Cold prevents the light induced inactivation of pineal N-acetyltransferase in the Djungarian hamster, Phodopus sungorus

Annual rhythms that underlie phenology: biological time-keeping meets environmental change

Seasonal recurrence of biological processes (phenology) and its relationship to environmental change is recognized as being of key scientific and public concern, but its current study largely overlooks the extent to which phenology is based on biological time-keeping mechanisms. We highlight the relevance of physiological and neurobiological regulation for organisms' responsiveness to environmental conditions. Focusing on avian and mammalian examples, we describe circannual rhythmicity of reproduction, migration and hibernation, and address responses of animals to photic and thermal conditions. Climate change and urbanization are used as urgent examples of anthropogenic influences that put biological timing systems under pressure. We furthermore propose that consideration of Homo sapiens as principally a 'seasonal animal' can inspire new perspectives for understanding medical and psychological problems.

Significance and application of melatonin in the regulation of brown adipose tissue metabolism: relation to human obesity

A worldwide increase in the incidence of obesity indicates the unsuccessful battle against this disorder. Obesity and the associated health problems urgently require effective strategies of treatment. The new discovery that a substantial amount of functional brown adipose tissue (BAT) is retained in adult humans provides a potential target for treatment of human obesity. BAT is active metabolically and disposes of extra energy via generation of heat through uncoupling oxidative phosphorylation in mitochondria. The physiology of BAT is readily regulated by melatonin, which not only increases recruitment of brown adipocytes but also elevates their metabolic activity in mammals. It is speculated that the hypertrophic effect and functional activation of BAT induced by melatonin may likely apply to the human. Thus, melatonin, a naturally occurring substance with no reported toxicity, may serve as a novel approach for treatment of obesity. Conversely, because of the availability of artificial light sources, excessive light exposure after darkness onset in modern societies should be considered a potential contributory factor to human obesity as light at night dramatically reduces endogenous melatonin production. In the current article, the potential associations of melatonin, BAT, obesity and the medical implications are discussed.

Seasonal and diurnal melatonin production in exercising sled dogs

Melatonin is a hormone that is released from the pineal gland into the blood stream and is controlled by nerve impulses from the suprachiasmatic nuclei. Melatonin synthesis, which is inhibited by light on the mammalian retina, peaks in plasma concentrations during the night. Though still a subject of intense research, melatonin in mammals is known to effect the reproductive system, thyroid function, and adaptations to seasonal changes. Sled dogs in Fairbanks, Alaska (65 degrees N) can be exposed to anywhere from 21 h of daylight in the summer to 4 h in the winter. While light may be the primary factor influencing melatonin production, we hypothesized that exercise may also affect melatonin production. In the current study, sled dogs were used to study seasonal and diurnal variation in melatonin production. Sled dogs by nature are elite athletes and therefore exercise was a focus in the study. Both exercise and non exercise dogs from 2 distinct latitudes were used. The peak in melatonin production was prolonged in high latitude dogs (65 degrees N), compared with lower latitude dogs (45 degrees N). Dogs at both latitudes show a reduction in peak melatonin levels with exercise, and winter melatonin levels in both locations were higher than the summer. Surprisingly, sled dogs in Alaska had lower melatonin levels than sled dogs in New York.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Seasonal aspects of sleep in the Djungarian hamster

BACKGROUND

Changes in photoperiod and ambient temperature trigger seasonal adaptations in the physiology and behaviour of many species, including the Djungarian hamster. Exposure of the hamsters to a short photoperiod and low ambient temperature leads to a reduction of the polyphasic distribution of sleep and waking over the light and dark period. In contrast, a long photoperiod enhances the daily sleep-wake amplitude leading to a decline of slow-wave activity in NREM sleep within the light period. It is unknown whether these changes can be attributed specifically to photoperiod and/or ambient temperature, or whether endogenous components are contributing factors. The influence of endogenous factors was investigated by recording sleep in Djungarian hamsters invariably maintained at a low ambient temperature and fully adapted to a short photoperiod. The second recording was performed when they had returned to summer physiology, despite the maintenance of the 'winter' conditions.

RESULTS

Clear winter-summer differences were seen in sleep distribution, while total sleep time was unchanged. A significantly higher light-dark cycle modulation in NREM sleep, REM sleep and waking was observed in hamsters in the summer physiological state compared to those in the winter state. Moreover, only in summer, REM sleep episodes were longer and waking bouts were shorter during the light period compared to the dark period. EEG power in the slow-wave range (0.75-4.0 Hz) in both NREM sleep and REM sleep was higher in animals in the summer physiological state than in those in the 'winter' state. In winter SWA in NREM sleep was evenly distributed over the 24 h, while in summer it decreased during the light period and increased during the dark period.

CONCLUSION

Endogenous changes in the organism underlie the differences in sleep-wake redistribution we have observed previously in hamsters recorded in a short and long photoperiod.

Low ambient temperature accelerates short-day responses in Siberian hamsters by altering responsiveness to melatonin

Exposure to low ambient temperatures (Ta) accelerates appearance of the winter phenotype in Siberian hamsters transferred from long to short day lengths. Because melatonin transduces the effects of day length on the neuroendocrine axis, the authors assessed whether low Ta promotes the transition to winterlike traits by accelerating the onset of increased nocturnal melatonin secretion or by enhancing responsiveness to melatonin in short day lengths. Male hamsters were transferred from 16L (16 h light/day) to 8L (8 h light/day) photoperiods and held at 5 degrees C or 22 degrees C. Locomotor activity was recorded continuously, and body mass, testis size, and pelage color were determined biweekly for 8 weeks. The duration of nocturnal locomotion (alpha), a reliable indicator of the duration of nocturnal melatonin secretion, lengthened significantly earlier in hamsters exposed to a Ta of 5 degrees C than 22 degrees C. Cold exposure increased the proportion of hamsters that were photoresponsive: gonadal regression in short days increased from 44% at 22 degrees C to 81% at 5 degrees C (p < 0.05); low Ta did not, however, accelerate testicular regression in animals that were photoresponsive. Nonphotoresponsive animals at 5 degrees C temporarily had longer alphas during the first 4 weeks in short days and significant decreases in body mass and testicular size that were reversed during the ensuing weeks when alpha decreased. In a 2nd experiment, pinealectomized male hamsters infused for 10 h/day with melatonin for 2 weeks had significantly lower body and testes masses when maintained at 5 degrees C but not 22 degrees C. Low-ambient temperature appears to accelerate the appearance of the winter phenotype primarily by increasing target tissue responsiveness to melatonin and to a lesser extent by augmenting the rate at which the duration of nocturnal melatonin secretion increases in short day lengths.

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.

Urinary 6-sulphatoxymelatonin excretion reflects pineal melatonin secretion in the Djungarian hamster (Phodopus sungorus)

To monitor pineal function in the Djungarian hamster (Phodopus sungorus), we measured the urinary excretion of the melatonin metabolite 6-sulphatoxymelatonin (aMT6s) at 3-hr intervals by radioimmunoassay. Hamsters maintained in either long photoperiod (LP, LD 16:8) or short photoperiod (SP, LD 8:16) showed marked daily rhythms in aMT6s excretion, with elevated levels during the dark phase. In both photoperiods, we found large interindividual differences, mainly in the amplitude of the signal. However, the amplitude as well as the duration of nocturnal aMT6s excretion was higher in SP than in LP. Light exposure at night (180 mW/m2, 30 min) caused a decrease in aMT6s excretion, indicating that the pineal gland is the major source of urinary aMT6s. Moreover, there was a significant correlation between nocturnal pineal/plasma melatonin contents and 24-hr aMT6s excretion. We conclude that, measurements of aMT6s provide a valid and quantitative index of pineal melatonin synthesis in this hamster species. As an advantage in determining pineal melatonin contents, this approach will allow noninvasive long-term studies of individual animals under varying environmental conditions.

Alterations of the circadian melatonin rhythm by the electromagnetic spectrum: a study in environmental toxicology

The nightly production and secretion of melatonin by the pineal gland, an endocrine organ near the anatomical center of the brain, provides important time-of-day and time-of-year information to the remainder of the body. In mammals, the circadian rhythm of melatonin (low levels during the day and high levels at night) is synchronized by the prevailing light:dark environment with the retinas of the eyes doing the photoreception required for the induction of this rhythm. The advent of artificial light sources has allowed animals or humans to be exposed to light at unusual times, i.e., during the night. Light falling on the retinas at night leads to a rapid depression in the production and secretion of melatonin by the pineal gland. The magnitude of the drop in circulating melatonin due to light exposure at night is related to the brightness (intensity) as well as the wavelength (color) of light to which humans are exposed. The lowered melatonin values following unusual light exposure at night provide erroneous information to a number of organs that respond to the melatonin message since the signal implies it is day when, in fact, it is still night. Besides visible light, certain ultraviolet wavelengths as well as extremely low frequency electric and magnetic fields may also disturb the melatonin rhythm. These nonvisible wavelengths may influence the circadian melatonin rhythm by mechanisms similar to those by which light causes disturbances of melatonin production and release.