Research reportSleep properties of CS mice with spontaneous rhythm splitting in constant darkness
Introduction
Although it is well known that several parameters of sleep are altered by genetic factors, the genes affecting sleep have not been identified in most cases. However, recent progress in molecular biology such as transgenic mice or knockout mice makes it possible to specify the gene involved in the mechanisms for the regulation of sleep. So far, several studies have been performed to examine alteration of sleep and sleep regulation in gene targeted mice. Of which, the most remarkable progress was made in orexin knockout mice which exhibit behavior similar to human narcolepsy [4].
In addition to this, evidence is increasing that the mechanisms underlying circadian and sleep regulation may be correlate with one another. For example, mice devoid of prion protein show alteration in both circadian activity rhythms and sleep [12], and the mutation of Clock which is involved in the core of the circadian clock mechanisms affects sleep homeostasis in mice [10]. A recent report also indicates that cryptochrome 1 and 2 genes (cry1 and 2), necessary for the generation of circadian rhythms, are implicated in sleep homeostatic regulation [18]. Therefore it is possible to speculate that mice with abnormal circadian rhythms also exhibit abnormal sleep properties.
CS strain of mice, which is an inbred line originally established from hybrids between the NBC and SII strain, has three remarkable characteristics in the circadian rhythm of locomotor activity; (1) high activity both during the day and night, (2) unstable freerunning activity with longer circadian period than 24 h and (3) spontaneous rhythm splitting under constant darkness (DD) [1]. CS is also sensitive to external stimuli and often shows a sudden cramp or a muscle attack. The circadian profiles of clock genes in the suprachiasmatic nucleus (SCN) during rhythm splitting are essentially the same as those observed under unsplit conditions [2], [3]. However, the mPer1 gene (one of the core clock genes) expression throughout the day is bimodal in the piriform and cingulate cortex, peaking in correspondence with two split components of locomotor activity. Thus it is interesting to examine sleep properties of CS mice. In this report, we attempted to characterize the sleep of CS mice comparing with other inbred strains (C3H/He and C57BL/6J) with normal circadian rhythmicity.
Section snippets
Animals and housing
Male CS (n=9), C57BL/6J (n=5) and C3H/He mice (n=4) were used. CS mice were maintained in our laboratory, and C57BL/6J and C3H/He were purchased from a dealer (Chubu-kagaku-shizai, Japan). All mice used in this experiment were between 8 and 12 weeks of age, and wheel-running activity (diameter of wheel, 10 cm) was recorded under light–dark (LD) 12:12-h cycles and DD. Chronobiology Kit (Stanford Software Systems) was used for the recording.
Surgery
All mice were deeply anesthetized with pentobarbital (50
Daily rhythmicity of Awake, SWS and PS in three strains
C57BL/6J and C3H/He mice exhibited a clear daily sleep–wake rhythm with large amounts of sleep during the light than during the dark under LD. The rhythmicity persisted under DD with essentially the similar pattern as that under LD (Fig. 2). The amount of Awake, SWS and PS was similar under LD and DD in these mice (Fig. 3).
The rhythmicity of CS mice under both LD and DD, however, were not so clear as other strains. In all sleep states, the daily rhythmicity became obscure (Fig. 2). These are
Discussion
Several studies have demonstrated genetic variations in sleep/wake architecture and EEG profiles in inbred mice [6], [7], [14], [16], [17]. For example, C57BL or C57BR mice are reported to show long PS sleep episodes, short SWS episodes and significant circadian variations under LD, while BALB/c mice have very short PS sleep episodes and unclear diurnal variations. Using CXB (derived from a cross between C57BL/6 and BALB/c) and BXD (derived from C57BL/6J and DBA/2) recombinant inbred lines,
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