A half-centre oscillator encodes sleep pressure

Abstract

SummaryOscillatory neural dynamics are an inseparable part of mammalian sleep. Characteristic rhythms are associated with different sleep stages and variable levels of sleep pressure, but it remains unclear whether these oscillations are passive mirrors or active generators of sleep. Here we report that sleep-control neurons innervating the dorsal fan-shaped body ofDrosophila(dFBNs) produce slow-wave activity (SWA) in the delta frequency band (0.2–1 Hz) that is causally linked to sleep. The dFBN ensemble contains one or two rhythmic cells per hemisphere whose membrane voltages oscillate in anti-phase between hyperpolarized DOWN and depolarized UP states releasing bursts of action potentials. The oscillations rely on direct interhemispheric competition of two inhibitory half-centres connected by glutamatergic synapses. Interference with glutamate release from these synapses disrupts SWA and baseline as well as rebound sleep, while the optogenetic replay of SWA (with the help of an intersectional, dFBN-restricted driver) induces sleep. Rhythmic dFBNs generate SWA throughout the sleep–wake cycle—despite a mutually antagonistic ‘flip-flop’ arrangement with arousing dopaminergic neurons—but adjust its power to sleep need via an interplay of sleep history-dependent increases in dFBN excitability and homeostatic depression of their efferent synapses, as we demonstrate transcriptionally, structurally, functionally, and with a simple computational model. The oscillatory format permits a durable encoding of sleep pressure over long time scales but requires downstream mechanisms that convert the amplitude-modulated periodic signal into binary sleep–wake states.

An integrative sensor of body states: how the mushroom body modulates behavior depending on physiological context

“…When the need to sleep is high, awake flies are in a tired state in which important functions of the brain like attention and MB-dependent memory consolidation are compromised ( Ganguly-Fitzgerald et al 2006 ; Seugnet et al 2008 ; Li et al 2009 ). During this state, specific networks in the CC start displaying sleep-like state properties like slow wave activity, which promotes sensory decoupling and sleep need signaling ( Raccuglia et al 2019 , 2022 ; Hasenhuetl et al 2024 ). This finding indicates that tiredness is experienced at a network level, a notion supported by local sleep events during the wake in mammals that affect circuit function and lead to task-specific impairments ( Vyazovskiy et al 2011 ; Nir et al 2017 ; Quercia et al 2018 ; Suárez-Grimalt and Raccuglia 2021 ).…”