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

Abstract: 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 ～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 ～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. The endogenous nature of melatonin secretion was tested further by exposing birds to LD 6:30 for 4 cycles and then releasing into darkness. The rhythm in melatonin secretion persisted for at least three cycles before beginning to damp-out. The circadian nature of the rhythm in melatonin secretion was also examined by subjecting quail to T-cycles and then releasing into darkness. Both under the T-cycles and darkness following T-cycle treatments, the phase of the melatonin rhythm was advanced by > 3 hr under T = 27 hr cycles (LD 3:24) compared with T = 24 hr cycles (LD 3:21). This property is consistent with the melatonin oscillator being a circadian rhythm.     

Changes in serotonin levels, N-acetyltransferase activity, hydroxyindole-O-methyltransferase activity, and melatonin levels in the pineal gland of the Richardson's ground squirrel in relation to the light-dark cycle

Pineal serotonin and melatonin levels and the activities of hydroxyindole-O-methyltransferase (HIOMT) and N-acetyltransferase (NAT) were studied over a 24-hour period in the pineal gland of the diurnally active Richardson's ground squirrel (Spermophilus richardsonii). Under alternating light-dark conditions (light:dark hours 14:10), pineal serotonin and melatonin levels exhibited a rhythm with high values occurring either during the day (serotonin) or during the night (melatonin). NAT activity was also markedly increased during darkness. HIOMT activity exhibited no 24-hour variation. Exposure of squirrels to constant light for 7 days exaggerated the serotonin rhythm, but obliterated the cycles of NAT and melatonin. Under constant darkness (for 7 days), the rhythms in serotonin, melatonin and NAT persisted, each having a period of about 24 h. In the second study, ground squirrels were exposed to light-dark cycles of either 8:16, 10:14 or 14:10. Under each of these photoperiodic environments, rhythms in pineal NAT and melatonin were apparent. Increasing the daily dark period from 10 to 14 h caused a prolongation of the elevated NAT and melatonin levels. However, a further prolongation of the daily dark period (to 16 h) did not further increase the duration of the rise in NAT and melatonin. The results show that continual light exposure (irradiance of 200 microW/cm2) for 7 days suppresses the pineal rhythms in both NAT activity and melatonin level in the Richardson's ground squirrel. Conversely, light exposure, rather than depressing the serotonin rhythm, actually exaggerates it. Constant darkness for 7 days has little influence on the 24-hour rhythms of either NAT or melatonin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of different photoperiods on concentrations of 5-methoxytryptophol and melatonin in the pineal gland of the Syrian hamster

Specific, sensitive and direct radioimmunoassays have been used to determine the daily patterns of 5-methoxytryptophol (ML) and melatonin in the pineal glands of Syrian hamsters kept in different photoperiods: 8 h light: 16 h darkness (8L:16D), 14L:10D and 16L:8D. A rhythm in pineal ML was evident in animals in all the photoperiods, with high daytime levels (641 +/- 35 (S.E.M.) fmol/gland; n = 162) which dropped to 119 +/- 16 fmol/gland (n = 44) 7.1-7.5 h after lights out. The duration of low night-time ML levels was proportional to the length of the dark phase (1.2 h in 16L:8D, 5.4 h in 14L:10D and 8.4 h in 8L:16D). A marked daily rhythm in melatonin was also present in hamsters in the different photoperiods, with daytime levels of 323 +/- 34 fmol/gland (n = 129) and night-time peak concentrations of 3676 +/- 336 fmol/gland (n = 22). The duration of high nocturnal melatonin levels was dependent upon the length of the dark phase (4.1 h in 16L:8D, 4.5 h in 14L:10D and 12.5 h in 8L:16D). Linear regression analysis revealed a statistically significant inverse relationship between pineal ML and melatonin levels in 8L:16D (P less than 0.001), 14L:10D normal (P less than 0.05) and 14L:10D shifted (P less than 0.001) photoperiods. After advancing the lighting schedule by 10 h (14L:10D, lights off at 04.00 h), pineal ML and melatonin rhythms became entrained to the new lighting regimen. The daily rhythms in pineal ML and melatonin in the Syrian hamster thus depend on the prevailing photoperiod, a reciprocal relationship existing between pineal ML and melatonin concentrations.     

Melatonin Rhythms in Quail: Regulation by Photoperiod and Circadian Pacemakers

The profile of melatonin in the eyes, pineal, and blood of Japanese quail was assessed in birds held under LD 16:8 and LD 6: 18 photoperiods. Melatonin levels in all three tissues showed a robust daily rhythm with higher levels occurring at night. The amplitude of the rhythm was depressed and its duration lengthened on LD 6: 18 relative to LD 16:8. The blood melatonin rhythm precisely reflected the rhythms shown by the pineal and eyes, supporting the idea that the blood rhythm is a result of melatonin secretion by both the eyes and pineal.
The ocular melatonin rhythm continued after sectioning of the optic nerve, was reentrainable to a shift in the phase of the LD cycle, and persisted for at least 2 days in constant darkness. It was concluded that either (1) an intraocular circadian clock drives the ocular melatonin rhythm, or (2) an extraocular clock drives the ocular melatonin rhythm via a route other than the efferent innervation (which enters the eye via the optic tract).     

Serum melatonin profiles and endocrine responses of ewes exposed to a pulse of light late in the dark phase

To determine whether effects of light pulses on the photoperiodic time measuring system involve changes in pineal gland function, melatonin profiles were determined in groups of ewes maintained under 10-h light, 14-h dark (10L:14D) or 10L:10D:1L:3D. Ewes exposed to 10L:14D had a significantly (P less than 0.01) longer duration of melatonin secretion (15.0 +/- 0.4 h, mean +/- SE) than ewes under 10L:10D:1L:3D (9.0 +/- 0.4 h). The 1-h pulse of light therefore acted as a dawn signal in the latter group. During a period of extended darkness imposed to study endogenous control of melatonin release, there was no change in the duration of elevated melatonin in control ewes (16.1 +/- 0.5 h), but a significant (P less than 0.05) lengthening occurred in pulsed ewes (13.2 +/- 1.4 h). PRL responses to a bolus iv injection of TRH (50 ng/kg BW) were significantly (P less than 0.01) smaller in control ewes (478 +/- 134 ng/ml) compared with pulsed ewes (1578 +/- 175 ng/ml), with responses in the latter group resembling those observed in ewes on long days. A 1-h pulse of light late in the dark phase, therefore, resulted in a melatonin pattern normally observed under long days in ewes, and this was associated with other endocrine functions also characteristic of sheep on long days. It is concluded that pulses of light modify activity of the pineal gland which in turn interacts with the photoperiodic time-measuring system via melatonin. The increase in duration of melatonin secretion observed in pulsed ewes under extended darkness suggests that the melatonin rhythm is under the control of two oscillators coupled to dusk and dawn, and that these oscillators interact more strongly when compressed by an interrupted dark phase.     

Pineal melatonin concentrations during day and night in the adult hedgehog: Effect of a light pulse at night and superior cervical ganglionectomy

The European hedgehog (Erinaceus europaeus L.) is a hibernating mammal and seasonal breeder in which numerous circadian and circannual rhythms are entrained and synchronized by photoperiod. The present study was undertaken in order to establish the involvement of the pineal gland in transducing the photoperiodic message in this species. Pineal melatonin concentrations were determined by radioimmunoassay in female hedgehogs kept under natural climatic conditions and killed during the light:dark (L:D) cycle in spring and autumn, after the interruption of darkness by a 45 min light pulse, and after bilateral superior cervical ganglionectomy (SCGx). Absolute melatonin concentrations were low (less than 100 pg/pineal) in the pineal gland. Under natural climatic conditions, in spring and in autumn, pineal melatonin concentrations exhibited a marked diurnal rhythmicity, with very low levels in the day (1200: less than 10 pg/pineal) and high levels during the night (2200: 71.9 +/- 18.6 pg/pineal; 0200: 42.5 +/- 15.6 pg/pineal). The 45 min light pulse during darkness depressed rapidly and significantly the melatonin content (dark + light [D + L]: less than 10 pg/pineal), but a subsequent return to darkness restored high melatonin content after approximately 2 h (D + L + D: 65.4 +/- 20.2 pg/pineal). After bilateral SCGx, melatonin concentrations were reduced and no increase during night could be observed, either in animals sacrificed 42 days after SCGx or in animals killed 2 years after SCGx. In the hedgehog, as in other mammals, pineal melatonin concentrations are related to the light:dark cycle. Such data indicate that during the year, in this species, the effects of light on seasonal endocrine rhythms may be mediated by the pineal gland.     

Adjustment of pineal melatonin and N-acetyltransferase rhythms to change from long to short photoperiod in the Djungarian hamster Phodopus sungorus

In the Djungarian hamster Phodopus sungorus, the daily temporal pattern of synthesis and release of pineal hormone melatonin, mainly the length of the period of elevated melatonin levels, may be involved in transferring the information on day length to the neuroendocrine-gonadal axis. The present study investigated the time course of adjustment of the rhythm in melatonin production and concentration to the change from long to short photoperiods. Adult female Djungarian hamsters, maintained on a regime of 16 h of light and 8 h of darkness per day (LD 16:8) were transferred to the LD regime 8:16 and the daily rhythms in the pineal melatonin concentration and in the pineal N-acetyltransferase activity, as an indicator of melatonin formation, were studied at various intervals following the transfer. Under LD 16:8, the nocturnal melatonin concentration was elevated for 4.8 h. After 3 days on LD 8:16, no extension of the period of high melatonin levels occurred. 2, 4 and 6 weeks after the transfer to LD 8:16, the period of elevated melatonin levels lasted for 8.1, 9.3 and 11.5 h, respectively. Extension of the melatonin pattern proceeded first predominantly into the morning hours. Only after this extension was completed, a considerable extension into the evening hours began. Extension of the N-acetyltransferase rhythm on short photoperiods proceeded in the same way as that of the melatonin rhythm. The data show that while a change in the photoperiod might be seen by hamsters within 2 weeks after the transfer to LD 8:16, the full shortening of the photoperiod might be recognized only within 6 weeks or later.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pineal melatonin concentrations in the Syrian hamster

Pineal melatonin concentrations exhibited a marked diurnal rhythmicity in gold hamsters maintained in a light-dark cycle of 15 h of light and 10 h of darkness. When tissue was collected at 3-h intervals throughout a 24-h period, daytime levels of 95--232 pg/pineal gland rose to concentrations of 760--1335 pg/pineal gland (P greater than 0.001 vs. all other values) at 0400 h, 8 h after the onset of darkness. When tissue was collected sequentially during the dark phase, pineal melatonin concentrations remained significantly elevated from 0200--0500 h (P less than 0.01 and P less than 0.001 vs. daytime values, respectively). Superior cervical ganglionectomy abolished the rhythm of pineal melatonin concentrations, and the concentrations were maintained at 60--105 pg/pineal gland throughout a 24-hr period.     

Examination of in vitro melatonin secretion from superfused trout (Salmo gairdneri) pineal organs maintained under diel illumination or continuous darkness

Melatonin secretion was measured from rainbow trout (Salmo gairdneri) pineal organs maintained individually under flow-through whole organ culture (superfusion) conditions. Radioimmunoassay of perfusate fractions collected during controlled photic conditions demonstrated that melatonin secretion in vitro remained basal during the photophase and underwent increases in titer during the scotophase. While amounts of melatonin (mel) secreted were characteristic of individual pineal organs, photophase values ranged between 0.25 and 0.75 ng mel/ml and scotophase values ranged from 6 to 10 ng mel/ml of perfusate. Diel melatonin secretion profiles reflected the illumination regimen, with light associated with low melatonin titer in the perfusate and darkness associated with high titer. Light pulses during a normal scotophase resulted in a depression in melatonin secretion regardless of whether it was administered early or late in the dark period. Pulses of darkness given early or late in a normal photophase resulted in increased melatonin secretion. Superfused trout pineal organs did not display endogenous rhythmicity in melatonin secretion when subjected to prolonged exposure to continuous darkness (DD), whether first exposed to entraining light/dark (LD) cycles prior to DD or exposed to DD at the initiation of superfusion. In both studies, elevated melatonin secretion gradually declined over time. But exposure to a 4:4LD cycle after DD resulted in decreased (with light) and increased (with darkness) melatonin secretion. These results demonstrate that the trout pineal organ can be maintained for extended periods of time in superfusion culture, that the trout pineal organ is very responsive to light or dark for regulating melatonin synthesis, and that an endogenous rhythm in melatonin synthesis when organs were maintained in DD was not detectable.     

Posthatch Day/Night Differences in Synaptic Ribbon Populations of the Chick Pineal

Pineal synaptic ribbon (SR) populations of the early posthatch white leghorn chick were counted to determine if they demonstrate a rhythm that is in accordance with the light/dark cycle. SRs were counted between day 7 and day 10 and on day 14 of posthatch development, with samples at midlight, middark (14L:10D), and constant darkness. SR populations did not exhibit significant changes on days 7 and 8 under cycled lighting conditions nor on days 9 and 10 under constant darkness. A second experiment demonstrated that the dark:light ratio of SR populations of day 14 chicks, under cycled lighting, was 3.4:1.0, indicating SR rhythmicity by that stage of development. In that a preliminary experiment had demonstrated a 4.2:1.0 dark:light ratio in SR populations in a predominantly day-10 population of chicks, we believe that SR rhythmicity begins on, or near, day 10 of posthatch development. To determine if the invasion of sympathetic fibers from the superior cervical ganglion (SCG) correlates with the initiation of SR light/dark population differences, we employed tyrosine hydroxylase immunofluorescence to reveal the distribution of catecholaminergic fibers in chick pineal follicles. Follicular innervation doubled over the day 7 to day 14 period, during which time light/dark differences in SR populations were established. There is a correlation, in time, between the invasion of the pineal by the sympathetic fibers and the initiation of SR light/dark differences. The circadian rhythm of pineal N-acetyltransferase (NAT) activity, the rate-limiting enzyme in the melatonin pathway, is established earlier (day 2) than the light/dark differences in SR populations (day 10). It is possible that SR rhythmicity is influenced by the ingrowth of the pineal sympathetic innervation, and that SRs respond to an extrapineal oscillator rather than the independent oscillators of the chick pineal responsible for the rhythm of NAT activity and melatonin synthesis.     

Entrainment of the circadian rhythm in the rat pineal N-acetyltransferase activity by melatonin is photoperiod dependent

Entraining effect of melatonin on the circadian rhythm in rat pineal N-acetyltransferase (NAT) activity was studied under various photoperiods. Melatonin administration prior to dark onset for 5 successive days phase-advanced the evening NAT rise under the light:dark (LD) cycle of either LD 10:14 or LD 8:16, but not under LD 12:12. It is assumed that under the latter regime, the end of a light period exhibited a phase-delaying effect on the NAT rise. The light exposure appeared to be a stronger Zeitgeber than melatonin itself. Data show that melatonin applied in the late light period advances the evening NAT rise under a short photoperiod only; under a longer photoperiod, the phase-advancing effect of melatonin may conflict with a phase-delaying effect of the end of a light period, and the effect of light exposure overrides that of melatonin.     

Entrainment of the Circadian Rhythm in Rat Pineal N-Acetyltransferase Activity Under Extremely Long and Short Photoperiods

Entrainment of a pacemaker driving the circadian rhythm in rat pineal N-acetyltransferase activity was studied under extremely long and short photoperiods. Adult male rats maintained under the light-dark regime (LD) 18:6 or under the regime LD 6:18 were exposed to a 1-min light pulse at different times at night, then they were released into darkness, and the next night phase-shifts of the evening N-acetyltransferase rise and of the morning N-acetyltransferase decline caused by light pulses were determined. The evening rise was phase-delayed by at most 0.5 h under LD 18:6, but by as much as 2.8 h under LD 6:18. The morning decline was phase-advanced by at most 1.9 h under LD 18:6, but by as much as 3.5 h under LD 6:18. Hence, the magnitude of phase-shifts and consequently patterns of phase-response curves, which show possibilities of discrete entrainment, depend on the photoperiods under which animals are maintained. A 1-min light pulse applied within 1 h before the end of the dark period phase-advanced the morning N-acetyltransferase decline under LD 18:6 as well as under LD 6:18, while a pulse applied within 1 h after the beginning of the dark period phase-delayed the evening N-acetyltransferase rise only in rats maintained under LD 18:6, but not in those kept under LD 6:18. It seems that under very long photoperiods, the N-acetyltransferase rhythm may be entrained by evening as well as by the morning light, while under very short photoperiods the rhythm may be synchronized by morning light only.     

Neuroendocrine responsiveness to light during the neonatal period in the sheep

Circulating prolactin concentrations were monitored during the early postnatal period in sheep to evaluate their response to photoperiod. In the first experiment, male and female lambs were exposed from 1 week of age, with their mothers, to either long days (16 h light: 8 h darkness; n = 15) or short days (8 h light: 16 h darkness; n = 16) to test whether they could discriminate different day lengths. In both sexes, serum prolactin concentrations were higher on long than on short days during the first 7 weeks after birth. In the second experiment, female lambs (n = 21) were raised on long days from 2 weeks of age. The superior cervical ganglia were removed bilaterally at 4 weeks of age from 14 lambs to lesion the sympathetic innervation to the pineal gland, and thus ablate the nocturnal increase in pineal melatonin secretion. After surgery, serum prolactin concentrations on long days were significantly lower in ganglionectomized lambs than in the intact controls. In the third experiment, the amplitude of the night-time melatonin rise was artificially increased in female lambs (n = 8) between 2 and 7 weeks of age to adult levels. Unrestrained lambs were infused during the 8-h dark phase of each day with melatonin by means of a self-contained, computerized syringe-pump. Concentrations of circulating prolactin did not differ from those in uninfused lambs (n = 8) with lower endogenous nocturnal melatonin. These results reveal that the sheep can discriminate photoperiod cues during the early postnatal period, and suggest that the low-amplitude melatonin rhythm in the neonatal lamb is sufficient to mediate this response.     

Rhythmic Melatonin Response of the Syrian Hamster Pineal Gland to Norepinephrine In Vitro and In Vivo

Norepinephrine (NE, 10(-6) M) stimulated melatonin accumulation in the incubation medium of rat (but not Syrian hamster) pineals taken at the end of the light phase. However, NE elevated melatonin accumulation in the medium of pineals taken after 20 min of light exposure of animals of either species at 6 h into the 10-h dark phase. A dose response to 10(-7)-10(-5) M NE was observed in both the medium and pineals upon incubation of pineals taken from rats at 4 h into the light phase and from hamsters after 20 min light exposure at 6 h into the dark phase. Approximately 95% of the melatonin present was in the medium. The incubation time was 4 h in all cases. Subcutaneous injection of 1 microgram/g NE (either at the end of the light phase or after 30 min of light at 6 h into the dark phase) did not stimulate in vivo Syrian hamster pineal melatonin content determined 1 or 2 h after injection, whether the hamsters were placed in light or darkness after the injection. However, after 30 min of light beginning at 6 h into dark, injection of 5 micrograms/g desipramine (DMI, a blocker of catecholamine uptake into nerve endings) allowed a dramatic hamster pineal melatonin response to additional injection of 1 microgram/g NE, observed at 1 and 2 h in light after injection. A small effect of DMI alone was seen. DMI also potentiated the effect of NE (each 10(-6) M) on melatonin accumulation in the medium of incubated hamster pineals taken after a short light exposure at night. No significant stimulatory effect of NE and/or DMI was seen in vivo or in vitro near the middle of the light phase. Measurement of melatonin in the incubation medium is a useful method for studying pineal function. The Syrian hamster pineal has rhythm of sensitivity to NE (sensitivity evident at night) and even at night is protected by neuronal uptake from circulating NE-induced stimulation of melatonin production. NE appears to be the neurotransmitter for stimulation of pineal melatonin production in the Syrian hamster. The sensitivity rhythm and uptake protection might provide specificity of control of the nightly melatonin signal by reducing the chance of a melatonin response during the day or a response to circulating catecholamines from general sympathetic stimuli.     

Phototransduction-related circadian changes in indoleamine metabolism in the chick pineal gland in vivo

Abstract: The purpose of this study was to examine the day/night levels of pineal melatonin and its rate limiting enzyme N-acetyltransferase (NAT) in relationship to the ratio of 11-cis-to all-trans-retinal. Three-week-old chicks were placed in 12:12 light: dark (LD 12:12) cycle for one week, pineals were collected during the light phase at 1500 (i.e., after 10 hr light), during the dark phase at 1900 (i.e., 2 hr after dark), at 2100 (i.e., 4 hr after dark), and at 2300 (i.e., 6 hr after dark) and after light extension to 1900. The results show that light-sensitive 11-cis-retinal in the chick pineal has the same diurnal rhythm as NAT and melatonin; all constituents increased within 2 hr of darkness onset (at 1900) and reached their peak after 4 hr of dark. All values were lowest during the light phase at 1500. Low values for 11-cis-retinal, NAT, and melatonin were also seen in the group of chicks which experienced light extension to 1900. The data indicate that in vivo light plays a major role in triggering rhodopsin-bound 11-cis-retinal production within 2–4 hr after darkness onset; this change likely serves as the signal for the subsequent formation of the hormonal product of the pineal gland, melatonin.     

Patterns of progesterone, melatonin and prolactin secretion in ewes maintained in four different photoperiods

Twenty-four-hour patterns of serum melatonin and prolactin levels were determined in ewes on nine occasions during a year. The sheep were maintained in four different photoperiods: room 1, simulated natural photoperiod; room 2, normal daylength extremes twice in 12 months, changes occurring in a regular fashion; room 3, alternating long (16 h) and short (8 h) days for 90 days; room 4, constant light. Cyclic ovarian activity, determined by twice-weekly determinations of serum progesterone, commenced in rooms 1, 2 and 3 after a transition from long to short daylength and terminated during long daylength. Thus in rooms 2 and 3 there were two periods of ovarian activity. In room 4 (constant light) ovarian activity began earlier than in room 1 and was of greater duration (240 days v. 190 days). Basal prolactin levels were highest (50-134 micrograms/l) during periods of long daylength and lowest (less than 10 micrograms/l) in short daylength. Ewes maintained in constant light had an intermediate level (21-62 micrograms/l) throughout the study. Melatonin secretion was lowest during daylight (less than 78 pmol/l) and highest during darkness. Night-time melatonin levels varied markedly from hour to hour and between individuals in rooms 1, 2 and 3. There was, however, no consistent seasonal change in the absolute levels of melatonin, although the duration of melatonin secretion did closely follow the length of the dark phase. There were no significant changes in melatonin levels during the oestrous cycle. Ewes kept in constant light had less than 78 pmol melatonin/l throughout the period of study. If the pineal gland is involved in transmitting photoperiodic information to the endocrine system, then it is most likely to be by means of an interaction between duration of melatonin secretion and an underlying change in sensitivity of end organs to melatonin.     

Immunohistochemical Assessment of Melatonin Binding in the Pineal Gland

Melatonin binding in the pineal gland of albino rats is estimated using an immunohistochemical procedure. Binding is saturable, has relatively high affinity (Apparent KD = 2.7 nM), and competition studies indicate binding of indoleamines possessing an N-acetyl group on the terminus of the side chain (N-acetylserotonin and melatonin). These data are consistent with the interpretation that immunohistochemically determined melatonin in unfixed pineal tissue is assessing binding of N-acetylated indolealkylamines to pineal cell components. In albino rats maintained on 12-hour light: 12-hour dark cycles, melatonin binding exhibits a diurnal rhythm with low levels of saturation (30%) early in the light and saturation by endogenous melatonin near the onset of darkness. An annual rhythm of melatonin binding was observed in albino rats with low levels during the summer and high levels during the winter. Other rats were maintained on 12-hour light:dark cycles and fed for 2 hours either early in the light period or early in the dark period. For both morning- and evening-fed animals, melatonin binding was high prior to feeding and dropped immediately after feeding. Changes in melatonin binding that occur in response to alterations of feeding and time of year suggest the possibility that this binding reflects a functional site for melatonin.     

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)     

Effects of neutralization of circulating melatonin and N-acetylserotonin on plasma prolactin levels

The effect of pinealectomy or immunization against melatonin (Mel) and N-acetylserotonin (NAS) on plasma prolactin (Prl) levels was studied in rats following pineal stimulation induced by blinding or exposure to short photoperiods (1 h light and 23 h darkness daily). Blinding alone or together with pinealectomy or immunization did not alter resting Prl levels or the response to novelty stimulation. Exposure to short photoperiods flattened the diurnal Rrl rhythm seen in control rats kept in 12 h of light and 12 h of darkness daily. Pinealectomy slightly lowered Prl levels but did not affect the diurnal rhythm. Immunization caused a significant reduction in Prl levels, although the diurnal Prl rhythm persisted. These data suggest that Mel and/or NAS may be involved in the maintenance of basal Prl levels.     

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.