The characteristics of the synchronizing action of melatonin on the dynamics of circadian mobility in rats

Morning (9.00-10.00 h), but not noon, repeated injections of the pineal hormone melatonin (1 or 10 mg/kg daily, during 2 weeks) synchronize the circadian locomotor rhythm of rats. As shown with inverted photoperiod, melatonin facilitates off and on light reactions. Some animals have low and some high hormonal susceptibility. This indicates the role of injections regime and rat's individual reactivity for expressiveness of melatonin synchronizing effect.     

Melatonin-induced phase and dose responses in a diurnal mammal,Funambulus pennantii

ABSTRACT

Melatonin, an essential pineal hormone, acts as a marker of the circadian clock that regulates biological rhythms in animals. The effects of exogenous melatonin on the circadian system of nocturnal rodents have been extensively studied; however, there is a paucity of studies on the phase-resetting characteristics of melatonin in diurnal rodents. We studied the phase shifting effects of exogenous melatonin as a single melatonin injection (1 mg/kg) at various phases of the circadian cycle on the circadian locomotor activity rhythm in the palm squirrel, Funambulus pennantii. A phase response curve (PRC) was constructed. Adult male squirrels (N = 10) were entrained to a 12:12 h light-dark cycle (LD) in a climate-controlled chronocubicle with food and water provided ad libitum. After stable entrainment, squirrels were transferred to constant dark condition (DD) for free-running. Following stable free run, animals were administered a single dose of melatonin (1 mg/kg in 2% ethanol-phosphate buffered saline (PBS) solution) or vehicle (2% ethanol-PBS solution) at circadian times (CTs) 3 h apart to evoke phase shifts. The phase shifts elicited at various CTs were plotted to generate the PRC. A dose response curve was generated using four doses (0.5, 1, 2 and 4 mg/kg) administered at the CT of maximum phase advance. Melatonin evoked maximum phase advances at CT0 (1.23 ± 0.28 h) and maximum phase delays at CT15 (0.31 ± 0.09 h). In the dose response experiment, maximal phase shifts were evoked with 1 mg/kg. In contrast, no significant shifts were observed in control groups. Our study demonstrates that the precise timing and appropriate dose of melatonin administration is essential to maximize the amelioration of circadian rhythm–related disorders in a diurnal model.     

Eventual entrainment of the human circadian pacemaker by melatonin is independent of the circadian phase of treatment initiation: clinical implications

About 15% of the legally blind completely lack light perception. Most of these individuals have abnormally phased circadian rhythms and many free-run. Light treatment is not an option for them. However, melatonin treatment can be highly effective. A daily dose of 0.5 mg of melatonin usually results in entrainment. It has been suggested that treatment in individuals with circadian periods > 24 h should be initiated on the advance zone of the melatonin phase response curve, which was based on findings in which melatonin initiated on the delay zone were less likely to result in entrainment, even though treatment continued across all circadian phases. In the present study, 7 totally blind people started low-dose melatonin treatment (0.5 mg; 1 person was given 0.05 mg) on the delay zone. All entrained as circadian phase free-ran and the advance zone of the melatonin phase response curve coincided with the time of melatonin administration. These results are consistent with studies in other mammals. It does not appear that low-dose melatonin treatment needs to be initiated on the advance zone to induce eventual entrainment in blind people with free-running rhythms > 24 h. Therefore, it is not essential that circadian phase be ascertained before starting low-dose melatonin treatment of blind people.     

The pineal gland influences rat circadian activity rhythms in constant light.

Mammalian circadian organization is believed to derive primarily from circadian oscillators within the hypothalamic suprachiasmatic nuclei (SCN). The SCN drives circadian rhythms of a wide array of functions (e.g., locomotion, body temperature, and several endocrine processes, including the circadian secretion of the pineal hormone melatonin). In contrast to the situation in several species of reptiles and birds, there is an extensive literature reporting little or no effect of pinealectomy on mammalian circadian rhythms. However, recent research has indicated that the SCN and circadian systems of several mammalian species are highly sensitive to exogenous melatonin, raising the possibility that endogenous pineal hormone may provide feedback in the control of overt circadian rhythms. To determine the role of the pineal gland in rat circadian rhythms, the effects of pinealectomy on locomotor rhythms in constant light (LL) and constant darkness (DD) were studied. The results indicated that the circadian rhythms of pinealectomized rats but not sham-operated controls dissociated into multiple ultradian components in LL and recoupled into circadian patterns only after 12-21 days in DD. The data suggest that pineal feedback may modulate sensitivity to light and/or provide coupling among multiple circadian oscillators within the SCN.     

Maternal Coordination of the Daily Rhythm of Malate Dehydrogenase Activity in Testes from Young Rats: Effect of Maternal Sympathetic Denervation of the Pineal Gland and Administration of Melatonin

Chronic sympathetic denervation of the pineal gland by bilateral removal of the superior cervical ganglia (SCG) was performed on female rats 30 days before impregnation. The offspring, maintained in the dark from birth, had disruption of the malate dehydrogenase circadian rhythm in the testes at 25 days of age. A daily injection of melatonin (1 mg/kg s.c. at 10:00 or 18:00 h) to denervated mothers from the 14th day of pregnancy up to the 10th day postpartum produced one daily phase in the enzyme activity of testes in the offspring. Entrainment of daily enzyme activity also was obtained when the hormone was administered orally to the pups during the postnatal period or when pups were reared by intact (not denervated) foster mothers. The results indicate the involvement of the maternal pineal gland in the maternal transfer of photoperiodic information necessary for the coordination of the circadian system in young rats.     

Maternal Coordination of the Daily Rhythm of Malate Dehydrogenase Activity in Testes from Young Rats: Effect of Maternal Sympathetic Denervation of the Pineal Gland and Administration of Melatonin

Chronic sympathetic denervation of the pineal gland by bilateral removal of the superior cervical ganglia (SCG) was performed on female rats 30 days before impregnation. The offspring, maintained in the dark from birth, had disruption of the malate dehydrogenase circadian rhythm in the testes at 25 days of age. A daily injection of melatonin (1 mg/kg s.c. at 10:00 or 18:00 h) to denervated mothers from the 14th day of pregnancy up to the 10th day postpartum produced one daily phase in the enzyme activity of testes in the offspring. Entrainment of daily enzyme activity also was obtained when the hormone was administered orally to the pups during the postnatal period or when pups were reared by intact (not denervated) foster mothers. The results indicate the involvement of the maternal pineal gland in the maternal transfer of photoperiodic information necessary for the coordination of the circadian system in young rats.     

Pineal-independent regulation of photo-nonresponsiveness in the Siberian hamster (Phodopus sungorus)

The pineal hormone melatonin influences circadian rhythms and also mediates reproductive responses to photoperiod. The authors tested whether pinealectomy influences circadian oscillators responsible for induction of nonresponsiveness to short day lengths by preventing normal short-day patterns of circadian entrainment. Adult male Siberian hamsters were pinealectomized or sham operated, maintained in either 18 h light per day (18L) or 15L for 10 weeks, and then tested for responsiveness to 10L. Because pinealectomized hamsters do not show gonadal regression in short day lengths, responsiveness was assessed by measuring phase angle of entrainment and the length of the nightly activity period following transfer to 10L. The incidence of nonresponsiveness was significantly higher in 18L hamsters than in 15L hamsters but was unaffected by pineal status. Fully 88% of 18L hamsters failed to entrain to 10L in the normal short-day manner; the duration of nightly activity remained compressed, and the phase angle of entrainment was large and negative relative to lights off. The 15L hamsters entrained normally to 10L. Exposure to constant light after 10L treatment was equally effective in inducing arrhythmicity in pinealectomized and intact hamsters. Changes in the period of morning and evening circadian oscillators subsequent to 18L treatment did not predict circadian responsiveness to short photoperiod. Long-day induction of photo-nonresponsiveness, which prevents winter responses to short day lengths, occurs independently of pineal melatonin feedback on the circadian system.     

Photic resetting of the circadian clock is correlated with photic habitat in <Emphasis Type="Italic">Anolis</Emphasis> lizards

Circadian rhythms are regulated by an internal clock, which is itself synchronized to environmental cues such as light and temperature. It is widely assumed that the circadian system is adapted to local cues, which vary enormously across habitats, yet the comparative data necessary for testing this idea are lacking. We examined photic and thermal resetting of the circadian clock in five species of Anolis lizards whose microhabitats differ in the amounts of sun and shade. The primary circadian oscillator in Anolis is the pineal gland, which produces the hormone melatonin. A flow-through culture system was employed to measure rhythmic melatonin output from individually cultured pineal glands. All species showed temperature-compensated circadian rhythms of pineal melatonin. Light caused significant phase delays of the melatonin rhythm, and this effect varied among species. Controlling for phylogenetic differences, the results indicate that the pineal glands of shade-dwelling species are more sensitive to photic resetting than species living in more brightly illuminated habitats. The differences were not due to variation in free-running period, but may be due to variation in oscillator phase and/or robustness. Surprisingly, thermal resetting was not statistically significant. Overall, the results suggest that the Anolis circadian system is adapted to photic habitat.     

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.     

The circadian system of ruin lizards: a seasonally changing neuroendocrine loop?

Previous studies have shown that the amplitude of daily melatonin production in cultured ruin lizard pineal organs explanted in the summer is significantly higher than that from organs explanted in the winter. To test whether seasonal photoperiodic changes are decoded autonomously by the pineal gland, pineals explanted in summer were cultured in vitro and exposed to changes between winter and summer photoperiods. The changes in photoperiod duration did not affect the daily profiles of in vitro melatonin production. The discrepancy between the present in vitro results and those from lizards exposed to winter or summer photoperiods before pineal explantation supports the view that circadian information entering the pineal gland via its innervation is involved in determining seasonal changes of melatonin production in ruin lizards. We further examined whether a central component of the circadian system of ruin lizards, specifically the retinae of the lateral eyes, expresses similar seasonal changes in function as does the pineal gland. We did not find any difference between summer and autumn-winter in the effectiveness of either bilateral retinalectomy or optic nerve lesion-at the level of the optic chiasm-in altering circadian locomotor behavior in constant conditions. Both surgical procedures mostly induced a shortening of the free-running period of the locomotor rhythm of similar magnitude in all seasons. Thus, the retinae do not appear to participate in the seasonal reorganization of the circadian system in ruin lizards.     

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)     

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)     

Circadian locomotor rhythms in the desert iguana I. The role of the eyes and the pineal

Summary The pineal and the eyes are known to be important components in the circadian system of some species of lizards; their effects may be mediated by the hormone melatonin. We examined the role played by these structures in the desert iguana (Dipsosaurus dorsalis). Surgical removal of the pineal had no effect on circadian locomotor rhythms, even though this procedure abolished the circadian rhythm of melatonin in the blood. Furthermore, when the isolated pineal of Dipsosaurus was studied in organ culture, it showed no circadian rhythm of melatonin secretion, as do pineals of some other lizard species, although it did produce large quantities of this hormone. Bilateral ocular enucleation had only small effects on the freerunning period of locomotor rhythms, without affecting melatonin levels in the blood. Behavioral circadian rhythms persisted in desert iguanas subjected to both enucleation and pinealectomy. These data suggest that neither the pineal nor the eyes are central components of the circadian pacemaking system in Dipsosaurus, nor is melatonin critically involved in maintaining its organization.Abbreviations CT circadian time - ZT zeitgeber time - LL constant light - LD light-dark cycle - DD constant darkness - freerunning circadian period     

Suppression of circadian rhythms of pineal and testicular hormones following lithium treatment in normal and reversed light-dark cycles, constant light and constant dark in rats

The aim of the current investigation was to study the effect of lithium on circadian rhythms of pineal - testicular hormones by quantitations of pineal and serum serotonin, N-acetylserotonin and melatonin, and serum testosterone at four time points (06.00, 12.00, 18.00 and 24.00) of a 24-hr period under normal photoperiod (L:D), reversed photoperiod (D:L), constant light (L:L) and constant dark phase (D:D) in rats. Circadian rhythms were observed in pineal hormones in all the combinations of photoperiodic regimens, except in constant light, and in testosterone levels in all the photoperiodic combinations. Pineal and serum N-acetylserotonin and melatonin levels were higher than serotonin at night (24.00 hr), in natural L:D cycle, in reversed L:D cycle or similar to normal L:D cycle in constant dark phase, without any change in constant light. In contrast, testosterone level was higher in light phase (12.00 hr through 18.00 hr) than in the dark phase (24.00 hr through 06.00 hr) in normal L:D cycle, in reversed L:D cycle, similar to normal L:D cycle in constant dark (D:D), and reversed to that of the normal L:D cycle in constant light (L:L). Lithium treatment (2 mEq/kg body weight daily for 15 days) suppressed the magnitude of circadian rhythms of pineal and serum serotonin, N-acetylserotonin and melatonin, and testosterone levels by decreasing their levels at four time points of a 24-hr period in natural L:D or reversed D:L cycle and in constant dark (D:D). Pineal indoleamine levels were reduced after lithium treatment even in constant light (L:L). Moreover, lithium abolished the melatonin rhythms in rats exposed to normal (L:D) and reversed L:D (D:L) cycles, and sustained the rhythms in constant dark. But testosterone rhythm was abolished after lithium treatment in normal (L:D)/reversed L:D (D:L) cycle or even in constant light/dark. The findings indicate that the circadian rhythm exists in pineal hormones in alternate light - dark cycle (L:D/D:L) and in constant dark (D:D), but was absent in constant light phase (L:L) in rats. Lithium not only suppresses the circadian rhythms of pineal hormones, but abolishes the pineal melatonin rhythm only in alternate light - dark cycles, but sustains it in constant dark. The testosterone rhythm is abolished after lithium treatment in alternate light - dark cycle and constant light/dark. It is suggested that (a) normal circadian rhythms of pineal hormones are regulated by pulse dark phase in normal rats, (b) lithium abolishes pineal hormonal rhythm only in pulse light but sustains it in constant dark phase, and (c) circadian testosterone rhythm occurs in both pulse light or pulse dark phase in normal rats, and lithium abolishes the rhythm in all the combinations of the photoperiod. The differential responses of circadian rhythms of pineal and testicular hormones to pulse light or pulse dark in normal and lithium recipients are discussed.     

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)     

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.     

Free-running rhythms of pineal circadian output

Circadian rhythms are self-sustaining oscillations that free-run in constant conditions with a period close to 24 h. Overt circadian rhythms have been studied mostly using onset phase as the marker for the underlying pacemaker. Using in vivo online pineal microdialysis, the authors have performed detailed analysis of free-running profiles of rat pineal secretory products, including N-acetylserotonin (NAS) and melatonin that have precisely defined onsets and offsets. When rats entrained in LD 12:12 were released into constant darkness (DD), both onset and offset phases of melatonin and NAS free-run. However, while onsets free-run with a period closer to a day (FRP(on) = 24-24.17 h) at the beginning, offset phases free-run with significantly larger FRPs (free-running periods) (FRP(off) = 24.24-24.42 h). This asymmetric free-running of onset and offset of NAS and melatonin in DD resulted in a 60- to 120-min increase of secretion duration of both NAS and melatonin. The rate of expansion of melatonin duration was 10 to 15 min per circadian cycle. The expansion of melatonin secretion duration ended for some within 4 days, while others were still expanding by the end of 10th day in DD. These results revealed that upon release into DD, the pacemaker's oscillation is initially driven by 2 forces, free running and decompression, before reaching a stable state of free running, and suggest that the circadian pacemaker may be an elastic structure that can decompress and compress under varying photic conditions. They also illustrate the importance of using both onset and offset of a given rhythm as phase markers, as compression/decompression, and transient disparity between FRP(on) and FRP(off) may be a common phenomenon of the circadian pacemaker.     

Parameters of the circadian rhythm of pineal melatonin secretion affecting reproductive responses in Siberian hamsters

The major function of the mammalian pineal gland appears to be its central role in photoperiodism. The pineal hormone, melatonin, is synthesized and secreted primarily at night, under the control of a circadian oscillator that is entrained to the light-dark cycle. Both the circadian phase and the duration of the nocturnal peak of melatonin secretion are established primarily by interactions between the endogenous circadian oscillator and the daily photic cycle. The duration of the melatonin peak varies inversely with day length, and this relationship between day length and the duration of each circadian melatonin peak appears to be an integral part of the photoperiodic mechanism. When pinealectomized animals are given daily melatonin infusions of long duration, they exhibit physiologic responses that normally are observed during exposure to short day photoperiods; when administered short-duration melatonin infusions, the animals display long photoperiod-type responses. In addition to the importance of the duration of each melatonin peak, certain other parameters appear to be significant. If a long-duration infusion of melatonin is interrupted by a period of 2 hours or more without melatonin (i.e., to produce two short duration infusions), the responses are those typical for long day-exposed animals. Thus, to elicit short day-type responses, each long-duration melatonin peak must be relatively continuous; responses are not determined simply by the total time of exposure to melatonin in each circadian cycle. Also, long-duration melatonin peaks may not be effective to elicit photoperiod-type responses unless they are present at frequencies of nearly once every 24 hours or more.     

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)