Circadian rhythmicity in leukocytes immune responses in the freshwater snake,Natrix piscator

Circadian rhythm is observed in most of the physiological functions including immune response. The use of animal models other than mammals is useful in understanding how the vertebrate circadian system is organized and how this biological clock has changed throughout the vertebrate evolution. The present study was aimed to examine the circadian variability in the innate immune responses of leukocytes in the freshwater snake, Natrix piscator. Leukocytes were isolated and processed for total and differential leukocyte count, leukocyte phagocytosis, NBT reduction, nitrite production, and lymphocyte proliferation. Experiments were conducted for seven time points at 24, 4, 8, 12, 16, 20, and 24 h in three seasons – summer, winter, and spring. Cosinor analysis revealed that among leukocytes, only lymphocyte count showed circadian variation in summer. Percent phagocytosis and phagocytic index had significant rhythm of 24 h in winter and summer season, respectively. The acrophase of NBT reduction and nitrite release were coming during the evening hours in summer and during morning hours in winter and had circadian rhythmicity. A significant phase shift in nitrite release was observed with a trend of delayed phase shift from winter to summer. Circadian rhythm was also observed in lymphocyte proliferation (basal and concanavalin A stimulated). It is evident from the present study that animals synchronize their immune activity according to the time of the day and season. Enhancement of immune function helps the individual cope with seasonal stressors that would otherwise jeopardize the survival of animal.     

Seasonal changes in the circadian activity rhythm of light-dark (LD) and completely dark (DD) regimen in the mouse submandibular gland in the presence of light-dark (LD) and completely dark (DD) regimen

Influence of seasons on circadian activity changes and the influence of one and six weeks of DD upon these changes of acid phosphatase (AP) and beta-acetylglucosaminidase (AM) was studied in the submandibular gland of sexually mature male mice. Total enzyme activity was determined in tissue homogenates at four-hour intervals in March, June, October, and February under standard LD12/12 conditions and after one and six weeks of the DD regime. The rhythms were analysed according to cosinor method. Under constant lighting conditions the seasonal differences in the AM circadian activity rhythm were found. AP activity was considerably less influenced by seasonal changes. Both enzyme activity changes were independent of each other and each rhythm was differently influenced by DD. In the case of AM the most pronounced circadian activity changes had the highest amplitude and mesor occurred in summer. The strongest influence of DD upon this enzyme activity rhythm was observed in spring and summer especially after the first week, after six weeks the acrophase returned to the LD group value (spring). In autumn and winter the reaction to DD was different to that of summer and spring. For AP the circadian changes of activity were non-rhythmic in spring, whereas in all other seasons the acrophases occurred almost at the same time in the afternoon. In DD the activity rhythm significantly changed after six weeks. In all seasons, except spring the circadian rhythm of activity was not observed after six weeks of DD. An attempt was made to explain the observed results by the certain kind of genetic memory present in laboratory animals the neurohormonal system of which is influenced by seasonal changes.     

Circadian and seasonal rhythms in the radiosensitivity of mongrel white rats

From experiments in albino mongrel rats it is shown that the radiosensitivity of gamma-irradiated (60Co) animals follows a daily rhythm. A synchronization of the daily rhythms in radiosensitivity was noted in winter and during the first spring month which was impaired in April. Established were the rhythms of radiosensitivity for three seasons, i. e. winter, spring and summer, with the extremes in the dependence upon mean annual values varying significantly.     

Seasonal changes of locomotor activity patterns in the desert isopod Hemilepistus reaumuri

The locomotor activity rhythm of the desert woodlouse Hemilepistus reaumurii was monitored as individuals over the four seasons, under entraining and constant darkness conditions. Results indicated that individuals exhibited an endogenous rhythm of locomotor activity with a circadian period longer in spring than the other seasons whatever the regimen. Due to the important inter-individual variability, no significant difference in period was found between seasons as well as between regimens. Periodogram analysis revealed also an ultradian period around 12 h. Activity was mostly confined to the hours of the photophase or to those of subjective day. The activity pattern showed two main peaks. This bimodal circadian rhythm persisted for as long as the experiments were run, and was clearer in summer and autumn than in winter. The most clearly defined rhythms were found in spring and summer recordings, respectively, under entraining and free running conditions. The variation of activity patterns according to the seasons will be discussed.     

Seasonal variation in endogenous serum melatonin profiles in goats: a difference between spring and fall?

The pineal hormone melatonin serves as a signal of day length in the regulation of annual rhythms of physiological functions and behavior. The duration of high melatonin levels in body fluids is proportional to the duration of the dark period of the day. Due to the direct suppression of melatonin by light, the overt melatonin rhythm may differ from the endogenous rhythm driven by the hypothalamic circadian clock. The aim of this study was to find out possible differences between the overt and endogenous melatonin rhythms in goats during the course of a year. Seven Finnish landrace goats (nonlactating females) were kept under artificial lighting that approximately simulated the annual changes of day length at 60 degrees N. Blood samples for melatonin measurements by radioimmunoassay were collected at 2-h intervals during six seasons: winter (light:dark 6:18 h), early spring (10:14), late spring (14:10), summer (18:6), early fall (14:10), and late fall (10:14). Melatonin profiles were determined for 2 consecutive days, first in light-dark (LD) conditions and then in continuous darkness (DD). In LD conditions, the profiles matched the dark period with one exception: In winter, the mean peak duration was significantly shorter than the scotoperiod. In DD conditions, two types of endogenous melatonin patterns were found: a "winter pattern" (peak duration 13-15 h) in winter, early spring, early fall, and late fall, and a "summer pattern" (duration about 11 h) in late spring and summer. Thus, in equal habitual LD conditions in late spring and early fall (LD 14:10), the endogenous melatonin rhythms were not quite similar: The pattern in late spring resembled that in summer, and the pattern in early fall that in winter. These results suggest that, in addition to the light-adjusted overt melatonin rhythm, the endogenous rhythm of melatonin secretion varies during the course of a year.     

Influence of seasons on circadian rhythm of acid phosphatase and beta-acetyl-glucosaminidase activity in the mouse salivary gland and liver

Influence of seasons on circadian changes in acid phosphatase and beta-acetylglucosaminidase activity was studied in the liver and submandibular gland of sexually mature mice. Seasonal differences in circadian rhythm was found in both examined enzymes in both organs. These changes were independent from each other and each rhythm was differently subjected to seasonal changes. The lowest seasonal influence was observed in acid phosphatase in the salivary gland. Activity of the other enzymes changed in different degrees, having their acrophases at different times of the day and different rhythm intensity. The highest circadian activity changes measured by amplitude and mesor were observed in spring and summer whereas in the autumn and winter their activity had much weaker rhythms or even they disappeared completely. An attempt was made to explain the observed changes by changes in hormonal background and a certain kind of genetic memory pertaining to laboratory animals.     

Seasonal variation in blood concentrations of interleukin-6, adrenocorticotrophic hormone,metabolites of catecholamine and cortisol in healthy volunteers

We investigated seasonal changes in blood concentrations of interleukin-6 (IL-6), adrenocorticotrophic hormone (ACTH), metabolites of catecholamine (VMA, HVA, and 5-HIAA) and cortisol in humans. Eight volunteers were investigated at four times during the year (February, May, August and October) at latitude 35° N. The mean ambient temperature at the collection periods was higher in the order of summer > autumn ≈ spring > winter. Changes in mood were also monitored by a profile of mood states (POMS) questionnaire. The concentration of IL-6 was significantly higher in winter and summer than in spring and autumn. The concentrations of ACTH, HVA and VMA were significantly higher in summer. No seasonal variation was detected in cortisol. There were significant differences among the seasons in subscale tension and anger in the POMS questionnaire; the tension subscale showed significant differences between spring and autumn, with a higher score in spring. The results demonstrate that Il-6, ACTH, HVA and VMA exhibit statistically significant seasonal rhythms, which might have important diagnostic and therapeutic implications.     

Seasonal modulation of the 8-and 24-hour rhythms of ondansetron tolerance in mice

Ondansetron (Zophren((R))) is a serotonin 5HT(3)-receptor antagonist used primarily to control nausea and vomiting caused by cytotoxic chemo-and radio-therapy. Tolerance to this drug shows both 24 and 8 h periodicities. In this framework, this study aimed to determine whether these ondansetron tolerance rhythms are modulated by season. The chronotoxic effect of a fixed dose (3.5 mg/kg, i.p.) of the drug was investigated with reference to both time of the day and year dependencies. Season-related studies were performed on 560 male Swiss mice, 10 to 12 wks old, synchronized with L:D=12:12 for three weeks. During a 1 yr span (2005), four 24 h studies were performed with a single dosing time at 1, 7, 13, and 19 hours after light onset (HALO), respectively. Tolerance was assessed daily during a 40-day span after acute ondansetron treatment. Both chi(2) test and cosinor methods were used to analyze the time series data. Statistically significant dosing time-dependent changes were validated in both yearly and daily time scales. The 24 h mean survival rate peaked in spring (92%) compared to fall (72%), the 20% difference being statistically significant (chi(2) test with p<0.05 and cosinor with p<0.0001 for seasonal rhythm detection and with a peak time, ?,=April 3+/-6.6 days). A 24 h rhythm was also detected in each of the seasonal time points. However, the curve pattern was monophasic in fall as well as spring. In fall, a large amplitude (A) circadian rhythm was detected that peaked at 19 HALO, while in the spring, a small circadian rhythm was detected that peaked at 1 HALO. The curve pattern was biphasic in summer (with large A) and in winter (with a small A). The existence of two peaks of equal magnitude in winter (100% survival rate) and in summer (100% and 90%) suggests the presence of both circadian and ultradian rhythms rather than an ultradian component of the 24 h period. The seasonal modulation of ondansetron circadian chronotolerance seems to involve several rhythm parameters: season-related changes in the 24 h mean (M), amplitude (A), acrophase location (?), as well as bimodal curve patterns including the coexistence of rhythms with respectively 24 and 8 h periods in winter and summer. In conclusion, tolerance to ondansetron varies not only according to the 24 and 8 h periods but also according to seasons, which suggests the complexity of ondansetron toxicity rhythms. Seasonal modulation of ondansetron tolerance may also influence the strategies of chemo-and chrono-therapy, and it is therefore necessary to take it into account in clinical drug-delivery protocols to minimize side effects of cytotoxic anticancer and antiemetic agents.     

Interseasonal variation in the circadian rhythms of locomotor activity and temperature selection in sleepy lizards, <Emphasis Type="Italic">Tiliqua rugosa</Emphasis>

Few studies in non-mammalian vertebrates have examined how various effectors of the circadian system interact. To determine if the daily locomotor and behavioural thermoregulatory rhythms of Tiliqua rugosa are both controlled by the circadian system in different seasons, lizards were tested in laboratory thermal gradients in four seasons and in constant darkness. Circadian rhythmicity for both rhythms was present in each season, being most pronounced in spring and summer and least evident in autumn. Most lizards displayed a unimodal locomotor activity pattern across all seasons. However, some individuals presented a bimodal locomotor activity pattern in spring and summer. Seasonal variations in the phase relationships of both rhythms to the light:dark (LD) cycle were demonstrated. No seasonal differences in the free-running period lengths of either rhythm were detected, raising the possibility that a single circadian pacemaker drives both rhythms in this species. Our present results demonstrate that both rhythms are similarly controlled by the circadian system in each season. Although seasonal variations in the thermal preferences of reptiles both in the field and laboratory have previously been well documented, this study is the first to demonstrate circadian rhythms of temperature selection in a reptile species in each season.     

Seasonal variation in haematological parameters in male and female Tinca tinca

This study was designed to investigate any seasonal (spring, summer, autumn and winter) changes in haematological parameters in the blood of Tinca tinca measuring the number of red blood cells (RBC), haematocrit, white blood cells, and total plasma proteins.The results show significant changes in RBC and haematocrit in males comparing spring and summer with autumn and winter, whereas in females the RBC remained constant for all 4 seasons but the haematocrit decreased in autumn and winter compared to spring and summer. The white blood cells of male and female animals were significantly lower in spring and winter compared to summer and autumn. In male fish total protein contents significantly decreased in autumn and winter compared to spring and summer, whereas in females protein output significantly decreased in winter compared to the other seasons. The results indicate marked seasonal variation in the blood of male and female Tinca tinca. This variation may play a important protective role for the survival of the animals.     

Seasonal Modulation of the 8‐and 24‐Hour Rhythms of Ondansetron Tolerance in Mice

Ondansetron (Zophren®) is a serotonin 5HT₃-receptor antagonist used primarily to control nausea and vomiting caused by cytotoxic chemo‐and radio‐therapy. Tolerance to this drug shows both 24 and 8 h periodicities. In this framework, this study aimed to determine whether these ondansetron tolerance rhythms are modulated by season. The chronotoxic effect of a fixed dose (3.5 mg/kg, i.p.) of the drug was investigated with reference to both time of the day and year dependencies. Season‐related studies were performed on 560 male Swiss mice, 10 to 12 wks old, synchronized with L:D=12:12 for three weeks. During a 1 yr span (2005), four 24 h studies were performed with a single dosing time at 1, 7, 13, and 19 hours after light onset (HALO), respectively. Tolerance was assessed daily during a 40‐day span after acute ondansetron treatment. Both χ² test and cosinor methods were used to analyze the time series data. Statistically significant dosing time‐dependent changes were validated in both yearly and daily time scales. The 24 h mean survival rate peaked in spring (92%) compared to fall (72%), the 20% difference being statistically significant (χ² test with p<0.05 and cosinor with p<0.0001 for seasonal rhythm detection and with a peak time, Ø,=April 3±6.6 days). A 24 h rhythm was also detected in each of the seasonal time points. However, the curve pattern was monophasic in fall as well as spring. In fall, a large amplitude (A) circadian rhythm was detected that peaked at 19 HALO, while in the spring, a small circadian rhythm was detected that peaked at 1 HALO. The curve pattern was biphasic in summer (with large A) and in winter (with a small A). The existence of two peaks of equal magnitude in winter (100% survival rate) and in summer (100% and 90%) suggests the presence of both circadian and ultradian rhythms rather than an ultradian component of the 24 h period. The seasonal modulation of ondansetron circadian chronotolerance seems to involve several rhythm parameters: season‐related changes in the 24 h mean (M), amplitude (A), acrophase location (Ø), as well as bimodal curve patterns including the coexistence of rhythms with respectively 24 and 8 h periods in winter and summer. In conclusion, tolerance to ondansetron varies not only according to the 24 and 8 h periods but also according to seasons, which suggests the complexity of ondansetron toxicity rhythms. Seasonal modulation of ondansetron tolerance may also influence the strategies of chemo‐and chrono‐therapy, and it is therefore necessary to take it into account in clinical drug‐delivery protocols to minimize side effects of cytotoxic anticancer and antiemetic agents.     

Behavioral rhythms of the Japanese newts, Cynops pyrrhogaster, under a semi-natural condition

 Locomotor activity rhythms of the Japanese newt, Cynops pyrrhogaster, were recorded under a semi-natural condition using phototransistor systems. The daily activity rhythm showed a seasonal change: the locomotor activity was mainly diurnal (active during the daytime) from spring to early summer; mainly nocturnal (active during the night-time) from summer to autumn; and showed either a diurnal or nocturnal pattern, depending on the ambient temperature, in winter. To analyze the daily activity in detail, we observed the behavior of a group of newts (three males, three females) throughout 24 h. Four types of behavior (respiration, feeding, mating, and resting on the land) were observed. Each behavior had daily rhythms and showed a seasonal change. The behavior on land showed mainly a nocturnal or bimodal pattern (activity rhythms with two peaks) throughout the year and was more frequently observed in summer. Mating behavior also showed a seasonal change: high activity in spring, with peaks in the early morning and evening, but no activity in summer. Except in winter, feeding and respiratory behavior showed no seasonal changes in either activity period or frequency. Coupling between behavior and the clock seems to be weak in the Japanese newt because of indistinct daily rhythms and frequent phase changes of locomotor activity in water. Physical factors such as humidity and temperature seem to affect strongly the daily activity of the newts. Received: 21 April 1997 / Accepted: 1 September 1997     

雌性黑线仓鼠免疫功能的季节变化

温带地区动物的免疫功能常表现出季节性变化.本研究以捕获于2014年秋季、冬季和2015年春季、夏季的雌性黑线仓鼠为研究对象,通过注射植物血球凝集素(PHA)后足垫的肿胀程度反映细胞免疫能力;注射抗原匙孔血蓝蛋白(KLH)后,利用酶联免疫吸附(EHSA)方法检测血清免疫球蛋白IgG和IgM的浓度,反映体液免疫功能;检验冬...     

Quantitative and qualitative numerical alterations in splenocyte subpopulations of the cotton rat (Sigmodon hispidus) across seasons

Abstract

Previous research in our laboratory has documented seasonal alterations in humoral and cell‐mediated immunity in cotton rat (Sigmodon hispidus) populations. Based on these observations, we hypothesized that these seasonal differences in immune function were attributable in part to qualitative and quantitative numerical changes in specific splenocyte subpopulations. Lymphocytes were harvested from spleens of 139 cotton rats collected from a tallgrass prairie in central Oklahoma from December 1991 to September 1992. Unique splenocyte subpopulations were identified using fluorescein conjugated cell surface markers (concanavalin‐A, peanut agglutinin, soybean agglutinin, Helix pomatia agglutinin, pokeweed mitogen, and rabbit‐anti‐rat immunoglobulin‐G). All subpopulations examined were more abundant in fall and winter than spring and summer. Several plausible explanations for seasonal variation in abundance of splenocyte subpopulations are discussed.     

Circannual rhythms in the blood picture of laboratory rats

Circannual rhythms of the erythrocyte, total leucocyte, absolute segmented neutrophil and lymphocyte counts in laboratory Wistar rats were found. The highest mean counts occurred in autumn and at the begining of winter. The lowest counts werte recorded in spring and at the begining of summer. A similar circannual rhythm of absolute eosinophil counts in females was also found, but it was not present in males. Circannual rhythms of the relative lymphocyte, segmented neutrophil and eosinophil were not found. The course of seasonal variations in erythrocyte, total leucocyte, absolute segmented neutrophil and lymphocyte counts of males was similar to those of females.     

Seasonal changes in the cardiorespiratory responses to hypercarbia and temperature in the bullfrog, Rana catesbeiana

We assessed the seasonal variations in the effects of hypercarbia (3 or 5% inspired CO2) on cardiorespiratory responses in the bullfrog Rana catesbeiana at different temperatures (10, 20 and 30 degrees C). We measured breathing frequency, blood gases, acid-base status, hematocrit, heart rate, blood pressure and oxygen consumption. At 20 and 30 degrees C, the rate of oxygen consumption had a tendency to be lowest during winter and highest during summer. Hypercarbia-induced changes in breathing frequency were proportional to body temperature during summer and spring, but not during winter (20 and 30 degrees C). Moreover, during winter, the effects of CO2 on breathing frequency at 30 degrees C were smaller than during summer and spring. These facts indicate a decreased ventilatory sensitivity during winter. PaO2 and pHa showed no significant change during the year, but PaCO2 was almost twice as high during winter than in summer and spring, indicating increased plasma bicarbonate levels. The hematocrit values showed no significant changes induced by temperature, hypercarbia or season, indicating that the oxygen carrying capacity of blood is kept constant throughout the year. Decreased body temperature was accompanied by a reduction in heart rate during all four seasons, and a reduction in blood pressure during summer and spring. Blood pressure was higher during winter than during any other seasons whereas no seasonal change was observed in heart rate. This may indicate that peripheral resistance and/or stroke volume may be elevated during this season. Taken together, these results suggest that the decreased ventilatory sensitivity to hypercarbia during winter occurs while cardiovascular parameters are kept constant.     

The effects of season,daylight saving and time of sunrise on serum cortisol in a large population

Cortisol is critical for maintenance of health and homeostasis and factors affecting cortisol levels are of clinical importance. There is conflicting information about the effects of season on morning cortisol and little information on the effects of sunlight on population cortisol assessment. The aim of this study was to assess whether changes in median serum cortisol occurred in a population in conjunction with changing seasons, daylight saving time (DST) or time of sunrise. We analysed serum cortisol results (n?=?27 569) from a single large laboratory over a 13-year period. Subjects with confounding medications or medical conditions were excluded and data analysed in 15-minute intervals. We assessed the influence of traditional seasons, seasons determined by equinox/solstice, DST and time of sunrise on median cortisol. The median time of cortisol collection did not vary significantly between seasons. Using traditional seasons, median cortisol was lowest in summer (386?nmol/L) and spring (384?nmol/L) with higher cortisol in autumn (406?nmol/L) and winter (414?nmol/L). Median cortisol was lowest in the summer solstice quarter with significant comparative increases in the spring equinox quarter (3.1%), the autumn equinox quarter (4.5%) and the winter solstice quarter (8.6%). When cortisol was modelled against time, with adjustment for actual sunrise time on day of collection, for each hour delay in sunrise there was a 4.8% increase in median cortisol (95% CI: 3.9–5.7%). In modelling to explain the variation in cortisol over the morning, sunrise time was better than season in explaining seasonal effects. A subtle cyclic pattern in median cortisol also occurred throughout the months of the year. A 3-year trial of DST allowed comparison of cortisol in DST and non DST periods, when clock time differed by one hour. There was modest evidence of a difference in acrophase between DST and non DST cortisol (p?=?0.038), with DST peak cortisol estimated to occur 58?minutes later than non-DST peak. In summary, we found that time of sunrise and time of cortisol collection were the most important factors influencing median cortisol. For each hour later that the sun rose there was an almost 5% increase in median cortisol. There was significant seasonal variability with lowest cortisol noted in summer coinciding with the earliest sunrise time. This is an important finding which is consistent with the understanding that light is the major zeitgeber in entrainment of the human circadian cortisol rhythm. Our data suggest this rhythm is resistant to the arbitrary changes in clock time with daylight saving.     

黑线仓鼠体重和能量代谢的季节性变化

为阐明小型哺乳动物体重和能量代谢的季节性变化以及生理调节机制,将黑线仓鼠驯化于自然环境下12个月,测定其体重、能量收支、身体组织器官和血清瘦素水平的季节性变化。黑线仓鼠能量摄入和支出的季节性变化显著,冬季摄入能、基础代谢率(BMR)、非颤抖性产热(NST)显著高于夏季。体重季节性变化不显著,但身体组织器官重量呈现显著的季节性变化,冬季肝脏、心脏、肾脏以及消化道重量显著高于夏季。体脂含量夏季最高,冬季最低,冬季显著低于夏、秋和春季(P <0.01)。血清瘦素水平的季节性变化显著,夏季瘦素水平比秋、冬季分别高88.2% 和52.4% (P <0.05)。结果表明,黑线仓鼠体重维持季节性稳定,与“调定点假说”的预测不同;但脂肪含量和血清瘦素季节性变化显著,符合该假说。夏季血清瘦素升高具有抑制能量摄入的作用,冬季血清瘦素可能是促进代谢产热的重要因子,瘦素对能量代谢和体重的调节作用与气候的季节性变化有关。       

Daily and annual variations of free fatty acid, glycerol and leptin plasma concentrations in goats (Capra hircus) under different photoperiods

The aim of the study was to differentiate the impact of lighting conditions and feeding times on the regulation of lipid metabolism of goats under different photoperiods throughout the year. Seven Finnish landrace goats were kept under artificial lighting that simulated the annual changes of photoperiod at 60 degrees N (the longest light period 18 h, the shortest 6 h). Ambient temperature and feeding regime were kept constant. Blood samples were collected six times a year at 2-h intervals for 2 days, first in light/dark (LD) conditions and then after 3 days in constant darkness (DD). Significant daily variations were detected in the concentrations of plasma free fatty acids (FFA) and glycerol throughout the year. The nocturnal decrease and morning rise of FFA levels were related to the photoperiod, while the trough levels of glycerol were associated with the concentrate meal times. In DD conditions, FFA and glycerol rhythms were unstable. A significant seasonal variation was detected in the overall FFA and glycerol levels suggesting decreased lipogenesis in winter, increased lipolysis in spring and high lipogenesis in summer and fall. There was no significant daily rhythm in serum leptin levels, nor did the profiles in LD and DD conditions differ. The leptin level was slightly lower in early fall than in the other seasons, paralleling a small decrease of body mass in the goats after the grazing season. The daily or annual variations of FFA and glycerol levels were not clearly related to leptin concentrations. The results suggest that lipid metabolism of goats is regulated by light even in constant temperature and feeding conditions; however, no significant contribution of leptin levels could be shown.     

Monthly day/night changes and seasonal daily rhythms of sexual steroids in Senegal sole (Solea senegalensis) under natural fluctuating or controlled environmental conditions

In this paper we attempted to investigate the existence of daily fluctuations on plasma sexual steroids (17beta-estradiol, E(2) and testosterone, T) in Senegal sole (Solea senegalensis) females. We described the monthly day/night concentrations and seasonal daily rhythms in animals reared under natural photo- and thermo-period. In addition, the influence of the natural annual fluctuation of the water temperature on the plasma concentration of these steroids was investigated, using one group of Senegal sole under a natural photoperiod, but with an attenuated thermal cycle (around 17-20 degrees C) for one year. Although no significant day/night differences were detected in monthly samplings, the existence of an annual rhythm of E(2) and T (p<0.01) with an acrophase in February was revealed by COSINOR analysis. Maximum values were reached in March for both steroids (6.1+/-1.7 ng mL(-1) at mid-dark, MD and 4.0+/-0.6 ng mL(-1) at mid-light, ML for E2 and 1.4+/-0.4 ng mL(-1) at MD and 0.8+/-0.1 ng mL(-1) at ML for T) in anticipation of the spawning season (May-June). As regards seasonal daily rhythms, the presence of daily oscillations was revealed. At the spring solstice (21st March) a daily rhythm was observed for both steroids (COSINOR, p<0.01), with an acrophase at 20:00 h (E(2)) and at 21:08 h (T). In summer, autumn and winter no daily rhythms were observed due to the low steroid levels at those seasons. When Senegal sole females were submitted to an attenuated annual thermal cycle, the steroid rhythm disappeared (there was no surge in spring, as in the control group) and these fish did not spawn, despite being subjected to natural photoperiod conditions. This result underlined the importance of the natural annual fluctuation of water temperature and photoperiod on the synchronization of the spawning season and on the onset of steroidogenesis.