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.     

Risperidone resets the circadian clock in mice

Risperidone is an atypical antipsychotic that is active at multiple dopamine and serotonin receptor subtypes. Based on its high affinity for serotonin receptors, we predicted that it might reset circadian rhythms in a nocturnal rodent. We report temporally differentiated and differential effects of various doses of risperidone on the voluntary locomotor activity rhythm in the Indian field mice, Mus booduga. Risperidone (0.5 mg/kg) elicited phase delays at phases between CT (circadian time) 12 to CT18 and CT0 to CT3, and phase advances at CT6, CT9 and CT21. However, mice injected at CT6 showed maximum advances (1.299 ± 0.286 h), whereas at CT15 showed maximum delays (?1.514 ± 0.312 h). Increasing the dose beyond 0.5 mg/kg at maximally responsive CTs (CT6 and CT15) resulted in progressively smaller but significant shifts. Thus, 0.5 mg/kg is the optimal dose in this species. The fact that risperidone resets the circadian rhythm in a mammal can be extended to clinical studies and used for optimal adjustment of the circadian rhythm in mental disorders. Conversely, risperidone administration for various treatments must be carefully timed to prevent unwanted phase shifts in patients.     

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.     

Phase and period responses to short light pulses in a wild diurnal rodent,Funambulus pennanti

Photic phase response curves (PRCs) have been extensively studied in many laboratory-bred diurnal and nocturnal rodents. However, comparatively fewer studies have addressed the effects of photic cues on wild diurnal mammals. Hence, we studied the effects of short durations of light pulses on the circadian systems of the diurnal Indian Palm squirrel, Funambulus pennanti. Adult males entrained to a light–dark cycle (12?h–12?h) were transferred to constant darkness (DD). Free-running animals were exposed to brief light pulses (250 lux) of 15?min, 3 circadian hours (CT) apart (CT 0, 3, 6, 9, 12, 15, 18 and 21). Phase shifts evoked at different phases were plotted against CT and a PRC was constructed. F. pennanti exhibited phase-dependent phase shifts at all the CTs studied, and the PRC obtained was of type 1 at the intensity of light used. Phase advances were evoked during the early subjective day and late subjective night, while phase delays occurred during the late subjective day and early subjective night, with maximum phase delay at CT 15 (?2.04?±?0.23?h), and maximum phase advance at CT 21 (1.88?±?0.31?h). No dead zone was seen at this resolution. The free-running period of the rhythm was concurrently lengthened (deceleration) during the late subjective day and early subjective night, while period shortening (acceleration) occurred during the late subjective night. The maximum deceleration was noticed at CT 15 (?0.40?±?0.09?h) and the maximum acceleration at CT 21 (0.39?±?0.07?h). A significant positive correlation exists between the phase shifts and the period changes (r?=?0.684, p?=?0.001). The shapes of both the PRC and period response curve (τRC) qualitatively resemble each other. This suggests that the palm squirrel’s circadian system is entrained both by phase and period responses to light. Thus, F. pennanti exhibits robust clock-resetting in response to light pulses.     

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     

Timely administration of melatonin accelerates reentrainment to phase-shifted light-dark cycles in the field mouse Mus booduga

The effect of melatonin on the rate of reentrainment after a 6h phase delay and a 6h phase advance in the light-dark (LD) cycle was assayed in the nocturnal field mouse Mus booduga. After a phase delay of 6h in the LD cycle, a single dose of melatonin (1 mg/kg) was administered for three consecutive days at about CT4 (circadian time 4). After a phase advance of 6h in the LD cycle, melatonin was administered for three consecutive days at about CT22. Melatonin was found to accelerate reentrainment in both cases. Melatonin-treated animals took significantly fewer cycles to reentrain compared to vehicle-treated (50% dimethylsulfoxide [DMSO]) and nontreated control animals.     

Circadian periods of sensitivity for ramelteon on the onset of running-wheel activity and the peak of suprachiasmatic nucleus neuronal firing rhythms in C3H/HeN mice

Ramelteon, an MT(1)/MT(2) melatonin receptor agonist, is used for the treatment of sleep-onset insomnia and circadian sleep disorders. Ramelteon phase shifts circadian rhythms in rodents and humans when given at the end of the subjective day; however, its efficacy at other circadian times is not known. Here, the authors determined in C3H/HeN mice the maximal circadian sensitivity for ramelteon in vivo on the onset of circadian running-wheel activity rhythms, and in vitro on the peak of circadian rhythm of neuronal firing in suprachiasmatic nucleus (SCN) brain slices. The phase response curve (PRC) for ramelteon (90?μg/mouse, subcutaneous [sc]) on circadian wheel-activity rhythms shows maximal sensitivity during the late mid to end of the subjective day, between CT8 and CT12 (phase advance), and late subjective night and early subjective day, between CT20 and CT2 (phase delay), using a 3-day-pulse treatment regimen in C3H/HeN mice. The PRC for ramelteon resembles that for melatonin in C3H/HeN mice, showing the same magnitude of maximal shifts at CT10 and CT2, except that the range of sensitivity for ramelteon (CT8-CT12) during the subjective day is broader. Furthermore, in SCN brain slices in vitro, ramelteon (10 pM) administered at CT10 phase advances (5.6?±?0.29?h, n?=?3) and at CT2 phase delays (-3.2?±?0.12?h, n?=?6) the peak of circadian rhythm of neuronal firing, with the shifts being significantly larger than those induced by melatonin (10 pM) at the same circadian times (CT10: 2.7?±?0.15?h, n?=?4, p?相似文献   

Melatonin ameliorates oxidative stress and induces cellular proliferation of lymphoid tissues of a tropical rodent, Funambulus pennanti, during reproductively active phase

Effect of melatonin treatment on free radical production was assessed with simultaneous investigation of hormonal level (melatonin and testosterone), blastogenic response, stimulation index, and histological observation of lymphoid organs (spleen, thymus, and bone marrow) in male Indian palm squirrel (Funambulus pennanti) during reproductively active phase (RAP). Low endogenous melatonin and high testosterone level were noted during RAP. Daily subcutaneous injection of melatonin (25 μg/100 g B wt.) at 17.30–18.00 h to squirrels for 60 consecutive days during May–June significantly decreased the thiobarbituric acid reactive substances (TBARS) level compared to control squirrels. Melatonin treatment significantly increased % stimulation ratio (%SR) of splenocytes and thymocytes against T cell mitogen concanavalin A. Pinealectomy (Px) led to a significant increase in TBARS level whereas a significant decrease was observed in blastogenic response and stimulation index was noted. Melatonin injection to Px squirrels showed restoration in %SR of thymocytes and splenocytes with a significant decrease in the TBARS level of the lymphoid tissues. Further, free radical load was induced by lipopolysaccharide (LPS; 400 μg/ml) in lymphatic tissue homogenates and noted that melatonin supplementation (2 mM/ml) led to a significant decrease in TBARS level compared to the control and LPS-supplemented groups. Histological observation showed dense cellularity of thymocytes and splenocytes. Acridine orange staining technique shows a significant increase in thymocyte apoptosis Px squirrels when compared with melatonin-treated squirrels. These findings suggest that endogenous and exogenous melatonin might be responsible for the maintenance of immune system to adapt this seasonal breeder for the rigors of the environmental changes.     

Probing the circadian pacemaker of a mouse using two light pulses

In two separate sets of experiments, the phases of the locomotor activity rhythm of the nocturnal field mouse Mus booduga were probed using two light pulses (LPs). In the first set of experiments, the circadian pacemaker underlying the locomotor activity rhythm was perturbed at circadian time 14 (CT 14) using a resetting light pulse LP1 of 1000 lux intensity and 15 min duration. The phases of the resetting pacemaker were then probed at all even CTs between CT 16 and CT 14 using a PRC probing light pulse LP2 of equal strength. The "LP2 PRC" thus obtained was then compared with the single light pulse PRC in terms of the area under delay (D) and advance (A) zones of the PRCs. The time course and waveform of the two LP PRCs suggest that the LP2 PRC resembled the single LP PRC, displaced by 2 h toward the right. The LP1 PRC had smaller D compared to the single LP PRC (p = 0.007), whereas both the PRCs had A of equal magnitude (p = 0.23). This suggests that the pacemaker phase shifts rapidly after LP perturbations. In the second set of experiments, the LP1 was administered at CT 14. The phase of the pacemaker was then perturbed on day 1 (next cycle after LP1) either 2 h after activity onset (at ca. CT 14 of the transient cycle) or 8 h after activity onset (at ca. CT 20 of the transient cycle) using an LP2 of equal strength. It was observed that the steady-state phase shifts evoked by positioning an LP2, 2 h after activity onset, were positively correlated with the phase shifts observed on day 1. The steady-state phase shifts observed, when the LP2 was positioned, 8 h after activity onset, were negatively correlated with the phase shifts observed on day 1. These results suggest that the transient cycles do not mirror the state of the pacemaker oscillator.     

MT2-like melatonin receptor modulates amplitude receptor potential in visual cells of crayfish during a 24-hour cycle

Retinular photoreceptors are structures involved in the expression and synchronization of the circadian rhythm of sensitivity to light in crayfish. To determine whether melatonin possesses a differential effect upon the receptor potential (RP) amplitude of retinular photoreceptors circadian time (CT)-dependent, we conducted experiments by means of applying melatonin every 2 h during a 24-hour cycle. Melatonin with 100 nM increased RP amplitude during subjective day to a greater degree than during subjective night. To determine whether MT₂ melatonin receptors regulate the melatonin-produced effect, we carried out two experiments, circadian times (CTs) 6 and 18, which included the following: (1) application of the MT₂ receptor selective agonist 8-M-PDOT and antagonist DH97, and (2) the specific binding of [¹²⁵I]-melatonin in eyestalk membranes. The amount of 10 nM of 8-M-PDOT increased RP amplitude in a similar manner to melatonin, and 1 nM DH97 abolished the increase produced by melatonin and 8-M-PDOT. Binding of [¹²⁵I]-melatonin was saturable and specific. Scatchard analysis revealed an affinity constant (K_d) of 1.1 nM and a total number of binding sites (B_max) of 6 fmol/mg protein at CT 6, and a K_d of 1.46 nM and B_max of 7 fmol/mg protein at CT 18. Our results indicate that melatonin increased RP amplitude of crayfish retinular photoreceptors through MT₂-like melatonin receptors. These data support the idea that melatonin is a signal of darkness for the circadian system in crayfish retinular cells.     

The effects of low-dose 0.5-mg melatonin on the free-running circadian rhythms of blind subjects

Exogenous melatonin (0.5-10 mg) has been shown to entrain the free-running circadian rhythms of some blind subjects. The aim of this study was to assess further the entraining effects of a daily dose of 0.5 mg melatonin on the cortisol rhythm and its acute effects on subjective sleep in blind subjects with free-running 6-sulphatoxymelatonin (aMT6s) rhythms (circadian period [tau] 24.23-24.95 h). Ten subjects (9 males) were studied, aged 32 to 65 years, with no conscious light perception (NPL). In a placebo-controlled, single-blind design, subjects received 0.5 mg melatonin or placebo p.o. daily at 2100 h (treatment duration 26-81 days depending on individuals' circadian period). Subjective sleep was assessed from daily sleep and nap diaries. Urinary cortisol and aMT6s were assessed for 24 to 48 h weekly and measured by radioimmunoassay. Seven subjects exhibited an entrained or shortened cortisol period during melatonin treatment. Of these, 4 subjects entrained with a period indistinguishable from 24 h, 2 subjects continued to free run for up to 25 days during melatonin treatment before their cortisol rhythm became entrained, and 1 subject appeared to exhibit a shortened cortisol period throughout melatonin treatment. The subjects who entrained within 7 days did so when melatonin treatment commenced in the phase advance portion of the melatonin PRC (CT6-18). When melatonin treatment ceased, cortisol and aMT6s rhythms free ran at a similar period to before treatment. Three subjects failed to entrain with initial melatonin treatment commencing in the phase delay portion of the PRC. During melatonin treatment, there was a significant increase in nighttime sleep duration and a reduction in the number and duration of daytime naps. The positive effect of melatonin on sleep may be partly due to its acute soporific properties. The findings demonstrate that a daily dose of 0.5 mg melatonin is effective at entraining the free-running circadian systems in most of the blind subjects studied, and that circadian time (CT) of administration of melatonin may be important in determining whether a subject entrains to melatonin treatment. Optimal treatment with melatonin for this non-24-h sleep disorder should correct the underlying circadian disorder (to entrain the sleep-wake cycle) in addition to improving sleep acutely.     

Melatonin Shifts Human Orcadian Rhythms According to a Phase-Response Curve

A physiological dose of orally administered melatonin shifts circadian rhythms in humans according to a phase-response curve (PRC) that is nearly opposite in phase with the PRCs for light exposure: melatonin delays circadian rhythms when administered in the morning and advances them when administered in the afternoon or early evening. The human melatonin PRC provides critical information for using melatonin to treat circadian phase sleep and mood disorders, as well as maladaptation to shift work and transmeridional air travel. The human melatonin PRC also provides the strongest evidence to date for a function of endogenous melatonin and its suppression by light in augmenting entrainment of circadian rhythms by the light-dark cycle.     

Melatonin Shifts Human Orcadian Rhythms According to a Phase-Response Curve

A physiological dose of orally administered melatonin shifts circadian rhythms in humans according to a phase-response curve (PRC) that is nearly opposite in phase with the PRCs for light exposure: melatonin delays circadian rhythms when administered in the morning and advances them when administered in the afternoon or early evening. The human melatonin PRC provides critical information for using melatonin to treat circadian phase sleep and mood disorders, as well as maladaptation to shift work and transmeridional air travel. The human melatonin PRC also provides the strongest evidence to date for a function of endogenous melatonin and its suppression by light in augmenting entrainment of circadian rhythms by the light-dark cycle.     

Daily variations of immunoreactive melatonin in the visual system of crayfish

In crustaceans, melatonin has been detected in the central nervous system and some other organs. The aim of this study was to analyze the melatonin content in the visual system of Procambarus clarkii, by means of radioimmunoassay, at different day-night phases. We have also studied the action of exogenous melatonin on the main properties of the electroretinogram (ERG) circadian rhythm. Experiments were conducted with 25 specimens maintained under controlled conditions of 16°C and 12 h of light alternating with 12 h of darkness. Eyes where cut in dim red light and shock frozen with liquid nitrogen and pulverized in a mortar until a homogenous powder was obtained. Melatonin was extracted with acetone, followed by centrifugation, diluted with an equal volume of equa bidest to ensure freezing at ?80°C for at least 90 min and lyophilization at the same temperature. Lyophilizates, after having been dissolved in RIA buffer, were used for determinations of melatonin. Long-term recordings of electrical responses to light (ERG) were obtained for 10 or more consecutive days. At the 5th day, a single dose of melatonin was injected and its effects on amplitude and period of the ERG circadian rhythm were measured. Melatonin concentrations differed considerably depending on the circadian time and attained a maximum during dark phase. Among the crustaceans, Procambarus clarkii represents the first case in which melatonin peaks during the night following the typical pattern known in the majority of organisms. After melatonin injection, period and amplitude of the ERG circadian rhythm were increased. This effect suggests the involvement of melatonin in the oscillators underlying the generation and expression of circadian rhythms in crayfish.     

Circadian Variation in the Response to Experimental Endotoxemia and Modulatory Effects of Exogenous Melatonin

Disturbances in circadian rhythms are commonly observed in the development of several medical conditions and may also be involved in the pathophysiology of sepsis. Melatonin, with its antioxidative and anti-inflammatory effects, is known to modulate the response to endotoxemia. In this paper, we investigated the circadian variation with or without melatonin administration in an experimental endotoxemia model based on lipopolysaccharide (LPS). Sixty male Sprague-Dawley rats were assigned to six groups receiving an intraperitoneal injection of either LPS (5?mg/kg), LPS?+?melatonin (1?mg/kg), or LPS?+?melatonin (10?mg/kg) at either daytime or nighttime. Superoxide dismutase (SOD) was analyzed in liver samples collected after decapitation. Furthermore, inflammatory plasma markers (cytokines interleukin [IL]-6, IL-10) and oxidative plasma markers (ascorbic acid [AA], dehydroascorbic acid [DHA], and malondialdehyde [MDA]) were analyzed before and 5?h after the onset of endotoxemia. There were significant higher levels of SOD (p?<?0.05), IL-6 (p?<?0.01), and IL-10 (p?<?0.05) during nighttime endotoxemia compared with daytime. At daytime, melatonin 1 and 10?mg reduced the levels of MDA and increased SOD, IL-6, IL-10, and DHA (p?<?0.05). At nighttime, melatonin reduced the levels of MDA and increased DHA (p?<?0.05). Additionally, 10?mg melatonin resulted in lower levels of AA during daytime (p?<?0.05). No dose relationship of melatonin was observed. The results showed that the response induced by experimental endotoxemia was dependent on time of day. Melatonin administration modulated the inflammatory and oxidative stress responses induced by endotoxemia and also resulted in higher levels of antioxidants during daytime. The effect of circadian time on the endotoxemia response and possible modulatory effects of melatonin need further investigations in a human endotoxemia model.     

The benefits of four weeks of melatonin treatment on circadian patterns in resistance-trained athletes

Exercise can induce circadian phase shifts depending on the duration, intensity and frequency. These modifications are of special meaning in athletes during training and competition. Melatonin, which is produced by the pineal gland in a circadian manner, behaves as an endogenous rhythms synchronizer, and it is used as a supplement to promote resynchronization of altered circadian rhythms. In this study, we tested the effect of melatonin administration on the circadian system in athletes. Two groups of athletes were treated with 100?mg?day^?1 of melatonin or placebo 30?min before bed for four weeks. Daily rhythm of salivary melatonin was measured before and after melatonin administration. Moreover, circadian variables, including wrist temperature (WT), motor activity and body position rhythmicity, were recorded during seven days before and seven days after melatonin or placebo treatment with the aid of specific sensors placed in the wrist and arm of each athlete. Before treatment, the athletes showed a phase-shift delay of the melatonin circadian rhythm, with an acrophase at 05:00?h. Exercise induced a phase advance of the melatonin rhythm, restoring its acrophase accordingly to the chronotype of the athletes. Melatonin, but not placebo treatment, changed daily waveforms of WT, activity and position. These changes included a one-hour phase advance in the WT rhythm before bedtime, with a longer nocturnal steady state and a smaller reduction when arising at morning than the placebo group. Melatonin, but not placebo, also reduced the nocturnal activity and the activity and position during lunch/nap time. Together, these data reflect the beneficial effect of melatonin to modulate the circadian components of the sleep–wake cycle, improving sleep efficiency.     

Rectal temperature in active and passive rats during desynchronosis and under melatonin treatment

Effects of phase shifts in circadian rhythms and of melatonin administration on rectal temperature in rats with different activity were studied in the open-field test on 176 Wistar rats kept under conditions of natural or shifted light-darkness period. Under normal light-darkness conditions, the amplitude of diurnal variation in rectal temperature was higher in active rats as compared with passive ones. A shift in the light-darkness conditions inverted the circadian rhythm of rectal temperature and augmented the difference between daytime and night time temperatures in passive and, particularly, in active rats. Melatonin effect depended on dose and time of administration. 1 mg/kg Melatonin enhanced the amplitude of diurnal rhythms of energy metabolism in behaviourally active rats. These changes seem to contribute to adaptive reconstruction in the organism during desynchronosis.     

Twenty-four-hour rhythm patterns of plasma melatonin in short-day and long-day breeders maintained under natural environmental conditions

ABSTRACT

Photoperiodic treatments have been of practical interest in controlling seasonal reproduction in sheep, goats and horses. Melatonin is the principal mediator of the environmental photoperiodic message. To investigate the intra- and inter-subject variability of melatonin 24 h rhythm, ten female Italian Saddle horses (8–10 yrs old, mean body weight 525 ± 30 kg), ten female Sarda breed sheep (2–3 yrs old, mean body weight 40.5 ± 2.8 kg) and ten female Sarda breed goats (3–4 yrs old, mean body weight 38.9 ± 4.1 kg), housed individually in a 4 × 4 m soundproof box equipped with 50 × 100 cm opening windows, were subjected to a natural photoperiod of the vernal equinox (sunrise 06:00 h; sunset 18:00 h). Blood samples were collected from each animal, every 3 h over a 48 h period starting at 00:00 h of day 1 and ending at 00:00 h of day 3. Plasma melatonin concentrations were determined by direct radioimmunoassay (MelatoninDirect RIA, Labor Diagnostika Nord GmbH, Nordhorn, Germany). The application of single cosinor method substantiated a circadian rhythm of melatonin with a nocturnal peak in all studied species. The application of two-way ANOVA on the rhythmic parameters indicated statistically significant differences between the three species in all of the cosinor analysis-derived parameters of MESOR, amplitude, acrophase and robustness of rhythm. Analyses of intra- and inter-subject variability indicate that organization of the melatonin 24 h rhythm is characterized by great accuracy of control within and between the individuals of a breed. In conclusion, features of the 24 h rhythm of melatonin among species; however, the 24 h rhythmicity of melatonin each species showed high stability within the various subjects and within the same subject. These findings must be taken into consideration when applying photoperiod and melatonin treatments for breeding purposes.     

Circadian rhythms of melatonin in haemolymph and optic lobes of Chinese mitten crab (Eriocheir sinensis) and Chinese grass shrimp (Palaemonetes sinensis)

This study is the first to examine the circadian rhythms of melatonin in Eriocheir sinensis and Palaemonetes sinensis, two economically important crustaceans. We collected haemolymph and optic lobes from both species every 4 h over a whole day cycle. Melatonin content was measured with high-performance liquid chromatography. E. sinensis haemolymph exhibited significant (p < 0.05) peaks in melatonin at 16:00 (0.180 ± 0.020 μg·mL^?1) and 24:00 (0.244 ± 0.055 μg·mL^?1), while eyestalks had significant peaks at 16:00 (72.377 ± 18.100 μg·eyestalk^?1) and 24:00 (98.756 ± 30.271 μg·eyestalk^?1). In P. sinensis, melatonin peaked significantly only at 16:00 in optic lobes (12.493 ± 1.475 μg·eyestalk^?1) (p < 0.05); no significant peaks were present in haemolymph. Thus, both E. sinensis and P. sinensis exhibit species-specific melatonin rhythms. Time of day should therefore be considered when examining the physiological status of both crustaceans, given the potential influence of fluctuating daily melatonin concentrations.