Network Structure of the Master Clock Is Important for Its Primary Function.

Changgui Gu,Jos Rohling,Huijie Yang,Jiahui Li,Jian Zhou

doi:10.3389/fphys.2021.678391

Abstract

A master clock located in the suprachiasmatic nucleus (SCN) regulates the circadian rhythm of physiological and behavioral activities in mammals. The SCN has two main functions in the regulation: an endogenous clock produces the endogenous rhythmic signal in body rhythms, and a calibrator synchronizes the body rhythms to the external light-dark cycle. These two functions have been determined to depend on either the dynamic behaviors of individual neurons or the whole SCN neuronal network. In this review, we first introduce possible network structures for the SCN, as revealed by time series analysis from real experimental data. It was found that the SCN network is heterogeneous and sparse, that is, the average shortest path length is very short, some nodes are hubs with large node degrees but most nodes have small node degrees, and the average node degree of the network is small. Secondly, the effects of the SCN network structure on the SCN function are reviewed based on mathematical models of the SCN network. It was found that robust rhythms with large amplitudes, a high synchronization between SCN neurons and a large entrainment ability exists mainly in small-world and scale-free type networks, but not other types. We conclude that the SCN most probably is an efficient small-world type or scale-free type network, which drives SCN function.

Highlights

Introduction to Complex NetworksSeveral typical types of the network structures have been widely studied in a lot of fields, including all-to-all connectivity networks, regularly connected networks, Newman-Watts (NW) small-world networks and scale-free networks (Vasalou et al, 2009; Hafner et al, 2012; Gu and Yang, 2016; Gu et al, 2019a)
The suprachiasmatic nucleus (SCN) rhythm originates from the oscillation of the individual neurons, but the function of the SCN depends on the collective behaviors from all of the neurons within the SCN network (Michel and Meijer, 2020)
The robust rhythm of the SCN depends on the synchronization between the neurons, the recovery from the jetlag desynchrony is related to the synchronization of the neuronal communities in the DM and the VL parts of the SCN, and the entrainment of the SCN to external T-cycles relies on the coupling between neurons

Summary

Introduction of the Master Clock

There are many rhythmic processes present in our body, one of which is the so-called circadian rhythm, which helps our body to adjust to and anticipate daily changes in our environment. In certain very specific circumstances these two parts can become desynchronized and the left and right SCN start to oscillate in anti-phase, meaning that a phase difference of the two components stabilizes to 12 h (or π) (de la Iglesia et al, 2000; Ohta et al, 2005) This phenomenon is called splitting and can even be observed in the behavior of the animal, where the animal shows two bouts of behavioral activity in each 24-h cycle. The amplitude of SCN rhythms differs between the short photoperiod, e.g., 8 h:16 h light:dark cycle (representing winter days: short days and long nights) and the long photoperiod, e.g., 16 h:8 h light:dark cycle (representing summer days) (VanderLeest et al, 2007; Meijer et al, 2010; Gu et al, 2016) This difference results from the degree of synchronization of the activity phase of the neurons and not from the amplitude of the single neurons. The conclusions and discussions are presented in section “Discussion and Conclusion.”

Introduction to Complex Networks

Findings

DISCUSSION AND CONCLUSION