Twenty-four hour rhythm in monoamine oxidase activity in specific areas of the rat brain stem

Diurnal variations in activities of both type A and type B monoamine oxidase (MAO) in specific areas of the brain stem were investigated in male rats maintained on a 12 h light-12 h dark schedule (06.00 to 18.00 h light). A significant diurnal variation in the activity of both type A and type B MAO was observed in the areas A1, A2 and in the locus coeruleus, but not in the cerebellar cortex. For both type A and type B MAO lowest activities were found during the morning, while highest activities occurred in the late afternoon and the beginning of the night. These results may reflect a role of MAO in the circadian variation of the concentrations of catecholamines in the brain stem.