Tetrodotoxin blocks the circadian effects of NMDA during the day but not at night

Abstract

Light resets the circadian clock in the suprachiasmatic nucleus (SCN) during the night and inhibits the ability of non-photic stimuli, such as injection of muscimol, to reset the clock during the day. Since activation of NMDA receptors appears to mediate the effects of light in the SCN, we investigated whether there are day-night differences in the SCN responses to NMDA. During the night, microinjection of NMDA mimicked the ability of light to induce phase delays, and these effects were not inhibited by blocking Na+-dependent action potentials with tetrodotoxin (TTX). During the day, NMDA mimicked the ability of light to inhibit muscimol-induced phase advances; however, this was inhibited by TTX. These data suggest that Na+-dependent action potentials within the SCN are required for NMDA to mimic the effects of light during the day but not during the night.