ABSTRACT
The essential but enigmatic functions of sleep1,2 must be reflected in molecular changes sensed by the brain's sleep-control systems. In the fruitfly Drosophila, about two dozen sleep-inducing neurons3 with projections to the dorsal fan-shaped body (dFB) adjust their electrical output to sleep need4, via the antagonistic regulation of two potassium conductances: the leak channel Sandman imposes silence during waking, whereas increased A-type currents through Shaker support tonic firing during sleep5. Here we show that oxidative byproducts of mitochondrial electron transport6,7 regulate the activity of dFB neurons through a nicotinamide adenine dinucleotide phosphate (NADPH) cofactor bound to the oxidoreductase domain8,9 of Shaker's KVß subunit, Hyperkinetic10,11. Sleep loss elevates mitochondrial reactive oxygen species in dFB neurons, which register this rise by converting Hyperkinetic to the NADP+-bound form. The oxidation of the cofactor slows the inactivation of the A-type current and boosts the frequency of action potentials, thereby promoting sleep. Energy metabolism, oxidative stress, and sleep-three processes implicated independently in lifespan, ageing, and degenerative disease6,12-14-are thus mechanistically connected. KVß substrates8,15,16 or inhibitors that alter the ratio of bound NADPH to NADP+ (and hence the record of sleep debt or waking time) represent prototypes of potential sleep-regulatory drugs.
Subject(s)
Drosophila melanogaster/physiology , Mitochondria/metabolism , Potassium Channels/chemistry , Potassium Channels/metabolism , Protein Subunits/metabolism , Sleep/physiology , Action Potentials , Animals , Drosophila Proteins/metabolism , Drosophila melanogaster/cytology , Electron Transport , Energy Metabolism , Female , Luminescent Proteins/metabolism , NADP/metabolism , Neurons/metabolism , Optogenetics , Oxidation-Reduction , Oxidative Stress , Oxidoreductases/metabolism , Protein Subunits/chemistry , Reactive Oxygen Species , Recombinant Fusion Proteins/metabolism , Shaker Superfamily of Potassium Channels/metabolism , Sleep Aids, Pharmaceutical , Time FactorsABSTRACT
Sleep disconnects animals from the external world, at considerable risks and costs that must be offset by a vital benefit. Insight into this mysterious benefit will come from understanding sleep homeostasis: to monitor sleep need, an internal bookkeeper must track physiological changes that are linked to the core function of sleep. In Drosophila, a crucial component of the machinery for sleep homeostasis is a cluster of neurons innervating the dorsal fan-shaped body (dFB) of the central complex. Artificial activation of these cells induces sleep, whereas reductions in excitability cause insomnia. dFB neurons in sleep-deprived flies tend to be electrically active, with high input resistances and long membrane time constants, while neurons in rested flies tend to be electrically silent. Correlative evidence thus supports the simple view that homeostatic sleep control works by switching sleep-promoting neurons between active and quiescent states. Here we demonstrate state switching by dFB neurons, identify dopamine as a neuromodulator that operates the switch, and delineate the switching mechanism. Arousing dopamine caused transient hyperpolarization of dFB neurons within tens of milliseconds and lasting excitability suppression within minutes. Both effects were transduced by Dop1R2 receptors and mediated by potassium conductances. The switch to electrical silence involved the downregulation of voltage-gated A-type currents carried by Shaker and Shab, and the upregulation of voltage-independent leak currents through a two-pore-domain potassium channel that we term Sandman. Sandman is encoded by the CG8713 gene and translocates to the plasma membrane in response to dopamine. dFB-restricted interference with the expression of Shaker or Sandman decreased or increased sleep, respectively, by slowing the repetitive discharge of dFB neurons in the ON state or blocking their entry into the OFF state. Biophysical changes in a small population of neurons are thus linked to the control of sleep-wake state.
Subject(s)
Drosophila melanogaster/physiology , Homeostasis , Sleep/physiology , Animals , Cell Membrane/metabolism , Dopamine/metabolism , Dopaminergic Neurons/metabolism , Drosophila melanogaster/anatomy & histology , Drosophila melanogaster/cytology , Electric Conductivity , Female , Male , Neurotransmitter Agents/metabolism , Optogenetics , Potassium/metabolism , Potassium Channels/chemistry , Potassium Channels/metabolism , Protein Transport , Receptors, Dopamine/metabolism , Shaker Superfamily of Potassium Channels/metabolism , Sleep Deprivation , Sleep Initiation and Maintenance Disorders/physiopathology , Time Factors , Wakefulness/physiologyABSTRACT
The neurochemical mechanisms that contribute to synaesthesia are poorly understood, but multiple models implicate serotonin and GABA in the development of this condition. Here we used psychophysical tasks to test the predictions that synaesthetes would display behavioral performance consistent with reduced GABA and elevated serotonin in primary visual cortex. Controls and synaesthetes completed the orientation-specific surround suppression (OSSS) and tilt-after effect (TAE) tasks, previously shown to relate to GABA and serotonin levels, respectively. Controls and synaesthetes did not differ in the performance parameter previously associated with GABA or in the magnitude of the TAE. However, synaesthetes did display lower contrast difference thresholds in the OSSS task than controls when no surround (NS) was present. These results are inconsistent with the hypothesized roles of GABA and serotonin in this condition, but provide preliminary evidence that synaesthetes exhibit enhanced contrast discrimination.
