The Proportion of Light-Responsive Neurons Determines the Limit Cycle Properties of the Suprachiasmatic Nucleus

Feb 5, 2014Journal of biological rhythms

The number of light-sensitive neurons influences the daily rhythm patterns of the brain's biological clock

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Abstract

The entrainment range of the suprachiasmatic nucleus (SCN) may be positively related to coupling strength when coupling is weak.

The SCN is responsible for regulating circadian rhythms and is influenced by light exposure.
Entrainment range can be measured by exposing animals to light-dark cycles of varying durations.
Coupling between SCN neurons affects their ability to synchronize and respond to external light stimuli.
When all neurons receive external light input, the SCN exhibits consistent limit cycle behavior.
A weak coupling strength among neurons is associated with a greater entrainment range.
Stronger coupling beyond a critical threshold may lead to a reduced entrainment range.

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In mammals, the central clock in the suprachiasmatic nucleus (SCN) controls physiological and behavioral circadian rhythms and is entrained to the external light-dark cycle. The ability of the SCN to entrain can be measured by exposing the animal to a light-dark cycle with a duration that deviates from 24 h (T-cycles); a wider entrainment range reflects a higher ability to entrain. The neurons of the SCN are either light responsive or light unresponsive and are mutually synchronized. The coupling and synchronization between individual SCN neurons and between groups of neurons within the SCN influence the SCN's ability to entrain. Some studies suggest that enhanced coupling decreases the entrainment range, whereas others suggest that enhanced coupling increases the entrainment range. The latter results are surprising, as they are not consistent with the prevalent assumption that the SCN is a limit cycle oscillator that has larger phase shifts when the amplitude is smaller. Here, we used the Poincaré and Goodwin models to test entrainment properties using various proportions of neurons that are responsive to an external stimulus. If all neurons receive external input, the SCN shows limit cycle behavior in all conditions. If all neurons do not receive light input, we found that the entrainment range of the SCN was positively related to coupling strength when coupling was weak. When coupling strength was stronger and above a critical value, the entrainment range was negatively correlated with coupling strength. The results obtained from our simulations were confirmed by analytical studies. Thus, the limit cycle behavior of the SCN appears to be critically dependent on the coupling strength among the neurons and the proportion of neurons that respond to the entraining stimulus.

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The Proportion of Light-Responsive Neurons Determines the Limit Cycle Properties of the Suprachiasmatic Nucleus

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