Influence of the seasons on the circadian rhythm of blood glucose and tissue glycogen in male Wistar rats

Adult, ad libitum fed male Wistar rats were adapted for three weeks to standard laboratory conditions and a 12:12 h light:dark regimen (7 a.m. to 7 p.m., 7 p.m. to 7 a.m.). In January, in April, in July and in October they were decapitated at 3-hour intervals and the glucose level was determined in their blood and the glycogen concentration in their liver, skeletal muscle, heart and white and brown adipose tissue. The influence of the seasons on circadian variation of the given parameters was not the same in all the tissues. Circadian variation of the liver and heart glycogen concentration was the least affected. The liver glycogen curves attained the maximum at the beginning of the light part of the day in all the seasons except the autumn; the rhythm was present everywhere. The rhythm was not demonstrated in the heart in the winter only and except for the summer the curves attained the maximum at the end of the dark part of the day. Pronounced seasonal changes were observed in the blood glucose and skeletal muscle glycogen concentration, whose curves reached the maximum in various parts of the day and where the rhythm was absent in the autumn - and in the case of skeletal muscle in the summer as well. The glycogen concentration in the two adipose tissues displayed the greatest seasonal changes. The shape of the circadian oscillation curves and their maxima in the various seasons were very different, with absence of the rhythm in the autumn. Determination of the influence of the photoperiodic reactions and of changes in hormones and the key enzymes regulating the main metabolic routes might help to explain the basis of circaannual variations in the metabolism of the laboratory rat.     

Circadian rhythm of blood glucose and tissue glycogen in fed and fasted rats

Blood glucose and tissue glucogen circadian rhythms were determined in male Wistar rats adapted 3 weeks to an artificial lighting regimen of 12 hours' light and 12 hours' darkness. Over a period of 24 hours we examined at 3-hour intervals the blood glucose concentration and the glycogen content of the liver, heart, skeletal muscle (quadriceps femoris) and white (epididymal) and brown (interscapular) adipose tissue of fed rats and rats fasted for 24 hours. The experiments were carried out in the autumn and the results were evaluated statistically by an analysis of variance and the cosinor test. The blood glucose level and the glycogen concentration in all the given tissues, in both fed and starved rats, displayed rhythmic oscillations with a 24- or 12-hour period in the course of the day, with the exception of glycogen in the white adipose tissue of fed rats, in which cosinor analysis failed to demonstrate any rhythm. One day's fasting did not affect the character of circadian rhythm.     

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.     

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.     

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.     

Metabolomics of the interaction between PPAR‐α and age in the PPAR‐α‐null mouse

Regulation between the fed and fasted states in mammals is partially controlled by peroxisome proliferator‐activated receptor‐α (PPAR‐α). Expression of the receptor is high in the liver, heart and skeletal muscle, but decreases with age. A combined ¹H nuclear magnetic resonance (NMR) spectroscopy and gas chromatography‐mass spectrometry metabolomic approach has been used to examine metabolism in the liver, heart, skeletal muscle and adipose tissue in PPAR‐α‐null mice and wild‐type controls during ageing between 3 and 13 months. For the PPAR‐α‐null mouse, multivariate statistics highlighted hepatic steatosis, reductions in the concentrations of glucose and glycogen in both the liver and muscle tissue, and profound changes in lipid metabolism in each tissue, reflecting known expression targets of the PPAR‐α receptor. Hepatic glycogen and glucose also decreased with age for both genotypes. These findings indicate the development of age‐related hepatic steatosis in the PPAR‐α‐null mouse, with the normal metabolic changes associated with ageing exacerbating changes associated with genotype. Furthermore, the combined metabolomic and multivariate statistics approach provides a robust method for examining the interaction between age and genotype.     

Regulation of Endogenous Glucose Production in Glucose Transporter 4 Over-Expressing Mice

Strategies to amplify whole-body glucose disposal are key therapies to treat type 2 diabetes. Mice that over-express glucose transporter 4 (Glut4) in skeletal muscle, heart, and adipose tissue (G4Tg) exhibit increased fasting glucose disposal and thus lowered blood glucose. Intriguingly, G4Tg mice also exhibit improved insulin-stimulated suppression of endogenous glucose production even though Glut4 is not present in the liver. It is unclear, however, if hepatic gluco-regulation is altered in G4Tg mice in the basal, non-insulin-stimulated state. The current studies were performed to examine fasting hepatic glucose metabolism in G4Tg mice and to determine whether gluco-regulatory adaptations exist in the non-insulin-stimulated condition. To test this question, phloridzin-glucose clamps were used to match blood glucose and pancreatic hormone levels while tracer dilution techniques were used to measure glucose flux. These techniques were performed in chronically-catheterized, conscious, and un-stressed 5h-fasted G4Tg and wild-type (WT) littermates. Results show reduced blood glucose, hepatic glycogen content, and hepatic glucokinase (GK) activity/expression as well as higher endogenous glucose production, glucose disposal, arterial glucagon, and hepatic glucose-6-phosphatase (G6Pase) activity/expression in G4Tg mice versus WT controls. Clamping blood glucose for 90 min at ∼115 mg/dLin G4Tg and WT mice normalized nearly all variables. Notably, however, net hepatic glycogen synthetic rates were disproportionately elevated compared to changes in blood glucose. In conclusion, these studies demonstrate that basal improvements in glucose tolerance due to increased uptake in extra-hepatic sites provoke important gluco-regulatory adaptations in the liver. Although changes in blood glucose underlie the majority of these adaptations, net hepatic glycogen synthesis is sensitized. These data emphasize that anti-diabetic therapies that target skeletal muscle, heart, and/or adipose tissue likely positively impact the liver.     

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.     

Circadian clock controlling egg hatching in the cricket (Gryllus bimaculatus)

Adult crickets (Gryllus bimaculatus) were maintained under a 12-h light:12-h dark cycle (LD 12:12). After oviposition, their eggs were incubated under different lighting regimens at 23 degrees C, and temporal profiles of egg hatching were examined. When the eggs were incubated in LD 12:12 or in DL 12:12 with a phase difference of 12h from LD 12:12, throughout embryogenesis, 88% to 97% of hatching occurred within 3 h of the dark-light transition on days 17 and 18 of embryogenesis; the phases of the egg-hatching rhythms in the LD 12:12 and DL 12:12 groups differed by about 12 h. In eggs incubated in constant darkness (DD) throughout embryogenesis, a circadian (about 24 h) rhythm of hatching was found, and the phase of the rhythm was similar to that seen in eggs incubated in LD 12:12, but not DL 12:12, throughout embryogenesis. When eggs that had been incubated in DD after oviposition were transferred to DL 12:12 in the middle or later stages of embryogenesis and were returned to DD after three cycles of DL 12:12, the rhythm of hatching synchronized (entrained) to DL 12:12. However, when eggs in the earlier stages of embryogenesis were transferred from DD to DL 12:12 and returned to DD after three cycles, 52% to 94% of hatching did not entrain to DL 12:12. To determine whether photoperiodic conditions to which the parents had been exposed influenced the timing of egg hatching, adult crickets were maintained in DL 12:12, and their eggs were incubated in LD 12:12, DL 12:12, or DD throughout embryogenesis. The egg-hatching rhythm was also found in the eggs incubated under these three lighting regimens. In DD, the phase of the rhythm was similar to that seen in eggs incubated in DL 12:12, not LD 12:12, throughout embryogenesis. The results indicate that in the cricket, the timing of egg hatching is under circadian control and that the circadian rhythm of hatching entrains to 24-h light:dark cycles, but only if the light:dark cycles are imposed midway through embryogenesis. Therefore, by midembryogenesis, a circadian clock has been formed in the cricket, and this is entrainable to light:dark cycles. In addition, the photoperiodic conditions to which the parents (probably the mothers) have been exposed influence the timing of hatching, suggesting that maternal factors may regulate the timing of egg hatching.     

Photoperiod,stress and the distribution of leukocytes in the peripheral blood ofNotophthalmus viridescens

Summary Adult newts,Notophthalmus viridescens, were maintained for 8 days at a constant temperature of 11.0±0.5 °C. In one series, the control animals were kept in constant darkness (DD), while the experimental newts were exposed to alternating 12-hour periods of light and dark (LD). In a second series, controlNotophthalmus lived in DD, and experimental animals lived in constant light (LL). In both series, the newts were sacrificed on the ninth day when blood smears were prepared. Differential counts of the leukocytes of animals that lived under the LD regimen were the same as those of controlNotophthalmus (Table 1). However, in newts that were maintained in LL, the neutrophils increased and the lymphocytes decreased relative to those types of cells in the controls (Table 1). Those changes indicate that continuous light constitutes stress for this species.Supported in part by a grant from the Committee on Research, Travel and Sabbatical Leaves, Colby College     

Diurnal variations of plasma lipids, tissue and plasma lipoprotein lipase, and VLDL secretion rates in the rat. A model for studies of VLDL metabolism

Circadian rhythms of plasma lipids and lipoproteins, lipoprotein lipase activities and VLDL secretion rates were studied in fed and food-deprived (12 h) male rats after a light/dark synchronization of 14 days. In ad libitum fed rats, a circadian rhythm of plasma triacylglycerol, blood glucose and liver glycogen was clearly identified. A rhythm was also identified for plasma cholesterol, but not phospholipids. The peak of plasma triacylglycerol occurred 2 h after the beginning of the light period (7.00 a.m.), and the nadir, 2 h after the beginning of the dark period (7.00 p.m.). The differences of plasma triacylglycerol at these two circadian stages were even more pronounced in food-deprived rats and were confined to the very-low-density lipoprotein (VLDL) fraction. Plasma post-heparin and heart and muscle lipoprotein lipase activities were 50-100% higher at 7.00 p.m., the time when plasma triacylglycerol were lowest, as compared to 7.00 a.m. Plasma post-heparin hepatic lipase and adipose tissue lipoprotein lipase activities, in contrast, did not change. VLDL secretion rates were somewhat higher at 7.00 a.m. compared to 7.00 p.m., but this difference was not significant. It is concluded that physiological variation of heart and muscle lipoprotein lipase together with small differences of VLDL secretion rates are responsible for normal range oscillations of plasma VLDL triacylglycerol levels.     

Abdominally entrained periodicities of testis and vas deferens activity in the Mediterranean flour moth

In a light-dark (LD) regimen, sperm, first apyrene and then eupyrene, start moving out of the fused testes of the Mediterranean flour moth, Anagasta kuehniella, toward the beginning of the scotophase. At 27° ± 2°C, the sperm mass remains in the proximal part of the vasa deferentia for 10 to 12 hr and then passes rapidly into the seminal vesicles, remains in these organs for about 5 hr, and is then transported to the ductus ejaculatoris duplex where it becomes available for ejaculation. The phases of sperm movement appear to be closely related to sperm development, and the reproductive activity of the moths. In isolated abdomens there is a significant reduction in the amount of sperm released from the testes, but normal periodicity of sperm release and movement continues in either LD or continuous dark (DD) regimens, and rapid phase shifting occurs when a LD regimen is reversed. All stages of sperm movement are disrupted in continuous light (LL), but normal periodicity is usually resumed when isolated abdomens of the LL moths are placed in LD or DD regimens. Normal periodicity also occurs in moths paralyzed with tetrodotoxin or procaine. Removal of any one of the four abdominal ganglia from LL moths does not prevent increased sperm release when the moths are placed in LD, though with each ganglion there is some disruption of the normal pattern of movement down the vasa deferentia. It is thought that the testes and vasa deferentia down to at least the seminal vesicles represent a semiautonomous complex in which periodicity is maintained by endogenous circadian activity in cells of the testes (and possibly the vasa deferentia) or more probably in a peripheral control center.     

Prolonged melatonin administration in 6-month-old Sprague-Dawley rats: metabolic alterations

The aim of this work was to evaluate the effect of prolonged melatonin administration on chosen metabolic and hormonal variables in male and female Sprague-Dawley rats. Melatonin was administered in tap water (4 microg/ml) daily from the 6th month of age. Rats were fed a standard type of diet ad libitum and were kept in a light regimen L:D--12:12h. The experiment was terminated after 12 weeks of melatonin administration. Melatonin decreased body mass during the whole experiment in females and from the 42nd day of the experiment in males. Relative heart muscle weight in females and absolute/relative thymus weight in males were increased after melatonin administration. Melatonin decreased glycaemia, heart muscle glycogen concentration in females and liver glycogen concentration in both sexes. Serum insulin concentration in males was decreased; serum corticosterone concentration was increased in both males and females. Serum triacylglycerol and heart muscle cholesterol concentration in females were decreased, however in males serum and heart muscle cholesterol concentration was increased. Liver phospholipid concentration in females was decreased and heart muscle phospholipid concentration in males was increased. Melatonin increased malondialdehyde concentration in heart muscle in males and in liver in both sexes. Melatonin induced prominent sex-dependent changes in both carbohydrate and lipid metabolism.     

Glycogen synthase kinases. Distribution in mammalian tissues of forms that are independent of cyclic AMP.

Extracts of rat tissues contain kinases which catalyze the conversion of glycogen synthease from the glucose 6-phosphate-independent (I) form to the glucose 6-phosphatate-dependent (D) form. These kinases were stimulated by adenosine 3':5' monophosphate (cyclic AMP). The glycogen synthase kinase activity ratio (activity in the absence of cyclic AMP divided by activity in the presence of cyclic AMP) varied from 0.28 to 0.97. The activity ratio for histone kinase in the same extracts ranged from 0.11 to 0.29. The levels of glycogen synthase kinase varied by a factor of 80 in the following rat tissues (given in order of decreasing enzyme activity): kidney, liver, stomach mucosa, lung, brain, heart, skeletal muscle, and adipose tissue. In the same tissues the levels of histone kinase varied by only a factor of 6 and did not correlate with the levels of glycogen synthase kinase. A modification of the method of Walsh et al. ((1971) J. Biol. Chem. 246, 1977-1985) was developed for purification of the heat-stable inhibitor of cyclic AMP-dependent protein kinases (inhibitor). The modified procedure resulted in good yields of highly purified inhibitor and was much simpler than the previously described procedure. This inhibitor completely inhibited cyclic AMP-dependent histone kinase activity of the extracts but much of the glycogen synthase kinase activity was not inhibited. The portion of glycogen synthase kinase that was insensitive to the inhibitor was: stomach mucosa, 95%; brain, 90%; liver, 82%; kidney, 81%; lung, 68%; adipose tissue, 65%; skeletal muscle, 63%; and heart, 54%. This histone kinase activity in the extracts and hte ratio of glycogen synthase kinase to histone kinase activity of purified catalytic subunit of the cyclic AMP-dependent protein kinase was used to calculate for each extract the glycogen synthase kinase activity contributed by the cyclic AMP-dependent protein kinase. Based on these calculations, the portion of the glycogen synthase kinase which was due to kinases independent of cyclic AMP was: kidney, 97%; liver, 91%; lung, 89%; brain, 87%, heart, 85%; stomach mucosa, 84%; adipose tissue, 38%; and skeletal muscle, 33%. A significant portion of the glycogen synthase kinase activity, but virtually none of the cyclic AMP-dependent histone kinase activity, of these extracts could be adsorbed to phosphocellulose columns. Liver extracts contained, in addition, a form of glycogen synthase kinase which was not adsorbed to phosphocellulose and which could be separated from the cyclic AMP-dependent protein kinase by additional chromatography. These studies demonstrate that kinases independent of cyclic AMP account for most of the glycogen synthase kinase activity of many tissues. The widespread distribution and high concentrations of these enzymes suggest that they are of physiological importance.     

Morphologic changes in the pineal gland of rats exposed to continuous darkness

We examined the pineal structure of rats exposed to constant darkness (DD) at light microscopic level. Two groups of rats were exposed to 12:12 light/dark cycle (LD) or DD from their prenatal ontogenesis and then for 3 months after birth. The gland structure of DD rats was observed to have an active appearance. Some of the observed pinealocytes with light nuclei from DD rats were determined to contain double nucleoli. Nuclear area and perimeter of both dark and light types were greater in rats kept in DD than in LD. Rats exposed to DD had more cells with light nuclei and lesser cells with dark ones than rats kept in LD. No significant differences in nuclear characteristics of intermediate type were found between rats kept in LD and those kept in DD. The activity of mammalian pineal can be altered by light conditions to which the animal is exposed.     

Societal synchronization in groups of rats or quail submitted to various lighting regimens

Groups of rats or of quail that had been previously synchronized in a light (L = 100 lux) dark (D) phase opposition (PO = LD and DL) were placed together in a L12:D12 or D12:L12 alternation or in continuous light (LL) or continuous darkness (DD). Emission of carbon dioxide (VCO2) which was continuously recorded in groups of individuals placed in respiratory chambers under controlled environmental conditions allows an index of their overall respiratory and metabolic exchanges to be found. In PO animals placed in LD or DL, the VCO2 circadian light dark synchronization comes back less quickly in rats than in quail, and the VCO2 variations at the light dark transitions (L-D and D-L) remain unchanged in rats, but are modified in quail. When PO animals are placed for 18 days in LL or DD, respiratory circadian rhythms disappear except in the grouped rats where they reappear after 4-5 days in DD.     

Effects of starvation on the tissular lipogenic rate in the obese Zucker rat.

The lipogenic rate of the obese rats was significantly higher than that of the lean rats in liver, white adipose tissue, skeletal muscle, heart and carcass. In the lean rats, a 24 h starvation period caused a significant decrease in the lipogenic rate of white adipose tissue and skeletal muscle while it increased that of heart, brain and brown adipose tissue. In the obese rats, starvation decreased the lipogenic rate in liver, skeletal muscle, white adipose tissue, brown adipose tissue and carcass. In spite of this, liver and skeletal muscle showed higher rates of lipid synthesis than the corresponding fed lean. It is concluded that starvation induces a qualitatively similar response in the obese versus the lean rat although the total lipogenic capacity of the animal is still higher.     

Effect of Different Light Regimes on Pre-Adult Fitness in Drosophila melanogaster Populations Reared in Constant Light for over Six Hundred Generations

Egg to eclosion development time and survivorship were assayed on four laboratory populations of Drosophila melanogaster that had been reared for over 600 generations in continuous light (LL) and constant temperature. The assays were performed in three environments: continuous light (LL), periodically varying light/dark cycles (LD 12:12 hr), and continuous darkness (DD). Development time in LL was significantly less than that in LD, which, in turn, was significantly less than that in DD, whereas survivorship did not differ significantly among the three treatments. The results indicate that individuals from Drosophila populations routinely maintained in LL do not suffer any deleterious effects of LL treatment on pre-adult fitness. Other studies on these populations have shown that free-running period (t) of the eclosion rhythm in DD is greater than that in LD. Our results are, thus, also consistent with the notion that development time may be a function of the free-running period.     

Effect of Different Light Regimes on Pre-Adult Fitness in Drosophila melanogaster Populations Reared in Constant Light for over Six Hundred Generations

Egg to eclosion development time and survivorship were assayed on four laboratory populations of Drosophila melanogaster that had been reared for over 600 generations in continuous light (LL) and constant temperature. The assays were performed in three environments: continuous light (LL), periodically varying light/dark cycles (LD 12:12 hr), and continuous darkness (DD). Development time in LL was significantly less than that in LD, which, in turn, was significantly less than that in DD, whereas survivorship did not differ significantly among the three treatments. The results indicate that individuals from Drosophila populations routinely maintained in LL do not suffer any deleterious effects of LL treatment on pre-adult fitness. Other studies on these populations have shown that free-running period (t) of the eclosion rhythm in DD is greater than that in LD. Our results are, thus, also consistent with the notion that development time may be a function of the free-running period.     

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.