NEURAL BASIS AND BIOLOGICAL FUNCTION OF MASKING BY LIGHT IN MAMMALS: SUPPRESSION OF MELATONIN AND LOCOMOTOR ACTIVITY

Light influences mammalian circadian rhythms in two different ways: (1) It entrains endogenous oscillators (clocks), which regulate physiology and behavior; and (2) it affects directly and often immediately physiology and behavior (these effects are also referred to as masking). Masking effects of light on pineal melatonin, locomotor activity, and the sleep-wake cycle in mammals and man are reviewed. They seem to represent a universal response in this group. The review reveals that the mechanism of photic inhibition of melatonin is fairly well understood, whereas only little is known about the influence of light on other circadian rhythm outputs, such as locomotor activity. (Chronobiology International, 18(5), 737–758, 2001)     

S20098 AFFECTS THE FREE-RUNNING RHYTHMS OF BODY TEMPERATURE AND ACTIVITY AND DECREASES LIGHT-INDUCED PHASE DELAYS OF CIRCADIAN RHYTHMS OF THE RAT

Mammalian endogenous circadian rhythms are entrained to the environmental day-night cycle by light exposure. Melatonin is involved in this entrainment by signaling the day-night information to the endogenous circadian pacemaker. Furthermore, melatonin is known to affect the circadian rhythm of body temperature directly. A striking property of the endogenous melatonin signal is its synthesis pattern, characterized by long-term elevated melatonin levels throughout the night. In the present study, the influence of prolonged treatment with the melatonin agonist S20098 during the activity phase of free-running rats was examined. This was achieved by giving S20098 in the food. The free-running body temperature and activity rhythms were studied. The present study shows that enhancement of the melatonin signal, using S20098, affected the free-running rhythm by gradual phase advances of the start of the activity phase, consequently causing an increase in length of the activity phase. A well-known feature of circadian rhythms is its time-dependent sensitivity for light. Light pulse exposure of an animal housed under continuous dark conditions can cause a phase shift of the circadian pacemaker. Therefore, in a second experiment, the influence of melatonin receptor stimulation on the sensitivity of the pacemaker to light was examined by giving the melatonin agonist S20098 in the food during 1 day prior to exposure to a 60-min light pulse of 0, 1.5, 15, or 150 lux given at circadian time (CT) 14. S20098 pretreatment caused a diminished lightpulse- induced phase shift when a light pulse of low light intensity (1.5 lux) was given. S20098 treatment via the food was sufficient to exert chronobiotic activity, and S20098 treatment resulting in prolonged overstimulation of melatonin receptors is able to attenuate the effect of light on the circadian timing system. (Chronobiology International, 18(5), 781-799, 2001)     

S20098 AFFECTS THE FREE-RUNNING RHYTHMS OF BODY TEMPERATURE AND ACTIVITY AND DECREASES LIGHT-INDUCED PHASE DELAYS OF CIRCADIAN RHYTHMS OF THE RAT

Mammalian endogenous circadian rhythms are entrained to the environmental day-night cycle by light exposure. Melatonin is involved in this entrainment by signaling the day-night information to the endogenous circadian pacemaker. Furthermore, melatonin is known to affect the circadian rhythm of body temperature directly. A striking property of the endogenous melatonin signal is its synthesis pattern, characterized by long-term elevated melatonin levels throughout the night. In the present study, the influence of prolonged treatment with the melatonin agonist S20098 during the activity phase of free-running rats was examined. This was achieved by giving S20098 in the food. The free-running body temperature and activity rhythms were studied. The present study shows that enhancement of the melatonin signal, using S20098, affected the free-running rhythm by gradual phase advances of the start of the activity phase, consequently causing an increase in length of the activity phase. A well-known feature of circadian rhythms is its time-dependent sensitivity for light. Light pulse exposure of an animal housed under continuous dark conditions can cause a phase shift of the circadian pacemaker. Therefore, in a second experiment, the influence of melatonin receptor stimulation on the sensitivity of the pacemaker to light was examined by giving the melatonin agonist S20098 in the food during 1 day prior to exposure to a 60-min light pulse of 0, 1.5, 15, or 150 lux given at circadian time (CT) 14. S20098 pretreatment caused a diminished lightpulse- induced phase shift when a light pulse of low light intensity (1.5 lux) was given. S20098 treatment via the food was sufficient to exert chronobiotic activity, and S20098 treatment resulting in prolonged overstimulation of melatonin receptors is able to attenuate the effect of light on the circadian timing system. (Chronobiology International, 18(5), 781–799, 2001)     

The circadian timing system and reproduction in mammals.

Circadian systems in a wide variety of organisms all appear to include three basic components: 1) biological oscillators that maintain a self-sustained circadian periodicity in the absence of environmental time cues; 2) input pathways that convey environmental information, especially light cues, that can entrain the circadian oscillations to local time; and 3) output pathways that drive overt circadian rhythms, such as the rhythms of locomotor activity and a variety of endocrine rhythms. In mammals, the circadian system is employed in the regulation of reproductive physiology and behavior in two very important ways. 1) In some species, there is a strong circadian component in the timing of ovulation and reproductive behavior, ensuring that these events will occur at a time when the animal is most likely to encounter a potential mate. 2) Many mammals exhibit seasonal reproductive rhythms that are largely under photoperiod regulation; in these species, the circadian system and the pineal gland are crucial components of the mechanism that is used to measure day length. The rhythm of pineal melatonin secretion is driven by a neural pathway that includes the circadian oscillator(s) in the suprachiasmatic nuclei. Melatonin is secreted at night in all mammals, and the duration of each nocturnal episode of melatonin secretion is inversely related to day length. The pineal melatonin rhythm appears to serve as an internal signal that represents day length and that is capable of regulating a variety of seasonal variations in physiology and behavior.     

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.     

Clock-controlled endogenous melatonin rhythms in Nile tilapia (Oreochromis niloticus niloticus) and African catfish (Clarias gariepinus)

The purpose of this work was to investigate the circadian melatonin system in two tropical teleost species characterized by different behavioral habits, Nile tilapia (diurnal) and African catfish (nocturnal). To do so, fish were subjected to either a control photoperiod (12L:12D), continuous light (LL) or darkness (DD), or a 6L:6D photoperiod. Under 12L:12D, plasma melatonin levels were typically low during the photophase and high during the scotophase in both species. Interestingly, in both species, melatonin levels significantly decreased prior to the onset of light, which in catfish reached similar basal levels to those during the day, demonstrating that melatonin production can anticipate photic changes probably through circadian clocks. Further evidence for the existence of such pacemaker activity was obtained when fish were exposed to DD, as a strong circadian melatonin rhythm was maintained. Such an endogenous rhythm was sustained for at least 18 days in Nile tilapia. A similar rhythm was shown in catfish, although DD was only tested for four days. Under LL, the results confirmed the inhibitory effect of light on melatonin synthesis already reported in other species. Finally, when acclimatized to a short photo-cycle (6L:6D), no endogenous melatonin rhythm was observed in tilapia under DD, with melatonin levels remaining high. This could suggest that the circadian clocks cannot entrain to such a short photocycle. Additional research is clearly needed to further characterize the circadian axis in teleost species, identify and localize the circadian clocks, and better understand the environmental entrainment of fish physiology.     

CONTRIBUTION OF CIRCADIAN PHYSIOLOGY AND SLEEP HOMEOSTASIS TO AGE-RELATED CHANGES IN HUMAN SLEEP

The circadian pacemaker and sleep homeostasis play pivotal roles in vigilance state control. It has been hypothesized that age-related changes in the human circadian pacemaker, as well as sleep homeostatic mechanisms, contribute to the hallmarks of age-related changes in sleep, that is, earlier wake time and reduced sleep consolidation. Assessments of circadian parameters in healthy young (～20–30 years old) and older people (～65–75 years old)—in the absence of the confounding effects of sleep, changes in posture, and light exposure—have demonstrated that an earlier wake time in older people is accompanied by about a 1h advance of the rhythms of core body temperature and melatonin. In addition, older people wake up at an earlier circadian phase of the body temperature and plasma melatonin rhythm. The amplitude of the endogenous circadian component of the core body temperature rhythm assessed during constant routine and forced desynchrony protocols is reduced by 20–30% in older people. Recent assessments of the intrinsic period of the human circadian pacemaker in the absence of the confounding effects of light revealed no age-related reduction of this parameter in both sighted and blind individuals. Wake maintenance and sleep initiation are not markedly affected by age except that sleep latencies are longer in older people when sleep initiation is attempted in the early morning. In contrast, major age-related reductions in the consolidation and duration of sleep occur at all circadian phases. Sleep of older people is particularly disrupted when scheduled on the rising limb of the temperature rhythm, indicating that the sleep of older people is more susceptible to arousal signals genernpated by the circadian pacemaker. Sleep-homeostatic mechanisms, as assayed by the sleep-deprivation–induced increase of EEG slow-wave activity (SWA), are operative in older people, although during both baseline sleep and recovery sleep SWA in older people remains at lower levels. The internal circadian phase advance of awakening, as well as the age-related reduction in sleep consolidation, appears related to an age-related reduction in the promotion of sleep by the circadian pacemaker during the biological night in combination with a reduced homeostatic pressure for sleep. Early morning light exposure associated with this advance of awakening in older people could reinforce the advanced circadian phase. Quantification of the interaction between sleep homeostasis and circadian rhythmicity contributes to understanding age-related changes in sleep timing and quality. (Chronobiology International, 17(3), 285–311, 2000)     

Circadian regulation of bird song, call, and locomotor behavior by pineal melatonin in the zebra finch

As both a photoreceptor and pacemaker in the avian circadian clock system, the pineal gland is crucial for maintaining and synchronizing overt circadian rhythms in processes such as locomotor activity and body temperature through its circadian secretion of the pineal hormone melatonin. In addition to receptor presence in circadian and visual system structures, high-affinity melatonin binding and receptor mRNA are present in the song control system of male oscine passeriform birds. The present study explores the role of pineal melatonin in circadian organization of singing and calling behavior in comparison to locomotor activity under different lighting conditions. Similar to locomotor activity, both singing and calling behavior were regulated on a circadian basis by the central clock system through pineal melatonin, since these behaviors free-ran with a circadian period and since pinealectomy abolished them in constant environmental conditions. Further, rhythmic melatonin administration restored their rhythmicity. However, the rates by which these behaviors became arrhythmic and the rates of their entrainment to rhythmic melatonin administration differed among locomotor activity, singing and calling under constant dim light and constant bright light. Overall, the study demonstrates a role for pineal melatonin in regulating circadian oscillations of avian vocalizations in addition to locomotor activity. It is suggested that these behaviors might be controlled by separable circadian clockworks and that pineal melatonin entrains them all through a circadian clock.     

Cardiovascular control by the suprachiasmatic nucleus: neural and neuroendocrine mechanisms in human and rat

The risk for cardiovascular incidents is highest in the early morning, which seems partially due to endogenous factors. Endogenous circadian rhythms in mammalian physiology and behavior are regulated by the suprachiasmatic nucleus (SCN). Recently, anatomical evidence has been provided that SCN functioning is disturbed in patients with essential hypertension. Here we review neural and neuroendocrine mechanisms by which the SCN regulates the cardiovascular system. First, we discuss evidence for an endogenous circadian rhythm in cardiac activity, both in humans and rats, which is abolished after SCN lesioning in rats. The immediate impact of retinal light exposure at night on SCN-output to the cardiovascular system, which signals 'day' in both diurnal (human) and nocturnal (rat) mammals with opposite effects on physiology, is discussed. Furthermore, we discuss the impact of melatonin treatment on the SCN and its potential medical relevance in patients with essential hypertension. Finally, we argue that regional differentiation of the SCN and autonomous nervous system is required to explain the multitude of circadian rhythms. Insights into the mechanisms by which the SCN affects the cardiovascular system may provide new strategies for the treatment of disease conditions known to coincide with circadian rhythm disturbances, as is presented for essential hypertension.     

MELATONIN CIRCADIAN RHYTHM IN THE RETINA OF MAMMALS

Melatonin has been traditionally considered to be derived principally from the pineal gland. However, several investigations have now demonstrated that melatonin synthesis occurs also in the retina (and in other organs as well) of several vertebrate classes, including mammals. As in the pineal, melatonin synthesis in the retina is elevated at night and reduced during the day. Since melatonin receptors are present in the retina and retinal melatonin does not contribute to the circulating levels, retinal melatonin probably acts locally as a neuromodulator. Melatonin synthesis in the retinas of mammals is under control of a circadian oscillator located within the retina itself, and circadian rhythms in melatonin synthesis and/or release have been described for several species of rodents. These rhythms are present in vivo, persist in vitro, are entrained by light, and are temperature compensated. The recent cloning of the gene responsible for the synthesis of the enzyme arylalkylamine N-acetyltransferase (the only enzyme unique to the melatonin synthetic pathway) will facilitate localizing the cellular site of melatonin synthesis in the retina and investigating the molecular mechanism responsible for the generation of retinal melatonin rhythmicity. Melatonin has been implicated in many retinal functions, and the levels of melatonin and dopamine appear to regulate several aspects of retinal physiology that relate to light and dark adaptation. In conclusion, it seems that retinal melatonin is involved in several functions, but its precise role is yet to be understood. (Chronobiology International, 17(5), 599–612, 2000)     

Nonvisual responses to light exposure in the human brain during the circadian night

The brain processes light information to visually represent the environment but also to detect changes in ambient light level. The latter information induces non-image-forming responses and exerts powerful effects on physiology such as synchronization of the circadian clock and suppression of melatonin. In rodents, irradiance information is transduced from a discrete subset of photosensitive retinal ganglion cells via the retinohypothalamic tract to various hypothalamic and brainstem regulatory structures including the hypothalamic suprachiasmatic nuclei, the master circadian pacemaker. In humans, light also acutely modulates alertness, but the cerebral correlates of this effect are unknown. We assessed regional cerebral blood flow in 13 subjects attending to auditory and visual stimuli in near darkness following light exposures (>8000 lux) of different durations (0.5, 17, 16.5, and 0 min) during the biological night. The bright broadband polychromatic light suppressed melatonin and enhanced alertness. Functional imaging revealed that a large-scale occipito-parietal attention network, including the right intraparietal sulcus, was more active in proportion to the duration of light exposures preceding the scans. Activity in the hypothalamus decreased in proportion to previous illumination. These findings have important implications for understanding the effects of light on human behavior.     

Acute and phase-shifting effects of ocular and extraocular light in human circadian physiology

Light can influence physiology and performance of humans in two distinct ways. It can acutely change the level of physiological and behavioral parameters, and it can induce a phase shift in the circadian oscillators underlying variations in these levels. Until recently, both effects were thought to require retinal light perception. This view was challenged by Campbell and Murphy, who showed significant phase shifts in core body temperature and melatonin using an extraocular stimulus. Their study employed popliteal skin illumination and exclusively considered phase-shifting effects. In this paper, the authors explore both acute effects and phase-shifting effects of ocular as well as extraocular light. Twelve healthy males participated in a within-subject design and received all of three light conditions--(1) dim ocular light/no light to the knee, (2) dim ocular light/bright extraocular light to the knee, and (3) bright ocular light/no light to the knee--on separate nights in random order. The protocol consisted of an adaptation night followed by a 26-h period of sustained wakefulness, during which a 4-h light pulse was presented at a time when maximal phase delays were expected. The authors found neither immediate nor phase-shifting effects of extraocular light exposure on melatonin, core body temperature (CBT), or sleepiness. Ocular bright-light exposure reduced the nocturnal circadian drop in CBT, suppressed melatonin, and reduced sleepiness significantly. In addition, the 4-h ocular light pulse delayed the CBT rhythm by -55 min compared to the drift of the CBT rhythm in dim light. The melatonin rhythm shifted by -113 min, which differed significantly from the drift in the melatonin rhythm in the dim-light condition (-26 min). The failure to find immediate or phase-shifting effects in response to extraocular light in a within-subjects design in which effects of ocular bright light are confirmed strengthens the doubts raised by other labs of the impact of extraocular light on the human circadian system.     

Melatonin and Human Rhythms

Melatonin signals time of day and time of year in mammals by virtue of its pattern of secretion, which defines 'biological night.' It is supremely important for research on the physiology and pathology of the human biological clock. Light suppresses melatonin secretion at night using pathways involved in circadian photoreception. The melatonin rhythm (as evidenced by its profile in plasma, saliva, or its major metabolite, 6-sulphatoxymelatonin [aMT6s] in urine) is the best peripheral index of the timing of the human circadian pacemaker. Light suppression and phase-shifting of the melatonin 24 h profile enables the characterization of human circadian photoreception, and circulating concentrations of the hormone are used to investigate the general properties of the human circadian system in health and disease. Suppression of melatonin by light at night has been invoked as a possible influence on major disease risk as there is increasing evidence for its oncostatic effects. Exogenous melatonin acts as a 'chronobiotic.' Acutely, it increases sleep propensity during 'biological day.' These properties have led to successful treatments for serveal circadian rhythm disorders. Endogenous melatonin acts to reinforce the functioning of the human circadian system, probably in many ways. The future holds much promise for melatonin as a research tool and as a therapy for various conditions.     

Melatonin and Human Rhythms

Melatonin signals time of day and time of year in mammals by virtue of its pattern of secretion, which defines ‘biological night.’ It is supremely important for research on the physiology and pathology of the human biological clock. Light suppresses melatonin secretion at night using pathways involved in circadian photoreception. The melatonin rhythm (as evidenced by its profile in plasma, saliva, or its major metabolite, 6‐sulphatoxymelatonin [aMT6s] in urine) is the best peripheral index of the timing of the human circadian pacemaker. Light suppression and phase‐shifting of the melatonin 24 h profile enables the characterization of human circadian photoreception, and circulating concentrations of the hormone are used to investigate the general properties of the human circadian system in health and disease. Suppression of melatonin by light at night has been invoked as a possible influence on major disease risk as there is increasing evidence for its oncostatic effects. Exogenous melatonin acts as a ‘chronobiotic.’ Acutely, it increases sleep propensity during ‘biological day.’ These properties have led to successful treatments for serveal circadian rhythm disorders. Endogenous melatonin acts to reinforce the functioning of the human circadian system, probably in many ways. The future holds much promise for melatonin as a research tool and as a therapy for various conditions.     

Melatonin and human rhythms

Melatonin signals time of day and time of year in mammals by virtue of its pattern of secretion, which defines 'biological night.' It is supremely important for research on the physiology and pathology of the human biological clock. Light suppresses melatonin secretion at night using pathways involved in circadian photoreception. The melatonin rhythm (as evidenced by its profile in plasma, saliva, or its major metabolite, 6-sulphatoxymelatonin [aMT6s] in urine) is the best peripheral index of the timing of the human circadian pacemaker. Light suppression and phase-shifting of the melatonin 24 h profile enables the characterization of human circadian photoreception, and circulating concentrations of the hormone are used to investigate the general properties of the human circadian system in health and disease. Suppression of melatonin by light at night has been invoked as a possible influence on major disease risk as there is increasing evidence for its oncostatic effects. Exogenous melatonin acts as a 'chronobiotic.' Acutely, it increases sleep propensity during 'biological day.' These properties have led to successful treatments for serveal circadian rhythm disorders. Endogenous melatonin acts to reinforce the functioning of the human circadian system, probably in many ways. The future holds much promise for melatonin as a research tool and as a therapy for various conditions.     

Duration of melatonin regulates seasonal changes in song control nuclei of the house sparrow, Passer domesticus: independence from gonads and circadian entrainment

Avian behavior and physiology are temporally regulated by a complex circadian clock on both a daily and an annual basis. The circadian secretion of the hormone melatonin is a critical component of the regulation of circadian/daily processes in passerine birds, but there is little evidence that the gland regulates annual changes in primary reproductive function. Here it is shown that locomotor rhythms of house sparrows, Passer domesticus, which are made arrhythmic by either pinealectomy or maintenance in constant light, can be synchronized by daily administration of melatonin of different durations to simulate the melatonin profiles indicative of long and short photoperiods. Pinealectomized male sparrows maintained in constant darkness were entrained by both melatonin regimens. In both cases, testes were regressed and the song control nuclei were small. Intact male house sparrows maintained in constant light were also entrained to both melatonin regimens. However, sparrows that received a long duration melatonin cycle exhibited small song control nuclei, while sparrows that received short duration melatonin or no melatonin at all exhibited large song control nuclei. The data indicate that seasonal changes in melatonin duration contribute to the regulation of song control nuclei.     

Seasonal variations in circadian rhythms of plasma melatonin in ruin lizards

We examined melatonin profiles of ruin lizards in different seasons (spring, summer, and autumn) under light:dark (LD) and concomitant responses when transferred to continuous darkness (DD) to determine the degree to which previously reported seasonally dependent effects of pinealectomy on locomotor behavior are related to melatonin secretion. The amplitude of the melatonin rhythm and the amount of melatonin produced over 24 h varied with season. In spring, the amount of melatonin produced was greatest and the amplitude 4- 5 times that found in summer or autumn. The degree of self-sustainment of the melatonin rhythm when transferred to DD also varied with season. In DD, melatonin levels remained high but did not exhibit circadian variation in spring. In summer, the melatonin profile persisted virtually unchanged in DD, showing the existence of a circadian rhythm. Finally, in the fall there was no circadian variation in DD and levels remained low. These responses correspond closely to previously reported effects of pinealectomy on locomotor behavior where there is little or no effect of pinealectomy in spring or fall but a profound alteration of locomotor behavior in summer. These results suggest that the seasonally dependent effects of pinealectomy on locomotor behavior in ruin lizards are related to a seasonally mediated change in the degree of self-sustainment of some component of the circadian pace-making system of which melatonin plays some role.     

Internal rhythms in humans

Human physiology and behavior are characterized by a daily internal temporal dimension. This so-called circadian rhythmicity is present for almost all variables studied to date, persists in the absence of external cycles, and is synchronized to the external 24-h world by an internally generated circadian rhythm of light sensitivity. The light-sensitive circadian pacemaker, presumably also in humans located in the suprachiasmatic nucleus of the hypothalamus, drives the endogenous circadian component of rhythmicity for a number of variables including plasma melatonin, alertness, sleep propensity and sleep structure. Overt rhythmicity and the consolidation of vigilance states are generated by a fine-tuned interaction of this circadian process with other regulatory processes such as sleep homeostasis.     

CIRCADIAN CONTROL OF MIGRATORY RESTLESSNESS AND THE EFFECTS OF EXOGENOUS MELATONIN IN THE BRAMBLING,FRINGILLA MONTIFRINGILLA

Circadian pacemakers control both “daytime” activity and nocturnal restlessness of migratory birds, and the daily rhythm of melatonin release from the pineal has been suggested to be involved in the control of migratory activity. To study the phase relations between the two activity components during entrainment and when free running, locomotor activity of bramblings (Fringilla montifringilla) was recorded continuously under a 12:12 “cool light” to “warm light” cycle (CL:WL, ca. 5000 K and ca. 2500 K, respectively) or blue light to red light cycle (BL:RL, maxima at 440 and 650 nm, respectively) at different irradiance ratios. Migratory activity was expressed primarily during the WL or RL phase of the light cycles. Under free-running conditions, the circadian periods τ correlated with the phase relations between day and night (migratory) activity components during preceding entrainment. Bramblings with migratory activity had significantly longer τ at constant light intensity than the same individuals without migratory activity. Birds with migratory activity reentrained faster after a 6h phase shift of the CL:WL cycle than birds without migratory activity. When exogenous melatonin was given in the drinking water (200 μg/mL 1% ethanol or 0.86 mM) to bramblings exposed to 12:12 CL:WL cycles with constant irradiance, the amounts of activity, which were initially higher during the WL phase of the light cycle, were suppressed to similar low levels during both light phases. The systematic changes in the amounts of activity during melatonin treatment were not correlated with consistent changes in entrainment status. The data support the hypothesis that changes in the amplitude and level of the daily melatonin cycle are involved in regulating migratory restlessness, by either allowing or inhibiting nocturnal activity. (Chronobiology International, 17(4), 471–488, 2000)     

Does the circadian pacemaker act through cyclic AMP to drive the melatonin rhythm in chick pineal cells?

Cyclic AMP is a key regulator of melatonin production in the chick pineal gland. Agents that raise cyclic AMP levels (such as forskolin), or cyclic AMP analogues (such as 8-bromocyclic AMP), increase melatonin synthesis and release, whereas agents that lower cyclic AMP levels (including light) decrease melatonin synthesis and release. A circadian oscillator in these cells also raises and lowers melatonin output. We have been investigating the relationships between cyclic AMP and the circadian pacemaker in the regulation of melatonin production. In the chick pineal (unlike certain neuronal systems), the weight of the evidence indicates that cyclic AMP is not on an entrainment pathway to the circadian pacemaker. Instead, cyclic AMP appears to act downstream from the pacemaker. The pacemaker might itself act directly through cyclic AMP, regulating melatonin content by raising and lowering cyclic AMP levels. If this were the case, and if the effects of cyclic AMP levels on melatonin output are saturable (as they must be), then, in the face of such saturating levels of cyclic AMP, the pacemaker should no longer raise or lower melatonin output. To test this prediction, maximally effective concentrations of forskolin and 8-bromocyclic AMP were determined. Both agents markedly increased melatonin output. After 36 hr, cells were refractory to additional stimulation of melatonin output by addition of both agents together, or by higher concentrations of forskolin (although cyclic AMP levels could still be raised further). Nonetheless, the circadian pacemaker continued to raise and lower melatonin output: The rhythm persisted in the face of saturating levels of cyclic AMP. It is therefore suggested that the circadian pacemaker in chick pineal cells acts with, not through, cyclic AMP to regulate melatonin synthesis. Cyclic AMP and the pacemaker act synergistically to regulate serotonin N-acetyltransferase activity and the melatonin rhythm, with cyclic AMP mediating acute effects and amplitude regulation.