Transient fluctuation of serum melatonin rhythm is suppressed centrally by vitamin B12

Vitamin B12 has been reported to improve sleep-wake rhythm disorders. Although the mechanism is still unclear, a change in the sensitivity of the circadian clock system to photic input is thought to be a possible mechanism of the effect. In this study, the effect of the vitamin B12 on the circadian aspect of the electroretinogram (ERG) and serum melatonin level was analyzed in rats. Vitamin B12, alpha-(5,6-dimethylbenzimidazolyl)-co-methyl-cobamide was daily administrated subcutaneously for 8 weeks to adult male Wister rats in the experimental group, and saline was given to the control group. The ERGs were recorded under dark adaptation during the night and day, and under light adaptation (0.1 lux) during the night. Blood was drawn before and after ERG recording. The amplitudes of the a-wave, b-wave, and trough-to-peak of both waves and latencies of ERG were analyzed following various exposures to stimuli of light intensity. These parameters in the group treated with vitamin B12 showed similar characteristics to the control group, and no significant difference was observed between the two groups. The melatonin levels of both groups before the measurement of ERG were similar under each measurement condition. The elevated serum melatonin concentration in the control group under dark adaptation at night was suppressed after the series of 10-msec light stimuli used for measurement of ERG. However, this suppressing effect of light pulses on melatonin level was significantly inhibited in the group treated with vitamin B12. Under light adaptation during the night and under dark adaptation during the day, melatonin levels after the measurement of ERG were not different between the groups. From these results, it is suggested that vitamin B12 if effective in suppressing melatonin rhythm disturbances introduced by transient light stimulation, and it affects the site more central than the retinal level.