Interneurons of fan-shaped body promote arousal in Drosophila
Sleep is required to maintain physiological functions and is widely conserved across species. To understand the sleep-regulatory mechanisms, sleep-regulating genes and neuronal circuits are studied in various animal species. In the sleep-regulatory neuronal circuits in Drosophila melanogaster, the dorsal fan-shaped body (dFB) is a major sleep-promoting region. However, other sleep-regulating neuronal circuits were not well identified. We recently found a novel sleep-regulatory circuit consisting of arousal-promoting T1 dopamine neurons and protocerebral bridge (PB) neurons innervating the ventral part of the FB, which we named "the PB-FB pathway". However, the post-synaptic target of the PB-FB pathway was still unknown. To identify it, we performed anterograde tracing, immunohistochemistry, and Ca2+ imaging analysis and found that the PB-FB pathway projects to FB interneurons, also known as pontine neurons. Besides, we found that cholinergic pontine neurons promote arousal. Moreover, we indicated that pontine neurons form an anatomical connection with sleep-promoting dFB neurons. Together, we showed that pontine neurons receive excitatory signals from the PB-FB pathway and cholinergic pontine neurons promote arousal. These results completed one of the output pathways from the PB-FB pathway.