Circadian rhythms in vitamin B₁₂ content of the rat brain

The whole brain content of vitamin B₁₂ (VB₁₂) in the rat was assayed by the chemiluminescent immunoassay at 4 h intervals in 12:12 h light-dark cycles (LD) and five different circadian times (CTs) under constant dim illumination (dim LL). In LD-entrained rats, the content of VB₁₂ exhibited a day-night rhythm with a peak 2 h after the dark onset time and a trough 2 h before the light onset time. In freerunning rats under dim LL, the content of VB₁₂ also exhibited a circadian variation with a peak ca. 2 h after the activity onset time (CT14) and a trough ca. 12 h after the activity onset time (CT 0). These findings clearly indicate that the brain VB₁₂ content decreased during the active phase and increased during the resting phase regardless of the lighting schedule. On the other hand, the drinking behavior, as an index of the intake activity, was observed less frequently in the light phase of LD cycles and the resting phase of dim LL. Since most of the digested VB₁₂ is known to be continuously stored in the liver, it is demonstrated that the rhythm of the brain content of VB₁₂ may be caused by brain consumption of VB₁₂ during the active phase and the transportation from the peripheral storage during the resting phase, independent of intake activities.