Local 5-HT signal bi-directionally regulates the coincidence time window of associative learning

Abstract

Temporal coincidence between the conditioned stimulus (CS) and unconditioned stimulus (US) is essential for associative learning across species. Despite its ubiquitous presence, the mechanism that may regulate this time window duration remains unclear yet. Using olfactory associative learning in Drosophila as a model, we find that suppressing or promoting serotonin (5-HT) signal could respectively shorten or prolong the coincidence time window of odor-shock associative learning and synaptic plasticity in mushroom body (MB) Kenyon cells (KCs). Capitalizing on GPCR-activation based (GRAB) sensors for 5-HT and acetylcholine (ACh), we characterized the in vivo 5-HT dynamics in MB lobes during odor and shock stimulations and further dissected this microcircuit. Interestingly, local KC-released ACh activates nicotinic receptors on the dorsal paired medial (DPM) neuron, and in turn the DPM neuron releases 5-HT to inhibit the ACh signal via the 5-HT1a receptor. Finally, we demonstrated that the DPM-mediated serotonergic feedback circuit is sufficient and necessary to regulate the coincidence time window. This work provides a model for studying the temporal contingency of environmental events and their causal relationship.