Feeding-related circadian variation in tele-methylhistamine levels of mouse and rat brains.

Circadian changes in the brain histamine (HA) and tele-methylhistamine (t-MH) levels were studied in mice and rats after adaptation to an alternating 12-h light/dark cycle (lights on at 0600). Although there was no significant circadian fluctuation of the brain HA levels, the levels of t-MH, a major metabolite of brain HA, showed a marked circadian variation. In mice, the t-MH levels were about 80 ng/g from 1200 to 1800 but about two times higher values were obtained from 2400 to 0600 of the next morning. In rats, the t-MH levels ranged from 24 to 28 ng/g at 0600 and 1200, slightly increased at 1800, and reached at 2400 a peak twice as high as the levels seen during the light period. The t-MH levels again rapidly decreased during the subsequent 3 h. In mice fasted from 1200, the t-MH levels did not increase during the period of darkness. When mice were fed at 1200 after a 24-h fast, a significant increase in the t-MH levels was observed at 1800. There was no significant circadian variation of the HA and t-MH levels in the plasma of mice and rats. These results suggest that circadian variation in brain t-MH levels is related to feeding and possible subsequent changes in elimination of t-MH from the brain and/or turnover of HA in the brain. This phenomenon should be given due attention when HA dynamics in the brain are being assessed.