, 2007). Similarly, even very large and complete lesions of the VLPO at most reduce sleep by about 50%, suggesting that there are other pathways capable of inhibiting the ascending arousal system in the absence of VLPO neurons. Thus, there are likely to be components of our proposed wake-sleep flip-flop switch that are as yet unknown, and evidence in support of this switch and its full understanding will require a more complete knowledge of its components. In addition, it will be important to gain a more complete understanding of the interactions of these components of both the proposed wake-sleep and NREM-REM sleep switches, and to test these circuit models rigorously. One challenge in

dissecting these circuits and testing their functions is that wake-promoting

and sleep-promoting cell populations may overlap spatially in some locations. Fortunately, OSI-906 nmr these neurons have different neurotransmitters and connections, which allow us to introduce novel genetic and transneuronal Raf inhibitor methods for manipulating them independently. Approaches might include testing the functional roles of these components with optogenetic or perhaps genetically driven, ligand-gated channels such as ivermectin-activated chloride channels (Adamantidis et al., 2007 and Lynagh and Lynch, 2010). It would be particularly valuable to record simultaneously from neurons in multiple components of a putative switch while driving one component and to examine their firing patterns across wake-sleep or NREM-REM sleep transitions. We also know little about the mechanisms by which homeostatic sleep drive is either accumulated or discharged. Current evidence points to a role of adenosine, but it is unlikely that this explains the entire process as adenosine levels in many parts of the brain do not increase with prolonged wakefulness (Strecker et al., 2000) and homeostatic sleep

drive persists even in the absence of adenosine receptors. It remains possible that slower, progressive changes as animals rest before sleep may contribute to modulating the flip-flop switch and are not necessarily in conflict with this model. Multiple neuronal dynamics with different time scales are likely to occur within the wake-sleep system. Such slower modulation Megestrol Acetate may contribute to reducing noise and stabilizing the switch and could help explain how putative flip-flop switches could accommodate noisy, unstable neuronal circuits and integrate multiple influences into sharp transitions. It will also be important to understand better how motivation and emotions influence the wake-sleep system. As these terms imply, these drivers for animal behavior involve movement, which is the antithesis of quiet sleep. Unraveling how the orexin neurons and other systems impel motivated behaviors, addictions, and reward will be important new vistas for understanding their overall role in the functions of the brain.