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Variations of core-temperature rhythms in unrestrained sheep
Physiology & Behavior, Vol. 48, pp. 467-473. e Pergamon Press plc, 1990. Printed in the U.S.A.
0031-9384/90 $3.00 + .00
Variations of Core-Temperature Rhythms in Unrestrained Sheep ELMAR MOHR AND HANSDIETER
KRZYWANEK
Institute o f Veterinary-Physiology, Freie Universitiit Berlin, DIO00 Berlin 33, Koserstr.20, F R G R e c e i v e d 29 D e c e m b e r 1989
MOHR, E. AND H. KRZYWANEK. Variations of core-temperature rhythms in unrestrained sheep. PHYSIOL BEHAV 48(3) 467-473, 1990.--Variations of core-temperature rhythms occurring during a "normal" day (24-hour period without extraordinary challenges for organism) were studied in 5 male sheep. To record the influence of the metabolic processes in different organic systems, core temperatures were measured at various locations at the same time. To minimise any influences due to measurement or behavior (e.g., effects of isolation), a telemetric system was used for registration and animals were kept without restraint in their habitual herd. Particularly biphasic circadian rhythms were found, and feeding schedule as zeitgeber is discussed. Beyond that, independent from point of measurement short-time rhythms with wavelengths of 140 and 90 min were found. Therefore, an origin in the central nervous system can be supposed. Rhythms with wavelengths of 3 hours, 75 min and 1 hour were not stable throughout a whole 24-hour period and did not occur at all measurement points in the same intensity. A comparison of anatomical placements of the various measurement points leads to the realization of distinct organic functions as sources for these rhythms. Body temperatures
Circadian rhythms
Ultradian rhythms
Various points of measurement
Unrestrained sheep
2) Examining ultradian rhythms it seems to be useful to have a model which is less influenced by circadian zeitgebers as the light-dark cycle or the light intensity: in ruminants monophasic as well as biphasic and polyphasic circadian patterns of core temperature are described but could not be associated with a distinct light-dark cycle (35). Therefore the influence of these circadian zeitgebers could be considered of less importance than in other mammalian [e.g., the distinct influence of light-dark cycles on core temperature rhythms in cats (18)]. Maybe this should be considered together with the very special sleep structure of ruminants (34) but this is beyond the scope of the present study. However, sheep are gregarious animals and therefore, to avoid an influence caused by "social problems" (e.g., effects of isolation), it was necessary to keep the animals without restraint in their habitual herd. Due to these circumstances a telemetric system (23) was designed and assembled in our laboratory and adapted to these specific requirements. So, the purpose of this study is to examine the oscillations of core temperature in sheep, maintained in surroundings which are as true to their natural habitat as possible.
IT is well known that there are oscillatory patterns in core temperature with wavelengths of about 24 hours (2). On these circadian rhythms, not only those of core temperature, usually rhythms with relatively higher frequencies are superimposed. Although there is a wide range of differing wavelengths, all rhythms with a period shorter than a circadian rhythm are summarized as ultradian (12). Unfortunately, and in contrast to circadian rhythms, this definition gives neither a clue to their functional significance (1 l) nor any indications of possible origins. However, numerous investigations in such short-time fluctuations have demonstrated that at least they cannot only be considered secondary to changes of internal or external conditions (7, 19, 24). However, it seems that ultradian rhythms are more influenced by endogenous and exogenous factors than circadian rhythms are (3). Even during a " n o r m a l " day (which means: a 24-hr period without extraordinary challenges for the organism), it seems probable that the intensity of some of these influencing factors will change. Therefore their control on the organism is not constant, which means varying metabolic processes, as well as direct modification of thermoregulatory centres, may alter the core temperature. Because of this, the following questions arise: 1) do the endogenous and/or exogenous changes which occur during a " n o r m a l " day modify the ultradian core temperature pattern and, if this happens, 2) is it possible to associate such modifications with any varying organic function? For two reasons we took sheep as a model to answer these questions: l) Because it seems to be helpful to measure temperature rhythms at various locations in one animal at the same time, the animal has to be large enough to expect a temperature gradient between the different points of measurement.
METHOD Five male sheep of mixed Merino lineage were used, aged between 1 and 2 years with body weights from 38 to 53 kg. All animals were kept in a temperature-controlled stable (3 m • 5 m) at 17-20°C. The humidity of the atmosphere ranged between 60% and 80%. Since the light-dark cycle seems to be of less significance, experiments with ruminants could be performed at different seasons: for animals A and D they took place in April and May; this implies a natural light-dark cycle of about 15:9 hours with sunrise at about 6:00 a.m. (April) and 5:00 a.m. (May). The
467
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MOHR AND KRZYWANEK
TABLE 1 MEAN CORE TEMPERATURES (°C) WITHIN A PERIOD OF 8 DAYS Animal
Period
Site of Probes
A B C D E
April Jan. Jan. May Feb.
mediastinum (Tin) mediastinum (Tm) mediastinum (Tm) beside liver (TI ) ventral abdomen (Tv)
experiments with animals B, C and E were run in January and February. During this time an artificial light-dark cycle of 12:12 hours with light period starting at 6:00 a.m. was standard. To avoid any behavioral influences all animals were kept without restraint in their familiar flock. The flock (6 animals; one of them not being used for the experiment) were fed once a day (between 7:30 and 8:00 a.m.) with about 3 - 4 kg of mixed grain and 8 kg of hay; water was available ad lib. Core temperatures in the abdomen were measured using a radio telemetric system (23). Equal thermoprobes were surgically implanted at various sites: 1) in the left caudal abdomen, fixed to middle of the mediastinum (animals A, B and C; temperature indicated as Tin); 2) beside the lateral surface of the liver, right behind the last rib, fixed to the parietal peritoneum (animal D; temperature indicated as T 0 and 3) in the ventral abdomen among the coils of intestine (animal E; temperature indicated as Tv). In animal D an additional thermoprobe was installed inside the rumen (Tr). In animal E, a chronically external silicon fistula was implanted which was equipped with a stainless steel tube to achieve a good thermal contact with the surrounding. It was fixed parallel to the V. cava caudalis and a thermistor was placed inside it. Additionally an extracorporal transmitter with the same sensitivity could be utilised. A complete healing of the external fistula without any local reactions was achieved by wrapping a piece of DACRON (surgical tissue used for implantations) around the silicon pipe. The temperature registered in the fistula is indicated as Te. Measurements started 14 days after implantation. At this time there were no clinical signs of systemic disturbance. In each animal core temperatures were measured for 4 weeks. During this time data were continuously transmitted, every 30 sec digitised, and stored on disk. After the end of experiment, evaluations were made in three different ways: a) To obtain the time course of core temperatures, a graphic description of the received values was computed. Then a curve derived from a moving average of 15 consecutive values was evaluated to estimate the basic rhythms. b) To get more precise information about the time course during a 24-hour period educed data were computed using a technic which is described in the literature as " P L E X O G R A M " (1,26): Temperature values of 8 days were divided into intervals of 24 hours. Taking into account the possible deviations which were caused by trend, the mean -+ SD of the sum of first values of each interval was calculated, followed by the mean of the second values and so on. This leads to a mean time course of the respective core temperature of a 24-hour period. Because corresponding values of each interval are superimposed there will be an increase of the mean value if the signal is periodic and in phase with the examined 24-hour period; at the same time the noise will be reduced by a
Mean _+ SD 39.48 38.94 38.82 38.85 39.70
-+ 0.24 _+ 0.27 -+ 0.25 + 0.22 + 0.25
Minimum
Maximum
AT
38.63 37.80 38.20 38.27 38.78
40.04 39.52 39.53 39.50 40.35
1.41 1.72 1.33 1.23 1.57
reciprocal extinction. To describe the basic temperature rhythms, the mean values were then submitted to FFT (Fast Fourier Transformation) (27); after that a temperature curve was calculated using the first three harmonics (18). Comparisons between the various Plexograms with reference to coherence or phase shift were made using SPSS/PC+ - C o m p u t e r program (SPSS Inc.). c) To analyse the intensity of the several frequencies (spectral densities of the core temperatures time courses during a 24-hour period), raw data were first reduced to 70% of the original value by a digital filter. This process eliminates the very little oscillations of the time series. Because of this, aliasing effects, which means the falsification of the intensity of the lower frequencies by the very fast-moving components (31), were reduced. After this, slow-changing trends were removed by a "zero phase-shift autoregressive filter" (21). The residual time series was tapered by a HANN-window (cosine tapering) to reduce the discontinuity at both ends of the time series. This minimised the additional power from adjacent frequency bands (leakage). FFT of the modified time series was performed after trend removal and tapering, and the power spectrum was calculated by S(i) = 2h/N • IX(i)l 2 where N is the total number of points and h the sampling interval of the time series. X(i) represent the discrete FFT components with i = 1, 2, 3 . . . . . (N/2) (21). This way the power spectra of 8 consecutive days were performed and summarized. To get a 3-dimensional presentation of variations in the spectral density with respect to time of day, discrete power spectra were computed by shifting the starting point of the applied time series for one hour. RESULTS
Time Course of Core Temperatures Table 1 presents mean core temperatures -_+SD of the different animals, periods of data collection, the minimal-maximal values and AT, the difference between minimum and maximum. Mean temperatures ranged from 38.82°C to 39.70°C. Although there are significant differences between the mean values, the SDs are nearly identical. Figure 1 shows the time courses of core temperatures in three animals (A, D, E) at various points of measurement for 8 consecutive days. One partition of the horizontal axis represents a single day from 6:00 a.m. to 6:00 a.m. The vertical axis presents °C. The thicker line shows fitted values derived from the moving average and represents the basic rhythms. A is a time course of T m of animal A recorded from a thermoprobe which was implanted in the mediastinum of the left caudal abdomen. D~ and D 2 are the time courses of animal D measured at the lateral side of the liver (D~, TO and inside the rumen (D 2, T~) at the same time. E l and E 2 are curves of animal E simultaneously registered in a ventral abdominal region (E l, Tv) and beside the V. cava caudalis (E 2,
VARIATIONS OF CORE-TEMPERATURE RHYTHMS
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atures were 39.70---0.25°C and 39.72 ± 0.24°C, respectively. The temperature of Tf ranged between 38.97 and 40.46°C. Notice the coherence of the basic rhythms ( r = .4065, p < 0 . 0 0 1 ) which were superimposed in the case of the fistula by distinct "high-frequenc y " oscillations. An improvement of r by shifting one time series could not be found.
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Tf). It is remarkable that in all animals the basic rhythms are superimposed by oscillations of much higher frequencies. D~ and D 2 of Fig. 1 present a comparison between T] and intraruminal temperature T r. In the course of the demonstrated 8-day experimental period T r ranged from 38.01 to 39.92°C with a mean of 38.90 ± 0.23°C. This mean value is significantly higher ( p < 0 . 0 1 ) than the mean of T[ (see Table 1). The larger difference between minimum and maximum of T r is produced by high oscillations in the morning time. The correlation coefficient between the basic rhythms is r = .4821, significant with p < 0 . 0 0 1 . It will increase to r = .5618 after shifting D 2 time series for + 185 min. This means there is a clear coherence between the basic rhythms but there is also a phase shift between the time courses of T~ and T~; the drift of the temperature occurs in T t 185 min later than in T r. E~ and E e of Fig. 1 show the time courses of core temperatures in animal E. They were measured in the ventral abdominal region among the coils of intestine (Tv) and, at the same time, beside the V. cava caudalis (external silicon fistula, Tf). The mean temper-
A continuous phase shift between the particular time series used to form a Plexogram could obliterate the underlying signal. To exclude this possibility we calculated the coherence between the single time series. A mean correlation coefficient of r = .856 --- .078 was deduced. This implies that there can only be slight phase differences and therefore it is admissible to compute Plexograms. Table 2 presents mean core temperatures---SD, minimalmaximal values and the differences from high to low of the averaged time courses. Figure 2 represents the different Plexograms of the previous time courses of core temperatures. During a 24-hour period there are multiple peaks and troughs caused by the fast-moving components. The fitted curves are derived from the first three harmonics. This means they represent the influence of the rhythms with a wavelength of 24, 12 and 8 hours. These slow moving components showed a monophasic rhythm in animals A and D (Fig. 2, A and D 0. In the other sheep there were more or less biphasic courses as presented in animal E (Fig. 2, El). In animal A the highest values occurred at about 3:00 a.m. In the other animals the highest values could be found at noon. Note that there is a trough of core temperatures in most animals in early morning. A comparison of T l (beside liver) and T r (intraruminal temperature) is demonstrated in Fig. 2 by D~ and D 2. There is a clear rise of T r immediately after feeding (7:30 a.m.). It increases by a mean value of 0.6°C to a mean temperature of 39.38°C. This rising is followed by drastic changes in T r which last for about 10 hours (until 6:00 p.m.). It is remarkable the T~ (more than Tr) starts rising 1/2 an hour before feeding time. The correlation coefficient between T l and Tr is r = .5206 (p<0.001) for the basic rhythms. It will increase to r = .9284 by shifting D 2 values for +207 min. This nearly identical phase difference between D~ and D 2 in Plexogram data and in time course data supports our opinion that it is admissible to calculate Plexograms. Figure 2 E l and E 2 demonstrate the differences due to various points of measurement in one animal. E] shows T v (ventral abdomen), E 2 the "fistula temperature" Tf. Although there is a difference in the amplitude of the basic rhythms (fitted curve), especially during night time, there is also a clear coherence in the time course ( r = .7535, p < 0 . 0 0 1 ) . The phase shift is noticeable. Minimal and maximal values occur in T v at 7:30 a.m. and 11:30 a.m., respectively. In Zf they are found at 6:20 a.m. and 10:30 a.m. This means that the time difference between minimum and
TABLE 2 MEAN CORE TEMPERATURES (°C) OF "ONE AVERAGE DAY" Animal A B C D E
Site of Probes mediastinum (Tm) mediastinum (Tin) mediastinum (Tin) beside liver (T~) ventral abdomen (Tv)
Mean ± SD 39.48 38.94 38.82 38.85 39.70
±- 0.12 --- 0.11 ± 0.13 ± 0.14 ± 0.11
Minimum
Maximum
AT
39.22 38.70 38.55 38.58 39.40
39.70 39.15 39.14 39.13 39.97
0.48 0.45 0.59 0.55 0.57
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The highest intensities were found between 5 and 22 cycles/ day, which means wavelengths of 288 rain and 65 rain. There were peaks at 10-11 cycles/day, 14-15 cycles/day and 20 cycles/ day, which correspond to wavelengths of about 137 rain, 99 rain and 72 rain. However, the highest peak only accounted for 7% of
the whole intensity, To get more information about the intensity modification of the several frequencies with respect to time of day, the plots shown in Fig. 4 were computed. The intensity (in %) of the different frequencies (cycles/day) was plotted against time of day. The first before T,.. line represents the spectrum starting at 6:00 a.m. For the successive lines the starting point was shifted for one hour. FFT Analysis The upper part presents the variations of core temperatures, The intensity of the several frequencies which are forming the exemplified in T~ of animal D. Nearly identical peaks as shown in time course of core temperature (T~) during a day are shown in animal C (Fig. 3) occur, but this plot shows that the intensity is Fig. 3. This is a power density spectrum of animal C, derived from varying during a 24-hour period. Note that the intensity of the 8 consecutive days. Similar spectra were achieved from the other 10-cycles/day frequency is nearly stable throughout the whole animals. The y-axis is graduated in percentages (of all intensities) period. In the frequencies of T~ (Fig. 4, lower part) which were and the x-axis represents the several frequencies (measured in recorded simultaneously, the same peaks as in T~ appear. Signifcycles/day) up to the 32nd harmonic (e.g., the highest presented icant differences between T~ and Tr occur only from 8:00 a.m. to frequency of 32 cycles/day equals a wavelength of 45 rain). 6:00 p.m.: the intensities of the l- and 2-cycles/day frequency of Tr are moving through a maximum within this time. A statistical test (t-test with p < 0 . 0 l ) between the corresponding intensity Intensity values of Tr and T~ could not indicate further differences. Figure 5 presents a comparison of Tv frequencies measured in a ventral abdominal region (upper part) and next to the V. cava caudalis (' 'fistula" temperature T~, lower part). In both cases there are again peaks at 10-I l cycles/day, 15 cycles/day (especially in the evening and early morning time) and 20-21 cycles/day. Additional to that, in the "intraabdominal" temperature there is a 1 10 20 30 peak at 24-25 cycles/day (wavelength of about 60 rain). In Tf the cycles/ day peak at 5-6 cycles/day (wavelength of about 265 rain) is more impressive than in T~. In general, Tf rhythms seem to be more stable than those measured in the ventral abdominal region. FIG. 3. Power density spectrum of animal C (computed over 8 consecutive days) (wavelength = 1440 rain/x).
maximum is about 4 hours in both cases. However, because of the phase shift, temperature changes occur in Tt. about one hour
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DISCUSSION The experiments reported above were performed in order to obtain precise information on the variations of core temperature in unrestrained sheep. Because of the proximity of the thermal regulatory centres to both thalamic and reticular systems, variations of core temperature might be influenced by physical exercises as well as by "psychic stress" (sensory input and vigilance). Earlier investigations showed that the manipulations during rectal measurement could modify core temperature in farm animals (23,33) and in laboratory animals (32); it is reported that insertion of a rectal thermoprobe for only one minute could elevate the animal's core temperature for 70 min (25). To minimise all these influences, a telemetric system for measuring was used, our animals were kept without restraint in their familiar flock and stable and there were no changes in feeding or housing conditions throughout observation time. It is well known that there are temperature gradients within the deeper body tissues. The best point to measure mean core temperature is where the blood leaves the left side of the heart (5). However, the temperature measured at the side of the abdominal aorta can also be considered as a good indicator of core temperature (17). Due to surgical difficulties, in our recent studies the temperature was measured next to the abdominal vena, but we assume that there will be a heat exchange between aorta and vena, which are located close to one another. We admit that this point of measurement might be more influenced by heated blood due to muscular activity of the rear legs. However, there was not much locomotive activity because the animals were kept in their accustomed stable during the whole experimental period. The clear coherence between time courses of Tv and "fistula temperature" Tf (Fig. 2, E~ and E2) supports the above considerations. All mean values of core temperatures (38.82-39.70°C, Table
471
1) were within the physiological range from 38.4 to 40.0°C for sheep over 1 year (16). The minimal differences to the lower values given especially for Merino rams (38.80-39.06°C) (13) can be explained by a temperature gradient between an intraabdominal and a rectal point of measurement. The daily mean difference (see Table 2) of 0.53°C corresponds well with the values given for cattle (0.45°C) (35) and for sheep (16). The nearly identical SDs of the core temperatures were astonishing. They indicate similar oscillations in all animals in spite of significant differences between the core temperatures. Contrary to other opinions (6,30), an association between the levels of core temperatures and the times of data collection was not possible. In our experiments seasonal effects might have been reduced by housing conditions (i.e., temperature-controlled stable for 24 hours each day). With regard to circadian rhythms, our findings are similar to those of other authors who found biphasic core temperature rhythms in cattle (29) and sheep (16). The demonstrated basic rhythm of E 1 (Fig. 1) and most of the Plexograms confirm this: a trough in the morning followed by a rise until early afternoon and another peak between early evening and early morning. On the other hand, our findings also support those results which describe monophasic and aphasic patterns in cows (4,35) and calves (28) (see Figs. 1 and 2A, D 0. Therefore our study cannot conclude definitely whether monophasic or biphasic basic rhythms are predominant in ruminants. The rise of T r of 0.6°C immediately after feeding (Fig. 2, D2) accounts for most of the daily mean difference of T r (0.73°C) and is therefore one of the major causes of variation of core temperature. However, this is only a small increase in comparison to other results found in cattle [1.63°C within 15 min after feeding of 8 pounds of mixed grain (9)]. We attribute this difference primarily to the diminished quantity of food in our experiments (4 kg of mixed grain to a flock of 6 animals, hay was not available to the flock the whole time). Because of this, heat production caused by intraruminal fermentation was reduced. During a 24-hour period highest values of T r were reached about one hour after feeding and were followed by dramatic variations of T r which lasted for about 10 hours (6:00 p.m.). They should be considered as an effect of drinking. Analogous effects were described by many authors (8-10). For the remainder of the day and until next feeding time, there were only smooth variations in T r. The correlation analysis indicates that T] was following the drift of Tr with a phase shift of about 3.2 hours. Furthermore, the phase difference of 1 hour between T v and Tf (Fig. 2, E l, E2) indicates that there was a source of heat closer to the measuring point of Tf than to that of Tv. The time of the rise of Ty(8:30 a.m.) in relation to feeding time (8:00 a.m.) suggests the rnmen as the most probable source. These two findings may support the hypothesis that feeding time influences the rhythms of core temperature [especially some biphasic pattems (29) correspond well with feeding times]. On the other hand, there were monophasic or polyphasic temperature patterns (35). Further studies should examine whether the feeding schedule can be a sort of zeitgeber for core temperature rhythms or not. Core temperature is a result of all controlling mechanisms which are involved in heat production and heat loss (2). We know that fluctuations of physiological functions--like core temperat u r e - a r e not only a response to changing environmental conditions nor simply produced by various internal activities (19). They are also caused by one or several internal oscillators. Different physiological systems exhibit ultradian oscillations with frequencies between 1 and 2 hours: cortisol rhythms with a predominant period of 85 to 90 rain in monkeys (14); alertness, renal excretions and gastric activities with a frequency of about 90 min (20); in dairy cows core temperature rhythms with wavelengths of 90 min (4); coherent ultradian rhythms of mean arterial pressure and heart
472
MOHR AND KRZYWANEK
rate with a period of 1-2 hours in dogs (21). Our Fourier analysis of core temperatures in sheep demonstrates that the main ultradian oscillations were also between the 1 and 3 hour frequencies: like in other physiological systems there is a dominant intensity at a wavelength of about 90 min and additionally, there are rhythms which are centred around 140 min. Regarding the intensity of ultradian rhythms with respect to time of day, one result of the present study is that independent of the point of measurement in all our animals the rhythms with 90 min (16 cycles/day) and especially those with 140 min (10 cycles/day) are very stable through an entire 24-hour period. Concerning those ultradian rhythms where the intensity is varying during a 24-hour period, it is necessary to take into account the diverse anatomical placement of the thermoprobes: In intrarnminal data (Fig. 4, loser case) most modifications occur between 8:00 a.m. and 6:00 p.m. During this time the 24-hour (1 cycle/day), 12-hour (2 cycles/day), 3-hour (8 cycles/ day) and the 75-min (18-20 cycles/day) rhythms have their greatest intensities. Taking into account the Plexogram results, it can be concluded that this is caused by food intake. Because the 3-hour and especially the 75-min rhythms show a similar variation in time, it seems that the origin of all these rhythms are the rumen and its microbial activity. [It should be mentioned that there is also an increase of the 75-min rhythm in the abdominal region (Fig. 5, upper case) starting at midnight. But this happened only in the demonstrated case and only during this experimental period. Therefore it was not taken into account.] Comparing the abdominal temperature rhythms (Fig. 5, upper part) with the others, there was another frequency (60-min rhythm = 24 cycles/day) which is very stable. This special rhythm could not be found with the same intensity (but sometimes with less intensity) at the other locations. Therefore the source of this rhythm may be associated with local processes in the ventral abdominal region which lasted throughout the whole 24-hour period. Probable candidates could be the various activities and/or the blood circulation of the gut. It is even more plausible if we take into consideration that the intestinal digestion of the ruminants is not depending on feeding-time. [Furthermore, there are findings that in unrestrained dogs the heart rate and the mean arterial pressure oscillate with frequencies between 1 and 2 hours (21).] Conclusions Based upon long-term continuous temperature monitoring at various sites of the abdomen, using a telemetric system we
conclude that the deep body temperature in unrestrained noninfluenced sheep is characterized by 1) a predominance of mostly biphasic circadian rhythms with different amplitudes (maximal mean value: 0.53°C) and 2) nearly stable ultradian rhythms with wavelengths of 140 and 90 rain which were not dependent on the point of measurement. Since not all ultradian rhythms are stable during a 24-hour period, different origins for the stable and nonstable rhythms seem probable. The oscillator/s of the stable rhythms is/are characterized by a) being only slightly influenced by time of day (and therefore, for example, by light cycles), b) acting on different deep body tissues at the same time and c) not being modified by local processes in the abdomen. Especially because of the observations b) and c), the origin of the stable rhythms can be presumed to be in the central nervous system. With regard to the different locations of measurement it seems possible to associate some of the other, nonstable ultradian rhythms with physiological processes: a) increased microbial activity in the rumen after feeding seems to produce two deep body temperature rhythms. One with a wavelength of about 3 hours and another of 75 min, both of which last for nearly 10 hours. (Nevertheless this hypothesis has to be verified by further investigations.) b) There is an additional stable rhythm with a wavelength of 1 hour which may be caused by intestinal acti~ty. The comparison of the various points of measurementlshowed that the registration of the core temperature next to one ofAhe main abdominal blood vessels gives not only the best reading of mean core temperature but also gives the best general view of the occurring ultradian rhythms. However. due to the fact of heat transport by blood convection, there is a difficulty that temperature variations measured at this point may be influenced by changes in heart rate and mean arterial pressure. It will be an aim of further investigations to elucidate the role of heart rate and mean arterial pressure regarding core temperature rhythms and also to verify the hypothesis that ruminal activity triggers the 3 hour- as well as the 75-min rhythm. ACKNOWLEDGEMENTS This study was supported by Deutsche Forschungsgemeinschaft Grant Mo 450/1-1. The authors are very indebted to Professor Dr. E. Hentschel, Institut ftir Veterin/ir-Anatomie, FU-Berlin for the performance of the surgical implantations. They also thank Professor Dr. C. Jessen, Physiologisches Institut der Universit/it Giessen, for his helpful comments on the technic of measurement. The authors are also very indebted to Mrs. K. Upmeyer and Mr. M. Weddington for revision for language.
REFERENCES 1. Arbogast, B. Methoden for die chronobiologische Analyse von Langzeit-Elektroenzephalogrammen und einige Anwendungen in der Paediatrie. Med. Diss. Muenchen; 1984. 2. Aschoff, J. Der Tagesgang der Krrpertemperatur beim Menschen. Klin. Wschr. 33:545-551; 1955. 3. Aschoff, J.; Gerkema, M. On diversity and uniformity of ultradian rhythms. In: Schulz, H.; Lavie, P., eds. Ultradian rhythms in physiology and behavior (Experimental brain search Supplementum 12). Berlin: Springer-Verlag; 1985:321-334. 4. Bitman, J.; Lefcourt, A.; Wood, D. L.; Stroud, B. Circadian and ultradian temperature rhythms of lactating dairy cows. J. Dairy Sci. 67:1014-1023; 1984. 5. Bligh, J. The relationship between the temperature in the rectum and of the blood in the bicarotic trunk of the calf during exposure to heat stress. J. Physiol. 136:393-403; 1957. 6. Bligh, J.; Ingram, D. L., Keynes, D.; Robinson, S. G. The deep body temperature of an unrestrained welsh mountain sheep recorded by a radiotelemetric technique during a 12-month period. J. Physiol. 176:136-144; 1965.
7. Bossom, J.; Natelson, B. H.; Levin, B. E.; Stokes, P. E. Ultradian rhythms in cognitive functions and their relationship to visceral processes. Physiol. Behav. 31:119-123; 1983. 8. Cunningham, M. D.; Martz, F. A.; Merlian, C. P. Effect of drinking-water temperature upon ruminant digestion, intraruminaI temperature, and water consumption of nonlactating dairy cows. J. Dairy Sci. 47:382-385; 1964. 9. Dale, H. E.; Stewart, R. E.; Brody, S. Rumen temperature I. Temperature gradients during feeding and fasting. Cornell Vet. 44:368-374; 1954. 10. Dracy, E.; Essler, W. O.; Jahn, J. R. Recording intrareticular temperatures by radiosonde equipment. J. Dairy Sci. 46:241-242; 1963. 11. Gerkema, M. P.; Daan, S. Ultradian rhythms in behavior: the case of the common vole (Microtus arvalis). In: Schulz, H.; Lavie, P., eds. Ultradian rhythms in physiology and behavior (Experimental brain research Supplementum 12). Berlin: Springer-Verlag; 1985:11-31. 12. Halberg, F.; Engeli, M.; Hamburger, C.; Hillman, V. D. Spectral resolution of low-frequency, small-amplitude rhythms in excreted
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14. 15.
16. 17. 18. 19. 20. 21. 22. 23.
17-ketosteroids: probably androgen-induced circaseptan desynchronization. Acta Endocrinol. 103:1-54; 1965. Hertrampf, B.; Pfeiffer, A.; Pourhassan, H.; Schlikker, P.; Bredenfeld, H.; Wischer, E.; Rosing, W. Untersuchung tiber die normalen rektalen Tagestemperaturen bei verschiedenen Schafrassen. Dtsch. Tier~ztl. Wochenschr. 85:189-272; 1978. Holaday, J. W.; Martinez, H. M.; Natelson, B. H. Synchronised ultradian cortisol rhythms in monkeys: Persistence during corticotropin infusion. Science 198:56-58; 1977. H6rnicke, H. Thermophysiologie. In: Scheunert, A.; Trautmann, A.; Wittke, G., eds. Lehrbuch der Veterin~-Physiologie 7. Aufl. Berlin: Verlag Paul Parey; 1987:142-159. Hunsaker, W. G.; Reiser, B.; Wolynetz, M. Vaginal temperature rhythms in sheep. Int. J. Chronobiol. 4:151-154; 1976. Jessen, C.; Feistkorn, G. Some characteristics of core temperature signals in the conscious goat. Am. J. Physiol. 247 (Regulatory Integrative Comp. Physiol. 16):R456-R464; 1984. Johnson, R. F.; Randall, W. Freernnning and entrained circadian rhythms in body temperature in the domestic cat. J. Interdiscipl. Cycle Res. 16:49-61; 1985. Kleitmann, N. Sleep and wakefulness. Chicago: University of Chicago Press; 1963. Lavie, P. Ultradian rhythmicity (cicra 90 min.) in physiologic processes--Evidence for multioscillatory phenomena. J. Interdiscipl. Cycle Res. 11:171-177; 1980. Livnat, A.; Zehr, J. E.; Broten, T. P. Ultradian oscillations in blood pressure and heart rate in free running dogs. Am. J. Physiol. 246 (Regul. Int. Comp. Physiol. 15):R817-R824; 1984. Mangold, E. Handbuch der Ern~hrung und des Stoffwechsels der landwirtschatlichen Nutztiere. Berlin: Springer-Verlag; 1932:49-84. Mohr, E.; Leuschner, T. Microcomputer-gesttitztes Telemetriesystem zur Erfassung ultradianer Temperaturrhythmik biem GroStier. (A microcomputer assisted telemetric system for recording of ultradian core temperature fluctuations in farm animals.) J. Vet. Med. A 36:331-339; 1989.
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24. Moore-Ede, M. C.; Sulzman, F. M.; Fuller, C. A. Circadian organisation in the squirrel monkey: the internal coupling between oscillators. In: Suda, M.; Hayaishi, O.; Nakagawa, H., eds. Biological rhythms and their central mechanism. Amsterdam: Elsevier/ North-Holland Biomedical Press; 1979:405-419. 25. Poole, S.; Stephenson, J. E. Core temperature: Some shortcomings of rectal temperature measurements. Physiol. Behav. 18:203-205; 1977. 26. Reinsberg, A.; Smolensky, M. H. Biological rhythms and medicine. New York: Springer-Verlag; 1983:40-41. 27. Schaefer, N.; Bertuch, M. Butterfly-Algorithmus. Theorie und Praxis der Fourier-Transformation. (Butterfly-algorithm. Theory and practice of Fourier-transformation.). c't 8:44-50; 1986. 28. Schmoldt, H. Thermoregulatorische Reaktionen von Trankk~ilbern auf unterschiedliche Lufttemperaturen am Beispiel der Rektaltemperatur. Mh. Vet. Med. 41:300-301; 1986. 29. Simmons, K. R.; Dracy, A. E.; Essler, W. O. Diurnal temperature patterns in unrestrained cows. J. Dairy Sci. 48:1490-1493; 1965. 30. Srivastava, H. N.; Raizada, B.; Pandey, M. D. Studies on physiological responses in Russian Merino sheep. Vet. Res. J. 3:100-104; 1980. 31. Stevens, C. F. Principles and applications of fluctuation analysis: a nonmathematical introduction. Fed. Proc. 34:1364-1369; 1975. 32. Von Kutzleben, R. Beeinflussung der K6rpertemperatur von Laborraten durch die bei Tierversuchen tiblichen Manipulationen. Diss. Med. Vet. Hannover; 1977. 33. Wirth, D. Einftihrung in die klinische Diagnostik der inneren Erkrankungen und Hautkrankheiten der Haustiere. 3. Aufl. Wien: Verlag Urban and Schwarzenberg; 1949:29-33. 34. Wittke, G. Physiologie des Nervensystems und der Sinnesorgane: 5.Schlaf. In: Scheunert, A.; Trautmann, A., eds. G. Wittke: Lehrbuch der Veterin~ir-Physiologie 7. Aufl. Berlin: Verlag Paul Parey; 1987: 644 645. 35. Wrenn, T. R.; Bitman, J.; Sykes, J. F. Diurnal patterns of bovine body temperature. J. Dairy Sci. 44:2077-2080; 1961.
0031-9384/90 $3.00 + .00
Variations of Core-Temperature Rhythms in Unrestrained Sheep ELMAR MOHR AND HANSDIETER
KRZYWANEK
Institute o f Veterinary-Physiology, Freie Universitiit Berlin, DIO00 Berlin 33, Koserstr.20, F R G R e c e i v e d 29 D e c e m b e r 1989
MOHR, E. AND H. KRZYWANEK. Variations of core-temperature rhythms in unrestrained sheep. PHYSIOL BEHAV 48(3) 467-473, 1990.--Variations of core-temperature rhythms occurring during a "normal" day (24-hour period without extraordinary challenges for organism) were studied in 5 male sheep. To record the influence of the metabolic processes in different organic systems, core temperatures were measured at various locations at the same time. To minimise any influences due to measurement or behavior (e.g., effects of isolation), a telemetric system was used for registration and animals were kept without restraint in their habitual herd. Particularly biphasic circadian rhythms were found, and feeding schedule as zeitgeber is discussed. Beyond that, independent from point of measurement short-time rhythms with wavelengths of 140 and 90 min were found. Therefore, an origin in the central nervous system can be supposed. Rhythms with wavelengths of 3 hours, 75 min and 1 hour were not stable throughout a whole 24-hour period and did not occur at all measurement points in the same intensity. A comparison of anatomical placements of the various measurement points leads to the realization of distinct organic functions as sources for these rhythms. Body temperatures
Circadian rhythms
Ultradian rhythms
Various points of measurement
Unrestrained sheep
2) Examining ultradian rhythms it seems to be useful to have a model which is less influenced by circadian zeitgebers as the light-dark cycle or the light intensity: in ruminants monophasic as well as biphasic and polyphasic circadian patterns of core temperature are described but could not be associated with a distinct light-dark cycle (35). Therefore the influence of these circadian zeitgebers could be considered of less importance than in other mammalian [e.g., the distinct influence of light-dark cycles on core temperature rhythms in cats (18)]. Maybe this should be considered together with the very special sleep structure of ruminants (34) but this is beyond the scope of the present study. However, sheep are gregarious animals and therefore, to avoid an influence caused by "social problems" (e.g., effects of isolation), it was necessary to keep the animals without restraint in their habitual herd. Due to these circumstances a telemetric system (23) was designed and assembled in our laboratory and adapted to these specific requirements. So, the purpose of this study is to examine the oscillations of core temperature in sheep, maintained in surroundings which are as true to their natural habitat as possible.
IT is well known that there are oscillatory patterns in core temperature with wavelengths of about 24 hours (2). On these circadian rhythms, not only those of core temperature, usually rhythms with relatively higher frequencies are superimposed. Although there is a wide range of differing wavelengths, all rhythms with a period shorter than a circadian rhythm are summarized as ultradian (12). Unfortunately, and in contrast to circadian rhythms, this definition gives neither a clue to their functional significance (1 l) nor any indications of possible origins. However, numerous investigations in such short-time fluctuations have demonstrated that at least they cannot only be considered secondary to changes of internal or external conditions (7, 19, 24). However, it seems that ultradian rhythms are more influenced by endogenous and exogenous factors than circadian rhythms are (3). Even during a " n o r m a l " day (which means: a 24-hr period without extraordinary challenges for the organism), it seems probable that the intensity of some of these influencing factors will change. Therefore their control on the organism is not constant, which means varying metabolic processes, as well as direct modification of thermoregulatory centres, may alter the core temperature. Because of this, the following questions arise: 1) do the endogenous and/or exogenous changes which occur during a " n o r m a l " day modify the ultradian core temperature pattern and, if this happens, 2) is it possible to associate such modifications with any varying organic function? For two reasons we took sheep as a model to answer these questions: l) Because it seems to be helpful to measure temperature rhythms at various locations in one animal at the same time, the animal has to be large enough to expect a temperature gradient between the different points of measurement.
METHOD Five male sheep of mixed Merino lineage were used, aged between 1 and 2 years with body weights from 38 to 53 kg. All animals were kept in a temperature-controlled stable (3 m • 5 m) at 17-20°C. The humidity of the atmosphere ranged between 60% and 80%. Since the light-dark cycle seems to be of less significance, experiments with ruminants could be performed at different seasons: for animals A and D they took place in April and May; this implies a natural light-dark cycle of about 15:9 hours with sunrise at about 6:00 a.m. (April) and 5:00 a.m. (May). The
467
468
MOHR AND KRZYWANEK
TABLE 1 MEAN CORE TEMPERATURES (°C) WITHIN A PERIOD OF 8 DAYS Animal
Period
Site of Probes
A B C D E
April Jan. Jan. May Feb.
mediastinum (Tin) mediastinum (Tm) mediastinum (Tm) beside liver (TI ) ventral abdomen (Tv)
experiments with animals B, C and E were run in January and February. During this time an artificial light-dark cycle of 12:12 hours with light period starting at 6:00 a.m. was standard. To avoid any behavioral influences all animals were kept without restraint in their familiar flock. The flock (6 animals; one of them not being used for the experiment) were fed once a day (between 7:30 and 8:00 a.m.) with about 3 - 4 kg of mixed grain and 8 kg of hay; water was available ad lib. Core temperatures in the abdomen were measured using a radio telemetric system (23). Equal thermoprobes were surgically implanted at various sites: 1) in the left caudal abdomen, fixed to middle of the mediastinum (animals A, B and C; temperature indicated as Tin); 2) beside the lateral surface of the liver, right behind the last rib, fixed to the parietal peritoneum (animal D; temperature indicated as T 0 and 3) in the ventral abdomen among the coils of intestine (animal E; temperature indicated as Tv). In animal D an additional thermoprobe was installed inside the rumen (Tr). In animal E, a chronically external silicon fistula was implanted which was equipped with a stainless steel tube to achieve a good thermal contact with the surrounding. It was fixed parallel to the V. cava caudalis and a thermistor was placed inside it. Additionally an extracorporal transmitter with the same sensitivity could be utilised. A complete healing of the external fistula without any local reactions was achieved by wrapping a piece of DACRON (surgical tissue used for implantations) around the silicon pipe. The temperature registered in the fistula is indicated as Te. Measurements started 14 days after implantation. At this time there were no clinical signs of systemic disturbance. In each animal core temperatures were measured for 4 weeks. During this time data were continuously transmitted, every 30 sec digitised, and stored on disk. After the end of experiment, evaluations were made in three different ways: a) To obtain the time course of core temperatures, a graphic description of the received values was computed. Then a curve derived from a moving average of 15 consecutive values was evaluated to estimate the basic rhythms. b) To get more precise information about the time course during a 24-hour period educed data were computed using a technic which is described in the literature as " P L E X O G R A M " (1,26): Temperature values of 8 days were divided into intervals of 24 hours. Taking into account the possible deviations which were caused by trend, the mean -+ SD of the sum of first values of each interval was calculated, followed by the mean of the second values and so on. This leads to a mean time course of the respective core temperature of a 24-hour period. Because corresponding values of each interval are superimposed there will be an increase of the mean value if the signal is periodic and in phase with the examined 24-hour period; at the same time the noise will be reduced by a
Mean _+ SD 39.48 38.94 38.82 38.85 39.70
-+ 0.24 _+ 0.27 -+ 0.25 + 0.22 + 0.25
Minimum
Maximum
AT
38.63 37.80 38.20 38.27 38.78
40.04 39.52 39.53 39.50 40.35
1.41 1.72 1.33 1.23 1.57
reciprocal extinction. To describe the basic temperature rhythms, the mean values were then submitted to FFT (Fast Fourier Transformation) (27); after that a temperature curve was calculated using the first three harmonics (18). Comparisons between the various Plexograms with reference to coherence or phase shift were made using SPSS/PC+ - C o m p u t e r program (SPSS Inc.). c) To analyse the intensity of the several frequencies (spectral densities of the core temperatures time courses during a 24-hour period), raw data were first reduced to 70% of the original value by a digital filter. This process eliminates the very little oscillations of the time series. Because of this, aliasing effects, which means the falsification of the intensity of the lower frequencies by the very fast-moving components (31), were reduced. After this, slow-changing trends were removed by a "zero phase-shift autoregressive filter" (21). The residual time series was tapered by a HANN-window (cosine tapering) to reduce the discontinuity at both ends of the time series. This minimised the additional power from adjacent frequency bands (leakage). FFT of the modified time series was performed after trend removal and tapering, and the power spectrum was calculated by S(i) = 2h/N • IX(i)l 2 where N is the total number of points and h the sampling interval of the time series. X(i) represent the discrete FFT components with i = 1, 2, 3 . . . . . (N/2) (21). This way the power spectra of 8 consecutive days were performed and summarized. To get a 3-dimensional presentation of variations in the spectral density with respect to time of day, discrete power spectra were computed by shifting the starting point of the applied time series for one hour. RESULTS
Time Course of Core Temperatures Table 1 presents mean core temperatures -_+SD of the different animals, periods of data collection, the minimal-maximal values and AT, the difference between minimum and maximum. Mean temperatures ranged from 38.82°C to 39.70°C. Although there are significant differences between the mean values, the SDs are nearly identical. Figure 1 shows the time courses of core temperatures in three animals (A, D, E) at various points of measurement for 8 consecutive days. One partition of the horizontal axis represents a single day from 6:00 a.m. to 6:00 a.m. The vertical axis presents °C. The thicker line shows fitted values derived from the moving average and represents the basic rhythms. A is a time course of T m of animal A recorded from a thermoprobe which was implanted in the mediastinum of the left caudal abdomen. D~ and D 2 are the time courses of animal D measured at the lateral side of the liver (D~, TO and inside the rumen (D 2, T~) at the same time. E l and E 2 are curves of animal E simultaneously registered in a ventral abdominal region (E l, Tv) and beside the V. cava caudalis (E 2,
VARIATIONS OF CORE-TEMPERATURE RHYTHMS
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atures were 39.70---0.25°C and 39.72 ± 0.24°C, respectively. The temperature of Tf ranged between 38.97 and 40.46°C. Notice the coherence of the basic rhythms ( r = .4065, p < 0 . 0 0 1 ) which were superimposed in the case of the fistula by distinct "high-frequenc y " oscillations. An improvement of r by shifting one time series could not be found.
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Tf). It is remarkable that in all animals the basic rhythms are superimposed by oscillations of much higher frequencies. D~ and D 2 of Fig. 1 present a comparison between T] and intraruminal temperature T r. In the course of the demonstrated 8-day experimental period T r ranged from 38.01 to 39.92°C with a mean of 38.90 ± 0.23°C. This mean value is significantly higher ( p < 0 . 0 1 ) than the mean of T[ (see Table 1). The larger difference between minimum and maximum of T r is produced by high oscillations in the morning time. The correlation coefficient between the basic rhythms is r = .4821, significant with p < 0 . 0 0 1 . It will increase to r = .5618 after shifting D 2 time series for + 185 min. This means there is a clear coherence between the basic rhythms but there is also a phase shift between the time courses of T~ and T~; the drift of the temperature occurs in T t 185 min later than in T r. E~ and E e of Fig. 1 show the time courses of core temperatures in animal E. They were measured in the ventral abdominal region among the coils of intestine (Tv) and, at the same time, beside the V. cava caudalis (external silicon fistula, Tf). The mean temper-
A continuous phase shift between the particular time series used to form a Plexogram could obliterate the underlying signal. To exclude this possibility we calculated the coherence between the single time series. A mean correlation coefficient of r = .856 --- .078 was deduced. This implies that there can only be slight phase differences and therefore it is admissible to compute Plexograms. Table 2 presents mean core temperatures---SD, minimalmaximal values and the differences from high to low of the averaged time courses. Figure 2 represents the different Plexograms of the previous time courses of core temperatures. During a 24-hour period there are multiple peaks and troughs caused by the fast-moving components. The fitted curves are derived from the first three harmonics. This means they represent the influence of the rhythms with a wavelength of 24, 12 and 8 hours. These slow moving components showed a monophasic rhythm in animals A and D (Fig. 2, A and D 0. In the other sheep there were more or less biphasic courses as presented in animal E (Fig. 2, El). In animal A the highest values occurred at about 3:00 a.m. In the other animals the highest values could be found at noon. Note that there is a trough of core temperatures in most animals in early morning. A comparison of T l (beside liver) and T r (intraruminal temperature) is demonstrated in Fig. 2 by D~ and D 2. There is a clear rise of T r immediately after feeding (7:30 a.m.). It increases by a mean value of 0.6°C to a mean temperature of 39.38°C. This rising is followed by drastic changes in T r which last for about 10 hours (until 6:00 p.m.). It is remarkable the T~ (more than Tr) starts rising 1/2 an hour before feeding time. The correlation coefficient between T l and Tr is r = .5206 (p<0.001) for the basic rhythms. It will increase to r = .9284 by shifting D 2 values for +207 min. This nearly identical phase difference between D~ and D 2 in Plexogram data and in time course data supports our opinion that it is admissible to calculate Plexograms. Figure 2 E l and E 2 demonstrate the differences due to various points of measurement in one animal. E] shows T v (ventral abdomen), E 2 the "fistula temperature" Tf. Although there is a difference in the amplitude of the basic rhythms (fitted curve), especially during night time, there is also a clear coherence in the time course ( r = .7535, p < 0 . 0 0 1 ) . The phase shift is noticeable. Minimal and maximal values occur in T v at 7:30 a.m. and 11:30 a.m., respectively. In Zf they are found at 6:20 a.m. and 10:30 a.m. This means that the time difference between minimum and
TABLE 2 MEAN CORE TEMPERATURES (°C) OF "ONE AVERAGE DAY" Animal A B C D E
Site of Probes mediastinum (Tm) mediastinum (Tin) mediastinum (Tin) beside liver (T~) ventral abdomen (Tv)
Mean ± SD 39.48 38.94 38.82 38.85 39.70
±- 0.12 --- 0.11 ± 0.13 ± 0.14 ± 0.11
Minimum
Maximum
AT
39.22 38.70 38.55 38.58 39.40
39.70 39.15 39.14 39.13 39.97
0.48 0.45 0.59 0.55 0.57
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FIG. ") Ptexograms recorded at various points of measurement, average of 8 consecutive days (exemplified in animals A, D and E). Presented are mean values ± SD (dotted lines) of core temperatures. Fitted curves are derived from the first three harmonic oscillations.
The highest intensities were found between 5 and 22 cycles/ day, which means wavelengths of 288 rain and 65 rain. There were peaks at 10-11 cycles/day, 14-15 cycles/day and 20 cycles/ day, which correspond to wavelengths of about 137 rain, 99 rain and 72 rain. However, the highest peak only accounted for 7% of
the whole intensity, To get more information about the intensity modification of the several frequencies with respect to time of day, the plots shown in Fig. 4 were computed. The intensity (in %) of the different frequencies (cycles/day) was plotted against time of day. The first before T,.. line represents the spectrum starting at 6:00 a.m. For the successive lines the starting point was shifted for one hour. FFT Analysis The upper part presents the variations of core temperatures, The intensity of the several frequencies which are forming the exemplified in T~ of animal D. Nearly identical peaks as shown in time course of core temperature (T~) during a day are shown in animal C (Fig. 3) occur, but this plot shows that the intensity is Fig. 3. This is a power density spectrum of animal C, derived from varying during a 24-hour period. Note that the intensity of the 8 consecutive days. Similar spectra were achieved from the other 10-cycles/day frequency is nearly stable throughout the whole animals. The y-axis is graduated in percentages (of all intensities) period. In the frequencies of T~ (Fig. 4, lower part) which were and the x-axis represents the several frequencies (measured in recorded simultaneously, the same peaks as in T~ appear. Signifcycles/day) up to the 32nd harmonic (e.g., the highest presented icant differences between T~ and Tr occur only from 8:00 a.m. to frequency of 32 cycles/day equals a wavelength of 45 rain). 6:00 p.m.: the intensities of the l- and 2-cycles/day frequency of Tr are moving through a maximum within this time. A statistical test (t-test with p < 0 . 0 l ) between the corresponding intensity Intensity values of Tr and T~ could not indicate further differences. Figure 5 presents a comparison of Tv frequencies measured in a ventral abdominal region (upper part) and next to the V. cava caudalis (' 'fistula" temperature T~, lower part). In both cases there are again peaks at 10-I l cycles/day, 15 cycles/day (especially in the evening and early morning time) and 20-21 cycles/day. Additional to that, in the "intraabdominal" temperature there is a 1 10 20 30 peak at 24-25 cycles/day (wavelength of about 60 rain). In Tf the cycles/ day peak at 5-6 cycles/day (wavelength of about 265 rain) is more impressive than in T~. In general, Tf rhythms seem to be more stable than those measured in the ventral abdominal region. FIG. 3. Power density spectrum of animal C (computed over 8 consecutive days) (wavelength = 1440 rain/x).
maximum is about 4 hours in both cases. However, because of the phase shift, temperature changes occur in Tt. about one hour
10
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,..
VARIATIONS OF CORE-TEMPERATURE RHYTHMS
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,
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~
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FIG. 5. Core temperature rhythms with respect to various points of measurement.
DISCUSSION The experiments reported above were performed in order to obtain precise information on the variations of core temperature in unrestrained sheep. Because of the proximity of the thermal regulatory centres to both thalamic and reticular systems, variations of core temperature might be influenced by physical exercises as well as by "psychic stress" (sensory input and vigilance). Earlier investigations showed that the manipulations during rectal measurement could modify core temperature in farm animals (23,33) and in laboratory animals (32); it is reported that insertion of a rectal thermoprobe for only one minute could elevate the animal's core temperature for 70 min (25). To minimise all these influences, a telemetric system for measuring was used, our animals were kept without restraint in their familiar flock and stable and there were no changes in feeding or housing conditions throughout observation time. It is well known that there are temperature gradients within the deeper body tissues. The best point to measure mean core temperature is where the blood leaves the left side of the heart (5). However, the temperature measured at the side of the abdominal aorta can also be considered as a good indicator of core temperature (17). Due to surgical difficulties, in our recent studies the temperature was measured next to the abdominal vena, but we assume that there will be a heat exchange between aorta and vena, which are located close to one another. We admit that this point of measurement might be more influenced by heated blood due to muscular activity of the rear legs. However, there was not much locomotive activity because the animals were kept in their accustomed stable during the whole experimental period. The clear coherence between time courses of Tv and "fistula temperature" Tf (Fig. 2, E~ and E2) supports the above considerations. All mean values of core temperatures (38.82-39.70°C, Table
471
1) were within the physiological range from 38.4 to 40.0°C for sheep over 1 year (16). The minimal differences to the lower values given especially for Merino rams (38.80-39.06°C) (13) can be explained by a temperature gradient between an intraabdominal and a rectal point of measurement. The daily mean difference (see Table 2) of 0.53°C corresponds well with the values given for cattle (0.45°C) (35) and for sheep (16). The nearly identical SDs of the core temperatures were astonishing. They indicate similar oscillations in all animals in spite of significant differences between the core temperatures. Contrary to other opinions (6,30), an association between the levels of core temperatures and the times of data collection was not possible. In our experiments seasonal effects might have been reduced by housing conditions (i.e., temperature-controlled stable for 24 hours each day). With regard to circadian rhythms, our findings are similar to those of other authors who found biphasic core temperature rhythms in cattle (29) and sheep (16). The demonstrated basic rhythm of E 1 (Fig. 1) and most of the Plexograms confirm this: a trough in the morning followed by a rise until early afternoon and another peak between early evening and early morning. On the other hand, our findings also support those results which describe monophasic and aphasic patterns in cows (4,35) and calves (28) (see Figs. 1 and 2A, D 0. Therefore our study cannot conclude definitely whether monophasic or biphasic basic rhythms are predominant in ruminants. The rise of T r of 0.6°C immediately after feeding (Fig. 2, D2) accounts for most of the daily mean difference of T r (0.73°C) and is therefore one of the major causes of variation of core temperature. However, this is only a small increase in comparison to other results found in cattle [1.63°C within 15 min after feeding of 8 pounds of mixed grain (9)]. We attribute this difference primarily to the diminished quantity of food in our experiments (4 kg of mixed grain to a flock of 6 animals, hay was not available to the flock the whole time). Because of this, heat production caused by intraruminal fermentation was reduced. During a 24-hour period highest values of T r were reached about one hour after feeding and were followed by dramatic variations of T r which lasted for about 10 hours (6:00 p.m.). They should be considered as an effect of drinking. Analogous effects were described by many authors (8-10). For the remainder of the day and until next feeding time, there were only smooth variations in T r. The correlation analysis indicates that T] was following the drift of Tr with a phase shift of about 3.2 hours. Furthermore, the phase difference of 1 hour between T v and Tf (Fig. 2, E l, E2) indicates that there was a source of heat closer to the measuring point of Tf than to that of Tv. The time of the rise of Ty(8:30 a.m.) in relation to feeding time (8:00 a.m.) suggests the rnmen as the most probable source. These two findings may support the hypothesis that feeding time influences the rhythms of core temperature [especially some biphasic pattems (29) correspond well with feeding times]. On the other hand, there were monophasic or polyphasic temperature patterns (35). Further studies should examine whether the feeding schedule can be a sort of zeitgeber for core temperature rhythms or not. Core temperature is a result of all controlling mechanisms which are involved in heat production and heat loss (2). We know that fluctuations of physiological functions--like core temperat u r e - a r e not only a response to changing environmental conditions nor simply produced by various internal activities (19). They are also caused by one or several internal oscillators. Different physiological systems exhibit ultradian oscillations with frequencies between 1 and 2 hours: cortisol rhythms with a predominant period of 85 to 90 rain in monkeys (14); alertness, renal excretions and gastric activities with a frequency of about 90 min (20); in dairy cows core temperature rhythms with wavelengths of 90 min (4); coherent ultradian rhythms of mean arterial pressure and heart
472
MOHR AND KRZYWANEK
rate with a period of 1-2 hours in dogs (21). Our Fourier analysis of core temperatures in sheep demonstrates that the main ultradian oscillations were also between the 1 and 3 hour frequencies: like in other physiological systems there is a dominant intensity at a wavelength of about 90 min and additionally, there are rhythms which are centred around 140 min. Regarding the intensity of ultradian rhythms with respect to time of day, one result of the present study is that independent of the point of measurement in all our animals the rhythms with 90 min (16 cycles/day) and especially those with 140 min (10 cycles/day) are very stable through an entire 24-hour period. Concerning those ultradian rhythms where the intensity is varying during a 24-hour period, it is necessary to take into account the diverse anatomical placement of the thermoprobes: In intrarnminal data (Fig. 4, loser case) most modifications occur between 8:00 a.m. and 6:00 p.m. During this time the 24-hour (1 cycle/day), 12-hour (2 cycles/day), 3-hour (8 cycles/ day) and the 75-min (18-20 cycles/day) rhythms have their greatest intensities. Taking into account the Plexogram results, it can be concluded that this is caused by food intake. Because the 3-hour and especially the 75-min rhythms show a similar variation in time, it seems that the origin of all these rhythms are the rumen and its microbial activity. [It should be mentioned that there is also an increase of the 75-min rhythm in the abdominal region (Fig. 5, upper case) starting at midnight. But this happened only in the demonstrated case and only during this experimental period. Therefore it was not taken into account.] Comparing the abdominal temperature rhythms (Fig. 5, upper part) with the others, there was another frequency (60-min rhythm = 24 cycles/day) which is very stable. This special rhythm could not be found with the same intensity (but sometimes with less intensity) at the other locations. Therefore the source of this rhythm may be associated with local processes in the ventral abdominal region which lasted throughout the whole 24-hour period. Probable candidates could be the various activities and/or the blood circulation of the gut. It is even more plausible if we take into consideration that the intestinal digestion of the ruminants is not depending on feeding-time. [Furthermore, there are findings that in unrestrained dogs the heart rate and the mean arterial pressure oscillate with frequencies between 1 and 2 hours (21).] Conclusions Based upon long-term continuous temperature monitoring at various sites of the abdomen, using a telemetric system we
conclude that the deep body temperature in unrestrained noninfluenced sheep is characterized by 1) a predominance of mostly biphasic circadian rhythms with different amplitudes (maximal mean value: 0.53°C) and 2) nearly stable ultradian rhythms with wavelengths of 140 and 90 rain which were not dependent on the point of measurement. Since not all ultradian rhythms are stable during a 24-hour period, different origins for the stable and nonstable rhythms seem probable. The oscillator/s of the stable rhythms is/are characterized by a) being only slightly influenced by time of day (and therefore, for example, by light cycles), b) acting on different deep body tissues at the same time and c) not being modified by local processes in the abdomen. Especially because of the observations b) and c), the origin of the stable rhythms can be presumed to be in the central nervous system. With regard to the different locations of measurement it seems possible to associate some of the other, nonstable ultradian rhythms with physiological processes: a) increased microbial activity in the rumen after feeding seems to produce two deep body temperature rhythms. One with a wavelength of about 3 hours and another of 75 min, both of which last for nearly 10 hours. (Nevertheless this hypothesis has to be verified by further investigations.) b) There is an additional stable rhythm with a wavelength of 1 hour which may be caused by intestinal acti~ty. The comparison of the various points of measurementlshowed that the registration of the core temperature next to one ofAhe main abdominal blood vessels gives not only the best reading of mean core temperature but also gives the best general view of the occurring ultradian rhythms. However. due to the fact of heat transport by blood convection, there is a difficulty that temperature variations measured at this point may be influenced by changes in heart rate and mean arterial pressure. It will be an aim of further investigations to elucidate the role of heart rate and mean arterial pressure regarding core temperature rhythms and also to verify the hypothesis that ruminal activity triggers the 3 hour- as well as the 75-min rhythm. ACKNOWLEDGEMENTS This study was supported by Deutsche Forschungsgemeinschaft Grant Mo 450/1-1. The authors are very indebted to Professor Dr. E. Hentschel, Institut ftir Veterin/ir-Anatomie, FU-Berlin for the performance of the surgical implantations. They also thank Professor Dr. C. Jessen, Physiologisches Institut der Universit/it Giessen, for his helpful comments on the technic of measurement. The authors are also very indebted to Mrs. K. Upmeyer and Mr. M. Weddington for revision for language.
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