Enhanced NMDA receptor activity in retinal inputs to the rat suprachiasmatic nucleus during the subjective night

Circadian oscillator mechanisms in the suprachiasmatic nucleus (SCN) can be reset by photic input, which is mediated by glutamatergic afferents originating in the retina. A key question is why light can only induce phase shifts of the biological clock during a restricted period of the circadian cycle, namely the subjective night. One of several possible mechanisms holds that glutamatergic transmission at retinosuprachiasmatic synapses would be altered, in particular the contribution of glutamate receptor subtypes to the postsynaptic response. By studying the contributions of NMDA and non-NMDA glutamate receptors to the retinal input to SCN in whole-cell patch-clamp recordings in acutely prepared slices, we tested the hypothesis that NMDA receptor current evoked by optic nerve activity is potentiated during the subjective night. During the day the NMDA component of the EPSC evoked by optic nerve stimulation was found less frequently and was significantly smaller in magnitude than during the night. In contrast, the non-NMDA component did not show a significant day-night difference. When the magnitude of the NMDA component was normalized to that of the non-NMDA component, the day-night difference was maintained, suggesting a selective potentiation of NMDA receptor conductance. In addition to contributing to electrically evoked EPSCs, the NMDA receptor was found to sustain a small, tonically active inward current during the night phase. No significant tonic contribution by NMDA receptors was detected during the day. These results suggest, first, a dual mode of NMDA receptor function in the SCN and, second, a clock-controlled type of receptor plasticity, which may gate the transfer of photic input to phase-shifting mechanisms operating at the level of molecular autoregulatory feedback loops.