Effects of melatonin feeding on smoltification in masu salmon (Oncorhynchus masou)

Influences of photoperiod on plasma melatonin profiles and effects of melatonin administration on long-day-induced smoltification in masu salmon (Oncorhynchus masou) were investigated in order to reveal the roles of melatonin in the regulation of smoltification in salmonids. Under light-dark (LD) cycles, plasma melatonin levels exhibited daily variation, with higher values during the dark phase than during the light phase. The duration of nocturnal elevation under short photoperiod (LD 8:16) was longer than that under long photoperiod (LD 16:8). Melatonin feeding (0.01, 0.1 and 1 mg/kg body weight) elevated plasma levels of melatonin in a dose-dependent manner for at least 7 h but not for 24 h. When masu salmon reared under short photoperiod were exposed to long photoperiod (LD 16:8) and fed melatonin (1 mg/kg body weight) 7 hours before the onset of darkness, a significantly smaller proportion of smolts appeared in the melatonin-fed group after 32 days than in the control group. However, after 59 days of the treatment, there was no difference in the proportion of smolts between the control and melatonin-treated groups. Thus, melatonin feeding mimicked the effects of short photoperiod, which delays but does not completely suppress smoltification. These results indicate that the day length is transduced into changes in the duration of nocturnal elevation in plasma melatonin levels, and that artificial modification of the plasma melatonin pattern possibly delays the physiological processes of smoltification induced by long-day photoperiodic treatment.     

A 15-minute light pulse during darkness prevents the antigonadotrophic action of afternoon melatonin injections in male hamsters

When adult male Syrian hamsters were maintained under 14 h light and 10 h darkness daily (lights on from 0600-2000 h), peak pineal melatonin levels (705 pg/gland) were attained at 0500 h. When the dark phase of the light:dark cycle was interrupted with a 15 min pulse of light from 2300–2315 h (3 h after lights out), the highest melatonin levels achieved was roughly 400 pg/gland. Finally, if the 15 min pulse of light was given at 0200–0215 h (6 h after lights out) the nocturnal rise in pineal melatonin was completely abolished. Having made these observations, a second experiment was designed to determine the ability of afternoon melatonin injections to inhibit reproduction in hamsters kept under an uninterrupted 1410 cycle or under the same lighting regimen where the dark phase was interrupted with a 15 min pulse of light (0200–0215 h). In the uninterrupted light:dark schedule the daily afternoon injection of 25 g melatonin caused the testes and the accessory sex organs to atrophy within 11 weeks. Conversely, if the dark phase was interrupted with light between 0200–0215 h, afternoon melatonin injections were incapable of inhibiting the growth of the reproductive organs. The findings suggest that exogenously administered melatonin normally synergizes with endogenously produced melatonin to cause gonadal involution in hamsters.     

Influence of propranolol,phenoxybenzamine or phentolamine on the in vivo nocturnal rise of pineal melatonin levels in the syrian hamster

In the Syrian hamster, pineal melatonin levels exhibit a 15-fold rise during the dark phase of the light: dark cycle. This rise is believed to be mediated by the release of norepinephrine from the postganglionic sympathetic fibers which terminate within the pineal. In order to determine the nature of the adrenergic receptor involved in the norepinephrine mediated nocturnal increase in melatonin, male hamsters were treated with either α- or β-adrenergic blockers just prior to lights out. Subsequently, radioimmunoassayable levels of melatonin were measured at 7, 8 and 9 hours (0300, 0400 and 0500 h, respectively) into the dark period. Propranolol (20 mg/kg) completely suppressed the nocturnal rise of melatonin while phentolamine (10 mg/kg) had no effect upon the increase. The minimum amount of propranolol necessary to block the nighttime rise of melatonin was determined to lie between 1 mg/kg and 10 mg/kg. Phenoxybenzamine (20 mg/kg) exhibited a slight, although statistically significant, blockade of the nocturnal melatonin rise.     

Effects of circadian phase and melatonin injection on anxiety-like behavior in nocturnal and diurnal rodents

Animals show daily rhythms in most bodily functions, resulting from the integration of information from an endogenous circadian clock and external stimuli. These rhythms are adaptive and are expected to be related to activity patterns, i.e., to be opposite in diurnal and nocturnal species. Melatonin is secreted during the night in all mammalian species, regardless of their activity patterns. Consequently, in diurnal species the nocturnal secretion of melatonin is concurrent with the resting phase, whereas in nocturnal species it is related to an increase in activity. In this research, we examined in three diurnal and three nocturnal rodent species whether a daily rhythm in anxiety-like behavior exists; whether it differs between nocturnal and diurnal species; and how melatonin affects anxiety-like behavior in species with different activity patterns. Anxiety-like behavior levels were analyzed using the elevated plus-maze. We found a daily rhythm in anxiety-like behavior and a significant response to daytime melatonin administration in all three nocturnal species, which showed significantly lower levels of anxiety during the dark phase, and after melatonin administration. The diurnal species showed either an inverse pattern to that of the nocturnal species in anxiety-like behavior rhythm and in response to daytime melatonin injection, or no rhythm and, accordingly, no response to melatonin.     

Nocturnal increase in plasma cGMP levels in humans.

The circadian dynamics of responses to cyclic guanosine 3',5'-monophosphate (cGMP) in in vitro experiments and the stimulating effects of the pineal hormone melatonin on cGMP levels both in vitro and in vivo provoked an investigation into the diurnal pattern of occurrence of this second messenger in human plasma and its correlation with plasma melatonin levels. Plasma cGMP levels were measured in 9 normal human subjects who were over 50 years of age. Samples were obtained hourly through a 20-h period (11 a.m. to 7 a.m.) that included the subjects' habitual hours of nocturnal sleep; physical activity was kept to a minimum during the daylight hours. The area under the time-plasma cGMP concentration curve showed a significant increase during the period of nocturnal sleep compared to that observed during the period of daytime wakefulness. The individual temporal pattern of the nocturnal rise in plasma cGMP differed among the subjects; however, the initial increase typically was observed soon after bedtime. No significant correlation was observed between individual nocturnal plasma melatonin levels and cGMP levels.     

Opposite effects of tryptophan intake on motor activity in ring doves (diurnal) and rats (nocturnal)

The role of l-tryptophan as precursor of serotonin and melatonin synthesis on activity-rest rhythm was studied in ring doves, Streptopelia risoria, as a representative of diurnal animals and rats, Rattus norvegicus, as a typical nocturnal one. The animals were housed in cages equipped for horizontal activity recording in a thermostatized chamber and submitted to a 12/12h light/dark photoperiod (lights on at 08:00 h). After acclimatization, the animals received vehicle (methylcellulose) and l-tryptophan (240 mg/kg) by esophagic cannula 2h before the onset of either light or dark phase. Also, oral melatonin (2.5mg/kg) was tested for comparative purposes. After nocturnal l-tryptophan administration, rats showed increased activity (149%), while the opposite occurred in ring doves (39% decrease). No significant changes were found after diurnal l-tryptophan intake in either species. Melatonin produced effects similar to those of l-tryptophan. These results suggest that the effects of l-tryptophan administration are dependent on the nocturnal/diurnal habits of the studied species and, most probably, are mediated by increased melatonin synthesis.     

Circadian rhythm of tryptophan hydroxylase activity in chicken retina

1. Retinal tryptophan hydroxylase activity in chickens (1-4 weeks old and embryos) was estimated by determination of levels of 5-hydroxytryptophan (5HTP) in retinas at defined intervals after inhibition of aromatic L-amino acid decarboxylase with m-hydroxybenzylhydrazine (NSD1015). 2. The relationship of tryptophan hydroxylase activity to photoperiod was explored. In chickens maintained on a 12-hr light: 12-hr dark cycle, a diurnal cycle in tryptophan hydroxylase activity was observed. Activity during middark phase was 4.4 times that seen in midlight phase. Cyclic changes in tryptophan hydroxylase activity persisted in constant darkness with a period of approximately 1 day, indicating regulation of the enzyme by a circadian oscillator. The phase of the tryptophan hydroxylase rhythm was found to be determined by the phase of the light/dark cycle. The relationship of the tryptophan hydroxylase rhythm to the light/dark cycle mirrors previously described rhythms of melatonin synthesis and serotonin N-acetyltransferase (NAT) activity in the retina. 3. Light exposure for 1 hr during dark phase suppressed NAT activity by 82%, while tryptophan hydroxylase activity was suppressed by only 30%. 4. Based on the differential responses of retinal NAT activity and tryptophan hydroxylase activity to acute light exposure during dark phase, it was predicted that exposure to light during dark phase would divert serotonin in the retina from melatonin biosynthesis to oxidation by MAO. In support of this, levels of 5-hydroxyindole acetic acid (5HIAA) in retina were found to be elevated approximately two-fold in chickens exposed to 30 min of light during dark phase. In pargyline-treated chickens, 2 hr of light exposure during dark phase was found to increase retinal serotonin levels by 64% over pargyline-treated controls. 5. Cyclic changes in tryptophan hydroxylase activity and NAT activity persisted for 2-3 days in constant light. Tryptophan hydroxylase activity at mid-night gradually decreased on successive days in constant light; on the first day of constant light, tryptophan hydroxylase activity at mid-night was 70% of activity seen during middark phase of the normal light/dark cycle and decreased further on subsequent days. In contrast, on each of 3 days of constant light, NAT activity at mid-night was approximately 15% of normal middark phase activity. 6. Cycloheximide completely inhibited the nocturnal increase in tryptophan hydroxylase activity when given immediately before light offset. The nocturnal increase in NAT activity was inhibited in a similar fashion. 7. Like the development of the NAT rhythm, cyclic changes of tryptophan hydroxylase activity in the retinas of chickens began on or immediately before the day of hatching. hatching.(ABSTRACT TRUNCATED AT 400 WORDS)     

Twice Daily Melatonin Peaks in Siberian but not Syrian Hamsters under 24 h Light:Dark:Light:Dark Cycles

The daily pattern of blood-borne melatonin varies seasonally under the control of a multi-oscillator circadian pacemaker. Here we examine patterns of melatonin secretion and locomotor activity in Siberian and Syrian hamsters entrained to bimodal LDLD8:4:8:4 and LD20:4 lighting schedules that facilitate novel temporal arrangements of component circadian oscillators. Under LDLD, both species robustly bifurcated wheel-running activity in distinct day scotophase (DS) and night scotophase (NS) bouts. Siberian hamsters displayed significant melatonin increases during each scotophase in LDLD, and in the single daily scotophase of LD20:4. The bimodal melatonin secretion pattern persisted in acutely extended 16 h scotophases. Syrian hamsters, in contrast, showed no significant increases in plasma melatonin during either scotophase of LDLD8:4:8:4 or in LD20:4. In this species, detectable levels were observed only when the DS of LDLD was acutely extended to yield 16 h of darkness. Established species differences in the phase lag of nocturnal melatonin secretion relative to activity onset may underlie the above contrast: In non-bifurcated entrainment to 24 h LD cycles, Siberian hamsters show increased melatonin secretion within ～2 h after activity onset, whereas in Syrian hamsters, detectable melatonin secretion phase lags activity onset and the L/D transition by at least 4?h. The present results provide new evidence indicating multi-oscillator regulation of the waveform of melatonin secretion, specifically, the circadian control of the onset, offset and duration of nocturnal secretion.     

Light/dark patterns of interleukin-6 in relation to the pineal hormone melatonin in patients with acute myocardial infarction

AIMS: Atherosclerosis is a chronic disease that, from its origin to its ultimate complications, involves inflammatory cells, inflammatory proteins, and inflammatory responses from vascular cells. It has been demonstrated that cytokine activities are under neuroendocrine control, in part exerted by the pineal gland through the circadian secretion of its main product melatonin. Melatonin is mainly released during the night, but the precise relationship between melatonin and the light/dark rhythm of interleukin-6 in patients with acute myocardial infarction is still unclear. METHODS AND RESULTS: The study included 60 patients diagnosed with acute myocardial infarction and 60 healthy volunteers whose venous blood samples were collected at 09:00 h (light period) and 02:00 h (dark period). Our results demonstrate that interleukin-6 concentrations presented a light/dark pattern with mean serum concentrations being higher in the acute myocardial infarction group than in the control group (101.26 +/- 13.43 and 52.67 +/- 7.73 pg/ml at 02:00 h, 41.93 +/- 5.90 and 22.98 +/- 4.49 pg/ml at 09:00 h, respectively, p < 0.05). Differences in the day/night changes in melatonin levels in control subjects (48.19 +/- 7.82 at 02:00 h, 14.51+/- 2.36 at 09:00 h, pg/ml) and acute myocardial infarction patients (25.97 +/- 3.90 at 02:00 h, 12.29 +/- 4.01 at 09:00 h, pg/ml) (p < 0.05) were a result of a reduced nocturnal elevation of melatonin in the acute myocardial infarction group. CONCLUSIONS: The current findings suggest that the circadian secretion of melatonin may be responsible at least in part for light/dark variations of endogenous interleukin-6 production in patients with acute myocardial infarction. In this study, the melatonin seems to have an anti-inflammatory effect.     

Changes in melatonin binding sites under artificial light-dark, constant light and constant dark conditions in the masu salmon brain

To test whether the affinity (Kd) and total binding capacity (Bmax) of melatonin receptors exhibit daily and circadian changes in teleost fish whose melatonin secretion is not regulated by intra-pineal clocks, we examined the changes in melatonin binding sites in the brains of underyearling masu salmon Oncorhynchus masou under artificial light-dark (LD), constant light (LL) and constant dark (DD) conditions. In Experiment 1, fish were reared under a long (LD 16:8) or short (LD 8:16) photoperiod for 69 days. Blood and brains were sampled eight times at 3 h intervals. Plasma melatonin levels were high during the dark phase and low during the light phase in both photoperiodic groups. The Bmax exhibited no daily variations. Although the Kd slightly, but significantly, changed under LD 8:16, this may be of little physiological significance. In Experiment 2, fish reared under LD 12:12 for 27 days were exposed to LL or DD from the onset of the dark phase under LD 12:12. Blood and brains were sampled 13 times at 4 h intervals for two complete 24 h cycles. Plasma melatonin levels were constantly high in the DD group and low in the LL group. No significant differences were observed in the Kd and the Bmax between the two groups, and the Kd and the Bmax exhibited no circadian variation either in the LL or DD groups. These results indicate that light conditions have little effect on melatonin binding sites in the masu salmon brain.     

Variability of plasma melatonin level in pony mares (Equus caballus), comparison with the hybrid: mules and with jennies (Equus asinus)

In long-day breeders like horses, the length of nocturnal melatonin secretion is the main messenger of photoperiod. Previous studies have shown that the nocturnal jugular melatonin concentration is lower in horses, than in mules but is unknown in donkeys. The aim of this study was to estimate the inter-animal variability of plasma melatonin concentration in domestic mares and to compare this concentration with those observed in domestic jennies and in their hybrid mules. In the autumn, blood samples were collected at 22 h, 23 h, 0 h and 1 h during 2 nights at 3 weeks intervals, in 110 pony mares, 10 jennies and 6 mules maintained under natural photoperiod. Melatonin was assayed by a validated RIA method. The statistical analysis of the measures was done with a specific unbalanced analysis of variance model. The effect of species and individuals (nested under species) was highly significant. The mean melatonin concentration was 24 pg.mL(-1) in mares and was significantly lower than in jennies and in mules which were 90 pg.mL(-1) and 169 pg.mL(-1) respectively. The melatonin plasma concentration was higher in jennies than in mares. These results suggest that the melatonin concentration is genetically determined.     

Daily cycles in plasma and ocular melatonin in demand-fed sea bass, Dicentrarchus labrax L.

Melatonin is regarded as an internal zeitgeber, involved in the synchronization to light of the daily and seasonal rhythms of vertebrates. To date, plasma and ocular melatonin in fish have been extensively surveyed almost solely in freshwater species – with the exception of some migrating species of salmonids. In the present paper, melatonin levels of a marine species (sea bass, Dicentrarchus labrax L) were examined. In addition, the daily rhythms of the demand-feeding activity of sea bass, a fish species characterized by a dual phasing capacity (i.e. the ability to switch between diurnal and nocturnal behaviour), were investigated before sampling. Sea bass, distributed in 12 groups of four fish and kept under constant water temperature and salinity, were exposed to a 12 h light:12 h dark cycle (200:0 lx, lights on at 0800 hours). After 4 weeks recording, the animals were killed at 0900, 1200, 1400, 1600, 1900, 2100, 2400, 0200, 0400, 0700 and 0900 hours. Actograms of demand-feeding records revealed a nocturnal feeding behaviour, with some cases of spontaneous inversions in phasing. Melatonin levels in plasma peaked in the middle of the dark phase, dropping after lights on. Melatonin in the eye, on the contrary, exhibited an inverse profile, with high levels during daytime and low levels at night. These results suggest that melatonin in the plasma and the eye may act independently on the flexible circadian system of sea bass. Accepted: 30 January 1997     

Melatonin in rat milk and the likelihood of its role in postnatal maternal entrainment of rhythms

The rhythm of melatonin in rat milk and the capacity of pups to synthesize and metabolize melatonin were studied. Melatonin was undetectable in milk in the light (< 21 pM), but increased rapidly 2-4 h after dark to peak at 357 +/- 66 pM at mid-dark. Oral or subcutaneous administration of melatonin to 5- and 10-day-old pups resulted in peak plasma melatonin levels 30 min after administration and rapid metabolism. Increases in pineal and plasma melatonin levels at night were detected at 5 and 6 days of age, respectively. Isoproterenol administration (2 microg/g body wt) at mid-light to day 10 pups increased plasma melatonin from 312 +/- 40 pM to 1,298 +/- 160 pM, whereas propranolol (2 microg/g body wt) suppressed nocturnal melatonin secretion from 1,270 +/- 128 pM to 395 +/- 66 pM. The rise of pineal and plasma melatonin in day 10 pups occurred 1 and 2 h after dark onset, respectively, preceding the onset in dams by 3 and 4 h, respectively. Propranolol administration to 2- and 5-day lactating dams inhibited plasma and milk melatonin at night but had no effect on their suckling pups. Transfer of melatonin via the milk is unlikely to provide an entraining signal for rat pups.     

Effects of VDT tasks with a bright display at night on melatonin, core temperature, heart rate, and sleepiness.

The effects of performing video display terminal (VDT) tasks with a bright display (BD) at night on nocturnal salivary melatonin concentration, rectal temperature, heart rate, and sleepiness were examined. Seven healthy male adults performed exciting VDT tasks with a BD and a dark display (DD) and boring VDT tasks with a BD and a DD from 2300 to 0200. The light intensities of the BD and DD were 45 and 15 lx at each subject's eye level, respectively. The exciting VDT task with both BD and DD significantly suppressed the nocturnal decrease in rectal temperature and heart rate and the nocturnal increase in sleepiness. The BD significantly suppressed the nocturnal decrease in rectal temperature during both exciting and boring VDT tasks. The nocturnal salivary melatonin concentration was significantly suppressed by the combination of the exciting task and BD. The results suggest that performing an exciting VDT task with a BD suppresses the nocturnal changes in melatonin concentration and other physiological indicators of human biological clocks.     

Suppression of nocturnal increases in levels of melatonin elicits an anticipatory behavior in male quail

Abstract

During the reproductive development of male Japanese quail the duration of daily activity is prolonged and the onset of the rhythm of activity is advanced relative to the light‐dark cycle. The neuroendocrine basis for these changes was investigated with focusing on plasma levels of melatonin and testosterone. By means of 4 additional hours of photic stimulation of the brain, after the environmental lights (8L: 16D, lights on at 1000 hr) were turned off, the increase in levels of melatonin after lights‐off was suppressed for a few days. Thereafter the early onset of daily locomotor activity was observed and the gonads began to develop. Similar behavioral changes occurred in castrated quail following direct brain‐illumination or testosterone implants. The testosterone implants also suppressed the increases in levels of melatonin after lights‐off, for a few days. Treatment with an antiserum raised against melatonin (anti‐M) for the first 3 days, to suppress the increases in levels of melatonin after lights‐off, elicited such an anticipatory behavior. These results suggest that suppression of the nocturnal rise in melatonin levels is important for the first steps toward reproductive activity in male Japanese quail.     

Change in duration of elevated concentrations of melatonin is the major factor in photoperiod response of the tammar, Macropus eugenii

In Exp. 1, 10 quiescent non-lactating tammars were exposed to 15L:9D (Days -41 to -1), 24L:0D (Days 0 to 14), 15L:9D (Days 15 to 34) and then to ambient increasing daylength from 13L:11D on Day 35. From Days 0 to 22 they received a s.c. injection of melatonin (400 ng/kg, N -5) on the arachis oil vehicle (N = 5) in the evening (19:30 h) 2.5 h before dark. Exposure to 24L:0D abolished the nocturnal plasma melatonin rise but this was reinstated by subsequent exposure to 15L:9D. Of 5 melatonin-treated tammars, 4 gave birth on Day 45, so had failed to respond to the melatonin injection alone but reactivated when this was combined with the endogenous melatonin rise during exposure to 15L:9D. Of 5 control tammars, 4 remained quiescent until reactivated by the decrease in daylength to 13L:11D, and gave birth significantly later (Day 63.7 +/- 2.2, mean +/- s.e.m., P less than 0.05). In Exp. 2, 6 tammars were exposed to 15L:9D (Days -15 to -1) and then to 12L:12D (Days 0 to 15) by extending the dark phase by 3 h in the morning. This extended the nocturnal melatonin rise by 2-3 h in the morning and all 6 tammars gave birth on Day 31.2 +/- 1.0. A transient pulse of peripheral plasma prolactin (81.5 +/- 31.0 ng/ml) was detected at dawn during 15L:9D in all 6 tammars but was not observed in any of them 5 days after exposure to 12L:12D. Together these results do not support the time of day hypothesis but indicate that increase in duration of the nocturnal melatonin rise mediates the effects of decreased daylength on reactivation of the corpus luteum, and that the first detectable result of this may be the abolition of a transient prolactin pulse at the end of the dark phase.     

Restricted daytime feeding attenuates reentrainment of the circadian melatonin rhythm after an 8-h phase advance of the light-dark cycle

It is well established that in the absence of photic cues, the circadian rhythms of rodents can be readily phase-shifted and entrained by various nonphotic stimuli that induce increased levels of locomotor activity (i.e., benzodiazepines, a new running wheel, and limited food access). In the presence of an entraining light-dark (LD) cycle, however, the entraining effects of nonphotic stimuli on (parts of) the circadian oscillator are far less clear. Yet, an interesting finding is that appropriately timed exercise after a phase shift can accelerate the entrainment of circadian rhythms to the new LD cycle in both rodents and humans. The present study investigated whether restricted daytime feeding (RF) (1) induces a phase shift of the melatonin rhythm under entrained LD conditions and (2) accelerates resynchronization of circadian rhythms after an 8-h phase advance. Animals were adapted to RF with 2-h food access at the projected time of the new dark onset. Before and at several time points after the 8-h phase advance, nocturnal melatonin profiles were measured in RF animals and animals on ad libitum feeding (AL). In LD-entrained conditions, RF did not cause any significant changes in the nocturnal melatonin profile as compared to AL. Unexpectedly, after the 8-h phase advance, RF animals resynchronized more slowly to the new LD cycle than AL animals. These results indicate that prior entrainment to a nonphotic stimulus such as RF may "phase lock" the circadian oscillator and in that way hinder resynchronization after a phase shift.     

Siberian hamsters that fail to reentrain to the photocycle have suppressed melatonin levels

Siberian hamsters readily reentrain to a 3-h phase delay of the photocycle (16 h light/day) but fail to reentrain to a 5-h phase delay. This study tested whether melatonin production was suppressed in animals that failed to reentrain. Melatonin was measured on the day before, day of, or several days after each phase shift. Melatonin levels measured 4 h after dark onset were approximately 83 microg/ml on the day before each phase delay and undetectable (<6 microg/ml) during the light phase on the day of the phase shift. Activity onsets regained their prior phase relationship to the photocycle 4 (3 h) or 5 (5 h) days after the phase shift; on that day, melatonin levels were measured 4 h after dark onset. Melatonin levels were unaffected by the 3-h phase delay (>57.6 microg/ml) but were undetectable after a 5-h phase delay (<8 microg/ml). Thus melatonin remained suppressed only after the phase delay to which hamsters also fail to reentrain. This relationship suggests that the propensity for reentrainment may be influenced by changes in melatonin production following a phase shift of the photocycle.