Abstract
In mammal, circadian rhythms are driven by a pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The pacemaker is entrained to environmental light-dark cycle via the retino-hypothalamic tract, which terminates predominantly in the ventral SCN. The main neurotransmitter of the tract is glutamate. Previous studies show that the ventral part of the SCN responds to light more quickly than the dorsal SCN. However, it is unknown that how the dorsal SCN is entrained to the ventral. The dorsal part of the SCN exhibits an endogenous rhythm of arginine vasopressin (AVP) in vivo and in vitro. Measuring AVP releasing rhythm in the SCN slice culture, we examined phase-shifting effect of N-methyl-D-aspartate (NMDA) on the rhythm of the dorsal SCN. The pulse application of NMDA induced phase delays at early subjective night and phase advances at late subjective night. Even in the presence of bicucullin, a GABA-A antagonist, NMDA also induced phase shift of the rhythm. On the other hand, VIP antagonist inhibited NMDA-induced phase shift. These results suggest that VIP but not GABA would concern the photic entrainment of the dorsal SCN. [J Physiol Sci. 2006;56 Suppl:S209]
