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.