Abstract
The suprachiasmatic nucleus (SCN) is the master circadian clock of mammals, generating and transmitting an internal representation of environmental time that is produced by the cell-autonomous transcriptional/post-translational feedback loops (TTFL) of the 10,000 neurons and 3,500 glial cells. Recently, we showed that TTFL function in SCN astrocytes alone is sufficient to drive circadian timekeeping and behaviour, raising questions about the respective contributions of astrocytes and neurons within the SCN circuit. We compared their relative roles in circadian timekeeping in mouse SCN explants, of either sex. Treatment with the glial-specific toxin fluorocitrate revealed a requirement for metabolically competent astrocytes for circuit-level timekeeping. Recombinase-mediated genetically complemented Cryptochrome (Cry) proteins in Cry1- and/or Cry2-deficient SCN, were used to compare the influence of the TTFLs of neurons or astrocytes in the initiation of de novo oscillation or in pacemaking. While neurons and astrocytes both initiated de novo oscillation and lengthened period equally, their kinetics were different: astrocytes taking twice as long. Furthermore, astrocytes could shorten period, but not as potently as neurons. Chemogenetic manipulation of Gi- and Gq-coupled signalling pathways in neurons acutely advanced or delayed ensemble phase, respectively. In contrast, comparable manipulations in astrocytes were without effect. Thus, astrocytes can initiate SCN rhythms and bi-directionally control SCN period, albeit with lower potency than neurons. Nevertheless, their activation does not influence SCN phase. The emergent SCN properties of high amplitude oscillation, initiation of rhythmicity, pacemaking and phase are differentially regulated: astrocytes and neurons sustain the ongoing oscillation, but its phase is determined by neurons.
Significance Statement:
The hypothalamic suprachiasmatic nucleus (SCN) encodes and disseminates time-of-day information to allow mammals to adapt their physiology to daily environmental cycles. Recent investigations have revealed a role for astrocytes, in addition to neurons, in regulation of this rhythm. Using pharmacology, genetic complementation and chemogenetics, we compared the abilities of neurons and astrocytes in determining the emergent SCN properties of high amplitude oscillation, initiation of rhythmicity, pacemaking and determination of phase. These findings parameterise the circadian properties of the astrocyte population in the SCN, and reveal the types of circadian information astrocytes and neurons can contribute within their heterogeneous cellular network.
Footnotes
The authors declare no financial interests or conflicts of interest.
This work was funded by MRC core funding (MC_U105170643) and the BBSRC grant award (BB/R016658/1). The Authors are grateful for excellent technical support from MRC Biomedical Facility (Ares) and the LMB Electronic and Mechanical Workshops.
This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license, which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.
