Lesions of the thalamic intergeniculate leaflet block activity-induced phase shifts in the circadian activity rhythm of the golden hamster.

A variety of stimuli, which are associated with acute increases in locomotor activity, induce similar phase-dependent shifts in the circadian rhythm of locomotor activity in golden hamsters. Treatment with the benzodiazepine, triazolam (Tz), or transfer of an animal normally housed without a running wheel to a new cage with 1 h of access to a wheel are both examples of such stimuli. Phase shifts normally induced by injections of Tz can be blocked by lesions of the intergeniculate leaflet (IGL) of the thalamus. Experiments were conducted to determine whether phase shifts induced by transfer to a new cage with a running wheel also require an intact IGL. Animals normally housed without running wheels were transferred to new cages with access to wheels for 1 h a few hours before the expected onset of activity. They then received either lesions of the IGL or sham lesions and, after recovery, they were again transferred to new cages with running wheels for 1 h. Lesions of the IGL blocked phase shifts normally induced by wheel access while sham lesions had no effect. The amount of wheel-running and total locomotor activity which occurred during access to the running wheel were significantly reduced by IGL lesions. These results indicate that the phase-shifting effect of a novel running wheel depends upon an intact IGL.