Circadian Behavioral Rhythms in Cry1/Cry2 Double-Deficient Mice Induced by Methamphetamine

JOURNAL OF BIOLOGICAL RHYTHMS, Vol. 23 No. 1, February 2008 91-94 DOI: 10.1177/0748730407311124 © 2008 Sage Publications CRYPTOCHROME (CRY) 1 and CRY2 proteins are considered to be indispensable for the circadian rhythm generation. In Cry1 and Cry2 double-deficient (Cry1/Cry2) mice, behavioral activity becomes arrhythmic under constant darkness (DD) and circadian rhythms in Per1 gene expression are not detected in single suprachiasmatic nucleus (SCN) cells, suggesting a lack of a functioning cellular clock in the SCN (van der Horst, 1999; Yamaguchi et al., 2003). Chronic treatment with methamphetamine (MAP) has been known to induce behavioral rhythms with a periodicity in the circadian range in rats and mice (Honma et al., 1986), which is not dependent on the SCN (Honma et al., 1987; Tataroǧlu et al., 2006). Those results suggest that an extra-SCN oscillator(s) for behavioral output is activated by MAP. To understand the mechanisms of MAP-induced behavioral rhythmicity, we examined the effect of MAP on Cry1/Cry2 mice. Wheel-running activity of Cry1/Cry2 mice exhibited significant 24-h periodicities in 12-h light and 12-h dark (LD) cycle (Fig. 1A) with the onset of activity at ZT 9.07 ± 2.81 (mean ± SD, n = 12), where ZT 12 was the time of lights-off. The activity onset was significantly earlier than that in the wild type mice (ZT 11.98 ± 0.06 h, n = 6). In continuous darkness (DD), none of the Cry1/Cry2 mice showed circadian periodicity. By chronic treatment with MAP via drinking water (0.0025% w/v), 10 of 12 Cry1/Cry2 mice exhibited significant wheel-running rhythms with a periodicity ranging from 23.8 to 32.3 h (27.7 ± 3.4 h, mean ± SD) (Fig. 1B, D). The remaining 2 mice did not show circadian rhythmicity up to 75 days of MAP treatment, but one of them exhibited a circabidian (about 48 h) rhythm (Fig. 1C, E). The buildup process of MAPinduced behavioral rhythms in Cry1/Cry2 mice was similar to that observed in the SCN-lesioned animals (Fig. 1B; Honma et al., 1987; Tataroǧlu et al., 2006). In some mice, the circadian rhythms persisted for a substantial period after the termination of MAP treatment (Fig. 2A). These findings indicate that CRY1 and CRY2 are not required for the MAP-induced behavioral rhythms. The circadian period of MAP-induced rhythm in the Cry1/Cry2 mice fluctuated during the course of MAP treatment as exemplified by large standard deviations of individual circadian period, that is, 1.3~4.8 h in Cry1/Cry2 mice, whereas 0.1 ~ 0.2 h in the wild-type mice (Fig. 2A, B). Signs of relative coordination, such as periodic changes in the circadian period with abrupt phase jumps, were detected in wheel-running rhythms of some Cry1/Cry2 mice as well as of the wild-type mice (Fig. 2B, C). In these animals, 2 significant periods, 1 around 24 h and the other around 30 h, were detected in the periodogram. Relative coordination suggests an interaction of 2 oscillations with different periods, which are not completely synchronized to each other (von Holst, 1939; Wever, 1979). Circabidian rhythms with periods of 43 to 50 h appeared in 5 Cry1/Cry2 mice (45.7 ± 0.9 h) (Fig. 1C, E). Circabidian rhythms have been observed exclusively in humans under temporal isolation, and LETTERS