Photoperiod-dependent correlation between light-induced scn c-fos
expression and resetting of circadian phase.
Tr[acute]avn[acute]ickov[acute]a, Zdena, Alena Sumov[acute]a, Robin
Peters, William J. Schwartz, Helena Illnerov[acute]a.
Institute of Physiology, Academy of Sciences of the Czech Republic,
142 20 Prague 4, Czech Republic, Department of Neurology, University
of Massachusetts Medical School, Worcester, MA 01655 U.S.A.
APStracts 3:0146R, 1996.
In rodents, brief light pulses that shift the phase of the circadian
locomotor rhythm also increase c-fos gene expression in the
suprachiasmatic nucleus (SCN), site of an endogenous clock that
regulates such rhythmicity. Since the magnitude of photic phase
shifts varies when light pulses are applied at different time points
over the course of the subjective night, we examined the degree of
SCN c-fos gene expression after single 30-min light pulses were
delivered at time points that spanned the early and late subjective
night in rats maintained in either short (LD 8:16) or long (LD 16:8)
photoperiods. The light- induced level of c-fos mRNA and the number
of cells expressing immunoreactive c-Fos protein were measured in the
SCN by in situ hybridization and immunohistochemistry, respectively,
and compared to the magnitude of the corresponding phase shifts of
the circadian rhythm of pineal N- acetyltransferase (NAT) activity.
We found a robust correlation between c-fos photoinduction and NAT
phase shifts, but this relationship was dependent on photoperiod. The
degree of c-fos gene expression was strongly correlated with the
magnitude of NAT phase advances and delays under the short
photoperiod, and with phase advances under the long photoperiod, but
not with phase delays under the long photoperiod. The data suggest
that c-fos gene expression in the SCN may be involved in the photic
resetting of the pineal NAT rhythm. Under the long photoperiod,
however, the magnitude of phase delays may be limited by the
functional state of the circadian pacemaking system.
Received 3 November 1995; accepted in final form 13 March 1996.
APS Manuscript Number R686-5.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 23 April 96