Daily pattern of melatonin secretion in an antarctic bird, the emperor penguin, Aptenodytes forsteri: seasonal variations, effect of constant illumination and of administration of isoproterenol or propranolol

The relationship between daily behavior, hormones, and a color dimorphism in a seabird under natural continuous light

The predictable oscillation between the light of day and the dark of night across the diel cycle is a powerful selective force that has resulted in anticipatory mechanisms in nearly all taxa. At polar latitude, however, this oscillation becomes highly attenuated during the continuous light of polar day during summer. A general understanding of how animals keep time under these conditions is poorly understood. We tested the hypothesis that the common murre (a seabird, Uria aalge) can use melatonin and corticosterone, hormones associated with timekeeping, to track the diel cycle despite continuous light. We also tested the assumption that common murres breeding during polar summer schedule their colony attendance by time of day and sex, as they do at subpolar latitude. In the Atlantic population, common murres have a plumage color dimorphism associated with fitness-related traits, and we investigated the relationship of this dimorphism with colony attendance, melatonin, and corticosterone. The common murres did not schedule their attendance behavior by time of day or sex, yet they had higher concentrations of melatonin and, to a more limited extent, corticosterone during "night" than "day". Melatonin also linked to behavioral state. The two color morphs tended to have different colony-attendance behavior and melatonin concentrations, lending support for balancing selection maintaining the plumage dimorphism. In common murres, melatonin can signal time of day despite continuous light, and the limited diel variation of corticosterone contributes to the mounting evidence that polar-adapted birds and mammals require little or no diel variation in circulating glucocorticoids during polar day.

Body Temperature and Activity Rhythms Under Different Photoperiods in High Arctic Svalbard ptarmigan (Lagopus muta hyperborea)

Organisms use circadian rhythms to anticipate and exploit daily environmental oscillations. While circadian rhythms are of clear importance for inhabitants of tropic and temperate latitudes, its role for permanent residents of the polar regions is less well understood. The high Arctic Svalbard ptarmigan shows behavioral rhythmicity in presence of light-dark cycles but is arrhythmic during the polar day and polar night. This has been suggested to be an adaptation to the unique light environment of the Arctic. In this study, we examined regulatory aspects of the circadian control system in the Svalbard ptarmigan by recording core body temperature (T _b) alongside locomotor activity in captive birds under different photoperiods. We show that T _b and activity are rhythmic with a 24-h period under short (SP; L:D 6:18) and long photoperiod (LP; L:D 16:8). Under constant light and constant darkness, rhythmicity in T _b attenuates and activity shows signs of ultradian rhythmicity. Birds under SP also showed a rise in T _b preceding the light-on signal and any rise in activity, which proves that the light-on signal can be anticipated, most likely by a circadian system.

Melatonin and corticosterone profiles under polar day in a seabird with sexually opposite activity-rhythms

The 24 h geophysical light-dark cycle is the main organizer of daily rhythms, scheduling physiology and behavior. This cycle attenuates greatly during the continuous light of summer at polar latitudes, resulting in species-specific and even individual-specific patterns of behavioral rhythmicity, but the physiological mechanisms underlying this variation are poorly understood. To address this knowledge gap and to better understand the roles of the hormones melatonin and corticosterone in rhythmic behavior during this 'polar day', we exploited the behavior of thick-billed murres (Uria lomvia), a charadriiform seabird with sexually opposite ('antiphase') activity-rhythms that have a duration of 24 h. Melatonin concentration in the plasma of inactive males was unexpectedly high around midday and subsequently fell during a sudden decrease in light intensity as the colony became shaded. Corticosterone concentration in plasma did not vary with time of day or activity in either sex. While the reasons for these unusual patterns remain unclear, we propose that a flexible melatonin response and little diel variation of corticosterone may be adaptive in thick-billed murres, and perhaps other polar birds and mammals, by stabilizing glucocorticoids' role of modulating energy storage and mobilization across the diel cycle and facilitating the appropriate reaction to unexpected stimuli experienced across the diel cycle while attending the colony.

Adaptations to polar life in mammals and birds

This Review presents a broad overview of adaptations of truly Arctic and Antarctic mammals and birds to the challenges of polar life. The polar environment may be characterized by grisly cold, scarcity of food and darkness in winter, and lush conditions and continuous light in summer. Resident animals cope with these changes by behavioural, physical and physiological means. These include responses aimed at reducing exposure, such as 'balling up', huddling and shelter building; seasonal changes in insulation by fur, plumage and blubber; and circulatory adjustments aimed at preservation of core temperature, to which end the periphery and extremities are cooled to increase insulation. Newborn altricial animals have profound tolerance to hypothermia, but depend on parental care for warmth, whereas precocial mammals are well insulated and respond to cold with non-shivering thermogenesis in brown adipose tissue, and precocial birds shiver to produce heat. Most polar animals prepare themselves for shortness of food during winter by the deposition of large amounts of fat in times of plenty during autumn. These deposits are governed by a sliding set-point for body fatness throughout winter so that they last until the sun reappears in spring. Polar animals are, like most others, primarily active during the light part of the day, but when the sun never sets in summer and darkness prevails during winter, high-latitude animals become intermittently active around the clock, allowing opportunistic feeding at all times. The importance of understanding the needs of the individuals of a species to understand the responses of populations in times of climate change is emphasized.

Diel pattern of corticosterone metabolites in Arctic barnacle goslings (Branta leucopsis) under continuous natural light

Here we describe the excretion pattern of corticosterone metabolites collected from droppings in barnacle goslings (Branta leucopsis) raised under 24 hours of continuous natural light in the Arctic. In lower latitudes, circulating corticosterone peaks around waking and shows a nadir between midnight and 4:00, whereas the peak and nadir are time-delayed slightly when measuring corticosterone metabolites from droppings. Photoperiod, along with other environmental factors, helps to entrain an animal's endogenous rhythm to that of the natural world. North of the Arctic Circle, photoperiod may not be a reliable cue as light is continuously absent during the winter and continuously present during the summer. Here, for the first time, we used droppings to describe a 24-hour excretion pattern of corticosterone metabolites (CORTm). By applying circular statistics for dependent data, we found a diel rhythmic pattern even under continuous natural light. We discuss potential alternative 'Zeitgeber' that may function even in the polar regions, focusing on melatonin. We propose a line of research to measure melatonin non-invasively from droppings. We also provide a validation of the adopted enzyme immunoassay (EIA) that was originally developed for greylag geese.

EVALUATION OF POTENTIAL RISK FACTORS ASSOCIATED WITH CATARACT IN CAPTIVE MACARONI (EUDYPTES CHRYSOLOPHUS) AND ROCKHOPPER PENGUINS (EUDYPTES CHRYSOCOME)

Complete ophthalmic examinations were performed on 160 Macaroni penguins ( Eudyptes chrysolophus ) and 90 Rockhopper penguins ( Eudyptes chrysocome ) at eight North American zoological institutions. Cataract prevalence in the Macaroni population was 46.5% (n = 74) of penguins and 42.3% (135/319) of eyes. Cataract prevalence in the Rockhopper population was 45.5% (n = 40) of penguins and 40.6% (73/180) of eyes. The mean age of Macaroni penguins without ocular disease was 7.4 ± 5.8 yr, while that of Rockhoppers was 9.8 ± 6.4 yr. Risk factors for cataract were examined through husbandry surveys completed by each institution and by evaluation of light intensity and ultraviolet (UV) light measurements acquired in each penguin exhibit. Risk factors associated with cataract in Macaroni penguins included age, dietary smelt, hand-feeding, and fluorescent exhibit lighting. Risk factors associated with cataract in Rockhopper penguins included age, dietary capelin, increasing population density, and increasing length of minimum photoperiod. Factors associated with decreased odds of cataract in Macaroni penguins included saltwater pool, monitoring of water quality for salinity, pH, and alkalinity; use of water additives; presence of pool filtration and sterilization systems; use of metal halide lightbulbs; increasing light intensity; and UV spectrum lighting. Factors associated with decreased odds of cataract in Rockhoppers included dietary herring and krill, increasing exhibit land area, pool temperature monitoring, increasing maximum photoperiod, and increasing minimum UV light.

Continuous activity and no cycling of clock genes in the Antarctic midge during the polar summer

The extreme seasonal shifts of day length in polar regions, ranging from constant light in the summer to constant darkness in the winter, pose an intriguing environment for probing activity rhythms and the functioning of circadian clocks. Here, we monitor locomotor activity during the summer on the Antarctic Peninsula and under laboratory conditions, as well as the accompanying patterns of clock gene expression in the Antarctic midge, the only insect endemic to Antarctica. Larvae and adults are most active during the warmest portion of the day, but at a constant temperature they remain continuously active regardless of the photoregime, and activity also persists in constant darkness. The canonical clock genes period, timeless, Clock, and vrille are expressed in the head but we detected no cycling of expression in either the field or under diverse photoregimes in the laboratory. The timekeeping function of the clock has possibly been lost, enabling the midge to opportunistically exploit the unpredictable availability of permissive thermal conditions for growth, development, and reproduction during the short summer in Antarctica.

Persistence, Entrainment, and Function of Circadian Rhythms in Polar Vertebrates

Polar organisms must cope with an environment that periodically lacks the strongest time-giver, or zeitgeber, of circadian organization–robust, cyclical oscillations between light and darkness. We review the factors influencing the persistence of circadian rhythms in polar vertebrates when the light-dark cycle is absent, the likely mechanisms of entrainment that allow some polar vertebrates to remain synchronized with geophysical time, and the adaptive function of maintaining circadian rhythms in such environments.

Revealing a circadian clock in captive arctic-breeding songbirds, lapland longspurs (Calcarius lapponicus), under constant illumination

Most organisms in temperate or tropic regions employ the light-dark (LD) cycle as the primary Zeitgeber to synchronize circadian rhythms. At higher latitudes (>66°33'), continuous illumination during the summer presents a significant time-keeping dilemma for polar-adapted species. Lapland longspurs (Calcarius lapponicus), arctic-breeding migratory songbirds, are one of the few recorded species maintaining an intact diel rhythm in activity and plasma melatonin titers during polar summer. However, it is unknown whether rhythms are endogenous and entrain to low-amplitude polar Zeitgeber signals, such as daily variations in light intensity and the spectral composition of the sun (as measured by color temperature). Wild-caught male and female longspurs were brought into captivity, and locomotor activity was assessed using infrared detection. To examine if rhythms were endogenous, birds were exposed to constant bright light (LL; 1300 lux) or constant darkness (DD; 0.1 lux). All birds exhibited free-running activity rhythms in LL and DD, suggesting the presence of a functional circadian clock. Mean periods in LL (22.86 h) were significantly shorter than those in DD (23.5 h), in accordance with Aschoff's rule. No birds entrained to diel changes in light intensity, color temperature, or both. To examine endogenous molecular clock function, the Per2 gene was partially cloned in longspurs (llPer2) and transcripts were measured in hypothalamic tissue punches, eye, and liver using competitive polymerase chain reaction. Ocular llPer2 gene expression was periodic in LL and elevated at ZT24 (CT24) for LD or constant conditions (LL and DD), but llPer2 rhythmicity was not detected in hypothalamus or liver. Plasma melatonin was significantly lower in LL compared with LD or DD. In conclusion, rhythmic ocular Per2 expression and melatonin secretion may maintain the circadian activity rhythm across the polar day.

Functional similarity in relation to the external environment between circadian behavioral and melatonin rhythms in the subtropical Indian weaver bird

The present study investigated whether the circadian oscillators controlling rhythms in activity behavior and melatonin secretion shared similar functional relationship with the external environment. We simultaneously measured the effects of varying illuminations on rhythms of movement and melatonin levels in Indian weaver birds under synchronized (experiment 1) and freerunning (experiment 2) light conditions. In experiment 1, weaverbirds were exposed to 12h light: 12h darkness (12L:12D; L = 20 lx, D = 0.1 lx) for 2.5 weeks. Then, the illumination of the dark period was sequentially enhanced to 1-, 5-, 10-, 20- and 100 lx at the intervals of about 2 to 4 weeks. In experiment 2, weaver birds similarly exposed for 2.5 weeks to 12L:12D (L = 100 lx; D = 0.1 lx) were released in constant dim light (LL(dim), 0.1 lx) for 6 weeks. Thereafter, LL(dim) illumination was sequentially enhanced to 1-, 3- and 5 lx at the intervals of about 2 weeks. Whereas the activity of singly housed individuals was continuously recorded, the plasma melatonin levels were measured at two time of the day, once in each light condition. The circadian outputs in activity and melatonin were phase coupled with an inverse phase relationship: melatonin levels were low during the active phase (light period) and high during the inactive phase (dark period). This phase relationship continued in both the synchronized and freerunning states as long as circadian activity and melatonin oscillators subjectively interpreted synchronously the daily light environment, based on illumination intensity and/or photophase contrast, as the times of day and night. There were dissociations between the response of the activity rhythms and melatonin rhythms in light conditions when the contrast between day and night was much reduced (20:10 lx) or became equal. We suggest that circadian oscillators governing activity behavior and melatonin secretion in weaverbirds are phase coupled, but they seem to independently respond to environmental cues. This would probably explain the varying degree to which the involvement of pineal/melatonin in regulation of circadian behaviors has been found among different birds.

Persistent diel melatonin rhythmicity during the Arctic summer in free-living willow warblers

Arctic environments are challenging for circadian systems. Around the solstices, the most important zeitgeber, the change between night and day, is reduced to minor fluctuations in light intensities. However, many species including songbirds nonetheless show clear diel activity patterns. Here we examine the possible physiological basis underlying diel rhythmicity under continuous Arctic summer light. Rhythmic secretion of the hormone melatonin constitutes an important part of the songbird circadian system and its experimental suppression, e.g., by constant light, usually leads to behavioral arrhythmia. We therefore studied melatonin patterns in a free-living migratory songbird, the willow warbler (Phylloscopus trochilus), that maintains diel activity during the Arctic summer. We compared melatonin profiles during late spring and summer solstice in two Swedish populations from the south (58 degrees N) and near the Arctic circle (66 degrees N). We found the northern Swedish population maintained clear diel changes in melatonin secretion during the summer solstice, although peak concentrations were lower than in southern Sweden. Melatonin levels were highest before midnight and in good accordance with periods of reduced activity. The maintenance of diel melatonin rhythmicity under conditions of continuous light may be one of the physiological mechanisms that enables continued functioning of the circadian system.

Photoperiod-Dependent Changes in Melatonin Synthesis in the Turkey Pineal Gland and Retina

The effect of photoperiod on melatonin content and the activity of the melatonin-synthesizing enzymes, namely, serotonin N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase, were investigated in the pineal gland and retina of turkeys. The birds were adapted to 3 different lighting conditions: 16L:8D (long photoperiod), 12L:12D (regular photoperiod), and 8L:16D (short photoperiod). Pineal, retinal, and plasma melatonin concentrations oscillated with a robust diurnal rhythm, with high values during darkness. The duration of elevated nocturnal melatonin levels in the turkey pineal gland, retina, and plasma changed markedly in response to the length of the dark phase, being longest during the short photoperiod with 16 h of darkness. These photoperiodic variations in melatonin synthesis appear to be driven by AANAT, because changes in the activity of this enzyme were closely correlated with changes in melatonin. By contrast, pineal and retinal hydroxyindole-O-methyltransferase activities failed to exhibit any significant 24-h variation in the different photoperiods. A marked effect of photoperiod on the level of melatonin production was also observed. Peak values of melatonin and AANAT activity in the pineal gland (but not in the retina) were highest during the long photoperiod. During the light phase, mean melatonin concentrations in the pineal gland and retina of turkeys kept under the long photoperiod were significantly higher compared with those from birds maintained under the regular and short photoperiods. In addition, mean circulating melatonin levels were lowest in the short photoperiod. Finally, the magnitude of the light-evoked suppression of nighttime pineal AANAT activity was also influenced by photoperiod, with suppression being smallest under the long photoperiod. These findings show that in the turkey, photoperiod plays an important role in regulating the melatonin signal.

Daytime light intensity affects seasonal timing via changes in the nocturnal melatonin levels

Daytime light intensity can affect the photoperiodic regulation of the reproductive cycle in birds. The actual way by which light intensity information is transduced is, however, unknown. We postulate that transduction of the light intensity information is mediated by changes in the pattern of melatonin secretion. This study, therefore, investigated the effects of high and low daytime light intensities on the daily melatonin rhythm of Afro-tropical stonechats (Saxicola torquata axillaris) in which seasonal changes in daytime light intensity act as a zeitgeber of the circannual rhythms controlling annual reproduction and molt. Stonechats were subjected to light conditions simulated as closely as possible to native conditions near the equator. Photoperiod was held constant at 12.25 h of light and 11.75 h of darkness per day. At intervals of 2.5 to 3.5 weeks, daytime light intensity was changed from bright (12,000 lux at one and 2,000 lux at the other perch) to dim (1,600 lux at one and 250 lux at the other perch) and back to the original bright light. Daily plasma melatonin profiles showed that they were linked with changes in daytime light intensity: Nighttime peak and total nocturnal levels were altered when transitions between light conditions were made, and these changes were significant when light intensity was changed from dim to bright. We suggest that daytime light intensity could affect seasonal timing via changes in melatonin profiles.

Pinealectomy shortens resynchronisation times of house sparrow (Passer domesticus) circadian rhythms

In many birds periodic melatonin secretion by the pineal organ is essential for the high-amplitude self-sustained output of the circadian pacemaker, and thus for the persistence of rhythmicity in 24 h oscillations controlled by it. The elimination of the pineal melatonin rhythm, or a reduction of its amplitude, renders the circadian pacemaker a less self-sustained, often highly damped, oscillatory system. A reduction in the degree of self-sustainment of a rhythm should not only increase its range of entrainment but also shorten the resynchronization times following phase-shifts of the zeitgeber. This hypothesis has not yet been directly tested. We therefore carried out the present study in which house sparrows (Passer domesticus) were subjected to both 6-h advance and 6-h delay phase-shifts of the light-dark cycle before and after the pinealectomy, and the rhythms in locomotion and feeding were recorded. The results indicate that following the delay, but not the advance, phase shift, resynchronization times were significantly shorter after pinealectomy. The dependence of resynchronization times on the presence or absence of the pineal organ is not only of theoretical interest but might also be of functional significance in the natural life of birds. A reduction or elimination of the amplitude of the melatonin secretion rhythm by the pineal organ might be responsible for faster adjustment to changes in zeitgeber conditions in nature.

Environmental control and adrenergic regulation of pineal activity in the diurnal tropical rodent, Arvicanthis ansorgei

Like nocturnal rodents, the diurnal tropical rodent Arvicanthis ansorgei shows a daily rhythm in pineal melatonin content. Seasonal and photoperiodic variations in the biosynthetic activity of the pineal gland: arylalkylamine-N-acetyltransferase (AA-NAT), hydroxyindole-O-methyltransferase (HIOMT) activities and melatonin content were measured in male and female A. ansorgei captured near Samaya, Mali, and kept either under artificial laboratory photoperiods [light-dark (LD) cycles: LD 14:10, LD 12:12 or LD 10:14 or caught in the field in Mali and killed at four different times of the year (January, April, June and November). Under artificial photoperiod, the duration of the nocturnal peak of AA-NAT activity and melatonin content increased with the duration of the dark period while the amplitude did not significantly change. In the field, annual variations in the amplitude of the nocturnal melatonin peak were observed with a maximum in April (highest temperature, low humidity and no grass availability, only seeds) and a minimum in November (high humidity, maximum green grass availability). The variations in the amplitude of the melatonin peak were not correlated with changes in AA-NAT HIOMT activities, suggesting that seasonal variations in the amplitude of the melatonin peak are not driven by these enzymes. Daytime injections of the beta-adrenergic agonist, isoproterenol, stimulated melatonin synthesis in January, April and June, but not in November. The annual differences in the amplitude of the melatonin peak as well as the seasonal differences in the response to an adrenergic stimulation suggest that environmental factors other than photoperiod, such as temperature, humidity and consequent food availability, could be important in the regulation of the annual variations in the pineal biosynthetic activity in this species.

The effects of electromagnetic fields from power lines on avian reproductive biology and physiology: a review

Electrical power lines are ubiquitous in the developed world and in urban areas of the developing world. All electrical currents, including those running through power lines, generate electric and magnetic fields (EMFs). Electrical power lines, towers,and distribution poles are used by birds for perching, hunting, and nesting. Therefore, many bird species, like humans, are exposed to EMFs throughout their lives. EMFs have been implicated in adversely affecting multiple facets of human health,including increasing the risks of life-threatening illnesses such as leukemia, brain cancer, amyotrophic lateral sclerosis, clinical depression, suicide, and Alzheimer's disease. A great deal of research and controversy exists as to whether or not exposure to EMFs affects the cellular, endocrine, immune, and reproductive systems of vertebrates. Laboratory work has used mice, rats, and chickens as models for this EMF research in an effort to understand better the possible implications of EMF exposure for humans. However, EMF exposure of wild birds may also provide insight into the impacts of EMFs on human health. This review focuses on research examining the effects of EMFs on birds; most studies indicate that EMF exposure of birds generally changes, but not always consistently in effect or in direction, their behavior, reproductive success, growth and development, physiology and endocrinology, and oxidative stress under EMF conditions. Some of this work has involved birds under aviary conditions, while other research has focused on free-ranging birds exposed to EMFs. Finally, a number of future research directions are discussed that may help to provide a better understanding of EMF effects on vertebrate health and conservation.

Convergent evolution of strigiform and caprimulgiform dark-activity is supported by phylogenetic analysis using the arylalkylamine N-acetyltransferase (Aanat) gene

Alternative hypotheses propose the sister order of owls (Strigiformes) to be either day-active raptors (Falconiformes) or dark-active nightjars and allies (Caprimulgiformes). In an effort to identify molecular characters distinguishing between these hypotheses we examined a gene, arylalkylamine N-acetyltransferase (Aanat), potentially associated with the evolution of avian dark-activity. Partial Aanat coding sequences, and two introns, were obtained from the genomic DNA of 16 species: Strigiformes (four species), Falconiformes (four species), Caprimulgiformes (five species), with outgroups: Ciconiiformes (one species), Passeriformes (one species), and Apterygiformes (one species). Phylogenetic trees derived from aligned, evolutionarily conserved Aanat regions did not consistently recover clades corresponding to orders Strigiformes and Falconiformes but did place a caprimulgiform clade more distant from the strigiform and falconiform species than the latter two groups are to each other. This finding was supported by spectral analysis. The taxonomic distribution of seven intronic indels is consistent with the Aanat derived phylogenetic trees and supports conventional family-level groupings within both Strigiformes and Caprimulgiformes. The phylogenetic analyses also indicate that Caprimulgiformes is a polyphyletic grouping. In conclusion the data support, but do not conclusively prove, the proposal that Falconiformes is the sister order to Strigiformes and therefore, that the dark-activity characteristic of Strigiformes and Caprimulgiformes arose by convergent evolution.

Diel changes in plasma melatonin and corticosterone concentrations in tropical Nazca boobies (Sula granti) in relation to moon phase and age

We investigated the effects of moon phases and age on diel rhythms of plasma melatonin and corticosterone in free-living Nazca boobies (Sula granti) on the Galápagos Islands, Ecuador. Melatonin and corticosterone secretion are regulated by the circadian system and the two hormones play a role in the control of locomotor activity and foraging, which can be influenced by moon phases. These seabirds have a long life span and in many vertebrates circadian function deteriorates with age. The functioning of the circadian system under different environmental conditions and changes related to age are poorly understood and hardly studied in wild birds. Nazca boobies had generally low plasma melatonin concentrations but showed a diel variation with higher concentrations at 00:00 and 16:00h. The diel variations in melatonin concentrations disappeared during full moon, suggesting that natural light levels at night can suppress melatonin secretion in Nazca boobies. Maximal melatonin concentrations tended to decline in older birds (10-19 years). Birds showed a clear diel variation in basal plasma corticosterone with a peak in the early morning, before the active period begins, and low concentrations throughout the day. As with melatonin, there were no diel variations in corticosterone at full moon, which may be due to different activity patterns in response to food availability or changes in the circadian system. While other studies have found a relationship between corticosterone and melatonin, we found no such correlation in Nazca boobies. The lunar cycle appears to affect the hormone titers of Nazca boobies both directly and indirectly. First, melatonin rhythms can be directly affected by the light intensity associated with full moon. Second, prey availability may change foraging patterns and can therefore indirectly alter corticosterone secretion in Nazca boobies.

Encoding time of day and time of year by the avian circadian system

The most important zeitgeber for seasonal rhythmicity of physiology and behaviour in birds is the annual cycle of photoperiod. Regulatory mechanisms are less well understood in birds than in mammals since photic information can be perceived by photoreceptors in the retina and the pineal gland, as well as in the brain, and photoperiodic time measurement might be performed with reference to at least three autonomous circadian systems, the retina, the pineal gland and a hypothalamic oscillator. In many bird species, the pineal melatonin rhythm plays a central role in circadian organization. Durations of elevated melatonin in the blood reflect night length when animals are kept under natural photoperiodic conditions, as well as under different light/dark schedules in the laboratory. In the house sparrow, time of year is encoded in a particular melatonin signal, being short in duration and high in amplitude in long photoperiods and being long in duration and low in amplitude in short photoperiods, independent of whether the light zeitgeber is natural or artificial or varies in strength. Specific features of the melatonin signal are retained in vivo as well as in vitro when birds or isolated pineal glands are transferred to constant conditions. To regulate daily and seasonal changes of behaviour and physiology, melatonin may act at various target sites, including a complex hypothalamic oscillator that, unlike that in mammals, is not confined to a single cell group in the house sparrow. There is increasing evidence that interactions between two or more components of the songbird circadian pacemaking system are essential to encode and store biologically meaningful information about time, and thus provide the basis for photoperiodic time measurements and after effects in birds.

Effect of polar day on plasma profiles of melatonin, testosterone, and estradiol in high-Arctic Lapland Longspurs

In polar habitats, continuous daylight (polar day) can prevail for many weeks or months around the summer solstice. In the laboratory, continuous light conditions impair or disrupt circadian rhythms in many animals. To determine whether circadian rhythms are disrupted under natural polar day conditions in a species that is only a summer resident in polar regions we analyzed diel rhythms in plasma concentrations of melatonin, testosterone (T), and 17-beta estradiol (E(2)) during the summer solstice in Arctic-breeding Lapland Longspurs (Calcarius lapponicus). We compared these profiles to those of conspecifics housed in outdoor aviaries at a mid-latitude site in Seattle, Washington, during spring, summer, fall, and winter. Under polar day conditions plasma melatonin concentrations of Lapland Longspurs were strongly suppressed, but still showed a significant diel rhythm. Likewise, plasma T in males, and E(2) in females, showed significant diel changes in Arctic birds. Lapland Longspurs housed at mid-latitude in Seattle showed high-amplitude melatonin cycles at all times of the year, and the duration of the nightly melatonin secretion was positively correlated with the duration of the dark phase. We found no diel changes in plasma T in Seattle males in May, but Seattle females showed significant day/night differences in plasma E(2) in May. The data suggest that even under polar day conditions diel rhythms can persist. The maintenance of hormone rhythms could provide a physiological basis to reports of rhythmic behavior in many birds during the Arctic summer.

Complex bird clocks

The circadian pacemaking system of birds comprises three major components: (i) the pineal gland, which rhythmically synthesizes and secretes melatonin; (ii) a hypothalamic region, possibly equivalent to the mammalian suprachiasmatic nuclei; and (iii) the retinae of the eyes. These components jointly interact, stabilize and amplify each other to produce a highly self-sustained circadian output. Their relative contribution to overt rhythmicity appears to differ between species and the system may change its properties even within an individual depending, for example, on its state in the annual cycle or its photic environment. Changes in pacemaker properties are partly mediated by changes in certain features of the pineal melatonin rhythm. It is proposed that this variability is functionally important, for instance, for enabling high-Arctic birds to retain synchronized circadian rhythms during the low-amplitude zeitgeber conditions in midsummer or for allowing birds to adjust quickly their circadian system to changing environmental conditions during migratory seasons. The pineal melatonin rhythm, apart from being involved in generating the avian pacemaking oscillation, is also capable of retaining day length information after isolation from the animal. Hence, it appears to participate in photoperiodic after-effects. Our results suggest that complex circadian clocks have evolved to help birds cope with complex environments.

Seasonal and daily variations in plasma melatonin in the high-arctic Svalbard ptarmigan (Lagopus mutus hyperboreus)

This study presents the daily rhythm of melatonin secretion throughout one year in a bird from the northern hemisphere, the Svalbard ptarmigan (Lagopus mutus hyperboreus), which lives naturally at 76-80 degrees N. Eight Svalbard ptarmigan were caged outdoors at 70 degrees N and blood sampled throughout one day each month for 13 months. At this latitude, daylight is continuous between May and August, but there is a short period of civil twilight around noon from late November to mid January. There was no daily rhythm in plasma melatonin in May-July. Plasma melatonin levels varied significantly throughout the day in all other months of the year, with the nighttime increase reflecting the duration of darkness. The highest mean plasma concentration occurred at midnight in March (110.1 +/- 16.5 pg/ml) and represented the annual peak in estimated daily production. Around the winter solstice, melatonin levels were significantly reduced at noon and elevated during the nearly 18 h of consecutive darkness, and the estimated mean daily production of melatonin was significantly reduced. Thus, at the times of the year characterized by light-dark cycles, melatonin may convey information concerning the length of the day and, therefore, progression of season. The nearly undetectable low melatonin secretion in summer and the reduced amplitude and production in midwinter indicate a flexible circadian system that may reflect an important adaptation to life in the Arctic.

Melatonin: generation and modulation of avian circadian rhythms

The pineal organ and its hormone melatonin are significant components of avian circadian pacemaking systems. In songbirds, pinealectomy results in the abolition or destabilization of overt circadian rhythms such as the rhythm of locomotor activity, feeding, or body temperature. A stable rhythmicity can be restored either by reimplanting a pineal organ, by periodic injections or infusions of melatonin, or by applying melatonin rhythmically through the drinking water. Several results suggest that the pineal melatonin rhythm acts on at least one other oscillator within the circadian pacemaking system, presumably the SCN, which in turn, feeds back to the pineal. As described by the "Neuroendocrine Loop" and "Internal Resonance" models, overall pacemaker output thus depends on the relative strengths of the oscillations in the pineal and the SCN. Investigations on migratory birds have shown that the amplitude of the 24-h plasma melatonin rhythm is reduced during the migratory seasons compared with the nonmigratory seasons. According to the models mentioned above, such a reduced melatonin amplitude should result in a reduction in the degree of self-sustainment of the pacemaker as a whole. This, in turn, should facilitate adjustment to the altered Zeitgeber conditions encountered by these birds as a result of their own migratory flights. A seasonal reduction in melatonin amplitude also occurs in some high-latitude birds during midsummer and midwinter. Under such conditions a less self-sustained circadian pacemaker may enhance entrainability to weak zeitgeber conditions. These examples suggest that the properties of the circadian system may be adjusted to match the changing requirements for synchronization, and that this is achieved by altering the melatonin amplitude.

Environmental control of the seasonal variations in the daily pattern of melatonin synthesis in the European hamster, Cricetus cricetus

Nocturnal patterns of pineal melatonin concentrations were measured at hourly intervals in the European hamster, Cricetus cricetus, maintained under different natural or experimental environmental conditions. There were pronounced variations in the night peak of pineal melatonin both in the duration and the amplitude of the melatonin peak and in the onset and decline of melatonin synthesis. The duration of the melatonin peak increased proportionally with increased dark period. The amplitude increased abruptly from LD 16/8 to LD 15/9 and remained constant in all other photoperiods. The onset of synthesis started 6:00 hours after the onset of darkness in LD 16/8, 15/9, and 14/10, while it started 4:00 hours after dark onset in shorter photoperiods (LD 12/12 and 10/14). This result is opposite to that observed in the rat. The decline of synthesis was delayed as darkness increased and was directly related to lights on in long photoperiods, while it was endogenous in short photoperiods. Temperature, under a long photoperiod, also seems to be implicated in the regulation of the amplitude of the melatonin peak.

Serotonin N-acetyltransferase activity as a target for temperature in the regulation of melatonin production by frog retina

The adaptive mechanisms of serotonin N-acetyltransferase (NAT) activity in the regulation of melatonin synthesis in frog retina in the face of chronic and acute temperature changes have been investigated. We performed thermal acclimation experiments to test different environmental temperatures at two seasons of the year (summer and winter), followed by the set-up of an eyecup culture system to investigate the acute effects of temperature on NAT activity and melatonin production daily rhythms. Low temperature induced a significant increase in NAT activity, independent of both the time of the photocycle (midday or midnight) and the season of the year (winter or summer). Acute cold-induced stimulation of NAT activity may be associated with lower decreases in the enzyme synthesis rate, rather than decreases in the degradation rate. In contrast, acclimation to warm temperature (25 degrees C) stimulated ocular melatonin production. Nocturnal melatonin production in eyecups cultured at 25 degrees C was significantly higher than in eyecups cultured at 5 degrees C. We suggest that this discrepancy in thermal regulation of melatonin synthesis can be justified by a seasonal variation in serotonin content within the photoreceptor cells, which determines the thermal response of melatonin production through changes in NAT kinetics.

Feeding rhythms in constant light and constant darkness: the role of the eyes and the effect of melatonin infusion

Exposure to constant light abolishes circadian behavioral rhythms of locomotion and feeding as well as circulating melatonin rhythms in pigeons (Columba livia). To determine if feeding rhythmicity could be maintained in pigeons exposed to constant light, periodic infusions; (10 h/day) of melatonin were administered to pinealectomized and bilaterally retinectomized/pinealectomized pigeons under conditions of both constant darkness and constant light. The infusions were sufficient to entrain rhythmicity in pinealectomized pigeons in constant darkness and to restore and maintain rhythmicity in bilaterally retinectomized/pinealectomized pigeons in constant darkness. On subsequent exposure to constant light, rhythmicity remained phase locked to the melatonin infusions in bilaterally retinectomized/pinealectomized pigeons but was abolished in sighted pinealectomized birds. These results suggest that while endogenous melatonin rhythms are both necessary and sufficient to maintain behavioral rhythms in DD, their effect can be overridden by constant light but only if perceived by the eyes. Thus, constant light may abolish behavioral rhythmicity in intact pigeons (and perhaps in other species) by a mechanism other than suppression of endogenous melatonin rhythmicity. Such a mechanism might involve direct stimulation of locomotor or feeding activity by retinally perceived (but not by extra-retinally perceived) light, or alternatively by suppression of a hypothalamic oscillator that receives its major light input from the retinae.

Melatonin rhythms in Arctic urban residents

The 24-hr rhythm of salivary melatonin was measured in persons living in the city of Tromsø (70 degrees N) at the following times of the year: in January at a day length of 2 hr of twilight, in June under continuous sunshine, and in March and September at about 12 hr light and 12 hr darkness. The hormone patterns varied widely between individuals, but, in general, they were consistent within most individuals between the seasons. Highest peak values occurred in January when the mean level was also significantly higher than at any other time of year. The lowest mean levels occurred in June. Although individual rhythms were not always apparent, the mean patterns showed significantly elevated melatonin concentrations during the night at all seasons. The June melatonin peak was similar to that in March and September, but appeared to be phase-delayed with increased melatonin concentrations from midnight until 0900. It is assumed that the delayed melatonin peak in June may be associated with a tendency among people to shift their activity/rest rhythm and that the pineal sensitivity to light is reduced in the morning in summer.

Seasonal variations in the daily rhythm of pineal gland and/or circulating melatonin and 5-methoxytryptophol concentrations in the European hamster, Cricetus cricetus

Day-night variations in pineal and/or circulating melatonin and 5-methoxytryptophol (5-ML) concentrations were measured monthly throughout the year in female European hamsters, Cricetus cricetus, maintained under natural conditions. Pronounced seasonal variations in the day-night rhythm of both melatonin and 5-ML were observed. As previously reported for melatonin, the daily rhythm of both methoxyindoles disappeared in spring and early summer, while a clear day-night rhythm occurred in autumn, winter, and early spring. The amplitude of the day-night variations appeared to be maximum from October until January. An inverse relationship existed between the rhythms of melatonin and 5-ML.