Effect of Single Low Intensity Light Pulse Exposure and Melatonin Treatment on the Circadian Variation of Melatonin in Indian Palm Squirrel Funambulus pennanti

The endogenous circadian rhythm of melatonin in mammals provides information regarding the resetting response of the mammalian circadian timing system in response to the changes in light dark cycle. Photoperiodic changes are reported to have acute and chronic effect on melatonin rhythm. Our aim in present experiment was to study the effect of single light pulse of low intensity on the circadian variation of melatonin in Indian palm squirrel. A short pulse of 5min was given to the animals at 22:55 h on day 16th in natural photoperiodic condition of long day length (LD ~ 13.55:10.05) and melatonin levels were estimated at every 4-h interval on ZT scale on day 17th (DD). Observations suggest that the light pulse given on day 16th suppressed the melatonin level on day 17th (DD). Besides this, it was also found that there was phase delay in the peak value of melatonin. Further, we tested the ability of single melatonin injection on the light pulse induced phase shift of acrophase of melatonin in this species F. pennanti. We injected the single physiological dose of melatonin (25 microgram/100 g body wt.) just 5 min prior to the commencement of light pulse (22:50 h) on day 16 and melatonin levels were estimated on day 17th as above. Injection of melatonin prior to light pulse altered the suppressing and phase shifting effect of light in terms of peak concentration of melatonin in squirrels. Above data may lead us to conclude that the biological clock mechanism controlling circadian rhythm of melatonin in this rodent is in response to the phase shifting effect of light and acute melatonin treatment. Further, we may suggest that single melatonin injection has the capability to entrain melatonin rhythm but a dose dependent study is required to facilitate the suggestion.     

Melatonin enhances the sensitivity of circadian pacemakers to light in the nocturnal field mouse Mus booduga

The effect of exogenous melatonin (1 mg/kg) on light pulse (LP) induced phase shifts of the circadian locomotor activity rhythm was studied in the nocturnal field mouse Mus booduga. Three phase response curves (PRCs: LP, control, and experimental) were constructed to study the effect of co-administration of light and melatonin at various circadian times (CTs). The LP PRC was constructed by exposing animals free-running in constant darkness (DD) to LPs of 100-lux intensity and 15-min duration, at various CTs. The control and experimental PRCs were constructed by using a single injection of either 50% DMSO or melatonin (1 mg/kg dissolved in 50% DMSO), respectively, administered 5 min before LPs, to animals free-running in DD. A single dose of melatonin significantly modified the waveform of the LP PRC. The experimental PRC had significantly larger areas under advance and delay regions of the PRC compared to the control PRC. This was also confirmed when the phase shifts obtained at various CTs were compared between the three PRCs. The phase delays at three phases (CT12, CT14, and CT16) of the experimental PRCs were significantly greater than those of the control and the LP PRCs. Based on these results we conclude that phase shifting effects of melatonin and light add up to produce larger responses.     

Phase shifts in the circadian rhythm in plasma concentrations of melatonin in rams induced by a 1-hour light pulse

The effect of a 1-hr light pulse, given at night, on the timing of the circadian rhythm in the plasma concentration of melatonin was examined in Soay rams to investigate the mechanisms involved in determining the duration of the nocturnal peak in melatonin secretion. Animals (n = 8) were housed under short days (LD 8:16) or long days (LD 16:8) and received a light pulse at various times of night. They were released into constant dim red light (DD) on day 1. Blood samples were collected hourly for 30 hr from 1000 hr on day 3, and the plasma concentration of melatonin was determined by radioimmunoassay to assess the timing of the melatonin peak. Control animals (n = 8) were maintained under the same conditions but received no light pulse. Under short days, a light pulse given early in the night caused a phase delay in the melatonin peak, and a light pulse given in the late night caused a phase advance. The mean duration of the melatonin peak was slightly reduced following a light pulse in the early or late night, and slightly increased following a pulse given near the middle of the night. Under long days, both light-pulse treatments given at night caused a phase delay in the melatonin peak, but there was no significant change in duration of the melatonin peak. The duration of the melatonin peak at day 3 under DD in the control animals was similar for all treatments, regardless of the previous entraining photoperiod (mean duration: 12.6-14.8 hr) and was similar to that under short days (14.6 hr), but was significantly longer than that under long days (8.2 hr). Information on the phase response curve in the Soay ram and on the period of the circadian oscillator governing the melatonin rhythm (c 23.0 hr under DD) predicts a close phase relationship between the end of the light phase and the onset of the melatonin peak as observed under normal 24-hr LD cycles. The current results also indicate that light acts to entrain the circadian rhythm influencing the onset and offset of melatonin secretion, and thus dictates the duration of the melatonin peak.     

Clock-controlled endogenous melatonin rhythms in Nile tilapia (Oreochromis niloticus niloticus) and African catfish (Clarias gariepinus)

The purpose of this work was to investigate the circadian melatonin system in two tropical teleost species characterized by different behavioral habits, Nile tilapia (diurnal) and African catfish (nocturnal). To do so, fish were subjected to either a control photoperiod (12L:12D), continuous light (LL) or darkness (DD), or a 6L:6D photoperiod. Under 12L:12D, plasma melatonin levels were typically low during the photophase and high during the scotophase in both species. Interestingly, in both species, melatonin levels significantly decreased prior to the onset of light, which in catfish reached similar basal levels to those during the day, demonstrating that melatonin production can anticipate photic changes probably through circadian clocks. Further evidence for the existence of such pacemaker activity was obtained when fish were exposed to DD, as a strong circadian melatonin rhythm was maintained. Such an endogenous rhythm was sustained for at least 18 days in Nile tilapia. A similar rhythm was shown in catfish, although DD was only tested for four days. Under LL, the results confirmed the inhibitory effect of light on melatonin synthesis already reported in other species. Finally, when acclimatized to a short photo-cycle (6L:6D), no endogenous melatonin rhythm was observed in tilapia under DD, with melatonin levels remaining high. This could suggest that the circadian clocks cannot entrain to such a short photocycle. Additional research is clearly needed to further characterize the circadian axis in teleost species, identify and localize the circadian clocks, and better understand the environmental entrainment of fish physiology.     

Melatonin and Human Rhythms

Melatonin signals time of day and time of year in mammals by virtue of its pattern of secretion, which defines ‘biological night.’ It is supremely important for research on the physiology and pathology of the human biological clock. Light suppresses melatonin secretion at night using pathways involved in circadian photoreception. The melatonin rhythm (as evidenced by its profile in plasma, saliva, or its major metabolite, 6‐sulphatoxymelatonin [aMT6s] in urine) is the best peripheral index of the timing of the human circadian pacemaker. Light suppression and phase‐shifting of the melatonin 24 h profile enables the characterization of human circadian photoreception, and circulating concentrations of the hormone are used to investigate the general properties of the human circadian system in health and disease. Suppression of melatonin by light at night has been invoked as a possible influence on major disease risk as there is increasing evidence for its oncostatic effects. Exogenous melatonin acts as a ‘chronobiotic.’ Acutely, it increases sleep propensity during ‘biological day.’ These properties have led to successful treatments for serveal circadian rhythm disorders. Endogenous melatonin acts to reinforce the functioning of the human circadian system, probably in many ways. The future holds much promise for melatonin as a research tool and as a therapy for various conditions.     

Melatonin rhythm observed throughout a three-cycle bright-light stimulus designed to reset the human circadian pacemaker.

Exposure to light and darkness can rapidly induce phase shifts of the human circadian pacemaker. A type 0 phase response curve (PRC) to light that has been reported for humans was based on circadian phase data collected from constant routines performed before and after a three-cycle light stimulus, but resetting data observed throughout the entire resetting protocol have not been previously reported. Pineal melatonin secretion is governed by the hypothalamic circadian pacemaker via a well-defined neural pathway and is reportedly less subject to the masking effects of sleep and activity than body temperature. The authors reasoned that observation of the melatonin rhythm throughout the three-cycle light resetting trials could provide daily phase-resetting information, allowing a dynamic view of the resetting response of the circadian pacemaker to light. Subjects (n = 12) living in otherwise dim light (approximately 10-15 lux) were exposed to a noncritical stimulus of three cycles of bright light (approximately 9500 lux for 5 h per day) timed to phase advance or phase delay the human circadian pacemaker; control subjects (n = 11) were scheduled to the same protocols but exposed to three 5-h darkness cycles instead of light. Subjects underwent initial and final constant routine phase assessments; hourly melatonin samples and body temperature data were collected throughout the protocol. Average daily phase shifts of 1 to 3 h were observed in 11 of 12 subjects receiving the bright light, supporting predictions obtained using Kronauer's phase-amplitude model of the resetting response of the human circadian pacemaker. The melatonin rhythm in the 12th subject progressively attenuated in amplitude throughout the resetting trial, becoming undetectable for >32 hours preceding an abrupt reappearance of the rhythm at a shifted phase with a recovered amplitude. The data from control subjects who remained in dim lighting and darkness delayed on average -0.2 h per day, consistent with the daily delay expected due to the longer than 24-h intrinsic period of the human circadian pacemaker. Both temperature and melatonin rhythms shifted by equivalent amounts in both bright light-treated and control subjects (R = 0.968; p<0.0001; n = 23). Observation of the melatonin rhythm throughout a three-cycle resetting trial has provided a dynamic view of the daily phase-resetting response of the human circadian pacemaker. Taken together with the observation of strong type 0 resetting in humans in response to the same three-cycle stimulus applied at a critical phase, these data confirm the importance of considering both phase and amplitude when describing the resetting of the human circadian pacemaker by light.     

Avian Melatonin Synthesis: Photic and Circadian Regulation of Serotonin N-Acetyltransferase mRNA in the Chicken Pineal Gland and Retina

Abstract: The circadian rhythms in melatonin production in the chicken pineal gland and retina reflect changes in the activity of serotonin N -acetyltransferase (arylalkylamine N -acetyltransferase; AA-NAT; EC 2.3.1.87). Here we determined that the chicken AA-NAT mRNA is detectable in follicular pineal cells and retinal photoreceptors and that it exhibits a circadian rhythm, with peak levels at night. AA-NAT mRNA was not detected in other tissues. The AA-NAT mRNA rhythm in the pineal gland and retina persists in constant darkness (DD) and constant lighting (LL). The amplitude of the pineal mRNA rhythm is not decreased in LL. Light appears to influence the phase of the clock driving the rhythm in pineal AA-NAT mRNA in two ways: The peak is delayed by ∼6 h in LL, and it is advanced by >4 h by a 6-h light pulse late in subjective night in DD. Nocturnal AA-NAT mRNA levels do not change during a 20-min exposure to light, whereas this treatment dramatically decreases AA-NAT activity. These observations suggest that the rhythmic changes in chicken pineal AA-NAT activity reflect, at least in part, clock-generated changes in mRNA levels. In contrast, changes in mRNA content are not involved in the rapid light-induced decrease in AA-NAT activity.     

The Paramecium circadian behavioral rhythm: light phase response curves and entrainment

The population of a ciliate protozoan, Paramecium multimicronucleatum, exhibits a circadian rhythm as measured by the number of the cells traversing an observation point ("traverse frequency," or TF). The present study examined phase shifting of the TF rhythm by administering 2-hr light pulses at different phases of the circadian cycle to cultures free-running in constant darkness (DD). The results were summarized in a phase response curve (PRC), categorized as Type 1. This PRC indicated a relatively narrow phase zone insensitive to the light pulse ("dead zone"). Entrainment of the rhythm to light pulses repeated at 24-hr intervals was also examined, and it was found that the rhythm gradually reached a steady state, following several transient cycles, with the pulses falling at a phase corresponding to the narrow dead zone. Such a steady-state rhythm, with a minimum at approximately 3 hr after the pulse and a maximum at approximately 12 hr after the pulse, was mathematically simulated by superimposing a response function to the pulse on a sinusoidal function representative of the free-running rhythm in DD.     

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

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

S20098 AFFECTS THE FREE-RUNNING RHYTHMS OF BODY TEMPERATURE AND ACTIVITY AND DECREASES LIGHT-INDUCED PHASE DELAYS OF CIRCADIAN RHYTHMS OF THE RAT

Mammalian endogenous circadian rhythms are entrained to the environmental day-night cycle by light exposure. Melatonin is involved in this entrainment by signaling the day-night information to the endogenous circadian pacemaker. Furthermore, melatonin is known to affect the circadian rhythm of body temperature directly. A striking property of the endogenous melatonin signal is its synthesis pattern, characterized by long-term elevated melatonin levels throughout the night. In the present study, the influence of prolonged treatment with the melatonin agonist S20098 during the activity phase of free-running rats was examined. This was achieved by giving S20098 in the food. The free-running body temperature and activity rhythms were studied. The present study shows that enhancement of the melatonin signal, using S20098, affected the free-running rhythm by gradual phase advances of the start of the activity phase, consequently causing an increase in length of the activity phase. A well-known feature of circadian rhythms is its time-dependent sensitivity for light. Light pulse exposure of an animal housed under continuous dark conditions can cause a phase shift of the circadian pacemaker. Therefore, in a second experiment, the influence of melatonin receptor stimulation on the sensitivity of the pacemaker to light was examined by giving the melatonin agonist S20098 in the food during 1 day prior to exposure to a 60-min light pulse of 0, 1.5, 15, or 150 lux given at circadian time (CT) 14. S20098 pretreatment caused a diminished lightpulse- induced phase shift when a light pulse of low light intensity (1.5 lux) was given. S20098 treatment via the food was sufficient to exert chronobiotic activity, and S20098 treatment resulting in prolonged overstimulation of melatonin receptors is able to attenuate the effect of light on the circadian timing system. (Chronobiology International, 18(5), 781-799, 2001)     

S20098 AFFECTS THE FREE-RUNNING RHYTHMS OF BODY TEMPERATURE AND ACTIVITY AND DECREASES LIGHT-INDUCED PHASE DELAYS OF CIRCADIAN RHYTHMS OF THE RAT

Mammalian endogenous circadian rhythms are entrained to the environmental day-night cycle by light exposure. Melatonin is involved in this entrainment by signaling the day-night information to the endogenous circadian pacemaker. Furthermore, melatonin is known to affect the circadian rhythm of body temperature directly. A striking property of the endogenous melatonin signal is its synthesis pattern, characterized by long-term elevated melatonin levels throughout the night. In the present study, the influence of prolonged treatment with the melatonin agonist S20098 during the activity phase of free-running rats was examined. This was achieved by giving S20098 in the food. The free-running body temperature and activity rhythms were studied. The present study shows that enhancement of the melatonin signal, using S20098, affected the free-running rhythm by gradual phase advances of the start of the activity phase, consequently causing an increase in length of the activity phase. A well-known feature of circadian rhythms is its time-dependent sensitivity for light. Light pulse exposure of an animal housed under continuous dark conditions can cause a phase shift of the circadian pacemaker. Therefore, in a second experiment, the influence of melatonin receptor stimulation on the sensitivity of the pacemaker to light was examined by giving the melatonin agonist S20098 in the food during 1 day prior to exposure to a 60-min light pulse of 0, 1.5, 15, or 150 lux given at circadian time (CT) 14. S20098 pretreatment caused a diminished lightpulse- induced phase shift when a light pulse of low light intensity (1.5 lux) was given. S20098 treatment via the food was sufficient to exert chronobiotic activity, and S20098 treatment resulting in prolonged overstimulation of melatonin receptors is able to attenuate the effect of light on the circadian timing system. (Chronobiology International, 18(5), 781–799, 2001)     

Melatonin or a melatonin agonist corrects age-related changes in circadian response to environmental stimulus

The effects of a melatonin agonist, S-20098, included in the diet were tested on a specific effect of aging in hamsters: the marked decline in the phase shifting effects of a 6-h pulse of darkness on a background of constant light. In contrast to young hamsters, old hamsters fed with the control diet showed little or no phase shifts in response to a dark pulse presented in the middle of their inactive or active period. Old hamsters fed with S-20098 showed phase shifts that were ~70% of the ones in young animals and significantly greater than those in old controls. The phase advancing response to a dark pulse presented during the inactive period was dose dependent and reversed after S-20098 discontinuation. Melatonin included in the diet showed comparable restorative effects on the phase shifting response to a dark pulse in old hamsters. Replacement therapy with melatonin or melatonin-related compounds could prove useful in treating, preventing, or delaying disturbances of circadian rhythmicity and/or sleep in older people.     

Locomotor Activity Rhythm in the Field Mouse Mus booduga Phase-shifts to Melatonin Injections in a Dose-dependent Manner

Melatonin is known to shift the phase of the locomotor activity rhythm in the field mouse Mus booduga in accordance with a type-I phase response curve (PRC), with phase delays during the subjective day and phase advances during late subjective night and the early subjective day. At CT4 (circadian time 4; i.e. 16 hr. after activity onset) and CT22 of the circadian cycle, a single dose of melatonin (1 mg/kg) is known to evoke maximum delay and maximum advance phase-shifts, respectively. We investigated the dose-dependent responses of the circadian pacemaker of these mice to a single dose of melatonin at the times for maximum delay and maximum advance. The circadian pacemaker responsible for the locomotor activity rhythm in these mice responded to various doses of melatonin in a dose-dependent manner with the magnitude of phase shifts increasing with dose.     

Locomotor Activity Rhythm in the Field Mouse Mus booduga Phase-shifts to Melatonin Injections in a Dose-dependent Manner

Melatonin is known to shift the phase of the locomotor activity rhythm in the field mouse Mus booduga in accordance with a type-I phase response curve (PRC), with phase delays during the subjective day and phase advances during late subjective night and the early subjective day. At CT4 (circadian time 4; i.e. 16 hr. after activity onset) and CT22 of the circadian cycle, a single dose of melatonin (1 mg/kg) is known to evoke maximum delay and maximum advance phase-shifts, respectively. We investigated the dose-dependent responses of the circadian pacemaker of these mice to a single dose of melatonin at the times for maximum delay and maximum advance. The circadian pacemaker responsible for the locomotor activity rhythm in these mice responded to various doses of melatonin in a dose-dependent manner with the magnitude of phase shifts increasing with dose.     

Differential regulation of arylalkylamine N-acetyltransferase activity in chicken retinal ganglion cells by light and circadian clock

Retinal ganglion cells (RGCs) contain circadian clocks driving melatonin synthesis during the day, a subset of these cells acting as nonvisual photoreceptors sending photic information to the brain. In this work, the authors investigated the temporal and light regulation of arylalkylamine N-acetyltransferase (AA-NAT) activity, a key enzyme in melatonin synthesis. The authors first examined this activity in RGCs of wild-type chickens and compared it to that in photoreceptor cells (PRs) from animals maintained for 48?h in constant dark (DD), light (LL), or regular 12-h:12-h light-dark (LD) cycle. AA-NAT activity in RGCs displayed circadian rhythmicity, with highest levels during the subjective day in both DD and LL as well as in the light phase of the LD cycle. In contrast, AA-NAT activity in PRs exhibited the typical nocturnal peak in DD and LD, but no detectable oscillation was observed under LL, under which conditions the levels were basal at all times examined. A light pulse of 30-60?min significantly decreased AA-NAT activity in PRs during the subjective night, but had no effect on RGCs during the day or night. Intraocular injection of dopamine (50 nmol/eye) during the night to mimic the effect of light presented significant inhibition of AA-NAT activity in PRs compared to controls but had no effect on RGCs. The results clearly demonstrate that the regulation of the diurnal increase in AA-NAT activity in RGCs of chickens undergoes a different control mechanism from that observed in PRs, in which the endogenous clock, light, and dopamine exhibited differential effects. (Author correspondence: mguido@fcq.unc.edu.ar ).     

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

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

Melatonin-induced phase and dose responses in a diurnal mammal,Funambulus pennantii

ABSTRACT

Melatonin, an essential pineal hormone, acts as a marker of the circadian clock that regulates biological rhythms in animals. The effects of exogenous melatonin on the circadian system of nocturnal rodents have been extensively studied; however, there is a paucity of studies on the phase-resetting characteristics of melatonin in diurnal rodents. We studied the phase shifting effects of exogenous melatonin as a single melatonin injection (1 mg/kg) at various phases of the circadian cycle on the circadian locomotor activity rhythm in the palm squirrel, Funambulus pennantii. A phase response curve (PRC) was constructed. Adult male squirrels (N = 10) were entrained to a 12:12 h light-dark cycle (LD) in a climate-controlled chronocubicle with food and water provided ad libitum. After stable entrainment, squirrels were transferred to constant dark condition (DD) for free-running. Following stable free run, animals were administered a single dose of melatonin (1 mg/kg in 2% ethanol-phosphate buffered saline (PBS) solution) or vehicle (2% ethanol-PBS solution) at circadian times (CTs) 3 h apart to evoke phase shifts. The phase shifts elicited at various CTs were plotted to generate the PRC. A dose response curve was generated using four doses (0.5, 1, 2 and 4 mg/kg) administered at the CT of maximum phase advance. Melatonin evoked maximum phase advances at CT0 (1.23 ± 0.28 h) and maximum phase delays at CT15 (0.31 ± 0.09 h). In the dose response experiment, maximal phase shifts were evoked with 1 mg/kg. In contrast, no significant shifts were observed in control groups. Our study demonstrates that the precise timing and appropriate dose of melatonin administration is essential to maximize the amelioration of circadian rhythm–related disorders in a diurnal model.     

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

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

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

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