Abstract
The mammalian central circadian pacemaker (the suprachiasmatic nucleus, SCN) contains thousands of neurons that are coupled through a complex network of interactions. In addition to the established role of the SCN in generating rhythms of ∼24 hours in many physiological functions, the SCN was recently shown to be necessary for normal self-similar/fractal organization of motor activity and heart rate over a wide range of time scales—from minutes to 24 hours. To test whether the neural network within the SCN is sufficient to generate such fractal patterns, we studied multi-unit neural activity of in vivo and in vitro SCNs in rodents. In vivo SCN-neural activity exhibited fractal patterns that are virtually identical in mice and rats and are similar to those in motor activity at time scales from minutes up to 10 hours. In addition, these patterns remained unchanged when the main afferent signal to the SCN, namely light, was removed. However, the fractal patterns of SCN-neural activity are not autonomous within the SCN as these patterns completely broke down in the isolated in vitro SCN despite persistence of circadian rhythmicity. Thus, SCN-neural activity is fractal in the intact organism and these fractal patterns require network interactions between the SCN and extra-SCN nodes. Such a fractal control network could underlie the fractal regulation observed in many physiological functions that involve the SCN, including motor control and heart rate regulation.
Highlights
In mammals, many physiological and behavioral variables, including heart rate and motor activity, exhibit temporal structures that are similar across widely different time scales, i.e. ‘‘fractal’’ or ‘‘scale-invariant’’ patterns [1,2]
By separately analyzing data collected from both mice and rats during light-dark cycles (LD: 12 h:12 h; Figure 1), we found that the fluctuation function F(n) of the in vivo multi-unit neural activity (MUA) possessed a powerlaw form (a straight line in the log-log plot: F(n),na) at time scales from,1 minute up to 10 hours, i.e., spanning a range of more than two orders of magnitude (Figure 2)
The fractal patterns of MUA were similar to those observed in motor activity of humans and rats [2,8] as well as to those observed in motor activity data collected from a subgroup of mice in this study
Summary
Many physiological and behavioral variables, including heart rate and motor activity, exhibit temporal structures that are similar across widely different time scales, i.e. ‘‘fractal’’ or ‘‘scale-invariant’’ patterns [1,2]. We recently discovered in rodents that the master clock of the circadian system (suprachiasmatic nucleus; SCN) [7] is essential for the overall expression of normal fractal patterns in motor activity fluctuations over a wide range of time scales from minutes to ,24 hours [8]. These fluctuation patterns cannot be generated by a simple superposition of independent oscillations at different time scales [9], and require feedback interactions between control nodes that affect a physiological system at multiple time scales [2,8]. Testing the last two hypotheses will allow us to determine whether genetic mouse models can be used in future studies to better understand the neural circuitry of fractal regulation
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