Microinjection of NMDA into the SCN region mimics the phase shifting effect of light in hamsters.

Although there is considerable data that glutamate is the primary transducer of photic information to the circadian clock in the suprachiasmatic nucleus (SCN), the ability of glutamate to mimic the phase-shifting effects of light has yet to be demonstrated in vivo. In the present study, microinjections of the glutamate agonist NMDA directly into the SCN of Syrian hamsters induced significant phase delays at circadian time (CT) 13.5 and phase advances at CT 19. These results support the hypothesis that glutamate is the primary neurotransmitter responsible for the transduction of photic information to the SCN.