Metaplastic sleep regulation in Drosophila determined by microscale circadian neural dynamics

The biophysical mechanisms by which circadian clock neurons integrate temporal coding signals to regulate sleep remain elusive. Here, using Drosophila , we identify Rabphilin (Rph) in DN1p clock neurons as a key stabilizer of the metaplasticity setpoint governing circadian regulation of sleep. Rph protein levels are elevated at night relative to daytime and modulate stochastic process of DN1p membrane potential dynamics linked to variability in synaptic activity at connections between DN1p neurons and their downstream postsynaptic partners. We find that Rph acts as a bidirectional regulator of synaptic plasticity thresholds. Under dim nocturnal light stimulation, Rph knockdown permits synaptic potentiation, whereas synthesized Rph introduction induces synaptic depression. In contrast, under optogenetic manipulation mimicking daytime spiking in DN1p neurons, these effects are reversed. We further show that spike timing dependent plasticity emerges when postsynaptic spiking is engaged, with nocturnal dim light conditions determining the direction of plasticity. Together, these findings establish a mechanistic link between microscale circadian neural dynamics and hierarchical metaplastic regulation, demonstrating how circadian regulation of sleep dynamically balances stability and adaptive flexibility through circadian setpoints and environmental nocturnal light interactions. ### Competing Interest Statement The authors have declared no competing interest. National Institute of Neurological Disorders and Stroke, R00NS101065 National Institute of General Medical Sciences, https://ror.org/04q48ey07, R35GM142490 Whitehall Foundation, https://ror.org/00her7k05 BrightFocus Foundation, A2021043S Japan Science and Technology Agency, https://ror.org/00097mb19, JPMJPR2386 Research Corporation for Science Advancement, SA-MBC-2024-080c