, 2002 and Vyazovskiy and Harris, 2013). Once a sensor reports a deviation

from the set point that calls for corrective action, the cv-c-dependent activity switch is thrown. To do so, the homeostat must compare a physiological variable representing the sleep balance of the organism to a reference representing the homeostatic set point. An obvious cellular mechanism for making such a comparison is the generation of an action potential, which classically SCH727965 solubility dmso involves the application of a fixed criterion to variable synaptic input. Unexpectedly, our recordings demonstrate that it is the ability of sleep-promoting neurons to generate action potentials, and not necessarily the magnitude of the synaptic drive they experience, that varies as a

function of sleep need ( Figure 7). Given that the excitability of dorsal FB neurons increases during waking, the barrage of synaptic impulses or the force of endogenous pacemaker currents will, at some point, suffice to push the cells across threshold, marking the onset of sleep and thus closing the regulatory loop. The mechanism we envisage, in which action potential generation by sleep-promoting neurons is a principal variable encoding the sleep balance of the animal, does not exclude that other sleep-related changes take place as well. For example, activity levels in the brain Autophagy Compound Library regions providing a synaptic reference signal to the dorsal FB might determine precisely when the transition to sleep occurs, creating a potential link between the intensity of waking experience and the induction of restorative sleep. Several genetic and pharmacological manipulations that Carnitine dehydrogenase target the mushroom bodies delay the onset of sleep, reduce sleep rebound, and enhance performance after extended waking (Donlea et al., 2012, Seugnet et al., 2008 and Seugnet et al., 2011), highlighting one possible source of relevant input signals to the dorsal FB. Arousal-promoting dopaminergic

projections to the dorsal FB constitute another (Liu et al., 2012 and Ueno et al., 2012). Of course, alternative scenarios are also possible in which sleep need is sensed entirely outside the dorsal FB and communicated to sleep-control neurons via synaptic, metabolic, endocrine, or even glial pathways that feed into cell-intrinsic signaling systems converging on Cv-c. The cell-intrinsic events downstream of Cv-c are already visible in outline. Our measurements of the electrical properties of dorsal FB neurons give a clear indication that sleep history alters the number or conductive properties of ion channels in the plasma membrane of these neurons. When sleep pressure is low, sleep-control neurons have low input resistances and short time constants that are diagnostic of the opening of transmembrane conductances (Figures 5 and 7). These conductances quickly dissipate depolarizing currents, limit the amplitude of membrane potential fluctuations, and oppose spiking.