Emergence of Circadian and Photoperiodic System Level Properties from Interactions among Pacemaker Cells

Jul 30, 2008Journal of biological rhythms

How daily and seasonal rhythms arise from interactions between internal timekeeping cells

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Abstract

Computer simulation suggests that mutual electrical activation between pacemaker neurons in the suprachiasmatic nucleus (SCN) accounts for seasonal variations in circadian behavior.

Mutual activation of pacemaker cells and light-induced activation may explain various circadian phenomena.
Self-excitation can lead to circadian rhythms even without light-dark cycles.
Rhythmicity can persist in constant darkness, while constant light may disrupt circadian outputs.
The system can adjust to different light-dark cycle patterns and day lengths.
Specific subsets of cells in the SCN may activate at particular times of day, influencing behavior.

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Daily patterns of behavior and physiology in animals in temperate zones often differ substantially between summer and winter. In mammals, this may be a direct consequence of seasonal changes of activity of the suprachiasmatic nucleus (SCN). The purpose of this study was to understand such variation on the basis of the interaction between pacemaker neurons. Computer simulation demonstrates that mutual electrical activation between pacemaker cells in the SCN, in combination with cellular electrical activation by light, is sufficient to explain a variety of circadian phenomena including seasonal changes. These phenomena are: self-excitation, that is, spontaneous development of circadian rhythmicity in the absence of a light-dark cycle; persistent rhythmicity in constant darkness, and loss of circadian rhythmicity in pacemaker output in constant light; entrainment to light-dark cycles; aftereffects of zeitgeber cycles with different periods; adjustment of the circadian patterns to day length; generation of realistic phase response curves to light pulses; and relative independence from day-to-day variation in light intensity. In the model, subsets of cells turn out to be active at specific times of day. This is of functional importance for the exploitation of the SCN to tune specific behavior to specific times of day. Thus, a network of on-off oscillators provides a simple and plausible construct that behaves as a clock with readout for time of day and simultaneously as a clock for all seasons.

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Emergence of Circadian and Photoperiodic System Level Properties from Interactions among Pacemaker Cells

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