Blocking the phase-shifting effect of neuropeptide Y with light.

Previous studies have indicated that the neuropeptide Y input from the intergeniculate leaflet of the lateral geniculate nucleus to the suprachiasmatic nucleus is the final part of a non-photic phase shifting pathway to pacemakers in hamsters, or that neuropeptide Y is necessary for other pathways to be effective. Experiments in which two stimuli are presented during the same circadian cycle have shown that phase shifts in response to at least two non-photic stimuli are attenuated by a subsequent light pulse during the subjective day. This study was conducted to investigate the neural site of the blocking effect of light on non-photic stimuli. Experiment 1 showed that phase shifts in response to induced wheel-running during the subjective day are greatly attenuated by a subsequent light pulse. Experiment 2 showed that phase shifts in response to injections of neuropeptide Y in the middle of the subjective day were also greatly reduced by a subsequent light pulse. These results provide some insight about the site of the blocking action of light on non-photic phase shifts. Because there is evidence indicating that neuropeptide Y may mediate phase shifts in response to induced activity, and because light was able to block phase shifts produced by neuropeptide Y, we conclude that, in blocking activity-induced shifts, light must act downstream from the release of neuropeptide Y into the suprachiasmatic nucleus.