Daily and circadian variation in the electroretinogram of the domestic fowl: effects of melatonin

Visual and circadian function are integrally related in birds, but the precise nature of their interaction is unknown. The present study determined whether visual sensitivity measured electroretinographically (ERG) in 7-week-old cockerels varies over the time of day, whether this rhythm persists in constant darkness (DD) and whether exogenous melatonin affects this ERG rhythmicity. ERG b-wave amplitude was rhythmic in LD and persisted in DD with peak amplitude during mid- to late afternoon in LD and mid-subjective day in DD, indicating that the ERG rhythm is endogenously generated. No daily or circadian variation in a-wave amplitude was observed, and ERG component latency and durations were not rhythmic. Intramuscular injection of 10 g/kg melatonin at ZT10 in LD significantly decreased b-wave amplitude but had no effect on a-wave. Intraocular injection of 600 pg melatonin, however, had no effect on any aspect of the ERG. These data indicate that a circadian clock regulates ocular sensitivity to light and that melatonin may mediate some or all of this effect. The level at which melatonin modulates retinal sensitivity is not known, but the present data suggest a central site rather than a direct effect of the hormone in the eye.Abbreviations DD constant darkness - ERG electroretinography - EW Edinger-Westphal nuclei - IMEL iodomelatonin - IO isthmooptic nucleus - LD light-dark cycle - SCG superior cervical ganglion - SCN suprachiasmatic nuclei - vSCN visual suprachiasmatic nucleus     

Daily variations of immunoreactive melatonin in the visual system of crayfish

In crustaceans, melatonin has been detected in the central nervous system and some other organs. The aim of this study was to analyze the melatonin content in the visual system of Procambarus clarkii, by means of radioimmunoassay, at different day-night phases. We have also studied the action of exogenous melatonin on the main properties of the electroretinogram (ERG) circadian rhythm. Experiments were conducted with 25 specimens maintained under controlled conditions of 16°C and 12 h of light alternating with 12 h of darkness. Eyes where cut in dim red light and shock frozen with liquid nitrogen and pulverized in a mortar until a homogenous powder was obtained. Melatonin was extracted with acetone, followed by centrifugation, diluted with an equal volume of equa bidest to ensure freezing at ?80°C for at least 90 min and lyophilization at the same temperature. Lyophilizates, after having been dissolved in RIA buffer, were used for determinations of melatonin. Long-term recordings of electrical responses to light (ERG) were obtained for 10 or more consecutive days. At the 5th day, a single dose of melatonin was injected and its effects on amplitude and period of the ERG circadian rhythm were measured. Melatonin concentrations differed considerably depending on the circadian time and attained a maximum during dark phase. Among the crustaceans, Procambarus clarkii represents the first case in which melatonin peaks during the night following the typical pattern known in the majority of organisms. After melatonin injection, period and amplitude of the ERG circadian rhythm were increased. This effect suggests the involvement of melatonin in the oscillators underlying the generation and expression of circadian rhythms in crayfish.     

Perturbation of a circadian rhythm by single and periodic signals and its mathematical simulation

Adaptive characteristics of circadian rhythm are based on their capacity to be synchronized by external signals, particularly light signals. The effect of both single and periodic light signals on the electroretinogram (ERG) circadian rhythm in crayfish is studied. In a previous work (Lara-Aparicioet al., Bull math. Biol. 55, 97–110, 1993) we developed a mathematical model simulating the emergence of the ERG circadian rhythm during the ontogeny of the crayfish. In the present work we have tested the familiar wave-shift behaviour of an oscillator with a single limit cycle. Two new facts, not present in a simpler model, now appear, which simulate adequately the experimental results, i.e. the presence of a transient stage and the shape of the perturbed wave which changes according to the characteristics of the external light signals.     

Human Retinal Light Sensitivity and Melatonin Rhythms Following Four Days in Near Darkness

The rods in the retina are responsible for night vision, whereas the cone system enables day vision. We studied whether rod function in humans exhibits an endogenous circadian rhythm and if changes occur in conditions of prolonged darkness. Seven healthy subjects (mean age±SD: 25.6±12.3 yr) completed a 4.5‐day protocol during which they were kept in complete darkness (days 1 and 4) and near darkness (<0.1 lux red light, days 2 and 3). Electroretinography (ERG) and saliva collections were done at intervals of at least 3 h for 27 h on days 1 and 4. Full‐field ERGs were recorded over 10 low‐intensity green light flashes known to test predominantly rod function. As a circadian marker, salivary melatonin concentration was measured by radioimmunoassay. The ERG data showed that rod responsiveness to light progressively diminished in darkness (significantly lower a‐ and b‐wave amplitudes, longer b‐wave implicit time). The decrease in amplitude (b‐wave) from day 1 to day 4 averaged 22±14%. After correction for the darkness‐related linear trend, the circadian variations in ERG indices were weak and usually non‐significant, with slightly higher responsiveness to light during the day than night. Rod sensitivity (by K index) tended to decrease. Strikingly, the overall amount of melatonin secretion (area under 24 h curve) also decreased from day 1 to day 4 by 33.1±18.9% (p=.017). The drift of the melatonin rhythm phase was within the normal range, less than 56 min over three days. There was no significant correlation between the changes in ERG responses and melatonin. In conclusion, scotopic retinal response to (low‐intensity) light and the amount of melatonin secreted are diminished when humans are kept in continuous darkness. Both processes may have a common underlying mechanism implicating a variety of neurochemicals known to be involved in the regulation of both photoreceptor and pineal gland function.     

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     

MT2-like melatonin receptor modulates amplitude receptor potential in visual cells of crayfish during a 24-hour cycle

Retinular photoreceptors are structures involved in the expression and synchronization of the circadian rhythm of sensitivity to light in crayfish. To determine whether melatonin possesses a differential effect upon the receptor potential (RP) amplitude of retinular photoreceptors circadian time (CT)-dependent, we conducted experiments by means of applying melatonin every 2 h during a 24-hour cycle. Melatonin with 100 nM increased RP amplitude during subjective day to a greater degree than during subjective night. To determine whether MT₂ melatonin receptors regulate the melatonin-produced effect, we carried out two experiments, circadian times (CTs) 6 and 18, which included the following: (1) application of the MT₂ receptor selective agonist 8-M-PDOT and antagonist DH97, and (2) the specific binding of [¹²⁵I]-melatonin in eyestalk membranes. The amount of 10 nM of 8-M-PDOT increased RP amplitude in a similar manner to melatonin, and 1 nM DH97 abolished the increase produced by melatonin and 8-M-PDOT. Binding of [¹²⁵I]-melatonin was saturable and specific. Scatchard analysis revealed an affinity constant (K_d) of 1.1 nM and a total number of binding sites (B_max) of 6 fmol/mg protein at CT 6, and a K_d of 1.46 nM and B_max of 7 fmol/mg protein at CT 18. Our results indicate that melatonin increased RP amplitude of crayfish retinular photoreceptors through MT₂-like melatonin receptors. These data support the idea that melatonin is a signal of darkness for the circadian system in crayfish retinular cells.     

Erg circadian rhythm in the course of ontogeny in crayfish

• 1.1. The objective of the present work was to study the ontogeny of the ERG circadian rhythm in crayfish.
• 2.2. Long-term recordings of ERG and shielding retinal pigments position measured from the instar, the second instar, the third instar and the adult crayfish were obtained.
• 3.3. In the youngest animals (1–8 days old) an ultradian rhythm (15min-4hr periods) in the ERG amplitude was detected.
• 4.4. Older animals showed a progressive increment in the period length before they exhibited a circadian pattern. This last appeared, the first time, in 30-day-old animals and showed noticeable differences in the adult crayfish. At the same time, the crayfish began to show photomotor reflex. Later on (140-day-old crayfish) the circadian rhythm attained its final parameters.
• 5.5. The SD was used as a measure of lability in periods. The 4 hr ultradian rhythm and the 22.4 hr circadian rhythm showed the lowest SD indicating that they are the most precise period values.
• 6.6. Our results support the idea that the ERG circadian rhythm results from the coupling among high frequency (ultradian) oscillators, particularly those of 4 hr periods and that the coupling depends on the action of neurosecretions released from the sinus gland.

     

Human Cone Light Sensitivity and Melatonin Rhythms Following 24-hour Continuous Illumination

This study investigates the possibility of an endogenous circadian rhythm in retinal cone function in humans. A full-field cone electroretinogram (ERG) was performed every 2?h for 24?h under continuous rod-saturating ambient white light (53 ±?30 lux; pupils dilated) in nine healthy subjects. Distinct circadian variations were superimposed upon a gradual decrease in cone responsiveness to light, demonstrated most reliably in the implicit times of b-wave and oscillatory potentials, and to a lesser extent in amplitude and a-wave implicit times. After mathematical correction of the linear trend, the cone response was found to be greatest around 20:00?h and least around 06:00?h. The phase of the ERG circadian rhythm was not synchronized with the phase of the salivary melatonin rhythm measured the previous evening. Melatonin levels measured under constant light on the day of ERG assessments were suppressed by 53% on average compared to melatonin profiles obtained previously under near-total darkness in seven participants. The progressive decline in cone responsiveness to light over the 24?h may reflect an adaptation of the cone-driven retinal system to constant light, although the mechanism is unclear. The endogenous rhythm of cone responsiveness to light may be used as an additional index of central or retinal circadian clock time. (Author correspondence: kvdani@mail.ru)     

Pigment dispersing hormone modulates spontaneous electrical activity of the cerebroid ganglion and synchronizes electroretinogram circadian rhythm in crayfish Procambarus clarkii

In crayfish, one very well-studied circadian rhythm is that of electroretinogram (ERG) amplitude. The cerebroid ganglion has been considered a plausible site for the circadian pacemaker of this rhythm and for the retinular photoreceptors, as the corresponding effectors. The pigment dispersing hormone (PDH) appears to synchronize ERG rhythm, but its characterization as a synchronizer cue remains incomplete. The main purposes of this work were a) to determine whether PDH acts on the cerebroid ganglion, and b) to complete its characterization as a non-photic synchronizer. Here we show that PDH increases the number of the spontaneous potentials of the cerebroid ganglion, reaching 149.92 ± 6.42% of the activity recorded in the controls, and that daily application of PDH for 15 consecutive days adjusts the ERG circadian rhythm period to 24.0 ± 0.2 h and the end of the activity period of the rhythm coincides with the injection of the hormone. In this work, we hypothesized that in crayfish, PDH transmits the “day” signal to the ERG circadian system and acts upon both the presumptive circadian pacemaker and the corresponding effectors to reinforce the synchronization of the system.     

Pigment dispersing hormone modulates spontaneous electrical activity of the cerebroid ganglion and synchronizes electroretinogram circadian rhythm in crayfish Procambarus clarkii

In crayfish, one very well-studied circadian rhythm is that of electroretinogram (ERG) amplitude. The cerebroid ganglion has been considered a plausible site for the circadian pacemaker of this rhythm and for the retinular photoreceptors, as the corresponding effectors. The pigment dispersing hormone (PDH) appears to synchronize ERG rhythm, but its characterization as a synchronizer cue remains incomplete. The main purposes of this work were a) to determine whether PDH acts on the cerebroid ganglion, and b) to complete its characterization as a non-photic synchronizer. Here we show that PDH increases the number of the spontaneous potentials of the cerebroid ganglion, reaching 149.92±6.42% of the activity recorded in the controls, and that daily application of PDH for 15 consecutive days adjusts the ERG circadian rhythm period to 24.0±0.2h and the end of the activity period of the rhythm coincides with the injection of the hormone. In this work, we hypothesized that in crayfish, PDH transmits the "day" signal to the ERG circadian system and acts upon both the presumptive circadian pacemaker and the corresponding effectors to reinforce the synchronization of the system.     

Transient Fluctuation of Serum Melatonin Rhythm is Suppressed Centrally by Vitamin B

Vitamin B₁₂ has been reported to improve sleep-wake rhythm disorders. Although the mechanism is still unclear, a change in the sensitivity of the circadian clock system to photic input is thought to be a possible mechanism of the effect. In this study, the effect of the vitamin B₁₂ on the circadian aspect of the electroretinogram (ERG) and serum melatonin level was analyzed in rats. Vitamin B₁₂, α-(5,6-dimethylbenzimidazolyl)-co-methyl-cobamide was daily administrated subcutaneously for 8 weeks to adult male Wister rats in the experimental group, and saline was given to the control group. The ERGs were recorded under dark adaptation during the night and day, and under light adaptation (0.1 lux) during the night. Blood was drawn before and after ERG recording. The amplitudes of the a-wave, fc-wave, and trough-to-peak of both waves and latencies of ERG were analyzed following various exposures to stimuli of light intensity. These parameters in the group treated with vitamin B₁₂ showed similar characteristics to the control group, and no significant difference was observed between the two groups. The melatonin levels of both groups before the measurement of ERG were similar under each measurement condition. The elevated serum melatonin concentration in the control group under dark adaptation at night was suppressed after the series of 10-msec light stimuli used for measurement of ERG. However, this suppressing effect of light pulses on melatonin level was significantly inhibited in the group treated with vitamin B₁₂. Under light adaptation during the night and under dark adaptation during the day, melatonin levels after the measurement of ERG were not different between the groups. From these results, it is suggested that vitamin B₁₂ is effective in suppressing melatonin rhythm disturbances introduced by transient light stimulation, and it affects the site more central than the retinal level. (Chronobiology International, 14(6), 549–560, 1997)     

Complex circadian regulation of pineal melatonin and wheel-running in Syrian hamsters

Circadian regulation of pineal melatonin content was studied in Syrian hamsters (Mesocricetus auratus), especially melatonin peak width and the temporal correlation to wheel-running activity. Melatonin was measured by radioimmunoassay in glands removed at different circadian times with respect to activity onset (= CT 12). Pineal melatonin peak width (h; for mean 125 pg/gland) and activity duration () were both 4–5 h longer after 12 or 27 weeks than after 5 or 6 days in continuous darkness (DD). Increased peak width was associated with a delay in the morning decline (M) of melatonin to baseline, correlated with a similar delay in wheel-running offset. In contrast, the evening rise (E) in melatonin occurred at approximately the same circadian phase regardless of the length of DD. Fifteen min light pulses produced similar phase-shifts in melatonin and activity. In a phase advance shift, M advanced at once, while E advanced only after several days of adjustment. Independent timing of shifts in the E and M components of the melatonin rhythm suggest that these events are controlled separately by at least two circadian oscillators whose mutual phase relationship determines melatonin peak width. This two-oscillator control of melatonin peak width is integral to the circadian mechanism of hamster photoperiodic time measurement.Abbreviations CT circadian time - DD continuous dark - L: D light: dark cycle - PMEL pineal melatonin - PRC phase response curve - RIA radioimmunoassay; , duration (h) of the active phase of the circadian wheel-running rhythm; , free-running period     

Protective action of melatonin against oxidative DNA damage—Chemical inactivation versus base-excision repair

Melatonin is a hormone-like substance that has a variety of beneficial properties as regulator of the circadian rhythm and as anti-inflammatory and anti-cancer agent. The latter activity can be linked with the ability of melatonin to protect DNA against oxidative damage. It may exert such action either by scavenging reactive oxygen species or their primary sources, or by stimulating the repair of oxidative damage in DNA. Since such type of DNA damage is reflected in oxidative base modifications that are primarily repaired by base-excision repair (BER), we tried to investigate in the present work whether melatonin could influence this DNA-repair system. We also investigated the ability of melatonin to inactivate hydrogen peroxide, a potent source of reactive oxygen species. Melatonin at 50 μM and its direct metabolite N¹-acetyl-N²-formyl-5-methoxykynuramine reduced DNA damage induced by hydrogen peroxide at approximately the same ratio. Melatonin stimulated the repair of DNA damage induced by hydrogen peroxide, as assessed by the alkaline comet assay. However, melatonin at 50 μM had no impact on the activity in vitro of three glycosylases playing a pivotal role in BER: Endo III, Fpg and ANPG 80. On the other hand, melatonin chemically inactivated hydrogen peroxide, reducing its potential to damage DNA. And finally, melatonin did not influence the repair of an a-basic (AP) site by cellular extracts, as was evaluated by a functional BER assay in vitro. In conclusion, melatonin can have a protective effect against oxidative DNA damage by chemical inactivation of a DNA-damaging agent as well as by stimulating DNA repair, but key factors in BER, viz. glycosylases and AP-endonucleases, do not seem to be affected by melatonin. Further study with other components of the BER machinery and studies aimed at other DNA-repair systems are needed to clarify the mechanism underlying the stimulation of DNA repair by melatonin.     

Retinally perceived light is not essential for photic regulation of pineal melatonin rhythms in the pigeon: studies with microdialysis

Using in vivo microdialysis, effects of retinally perceived light on pineal melatonin release and its rhythmicity was examined in the pigeon. In the first experiment, light-induced suppression of pineal melatonin release was studied. Although light given to the whole body during the dark strongly suppressed pineal melatonin release to a daytime level, light exclusively delivered to the eyes did not remarkably inhibit melatonin release. In the second experiment, in order to determine whether retinally perceived light has phase-shifting effects on pineal melatonin rhythms, pigeons were given a single light pulse of 2 h at circadian time (CT) 18 and the phases of the second cycle after the light pulse were compared with those of control pigeons without the light pulse. In this experiment, phase advances of pineal melatonin rhythms were observed when the light was given to the whole body but not when only the eyes were illuminated. In a third experiment, after entrainment to light-dark 12:12 (LD 12:12) cycles, birds whose heads were covered with black tapes were transferred into constant light (LL) conditions and only the eyes were exposed to new LD cycles for 7 days (the phase was advanced by 6 h from the previous cycles) using a patching protocol. This procedure, however, could not entrain pineal melatonin rhythms to the retinal LD cycles. These results indicate that the eyes are not essential for photic regulation of pineal melatonin release and its rhythmicity in the pigeon.Abbreviations CT circadian time - LD light-dark - LL constant light - SCN suprachiasmatic nucleus - LLdim constant dim light - NE norepinephrine - SCG superior cervical ganglia - WB whole body - E eye - EX extraretina - C control     

Changes in light and dark periods affect the arylalkylamine N-acetyl transferase,melatonin activities and redox status in the head and hemolymph of nocturnal insect Spodoptera litura

The present study was conducted to describe the impact of circadian rhythm on melatonin levels and redox statusunder three photoperiods (12L:12D, 0L:24D, and 24L:0D) in head and hemolymph of Spodoptera litura. Melatonin is an powerful antioxidant and controls the reproduction of organisms. In this study, melatonin levels, Arylalkylamine N-acetyltransferase(AA-NAT), and antioxidant enzyme activities were analyzed. Results showed melatonin, AA-NAT levels in hemolymph were significantly (p < 0.05) higher during the dark period than during LL regime. HPLC chromatogram of the insect head and hemolymph showed 5 peaks while hemolymph showed 6 peaks in LD, and LLregimes. The day–night changes of melatonin increased the antioxidant enzymes (GST, CAT, POX) persisted in the insect hemolymph, but were suppressed by constant light. The present study leads us to speculate that synthesis and release of melatonin in the S.litura head occur as circadian rhythm and light has an inhibitory effect on melatonin synthesis.     

AGING AND THE CIRCADIAN RHYTHM OF MELATONIN: A CROSS-SECTIONAL STUDY OF CHINESE SUBJECTS 30–110 YR OF AGE

Although previous reports indicate that nocturnal plasma melatonin secretion declines with age, some recent findings do not support this point. In the present cross-sectional study, we documented serum melatonin concentrations at two time points, 02:00 and 08:00h, in 144 persons aged 30–110 yr and found a significant age-related decline. It began around the age of 60 and reached a very significantly lower level in subjects in their 70s and over 80 yr of age (P<0.01, when compared with age <60 yr). Nocturnal melatonin levels were higher among (post-menopausal only) women than men overall (P<0.05). In the older age-groups, nocturnal melatonin levels did not differ between healthy controls and subjects with high blood pressure or ischemic heart disease. To further check these results, we also assessed the circadian pattern of serum melatonin in four subgroups of healthy men, aged 30–39, 40–49, 50–59, and 60–69 yr: blood samples were taken at 2h intervals from 08:00 to 22:00h and hourly from 22:00 to 08:00h. Our results showed generally similar circadian melatonin patterns that peaked at night with very low levels during the daytime. No significant difference was found among the three younger groups, but nocturnal melatonin levels were significantly lower in the men in their 60s.     

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.     

Effects of physiological cycles of infused melatonin on circadian rhythmicity in pigeons

Summary The role of the hormone melatonin in the circadian system of pigeons (Columba livia) was investigated. Using an automatic infusion system, melatoni at physiological levels was delivered for 10 h each day to cannulated, pinealectomized (P-X) pigeons in constant darkness. These cyclic infusions of melatonin entrained feeding rhythms in P-X pigeons while vehicle infusions were ineffective entraining agents. When the retinae of P-X pigeons were removed (E-X), feeding rhythms were abolished in constant darkness. When cyclic melatonin infusions were delivered to these birds (E-X and P-X), feeding rhythmicity was restored whereas vehicle infusions alone did not restore rhythmicity. When melatonin infusions were terminated in E-X/P-X pigeons, feeding rhythms persisted for several days but eventually decayed. Blood melatonin levels were measured in both P-X and E-X/P-X birds infused cyclically with exogenous melatonin and were found to be within the physiological range both in level and pattern. These results strongly suggest that endogenous melatonin, released by the pineal gland and the retinae, regulates the timing of feeding rhythms by entraining other oscillators in the circadian system of the pigeon.Abbreviations P-X pinealectomized - E-X bilaterally enucleated - T period of infusion cycle - LD light: dark cycle - DD constant darkness     

Daily Oscillation in Melatonin Synthesis in The Turkey Pineal Gland and Retina: Diurnal and Circadian Rhythms

The aim of the present study was to examine arylalkylamine N‐acetyltransferase (AANAT) activity and melatonin content in the pineal gland and retina as well as the melatonin concentration in plasma of the turkey (Meleagris gallopavo), an avian species in which several physiological processes, including reproduction, are controlled by day length. In order to investigate whether the analyzed parameters display diurnal or circadian rhythmicity, we measured these variables in tissues isolated at regular time intervals from birds kept either under a regular light‐dark (LD) cycle or under constant darkness (DD). The pineal gland and retina of the turkey rhythmically produced melatonin. In birds kept under a daily LD cycle, melatonin levels in the pineal gland and retina were high during the dark phase and low during the light phase. Rhythmic oscillations in melatonin, with high night‐time concentrations, were also found in the plasma. The pineal and retinal melatonin rhythms mirrored oscillations in the activity of AANAT, the penultimate enzyme in the melatonin biosynthetic pathway. Rhythmic oscillations in AANAT activity in the turkey pineal gland and retina were circadian in nature, as they persisted under conditions of constant darkness (DD). Transferring birds from LD into DD, however, resulted in a potent decline in the amplitude of the AANAT rhythm from the first day of DD. On the sixth day of DD, pineal AANAT activity was still markedly higher during the subjective dark than during the subjective light phase; whereas, AANAT activity in the retina did not exhibit significant oscillations. The results indicate that melatonin rhythmicity in the turkey pineal gland and retina is regulated both by light and the endogenous circadian clock. The findings suggest that environmental light may be of primary importance in the maintenance of the high‐amplitude melatonin rhythms in the turkey.