Pergamon 0306-4565(94)E0011-6

J. therm. Biol. Vol. 19, No. 4, pp. 269-275, 1994 Copyright © 1994 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0306-4565/94 $7.00 + 0.00

CIRCADIAN MODULATION OF ULTRADIAN OSCILLATION IN THE BODY TEMPERATURE OF THE

GOLDEN HAMSTER

ROBERTO REFINETTI

Department of Psychology, College of William and Mary, Williamsburg, VA 23187, U.S.A.

(Received 23 December 1993; accepted in revised form 5 March 1994)

Abstract--1. The body temperature rhythm of 52 golden hamsters maintained under a 14L:IOD light-dark cycle or constant illumination in the laboratory was recorded by telemetry.

2. Daily or circadian rhythmicity was observed in all animals. High-frequency (ultradian) oscillations were observed in most animals, although they were less regular than circadian oscillations.

3. The amplitude of ultradian oscillations was greater during the dark phase of the light,lark cycle than during the light phase.

4. As determined by chi-square periodogram analysis of 10-day blocks of data, the most common period of ultradian oscillations was 8 h for hamsters maintained under a 14:10 h light-dark cycle and 12 h for hamster maintained under constant light or constant darkness. In animals whose circadian rhythmicity was eliminated by suprachiasmatic lesions, the most common period of ultradian oscillations was 5 h.

5. It is concluded that the body temperature rhythm of golden hamsters has a significant ultradian component that is only under partial control of the circadian system.

Key Word Index: Body temperature; circadian rhythm; ultradian oscillation; Mesocricetus auratus

INTRODUCTION

The meaning of homeothermy is that body tempera- ture is approximately constant and that, indepen- dently of thermal disturbances caused by activity and changes in ambient temperature, there are small spontaneous oscillations of body temperature around the normothermic level (Cabanac and Simon, 1987). Some of these small spontaneous oscillations are regular enough to be called rhythms. A regular oscillation with a period of approximately 24 h is called a circadian rhythm, and there is a vast litera- ture on the circadian rhythm of body temperature (Refinetti and Menaker, 1992). Ultradian oscillations in body temperature (i.e., oscillations with a higher frequency or shorter period than circadian oscil- lations) have received much less attention. Studies in rats have detected oscillations in body temperature with periods of 2 to 4h (Eastman et al., 1984; Eastman and Rechtschaffen, 1983; Honma and Hi- roshige, 1978; Ruis et al., 1987), but formal analyses were not conducted to verify the regularity of these putative rhythms. In formal studies of ultradian oscillations, Wollnik and Turek (1988) found regular oscillations with periods from 4 to 6 h in the running- wheel activity of female rats, whereas Stupfel et al.

(1987) found very little regularity in the high-fre-

quency oscillations of oxygen consumption in several species of mammals and birds.

In view of the paucity of information on ultradian rhythms, especially regarding body temperature, the present study was conducted to investigate the presence of regular ultradian oscillations in the body temperature of the golden hamster.

MATERIALS AND METHODS

Fifty-two male golden hamsters (Mesocricetus

auratus) were used in the experiment. At the age of 8-12 weeks, the animals were transferred to individ- ual plastic cages (21 × 30 × 20 cm) lined with wood shavings and were fed Purina Lab Chow and water ad libitum. Radio transmitters for the monitoring of body temperature (Model VM-FH, Mini-Mitter Co., Sunriver, OR) were implanted intraperitoneally under sodium pentobarbital anesthesia (80mg/kg i.p.). The animal cages were placed on top of radio receivers (Model RA-1010, Mini-Mitter Co., Sun- river, OR) located in a temperature-controlled chamber. The radio receivers were attached to a computerized data acquisition system (Dataquest III,

269

270 ROBERTO REFINETTI

3 8 . 5

37.5

3 6 . 5

35 .5 ca

36.5

,'a o 37.5

I I I I I I

I I I I I I 0 2 4 6

Days

I

I

1 I 8 i0

1 I I I I I I I I I I

3 6 . 5

3 5 . 5 I t I I I I I I I I I 0 4 8 12 16 20 2 4

H o u r s

Fig. 1. Body temperature records of a male golden hamster maintained under a 14:10 h light-dark cycle. Data were collected in 6-min bins. The top panel shows 10 days of continuous records. The bottom panel shows the data from the first day on an expanded time scale. Lights were on daily from 0300 to 1700 h.

Animal ID: SE7.

Data Sciences, St. Paul, MN), so that body tempera- ture could be~ recorded in 6-min bins for 15 or more days. Ambient temperature (22-24°C) was main- rained constant throughout the experiment. Half of the animals (26 hamsters) were maintained under a 14:10 h l ight-dark cycle (LD) and half under con- stant conditions of illumination [15 animals under constant light of 200 lux (LL) and 11 animals under constant darkness (DD)].

Sixteen animals (8 from the LD group and 8 from the DD group) subsequently received histologically- confirmed complete lesions of the suprachiasmatic nuclei of the hypothalamus (SCN) and were studied for an additional interval of 10 or more days. Electro- lytic lesions (4mA, 10-15s) were performed under sodium pentobarbital anesthesia (80 mg/kg i.p.) and were stereotaxically placed along the midline between the base of the third ventricle and the optic chiasm (AP + 0.6, V - 8.3, L 0.0 mm).

Means and standard deviations of body tempera- ture readings over various segments of the data sets, as well as t tests for comparison of means, were computed by standard procedures. Time series analy- sis was conducted by the chi-square periodogram procedure (Sokolove and Bushell, 1978) using 10 consecutive days (2,400 bins) for each animal. Although the chi-square periodogram procedure is an excellent method for the analysis of biological time series (Refinetti, 1993), it is incapable of discriminat-

ing harmonics (Sokolove and Bushell, 1978; Wollnik and Turek, 1988). This means that, if the data contain a rhythm with a period of 3 h, the periodogram will have a peak not only at 3 h but also at 6, 12, 15 h, and so on. A simple solution would be to read the periodogram from left to right and disregard all harmonics of a previously detected period. Unfortu- nately, this might lead to the rejection of true period- icities (including the circadian one, since 3 x 8 = 24 h). Thus, the rejection rule was applied only if the spectral energy (the value of the Qp statistic) associated with a harmonic was less than three times as large as the spectral energy of the fundamental. To further insure the appropriate hand- ling of harmonics, the data from several animals were analyzed also by Fourier analysis (procedure Spectra in SPSS/PC+, SPSS Inc., Chicago, IL) and the resulting periodogram (which does not include false harmonics) was compared to the chi-square periodogram.

RESULTS

Body temperature records for two representative animals are shown in Figs 1 and 2. In both figures, the top panel shows 10 consecutive days of body temperature data. The most evident oscillation is the circadian one (period of 24 h). High frequency oscil- lations can also be seen but are more evident in the

Ultradian oscillations

3 8 . 5 -'

3 7 . 5

3 6 . 5

~ 3 5 . 5

36.5

o 3 7 . 5

I i i i l l l 1 l

i I I I I I I i I 0 2 4 6 8 10

D a y s

36.5

35.5

I I I I I I I I I I I

I I I I I I I I I I I 0 4 8 12 16 2 0 24

H o u r s

Fig. 2. Body temperature records of a male golden hamster maintained in constant light (200 lux). Data were collected in 5-min bins. The top panel shows l0 days of continuous records. The bottom panel shows

the data from the first day on an expanded time scale. Animal ID: CH11.

271

lower panels, which correspond to the first 24 h of the

upper panels re-plotted over an expanded time scale.

Visual inspection of the plots suggest the presence of

a relatively regular oscillation with period of approxi-

mately 2 h in addition to other irregular oscillations.

It also seems that the amplitude of the oscillations is

0 v

Q)

Q)

38 .5 1 I I I I i

37 .5

36 .5

85 .5

88 .5 : [ I I i I

8 7 . 5

36.5

3 5 . 5 I I I I

t 1 S 3 4 5

Days

Fig. 3. Body temperature records of a male golden hamster maintained under a 14:10 h light-dark cycle. Data were collected in 6-min bins. The top panel shows 5 days of continuous records. The middle panel shows the same data after filtering by a 12 h moving-averages procedure. The bottom panel shows the

difference between the two preceding data sets. Animal ID: SE7.

272 ROBERTO REFINETTI

r,)

88.0 ~ 37.0 36.0

39.0~ I I I I

o .o u . ,.. J..~.. ~ .A,, ~ ,._ . I ....

- , . 0 fB"# ~'~T~"~IY~'U'~r''l'/'er"vYl~t'~'P"'~V"~"m~A' w t r " t ~ , l I ,

1 2 3 4 5 Days

Fig. 4. Body temperature records of a male golden hamster maintained under constant darkness. Data were collected in 6-mJn bins. The top panel shows 5 days of continuous records. The middle panel shows the same data after filtering by a 12-h moving-averages procedure. The bottom panel shows the difference

between the two preceding data sets. Animal ID: D02.

greater during the circadian phase of high body temperature than during the phase of lower body temperature, especially in Fig. 2.

To examine in more detail the circadian difference in amplitude of ultradian oscillation, the circadian

component of the temperature rhythm was elimi- nated by subtracting the original data points from a filtered version of the data (12-h moving averages), as illustrated in Figs 3 and 4. In both figures, the top panel shows 5 days of the original data set, the middle panel shows the filtered version, and the bottom panel shows the data after elimination of the circa- dian oscillation. It can be seen that the amplitude of ultradian oscillations has a circadian oscillation, greater amplitudes occurring during subjective night (i.e. during the time when the smoothed curve is higher, which corresponds to the dark phase of the l ight , lark cycle for animals in the LD group). To obtain a quantitative measure of ultradian amplitude, the means and standard deviations of temperature readings over the 10 days were computed for each animal in the LD group for the light and the dark phases of the light-dark cycle. Not surprisingly, the mean of the 26 means during the dark phase (37.06°C) was significantly higher than the mean of the means during the light phase (36.56°C): t(19) = 13.95, P < 0.0001. The higher amplitude of ultradian oscillation during the dark phase is indi- cated by the fact that the mean standard deviation

2000.

1500.

1000-

500- ~>~

o

400. 0

rn 300-

200

100-

I I I I I Chi Square

I I I I

Four ier

0 ~, A ~' t I 0 5 10 15 20 25 30

Per iod (h)

Fig. 5. Spectral analysis of body temperature data of a male golden hamster maintained under a 14:10h light~:lark cycle. The data were collected at 6-min bins over 10 days. The same data were analyzed by the chi-square periodogram procedure (top) and Fourier analysis {bottom). The dashed line corresponds to the critical values of chi-square used to determine statistical significance (P < 0.01 per comparison).

Animal ID: SN2.

Ultradian oscillations 273

0

2000

1500

1000

500-

0

i i i

squaro I I

200 I I i i Fourier

150. ~

100

5 0 ~

0 5 10 15 20 25 30 P e r i o d (h)

Fig. 6. Spectral analysis of body temperature data of a male golden hamster maintained under constant light (200 lux). The date were collected at 6-min bins over l0 days. The same data were analyzed by the chi-square periodogram procedure (top) and Fourier analysis (bottom). The dashed line corresponds to the critical values of chi square used to determine statistical significance (P < 0.01 per comparison).

Animal ID: CH11.

during the dark phase (0.53°C) was significantly higher than the mean standard deviation during the

light phase (0.48°C): t (19) = 2.77, P = 0.01. Figures 5 and 6 show the results of the time series

analysis of the data from two representative animals. In Fig. 5, the chi-square periodogram indicates 4

periods with spectral energy above the chance level: 6, 8, 16, and 24 h. Since 16 is a harmonic of 8 and its spectral energy is only slightly greater than that of the fundamental , the peak at 16 h was discarded as an artifact. Inspection of the Fourier periodogram confirms the absence of 16-h periodicity. In Fig. 6, the chi-square periodogram again indicates 4 statistically significant periods: 6, 8, 12, and 24 h. Although 12 is a harmonic of 6, its spectral energy is more than three times as large as that of the fundamental and, conse- quently, the peak was not discarded. The Fourier periodogram supports the decision to accept the 12-h

periodicity. Analysis of the data from all 52 animals showed

considerable inter-individual variability in ultradian periodicities. All animals had significant periodicity in the circadian range (24.0h in the LD group, 23.6-24.4h in free-running group), but ultradian

periodicities varied from animal to animal. Five of the 52 hamsters had no significant periodicities out- side the circadian range, whereas the remaining ani- mals had 1 to 4 ultradian periodicities. The number of significant ultradian periodicities was not signifi- cantly correlated with the spectral energy in the circadian range (Q240) for the animals in the LD group: r = 0.13, P > 0.10. However, dependence of

uitradian periodicity on the strength of circadian periodicity was found in the free-running group, as the number of significant ultradian periodicities was significantly correlated with circadian spectral energy

in this group: r = 0.51, P < 0.01. Frequency polygons of ultradian periodicities for

the two groups of animals are shown in Fig. 7 (top two panels). The two sub-groups in the free-running

100

75

50

25

1002

{D 75-

~ 50-

q )

O

O

I

LD 14:10 I I I

• A

4 8 12 I

LL and DD

25-

0 0 4 8

I I

100- SeN lesion

"/5.

I I

1 12

I

5 0

2 5

, A ,

16 20 24 I I

1~6 210 24 I I I

o o 4 8 12 16 20 24

P e r i o d (h)

Fig. 7. Frequency polygons of significant ultradian period- icities in the body temperature records of male golden hamsters. The data were collected at 6-rain bins over l0 days and analyzed by the chi-square periodogram procedure. The three panels refer to three groups of animals: unoperated hamster maintained under a 14:l0 h light,lark cycle (LD 14:10, n = 26), unoperated hamsters maintained under con- stant conditions (LL and DD, n = 26), and hamsters sus-

taining complete SCN lesions (n = 16).

274 Roar~'ro R.~l,~"r"rl

41.0 O

(D

3 9 . 0

37.0

o 3 5 . 0 I:Z:I

I I I

1 I I

1 3

I I I I I I

I I I I I

5 7 9 Days

Fig. 8. Body temperature records of a male golden hamster for 3 days prior to and 6 days after complete SCN lesion (arrow). Data were collected in 6-min bins. Animal ID: SE16.

group (LL and DD) did not seem to differ from each other and were analyzed together. In the LD group, 73% of the animals had a significant 8-h periodicity and 35% had a 6-h periodicity. In the free-running group, only 46% of the animals had a significant 8-h periodicity but 69% had a 12-h rhythmicity.

Animals with complete SCN lesions had no signifi- cant periodicity in the circadian range, and 5 of the 16 animals had no significant periodicities at all. (These 5 animals were not the same 5 animals that had no ultradian periodicities previous to the lesions.) As shown in Fig. 7 (bottom panel), 50% of the animals had a 5-h periodicity. The raw data for a representative animal are shown in Fig. 8.

DISCUSSION

The results showed consistent oscillations in the body temperature of golden hamsters. The circadian oscillation was the most regular and robust oscil- lation but several regular ultradian oscillations were also found. Evidence was obtained of a partial circa- dian modulation of ultradian oscillations. Thus, the amplitude of ultradian oscillation was found to be greater during subjective night than during subjective day, which is probably due, at least in part, to the nocturnal increase in arousal and activity. Also, at least in the free-running condition, animals with stronger circadian rhythmicity tended to have a higher number of significant ultradian periodicities (r = 0.51). On the other hand, the involvement of other factors in the determination of ultradian rhyth- micity was indicated by the finding that the most common ultradian oscillation had a period of 8 h in animals maintained under a light-dark cycle but a period of 12 h in free-running animals. It is not clear

why the presence of a light-dark cycle should affect the period of ultradian oscillations, but this effect suggests an independence of ultradian oscillation from the circadian system. This independence is further supported by the fact that animals whose circadian rhythmicity was eliminated by SCN lesions had significant ultradian oscillations, albeit with a shorter period (5 h) than unoperated animals.

Ultradian oscillations in body temperature were much less consistent than the circadian oscillation. However, they were clearly not simple random vari- ation. How these regular ultradian oscillations are produced remains unknown. The existence of an ultradian pacemaker responsible for ultradian feed- ing rhythms in common voles (Microtus arvalis) has been recently suggested (Gerkema et al., 1993), and it is possible that an ultradian pacemaker is also responsible for the ultradian oscillations in body temperature described in this paper. Such pacemaker might control body temperature directly by affecting the thermoregulatory system or indirectly via changes in thermogenesis resulting from variations in food ingestion and activity levels.

REFERENCES

Cabanac M. and Simon E. (1987) Glossary of terms for thermal physiology. Pfluegers Arch. 410, 567-587.

Eastman C., Mistlbcrger R. E. and Rechtschaffen A. (1984) Suprachiasmatic nuclei lesions eliminate circadian tem- perature and sleep rhythms in the rat. Physiol. Behav. 32, 357-368.

Eastman C. and Rechtschaffen A. (1983) Circadian tem- perature and wake rhythms of rats exposed to prolonged continuous illumination. Physiol. Behav. 31, 417-427.

Gerkema M. P., Daan S., Wilbrink M., Hop M. W. and van der Leest F. (1993) Phase control of ultradian feeding rhythms in the common vole (Microtus arvalis): The roles

Ultradian oscillations 275

of light and the circadian system. J. Biol. Rhythms 8, 151-171.

Honma K. and Hiroshige T. (1978) Simultaneous determi- nation of circadian rhythms of locomotor activity and body temperature in the rat. Jap. J. Physiol. 28, 159-169.

Refinetti R. (1993) Comparison of six methods for the determination of the period of circadian rhythms. Physiol. Behav, 54, 869-875.

Refinetti R. and Menaker M. (1992) The circadian rhythm of body temperature. Physiol. Behav. 51, 613-637.

Ruis J. F., Rietveld W. J. and Buys P. J. (1987) Effects of suprachiasmatic nuclei lesions on circadian and ultradian

rhythms in body temperature in ocular enucleated rats. J. Interdiscipl. Cycle Res. 18, 259-273.

Sokoiove P. G. and Bushell W. N. (1978) The chi-square periodogram: Its utility for analysis of circadian rhythms. J. Theor. Biol. 72, 131-160.

Stupfel M., Gourlet V., Court L., Mestries J., Perramon A. and Merat P. (1987) Periodic analysis of ultradian respir- atory variations in laboratory vertebrates of various circadian activities. Chronobiologia 14, 365-375.

Wollnik F. and Turek F. W. (1988) Estrous correlated modulations of circadian and ultradian wheel-running activity rhythms in LEW/Ztm rats. Physiol. Behav. 43, 389-396.