Editors' ChoiceNeuroscience

Daily Dance of Synaptic Partners

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Sci. Signal.  30 Sep 2014:
Vol. 7, Issue 345, pp. ec265
DOI: 10.1126/scisignal.2005959

Circadian clocks control daily rhythms in physiology, metabolism, and behavior. They also affect the consolidation of memory by influencing synaptic plasticity. In the fruit fly Drosophila melanogaster, daily locomotor rhythms are coordinated by a group of circadian pacemaker neurons, the small ventral lateral (sLNv) neurons. The axonal projections of these neurons undergo daily remodeling, and a report from Gorostiza et al. indicates that these pacemaker cells also form and destroy new synapses daily. Over the course of a day, arborizations (branches) of the dorsal termini of sLNv neurons expressing an inducible fluorescent marker decreased in complexity in vivo and in cultured brains. The arborizations remained unchanged overnight, but the axons decreased in length before returning to maximal length early the following day. The distribution of fluorescently tagged synaptic vesicle and active zone markers showed that the synapses formed by these neurons were larger and had more active zones in the early morning than in the early night. The authors visualized synapses between sLNVs and other neurons in adult flies by expressing a membrane-tethered fragment of green fluorescent protein (GFP) in sLNvs and the complementary membrane-tethered GFP fragment in various subsets of neighboring neurons. These fragments reconstitute GFP fluorescence when they are expressed on cells that form synapses with one another. Using this technique, the authors found that sLNvs formed synapses with different partners over the course of a day. They identified clusters of neurons preferentially contacted by sLNvs in the morning and other clusters preferentially contacted at night. Some of the neurons implicated in these rhythmic contacts with sLNvs control sleep and arousal. Depolarizing some of these groups of neurons altered daily locomotor rhythms, suggesting that some of these transient contacts control circadian locomotor activity. Thus, circadian signals direct daily changes in the morphology and connectivity of circadian pacemaker neurons that control daily behavioral rhythms.

E. A. Gorostiza, A. Depetris-Chauvin, L. Frenkel, N. Pírez, M. F. Ceriani, Circadian pacemaker neurons change synaptic contacts across the day. Curr. Biol. 24, 2161–2167 (2014). [PubMed]