SummaryThe suprachiasmatic nucleus (SCN) of the hypothalamus orchestrates daily rhythms of physiology and behavior in mammals. Its circadian (∼24 hr) oscillations of gene expression and electrical activity are generated intrinsically and can persist indefinitely in temporal isolation. This robust and resilient timekeeping is generally regarded as a product of the intrinsic connectivity of its neurons. Here we show that neurons constitute only one “half” of the SCN clock, the one metabolically active during circadian daytime. In contrast, SCN astrocytes are active during circadian nighttime, when they suppress the activity of SCN neurons by regulating extracellular glutamate levels. This glutamatergic gliotransmission is sensed by neurons of the dorsal SCN via specific pre-synaptic NMDA receptor assemblies containing NR2C subunits. Remarkably, somatic genetic re-programming of intracellular clocks in SCN astrocytes was capable of remodeling circadian behavioral rhythms in adult mice. Thus, SCN circuit-level timekeeping arises from interdependent and mutually supportive astrocytic-neuronal signaling.

By screening N-ethyl-N-nitrosourea-mutagenized animals for alterations in rhythms of wheel-running activity, we identified a mouse mutation, after hours (Afh). The mutation, a Cys(358)Ser substitution in Fbxl3, an F-box protein with leucine-rich repeats, results in long free-running rhythms of about 27 hours in homozygotes. Circadian transcriptional and translational oscillations are attenuated in Afh mice. The Afh allele significantly affected Per2 expression and delayed the rate of Cry protein degradation in Per2::Luciferase tissue slices. Our in vivo and in vitro studies reveal a central role for Fbxl3 in mammalian circadian timekeeping.