Summary Intercellular signaling is important for accurate circadian rhythms. In Drosophila, the small ventral lateral neurons (s-LNvs) are the dominant pacemaker neurons and set the pace of most other clock neurons in constant darkness. Here we show that two distinct G-protein signaling pathways are required in LNvs for 24hr rhythms. Reducing signaling in LNvs via the G-alpha subunit Gs, which signals via cAMP, or via the G-alpha subunit Go, which we show signals via Phospholipase 21c, lengthens the period of behavioral rhythms. In contrast, constitutive Gs or Go signaling makes most flies arrhythmic. Using dissociated LNvs in culture, we found that Go and the metabotropic GABAB-R3 receptor are required for the inhibitory effects of GABA on LNvs and that reduced GABAB-R3 expression in vivo lengthens period. Although no clock neurons produce GABA, hyper-exciting GABAergic neurons disrupts behavioral rhythms and s-LNv molecular clocks. Therefore, s-LNvs require GABAergic inputs for 24hr rhythms.

Circadian rhythms offer probably the best understanding of how genes control behavior, and much of this understanding has come from studies in Drosophila. More recently, genetic manipulation of clock neurons in Drosophila has helped identify how daily patterns of activity are programed by different clock neuron groups. Here, we review some of the more recent findings on the fly molecular clock and ask what more the fly model can offer to circadian biologists.