The Journal of Neuroscience, January 1, 1999, 19(1):372-380
The Role of the Intergeniculate Leaflet in Entrainment of
Circadian Rhythms to a Skeleton Photoperiod
Kim
Edelstein and
Shimon
Amir
Center for Studies in Behavioral Neurobiology, Department of
Psychology, Concordia University, Montreal, Quebec, Canada H3G 1M8
Mammalian circadian rhythms are synchronized to environmental
light/dark (LD) cycles via daily phase resetting of the
circadian clock in the suprachiasmatic nucleus (SCN). Photic
information is transmitted to the SCN directly from the retina via the
retinohypothalamic tract (RHT) and indirectly from the retinorecipient
intergeniculate leaflet (IGL) via the geniculohypothalamic tract (GHT).
The RHT is thought to be both necessary and sufficient for photic
entrainment to standard laboratory light/dark cycles. An obligatory
role for the IGL-GHT in photic entrainment has not been demonstrated.
Here we show that the IGL is necessary for entrainment of circadian rhythms to a skeleton photoperiod (SPP), an ecologically relevant lighting schedule congruous with light sampling behavior in nocturnal rodents. Rats with bilateral electrolytic IGL lesions entrained normally to lighting cycles consisting of 12 hr of light followed by 12 hr of darkness, but exhibited free-running rhythms when housed under an
SPP consisting of two 1 hr light pulses given at times corresponding to
dusk and dawn. Despite IGL lesions and other damage to the visual
system, the SCN displayed normal sensitivity to the entraining light,
as assessed by light-induced Fos immunoreactivity. In addition, all
IGL-lesioned, free-running rats showed masking of the body temperature
rhythm during the SPP light pulses. These results show that the
integrity of the IGL is necessary for entrainment of circadian rhythms
to a lighting schedule like that experienced by nocturnal rodents in
the natural environment.
Key words:
suprachiasmatic nucleus; neuropeptide Y; immediate early
gene; geniculohypothalamic tract; retinohypothalamic tract; body
temperature
Copyright © 1999 Society for Neuroscience 0270-6474/99/191372-09$05.00/0
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