only activated the lipid storage program while inhibiting the lipid transport program. The factors governing lipid mobilization and transport remain to be elucidated.

Genetic Control of Avian Migration: Insights from Studies in Latitudinal Passerine Migrants

Twice-a-year, large-scale movement of billions of birds across latitudinal gradients is one of the most fascinating behavioral phenomena seen among animals. These seasonal voyages in autumn southwards and in spring northwards occur within a discrete time window and, as part of an overall annual itinerary, involve close interaction of the endogenous rhythm at several levels with prevailing photoperiod and temperature. The overall success of seasonal migrations thus depends on their close coupling with the other annual sub-cycles, namely those of the breeding, post-breeding recovery, molt and non-migratory periods. There are striking alterations in the daily behavior and physiology with the onset and end of the migratory period, as shown by the phase inversions in behavioral (a diurnal passerine bird becomes nocturnal and flies at night) and neural activities. Interestingly, there are also differences in the behavior, physiology and regulatory strategies between autumn and spring (vernal) migrations. Concurrent molecular changes occur in regulatory (brain) and metabolic (liver, flight muscle) tissues, as shown in the expression of genes particularly associated with 24 h timekeeping, fat accumulation and the overall metabolism. Here, we present insights into the genetic basis of migratory behavior based on studies using both candidate and global gene expression approaches in passerine migrants, with special reference to Palearctic-Indian migratory blackheaded and redheaded buntings.

Endocrine effects of exposure to artificial light at night: A review and synthesis of knowledge gaps

Animals have evolved with natural patterns of light and darkness, such that light serves as an important zeitgeber, allowing adaptive synchronization of behavior and physiology to external conditions. Exposure to artificial light at night (ALAN) interferes with this process, resulting in dysregulation of endocrine systems. In this review, we evaluate the endocrine effects of ALAN exposure in birds and reptiles, identify major knowledge gaps, and highlight areas for future research. There is strong evidence for ecologically relevant levels of ALAN acting as an environmental endocrine disruptor. However, most studies focus on the pineal hormone melatonin, corticosterone release via the hypothalamus-pituitary-adrenal axis, or regulation of reproductive hormones via the hypothalamus-pituitary-gonadal axis, leaving effects on other endocrine systems largely unknown. We call for more research spanning a diversity of hormonal systems and levels of endocrine regulation (e.g. circulating hormone levels, receptor numbers, strength of negative feedback), and investigating involvement of molecular mechanisms, such as clock genes, in hormonal responses. In addition, longer-term studies are needed to elucidate potentially distinct effects arising from chronic exposure. Other important areas for future research effort include investigating intraspecific and interspecific variability in sensitivity to light exposure, further distinguishing between distinct effects of different types of light sources, and assessing impacts of ALAN exposure early in life, when endocrine systems remain sensitive to developmental programming. The effects of ALAN on endocrine systems are likely to have a plethora of downstream effects, with implications for individual fitness, population persistence, and community dynamics, especially within urban and suburban environments.

Ghrelin, not corticosterone, is associated with transitioning of phenotypic states in a migratory Galliform

In both captive and free-living birds, the emergence of the migratory phenotype is signalled by rapid and marked increases in food intake and fuelling, as well as changes in amount of nocturnality or migratory restlessness. The metabolic hormone corticosterone and, as more recently suggested, the gut-derived hormone ghrelin have been suggested to play a role in mediating such phenomenal phenotypic flexibility given that they both regulate fuel metabolism and locomotion across vertebrate taxa. Here, using the Common quail (Coturnix coturnix) as our study species, we induced autumn migration followed by a non-migratory wintering phase through controlled changes in daylight. We thus compared plasma corticosterone and ghrelin concentrations between the two sampling phases and assessed whether these hormones might reflect the migratory state. While we found no differences in plasma corticosterone between the two sampling phases and no link of this hormone with changes in body mass, levels of food intake or migratory restlessness, the migratory birds had substantially higher levels of plasma ghrelin relative to the non-migratory birds. Furthermore, while ghrelin did not correlate with the gain in body mass over the entire pre-migratory fuelling phase (over an average of nine weeks preceding blood sampling), plasma ghrelin did positively correlate with the gain in body mass observed during the final fattening stages (over an average of three weeks preceding blood sampling). Again, variation in plasma ghrelin also reflected the amount of body mass depleted over both the long- and short-time frame as birds returned to their non-migratory baseline - lower levels of plasma ghrelin consistently correlated with larger losses in body mass. Thus, while our data do not highlight a role of the hormone corticosterone in sustaining pre-migratory fattening as shown in other bird species, they do add evidence for a potential role of ghrelin in mediating migratory behaviour and further suggest that this hormone might be important in regulating the transitioning of migratory states, possibly by promoting fuel mobilisation and usage.

Leptin administration does not influence migratory behaviour in white-throated sparrows (Zonotrichia albicollis)

Migratory flights by birds are among the most energetically demanding forms of animal movement, and are primarily fueled by fat as an energy source. Leptin is a critical fat-regulation hormone associated with energy balance in non-avian species but its function in birds is highly controversial. Prior research indicated the effects of leptin differed between birds in migratory condition or not, but no research has assessed the effect of leptin on migratory behaviour itself. In this study, our objective was to determine if leptin affects migratory restlessness and fat deposition in migratory songbirds. We used photoperiod manipulation to induce spring migratory condition, and measured migratory restlessness in leptin-injected and saline-injected white-throated sparrows (Zonotrichia albicollis). Leptin treatment had no effect on migratory restlessness nor fat deposition, providing evidence that leptin does not influence avian migratory motivation or behaviour. Our results also further support the idea that birds in a hyperphagic migratory condition may be insensitive to leptin.

Systemic infusion of exogenous ghrelin in male broiler chickens (Gallus gallus domesticus). The effect of pulse frequency, doses, and ghrelin forms on feed intake, average daily gain, corticosterone, and growth hormone concentrations

There is limited information on the effect of exogenous ghrelin infusion on feed intake (FI) in chickens. Therefore, male broilers were used in 3 factorial experiments to determine the relationships between doses (0, 1, or 4 nM; Dose), frequency (once every two h; 2 h), once every 4th h (4 h) or continuous infusion, and ghrelin forms including acylated-ghrelin (AG) and desacylated-ghrelin (DAG) on FI, ADG, and concentrations of corticosterone and Growth Hormone (GH). Treatments were delivered via a jugular cannula, using programmable pumps for 11 consecutive days. FI and ADG were recorded, and plasma was collected. Data were analyzed using a factorial design. In Experiment 1 the effect of AG pulse frequency and doses were evaluated. There was a linear decrease in FI (P = 0.002) and a linear increase in corticosterone (P = 0.033) and GH (P = 0.011) concentrations when AG was infused. However, ADG decreased with doses (P = 0.011) only when AG was given at 2 h. In Experiment 2 the effect of ghrelin forms and doses given at 2 h was evaluated. There was a linear decrease in FI when AG was infused and a linear increase in FI when DAG was infused (P < 0.05). Birds infused with DAG gained more weight than those infused with AG. There was a linear increase in corticosterone and GH concentrations only when AG was infused (P < 0.01). In Experiment 3 the effect of continuous infusion of 2 doses (0 and 1 nM) of AG and DAG were evaluated. There was a linear decrease in FI and ADG when AG (P < 0.001) was infused and a linear increase in FI and ADG when DAG was infused (P < 0.05). There was an increase in corticosterone concentrations only when AG was infused (P = 0.022). However, GH concentrations were not affected by treatments. We concluded that AG and DAG pulse frequency and doses had a differential effect on FI, ADG, corticosterone, and GH concentrations in broiler chickens.

Experimental ghrelin administration affects migratory behaviour in a songbird

Twice a year, billions of birds take on drastic physiological and behavioural changes to migrate between breeding and wintering areas. On migration, most passerine birds regularly stop over along the way to rest and refuel. Endogenous energy stores are not only the indispensable fuel to complete long distance flights, but are also important peripheral signals that once integrated in the brain modulate crucial behavioural decisions, such as the decision to resume migration after a stopover. A network of hormones signals metabolic fuel availability to the brain in vertebrates, including the recently discovered gut-hormone ghrelin. Here, we show that ghrelin takes part in the control of migratory behaviour during spring migration in a wild migratory passerine. We manipulated blood concentrations of ghrelin of 53 yellow-rumped warblers (Setophaga coronata coronata) caught during stopover and automatically radio-tracked their migratory behaviour following release. We found that injections of acylated and unacylated ghrelin rapidly induced movements away from the release site, indicating that the ghrelin system acts centrally to mediate stopover departure decisions. The effects of the hormone manipulation declined within 8 h following release, and did not affect the overall rate of migration. These results provide experimental evidence for a pivotal role of ghrelin in the modulation of behavioural decisions during migration. In addition, this study offers insights into the regulatory functions of metabolic hormones in the dialogue between gut and brain in birds.

Form and Function of the Vertebrate and Invertebrate Blood-Brain Barriers

The need to protect neural tissue from toxins or other substances is as old as neural tissue itself. Early recognition of this need has led to more than a century of investigation of the blood-brain barrier (BBB). Many aspects of this important neuroprotective barrier have now been well established, including its cellular architecture and barrier and transport functions. Unsurprisingly, most research has had a human orientation, using mammalian and other animal models to develop translational research findings. However, cell layers forming a barrier between vascular spaces and neural tissues are found broadly throughout the invertebrates as well as in all vertebrates. Unfortunately, previous scenarios for the evolution of the BBB typically adopt a classic, now discredited 'scala naturae' approach, which inaccurately describes a putative evolutionary progression of the mammalian BBB from simple invertebrates to mammals. In fact, BBB-like structures have evolved independently numerous times, complicating simplistic views of the evolution of the BBB as a linear process. Here, we review BBBs in their various forms in both invertebrates and vertebrates, with an emphasis on the function, evolution, and conditional relevance of popular animal models such as the fruit fly and the zebrafish to mammalian BBB research.

Limited flexibility in departure timing of migratory passerines at the East-Mediterranean flyway

The rapid pace of current global warming lead to the advancement of spring migration in the majority of long-distance migratory bird species. While data on arrival timing to breeding grounds in Europe is plentiful, information from the African departure sites are scarce. Here we analysed changes in arrival timing at a stopover site in Israel and any links to Enhanced Vegetation Index (EVI) on the species-specific African non-breeding range in three migratory passerines between 2000-2017. Differences in wing length between early and late arriving individuals were also examined as a proxy for migration distance. We found that male redstart, but not females, advanced arrival to stopover site, but interestingly, not as a response to EVI phenology. Blackcap and barred warbler did not shift arrival timing significantly, although the arrival of blackcap was dependent on EVI. Barred warbler from the early arrival phase had longer wings, suggesting different populations. Our study further supports the existence species-specific migration decisions and inter-sexual differences, which may be triggered by both exogenous (local vegetation condition) and endogenous cues. Given rapid rate of changes in environmental conditions at higher latitudes, some migrants may experience difficulty in the race to match global changes to ensure their survival.

Unexpected Association of Desacyl-Ghrelin with Physical Activity and Chronic Food Restriction: A Translational Study on Anorexia Nervosa

Anorexia nervosa (AN) is a severe metabopsychiatric disorder characterised by caloric intake restriction and often excessive physical exercise. Our aim is to assess in female AN patients and in a rodent model, the co-evolution of physical activity and potential dysregulation of acyl—(AG) and desacyl—(DAG) ghrelin plasma concentrations during denutrition and weight recovery. AN inpatients were evaluated at inclusion (T0, n = 29), half—(T1) and total (T2) weight recovery, and one month after discharge (T3, n = 13). C57/Bl6 mice with access to a running wheel, were fed ad libitum or submitted to short—(15 days) or long—(50 days) term quantitative food restriction, followed by refeeding (20 days). In AN patients, AG and DAG rapidly decreased during weight recovery (T0 to T2), AG increased significantly one-month post discharge (T3), but only DAG plasma concentrations at T3 correlated negatively with BMI and positively with physical activity. In mice, AG and DAG both increased during short- and long-term food restriction. After 20 days of ad libitum feeding, DAG was associated to persistence of exercise alteration. The positive association of DAG with physical activity during caloric restriction and after weight recovery questions its role in the adaptation mechanisms to energy deprivation that need to be considered in recovery process in AN.

Energy Stores, Oxidative Balance, and Sleep in Migratory Garden Warblers (Sylvia borin) and Whitethroats (Sylvia communis) at a Spring Stopover Site

Little is known about how songbirds modulate sleep during migratory periods. Due to the alternation of nocturnal endurance flights and diurnal refueling stopovers, sleep is likely to be a major constraint for many migratory passerine species. Sleep may help to increase the endogenous antioxidant capacity that counteracts free radicals produced during endurance flight and reduces energy expenditure. Here, we investigated the relationship between sleep behavior, food intake, and two markers of physiological condition-the amount of energy reserves and oxidative status-in two migratory songbird species, the garden warbler (Sylvia borin) and the whitethroat (Sylvia communis). In garden warblers, birds with high energy stores were more prone to sleep during the day, while this condition-dependent sleep pattern was not present in whitethroats. In both species, birds with low energy stores were more likely to sleep with their head tucked in the feathers during nocturnal sleep. Moreover, we found a positive correlation between food intake and the extent of energy reserves in garden warblers, but not in whitethroats. Finally, we did not find significant correlations between oxidative status and sleep, or oxidative status and energy stores. Despite our study was not comparative, it suggests that different species might use different strategies to manage their energy during stopover and, additionally, it raises the possibility that migrants have evolved physiological adaptations to deal with oxidative damage produced during migration.

The physiology of movement

Movement, from foraging to migration, is known to be under the influence of the environment. The translation of environmental cues to individual movement decision making is determined by an individual's internal state and anticipated to balance costs and benefits. General body condition, metabolic and hormonal physiology mechanistically underpin this internal state. These physiological determinants are tightly, and often genetically linked with each other and hence central to a mechanistic understanding of movement. We here synthesise the available evidence of the physiological drivers and signatures of movement and review (1) how physiological state as measured in its most coarse way by body condition correlates with movement decisions during foraging, migration and dispersal, (2) how hormonal changes underlie changes in these movement strategies and (3) how these can be linked to molecular pathways. We reveale that a high body condition facilitates the efficiency of routine foraging, dispersal and migration. Dispersal decision making is, however, in some cases stimulated by a decreased individual condition. Many of the biotic and abiotic stressors that induce movement initiate a physiological cascade in vertebrates through the production of stress hormones. Movement is therefore associated with hormone levels in vertebrates but also insects, often in interaction with factors related to body or social condition. The underlying molecular and physiological mechanisms are currently studied in few model species, and show -in congruence with our insights on the role of body condition- a central role of energy metabolism during glycolysis, and the coupling with timing processes during migration. Molecular insights into the physiological basis of movement remain, however, highly refractory. We finalise this review with a critical reflection on the importance of these physiological feedbacks for a better mechanistic understanding of movement and its effects on ecological dynamics at all levels of biological organization.

The amount of available food affects diurnal locomotor activity in migratory songbirds during stopover

Migratory passerine birds fly long distances twice a year alternating nocturnal flights with stopovers to rest and replenish energy stores. The duration of each stopover depends on several factors including internal clocks, meteorological conditions, and environmental factors such as availability of food. Foraging entails energetic costs, and if birds need to refuel efficiently, they should modulate their activity in relation to food availability. We investigated how food availability influences locomotor activity in migrating birds of six passerine species at a spring stopover site in the central Mediterranean Sea. We selected birds with low fat scores which we expected to be strongly motivated to refuel. We simulated stopover sites of different quality by providing temporarily caged birds with different amounts of food to simulate scarce to abundant food. We analysed the diurnal locomotory activity as a proxy for food searching effort. Low food availability resulted in an increased diurnal locomotor activity in almost all species, while all birds showed low intensity of nocturnal migratory restlessness. In conclusion, our study shows that food availability in an important determinant of behaviour of migratory birds at stopover sites.

Nonbreeder birds at colonies display qualitatively similar seasonal mass change patterns as breeders

The difficulty in studying nonbreeding birds means that little is known about them or their resource requirements, despite forming a large and significant component of a population. One way to assess food requirements is to examine changes in body mass, because it indicates the amount of food acquired. In terms of body mass changes, our expectation is that nonbreeders will either (a) be in poorer condition than the breeders which potentially explains why they do not breed or (b) remain at a stable higher mass as they are unconstrained by the physiological costs associated with rearing chicks. Here, we interrogate body mass datasets of breeding and nonbreeding birds of two penguin species to assess these predictions and determine whether differences in mass exist between these two groups throughout the breeding season. The first dataset is from a wild Adélie penguin population, where bird mass was recorded automatically and breeding status determined from a resighting program. A second population of captive gentoo penguins were weighed regularly each breeding season. We demonstrate that although there were times in each year when breeders were heavier than their nonbreeding counterparts for both populations, the mass changes showed qualitatively similar patterns throughout the breeding season irrespective of breeding status. Heavier breeders at times during the breeding season are not unexpected but the overall similar pattern of mass change irrespective of breeding status is in contrast to expectations. It appears that breeding status per se and the constraints that breeding places on birds are not the only driver of changes in mass throughout the breeding season and, although not explicitly studied here, the role of hormones in driving changes in appetite could be key to explain these results. These results present a significant step toward understanding food requirements of nonbreeders in avian populations.

Immune function and blood parasite infections impact stopover ecology in passerine birds

Stopovers play a crucial role for the success of migrating animals and are key to optimal migration theory. Variation in refuelling rates, stopover duration and departure decisions among individuals has been related to several external factors. The physiological mechanisms shaping stopover ecology are, however, less well understood. Here, we explore how immune function and blood parasite infections relate to several aspects of stopover behaviour in autumn migrating short- and long-distance migrating songbirds. We blood sampled individuals of six species and used an automated radio-telemetry system in the stopover area to subsequently quantify stopover duration, 'bush-level' activity patterns (~ 0.1-30 m), landscape movements (~ 30-6000 m), departure direction and departure time. We show that complement activity, the acute phase protein haptoglobin and blood parasite infections were related to prolonged stopover duration. Complement activity (i.e., lysis) and total immunoglobulins were negatively correlated with bush-level activity patterns. The differences partly depended on whether birds were long-distance or short-distance migrants. Birds infected with avian malaria-like parasites showed longer landscape movements during the stopover than uninfected individuals, and birds with double blood parasite infections departed more than 2.5 h later after sunset/sunrise suggesting shorter flight bouts. We conclude that variation in baseline immune function and blood parasite infection status affects stopover ecology and helps explain individual variation in stopover behaviour. These differences affect overall migration speed, and thus can have significant impact on migration success and induce carry-over effects on other annual-cycle stages. Immune function and blood parasites should, therefore, be considered as important factors when applying optimal migration theory.

Peripherally injected ghrelin and leptin reduce food hoarding and mass gain in the coal tit (Periparus ater)

In birds little is known about the hormonal signals that communicate nutritional state to the brain and regulate appetitive behaviours. In mammals, the peptide hormones ghrelin and leptin elevate and inhibit consumption and food hoarding, respectively. But in birds, administration of both ghrelin and leptin inhibit food consumption. The role of these hormones in the regulation of food hoarding in avian species has not been examined. To investigate this, we injected wild caught coal tits (Periparus ater) with leptin, high-dose ghrelin, low-dose ghrelin and a saline control in the laboratory. We then measured food hoarding and mass gain, as a proxy of food consumption, every 20 min for 2 h post-injection. Both high-dose ghrelin and leptin injections significantly reduced hoarding and mass gain compared with controls. Our results provide the first evidence that hoarding behaviour can be reduced by both leptin and ghrelin in a wild bird. These findings add to evidence that the hormonal control of food consumption and hoarding in avian species differs from that in mammals. Food hoarding and consumptive behaviours consistently show the same response to peripheral signals of nutritional state, suggesting that the hormonal regulation of food hoarding has evolved from the consumption regulatory system.

Cues, corticosterone and departure decisions in a partial migrant

Most migrating birds make multiple stopovers to fuel and/or rest. The decision to resume migration from stopover is based on various cues, such as time within the season and wind conditions. There are hints that the strength of these departure cues shapes corticosterone level, which in its turn appears to regulate the timing of departure. We here provide results that very strongly indicate that indeed departure cues jointly shape corticosterone level of migrants at stopover. We compared corticosterone level between migrating and sedentary common blackbirds (Turdus merula) sampled simultaneously at the same location during autumn migration. As expected, in migrating individuals corticosterone level was positively associated with time within the season and with current wind conditions. The latter was only apparent in adult birds and not in 1st year migrants, thus matching the observation that 1st year autumnal migrants are less wind selective than adults. In contrast to the migrants, in sedentary blackbirds these "cues" did not explain variation in corticosterone level. Furthermore, stopover departure seemed more likely and to occur earlier in the night in migrants with high corticosterone level. Our unique comparative study thus supports the newly developed concept that corticosterone mediates between departure cues and stopover departure timing in avian migrants.

Comparative transcriptome analysis of hypothalamus-regulated feed intake induced by exogenous visfatin in chicks

Background

The intracerebroventricular injection of visfatin increases feed intake. However, little is known about the molecular mechanism in chicks. This study was conducted to assess the effect of visfatin on the feeding behavior of chicks and the associated molecular mechanism.

Results

In response to the intraventricular injection of 40 ng and 400 ng visfatin, feed intake in chicks was significantly increased, and the concentrations of glucose, insulin, TG, HDL and LDL were significantly altered. Using RNA-seq, we identified DEGs in the chick hypothalamus at 60 min after injection with various doses of visfatin. In total, 325, 85 and 519 DEGs were identified in the treated chick hypothalamus in the LT vs C, HT vs C and LT vs HT comparisons, respectively. The changes in the expression profiles of DEGs, GO functional categories, KEGG pathways, and PPI networks by visfatin-mediated regulation of feed intake were analyzed. The DEGs were grouped into 8 clusters based on their expression patterns via K-mean clustering; there were 14 appetite-related DEGs enriched in the hormone activity GO term. The neuroactive ligand-receptor interaction pathway was the key pathway affected by visfatin. The PPI analysis of DEGs showed that POMC was a hub gene that interacted with the maximum number of nodes and ingestion-related pathways, including POMC, CRH, AgRP, NPY, TRH, VIP, NPYL, CGA and TSHB.

Conclusion

These common DEGs were enriched in the hormone activity GO term and the neuroactive ligand-receptor interaction pathway. Therefore, visfatin causes hyperphagia via the POMC/CRH and NPY/AgRP signaling pathways. These results provide valuable information about the molecular mechanisms of the regulation of food intake by visfatin.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4644-7) contains supplementary material, which is available to authorized users.

Obese super athletes: fat-fueled migration in birds and bats

Migratory birds are physiologically specialized to accumulate massive fat stores (up to 50-60% of body mass), and to transport and oxidize fatty acids at very high rates to sustain flight for many hours or days. Target gene, protein and enzyme analyses and recent -omic studies of bird flight muscles confirm that high capacities for fatty acid uptake, cytosolic transport, and oxidation are consistent features that make fat-fueled migration possible. Augmented circulatory transport by lipoproteins is suggested by field data but has not been experimentally verified. Migratory bats have high aerobic capacity and fatty acid oxidation potential; however, endurance flight fueled by adipose-stored fat has not been demonstrated. Patterns of fattening and expression of muscle fatty acid transporters are inconsistent, and bats may partially fuel migratory flight with ingested nutrients. Changes in energy intake, digestive capacity, liver lipid metabolism and body temperature regulation may contribute to migratory fattening. Although control of appetite is similar in birds and mammals, neuroendocrine mechanisms regulating seasonal changes in fuel store set-points in migrants remain poorly understood. Triacylglycerol of birds and bats contains mostly 16 and 18 carbon fatty acids with variable amounts of 18:2n-6 and 18:3n-3 depending on diet. Unsaturation of fat converges near 70% during migration, and unsaturated fatty acids are preferentially mobilized and oxidized, making them good fuel. Twenty and 22 carbon n-3 and n-6 polyunsaturated fatty acids (PUFA) may affect membrane function and peroxisome proliferator-activated receptor signaling. However, evidence for dietary PUFA as doping agents in migratory birds is equivocal and requires further study.

Endocrine regulation of migratory departure from stopover: Evidence from a longitudinal migratory restlessness study on northern wheatears

Most migrating birds make stopovers to replenish fuel stores. The decision to resume migration from stopover to a large extent shapes the temporal organization of migration. This decision is known to be shaped by a suite of intrinsic and extrinsic factors such as the bird's fuel stores and current weather conditions. However, how departures from stopover are physiologically regulated is largely unknown. We here present data that strongly indicate that corticosterone, a hormone with a stimulatory effect on locomotion, acts as a mediator between fuel stores and departure from stopover. In migrating northern wheatears (Oenanthe oenanthe) temporarily caged at stopover, we observed a positive relationship between the change in fuel stores and the concurrent change in glucocorticoid metabolite (GCM) levels measured in the birds' droppings. We also found a positive relationship between the change in GCM levels and the change in the intensity of nocturnal migratory restlessness. As in northern wheatears nocturnal migratory restlessness is an accurate proxy for stopover departure likelihood, our results indicate that corticosterone mediates between fuel stores and the decision to resume migration. Our unique longitudinal study represents a considerable advance in our understanding of the endocrine regulation of avian migration.

Understanding variation in migratory movements: A mechanistic approach

Spatial and temporal fluctuations in resource availability have led to the evolution of varied migration patterns. In order to appropriately time movements in relation to resources, environmental cues are used to provide proximate information for timing and the endocrine system serves to integrate these external cues and behavioral and physiological responses. Yet, the regulatory mechanisms underlying migratory timing have rarely been compared across a broad range of migratory patterns. First, we offer an updated nomenclature of migration using a mechanistic perspective to clarify terminology describing migratory types in relation to ecology, behavior and endocrinology. We divide migratory patterns into three types: obligate, nomadic, and fugitive. Obligate migration is characterized by regular and directed annual movements between locations, most commonly for breeding and overwintering, where resources are predictable and sufficient. Nomadic migrations occur less predictably than do obligate migrations as animals make use of potentially rich but ephemeral resources that occur unpredictably in space or time. Fugitive migrations move animals away from an area in response to severe disruption of environmental conditions and occur as part of an emergency life history stage. We also consider partially migratory populations, which include a mix of sedentary and migratory individuals; the movement patterns of partial migrants are expected to fall into one of the three types above. For these various forms of migration, we review our understanding of the environmental cues and endocrine mechanisms that underlie the expression of a migratory state. Several common hormonal mechanisms exist across the varied migratory forms, but there are also important areas where further investigations are needed in order to gain broad insight into the origin of movements and the diversity of migratory patterns. We propose that taking a comparative approach across the migratory types that considers endocrine mechanisms will advance a new understanding of migration biology.

Timing avian long-distance migration: from internal clock mechanisms to global flights

Migratory birds regularly perform impressive long-distance flights, which are timed relative to the anticipated environmental resources at destination areas that can be several thousand kilometres away. Timely migration requires diverse strategies and adaptations that involve an intricate interplay between internal clock mechanisms and environmental conditions across the annual cycle. Here we review what challenges birds face during long migrations to keep track of time as they exploit geographically distant resources that may vary in availability and predictability, and summarize the clock mechanisms that enable them to succeed. We examine the following challenges: departing in time for spring and autumn migration, in anticipation of future environmental conditions; using clocks on the move, for example for orientation, navigation and stopover; strategies of adhering to, or adjusting, the time programme while fitting their activities into an annual cycle; and keeping pace with a world of rapidly changing environments. We then elaborate these themes by case studies representing long-distance migrating birds with different annual movement patterns and associated adaptations of their circannual programmes. We discuss the current knowledge on how endogenous migration programmes interact with external information across the annual cycle, how components of annual cycle programmes encode topography and range expansions, and how fitness may be affected when mismatches between timing and environmental conditions occur. Lastly, we outline open questions and propose future research directions.This article is part of the themed issue 'Wild clocks: integrating chronobiology and ecology to understand timekeeping in free-living animals'.

Ghrelin enhances food intake and carbohydrate oxidation in a nitric oxide dependent manner

In the present study we sought to investigate interactions between hypothalamic nitric oxide (NO) and ghrelin signaling on food intake and energy substrate utilization as measured by the respiratory exchange ratio (RER). Guide cannulae were unilaterally implanted in either the arcuate (ArcN) or paraventricular (PVN) nuclei of male Sprague-Dawley rats. Animals were pretreated with subcutaneous (2.5-10mg/kg/ml) or central (0-100pmol) N-nitro-l-Arginine methyl ester (l-NAME) followed by 50pmol of ghrelin administered into either the ArcN or PVN. Both l-NAME and ghrelin were microinjected at the onset of the active cycle and food intake and RER were assessed 2h postinjection. RER was measured as the ratio of the volume of carbon dioxide expelled relative to the volume of oxygen consumed (VCO₂/VO₂) using an open-circuit indirect calorimeter. Our results demonstrated that peripheral and central l-NAME pretreatment dose-dependently attenuated ghrelin induced increases in food intake and RER in either the ArcN or PVN. In fact the 100pmol dose largely reversed the metabolic effects of ghrelin in both anatomical regions. These findings suggest that ghrelin enhancement of food intake and carbohydrate oxidation in the rat ArcN and PVN is NO-dependent.

Regulation of Agouti-Related Protein and Pro-Opiomelanocortin Gene Expression in the Avian Arcuate Nucleus

The arcuate nucleus is generally conserved across vertebrate taxa in its neuroanatomy and neuropeptide expression. Gene expression of agouti-related protein (AGRP), neuropeptide Y (NPY), pro-opiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcript (CART) has been established in the arcuate nucleus of several bird species and co-localization demonstrated for AGRP and NPY. The proteins encoded by these genes exert comparable effects on food intake in birds after central administration to those seen in other vertebrates, with AGRP and NPY being orexigenic and CART and α-melanocyte-stimulating hormone anorexigenic. We have focused on the measurement of arcuate nucleus AGRP and POMC expression in several avian models in relation to the regulation of energy balance, incubation, stress, and growth. AGRP mRNA and POMC mRNA are, respectively, up- and downregulated after energy deprivation and restriction. This suggests that coordinated changes in the activity of AGRP and POMC neurons help to drive the homeostatic response to replace depleted energy stores in birds as in other vertebrates. While AGRP and POMC expression are generally positively and negatively correlated with food intake, respectively, we review here situations in some avian models in which AGRP gene expression is dissociated from the level of food intake and may have an influence on growth independent of changes in appetite. This suggests the possibility that the central melanocortin system exerts more pleiotropic functions in birds. While the neuroanatomical arrangement of AGRP and POMC neurons and the sensitivity of their activity to nutritional state appear generally conserved with other vertebrates, detailed knowledge is lacking of the key nutritional feedback signals acting on the avian arcuate nucleus and there appear to be significant differences between birds and mammals. In particular, recently identified avian leptin genes show differences between bird species in their tissue expression patterns and appear less closely linked in their expression to nutritional state. It is presently uncertain how the regulation of the central melanocortin system in birds is brought about in the situation of the apparently reduced importance of leptin and ghrelin compared to mammals.