Presence of an intrapineal circadian oscillator in the teleostean family Poeciliidae

In most fish, rhythmic melatonin production is controlled by circadian oscillators located within the pineal (=pineal clocks) that are reset daily by the ambient light:dark (LD) cycle. However, one question that has yet to be addressed concerns the phylogenetic distribution of the pineal clock within fish families. We tested whether a pineal clock identified in the sailfin molly (Poecilia velifera) in an earlier study is also present in some other representatives of the teleostean family Poeciliidae. Isolated pineals from adults belonging to the genus Poecilia (P. velifera albino, P. reticulata, and P. sphenops), genus Xiphophorus (X. helleri and X. maculatus), and genus Limia (L. vittata) were obtained and cultured under LD and/or continuous darkness (DD) at constant temperature (27 degrees C). With one exception, free-running rhythms in melatonin release with circadian periodicities ranging from 19.5 to 27.4 h (n = 26) were detected in isolated pineals from all poeciliid representatives tested under DD exposure. In addition, rhythmic melatonin production was also observed in isolated pineals of some representatives tested from all three genera under LD exposure, suggesting the property of direct photosensitivity. Taken together, these data suggest that a circadian oscillator residing in the pineal of the sailfin molly also appears to be present in all of the poeciliid representatives tested in our system, supporting the notion that the presence of a pineal clock occurs at the family level of taxonomic organization.     

Circadian rhythms of melatonin secretion from superfused goldfish (Carassius auratus) pineal glands in vitro

A flow-through, whole-organ culture (superfusion) system was developed, and goldfish pineal glands were maintained at 25 degrees under light-dark (LD) 12:12 cycles, reversed LD 12:12 cycles, continuous dark (DD), or continuous light (LL) conditions for 48 hr. Under LD 12:12 and reversed LD 12:12 cycles, superfused pineal glands showed a rhythmic melatonin secretion: Scotophase was associated with high titers and photophase with low titers. The melatonin secretion rhythms persisted for two cycles under DD conditions, whereas nocturnal rises were suppressed under LL conditions. After the transition from LL to DD conditions on the third day, melatonin showed a nocturnal increase. These results indicate that melatonin secretion from the superfused goldfish pineal gland is directly photosensitive and that the goldfish pineal gland harbors a circadian oscillator which generates melatonin secretion rhythms.     

Properties of the melatonin-generating system of the sailfin molly, Poecilia velifera

The properties of the melatonin-generating system of a tropical teleost, the sailfin molly (Poecilia velifera), were investigated in vitro in a series of experiments using static or perifusion culture techniques. The properties examined included photic entrainment, circadian rhythmicity under continuous light (LL) and continuous darkness (DD), functionality of the melatonin-generating system at birth, and presence of multiple circadian oscillators in the molly pineal. Pineal glands or skull caps with the pineal gland firmly attached were dissected from adult and new-born fishes, respectively, and placed into static or perifusion culture at constant temperature (27 degrees C) depending upon the experiment. Melatonin release in samples was quantified by RIA. Rhythmic melatonin release was observed from isolated adult pineals under 12L:12D and 14L:10D, with low amounts of melatonin released during the light and high amounts during the dark. Melatonin release was inhibited by LL. However, under DD, melatonin release was robust and rhythmic with a circadian period (Tau) that ranged between 21.3 and 27.0 h (n = 21). Pineals from new-born (1-day old) mollies released melatonin rhythmically under a light:dark cycle and DD in both static and perifusion culture. Melatonin release from half and quarter pineals of adult mollies under DD was robust and rhythmic with circadian periods that ranged between 22.5 and 29.0 h (n = 19). Taken together, these data show that the molly pineal is photosensitive, fully functional from birth, and contains multiple circadian oscillators (at least four) regulating melatonin production.     

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.     

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.     

Old rats are more sensitive to photoperiodic changes. A study on pineal melatonin

After having previously demonstrated that beta-adrenergic stimulation of melatonin under a standard light:dark (LD) cycle regimen of 12:12 is more effective in young than in old pineal glands, we have now studied how the daylength change LD 18:6 affects pineal melatonin secretion and its regulation by the beta-adrenergic system. Young (10 weeks) and old (22 months) male Wistar rats were synchronized with either a standard LD 12:12 for 4 weeks, or acclimatized under the same LD conditions for 4 weeks, then subjected to a long LD 18:6 photoperiod for 1 week. The rats were sacrificed at three time samplings: 0, 4, and 7h after dark onset (HADO) for LD 12:12 or 0, 2, and 3.5 HADO for LD 18:6. Pineal glands were collected and perifused for 480 min. Isoproterenol (10(-4)M) was infused for 20 min, 4h 10 min after the beginning of the perifusion. Basal levels of melatonin production in the young rats displayed a 1.5-2.5-fold increase compared to those in the old rats. Interestingly, mean basal melatonin levels in old rats under standard LD 12:12 conditions were significantly higher (P<0.05) than mean levels at the same relative dark phase intervals under LD 18:6 conditions. Isoproterenol stimulated melatonin production in both young and old rat pineal glands, regardless of time sampling or photoperiodic conditions. The magnitude of the response to 10(-4)M isoproterenol infusion in old pineals was approximately half that found in young glands (P<0.001), and tended to be higher under LD 12:12, in both young and old rat pineal glands, although no significant difference was found in melatonin response between the two photoperiods (P>0.05). This study shows that basal pineal melatonin levels in old rats are more sensitive to photoperiod changes than in young rats. These results also demonstrate that isoproterenol can stimulate both young and old rat pineal glands irrespective of time or photoperiod and confirm previous findings, showing that the melatonin response to isoproterenol is age-dependent and that pineal gland response to isoproterenol is not photoperiod-dependent, at least under our experimental conditions.     

Lack of circadian regulation of in vitro melatonin release from the pineal organ of salmonid teleosts

In many teleost species, the photoreceptive pineal organ harbors the circadian clock that regulates melatonin release in the pineal organ itself. However, the pineal organ of three salmonids (rainbow trout Oncorhynchus mykiss, masu salmon Oncorhynchus masou, and sockeye salmon Oncorhynchus nerka) did not exhibit circadian rhythms in melatonin release when maintained under constant darkness (DD) in vitro, suggesting that the pineal organs of all salmonids lack the circadian regulation of melatonin production. To test this hypothesis, the pineal organ of seven salmonids (common whitefish Coregonus lavaretus, grayling Thymallus thymallus, Japanese huchen Hucho perryi, Japanese charr Salvelius leucomaenis pluvius, brook trout Salvelius fontinalis, brown trout Salmo trutta and chum salmon Oncorhynchus keta) and closely related osmerids (ayu Plecoglossus altivelis altivelis and Japanese smelt Hypomesus nipponensis) were individually maintained in flow-through culture at 15 degrees C under several light conditions. Under light-dark cycles, the pineal organ of all species showed a rhythmic melatonin release with high rates during the dark phase. Under DD, the osmerid pineal organs exhibited circadian rhythms in melatonin release with high rates only during the subjective-night but the salmonid pineal organs constantly released melatonin at high rates. Under constant light, melatonin release was suppressed in all species. The pineal organ of rainbow trout maintained at different temperature (15, 20 or 25 degrees C) under DD released melatonin with high rates but the amount of melatonin released was temperature-sensitive (highest at 20 degrees C). Thus, melatonin release from the pineal organ of osmerids is regulated by both light and circadian clock but the circadian regulation is lacking in salmonids. These results indicate that ancestral salmonids lost the circadian regulation of melatonin production after the divergence from osmerid teleosts.     

Diurnal and circadian rhythms in melatonin synthesis in the turkey pineal gland and retina

The pineal gland and retina of the turkey rhythmically produce melatonin. In birds kept under a daily light-dark (LD) illumination cycle melatonin concentrations in the pineal gland and retina were low during the light phase and high during the dark phase. A similar melatonin rhythm with high night-time values was also observed in the plasma. The pineal and retinal melatonin rhythms mirror oscillations in the activity of serotonin N-acetyltransferase (AANAT; the penultimate enzyme in the melatonin biosynthetic pathway). In contrast, in both the pineal gland and retina the activity of the enzyme hydroxyindole-O-methyltransferase (HIOMT) did not exhibit significant changes throughout the 24-h period. Acute exposure of turkeys to light at night dramatically decreased melatonin levels in the pineal gland, retina and plasma. The rhythms in AANAT activity and melatonin concentrations in the turkey pineal gland and retina were circadian in nature as they persisted under conditions of constant darkness (DD). Under DD, however, the amplitudes of AANAT and melatonin rhythms were significantly lower (by 50-80%) than those found under the LD cycle. The findings indicate that melatonin rhythmicity in the turkey pineal gland and retina is regulated both by light and the endogenous circadian clock. The rapid dampening of the rhythms under DD suggests that of these two regulatory factors, environmental light may be the primary stimulus in the maintenance of the high amplitude melatonin rhythms in the turkey.     

Melatonin Profile in Marmots: The Influence of Catecholamines, Hibernation, and Light

The aim of this study was to determine the effects of circulating catecholamines and light on the daily melatonin rhythm in the marmot. Endogenous levels of circulating catecholamines and plasma melatonin were measured during arousal from hibernation in light and studies were performed on the circadian melatonin rhythm in two photoperiods (LD 4:20 and LD 8:16). In addition, studies were done on the capacity of broad-band white light at normal room intensities (32 muW/cm2 or 108 Ix) and of low-intensity monochromatic green light (500 nm; 1.4muW/cm2 or 3.1 Ix) to suppress high nocturnal melatonin levels. We conclude that high levels of plasma catecholamines that occur during arousal from hibernation do not influence the production and secretion of pineal melatonin. During the nocturnal portion of its light/dark cycle, the marmot plasma melatonin rhythm is suppressed by both white light and low-intensity green light.     

Ontogeny of pineal melatonin rhythm in rats under 12: 12-hr and 14: 14-hr lightdark conditions

Abstract: The aim of the study was to determine whether a discrepancy between the genetically determined endogenous circadian period and an abnormally long Zeitgeber period disturbs the development of melatonin synthesis. Breeding pairs of rats were kept under 12: 12- or 14: 14-hr light: dark (LD) conditions. Pineal melatonin contents in the offspring were measured by radioimmunoassay. At 2 weeks of age high melatonin contents were found from lights-off to lights-on in both conditions suggesting dominance of the photic regulation. At 3 weeks of age the signs of the circadian regulation in the melatonin profiles were evident: a lag period after the light offset in control conditions and a significant decline before the light onset in both conditions. However, in 14: 14-hr LD conditions the melatonin content did not decrease to daytime levels until the lights were on. This could suggest incomplete maturation of the circadian system. The phase relationships between the melatonin peak and LD cycle were different in the two conditions. A statistically significant LD difference was first found at the age of 8–10 days in male pups and at 14 days in female pups under both lightings. The results suggest that the abnormally long LD cycle did not cause any major disorders in the development of photic or circadian regulation of the 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.     

Regulation of melatonin production by catecholamines and adenosine in a photoreceptive pineal organ. An in vitro study in the pike and the trout

The pineal organ of fish contains photoreceptor cells with structural and functional analogies to retinal photoreceptors. In these cells, the light/dark (LD) cycle influences the production of melatonin by controlling the activity of one of its synthetizing enzymes, serotonin N-acetyltransferase (NAT). The daily rhythm in NAT activity is generated endogenously in the pike but not in the trout pineal. We report here that in addition to the LD information, chemical factors are also involved in the control of melatonin production. Adenosine and two of its analogs stimulated or inhibited NAT activity and melatonin release in cultured pike and trout pineals, depending on the experimental conditions. It is believed that the nucleoside, produced locally, exerts a modulatory role on the neurohormonal output via still enigmatic mechanisms, involving a transmembranous carrier. Nocturnal melatonin production in cultured pike pineals was inhibited by alpha-adrenergic agonists and stimulated by a beta-adrenergic agonist. No effect could be induced in trout pineals cultured under similar conditions. Because melatonin production by pineal photoreceptors is apparently regulated by both light and chemical inputs, we propose they might be multieffector cells.     

6-Sulphatoxymelatonin secretion in different locomotor activity types of the blind mole rat Spalax ehrenbergi

Abstract: 6-Sulphatoxymelatonin (aMT6S) excretion was examined in the urine of rhythmic and arrhythmic blind subterranean mole rats ( Spalax ehrenbergi ) to test the correlation between melatonin secretion (as represented by aMT6S) and variability in circadian locomotor activity. Activity pattern was tested in four males, first for a week under short photoperiod [light: dark (LD) 10: 14], followed by 10 days in constant darkness (DD). After several months the experiment was repeated under long photoperiod (LD 14: 10), followed by DD conditions. Under LD conditions all animals exhibited aMT6S excretion during the dark phase, with a decline just before the onset of light. No correlation was found between activity pattern and melatonin secretion. The animal with the highest melatonin secretion both under LD and DD had an arrhythmic locomotor pattern. The results suggest that in mole rats melatonin secretion and circadian locomotor activity are controlled by two different mechanisms. There were large differences in the aMT6S levels among individuals, suggesting the importance of duration of melatonin secretion over amplitude for gonadal development and thermoregulatory changes. During summer, i.e., before the breeding season, the animals keep a more stable aMT6S secretion than in winter, and the amplitude of secretion is higher under DD vs. LD conditions.     

Bovine large luteal cells show increasing de novo DNA methylation of the main ovarian CYP19A1 promoter P2

Wrasse species exhibit a definite daily rhythm in locomotor activity and bury themselves in the sand at the bottom of the ocean at night. It remains unclear how their behavior in locomotor activity is endogenously regulated. The aim of the present study was to clarify the involvement of melatonin and clock genes (Per1, Per2, Bmal1, and Cry1) in daily and circadian rhythms of the threespot wrasse, Halichoeres trimaculatus, which is a common species in coral reefs. Daily and circadian rhythms in locomotor activity were monitored under conditions of light-dark cycle (LD=12:12), constant light (LL), and darkness (DD). Daily rhythms in locomotor activity were observed under LD and persisted under LL and DD. Melatonin from a cultured pineal gland showed daily variations with an increase during the nighttime and a decrease during daytime, which persisted under DD. Melatonin treatment induced decreases in locomotor activity and respiratory rate, suggesting that melatonin has a sleep-inducing effect. Per1 and Per2 mRNA abundance in the brain under LD showed daily rhythms with an increase around lights on. Robust oscillation of Per1 and Per2 mRNA expression persisted under DD and LL, respectively. Expression of Bmal1 and Cry1 mRNA also showed daily and circadian patterns. These results suggest that clock genes are related to circadian rhythms in locomotor activity and that melatonin plays a role in inducing a sleep-like state after fish bury themselves in the sand. We conclude that the sleep-wake rhythm of the wrasse is regulated by a coordination of melatonin and clock genes.     

Circadian rhythms of ocular melatonin in the wrasse Halichoeres tenuispinnis, a labrid teleost

Using in vivo and in vitro methods we studied the regulation of ocular melatonin rhythms in the wrasse Halichoeres tenuispinnis, by either light or the circadian clock. Rhythmic changes in ocular melatonin levels under light-dark (LD) cycles were persistent under constant darkness (DD), and had a circadian periodicity of approximately 24h. However, ocular melatonin levels remained low under constant light conditions. When wrasse were exposed to a single 6-h light pulse at three different circadian phases under DD, phase-dependent phase shifts in the circadian rhythms of ocular melatonin were observed. When eyecups were prepared during mid-light periods or at the onset of darkness, and incubated in vitro in either light or dark periods, both time and light conditions affected melatonin release. These results indicate that the melatonin rhythms in the wrasse eye are driven by an ocular circadian clock that is entrained to LD cycles via local photoreceptors.     

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.     

The effect of light,dark or altered circadian cycle on the lifespan of the rotifer Asplanchna brightwelli

Rotifers were maintained in various light conditions for at least 20 generations. In the first set of experiments lifespan and fecundity data were compared for groups of rotifers maintained under LL (continuous light), DD (continuous dark) or LD 12:12 (control, 12hr. light 12 hr. dark). The mean lifespan of the rotifers cultured in DD conditions was significantly increased as compared to the LL or LD 12:12 groups but there were no fecundity differences. There was no alteration in lifespan or fecundity of the LL group as compared to the LD 12:12 control. In a second set of experiments, an LD 6:18 cycle was imposed to determine whether a shift in the circadian cycle would influence lifespan. Rotifers exposed to the LD 6:18 cycle and to the DD conditions showed an 18% and 22% increase in mean lifespan respectively. The results are interpreted to demonstrate that lifespan is influenced by the circadian cycle.     

Effects of a one-hour light pulse on the timing of the circadian rhythm in melatonin secretion in rams

Abstract: The effects of a 1-hr light pulse on the timing of the circadian rhythm in the blood plasma concentration of melatonin were documented in Soay rams. Groups of 5 to 6 animals were transferred from short days (LD 8: 16) to constant dim red light (DD) for 6 days, and were exposed to a 1-hr light pulse at one of 16 different times throughout 24 hr on day 3. Blood samples were collected hourly for 30 hr before (day 2–3) and after the light pulse (day 5–6), and the plasma concentrations of melatonin were measured by radioimmunoassay. The animals were allocated to experimental groups based on the circadian time (CT) when the light pulse was given using two hourly blocks through the circadian day; the onset of enhanced melatonin secretion (melatonin peak) was designated as CT 12. Under DD there was a clearly defined plasma melatonin rhythm in all animals. The mean duration of the melatonin peak was 13.24 ± 0.16 hr (n = 91) and the mean period between the onset of successive melatonin peaks was 23.55 ± 0.10 hr (n = 21). The effect of the 1-hr light pulse on the time of onset of the melatonin peak varied significantly with the circadian time when the light pulse was given (ANOVA, P= 0.031). Light-induced significant (pre- vs post-pulse onset, Students t-test, P < 0.05) phase delays in the onset of the melatonin peak in the early subjective day at CT 2.5 hrs (mean ø: -1.9 hr), and in the early subjective night at CT 12.5 and 14.5 (mean ø: -2.0 hrs), but not at other times. The light pulse never induced significant phase advances. The effects of the light pulse on the offset of plasma melatonin peak did not vary significantly with the time of the light pulse (ANOVA, P= 0.780), although significant differences in the pre- and post-pulse offset occurred at CT 14.5 and 18.5 (mean ø: -1.5 hr). The differential changes in the onset and the offset of the melatonin peak resulted in changes in the duration of the peak (maximum difference between means: 3.8 hr). The results indicate that entrainment occurs under natural 24 hr LD cycles when light impinges on the early subjective night and induces a net phase delay, thus extending the period of the melatonin rhythm to 24 hr. This causes a close phase relationship between the end of the light period and the onset of the melatonin peak as occurs in sheep under natural cycles. The results are also consistent with a multiple oscillator governing melatonin secretion, and that differential entrainment of the component oscillators by light affects the duration of the melatonin peak.     

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.     

Effects of nightbreak, T-cycle, and resonance lighting schedules on the pineal melatonin rhythm of the lizard Anolis carolinensis: Correlations with the reproductive response

Abstract: Experiments were conducted to determine if a correlation exists between any aspect of the pineal melatonin rhythm (such as its duration or phase) in the lizard Anolis carolinensis and the reproductive response to photoperiod. The rhythm of pineal melatonin content was determined in anoles exposed to nightbreak lighting protocols (10L:5D:1L:8D, 10L:10D:1L:3D), resonance lighting cycles (LD 11:13, LD 11:25), and T-cycle lighting protocols (LD 11:7, LD 11:9, LD 11:13, LD 11:15, LD 11:19) and compared with the testicular response to these lighting protocols as determined previously [Underwood and Hyde, (1990) J. Comp. Physiol. (A) 167:231–243]. Different T-cycles and nightbreak cycles elicited changes in both the duration of the melatonin peak and the phase of the melatonin peak relative to these light cycles. The response to the resonance cycle (LD 11:25) was complex, probably due to the overlapping patterns of two groups whose pineal melatonin rhythms were entrained approximately 12 hr out of phase with each other. No correlation was observed between the duration, or the amplitude, of the nocturnal melatonin peaks seen on the various light cycles and the reproductive response to these cycles. A correlation was observed between the phase of the pineal melatonin rhythm and the reproductive response. Light cycles were inductive (stimulated testicular growth) when the entrained melatonin rhythm peaked near the light-to-dark or the dark-to-light transition, but they were not inductive when the melatonin rhythm peaked during the middle third of the night. These results suggest that if melatonin is involved in the transduction of photoperiodic information in Anolis , neither the duration nor amplitude of the nocturnal melatonin pulse is involved in the measurement of day length. Instead, the phase-relationship of the melatonin rhythm to the rest of the circadian system may determine photoperiodic responsiveness.