Biological Rhythm Research 2002, Vol. 33 No. 2. pp. 199–212
0929-1016/02/3302-199$16.00 © Swets & Zeitlinger
The Circadian Activity and Body Temperature Rhythms of Mice During Their Last Days of Life H. Weinert1, D. Weinert1 and J. Waterhouse2 1
Institute of Zoology, Martin-Luther-University Halle-Wittenberg, 06108 Halle, Germany; 2Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, L3 3AF Liverpool, UK
Abstract Motor activity and body temperature rhythms have been investigated telemetrically in old mice until their death. The present paper is based mainly on the data of two animals which could be monitored over a sufficient length of time (more than three weeks), though the other three animals showed similar results. The daily body temperature rhythm of old mice was more stable as compared to the activity rhythm and was detectable until the last day of life. Its magnitude, defined as the difference between maximum and minimum, was similar to that obtained in middle-aged mice. By contrast, the activity rhythm disappeared or started to become erratic earlier. Unlike the position in younger animals, the body temperature rhythm was phase delayed with respect to the activity rhythm. In one mouse, the increased instability of the phase position two weeks before its death led to a free run with a period length of about 23.7 h. Both activity and temperature rhythms were fragmented in old mice. In the case of the body temperature this was obviously caused by masking. After purifying the raw temperatures, the fragmentation disappeared. On the other hand, the free-run condition was not caused by masking. The sensitivity of body temperature to motor activity was different from younger mice, and this probably reflects changes/deteriorations in the physiology of thermoregulation during the last days of life. In one mouse, during the last 4 days of life, a sharp, torpor-like decrease of body temperature corresponding with the time of the daily minimum was observed. This phenomenon was not found in other mice, though all of them died during the falling period of the temperature rhythm. The results confirm our hypothesis that the endogenous clock may work even during the very last days of life. The ability to synchronize with the periodic environment deteriorates earlier. Also they suggest that these phenomena are not only typical for the activity rhythm but apply also to the body temperature rhythm.
Address correspondence to: D. Weinert, Martin-Luther-Universität Halle-Wittenberg, Institut für Zoologie, Domplatz 4, D-06108 Halle, Germany. Tel.: +49-345-5526464; Fax: +49-345-5527152; E-mail:
[email protected]
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Keywords: Circadian rhythm, motor activity, body temperature, ageing, mice.
Introduction The circadian system undergoes pronounced age-dependent changes. These concern all rhythm characters, as has been frequently described (Aschoff, 1994; Turek et al., 1995; Weinert, 2000). Attempts have been made to reveal the putative causes of these changes and to use this knowledge for the treatment of rhythm disturbances (KosterVan Hoffen et al., 1993; Labyak et al., 1998; Van Someren, 2000). In our own investigations on laboratory mice, we have shown that, even though all rhythm characteristics change with age, they do not do so at the same time (Weinert, 2000; Sturm et al., 2000). In particular, the ability to synchronize with the periodic environment becomes lost earlier than circadian rhythmicity per se. This was shown for the daily activity rhythm of laboratory mice (Weinert & Weinert, 1998), in which, during their last weeks of life, the activity rhythm phase was advanced and the phase stability progressively decreased, leading some animals to free run. A circadian component was detectable in many cases even during the very last days of life. However, there were also mice who did not show a pronounced daily activity rhythm during the last weeks of life. Several reasons might be responsible for the observed age-dependent rhythm changes, which arise because of the high complexity of a circadian system which includes oscillator(s), mechanisms of external synchronization and internal coupling (Moore & Silver, 1998). Deterioration in the function of any of these components may result in changes of circadian rhythms in old age. In addition, one must bear in mind that not all changes in the overt rhythm — consisting of endogenous (due to the body clock) and exogenous (masking) components — are caused by changes in the circadian system. For example, temperature rhythms tend to be more statistically robust than activity rhythms even in young animals, and appear to be more resistant to disruption by LL than are activity rhythms (Refinetti & Menaker, 1992; Weinert & Waterhouse, 1999). Therefore the question arises whether the obtained agedependent changes reflect changes in the suprachiasmatic nucleus (SCN) or are caused by changes downstream from the central clock. As the body temperature rhythm is believed to be a better estimate of the endogenous clock, motor activity and body temperature were simultaneously investigated in old mice (Weinert & Waterhouse, 1999), and it was shown that, the rhythms did not change simultaneously with age. Whereas the magnitude of the activity rhythm decreased, the magnitude of the temperature rhythm remained high. Also, the activity rhythm showed a phase advance, but the phase of the body temperature rhythm remained unchanged. Accordingly, the phase relation between the two rhythms changed with age. The results may be discussed in terms of different oscillators regulating both rhythms. The literature provides evidence both for the view that the rhythms are produced by different oscillators or by the same oscillator (for a review, see Weinert & Waterhouse, 1999). Particularly in the second case, at least some of the changes of
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the activity rhythm could then be caused by mechanisms downstream from the SCN. This issue needs to be investigated. Another area of interest is whether a phase advance, a decreased phase stability or even a free-running state is typical for other rhythms, particularly the body temperature rhythm. That is, will an impaired synchronization of the body temperature rhythm be characteristic of very old mice and will the temperature rhythm itself remain longer? Finally, the problem of masking raises the issue whether the sensitivity of body temperature to activity changes in old age. To clarify these questions it was necessary to investigate motor activity and body temperature until death due to old age. Such a study has several problems. The most important is caused by the limited life span of the transmitters used for telemetric measurements. Also, one cannot predict when an animal will die. The mean life span of our mice is 85 weeks but mice can reach 125 weeks or more (Weinert & Weinert, 1998). The guaranteed life span of the transmitters is 3 months, and they should be implanted when the animals are young. Therefore, transmitters must be switched off for several months, and certain probability exists that a mouse dies before or just after its transmitter will be switched on again, or that the transmitter fails before the animal dies. We have so far been able to record activity and body temperature until the last day of life in five mice; however, only in two cases did the records last for several weeks. These results will be presented here.
Methods Experiments were carried out on female laboratory mice of our own outbred stock (HaZ : ICR). Animals were bred and housed in air-conditioned rooms at an ambient temperature of 22+/-2°C and a relative humidity of 55–65%. They were exposed to an artificial light-dark cycle (LD) of 12 : 12 h with lights on from 07:00 to 19:00 h Central European Time. Standardized food (Altromin®) and drinking water were available ad libitum. Spontaneous locomotor activity and the body temperature were continuously recorded using an integrated telemetry and data acquisition system (Dataquest III, Data Sciences Inc., USA). Transmitters (TA10TA-F20) were implanted in the peritoneal cavity. For this purpose the animals were anesthetized by exposure to an atmosphere of enflurane (Ethrane®, Abott GmbH) followed by an i.p. injection of 125 mg/kg ketamin hydrochloride (Ketanest®, Parke, Davis & Co.). An abdominal incision was made, the transmitter implanted and the incision closed using sutures (For further details, see Weinert et al., 1994). Transmitter implantation was performed when animals were between 20 and 35 weeks old. Following some days of activity and temperature recording, the transmitters were switched off by means of an external magnet, and the animals were kept under standardized conditions as described above. One year later the transmitters were switched on again. At this time 5 mice with intact transmitters were alive, and two of them could be investigated for several weeks. Locomotor activity and body temperature were recorded continuously at 10 min intervals until death. Activity impulses
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were summed over 10 minutes, and the temperature, averaged over a 10 sec period, was taken at the end of the 10 min intervals. Missing or obvious artefact temperature values were replaced by interpolated values (these constituted
