N-Methyl-D-aspartate microinjected into the suprachiasmatic nucleus mimics the phase-shifting effects of light in the diurnal Nile grass rat (Arvicanthis niloticus).

Mammals exhibit circadian rhythms in behavior generated by the suprachiasmatic nucleus (SCN). Exposure to light synchronizes the circadian clock to the environmental light:dark cycle through the release of glutamate into the SCN. In nocturnal animals such as Syrian hamsters, direct application of NMDA to the SCN results in phase shifts similar to those produced by exposure to light. This study was designed to determine if light phase shifts the circadian pacemaker of diurnal Nile grass rats (Arvicanthis niloticus) housed in constant darkness by acting on NMDA-type glutamate receptors in the suprachiasmatic nucleus (SCN). N-Methyl-D-aspartate (NMDA; 0, 1, 10, 50, and 100 mM) was administered through guide cannulae aimed at the SCN at circadian times when light induces phase shifts. Maximal phase delays were attained with 50 mM NMDA, and maximal phase advances were seen after 100 mM NMDA. A phase-response curve (PRC) for NMDA, determined by administering NMDA at each hour over the circadian cycle, resembled the PRC to light in this species. These data support the hypothesis that NMDA-type glutamate receptors play a critical role in mediating the phase shifting effects of light in diurnal, as well as nocturnal, animals. In addition, these data suggest that diurnal grass rats may be less sensitive to the phase shifting properties of NMDA than nocturnal rodents.