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-lesioned Syrian hamsters with a precise onset and offset of circadian wheel-running activity. Light pulses applied to animals in constant darkness during the early subjective night induced phase delays in both activity onset and offset, while light pulses during the late subjective night induced phase advances in the onset and offset of activity. Despite the fact that the direction of onset and offset shifts were similar, differences were found in the magnitude of light-induced phase shifts. Steady state phase delays were larger in the activity onset, while steady state phase advances were largest in the offset of activity. We found phase delays and phase advances within one cycle after the presentation of a light pulse in both activity onset and offset. Differences in magnitude of these immediate phase shifts in activity onset and offset resulted in a compression of activity time for a number of cycles following a light pulse. Similar results were obtained in a selected group of intact animals indicating that intergeniculate leaflet- and ventral lateral geniculate nucleus-lesioned hamsters provide a good model to investigate the effects of light on circadian onset and offset of running-wheel activity.