Reentrainment To 8hr Advanced Ld Cycle

Male SAMP8 and SAMRl (4, 12 and 24 months of age) were used in the re-entrainment experiment. All mice were transferred to individual mouse cages (32 x 20 x 13 cm) for measuring locomotor activity rhythm. These cages were kept in a box equipped with an infrared sensor (F5B, Omron, Osaka, Japan) and a fluorescent lamp. The movements of each mouse in its cage were counted by the infrared sensor, and total activity counts were collected every 6 min by a computer. For the examination of the effect of melatonin on the speed of re-entrainment to the 8-h advanced LD, the drinking water of some of the SAMP8 and SAMR1 was supplemented with melatonin (13.3^g/ml with 0.2% ethanol as the vehicle). All of the mice had free access to the standard mouse chow and drinking water with or without melatonin. One week after the start of their access to melatonin-water or vehicle-water, the LD cycle was advanced for 8 hr for some mice. The phase-advanced time (hr) was defined as the advanced time of activity onset on Day 3 after the LD shift.

In the first experiment, we compared the phase-advanced time three days after the initiation of the LD shift in SAMP8 with that in SAMR1. The phase-advanced times for 12- and 24-month-old mice were lower than that for the 4-month-old mice (p < 0.05 and p < 0.01, ANOVA, respectively). In the comparison of the shift in the phase-advanced (i.e., re-entrainment) time among 4-, 12- and 24-month-old mice, the re-entrainment in the SAMP8 (2.62hr) occurred about twice as fast as it did in the SAMR1 (1.16 hr). Thus, aging may affect SAMP8 rather than SAMR1 (Figure 1).

In the next experiment, we examined the effect of the drinking water administration of melatonin on an impairment of re-entrainment to a new LD cycle in aging mice. In the SAMR1, melatonin significantly ameliorated the impairment of re-entrainment in 4-, 12- and 24-month-old mice (p < 0.05, Student's t-test), whereas in the SAMP8, it did so only in 12-month-old mice (p < 0.01, Student's t-test) (Figure 1).

Control Melatonin

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Time of Day

Time of Day

Figure 1. Re-entrainment of locomotor activity rhythm to 8-hr advanced light-dark cycles and effect of melatonin drinking on re-entrainment in SAMP8 and SAMR1 mice. Top vertical bar shows the schedule of the light-dark cycle. The light-dark cycle was advanced on Day8. Re-entrainment occurred more quickly in the SAMR1 than in the SAMP8 (left panel). Melatonin drinking facilitated the re-entrainment to the advanced light-dark cycle (right panel).

Sleeping Sound

Sleeping Sound

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