How quickly does light reset the circadian clock

• Main Author: Best, J.
• Published: University of Cambridge 1998
• Subjects: 591.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596605

The first experiment used the male Syrian hamster as the subject, as it has good wheel-running rhythms with consistent and robust activity onsets and also a well defined phase delay and advance region of the phase response curve (PRC). In this study, a double light pulse paradigm was used to examine if the clock could reset within 2 h. The second light pulse administered 2 h after an initial pulse was used to map the effects of the first light pulse on the resetting behaviour to determine whether the clock had reset to primary light pulse before receiving the second one. The results obtained in both the phase delay and phase advance portions of the PRC were consistent with the hypothesis that the clock of the Syrian hamster could reset within 2 h of a light pulse. The next experiment investigated whether the immediate early genes (IEGs) are a reliable marker of clock resetting and if they have a role in clock resetting. The same double light pulse paradigm employed in the first experiment was used to ascertain if <I>c-fos</I> and <I>egr-l</I> could be induced to two delaying light pulses. The study revealed that both IEGs were induced to each light pulse demonstrating that the clock could differentiate between the two pulses of light and that the immediate early genes may play a role in phase resetting to light. Having established that the clock of the Syrian hamster was reset in 2 h the outbred mouse was used to ascertain if this rapid resetting is common to other mammalian species. The results obtained using multiple light pulses revealed that the mouse clock is also reset within 2 h, supporting the findings in the Syrian hamster and demonstrating cross-species homology. In addition, investigations on CREB phosphorylation, c-Fos induction and AP-l binding activity revealed that all three transcriptional regulators were induced in response to a light pulse during subjective night and that there was a strong correlation with photic phase resetting of the mammalian clock.