Vitamin B₁₂ enhances GABA content but reduces glutamate content in the rat suprachiasmatic nucleus

The present study attempted to elucidate effects of vitamin B₁₂ (VB₁₂) on the rat suprachiasmatic nucleus (SCN), which is the locus of the circadian clock in mammals, by evaluating the contents of its major neurotransmitters, γ-aminobutyric acid (GABA) and glutamate (Glu). First, contents of these amino acids were analyzed from tissue samples prepared at three different circadian times (CTs) in free-running rats under constant dim illumination. The content of GABA was highest in the middle of the resting phase (CT 6), intermediate early in the active phase (CT 14), and lowest late in the active phase (CT 20), whereas Glu content indicated inverse circadian variations. Subsequently, effects of intracerebroventricularly infused VB₁₂ were assayed at CT 20. Compared with the saline-infused control, the infusion of VB₁₂ (30 nmol/30 μl) at CT 12-15 significantly increased GABA content but decreased Glu content. The quantitative balance of these amino acids after VB₁₂ infusion was similar to that at CT 6 in noninfused rats. These results suggest that infusion of VB₁₂ to active rats shifts the ratio of SCN neurotransmitters toward that observed in resting rats.