Spontaneous fos expression in the suprachiasmatic nucleus of young and old mice.

The senescence-accelerated mouse (SAMP8) is an animal model of aging that displays an array of circadian rhythm disruptions as early as 7 months of age. The present study explored the physiological basis for age-related changes in circadian rhythms by measuring c-Fos immunostaining. Cellular activity in the SCN "core" and "shell" was examined for 2-, 7-, and 12-month-old SAMP8 at circadian times (CTs) 2 and 14. Consistent with previous studies in rats, we observed higher levels of cellular activity at CT2 than at CT14, and higher levels of activity in the "shell" than in the "core" of the SCN. However, there was no effect of age on the pattern of cellular activity in either the "core" or the "shell" of the SCN. These results are discussed in the context of current research on spontaneous and light-induced c-Fos expression in the SCN of rodents.