Influence of the Host Photoperiodicity on the Schizogonic Cycle and the Synchronism of the Rodent Malaria Plasmodium chabaudi chabaudi

This study intended to determine whether LD (Light:Dark) regimens different from the usual 12:12 hours could impair the schizogonic cycle and the synchronism of the rodent malaria parasite Plasmodium chabaudi chabaudi. Five illumination regimens of 12:12 LD, 5:5 LD, 18:18 LD, DD (constant dark) and LL (constant light) were used. Mice were kept in these regimens three months prior to and throughout the experiment. The total and the differential parasitaemia were checked every hour, during more than 24 hours. The parasitaemia data indicated that changes in the LD regimen, except for the LD 18:18, do not affect the length of the developmental cycle of this malaria parasite which remains 24 hours. In both the LL and 18:18 LD regimens, the synchronisation of parasites were impaired, mostly in the LL regimen. Also, we noticed that the schizogony which usually occurs in the dark part of the cycle may happen in the light part too. A circadian rhythm in the frequency of the schizogonic cycle and a time dependent occurrence of ring forms, trophozoites and schizonts were observed. At high parasitaemia, the infection was desynchronised. The total parasitaemia curves displayed a plateau region, followed by a drop at the end of the plateau, and an increase after the schizogony to reach the next plateau level.     

Rhythmic changes in metabolism of dopamine in the chick retina: the importance of light versus biological clock

Rhythmic changes in dopamine (DA) content and metabolism were studied in retinas of chicks that were adapted to three different lighting conditions: 12-h light : 12-h dark (LD), constant darkness (DD) and continuous light (LL). Retinas of chicks kept under LD conditions exhibited light-dark-dependent variations in the steady-state level of DA and the two metabolites of DA, i.e. 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA). Concentrations of DA, DOPAC and HVA were high in light hours and low in dark hours of the LD illumination cycle. In retinas of chicks kept under DD, the content of DA, DOPAC and HVA oscillated in a rhythmic manner for 2 days, with higher values during the subjective light phase than during the subjective dark phase. The amplitudes of the observed oscillations markedly and progressively declined compared with the amplitudes recorded under the LD cycle. In retinas of chicks kept under LL conditions, levels of DA, DOPAC and HVA were similar to those found during the light phase of the LD cycle. Changes in the retinal contents of DA and HVA did not exhibit pronounced daily oscillations, while on the first day of LL the retinal concentrations of DOPAC were significantly higher during the subjective light phase than during the subjective dark phase. Acute exposure of chicks to light during the dark phase of the LD cycle markedly increased DA and DOPAC content in the retina. In contrast, light deprivation during the day decreased the retinal concentrations of DA and DOPAC. It is suggested that of the two regulatory factors controlling the level and metabolism of DA in the retina of chick, i.e. light and biological clock, environmental lighting conditions seem to be of major importance, with light conveying a stimulatory signal for the retinal dopaminergic cells.     

Influence of light regimens on circadian changes in the blood glucose and tissue glycogen concentration in the rat

Adult male Wistar rats allowed food and drinking water ad libitum were kept 2- 5 weeks under standard conditions, but with a different artificial light regimens. The standard regimen was 12:12 h light and dark (LD) and the other variants were 12:12 h dark and light (DL), continuous darkness (DD) and continuous light (LL). The blood glucose level and the liver, skeletal muscle, heart and brown and white adipose tissue glycogen concentration were tested during the day at 3-hour intervals. The experiments were carried out during the winter and were evaluated by an analysis of variance and the cosinor test. Circadian variation of the blood glucose level and the liver and both the adipose tissue glycogen concentrations was only slightly affected by changes in the light regimen. Conversely, the oscillations of the skeletal muscle and heart glycogen concentration reacted markedly to the variants of the light regimen, their reaction being manifested in the localization of the acrophases in different parts of the day, especially when comparing the DD and LD regimens.     

Locomotor activity rhythm in two wrasses,Halichoeres tenuispinnis andPteragogus flagellifera,under various light conditions

The locomotor activity rhythms were examined by using an actograph with infra-red photo-electric switches for two species of wrasses, (Halichoeres tenuispinnis andPteragogus flagellifera) under various light conditions. InH. tenuispinnis, the locomotor activity of almost all fish under light-dark cycle regimen (LD12:12; 06:00–18:00 light, 18:00–06:00 dark) commenced somewhat earlier than the beginning of light period and continued till somewhat earlier than the beginning of the dark period. This species clearly showed free-running activity rhythms under both constant illumination (LL) and constant darkness (DD). Therefore,H. tenuispinnis appeared to have a circadian rhythm. The length of the circadian period ranged from 23 hr. 30 min. to 23 hr. 44 min. under LL, and was from 23 hr. 39 min. to 24 hr. 18 min. under DD. On the other hand, the locomotor activity ofP. flagellifera occurred mostly in the light period under LD 12:12. The activity of this species continued through LL, but was greatly suppressed in DD, so that none of the fish had any activity rhythm in both constant conditions. It was known from field observations thatH. tenuispinnis burrowed and lay in sandy bottoms, whileP. flagellifera hid and rested in bases of seagrasses and shallow crevices of rocks during the night. In the present two wrasses, it seemed that the above-mentioned difference of noctural behavior was closely related to the intensity of the endogenous factor in the activity rhythm.     

Exposure to light at night accelerates aging and spontaneous uterine carcinogenesis in female 129/Sv mice

The effect of the constant illumination on the development of spontaneous tumors in female 129/Sv mice was investigated. Forty-six female 129/Sv mice starting from the age of 2 mo were kept under standard light/dark regimen [12 h light (70 lx):12hr dark; LD, control group], and 46 of 129/Sv mice were kept under constant illumination (24 h a day, 2,500 lx, LL) from the age of 5 mo until to natural death. The exposure to the LL regimen significantly accelerated body weight gain, increased body temperature as well as acceleration of age-related disturbances in estrous function, followed by significant acceleration of the development of the spontaneous uterine tumors in female 129/Sv mice. Total tumor incidence as well as a total number of total or malignant tumors was similar in LL and LD group (p > 0.05). The mice from the LL groups survived less than those from the LD group (χ² = 8.5; p = 0.00351, log-rank test). According to the estimated parameters of the Cox’s regression model, constant light regimen increased the relative risk of death in female mice compared with the control (LD) group (p = 0.0041). The data demonstrate in the first time that the exposure to constant illumination was followed by the acceleration of aging and spontaneous uterine tumorigenesis in female 129/Sv mice.     

State-dependent effects of light-dark cycle on somatosensory and visual cortex EEG in rats

Somatosensory (SSctx) and visual cortex (Vctx) EEG were evaluated in rats under a 12:12-h light-dark (LD) cycle and under constant light (LL) or constant dark (DD) in each sleep or wake state. Under LD conditions during light period, relative Vctx EEG slow-wave activity (SWA) was higher than that of the SSctx, whereas during dark period, relative Vctx EEG SWA was lower than in the SSctx. These effects were state specific, occurring only during non-rapid eye movement sleep (NREMS). Under LL conditions, the duration of REMS and NREMS during the period that would have been dark if the LD cycle had continued (subjective dark period) was greater than under LD conditions. DD conditions had little effect on the duration of NREMS and REMS. SSctx and Vctx EEG SWA were suppressed by LL during the subjective dark period; however, the degree of Vctx SWA suppression was smaller than that of the SSctx. DD conditions during the subjective light period enhanced SSctx SWA, whereas Vctx SWA was suppressed. Under LL conditions during the subjective dark period, Vctx EEG power was higher than that of the SSctx across a broad frequency range during NREMS, REMS, and wakefulness. During DD, SSctx EEG power during NREMS was higher than that of the Vctx in the delta wave band, whereas SSctx power during REMS and wakefulness was higher than that of the Vctx in frequencies higher than 8 Hz. We concluded that the SSctx and Vctx EEGs are differentially affected by light during subsequent sleep. Results provide support for the notion that regional sleep intensity is dependent on prior regional afferent input.     

Influence of the light regimen on the circadian rhythm of serum lipids in the laboratory rat

Young male rats of a conventional Wistar breed were adapted for several weeks (3-6) to a 12:12 h light:dark (7 a.m. - 7 p.m., 7 p.m. - 7 a.m.) regimen (LD), to inversion of this standard regimen (DL), to continuous darkness (DD) or to continuous light (LL). After adaptation, groups of animals were killed at 3-hour intervals during the day and the basic lipid fractions were determined in their serum. Under the standard light regimen the cosinor test demonstrated a rhythm in all the indicators (triacylglycerols, total cholesterol, phospholipids) except for non-esterified fatty acids. When the regimen was inverted (DL), the peaks of the circadian non-esterified fatty acid and triacylglycerol curves shifted to the antiphase. The acrophases of "free-running" serum lipid rhythm under the DD regimen of the standard regimen rhythms differed in the case of cholesterol and phospholipids. In the case of triacylglycerols and phospholipids there was disagreement between the DD and LL regimen curves. With reference to the localization of the acrophase under the DD and LD regimens it was assumed that the influence of the light regimen on the synchronization of circadian rhythms is small in the case of serum non-esterified fatty acids and triacylglycerols and that it is greater in the case of serum cholesterol and phospholipids.     

Circadian Rhythm of Blood Ethanol Clearance Rates in Rats: Response to Reversal of the L/D Regimen and to Continuous Darkness and Continuous Illumination

Phase relationships of the circadian rhythms of blood ethanol clearance (metabolic) rates and body temperature were studied in rats successively exposed to 4 illumination regimens: LD (light from 0800-2000 hr), DL (light from 2000-0800 hr), constant darkness (DD) and, lastly, constant light (LL). After a 4-wk standardization to each regimen, body temperatures were taken at 9 × 4-hr intervals to establish baseline circadian profiles. One week later, groups (N = 8) received 1.5 g/kg ethanol (i.p.) at 6 equally spaced timepoints during a 24-hr span, when temperatures were again measured. Ethanol clearance rates were estimated from decreasing blood ethanol levels sampled every 20 min from 60-200 min after dosing, and the resultant elimination curves were subjected to cosinor analysis. These studies show for the first time that the high amplitude circadian rhythm in ethanol metabolism persists under constant conditions of illumination (DD and LL), demonstrating that it may well be a truly internal circadian rhythm and not a response to exogenous cues of the light/dark cycle. During both LD and DL, maximal and minimal ethanol clearance rates fell near the end of the dark and light phases, respectively, and followed circadian peak and trough control temperatures by approximately 6 hr. A fixed internal phase relationship between the core body temperature and the circadian rhythm in ethanol metabolism is demonstrated, thus establishing the rhythm in body temperature as a suitable and convenient internal marker rhythm for studies of the metabolism of low-to-moderate ethanol doses. These studies demonstrate that the phase relationships of blood ethanol clearance rate and body temperature can be manipulated by the illumination regimen selected, an observation of both basic and practical importance.     

Phase of the circadian clock is accurately transferred from mother to daughter cells in the dinoflagellate Gonyaulax polyedra.

In the first cycle following transfer from a 12 h light-12 h dark cycle (LD12:12) to constant darkness (DD), the standard deviation in circadian phase among individual clocks in populations of Gonyaulax polyedra is approximately 60 min. When a culture is transferred to constant light conditions (LL) from an LD 12:12 cycle, the standard deviation increases in the first 2-3 d, but then remains unchanged, suggesting a lack of observable desynchronization in LL after the transient period. The synchrony in a cell population is preserved even after several cell divisions. The results indicate that variations in period among cells are small, that the period of an individual clock does not fluctuate randomly from day to day, and that the circadian phase of a mother cell is faithfully passed to the clocks of the daughter cells.     

Circadian Rhythm in Pineal N-Acetyltransferase Activity: Rapid Phase Reversal and Response to Shorter than 24-Hour Cycles (IV)

N-Acetyltransferase (NAT) is an enzyme whose rhythmic activity in the pineal gland and retina is responsible for circadian rhythms in melatonin. The NAT activity rhythm has circadian properties such as persistence in constant conditions and precise control by light and dark. Experiments are reported in which chicks (Gallus domesticus), raised for 3 weeks in 12 h of light alternating with 12 h of dark (LD12:12), were exposed to 1-3 days of light-dark treatments during which NAT activity was measured in their pineal glands. (a) In LD12:12, NAT activity rose from less than 4.5 nmol/pineal gland/h during the light-time to 25-50 nmol/pineal gland/h in the dark-time. Constant light (LL) attenuated the amplitude of the NAT activity rhythm to 26-45% of the NAT activity cycle in LD12:12 during the first 24 h. (b) The timing of the increase in NAT activity was reset by the first full LD12:12 cycle following a 12-h phase shift of the LD12:12 cycle (a procedure that reversed the times of light and dark by imposition of either 24 h of light or dark). This result satisfies one of the criteria for NAT to be considered part of a circadian driving oscillator. (c) In less than 24-h cycles [2 h of light in alternation with 2 h of dark (LD2:2), 4 h of light in alternation with 4 h of dark (LD4:4), and 6 h of light in alternation with 6 h of dark (LD6:6)], NAT activity rose in the dark during the chicks' previously scheduled dark-time but not the previously scheduled light-time of LD12:12. In a cycle where 8 h of light alternated with 8 h of dark (LD8:8), NAT activity rose in both 8-h dark periods, even though the second one fell in the light-time of the prior LD12:12 schedule.(ABSTRACT TRUNCATED AT 250 WORDS)     

The influence of light on cone disk shedding in the lizard, Sceloporus occidentalis

The lizard, Sceloporus occidentalis has an all-cone retina. In lizards maintained on a 12-h light:12-h dark (12L:12D) cycle, a burst of cone outer segment (COS) shedding occurs 2 h after light offset (1400 h circadian time) (Young, R.W., 1977, J. Ultrastruct. Res. 61:172-72). In this investigation, we studied the effect of different lighting regimes on the pattern of cone disk shedding in this species. When lizards entrained to a 12L:12D cycle are kept in constant darkness (DD), the shedding peak is advanced approximately 2 h and the magnitude of shedding is reduced to 30% of control. COS increased in mean length from 12 micron in controls to 14 micron after one cycle in DD and maintained this length during a second cycle in DD. In constant light (LL), disk shedding was damped to approximately 10% of control values. Shedding synchrony in LL was also perturbed and therefore cyclic shedding bursts could not be distinguished. During LL there was a much larger increase in COS mean length than in DD. After one cycle of LL, COS length was 15 micron and after two cycles COS length exceeded 17 micron. When lizards entrained to 12L:12D are shifted to a 6L:18D regimen, the first shedding cycle is biphasic. The first peak of 5% shedding occurs 2 h after light offset whereas a second larger peak (13%) occurs according to the entrained schedule (1400 h). This manipulation separates out a dark-triggered and circadian shedding component, which is normally superimposed in lizards entrained to a 12L:12D cycle. When entrained lizards are placed in 36 h of LL followed by light offset, the peak shedding response after light offset is double the control response (53% vs. 27%). After 30 h of LL (lights off 90 degrees out of phase), there is a biphasic shedding response similar to the 6L:18D regimen although this time the dark-triggered shedding component is greater in magnitude then the circadian component. COS turnover is estimated by extrapolating from COS mean length increases during LL. From this method we obtained a 2.7-micron increase in COS length during each day in LL. If COS growth is not augmented during LL, this would yield a 4-5-d turnover time for the average 12.5-micron COS.     

Selection for Phase Angle Difference of the Adult Locomotor Activity in Drosophila rajasekari Affects the Activity Pattern,Free-running Period,Phase Shifts and Sensitivity to Light

Laboratory selection for the phase angle difference (Ψ, the time from lights-off in a 24 h light–dark cycle to an activity onset) of the adult locomotor activity in Drosophila rajasekari reared in LD (light:dark cycles) of 12:12 h for 59 generations resulted in the early and late strains which differed in Ψ value by about 8 h. The selection affected the activity pattern in LD 12:12; in contrast to the wild-type, which had a broad plateau of activity pattern, the early strain exhibited a biphasic activity pattern with morning and evening peaks, whereas the late strain had a single evening peak which extended for 6 h in the dark. The selection significantly decreased and increased the activity level per cycle in LD 12:12, continuous darkness (DD) and continuous light (LL) in the early and late strains respectiv ely when compared to that of the wild-type (P < 0.01). The free running period (τ) in DD was shortened in the early strain and lengthened in the late strain by the shortening and lengthening of the activity phases respectively, the rest phases remained unchanged in these strains from that of the wild-type. Phase response curves (PRCs) were measured for light pulses in all strains, the PRC for the early strain was characterized by larger phase shifts when compared to the PRC for the late or for the wild type flies. The ability of τ to be progressively lengthened by increasing intensity of LL was increased and decreased in the early and late strains respectively. Moreover, the threshold light intensity of LL to generate arrhythmicity was apparently decreased in the early strain and increased in the late strain, suggesting that the selection for Ψ had differently affected the subjective light sensitivity in these strains.     

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

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

Alterations of lecithin-cholesterol acyltransferase activity during Plasmodium chabaudi rodent malaria

In non fatal and synchronous P. chabaudi rodent malaria, we observed at the stage of parasitaemia peak, an alteration (50 % decrease) in LCAT activity. This decrease could be related partly to hepatic dysfunction, and mainly to circulating inhibitors released into blood from parasitized red blood cells at each end of a schizogonic cycle. This decrease in LCAT activity, at this step of the infection, accounts for part of the dyslipoproteinemia previously observed (i.e., increase in cholesterol and phospholipids into VLDL-LDL and decrease in the EC series and delayed conversion of Tg-rich lipoproteins into LDL-HDL. At a prepatent step of infection and after the parasitaemia peak, the alterations observed in LCAT activity, (respectively, increase and then decrease), would be related to similar changes in levels of cholesterol of HDL associated to complex changes in triacylglyceride transport and metabolism.     

Advancing Schedules and Constant Light Produce Faster Resynchronization of Circadian Rhythms

House sparrows, Passer domesticus, were subjected to rotated light-dark (LD) cycles that consisted of repeated 8-hr advances or delays of 5 days of LD 8:16, with intervening 40-80 hr of constant dark or light. Sparrows reset the fastest (by the second cycle) when they were advanced with intervening constant light (LL). They reset the slowest (taking six cycles) when they were delayed with intervening constant dark (DD).     

Effect of Changing the Light/Dark Schedule, the Time of Onset of the Light or Dark Period, or the Daylength, on Rhythms of Epidermal Cell Proliferation

Rhythms of labeling and mitotic indices were studied in the hindlimb epidermis of the anuran tadpole Rana pipiens under different light/dark (LD) cycles and daylengths in order to examine the role of the various parameters of the lighting regimen in setting the periods of the rhythms and the timing of the cell proliferation peaks. Altering the time of, or inverting, the 12 h light period on a 24 h day resulted in phase shifting of basically bimodal circadian rhythms with peaks in the light and dark. Thus the cell proliferation rhythms were entrained to the LD cycle. These rhythms also entrained to noncircadian schedules since they lengthened on a 15L : 15D cycle and shortened on a 9L : 9D cycle, although the bimodal characteristic of a peak in the light and a peak in the dark remained. Studies of 18L: 6D and 6L : 18D cycles in which either the time of onset of light or dark was changed relative to the 12L: 12D control indicated that the onset of dark may regulate the timing of the labeling index peaks while the onset of light may determine the time of occurrence of mitotic index peaks. Control of the timing of labeling and mitotic index peaks by different parameters of the LD cycle suggests a mechanism for cell cycle regulation by the environmental lighting schedule. Analysis of the rhythms on all the cycles studied suggested that labeling index rhythms equal the length of, or twice the length of, the dark period. Mitotic index rhythms equal the daylfength or a multiple of the length of the dark period.     

The effect of dorsal raphe nucleus (DRN) lesions on the locomotor activity rhythm in mice.

The study employed electrical lesions of dorsal raphe nucleus (DRN) to determine the functional significance of those nuclei in the regulation of wheel-running activity rhythm in mice in light/dark (LD 12:12), constant light (LL), and constant dark (DD) conditions. The wheel-running records showed that raphe nucleus lesions resulted in few days' decrease in common activity and amplitude in LD. The activity phase was not compact but in fragmentary form, especially in DD condition. In some animals an earlier onset of activity after DRN lesion in LD was observed. In LL extension of the rhythm period occurred. Destruction of DRN only slightly modulates the wheel-running circadian rhythm in mice.     

Circadian organization of a subarctic rodent, the northern red-backed vole (Clethrionomys rutilus)

Arctic and subarctic environments are exposed to extreme light: dark (LD) regimes, including periods of constant light (LL) and constant dark (DD) and large daily changes in day length, but very little is known about circadian rhythms of mammals at high latitudes. The authors investigated the circadian rhythms of a subarctic population of northern red-backed voles (Clethrionomys rutilus). Both wild-caught and third-generation laboratory-bred animals showed predominantly nocturnal patterns of wheel running when exposed to a 16:8 LD cycle. In LL and DD conditions, animals displayed large phenotypic variation in circadian rhythms. Compared to wheel-running rhythms under a 16:8 LD cycle, the robustness of circadian activity rhythms decreased among all animals tested in LL and DD (i.e., decreased chi-squared periodogram waveform amplitude). A large segment of the population became noncircadian (60% in DD, 72% in LL) within 8 weeks of exposure to constant lighting conditions, of which the majority became ultradian, with a few individuals becoming arrhythmic, indicating highly labile circadian organization. Wild-caught and laboratory-bred animals that remained circadian in wheel running displayed free-running periods between 23.3 and 24.8 h. A phase-response curve to light pulses in DD showed significant phase delays at circadian times 12 and 15, indicating the capacity to entrain to rapidly changing day lengths at high latitudes. Whether this phenotypic variation in circadian organization, with circadian, ultradian, and arrhythmic wheel-running activity patterns in constant lighting conditions, is a novel adaptation to life in the arctic remains to be elucidated.     

Presence of circadian rhythms in the locomotor activity of a cave-dwelling millipede Glyphiulus cavernicolus sulu (Cambalidae, Spirostreptida)

The locomotor activity of the millipede Glyphiulus cavernicolus (Spirostreptida), which occupies the deeper recesses of a cave, was monitored in light-dark (LD) cycles (12h light and 12h darkness), constant darkness (DD), and constant light (LL) conditions. These millipedes live inside the cave and are apparently never exposed to any periodic factors of the environment such as light-dark, temperature, and humidity cycles. The activity of a considerable fraction of these millipedes was found to show circadian rhythm, which entrained to a 12:12 LD cycle with maximum activity during the dark phase of the LD cycle. Under constant darkness (DD), 56.5% of the millipedes (n = 23) showed circadian rhythms, with average free-running period of 25.7h ± 3.3h (mean ± SD, range 22.3h to 35.0h). The remaining 43.5% of the millipedes, however, did not show any clear-cut rhythm. Under DD conditions following an exposure to LD cycles, 66.7% (n = 9) showed faint circadian rhythm, with average free-running period of 24.0h ± 0.8h (mean ± SD, range 22.9h to 25.2h). Under constant light (LL) conditions, only 2 millipedes of 11 showed free-running rhythms, with average period length of 33.3h ± 1.3h. The results suggest that these cave-dwelling millipedes still possess the capacity to measure time and respond to light and dark situations. (Chronobiology International, 17(6), 757-765, 2000)     

Circadian rhythm of physiological color change in the amphibian Bufo ictericus under different photoperiods

Ectothermic vertebrates can exhibit chromatic adaptation to the environment. The aim of this work was to characterize the rhythm of color change of the amphibian Bufo ictericus, submitted to different photoperiodic regimens, as quantified by skin reflectance values. Adult males were maintained under a 12:12 Light/Dark (LD) cycle during seven days before every experiment. During the experiments, animals were kept in individual boxes for 8 days, under the following photoperiodic regimens: LD 12:12, LD 14:10, DD and LL. In the last 3 days of the treatments, the reflectance of the toad dorsal skins was measured at 3-h intervals, with the aid of a reflectometer. A 3-day time series consisting of 8 data points per day was obtained, which was analyzed by the Cosinor method. The analysis demonstrated that the reflectance values exhibited significant circadian oscillations in the regimens LD 12:12, LD 14:10 and DD, suggesting that the specie B. ictericus shows an effective circadian rhythm of color change. The reflectance values did not exhibit a significant circadian rhythm in the LL regimen showing that this is a condition not permissive for the expression of the color change rhythm.