Editors' ChoiceCircadian Rhythms

GABA, the time keeper

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Sci. Signal.  04 Aug 2015:
Vol. 8, Issue 388, pp. ec213
DOI: 10.1126/scisignal.aad1082

Neurons in the suprachiasmatic nuclei (SCN) of brain play the role of central timekeepers, controlling multiple aspects of physiology and behavior on a circadian cycle entrained by light. For most plants and animals, the daily light cycle changes with the seasons. The neurotransmitter γ-aminobutryic acid (GABA) is a key signal in neuronal circuits of the SCN, and DeWoskin et al. and Myung et al. used mathematical modeling and experimental observations to investigate the role of GABAergic transmission in SCN clock neurons. Mathematical simulations of GABA-mediated postsynaptic currents (PSCs) along with experimental validation in SCN neurons in mouse brain slices by DeWoskin et al. showed that GABA signaling in SCN neurons had two components: a phasic (fast) and a tonic (slow) component. Phasic GABA-mediated PSCs either increased (when the stimuli were given shortly after the firing of an action potential) or decreased (when the stimuli were given at later times in the interspike period between action potentials) the neuronal firing rate but did not shift the phases in the oscillations of the simulated amounts of the transcription of a gene encoding a component of the circadian clock, PER2. The tonic GABA-mediated PSCs, which occurred when the neurons were depolarized, shifted the phases of PER2 oscillation and desynchronized the activity of neurons in the SCN. Changing the proportion of neurons depolarized by GABA to neurons hyperpolarized by GABA in the model of the SCN network changed the firing-rate rhythm but not the amount of synchronous activity, suggesting that the balance of neurons excited by or inhibited by GABA can shift the circadian rhythm of the master clock.

Using time-lapse imaging, Myung et al. investigated the activity of a Bmal1-driven bioluminescent reporter (Bmal1-ELuc) in individual neurons in SCN explants from mice trained under long-day (LD) or short-day (SD) light-dark cycles. Oscillations in the signal from the reporter of neurons in the dorsal SCN (D-SCN) had a phase gap relative to the oscillations of ventral SCN (V-SCN) neurons. This phase gap increased linearly with increasing day length. Mathematical analysis also predicted that the SCN neurons are comprised of two clusters defined by day length. One cluster is predicted to have “positive coupling,” which enabled the oscillations to come into phase and another with “negative coupling,” which allowed the oscillations to go out of phase. The model predicted that “tunable coupling”—an adjustment of the negative coupling strength between D-SCN and V-SCN—encoded the day length. Application of chloride cotransporter channel inhibitor, furosemide, to the D-SCN or V-SCN individually increased the period of Bmal1-ELuc oscillations in the D-SCN, but not in the V-SCN. Coapplication with ionotropic GABAA receptor inhibitor, gabazine, prevented this effect of furosemide, suggesting that the duration of the period involved GABAA receptor mediation and is a consequence of GABA excitation under high intracellular concentration chloride concentration in the D-SCN. When furosemide was applied to the whole SCN, period lengthening of Bmal1-ELuc oscillations occurred similar to that seen when the drug was only applied to the D-SCN, suggesting that the V-SCN follows D-SCN and that differences in intracellular chloride concentration contributed to the “tunable coupling.” Thus, both these studies suggest differential response of the V-SCN and D-SCN to GABAergic neurotransmission determines the phase relationship and period difference between D-SCN and V-SCN, resulting in seasonal day-length encoding of the SCN clock.

D. DeWoskin, J. Myung, M. D. C. Belle, H. D. Piggins, T. Takumi, D. B. Forger, Distinct roles for GABA across multiple timescales in mammalian circadian timekeeping. Proc. Natl. Acad. Sci. U.S.A. 112, 3911–3919 (2015). [PubMed]

J. Myung, S. Hong, D. DeWoskin, E. De Schutter, D. B. Forger, T. Takumi, GABA-mediated repulsive coupling between circadian clock neurons in the SCN encodes seasonal time. Proc. Natl. Acad. Sci. U.S.A. 112, 3920–3929 (2015). [PubMed]