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 supr achiasmatic nucleus is the final part of a non-photic phase shifting p athway to pacemakers in hamsters, or that neuropeptide Y is necessary for other pathways to be effective. Experiments in which two stimuli a re presented during the same circadian cycle have shown that phase shi fts in response to at least two non-photic stimuli are attenuated by a subsequent light pulse during the subjective day. This study was cond ucted to investigate the neural site of the blocking effect of light o n non-photic stimuli. Experiment 1 showed that phase shifts in respons e to induced wheel-running during the subjective day are greatly atten uated by a subsequent light pulse. Experiment 2 showed that phase shif ts in response to injections of neuropeptide Y in the middle of the su bjective day were also greatly reduced by a subsequent light pulse. Th ese results provide some insight about the site of the blocking action of light on non-photic phase shifts. Because there is evidence indica ting that neuropeptide Y may mediate phase shifts in response to induc ed activity, and because light was able to block phase shifts produced by neuropeptide Y, we conclude that, in blocking activity-induced shi fts, light must act downstream from the release of neuropeptide Y into the suprachiasmatic nucleus.