g., muscarinic antagonists, H1-histamine antagonists, or α2-adrenergic agonists) cause acute sleepiness, but chronic ablation of the basal forebrain cholinergic neurons (Kaur et al., 2008), tuberomammillary histaminergic neurons (Gerashchenko et al., 2004), the LC and pontine cholinergic neurons (Lu et al., 2006a, Shouse and Siegel, 1992 and Webster selleck inhibitor and Jones, 1988), or combinations of these structures (Blanco-Centurion et al., 2007) have minimal effects on the amount of wakefulness. One possible reason for this puzzling result is that the arousal system may contain sufficient redundancy that remaining wake-promoting systems may be able to compensate for the chronic (but perhaps

not acute) loss of one or even a few components, e.g., by increasing activity or receptor sensitivity in intact arousal systems. A related issue is which of these wake-promoting cell groups participate in the switching between sleep and wakefulness, as opposed to the maintenance of the waking state. This issue will be taken up in the section of this review on switching circuitry. During the epidemic of encephalitis lethargica around click here the time of the World War I, Von Economo (1930) reported that patients with lesions in the preoptic region around the rostral

end of the third ventricle demonstrated profound insomnia. Experimental lesions of the preoptic-basal forebrain region reduced sleep in rats and cats (McGinty and Sterman, 1968 and Nauta, 1946), but the exact population of sleep-promoting neurons was unknown. Sherin and colleagues (Sherin et al., 1996) subsequently identified a population of neurons in the ventrolateral preoptic nucleus (VLPO) that innervate the histaminergic TMN and that express Fos protein selectively during sleep but not wakefulness. VLPO neurons, containing the inhibitory neurotransmitters GABA and galanin,

innervate other components of the ascending arousal system as well, including the LC, raphe system, periaqueductal gray matter, parabrachial nucleus, and lateral hypothalamic area (Sherin et al., 1998). The VLPO was Sodium butyrate found to consist of a dense core of sleep-active, galanin-positive neurons that project heavily to the TMN. However, this is surrounded dorsally and medially by a more diffuse population of sleep-active, galanin-positive neurons, the extended VLPO, which more extensively targets the dorsal raphe and LC (Lu et al., 2000 and Sherin et al., 1998). As is true for many cell groups in the hypothalamus that are defined on the basis of common neurotransmitter, connections and physiology, the VLPO neurons are mixed in among other cell types. In addition, while there are other galaninergic neurons laterally in the basal forebrain and medially in the preoptic area, none of these are sleep-active or project to the TMN, LC, or dorsal raphe (Sherin et al., 1998 and Gaus et al., 2002).