Research Article: Intrinsic Regulation of Spatiotemporal Organization within the Suprachiasmatic Nucleus

Date Published: January 7, 2011

Publisher: Public Library of Science

Author(s): Jennifer A. Evans, Tanya L. Leise, Oscar Castanon-Cervantes, Alec J. Davidson, Shin Yamazaki.

Abstract: The mammalian pacemaker in the suprachiasmatic nucleus (SCN) contains a population of neural oscillators capable of sustaining cell-autonomous rhythms in gene expression and electrical firing. A critical question for understanding pacemaker function is how SCN oscillators are organized into a coherent tissue capable of coordinating circadian rhythms in behavior and physiology. Here we undertake a comprehensive analysis of oscillatory function across the SCN of the adult PER2::LUC mouse by developing a novel approach involving multi-position bioluminescence imaging and unbiased computational analyses. We demonstrate that there is phase heterogeneity across all three dimensions of the SCN that is intrinsically regulated and extrinsically modulated by light in a region-specific manner. By investigating the mechanistic bases of SCN phase heterogeneity, we show for the first time that phase differences are not systematically related to regional differences in period, waveform, amplitude, or brightness. Furthermore, phase differences are not related to regional differences in the expression of arginine vasopressin and vasoactive intestinal polypeptide, two key neuropeptides characterizing functionally distinct subdivisions of the SCN. The consistency of SCN spatiotemporal organization across individuals and across planes of section suggests that the precise phasing of oscillators is a robust feature of the pacemaker important for its function.

Partial Text: The mammalian circadian system controlling daily rhythms in behavior and physiology is an assembly of oscillators regulated by a central pacemaker within the suprachiasmatic nucleus (SCN) of the anterior hypothalamus [1]. The SCN displays robust electrical and biochemical rhythms that persist in individual neurons after synaptic communication is disrupted [2], [3]. Within cells of both the SCN and peripheral tissues, transcriptional-translational feedback loops regulate the rhythmic expression of clock genes and their protein products [4]. While the SCN and peripheral clocks appear to operate in a similar fashion at the molecular level, the SCN has unique network properties that synchronize oscillators within the population to form a functional pacemaker [5]. A critical question for understanding pacemaker function concerns how the numerous oscillators within the SCN are organized into a coherent tissue.

In mammals, the function of the central pacemaker depends on coordinated activity across a heterogeneous population of neurons. Here we have developed novel analytical tools and expanded understanding of SCN function by providing comprehensive phase maps under both entrained and free-running conditions. We find regional phase differences across all three dimensions of the SCN; however, the pattern, magnitude, and complexity depend greatly on the rostrocaudal position of the slice. Differences in slice position and regional definitions may account for discrepant descriptions of SCN spatiotemporal organization across previous studies, although the influence of other potential factors cannot be discounted (e.g., different output measures). To map SCN spatiotemporal organization, we have simultaneously imaged consecutive slices collected from the same animal and designed extensive computational analyses that investigate oscillatory function across the SCN without a priori regional definitions, which are techniques unique to the present study. While the resulting phase maps underscore the sophistication of SCN spatiotemporal organization, the most salient features that emerge are the early phase of the caudal SCN and the late-peaking node of the rostral SCN. The consistency of these findings despite methodological differences across experiments (e.g., entrainment condition, plane of section) demonstrates the robustness of SCN phase heterogeneity.