The hamster circadian rhythm system includes nuclei of the subcortical visual shell.

The clock regulating mammalian circadian rhythmicity resides in the suprachiasmatic nucleus. The intergeniculate leaflet, a major component of the subcortical visual system, has been shown to be essential for certain aspects of circadian rhythm regulation. We now report that midbrain visual nuclei afferent to the intergeniculate leaflet are also components of the hamster circadian rhythm system. Loss of connections between the intergeniculate leaflet and visual midbrain or neurotoxic lesions of pretectum or deep superior colliculus (but not of the superficial superior colliculus) blocked phase shifts of the circadian activity rhythm in response to a benzodiazepine injection during the subjective day. Such damage did not disturb phase response to a novel wheel stimulus. The amount of wheel running or open field locomotion were equivalent in lesioned and control groups after benzodiazepine treatment. Electrical stimulation of the deep superior colliculus, without its own effect on circadian rhythm phase, greatly attenuated light-induced phase shifts. Such stimulation was associated with increased FOS protein immunoreactivity in the suprachiasmatic nucleus. The results show that the circadian rhythm system includes the visual midbrain and distinguishes between mechanisms necessary for phase response to benzodiazepine and those for phase response to locomotion in a novel wheel. The results also refute the idea that benzodiazepine-induced phase shifts are the consequence of induced locomotion. Finally, the data provide the first indication that the visual midbrain can modulate circadian rhythm response to light. A variety of environmental stimuli may gain access to the circadian clock mechanism through subcortical nuclei projecting to the intergeniculate leaflet and, via the final common path of the geniculohypothalamic tract, from the leaflet to the suprachiasmatic nucleus.