JOURNAL OF BIOLOGICAL RHYTHMS / June 2002Morin and Pace / NEURAL CONTROL OF TauLL The Intergeniculate Leaflet, but Not the Visual Midbrain, Mediates Hamster Circadian Rhythm Response to

Abstract Several studies have demonstrated a variety of effects of intergeniculate leaflet (IGL) lesions on circadian rhythm regulation. Recent studies have sug-gested the possibility that certain rhythm functions attributed to the IGLare actu-ally controlled by retinorecipient midbrain nuclei or other brain areas connected to the IGL. The present investigations evaluated whether midbrain lesions previ-ously shown to block the phasic actions of benzodiazepine would also reduce or eliminate the period-lengthening effect of constant light (LL), a function that has been attributed to the IGL. Experiment 1 established that the circadian period of controls lengthened by about 0.57 h when the animals were transferred from con-stant dark (DD) to LL, but the magnitude of change was attenuated by about 50% in animals with IGL lesions caused by the neurotoxin N-methyl-D-aspartate (NMDA). In experiment 2, controls were compared to groups receiving either NMDAlesions of the pretectum or tectum or knife cuts designed to sever connec-tions between the IGL and more medial retinorecipient nuclei. As in experiment 1, there were no differences between groups with respect to circadian period i