Development of post-hatching melatonin rhythm in zebra finches (Poephila guttata)

Evidence That Artificial Light at Night Induces Structure-Specific Changes in Brain Plasticity in a Diurnal Bird

We recently reported that artificial light at night (ALAN), at ecologically relevant intensities (1.5, 5 lux), increases cell proliferation in the ventricular zone and recruitment of new neurons in several forebrain regions of female zebra finches (Taeniopygia guttata), along with a decrease of total neuronal densities in some of these regions (indicating possible neuronal death). In the present study, we exposed male zebra finches to the same ALAN intensities, treated them with 5′-bromo-2′-deoxyuridine, quantified cell proliferation and neuronal recruitment in several forebrain regions, and compared them to controls that were kept under dark nights. ALAN increased cell proliferation in the ventricular zone, similar to our previous findings in females. We also found, for the first time, that ALAN increased new neuronal recruitment in HVC and Area X, which are part of the song system in the brain and are male-specific. In other brain regions, such as the medial striatum, nidopallium caudale, and hippocampus, we recorded an increased neuronal recruitment only in the medial striatum (unlike our previous findings in females), and relative to the controls this increase was less prominent than in females. Moreover, the effect of ALAN duration on total neuronal densities in the studied regions varied between the sexes, supporting the suggestion that males are more resilient to ALAN than females. Suppression of nocturnal melatonin levels after ALAN exhibited a light intensity-dependent decrease in males in contrast to females, another indication that males might be less affected by ALAN. Taken together, our study emphasizes the importance of studying both sexes when considering ALAN effects on brain plasticity.

Artificial Light at Night Increases Recruitment of New Neurons and Differentially Affects Various Brain Regions in Female Zebra Finches

Despite growing evidence that demonstrate adverse effects of artificial light at night (ALAN) on many species, relatively little is known regarding its effects on brain plasticity in birds. We recently showed that although ALAN increases cell proliferation in brains of birds, neuronal densities in two brain regions decreased, indicating neuronal death, which might be due to mortality of newly produced neurons or of existing ones. Therefore, in the present study we studied the effect of long-term ALAN on the recruitment of newborn neurons into their target regions in the brain. Accordingly, we exposed zebra finches (Taeniopygia guttata) to 5 lux ALAN, and analysed new neuronal recruitment and total neuronal densities in several brain regions. We found that ALAN increased neuronal recruitment, possibly as a compensatory response to ALAN-induced neuronal death, and/or due to increased nocturnal locomotor activity caused by sleep disruption. Moreover, ALAN also had a differential temporal effect on neuronal densities, because hippocampus was more sensitive to ALAN and its neuronal densities were more affected than in other brain regions. Nocturnal melatonin levels under ALAN were significantly lower compared to controls, indicating that very low ALAN intensities suppress melatonin not only in nocturnal, but also in diurnal species.

Natural melatonin fluctuation and its minimally invasive simulation in the zebra finch

Melatonin is a key hormone in the regulation of circadian rhythms of vertebrates, including songbirds. Understanding diurnal melatonin fluctuations and being able to reverse or simulate natural melatonin levels are critical to investigating the influence of melatonin on various behaviors such as singing in birds. Here we give a detailed overview of natural fluctuations in plasma melatonin concentration throughout the night in the zebra finch. As shown in previous studies, we confirm that “lights off” initiates melatonin production at night in a natural situation. Notably, we find that melatonin levels return to daytime levels as early as two hours prior to the end of the dark-phase in some individuals and 30 min before “lights on” in all animals, suggesting that the presence of light in the morning is not essential for cessation of melatonin production in zebra finches. Thus, the duration of melatonin production seems not to be specified by the length of night and might therefore be less likely to directly couple circadian and annual rhythms. Additionally, we show that natural melatonin levels can be successfully simulated through a combination of light-treatment (daytime levels during subjective night) and the application of melatonin containing skin-cream (nighttime levels during subjective day). Moreover, natural levels and their fluctuation in the transition from day to night can be imitated, enabling the decoupling of the effects of melatonin, for example on neuronal activity, from sleep and circadian rhythmicity. Taken together, our high-resolution profile of natural melatonin levels and manipulation techniques open up new possibilities to answer various melatonin related questions in songbirds.

Circadian melatonin production develops faster in birds than in mammals

The development of circadian rhythmicity of melatonin biosynthesis in the pineal gland starts during embryonic period in birds while it is delayed to the postnatal life in mammals. Daily rhythms of melatonin in isolated pinealocytes and in intact pineal glands under in vivo conditions were demonstrated during the last third of embryonic development in chick embryos, with higher levels during the dark (D) than during the light (L) phase. In addition to the LD cycle, rhythmic temperature changes with the amplitude of 4.5°C can entrain rhythmic melatonin biosynthesis in chick embryos, with higher concentrations found during the low-temperature phase (33.0 vs 37.5°C). Molecular clockwork starts to operate during the embryonic life in birds in line with the early development of melatonin rhythmicity. Expression of per2 and cry genes is rhythmic at least at day 16 and 18, respectively, and the circadian system operates in a mature-like manner soon after hatching. Rhythmic oscillations are detected earlier in the central oscillator (the pineal gland) than in the peripheral structures, reflecting the synchronization of individual cells which is necessary for detection of the rhythm. The early development of the circadian system in birds reflects an absence of rhythmic maternal melatonin which in mammals synchronizes physiological processes of offspring. Developmental consequences of modified development of circadian system for its stability later in development are not known and should be studied.

Epigenetic maternal effects on endogenous rhythms in precocial birds

Development involves interactions between genetic and environmental influences. Vertebrate mothers are generally the first individuals to encounter and interact with young animals. Thus, their role is primordial during ontogeny. The present study evaluated non-genomic effects of mothers on the development of rhythms of precocial Japanese quail (Coturnix c. japonica). First, we investigated the influence of mothering on the ontogeny of endogenous rhythms of young. We compared circadian and ultradian rhythms of feeding activity of quail reared with or without adoptive mothers. More brooded than non-brooded quail presented a circadian and/or an ultradian rhythm. Thus, the presence of the mother during the normal brooding period favors, in the long term, expression of rhythms in the young. Second, we investigated the influence of rhythmic phenotype of the mother on the development of endogenous rhythms of young by comparing quail brooded by circadian-rhythmic adoptive mothers (R) to quail brooded by circadian-arrhythmic adoptive mothers (A). More R-brooded than A-brooded quail expressed circadian rhythmicity, and circadian rhythm clarities were greater in R-brooded than A-brooded quail. Ultradian rhythmicity did not differ between R- and A-brooded quail, nor between R and A adoptive mothers. Thus, the rhythmic phenotypes of quail mothers influence the rhythmic phenotypes of their young. Our results demonstrate that mothers of precocial birds influence epigenetically the ontogeny of endogenous rhythms of the young they raise.

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.

Melatonin affects the temporal organization of the song of the zebra finch

In birds and mammals, including humans, melatonin-binding sites are abundant in brain areas that have no known clock function. Although the role of such binding sites is still unclear, it is assumed that these sites link neural functions to circadian or circannual demands of neuroendocrine homeostasis and reproduction. To investigate a possible direct role of melatonin in motor control, we studied the song and neural song system of the zebra finch. Neurons of two sensory-motor areas of the descending song control circuit that are crucial for the organization of the song pattern, the HVC and RA, express the melatonin-1B receptor (Mel1B), while the hypoglossal motor neurons of the song circuit express melatonin-1C receptors (Mel1C). Application of melatonin to brain slices decreases the firing-rate of RA-neurons. Systemic administration of a Mel1B antagonist at the beginning of the night shortens the song and motif length and affects the song syllable lengths produced the next day. The temporal pattern of the song, however, does not undergo daily changes. Thus, melatonin is likely to affect a non-circadian motor pattern by local modulation of song control neurons and in consequence alters a sexual signal, the song of the zebra finch.

Posthatching developmental changes in noradrenaline content in the chicken pineal gland

Noradrenaline (NA) levels in pineal gland of chickens at various posthatching stages (P2, P4, P8, P15, P30 and P57) were determined by high-performance liquid chromatography with electrochemical detection. Pineal NA content markedly increased between P2 and P30. P30 and P57 chickens, kept from the day of hatching under a 12:12 hr light-dark (LD) illumination cycle, exhibited rhythmic changes in pineal NA, with levels in the dark period being markedly higher than in the light period. In younger birds pineal NA concentrations did not show pronounced daily variations. In 4-wk-old chickens (P28-30) kept under constant darkness (DD), the rhythmic pattern of pineal NA persisted for 1 day (with higher values during the subjective dark phase than during the subjective light phase), but this disappeared 24 hr after the introduction of DD. In contrast, NA content in pineal glands isolated from birds maintained for 2 days under continuous light was similar to that found during the light phase of the LD cycle, and did not exhibit significant rhythmicity. In P30 chickens, pretreated with alpha-methyl-p-tyrosine (AMPT, an inhibitor of tyrosine hydroxylase, the key regulatory enzyme in the biosynthesis of catecholamines), pineal NA content declined slowly and monophasically during the light phase. During the dark phase the AMPT-induced decay of NA was biphasic--namely an initial rapid decline over the first 15 min which was followed by a slow-rate decline--an observation indicating that NA turnover was higher in the dark. Acute exposure of the dark-adapted P30 and P57 chickens to light significantly decreased pineal NA content, but did not affect pineal NA concentrations in younger birds. Our results suggest that the NA rhythm in the chicken pineal gland and its sensitivity to light regulation progressively develop during the first month of life.

Hormone-Dependent Neural Plasticity in the Juvenile and Adult Song System: What Makes a Successful Male?

The sexual quality of adult song is the result of genetic and epigenetic mechanisms shaping the neural song system throughout life. Genetic brain-intrinsic mechanisms determine the neuron pools that develop into forebrain song control areas independent of gonadal steroid hormones, androgens and estrogens. One fate of these neurons is the potential to express sex steroid receptors, such as androgen and estrogen receptors. Genetic brain-intrinsic mechanisms, too, determine the activity of hypothalamic-pituitary-gonad (HPG) axis, i.e., the working range and responsiveness of HPG axis to produce gonadal hormones. The epigenetic action of gonadal steroid hormones (androgens and estrogens) on determined vocal neurons is required to maintain and increase the pool of determined vocal neurons and to complete the connections of the vocal system, i.e., to make it function motorically. The subsequent influence of environmental information, including both external (socio-sexual and physical) and internal (body physiology) signals, specify the further neural phenotype of vocal areas either through acting on the HPG axis and differential release of gonadal hormones or through non-gonadal hormone systems, both of which have target neurons in the functional vocal system. Despite the clear evidence of hormone dependency of the development of both the adult song phenotype and song system phenotype, their causal relation is complex.

Prehatch entrainment of circadian rhythms in the domestic chick using different light regimes

The onset of circadian rhythms in many animals occurs during prenatal development. We conducted four experiments, using the domestic chick as a model, to assess when these rhythms can first be entrained and the type of light zeitgeber necessary. In Experiment 1, the presence of circadian rhythms was assessed using tonic immobility, an antipredator behavior, whereas in Experiments 2 to 4 body temperature was studied. We demonstrate that (a) circadian rhythms can be entrained during the late stage of the chick's 21-day incubation period (prehatch Days 13-18), (b) only 1 day of light cues [12:12 hr light:dark (12L:12D)] on prehatch Day 13 is necessary for entrainment, and (c) short bouts of light, which simulate the light cues embryos typically experience during natural incubation, can act as zeitgebers although they are not as effective as 12L:12D. The onset of entrainment is earlier than predicted and suggests that the brain structures mediating circadian rhythms mature sooner than proposed by previous research.

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.

Development of rhythmic melatonin secretion from the pineal gland of embryonic mummichog (Fundulus heteroclitus)

The pineal gland of vertebrates produces and secretes the hormone melatonin in response to changes in the light-dark cycle, with high production at night and low production during the day. Melatonin is thought to play an important role in synchronizing daily and/or seasonal physiological, behavioral, and developmental rhythms in vertebrates. In this study, the functional development of the pineal melatonin-generating system was examined in the mummichog, Fundulus heteroclitus, an euryhaline teleost. In this species, the pineal gland contains an endogenous oscillator, ultimately responsible for timing the melatonin rhythm. Oocytes from gravid females were collected and fertilized in vitro from sperm collected from mature males. Skull caps containing attached pineal glands were obtained from F. heteroclitus embryos at different embryonic stages and placed in static or perfusion culture under various photoperiodic regimes. Rhythmic melatonin secretion from pineal glands of embryonic F. heteroclitus embryos exposed to a 12L:12D cycle in static culture was observed at five days post-fertilization. The ontogeny of circadian-controlled melatonin production from F. heteroclitus pineal glands exposed to constant darkness for five days was also seen at day five post-fertilization. These data show that early development of the pineal melatonin-generating system in this teleost occurs prior to hatching. Pre-hatching development of the melatonin-generating system may confer some selective advantage in this species in its interactions with the environment.

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.

Ontogeny of the daily profile of plasma melatonin in European starlings raised under long or short photoperiods

Photoperiodic manipulation of young European starlings suggests that their reproductive physiology is incapable of responding to a short photoperiod until they are fully grown. This study aimed to determine whether the lack of response to a short photoperiod is reflected in the daily profile of plasma melatonin concentrations. Five-day-old starlings taken from nest boxes showed a significant (p < 0.0001) rhythm in plasma melatonin concentrations, with high values during night. In nestlings hand-reared from 5 days of age on a long photoperiod (LD 16:8), equivalent to natural photoperiod at the time, the amplitude of the daily rhythm in melatonin increased significantly (p < 0.01) with age until birds were fully grown (20 days old). In nestlings reared on a short photoperiod (LD 8:16), the daily melatonin profile remained almost identical to that of long photoperiod birds until they were fully grown. However, after 20 days old, the duration of elevated nighttime melatonin began to extend to encompass the entire period of darkness. In contrast, fully grown starlings transferred from a long to a short photoperiod had partially adapted to the short photoperiod after 5 days; by 10 days, the daily melatonin profile was identical to that of birds held chronically on a short photoperiod. Thus, consistent with responses of reproductive physiology, the pineal of young birds appears to be incapable of perceiving, or adapting to, a short photoperiod.

Comparative ultrastructure and cytochemistry of the avian pineal organ

The breeding of birds is expected to solve problems of nourishment for the growing human population. The function of the pineal organ synchronizing sexual activity and environmental light periods is important for successful reproduction. Comparative morphology of the avian pineal completes data furnished by experiments on some frequently used laboratory animals about the functional organization of the organ. According to comparative histological data, the pineal of vertebrates is originally a double organ (the "third" and the "fourth eye"). One of them often lies extracranially, perceiving direct solar radiation, and the other, located intracranially, is supposed to measure diffuse brightness of the environment. Birds have only a single pineal, presumably originating from the intracranial pineal of lower vertebrates. Developing from the epithalamus, the avian pineal organ histologically seems not to be a simple gland ("pineal gland") but a complex part of the brain composed of various pinealocytes and neurons that are embedded in an ependymal/glial network. In contrast to organs of "directional view" that develop large photoreceptor outer segments (retina, parietal pineal eye of reptiles) in order to decode two-dimensional images of the environment, the "densitometer"-like pineal organ seems to increase their photoreceptor membrane content by multiplying the number of photoreceptor perikarya and developing follicle-like foldings of its wall during evolution ("folded retina"). Photoreceptor membranes of avian pinealocytes can be stained by antibodies against various photoreceptor-specific compounds, among others, opsins, including pineal opsins. Photoreceptors immunoreacting with antibodies to chicken pinopsin were also found in the reptilian pineal organ. Similar to cones and rods representing the first neurons of the retina in the lateral eye, pinealocytes of birds possess an axonal effector process which terminates on the vascular surface of the organ as a neurohormonal ending, or forms ribbon-containing synapses on pineal neurons. Serotonin is detectable immunocytochemically on the granular vesicles accumulated in neurohormonal terminals. Pinealocytic perikarya and axon terminals also bind immunocytochemically recognizable excitatory amino acids. Peripheral autonomic fibers entering the pineal organ through its meningeal cover terminate near blood vessels. Being vasomotor fibers, they presumably regulate the blood supply of the pineal tissue according to the different levels of light-dependent pineal cell activity.

Exogenous melatonin reduces the resynchronization time after phase shifts of a nonphotic zeitgeber in the house sparrow (Passer domesticus)

Continuous melatonin administration via silastic implants accelerates the resynchronization of the circadian locomotor activity rhythm in house sparrows (Passer domesticus) after exposure to phase shifts of a weak light-dark cycle. Constant melatonin might induce this effect either by increasing the sensitivity of the visual system to a light zeitgeber or by reducing the degree of self-sustainment of the circadian pacemaker. To distinguish between these two possible mechanisms, two groups of house sparrows, one carrying melatonin implants and the other empty implants, were kept in constant dim light and subjected to advance and delay shifts of a 12-h feeding phase. The resynchronization times of their circadian feeding rhythm following the phase shifts were significantly shorter when the birds carried melatonin implants than when they carried empty implants. In a second experiment, melatonin-implanted and control birds were released into food ad libitum conditions 2 days after either a delay or an advance phase shift. The number of hours by which the activity rhythms had been shifted on the second day in food ad libitum conditions was assessed. Melatonin-implanted house sparrows had significantly larger phase shifts in their circadian feeding rhythm than control birds. This is in accordance with the first experiment since a larger phase shift at a given time reflects accelerated resynchronization. Additionally, the second experiment also excludes any possible masking effects of the nonphotic zeitgeber. In conclusion, constant melatonin accelerates resynchronization even after phase shifts of a nonphotic zeitgeber, indicating that constant high levels of melatonin can reduce the degree of self-sustainment of the circadian pacemaker independent of any effects on the photoreceptive system.

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.

Influence of pinealectomy and pineal stalk deflection on circadian gastrointestinal tract melatonin rhythms in zebra finches (Taeniopygia guttata)

The authors examined levels of melatonin in the plasma and various tissues in intact, pinealectomized, and pineal stalk-deflected zebra finches kept under 12:12 LD to determine if the melatonin found in the gastrointestinal tract is secreted in a circadian manner. In intact and pineal stalk-deflected birds, there is a clear day-night rhythm in melatonin content of the plasma, pineal gland, eyes, proventriculus, crop, duodenum, jejunum/ileum, colon, heart, and liver. In contrast, pinealectomy abolished the day-night rhythm. These results indicate that most of the melatonin present in the gastrointestinal tract of zebra finches is of pineal origin. However, some melatonin remained. This suggests that this melatonin may be locally synthesized and has paracrine and/or autocrine functions. Nonetheless, the results do not lend support to the contention that this putative melatonin secretion by the gastrointestinal tract is circadian.

Cellular circadian clocks in the pineal

Daily rhythms are a fundamental feature of all living organisms; most are synchronized by the 24 hr light/dark (LD) cycle. In most species, these rhythms are generated by a circadian system, and free run under constant conditions with a period close to 24 hr. To function properly the system needs a pacemaker or clock, an entrainment pathway to the clock, and one or more output signals. In vertebrates, the pineal hormone melatonin is one of these signals which functions as an internal time-keeping molecule. Its production is high at night and low during day. Evidence indicates that each melatonin producing cell of the pineal constitutes a circadian system per se in non-mammalian vertebrates. In addition to the melatonin generating system, they contain the clock as well as the photoreceptive unit. This is despite the fact that these cells have been profoundly modified from fish to birds. Modifications include a regression of the photoreceptive capacities, and of the ability to transmit a nervous message to the brain. The ultimate stage of this evolutionary process leads to the definitive loss of both the direct photosensitivity and the clock, as observed in the pineal of mammals. This review focuses on the functional properties of the cellular circadian clocks of non-mammalian vertebrates. How functions the clock? How is the photoreceptive unit linked to it and how is the clock linked to its output signal? These questions are addressed in light of past and recent data obtained in vertebrates, as well as invertebrates and unicellulars.

Perinatal development of circadian melatonin production in domestic chicks

In contrast to the situation in mammals, in which circadian melatonin production by the pineal gland does not begin until some time after birth, the development of pineal gland rhythmicity is an embryonic event in the precocial domestic fowl. A distinct melatonin rhythm was found in 19-d-old chick embryos maintained under light:dark (LD) 16:8. No significant variation in melatonin levels was detected in embryos exposed to LD 8:16. The melatonin rhythm in the pineal gland and plasma of chick embryos incubated for 18 d in LD 12:12 persisted for 2 d in constant darkness indicating that melatonin production is under circadian control at least from the end of embryonic life. A 1-d exposure to a LD cycle during the first postembryonic day was sufficient to entrain the melatonin rhythm, and previous embryonic exposure to either LD or constant darkness (DD) neither modified this rapid synchronization nor did it affect the melatonin pattern during the two subsequent days in DD. It is suggested that, in contrast to the situation in mammals, the avian embryo has evolved its own early circadian melatonin-producing system because, as a consequence of its extrauterine development, it cannot use the system of its mother.

Synchronization by low-amplitude light-dark cycles of 24-hour pineal and plasma melatonin rhythms of hatchling European starlings (Sturnus vulgaris)

In young European starlings, as in other avian species, high-amplitude 24-hr rhythms in plasma and pineal melatonin are already present around the time of hatching. In chickens this rhythmicity results at least partly from the light sensitivity of the melatonin-producing and -secreting system. In contrast to the chicken, the starling is a hole-nesting bird, and it seemed questionable whether the low light intensities in the nest are sufficient to synchronize perinatal melatonin rhythms. We therefore exposed starling eggs to light cycles roughly simulating those measured in nest-boxes, i.e., an 11-hr phase of complete darkness and a 13-hr phase consisting of 15 min of dim light (10 lux) alternating with 30 min of darkness. For one group the photophase lasted from 0600 to 1900 hr; for the other group the photophase lasted from 1800 to 0700 hr. In approximately 10-hr-old hatchlings of both groups, plasma and pineal melatonin concentrations were high during the dark phase and low during the light phase. We conclude that perinatal low-amplitude light intensity changes of the kind experienced by hatching starlings in the field are sufficient for synchronizing the melatonin-producing and -secreting system in the pineal and possibly other organs.