Astrocytes keep time

The mammalian suprachiasmatic nucleus (SCN) can autonomously generate circadian oscillations in gene expression and neuronal activity, enabling it to fulfil its role as the brain's 'master circadian clock'. Although the contributions of specific neuronal populations to SCN function have begun to be elucidated, the potential influences of SCN astrocytes are relatively unexplored. Brancaccio et al. now reveal an important role for astrocyte–neuron signalling in SCN timekeeping.

Credit: Mel Curtis/Getty

Long-term imaging revealed the presence of circadian oscillations in astrocytic [Ca²⁺]_i

SCN neurons exhibit circadian oscillations in their intracellular calcium level ([Ca²⁺]_i), peaking during the circadian 'day'. To determine whether similar fluctuations in activity are observed in astrocytes, the authors expressed a genetically encoded reporter of astrocytic [Ca²⁺]_i in organotypic SCN slices. Long-term imaging revealed the presence of circadian oscillations in astrocytic [Ca²⁺]_i, which was at its highest during the circadian 'night' and thus was anti-phasic to that of neurons.

Astrocytes release 'gliotransmitters', including glutamate, in response to an increase in [Ca²⁺]_i. When the authors expressed a genetically encoded sensor of the extracellular glutamate concentration ([Glu]_e) in SCN slices, they observed circadian oscillations in [Glu]_e that were in phase with astrocytic [Ca²⁺]_i. oscillations. That astrocytes were the source of the measured [Glu]_e was supported by the fact that the pharmacological inhibition of astrocytic glutamate catabolism or the genetic ablation of astrocytes, respectively, increased or reduced [Glu]_e.

The authors found that manipulations that altered [Glu]_e disrupted oscillations in neuronal [Ca²⁺]_i and the expression of the 'clock' gene period circadian protein homologue 2 (PER2). Similar effects were observed when slices were treated with an antagonist of NMDA receptors (NMDARs) containing the subunit NR2C, and the effects were particularly prominent in the dorsal SCN. Furthermore, antagonism of NR2C-containing NMDARs resulted in increased neuronal activity during the circadian 'night', suggesting that astrocytic glutamate release usually acts to inhibit neuronal activity. These findings thus suggested that oscillations in astrocytic glutamate release act via NR2C-containing receptors (which the authors showed are located presynaptically and drive inhibitory neurotransmitter release) to maintain circadian timekeeping in the dorsal SCN.

To further assess the importance of these astrocyte–neuron interactions, the authors sought to 'decouple' the periods of circadian oscillations in neurons and astrocytes. In slices, a genetic manipulation that lengthened the period of astrocytic [Ca²⁺] _i oscillations relative to those of neurons disrupted the spatiotemporal characteristics of neuronal clock gene expression in the dorsal SCN. When a similar manipulation was performed in adult mice, the authors observed a lengthening in the circadian period of locomotor activity, demonstrating the influence of SCN astrocytes on circadian behaviour.

The authors propose a model in which astrocyte activity during the circadian night acts to maintain inhibitory tone across the SCN circuit and suggest that this is essential for SCN timekeeping. More broadly, these findings add to our growing understanding of the important roles that astrocytes have in neural circuit function throughout the brain.