Astrocytic control of extra-cellular GABA drives circadian time-keeping in the suprachiasmatic nucleus

Abstract

The hypothalamic suprachiasmatic nucleus (SCN) is the master mammalian circadian clock. Its cell-autonomous timing mechanism, a transcriptional/translational feedback loop (TTFL), drives daily peaks of neuronal electrical activity. Intercellular signals synchronize and amplify TTFL and electrical rhythms across the circuit. SCN neurons are GABAergic, but the role of GABA in circuit-level time-keeping is unclear. SCN slices expressing the GABA sensor iGABASnFR demonstrate a circadian oscillation of extracellular GABA ([GABA]e) that, counter-intuitively, runs in antiphase to neuronal activity, peaking in circadian night. Resolving this paradox, we found that [GABA]e is regulated by GABA transporters (GATs), uptake peaking during circadian day. This is mediated by the circadian-regulated, astrocytically expressed GAT3 (Slc6a11). Clearance of [GABA]e in circadian day facilitates neuronal firing, neuropeptide release and TTFL rhythmicity. Moreover, genetic complementation demonstrated that the astrocytic TTFL can alone drive [GABA]e rhythms. Thus, astrocytic clocks maintain SCN circadian time-keeping by temporally controlling GABAergic inhibition of SCN neurons.