A phosphate transporter in VIPergic neurons of the suprachiasmatic nucleus gates locomotor activity during the light/dark transition in mice.

The suprachiasmatic nucleus (SCN) encodes time of day through changes in daily firing; however, the molecular mechanisms by which the SCN times behavior are not fully understood. To identify factors that could encode day/night differences in activity, we combine patch-clamp recordings and single-cell sequencing of individual SCN neurons in mice. We identify PiT2, a phosphate transporter, as being upregulated in a population of Vip+Nms+ SCN neurons at night. Although nocturnal and typically showing a peak of activity at lights off, mice lacking PiT2 (PiT2-/-) do not reach the activity level seen in wild-type mice during the light/dark transition. PiT2 loss leads to increased SCN neuronal firing and broad changes in SCN protein phosphorylation. PiT2-/- mice display a deficit in seasonal entrainment when moving from a simulated short summer to longer winter nights. This suggests that PiT2 is responsible for timing activity and is a driver of SCN plasticity allowing seasonal entrainment.

Circadian VIPergic Neurons of the Suprachiasmatic Nuclei Sculpt the Sleep-Wake Cycle.

Although the mammalian rest-activity cycle is controlled by a "master clock" in the suprachiasmatic nucleus (SCN) of the hypothalamus, it is unclear how firing of individual SCN neurons gates individual features of daily activity. Here, we demonstrate that a specific transcriptomically identified population of mouse VIP+ SCN neurons is active at the "wrong" time of day-nighttime-when most SCN neurons are silent. Using chemogenetic and optogenetic strategies, we show that these neurons and their cellular clocks are necessary and sufficient to gate and time nighttime sleep but have no effect upon daytime sleep. We propose that mouse nighttime sleep, analogous to the human siesta, is a "hard-wired" property gated by specific neurons of the master clock to favor subsequent alertness prior to dawn (a circadian "wake maintenance zone"). Thus, the SCN is not simply a 24-h metronome: specific populations sculpt critical features of the sleep-wake cycle.

Spatiotemporal single-cell analysis of gene expression in the mouse suprachiasmatic nucleus.

Mammalian circadian behaviors are orchestrated by the suprachiasmatic nucleus (SCN) in the ventral hypothalamus, but the number of SCN cell types and their functional roles remain unclear. We have used single-cell RNA-sequencing to identify the basic cell types in the mouse SCN and to characterize their circadian and light-induced gene expression patterns. We identified eight major cell types, with each type displaying a specific pattern of circadian gene expression. Five SCN neuronal subtypes, each with specific combinations of markers, differ in their spatial distribution, circadian rhythmicity and light responsiveness. Through a complete three-dimensional reconstruction of the mouse SCN at single-cell resolution, we obtained a standardized SCN atlas containing the spatial distribution of these subtypes and gene expression. Furthermore, we observed heterogeneous circadian gene expression between SCN neuron subtypes. Such a spatiotemporal pattern of gene regulation within the SCN may have an important function in the circadian pacemaker.