Dopamine signaling in wake promoting clock neurons is not required for the normal regulation of sleep in Drosophila

Abstract

Dopamine is a wakefulness promoting neuromodulator in mammals and fruit flies. In D. melanogaster, the network of clock neurons that drives sleep/activity cycles comprises both wake and sleep promoting cell types, indicating that the sleep-wake circuitry is intimately linked to the circadian clock. The large and small ventrolateral neurons (l-LNvs and s-LNvs) have been identified as wake-promoting neurons within the clock neuron network. The l-LNvs are innervated by dopaminergic neurons, and earlier work proposed that dopamine signaling raises cAMP levels in the l-LNvs and thus induces excitatory electrical activity (action potential firing), which results in wakefulness and inhibits sleep. Here, we test this hypothesis by combining cAMP imaging and patch-clamp recordings in isolated brains. We find that dopamine application indeed increases cAMP levels and depolarizes the l-LNvs, but surprisingly, it does not result in increased firing rates. Down-regulation of the excitatory dopamine receptor, Dop1R1 in the l-and s-LNvs, but not of Dop1R2, abolished the depolarization of l-LNvs in response to dopamine. This indicates that dopamine signals via Dop1R1 to the l-LNvs. Down-regulation of Dop1R1 or Dop1R2 receptors in the l- and s-LNvs does not affect sleep. Unexpectedly, we find a moderate decrease of daytime sleep with down-regulation of Dop1R1 and of nighttime sleep with down-regulation of Dop1R2. Since the l-LNvs do not utilize Dop1R2 receptors and the s-LNvs respond also to dopamine, we conclude that the s-LNvs are responsible for the observed decrease in nighttime sleep. In summary, dopamine signaling in the wake-promoting LNvs is not required for daytime arousal, but likely promotes nighttime sleep via the s-LNvs. Significance statementIn insect and mammalian brains, sleep promoting networks are intimately linked to the circadian clock, and the mechanisms underlying sleep and circadian timekeeping are evolutionarily ancient and highly conserved. Here we show that dopamine, one important sleep modulator in flies and mammals, plays surprisingly complex roles in the regulation of sleep by clock containing neurons. Dopamine inhibits neurons in a central brain sleep center to promote sleep and excites wake-promoting circadian clock neurons. It is therefore predicted to promote wakefulness through both of these networks. Nevertheless, our results reveal that dopamine acting on wake promoting clock neurons promotes sleep, revealing a previously unappreciated complexity in the dopaminergic control of sleep.