Aspartate injections into the suprachiasmatic region of the Syrian hamster do not mimic the effects of light on the circadian activity rhythm.

The excitatory amino acids glutamate and aspartate are thought to be involved in the photic entrainment of the circadian pacemaker of the suprachiasmatic nuclei (SCN). When applied to the SCN region glutamate imitates the effects of dark pulses on the circadian activity rhythm rather than those of light. We have now injected aspartate into the SCN region of Syrian hamsters. These injections mimicked the effects of dark pulses as well, in so far as slight advances of the activity rhythm were obtained during the subjective day. However, the mean phase shift was not significantly different from the shift obtained with control injections. It is concluded that (1) aspartate has little or no effect on the phase of the circadian activity rhythm and (2) none of the putative transmitters of the photic afferents of the SCN produces the effects of light upon injection.