Photoperiodic regulation of plasma melatonin levels in the laying chicken (Gallus domesticus)1

Influence of Different Light Spectra on Melatonin Synthesis by the Pineal Gland and Influence on the Immune System in Chickens

It is well known that the pineal gland in birds influences behavioural and physiological functions, including those of the immune system. The purpose of this research is to examine the endocrine-immune correlations between melatonin and immune system activity. Through a description of the immune-pineal axis, we formulated the objective to determine and describe: the development of the pineal gland; how light influences secretory activity; and how melatonin influences the activity of primary and secondary lymphoid organs. The pineal gland has the ability to turn light information into an endocrine signal suitable for the immune system via the membrane receptors Mel1a, Mel1b, and Mel1c, as well as the nuclear receptors RORα, RORβ, and RORγ. We can state the following findings: green monochromatic light (560 nm) increased serum melatonin levels and promoted a stronger humoral and cellular immune response by proliferating B and T lymphocytes; the combination of green and blue monochromatic light (560-480 nm) ameliorated the inflammatory response and protected lymphoid organs from oxidative stress; and red monochromatic light (660 nm) maintained the inflammatory response and promoted the growth of pathogenic bacteria. Melatonin can be considered a potent antioxidant and immunomodulator and is a critical element in the coordination between external light stimulation and the body's internal response.

Melatonin in Health and Disease: A Perspective for Livestock Production

Mounting evidence in the literature indicates an important role of endogenous and exogenous melatonin in driving physiological and molecular adaptations in livestock. Melatonin has been extensively studied in seasonally polyestrous animals whereby supplementation studies have been used to adjust circannual rhythms in herds of animals under abnormal photoperiodic conditions. Livestock undergo multiple metabolic and physiological adaptation processes throughout their production cycle which can result in decreased immune response leading to chronic illness, weight loss, or decreased production efficiency; however, melatonin’s antioxidant capacity and immunostimulatory properties could alleviate these effects. The cardiovascular system responds to melatonin and depending on receptor type and localization, melatonin can vasodilate or vasoconstrict several systemic arteries, thereby controlling whole animal nutrient partitioning via vascular resistance. Increased incidences of non-communicable diseases in populations exposed to circadian disruption have uncovered novel pathways of neurohormones, such as melatonin, influence health, and disease. Perturbations in immune function can negatively impact the growth and development of livestock which has been examined following melatonin supplementation. Specifically, melatonin can influence nutrient uptake, circulating nutrient profiles, and endocrine profiles controlling economically important livestock growth and development. This review focuses on the physiological, cellular, and molecular implications of melatonin on the health and disease of domesticated food animals.

Diurnal Rhythm of Plasma Melatonin Concentration in the Domestic Turkey and Its Regulation by Light and Endogenous Oscillators

Simple Summary

Environmental light regulates a wide range of phenomena in almost all organisms on Earth. Daily and seasonal changes in the photoperiod duration are the most important factors controlling the secretion of melatonin (MLT), a pineal hormone that affects many physiological processes in birds. The results of previous studies on the effect of MLT on the productivity and health of poultry have been promising. However, there are very few studies on the daily profiles of plasma MLT concentrations in domestic birds; therefore, we decided to examine plasma MLT levels in 10-week-old domestic turkeys exposed to different light conditions. The results demonstrated that plasma MLT concentration in turkeys kept under a 12 h light: 12 h dark cycle showed a prominent diurnal rhythm. Night-time light exposure caused a rapid decrease in plasma MLT concentrations. The housing of turkeys in continuous dim red light revealed endogenously generated diurnal rhythm of MLT secretion. The rhythm of the plasma MLT level in a reversed cycle of 12 h dark: 12 h light adapted quickly to the new lighting condition.

Abstract

The aim of this study was to characterize the diurnal rhythm of plasma melatonin (MLT) concentration and its regulation by light and endogenous oscillators in 10-week-old domestic turkeys. Three experiments were performed to examine (i) the course of daily changes in plasma MLT concentration in turkeys kept under a 12 h light: 12 h dark (12L:12D) cycle; (ii) the influence of night-time light exposure lasting 0.5, 1, 2, or 3 h on the plasma MLT level; and (iii) the occurrence of circadian fluctuations in plasma MLT levels in birds kept under continuous dim red light and the ability of turkeys to adapt their pineal secretory activity to a reversed light-dark cycle (12D:12L). The plasma MLT concentration was measured with a direct radioimmunoassay. The plasma MLT concentration in turkeys kept under a 12L:12D cycle changed significantly in a daily rhythm. It was low during the photophase and increased stepwise after the onset of darkness to achieve the maximal level in the middle of the scotophase. Next, it decreased during the second half of the night. The difference between the lowest level of MLT and the highest level was approximately 18-fold. The exposure of turkeys to light during the scotophase caused a rapid, large decrease in plasma MLT concentration. The plasma MLT concentration decreased approximately 3- and 10-fold after 0.5 and 1 h of light exposure, respectively, and reached the day-time level after 2 h of exposure. In turkeys kept under continuous darkness, the plasma MLT level was approximately 2.5-fold higher at 02:00 h than at 14:00 h. In birds kept under 12D:12L, the plasma MLT level was significantly higher at 14:00 h than at 02:00 h. The results showed that plasma MLT concentrations in 10-week-old turkeys have a prominent diurnal rhythm, which is endogenously generated and strongly influenced by environmental light.

Effect of melatonin suppression on scoliosis development in chickens by either constant light or surgical pinealectomy

STUDY DESIGN

This study was designed to compare the effect of suppression of melatonin secretion by bright light in chickens with that of surgical pinealectomy.

OBJECTIVE

To determine whether suppression of melatonin secretion without surgery in chickens can result in scoliosis development.

SUMMARY OF BACKGROUND DATA

Pinealectomy in chickens consistently produces scoliosis with anatomic characteristics similar to those of human idiopathic scoliosis. Conversely, cutting of the pineal stalk without removal of the pineal gland will also result in scoliosis. This study addresses the question of whether constant bright light can induce scoliosis formation, because it is well known that 24-hour bright lighting conditions can suppress the secretion of melatonin to an equivalent level as pinealectomy.

MATERIALS AND METHOD

Seventy-seven newborn Nihon chickens were separated into three groups. A control group (n = 21) with no surgery performed; a pinealectomy group (n = 15) that served as surgical controls; and a constant light group (n = 41). The first two groups were kept together in a strict 12-hour light-dark cycle, whereas the third group was separately kept with constant lighting conditions (>100 lux). All the chickens were radiographed at two weekly intervals, and blood was taken during the middle of the light and dark cycles for serum melatonin assay using ELISA.

RESULTS

Fifty-four percent of the pinealectomized chickens had scoliosis develop by 6 weeks. None of the constant-light chickens or controls had scoliosis develop for up to 11 weeks. Measurements of serum melatonin levels of the constant light group confirm that secretion is suppressed.

CONCLUSION

This study suggests that for scoliosis to develop in chickens, the surgical operation itself is important and challenges the role of melatonin as an isolated etiological factor in the development of scoliosis.

Role of melatonin in photoperiodic time measurement in the migratory redheaded bunting (Emberiza bruniceps) and the nonmigratory Indian weaver bird (Ploceus philippinus)

In the present study, we asked the question whether physiological responses to day length of migratory redheaded bunting (Emberiza bruniceps) and nonmigratory Indian weaver bird (Ploceus philippinus) are mediated by the daily rhythm of melatonin. Melatonin was given either by injection at certain times of the day or as an implant. In series I experiments on the redheaded bunting, melatonin was administered by subcutaneous injections daily at zeitgeber time (ZT) 4 (morning) or ZT10 (evening) and by silastic capsules in photosensitive unstimulated buntings that were held in natural day lengths (NDL) at 27 degrees N beginning from mid February, and in artificial day lengths (ADL, 12L:12D and 14L:10D). Melatonin did not affect the photoperiod-induced cycles of gain and loss in body mass and testicular growth-involution, but there was an effect on temporal phasing of the growth-involution cycle of testes in some groups. For example, the rate of testicular growth and development was faster in birds that received melatonin injection at ZT4 in NDL, and was slower in birds that carried melatonin implants both in NDL and ADL. In series II experiments on Indian weaver birds, melatonin was given in silastic capsules in the first week of September when they still had large gonads. Birds were exposed for 12 weeks to short day length (8L:16D; group 1), to long day length (eight weeks of 16L:8D and four weeks of 18L:6D; group 2), or to both short and long day lengths (four weeks each of 8L:16D, 16L:8D, and 18L:6D; groups 3 and 4). Whereas groups 1 to 3 carried melatonin or empty implant from the beginning, group 4 received one after four weeks. All birds underwent testicular regression during the first four weeks irrespective of the photoperiod they were exposed to or the implant they carried in, and there was a slight re-initiation of testis growth in some birds during the next eight weeks of long day lengths. However, with the exception of group 2, there was no difference in mean testis volume during the period of experiment between the melatonin- and empty-implant birds. The data on androgen-dependent beak color also supported the observations on testes. Together, these results do not support the idea that the daily rhythm of melatonin is involved in the photoperiodic time measurement in birds. However, there may still be a role of melatonin in temporal phasing of the annual reproductive cycle in birds.

A method of achieving physiological plasma levels of melatonin in the chicken by oral administration

In avian species it has been difficult to elucidate the precise role of melatonin in the control of reproductive cycles. We have investigated ways of administering melatonin to immature chickens and laying hens to achieve physiological levels and patterns in blood simulating either short or long photoperiods. Melatonin was administered orally using different doses and various ways of applying melatonin to the feed. For subcutaneous injections, melatonin was suspended in propylene glycol or grape seed oil. Melatonin always appeared in the first blood samples taken within an hour of administration. When melatonin was absorbed into feed pellets or whole wheat, a high initial plasma concentration was reached, followed by a rapid decrease over the ensuing 2-3 hr, but was still detectable as long as 24 hr after administration. For example, doses of 300 microg/kg produced 15 nM, which is more than ten times higher than the nocturnal peak concentration. When melatonin was absorbed into cracked wheat grains that were subsequently washed with ethanol, the initial transitory peak was eliminated, levels in plasma were sustained for at least 12 hr in the normal nocturnal range (750 pM), and no melatonin (< 60 pM) was present 18 hr later. When injected (2 microg/bird), concentrations peaked (610 pM) within 30 min and decreased rapidly over the next 2-3 hr. It was concluded that melatonin-treated, ethanol-rinsed cracked wheat grains can be used to experimentally mimic long-night plasma melatonin patterns. Injections may be useful for mimicking the melatonin patterns of very short nights in chickens experiencing constant light.

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.

Oviposition patterns and plasma melatonin rhythms in response to manipulations of the light:dark cycle

1. In 2 experiments with Single Comb White Leghorn hens, the effects of different light:dark cycles (LD-cycles) upon oviposition patterns and plasma melatonin rhythms were studied. In experiment 1, a 28-h ahemeral LD-cycle (12L:16D) was used. In experiment 2, a normal 24-h LD-cycle (16L:8D) was applied and the effects of a sudden 8-h forward or backward shift of the 8-h dark period (that is phase-advanced or phase-delayed LD-cycle) were studied. 2. The oviposition patterns as well as the plasma melatonin rhythms were fully synchronised with both LD-cycles (24-h or 28-h). The 2 rhythms were gradually re-synchronised after phase shifts, and the melatonin response phase-led the oviposition response by 2 cycles. Thus, the change of the melatonin rhythm coincided with the change of the (presumed) open period for LH-release. 3. In the unchanged 24-h LD-cycle, ovipositions occurred almost exclusively (98.9%) during light hours, whereas in the 28-h LD-cycle, ovipositions occurred primarily (84.5%) during the last 9 h of the dark period. 4. In both LD-cycles and after changes of the LD-cycle, light always suppressed plasma melatonin, regardless of previous light history. During dark periods, concentrations were elevated but, interestingly, only if darkness had also been experienced during the same time period 24 h earlier. This indicates that light has a direct inhibiting effect upon pineal melatonin release, while actual melatonin release during darkness is controlled by an endogenous clock.

Thyroidectomy does not affect the daily or free-running rhythms of plasma melatonin in European starlings

Thyroidectomy results in the suppression of reproductive photoperiodic responses in starlings. Could this be a consequence of an effect on perception of daylength or on circadian pacemakers? Daily changes in plasma melatonin concentrations were monitored in intact and thyroidectomized starlings held in long days (LD 16:8) and short days (LD 8:16), and in intact and thyroidectomized starlings allowed to free-run in constant darkness from long days or short days. In long days and short days, melatonin was low during the light period and high during darkness. There was no difference between intact and thyroidectomized birds. In free-running birds, the melatonin profile of the preceding long day or short day was retained during the first day of constant darkness, with peak levels occurring at the same time they did during the light-dark cycles. Again there was no difference between intact and thyroidectomized birds. These data demonstrate that either the photoreceptive and circadian mechanisms driving melatonin secretion are independent of those concerned with reproductive photoperiodic responses, or that thyroidectomy affects reproduction "downstream" from the photoreceptive-circadian apparatus.

Effects of sex-linked imperfect albinism in the chicken (sal-c) on plasma luteinising hormone concentrations and early egg production

1. When measured before and after the onset of darkness, plasma LH concentrations in 40-day-old sex-linked albino pullets (sal-c) were slightly lower than those of nonalbinos (s+). 2. This finding prompted an experiment in which plasma LH concentrations were measured between 12 and 33 weeks of age when daylength was increased at 15 or 21 weeks. Egg production of the early and late maturing albino hens was measured. 3. Plasma LH concentrations overall and at 17 weeks were lower for albinos than for nonalbinos. In the early maturing group egg production of albinos was higher than that of nonalbinos. 4. Results suggest that increased egg production of albinos is not the direct result differences in plasma LH concentrations but may be a consequence of differences in the control of LH secretion.

The circadian nature of melatonin secretion in Japanese quail (Coturnix coturnix japonica)

Plasma melatonin concentrations were measured in Japanese quail held under different photoperiods and constant darkness (< 1 lux). When subjected to LD6:18 (6 hr light: 18 hr darkness), levels rose approximately 2 hr after lights-off, attained a peak level 8 hr after lights off, and subsequently declined to low daytime levels before the next lights-on signal. This generated a distinct daily rhythm in melatonin secretion with a duration of approximately 13 h. On exposing quail to a range of photoperiods, containing 6, 9, 11, 12, 13, 15, 18, or 20 hr of light per day, the onset of melatonin secretion remained essentially similar with the rise occurring soon after lights-off. However, the offset of melatonin secretion was suppressed by the light of the next day and thus a much truncated rhythm was produced under long (> 12 hr) photoperiods. Importantly, between night lengths of 4 to 18 hr (i.e., LD 20:4 to LD 6:18) a linear relationship existed between the duration of night-length and secretion of melatonin with the duration increasing by about 0.8 hr for each additional hour of darkness. If quail were released into darkness following a short (LD 6:18) or long (LD 20:4) day schedule, the rhythm persisted for at least two cycles with peaks occurring at about 24 hr intervals. In those quail coming into darkness from long days (LD 20:4), the rhythm of melatonin secretion decompressed rapidly on both sides of the peak, indicating that both the onset and offset of melatonin secretion were suppressed under long days.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of light on melatonin and two enzymes leading to its production in albino (s(al-c)) and nonalbino chickens

A gene for sex-linked imperfect albinism in the chicken (s(al)) has been associated with increased egg production with an implication that environmental light may play a role. In this study, levels of melatonin and hydroxyindole O-methyltransferase (HIOMT) and N-acetyltransferase (NAT), two enzymes leading to melatonin production, were studied in young albino and nonalbino chickens in relation to the daily light cycle, and after 19 days of constant light or dark. Differences between genotypes were found in the levels of HIOMT activity in the pineals and retinas of birds kept in constant light for 19 days. Other measurements were not significantly different. This study would appear to show that the visual system of imperfect albino chickens reacts differently to light than that of nonalbinos, but not with changes in the daily cycle of plasma melatonin or in NAT activity, which is the enzyme primarily responsible for the control of the melatonin level in the body.

Avian reproductive endocrinology

Hypothalamic-releasing factors regulate the secretion of anterior pituitary hormones. The anterior pituitary gland secretes the same six hormones as found in mammals: FSH, LH, prolactin, GH (somatotropic hormone), ACTH, and TSH, plus the melanotropic hormone. The endocrine hormones of the avian posterior pituitary gland concerned with reproduction are mesotocin and AVT. The pineal gland, through the secretion of the hormone melatonin, modulates the periodic autonomic functions of the central nervous system. The ovary produces estrogens, progestogens, and androgenic compounds. The testes produce testosterones and progesterone. The thyroid glands produce two hormones, T4 and T3. The avian adrenal glands produce corticosterone and aldosterone. The bursa of Fabricius is considered an endocrine organ since it is involved in the production of humoral factors. The male reproductive system undergoes hormonal changes associated with puberty, the breeding season, and molt. Some avian species undergo a type of disintegration and seasonal reconstruction of the testis and epididymis. The relationship of the ovarian follicular hormones and the plasma hormones varies depending on the stage of the reproductive cycle and the seasonal photostimulation. Female birds may conceive in the absence of a mate as a result of the fertile period phenomena. The blood chemistry of laying birds is different from that seen in nonlaying hens. Domestication has had a definite influence on the hormone cycles of some avian species. This may lead to certain reproductive problems.

Circadian rhythms of plasma melatonin in the Adelie penguin (Pygoscelis adeliae) in constant dim light and artificial photoperiods

The response of plasma melatonin in Adelie penguins (Pygoscelis adeliae) to constant dim light and to light/dark cycles was measured to determine the capacity of the pineal gland to secrete melatonin after exposure to continuous daylight for 2 months. Penguins were moved in mid-summer from the natural photoperiod to either constant dim light (n = 10), to a 12L:12D light/dark cycle (n = 5), or to a 12L:12D light/dark cycle with a 30 min light pulse (50-155 lux) on the third (n = 4) or sixth (n = 5) "night." Blood samples were collected regularly through cannulae for up to 33 h. The birds in dim light were sampled after 2 days, with samples obtained over at least 24 h from 7 birds. Three of these birds had melatonin rhythms (peak levels 66.7-130.2 pg/ml) whereas the other 4 birds had constant low levels (less than 44 pg/ml). The phase of the rhythm was similar for all 3 birds. This is consistent with the pacemaker that regulates the circadian rhythm of melatonin secretion being entrained to a period of 24 h when the penguins were exposed to the natural photoperiod. Mean melatonin levels (42.7 +/- 2.5 pg/ml) were elevated compared to those previously reported in penguins under natural daylight. All penguins held under a 12L:12D light/dark cycle had melatonin rhythms. The phase and form of these rhythms were similar to those reported for other birds, and they appeared to be circadian rhythms entrained by the light/dark cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma melatonin in the Adelie penguin (Pygoscelis adeliae) under conditions daylight in Antarctica

Circadian rhythms of melatonin secretion in birds are influenced by daylength and light intensity. Daily patterns of melatonin secretion were examined in Adelie penguins (Pygoscelis adeliae) under natural continuous daylight at Cape Bird, Antarctica (77 degrees S). Although daylight is continuous during the Antarctic summer there was a marked daily cycle of light intensity. However, there was no relationship between mean plasma melatonin levels and time of day in groups of 2-10 penguins sampled at 2-3 h intervals in November, December, or January. Mean melatonin levels over 24 h in groups of birds from which single samples were collected, or in groups of birds sampled repeatedly through cannulae, were low (12.4 +/- 1.2 pg/ml-28.8 +/- 4.4 pg/ml for 4 sampling periods; n = 22-163). Levels in individual birds were, however, quite variable and ranged from 5.0-68.1 pg/ml. Some birds had periods of increased melatonin levels that tended to occur during the time of day when light intensity was least. One bird had a clear low amplitude melatonin rhythm with a peak during the time of least light intensity. These results, the first for any bird under a natural photoperiod, indicate that melatonin secretion is inhibited by natural continuous daylight, but that it is not abolished.

Phylogenetic distribution and function of arylalkylamine N-acetyltransferase

Amine acetylation is a diverse topic with importance to the regulation of several physiological processes as well as the metabolism of drugs and environmental chemicals. Arylalkylamine N-acetyltransferase is widely distributed in several species, where this enzyme plays an important role in the seasonal regulation of reproduction and photoperiodism in vertebrates through the pathway of melatonin formation. In insects, this enzyme is involved in monoamine neurotransmitter inactivation and the formation of catecholamine intermediates necessary for sclerotization of the insect cuticle.

Circadian rhythms and influence of light on serotonin, norepinephrine, and epinephrine contents in the pineal-paraphyseal complex of soft-shelled turtles (Lissemys punctata punctata)

The aims of the current investigation were to examine the circadian rhythms in the pineal-paraphyseal amines and indoleamine in juvenile turtles. An attempt was also made to study the influence of photoperiod on pineal activities in adult turtles. Serotonin, norepinephrine, and epinephrine levels were studied at four different time intervals (0600, 1200, 1800, and 2400 hr) of a 24-hr period. Increased serotonin level was observed during day and trough values during night, but the norepinephrine and epinephrine levels showed reverse changes. Continuous light (24L:0D) or long photoperiod (22L:2D) resulted in an elevation of serotonin level and diminution of norepinephrine level, but continuous darkness (0L:24D) or short photoperiod (2L:22D) showed reverse changes. Epinephrine level altered in parallel to that of norepinephrine with long (22L:2D) or short (2L:22D) photoperiod, but remained unaltered with continuous light (24L:0D) or continuous darkness (0L:24D). It is suggested that circadian rhythms exist in pineal-paraphyseal amines and indoleamine in juvenile turtles like that of adult turtles. It is also suggested that light greatly modulates serotonin, norepinephrine, and epinephrine contents in the pineal-paraphyseal complex of the soft-shelled turtle.

Rapid adjustment of the pineal N-acetyltransferase rhythm to change from long to short photoperiod in the Japanese quail (Coturnix coturnix japonica)

The dynamics of adjusting the pineal N-acetyltransferase rhythm from long to short photoperiod was assessed in the Japanese quail (Coturnix coturnix japonica). The transition from LD 16:8 to LD 8:16 was accomplished by symmetrical prolongation of the dark period. In LD 16:8, the period of elevated nocturnal activity lasted approximately 7 hours. During the first prolonged night, the evening N-acetyltransferase rise advanced by almost 3 hours relative to the rise in LD 16:8 and occurred at the same time as during the 3rd, 7th, and 14th day after the transition. The morning N-acetyltransferase decline did not shift during the first long night; during the third night it was delayed relative to the decline in LD 16:8 by more than 2 hours and occurred at the same time as during the 7th and 14th night following the LD 16:8 to LD 8:16 transition. Three, 7, and 14 days after the transition, the period of elevated N-acetyltransferase activity lasted approximately 12 hours. Hence extension of the N-acetyltransferase rhythm profile proceeded first into the evening and then only into the morning hours, and it was accomplished within 2 to 3 days.

Influence of pinealectomy on plasma and extrapineal melatonin rhythms in young chickens (Gallus domesticus)

A specific radioimmunoassay was validated for the quantitative measurement of melatonin (MT) levels in plasma and homogenates of the pineal gland, Harderian gland, or retinae of young chickens. Single-comb White Leghorn (SCWL) cockerels were raised under a 12L:12D light/dark cycle for two experiments. In Experiment 1, 12 birds were bled and immediately killed for their pineal glands at 4-hr intervals during a single light/dark cycle at 25 days of age (25 da) for simultaneous determination of changes in MT levels in the plasma and pineal gland. Plasma MT levels were low during photophase (100 pg/ml) and reached a distinct peak (390 pg/ml) at mid-scotophase. A parallel MT rhythm was found in pineal homogenates where the average MT content during scotophase (7.4 ng/gland) was 10 times higher than the average MT content of pineal glands obtained during photophase. In Experiment 2, SCWL cockerels were either pinealectomized or sham-operated (PN) at 8 to 10 da. At 25 da, six birds from each surgical treatment group, including unoperated controls (C), were bled at 4-hr intervals, corresponding to those in Experiment 1, during a single light/dark cycle. Immediately after being bled, each bird was killed and the eyes and Harderian glands were removed for measurement of their MT contents. Pinealectomy completely abolished the plasma MT rhythm which in intact chicks (PN and C) reached a sharp peak (298 pg/ml) at mid-scotophase. Although not affected by surgical treatment, retinal MT levels showed a higher amplitude rhythm with a prominent peak (4 ng/retina) at mid-scotophase that was 15 times higher than the average retinal MT content during photophase. A modest nocturnal MT rhythm was found in the Harderian gland where the average MT level for all surgical treatment groups during scotophase (89 pg/100 mg wet wt) was only 51% higher than that observed for photophase. These data indicate that the plasma MT rhythm in chickens is derived solely from MT secreted into blood by the pineal gland and that the extrapineal MT produced rhythmically in both the retina and Harderian gland does not contribute to the plasma MT rhythm.