A Wake-Promoting Circadian Output Circuit in Drosophila

Oct 2, 2018Current biology : CB

A Daily Body Clock Circuit That Promotes Wakefulness in Fruit Flies

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Abstract

DN1p clock neurons in fruit flies can be divided into two distinct subsets, with one subset influencing sleep regulation.

One subset of DN1p neurons projects around the pars intercerebralis, a known circadian output region.
The second subset sends projections to the anterior optic tubercle (AOTU), a visual processing center.
DN1p neurons inhibit tubercular-bulbar (TuBu) neurons, which are associated with promoting consolidated sleep.
TuBu neurons form synapses with R neurons in the ellipsoid body, a region involved in various functions including sleep homeostasis.

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Circadian clocks play conserved roles in gating sleep and wake states throughout the day-night cycle [1-5]. In the fruit fly Drosophila melanogaster, DN1p clock neurons have been reported to play both wake- and sleep-promoting roles [6-11], suggesting a complex coupling of DN1p neurons to downstream sleep and arousal centers. However, the circuit logic by which DN1p neurons modulate sleep remains poorly understood. Here, we show that DN1p neurons can be divided into two morphologically distinct subsets. Projections from one subset surround the pars intercerebralis, a previously defined circadian output region [12]. In contrast, the second subset also sends presynaptic termini to a visual processing center, the anterior optic tubercle (AOTU) [13]. Within the AOTU, we find that DN1p neurons inhibit a class of tubercular-bulbar (TuBu) neurons that act to promote consolidated sleep. These TuBu neurons in turn form synaptic connections with R neurons of the ellipsoid body, a region linked to visual feature detection, locomotion, spatial memory, and sleep homeostasis [14-17]. Our results define a second output arm from DN1p neurons and suggest a role for TuBu neurons as regulators of sleep drive.

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A Wake-Promoting Circadian Output Circuit in Drosophila

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