Effects of pinealectomy and subsequent melatonin implants on activity rhythms in the house finch (Carpodacus mexicanus)

Avian photoreceptors and their role in the regulation of daily and seasonal physiology

Birds time their activities in synchronization with daily and seasonal periodicities in the environment, which is mainly provided by changes in day length (=photoperiod). Photoperiod appears to act at different levels than simply entraining the hypothalamic clock via eyes in birds. Photoreceptor cells that transmit light information to an avian brain are localized in three independent structures, the retina of eyes, pineal gland and hypothalamus, particularly in the paraventricular organ and lateral septal area. These hypothalamic photoreceptors are commonly referred to as encephalic or deep brain photoreceptors, DBPs. Eyes and pineal are known to contribute to the circadian regulation of behavior and physiology via rhythmic melatonin secretion in several birds. DBPs have been implicated in the regulation of seasonal physiology, particularly in photoperiod induced gonadal growth and development. Here, we briefly review limited evidence that is available on the roles of these photoreceptors in the regulation of circadian and seasonal physiology, with particular emphasis placed on the DBPs.

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.

Entrainment of circadian locomotor activity rhythm of the nocturnal field mouse Mus booduga using daily injections of melatonin

In this paper, we report the effects of daily injections of melatonin on the locomotor activity rhythm of the nocturnal field mouse Mus booduga. The locomotor activity rhythm of 45 animals was first monitored in constant darkness (DD) of the laboratory for about 15 days. The animals were then divided into three groups (experimental, vehicle-treated control, and the nontreated control groups) and subjected to three different treatments. The animals from the experimental group (n=19) were administered daily a single subcutaneous (s.c.) injection of melatonin (1 mg/kg) for about 45 days. The vehicle treated controls (n=13) were administered daily injections of 50% dimethyl sulfoxide (DMSO) for about 45 days, and the nontreated controls (n=13) were handled similar to the other two groups without being administered injections. Following the treatments, the animals were maintained in DD for about 20 days, after which the experiments were terminated. A significantly larger percentage of animals from the experimental group either entrained or showed phase control to daily treatments, compared to the animals from the two control groups. These results suggest that externally administered melatonin can influence the phase of the circadian locomotor activity rhythm of M. booduga. The fact that none of the nontreated controls showed any sign of phase control to daily handling, clearly demonstrates that the entrainment or phase control in the melatonin treated group of animals is caused by melatonin alone and not due to handling.

Circadian organization and the role of the pineal in birds

All organisms exhibit significant daily rhythms in a myriad of functions from molecular levels to the level of the whole organism. Significantly, most of these rhythms will persist under constant conditions, showing that they are driven by an internal circadian clock. In birds the circadian system is composed of several interacting sites, each of which may contain a circadian clock. These sites include the pineal organ, the suprachiasmatic nucleus (SCN) of the hypothalamus, and, in some species, the eyes. Light is the most powerful entraining stimulus for circadian rhythms and, in birds, light can affect the system via three different pathways: the eyes, the pineal, and extraretinal photoreceptors located in the deep brain. Circadian pacemakers in the pineal and in the eyes of some avian species communicate with the hypothalamic pacemakers via the rhythmic synthesis and release of the hormone melatonin. Often the hypothalamic pacemakers are unable to sustain persistent rhythmicity in constant conditions in the absence of periodic melatonin input from the pineal (or eyes). It has also been proposed that pineal pacemakers may be unable to sustain rhythmicity in constant conditions without periodic neural input from the SCN. Significant variation can occur among birds in the relative roles that the pineal, the SCN, and the eyes play within the circadian system; for example, in the house sparrow pacemakers in the pineal play the predominant role, in the pigeon circadian pacemakers in both the pineal and eyes play a significant role, and in Japanese quail ocular pacemakers play the predominant role.

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 neuroplasticity in the songbird telencephalon: a role for melatonin

Neuroplasticity in the vocal control system of songbirds is strongly influenced by seasonal fluctuations in circulating testosterone. These seasonally plastic telencephalic structures are implicated in the learning and production of song in songbirds. The role of the indoleamine melatonin in seasonal adaptations in birds has remained unclear. In this experiment, European starlings were castrated to remove the neuromodulating activity of gonadal steroids and were exposed to different photoperiods to induce reproductive states characteristic of different seasonal conditions. Long days increased the volume of the song-control nucleus high vocal center compared with its volume on short days. Exogenous melatonin attenuated the long-day-induced volumetric increase in high vocal center and also decreased the volume of another song-control nucleus, area X. This effect was observed regardless of reproductive state. To our knowledge, this is the first direct evidence of a role for melatonin in functional plasticity within the central nervous system of vertebrates.

Melatonin, immunity and cost of reproductive state in male European starlings

The effects of reproductive condition and exogenous melatonin on immune function were investigated in castrated European starlings, Sturnus vulgaris. Photorefractory and photostimulated starlings exposed to long days were implanted with melatonin or with blank capsules. Photostimulated starlings with blank capsules exhibited reduced splenocyte proliferation in response to the T-cell mitogen, concanavalin A, compared with the other long-day birds. Exogenous melatonin prevented the suppression of immune function by photostimulation. Photorefractory starlings, with or without melatonin implants, exhibited enhanced immune function compared with photostimulated starlings implanted with blanks. This enhancement was not mediated by endogenous melatonin, but appeared to be related to changes in reproductive state. In addition to the traditional costs of reproduction in birds (e.g. raising of young), there may be a cost of the reproductive state of starlings (i.e. whether they are photorefractory or photostimulated). These data are, we believe, the first to indicate a direct effect of reproductive state on immune function that is independent of both photoperiod (i.e., changes in the duration of melatonin secretion) and gonadal steroids.

The circadian rhythm of thermoregulation in Japanese quail: III. Effects of melatonin administration

Recent studies indicate that the circadian pacemakers in the eyes of Japanese quail are coupled to the rest of the circadian system by both neural and hormonal outputs. The effects of exogenous melatonin administration on circadian body temperature and activity rhythms of quail were tested to determine whether melatonin could be the hormonal link involved. Continuous melatonin administration caused arrhythmicity or period changes in the body temperature and activity rhythms of pinealectomized and sham-pinealectomized birds held in constant darkness and also significantly decreased the amplitude of the body temperature rhythms of normal birds held on light:dark 12:12. Further, melatonin entrained the body temperature and activity rhythms of normal birds when administered daily via the drinking water. The results show that melatonin can affect the circadian system of quail and support the hypothesis that melatonin is importantly involved in linking pacemakers in the eyes to the rest of the circadian system.

The circadian rhythm of thermoregulation in Japanese quail. II. Multioscillator control

Most biochemical, physiological, and behavioral processes in vertebrates show significant daily rhythms. Under constant conditions, these rhythms exhibit an endogenous periodicity around 24 h showing that they are driven by an internal circadian clock. In Japanese quail, the circadian clock driving activity and body temperature rhythms is functionally organized as a dual-oscillator system. Under certain conditions, such as switching birds from light:dark (LD) 12:12 to continuous darkness (DD), the body temperature rhythm splits into two circadian components that free-run independently before recoupling in a normal phase-relationship. The behavior of the activity rhythm parallels that of the body temperature rhythm, supporting the hypothesis that both rhythms are driven by the same set of oscillators. In some instances, recoupling fails to occur and birds continue to exhibit two circadian components that free-run independently. Dual-oscillator control of body temperature was observed in normal birds, pinealectomized birds, and optic nerve sectioned birds. However, birds were rendered arrhythmic by complete eye removal. It is proposed that the central circadian system (suprachiasmatic nuclei?) acts as a complex pacemaker that is functionally organized as two sets of oscillators and that circadian input from the eyes is necessary to preserve the integrity of this complex pacemaker.

The effect of pinealectomy on circadian plasma melatonin levels in house sparrows and European starlings

We determined 24-hr plasma melatonin profiles in intact, sham-pinealectomized, and pinealectomized European starlings (Sturnus vulgaris) and house sparrows (Passer domesticus) in a light-dark (LD) cycle and in constant darkness (DD). In the intact and sham-pinealectomized birds of both species, a melatonin rhythm was found, with low levels during the day and high levels during the night. Pinealectomy abolished the nighttime peak of melatonin in both species; hence, levels were low at all times sampled. This uniform response of plasma melatonin to pinealectomy contrasts with the differential response of circadian activity rhythms to pinealectomy for these two species. In DD, locomotor activity in pinealectomized house sparrows is usually arrhythmic, whereas in starlings a rhythm usually persists. This suggests that in the latter species free-running circadian rhythms are not necessarily dependent on a rhythm in plasma melatonin. The same is true for the synchronized activity rhythm observed in pinealectomized birds of both species in LD, as well as for the damped rhythm that persists in pinealectomized house sparrows following an LD-to-DD transfer. The results are consistent with the hypothesis that the pineal and its periodic output of melatonin constitute only one component in a system of at least two coupled pacemakers. They also suggest that there are species differences in the relative role played by the pineal and other pacemakers in controlling circadian rhythms in behavior.

The superior cervical ganglia are not necessary for entrainment or persistence of the pineal melatonin rhythm in Japanese quail

The avian pineal exhibits a daily rhythm in the synthesis and secretion of the hormone, melatonin, which is involved in maintaining temporal order within the circadian system of some species. The pineal is richly innervated by sympathetic nerves which originate in the superior cervical ganglia (SCG) and, in the chicken, these nerves play a role in generating the melatonin rhythm. In the Japanese quail, the pineal melatonin rhythm can be entrained by light perceived directly by the pineal or by light perceived by the eyes. The role of the sympathetic innervation of the pineal was investigated in the Japanese quail by subjecting birds to bilateral superior cervical ganglionectomy (SCGX) and determining if SCGX either abolished the ability of retinally perceived light to entrain the pineal melatonin rhythm or if it disrupted the rhythm under constant darkness (DD). The results show that SCGX neither prevented entrainment of the pineal melatonin rhythm by retinally perceived light nor affected the rhythm expressed in DD. An entrainment pathway between the eyes and pineal exists in quail which does not involve the SCG.

Retinally perceived light can entrain the pineal melatonin rhythm in Japanese quail

The avian pineal organ contains a circadian oscillator that can drive a daily rhythm of melatonin synthesis. In some avian species the pineal organ may act, via the cyclic release of melatonin, as a pacemaker within a multioscillator circadian system. The routes by which light entrains the pineal melatonin rhythm were investigated in the Japanese quail. A 'patching' protocol was used to expose directly either the eyes or the pineal to a light-dark cycle while the rest of the bird was exposed to constant light. The results show that the pineal melatonin rhythm can be entrained (1) by light perceived directly or (2) by light perceived by the eyes. Furthermore, the pathway by which light entrains the pineal melatonin rhythm includes the optic nerves because transection of the optic nerve eliminates the ability of ocularly perceived light to entrain the pineal melatonin rhythm.

The circadian activity rhythms of rats with mid- and parasagittal 'split-SCN' knife cuts and pinealectomy

To explore the multioscillator nature of the neurohumoral circuitry controlling the expression of circadian rhythmicity, rats' wheel running circadian activity rhythms were compared following sagittal knife cuts in the region of the suprachiasmatic nucleus (SSCN), following pinealectomy (PX) and following the combination of SSCN and PX. 25% of animals with knife cuts that passed through one SCN had disturbed running activity under constant illumination; rhythmic disturbances were seen neither in animals with sham knife cuts nor in rats with knife cuts on the midline or lateral to the SCN. Animals with both SSCN and PX were twice as likely to show severe rhythmic disruptions under free-running conditions as rats with SSCN and sham PX. Rats with PX and sham SSCN did not display disrupted activity rhythms. When animals with PX alone or SSCN alone were first observed under free-running conditions and then subjected to a second surgical procedure so that all animals underwent both PX and SSCN, all PX and most SSCN animals demonstrated coherent activity rhythms after the first operation, but 35% showed disruptions in circadian activity patterns only following the second surgery. The activity rhythms of rats with knife cuts placed either on the midline or lateral to the SCN did not deteriorate when combined with PX. Rats with coherent rhythms following knife cuts damaging one SCN had rhythm disruptions after the addition of PX. The effects of pinealectomy may indicate that the pineal gland plays a role in maintaining the coupling relationships in the multioscillator system controlling circadian activity rhythms. The results of this study also suggest that neither the direct commissural connection of the SCNs nor the humoral output of the pineal gland is indispensable for the expression of coherent circadian activity rhythms in the rat.

A new mutation at the period locus of Drosophila melanogaster with some novel effects on circadian rhythms

A new period mutation has been induced and characterized in D. melanogaster. It causes flies to be apparently arrhythmic in tests of locomotor activity and thus is superficially similar to the original per01 mutant. Yet, the new "zero" allele, per04, has some novel properties and effects: Behaviorally, per04 adults often exhibit weak, long-period rhythms of locomotor activity in constant darkness; this low-frequency rhythmicity usually was not obvious in the analog behavioral records but was readily revealed by spectral analyses. These treatments of the data also extracted hidden high-frequency (ultradian) rhythms in many of the behavioral records, of the type associated with per01 and other per-nulls. The wide range of periodicities exhibited by different per04-expressing flies implies the expression of multiple oscillatory modes by this mutant. The new mutation also leads to a tendency for flies to be hyperactive during activity monitoring and is thus dissimilar to the other arrhythmic variants in the per gene but similar to the effects of a deletion of the locus. During light:dark cycling, per04 adults once more behave differently from other per0's and in fact tend to resemble wild-type flies in these conditions. The new mutation is not caused by the same nucleotide substitution that created a stop codon in the original arrhythmic per mutant and, as it turns out, per02 and per03 as well. per04 is also not a null variant at the transcriptional level; but it leads to an anomalous form of per mRNA, which is smaller than the normal 4.5 kb species encoded by this clock gene.

Differential effects of pinealectomy on circadian rhythms of feeding and perch hopping in the European starling

To study the effects of pinealectomy on the circadian rhythms of locomotor activity and feeding. European starlings (Sturnus vulgaris) were held in constant light (0.2 lux and 200 lux) and under constant temperature conditions. Locomotor activity was measured by means of perches with microswitches mounted underneath, and feeding with an infrared photocell system at the feeder. Pinealectomy consistently led to disturbances in perch-hopping rhythms and often to a complete loss of rhythmicity as revealed by periodogram analysis. In some birds, perch-hopping rhythms recovered following a period of initial arrhythmicity. When a perch-hopping rhythm was present, its period was usually shorter than it had been before pinealectomy. In contrast to its effects on perch hopping, pinealectomy had no effect on the persistence of feeding rhythmicity, although its period, like that of the hopping rhythm, decreased after this operation. These results support the hypothesis derived from previous studies that the circadian organization of feeding is different from that of perch hopping. Different circadian pacemakers may be involved, but other models may possibly explain the data just as well.

Structure and function in circadian timing systems: evidence for multiple coupled circadian oscillators

Considerable progress has been made in elucidating the mechanisms underlying the generation of circadian rhythmicity. This review describes several distinct lines of evidence which converge on the general hypothesis that circadian timing systems consist of multiple circadian oscillators, coordinated by both hierarchical and non-hierarchical coupling relationships. Such a view is supported by the complex phenomenology of circadian systems, as well as by physiological considerations. We have attempted wherever possible to integrate these two sources of evidence, in order to define the current "state of the art" in bridging the gap between structure and function in the analysis of circadian timing systems. While we concentrate mainly on the mammalian, and particularly the rodent, circadian system, we also incorporate comparative evidence obtained from a variety of vertebrate and invertebrate species.

Effects of melatonin implants on structures and behaviors of the house finch (Carpodacus mexicanus) eye

This study aimed to determine the extraretinal effects of melatonin upon the eyes of an avian species, the House Finch (Carpodacus mexicanus). Twelve birds (full-grown, second-year males) each received a Silastic tubing intraperitoneal implant, six containing melatonin (average release = 24 micrograms/d/bird; = M birds) and six being empty (= C birds). Microscopic study of pupillary and palpebral behaviors during the final week demonstrated lesser pupillary diameters and interpalpebral distances in M birds under all test conditions. These effects could have diminished mean light levels reaching parts of the retina. Characteristics of the relative miosis and ptosis of M birds resemble signs in some CNS disorders, such as altered inhibition of the Edinger-Westphal nucleus, and especially lesions in, or lowered activity of, higher sympathetic centers (a subtype of Horner's syndrome). Weights of eyes and their parts were the same in M and C birds, contrasting with previously reported results from male Golden Hamsters, possibly due to species differences and/or preexperimental attainment of full growth in the finches. Effects of melatonin on pupillary and palpebral behaviors, demonstrated here for the first time, foster caveats for simplistic experimental designs and interpretations with melatonin when sensory-neural-behavioral interactions are affected. Quantitative changes in pupillary and palpebral behaviors may, nevertheless, provide a window for monitoring central actions of melatonin in living test subjects in chronic studies.

Is the avian circadian system a neuroendocrine loop?

Avian circadian organization is a result of a complex interaction of photoreceptive and oscillatory components. The known components include the pineal gland, the lateral eyes, the suprachiasmatic nuclei (SCN), and extraocular brain photoreceptors. The pathways by which these components integrate circadian rhythmicity suggest a neuroendocrine loop in which the SCN inhibits pineal and ocular oscillators during the course of subjective day via a multisynaptic neuronal pathway which includes the superior cervical ganglia (SCG). During the night, the pineal in turn inhibits SCN activity via its secretion of the hormone melatonin into the blood circulation. This neuroendocrine loop, it is proposed, synchronizes multiple oscillators within each component and maintains the stability and precision of the system.

Daily rhythms of serotonin metabolism in the medial hypothalamus of the chicken: effects of pinealectomy and exogenous melatonin

Indoleamine levels in punches of the medial hypothalamus containing the suprachiasmatic nuclei (SCN) of 4-week-old cockerels were determined by HPLC-EC. Melatonin levels in punches were determined by radioimmunoassay (RIA). Daily rhythms of serotonin (5-HT) and of its metabolite 5-hydroxy-3-indoleacetic acid (5-HIAA) were observed; levels were higher at midnight than at midday. A daily rhythm with the same phase in punch melatonin content was also observed. Pinealectomy at 1 week after hatching abolished the 5-HIAA and melatonin rhythm in 4-week-old birds but did not abolish the 5-HT rhythm. Injections of melatonin (0.5 mg/kg) increased 5-HT, 5-HIAA and melatonin levels in the hypothalamic punches. These results indicate that circulating melatonin of pineal origin may act to increase 5-HT turnover and/or release in the SCN. They suggest a link between the circadian secretion of pineal melatonin and the regulation of 5-HT projections to the hypothalamus from the raphe nuclei in the brainstem of the chicken. We have previously shown that the rhythmic secretion of melatonin by the pineal is influenced by oscillators in the brain via the superior cervical ganglia. The results reported here indicate that melatonin in turn may regulate brain oscillators, suggesting a neuroendocrine loop within the avian circadian system.