LIGHT-INDUCED PHASE-SHIFTS IN ONSET AND OFFSET OF RUNNING-WHEEL ACTIVITY IN THE SYRIAN-HAMSTER

The effects of Light pulses on activity onset and offset were assessed in intergeniculate leaflet- and ventral lateral geniculate nucleus-le sioned Syrian hamsters with a precise onset and offset of circadian wh eel-running activity. Light pulses applied to animals in constant dark ness during the early subjective night induced phase delays in both ac tivity onset and offset, while Light pulses during the late subjective night induced phase advances in the onset and offset of activity. Des pite the fact that the direction of onset and offset shifts were simil ar, differences were found in the magnitude of light-induced phase shi fts. Steady state phase delays were larger in the activity onset, whil e steady state phase advances were largest in the offset of activity. We found phase delays and phase advances within one cycle after the pr esentation of a Light pulse in both activity onset and offset. Differe nces in magnitude of these immediate phase shifts in activity onset an d offset resulted in a compression of activity time for a number of cy cles following a light pulse. Similar results were obtained in a selec ted group of intact animals indicating that intergeniculate leaflet- a nd ventral lateral geniculate nucleus-lesioned hamsters provide a good model to investigate the effects of light on circadian onset and offs et of running-wheel activity.