Roberto Refinetti

Laboratory instrumentation and computing : comparison of six methods for the determination of the period of circadian rhythms

Data sets with known periodicity were used to compare the accuracy and noise tolerance of six methods of circadian period analysis: Fourier analysis, autocorrelation, Enrights (chi-square) periodogram, linear regression of onsets, interonset averaging, and acrophase counting. All methods except acrophase counting accurately detected the period of rhythms with pure waveform (cosine and square wave), whereas Fourier analysis and Enrights periodogram were superior to the other methods in the analysis of more complex waveforms (which more closely resemble actual circadian rhythms). The sensitivity of all methods was reduced by the insertion of random noise into the original data sets, but the methods of autocorrelation and Enrights periodogram were more tolerant of low signal-to-noise ratios than the remaining methods. Although particular situations may require particular methods, the results suggest that Enrights periodogram is the best choice as a general method for the determination of the period of circadian rhythms.

Contribution of locomotor activity to the generation of the daily rhythm of body temperature in golden hamsters

The locomotor activity and body temperature of 40 golden hamsters maintained under a 14L:10D light:dark cycle were studied by telemetry. Body temperature was found to be highly correlated with activity. On average, an increase from 0 to 200 units of activity was associated with a 0.7 degrees C increase in body temperature. However, body temperature during the dark phase of the light:dark cycle was 0.3 degrees C higher than during the light phase, irrespective of the activity level. These results indicate that, although activity can affect body temperature, the increase in activity during the dark phase is not the cause of the temperature rhythm. At least 30% of the total daily variation in body temperature is independent of variations in the activity level.

Comparison of the body temperature rhythms of diurnal and nocturnal rodents

The daily rhythm of body temperature of six rodent species, three of them nocturnal (Rattus norvegicus, Mesocricetus auratus, and Meriones unguiculatus) and three diurnal (Octodon degus, Spermophilus tridecemlineatus, and S. richardsoni), was studied by telemetry under a 14L:10D light-dark cycle. Significant interspecies differences were found in the mean level (range: 36.0-37.4 degrees C), amplitude (range: 2.1 degrees-4.3 degrees C), robustness (range: 115-185 Qp), and acrophase (range: 8.5-16.0 hr) of the temperature rhythm. In addition, a positive correlation (r = 0.79) was found between body mass and robustness of the rhythm across species. However, the only consistent difference between the diurnal and nocturnal species was the acrophase, which occurred during the day in the diurnal species (range: 8.5-11.5 hr) and during the night in the nocturnal species (range: 13.5-16.0 hr).

Homeostatic and circadian control of body temperature in the fat-tailed gerbil.

The interplay of homeostasis and circadian rhythmicity in the control of body temperature was studied in the fat-tailed gerbil (Pachyuromys duprasi). In a first study, the body temperature rhythm of 8 gerbils maintained at 24 degrees C under a 14L:10D light-dark cycle was studied by telemetry. Data from 9 other species of small mammals were also obtained for comparison. The gerbils were found to exhibit a robust rhythm of body temperature (the most robust of the 10 species) with a high plateau during the dark phase of the light-dark cycle and a low plateau during the light phase. In a second experiment, 5 gerbils were allowed to select the temperature of their environment by moving along a thermal gradient. The animals consistently selected higher ambient temperatures during the light phase of the light-dark cycle (when their body temperature was at the low plateau). In a third experiment, the metabolic response of 8 gerbils to an acute cold exposure was determined by indirect calorimetry. Greater cold-induced thermogenesis was observed during the light phase. The fact that the animals selected higher ambient temperatures and displayed greater cold-induced thermogenesis when their body temperature was lower contradicts the hypothesis that the body temperature rhythm is caused by a rhythmic oscillation of the thermoregulatory set point.

Rhythms of body temperature and temperature selection are out of phase in a diurnal rodent, Octodon degus

Individual Chilean degus (Octodon degus) were maintained in a thermal gradient (14 degrees C to 33 degrees C) for two or more weeks under a 14L:10D light-dark cycle. All animals showed robust daily rhythms of body temperature and locomotor activity consistent with the diurnal habits of the species. They also showed a robust daily rhythm of temperature selection 180 degrees out of phase with the rhythms of body temperature and locomotor activity. These results in a diurnal species extend previous findings of a 180 degrees phase difference between the rhythms of body temperature and temperature selection in nocturnal rodents. The asynchrony between these two rhythms implies an opposition between the circadian system (responsible for the generation of the body temperature rhythm) and the homeostatic system (responsible for the behavioral response of temperature selection that opposes the body temperature rhythm.

Homeostasis and Circadian Rhythmicity in the Control of Body Temperature

The concept of homeostasis is perhaps the most important concept in physiology. As described by Claude Bernard over a century ago, the constancy of the internal environment is a precondition for an independent life.] Yet, research conducted over the last fifty years has clearly indicated that practically all physiological variables are not regulated at a stable set point but are subject to regular daily or circadian oscillations.2 In thermal physiology, the double modulation of body temperature by a homeostatic and a circadian process has been interpreted as an indication that the daily rhythm of body temperature results from the concerted action of the two processes. Thus, the daily temperature rise has been explained as the result of a regulated elevation in the thermoregulatory ~et-point.~-’ This concept is diagrammed in FIGURE l(A). On the other hand, there is no a priori reason why the homeostatic control and the circadian control must be integrated. The two processes could have evolved separately, so that the thermoregulatory system and the circadian system could have independent control of the effector organs that regulate heat production and heat dissipation, as suggested in FIGURE l(B). Indeed, my research during the last six years provides strong support for the hypothesis of the separate control of body temperature by the homeostatic and the circadian system.

Circadian Rhythm of Temperature Selection in a Nocturnal Lizard

We recorded body temperature and locomotor activity of Tokay geckos (Gekko gecko) with free access to a heat source under a 14:10 light-dark cycle and in constant darkness. Under the light-dark cycle, the lizards selected higher temperatures during the light phase, when locomotor activity was less intense. Rhythmicity in temperature selection was transiently disrupted but later resumed when the animals were placed in constant darkness. These results demonstrate the existence of a circadian rhythm of temperature selection in nocturnal ectotherms and extend previous findings of a temporal mismatch between the rhythms of locomotor activity and temperature selection in nocturnal rodents.

Body temperature and behavior of tree shrews and flying squirrels in a thermal gradient

The daily rhythms of body temperature, temperature selection, and locomotor activity of tree shrews and flying squirrels were studied in a thermal gradient. In accordance with previous observations in other mammalian species, the rhythm of temperature selection was found to be 180 degrees out of phase with the body temperature rhythm in both species. Comparison of the amplitude of the body temperature rhythm in the presence and absence of the ambient temperature gradient indicated that behavioral temperature selection reduces the amplitude of the body temperature rhythm. This provides support for the hypothesis that the homeostatic control of body temperature opposes-rather than facilitates-the circadian oscillation in body temperature.

Daily rhythms of metabolic heat production, body temperature, and locomotor activity in golden hamsters

Abstract 1. 1. The body temperature, locomotor activity, and metabolic heat production of golden hamsters maintained under a 14L:10D light-dark cycle were measured at 6-min intervals over several days. 2. 2. The results confirm previous observations of a daily rhythm of locomotor activity and body temperature in this species and provide a record of daily rhythmicity in metabolic heat production. 3. 3. Data analysis indicates that the rhythm of locomotor activity affects but does not determine the rhythm of body temperature and that day-night differences in heat production are significant but play a much smaller role than differences in heat loss in the generation of the body temperature rhythm.

Rhythms of temperature selection and body temperature are out of phase in the golden hamster.

Body temperature, locomotor activity, and thermoregulatory behavior of freely moving golden hamsters maintained in a spatial thermocline were measured over several weeks. The thermoregulatory behavior of temperature selection exhibited a robust daily rhythm 180 degrees out of phase with the rhythms of body temperature and locomotor activity. However, the parameters of the body temperature rhythm (rhythmicity, mean level, and amplitude) were not significantly affected by the thermoregulatory behavior. Although the observed phase difference between the rhythms of temperature selection and body temperature might suggest that thermoregulatory behavior is modulated to oppose (rather than to defend) the rhythm of body temperature, the absence of effect of temperature selection on the parameters of the body temperature rhythm fails to reveal the physiological significance of such opposition. Further studies are necessary to establish the physiological significance of the rhythm of temperature selection.