Optic nerve stimulation has been reported to evoke a field potential (FP) in rat suprachiasmatic nucleus (SCN) slices. Methylcobalamin,δ-(5,6-dymethylbenzimidazolyl)-Co-methyl-cobamide (Me-B12) enhanced this FP and the enhancement lasted more than 1 h after washing out. Maximal enhancement (143.6 ± 9.8%) was achieved at a concentration of 10 μM. By contrast, cyanocobalamin containing CN- instead of CH3- showed no enhancement of the amplitude in the FP. Me-B12 induced enhhncement of FP was strongly blocked by an N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonovaleric acid (APV). These results indicate that CH3- in the Me-B12 is required to modulate the FP amplitude and the NMDA receptor is involved in the long-lasting FP enhancement induced by Me-B12. The present results suggest that Me-B12 modifies the photic entrainment of the circadian clock in the suprachiasmatic nucleus via an activation of NMDA receptors.
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