Eg. Marchant et Lp. Morin, The hamster circadian rhythm system includes nuclei of the subcortical visual shell, J NEUROSC, 19(23), 1999, pp. 10482-10493
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.