The scientists found a substantial difference between the summer-born and winter-born groups.
The summer-born mice behaved the same whether they had been kept on the summer cycle or switched to the winter cycle. They started running at the time of dusk (as determined by their former day/night cycle), continued for ten hours and then rested for 14 hours.
The behavior of the winter-born mice was much different. Those who had been kept on the winter light cycle through maturation showed basically the same pattern as their summer cousins: They became active at the time of dusk and continued for 10 hours before resting. However, those who had been switched to a summer cycle remained active for an extra hour and a half.
When they looked at what was happening in the brains of the different groups, they found a strikingly similar pattern.
In the summer-born mice, the activity of the neurons in the SCN peaked at the time of dusk and continued for 10 hours. When the winter-born mice were matured in the winter cycle, their neuronal activity peaked one hour after the time of dusk and continued for 10 hours. But, in the winter-born mice switched to a summer cycle, the master bioclock's activity peaked two hours before the time of dusk and continued for 12 hours.
When they looked at the equinox group, the scientists found variations that fell midway between the summer and winter groups. Those subjected to a summer cycle when they matured had biological clocks that peaked one hour before the time of dusk and the biological clocks of those subjected to a winter cycle peaked a half hour after the time of dusk. In both cases the duration of SCN activity was 11 hours.
Their analysis showed that these variations are caused by alterations in the activity patterns of the individual neurons, rat
|Contact: David F. Salisbury|