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

The clock regulating mammalian circadian rhythmicity resides in the suprach iasmatic nucleus. The intergeniculate leaflet, a major component of the sub cortical visual system, has been shown to be essential for certain aspects of circadian rhythm regulation. We now report that midbrain visual nuclei a fferent to the intergeniculate leaflet are also components of the hamster c ircadian 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 subje ctive day. Such damage did not disturb phase response to a novel wheel stim ulus. The amount of wheel running or open field locomotion were equivalent in lesioned and control groups after benzodiazepine treatment. Electrical s timulation of the deep superior colliculus, without its own effect on circa dian rhythm phase, greatly attenuated light-induced phase shifts. Such stim ulation was associated with increased FOS protein immunoreactivity in the s uprachiasmatic nucleus. The results show that the circadian rhythm system i ncludes the visual midbrain and distinguishes between mechanisms necessary for phase response to benzodiazepine and those for phase response to locomo tion in a novel wheel. The results also refute the idea that benzodiazepine -induced phase shifts are the consequence of induced locomotion. Finally, t he 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 pro jecting to the intergeniculate leaflet and, via the final common path of th e geniculohypothalamic tract, from the leaflet to the suprachiasmatic nucle us.