Circadian Structural Plasticity Drives a Timed Switch in the Hierarchy Among Key Pacemaker Neurons

Abstract

Behavioral outputs arise as a result of highly regulated, yet flexible, communication among neurons. In Drosophila, two neuronal clusters, the morning (M) and evening (E) cells, are essential for the characteristic bimodal pattern of daily locomotor activity. However, the precise interaction between these two clusters has not been fully described. Here we show the existence of synaptic input provided by the evening oscillator onto the M-cells. In addition, both structural and functional approaches revealed that E- to M-cell connectivity changes across the day, with higher excitatory input taking place before the day to night transition. We identified two different neurotransmitters, acetylcholine and glutamate, released by E-cells that are relevant for robust circadian output. Indeed, we show that acetylcholine is responsible for the excitatory input from E-cells to M-cells, which show preferential responsiveness to acetylcholine during the evening. Our findings provide evidence of an excitatory feedback between circadian clusters and unveil an important plastic remodeling of the E-cells synaptic connections.