Drugs for sleep disorders: mechanisms and therapeutic prospects

The past decade has witnessed an explosion of knowledge about the neural mechanisms that control sleep and arousal, triggered by two discoveries relating to the sleep disorder narcolepsy. Narcolepsy is caused by the loss of orexin-containing neurons in the hypothalamus, and a novel nonstimulant wakefulness-promoting drug, modafinil, alleviates excessive day-time sleepiness associated with the disorder. The level of arousal is controlled by an intricate interplay between distinct wakefulness- and sleep-promoting nuclei situated in the hypothalamus and brainstem and the interconnections between the nuclei and the neurotransmitters involved have been mapped. Wakefulness-promoting nuclei include the orexinergic lateral hypothalamic/perifornical area, the histaminergic tuberomammillary nucleus, the cholinergic pedunculopontine tegmental nucleus, the noradrenergic locus coeruleus, the 5-hydroxytryptaminergic raphe nuclei and possibly the dopaminergic ventral tegmental area. The major sleep-promoting nucleus is the GABAergic ventrolateral preoptic nucleus of the hypothalamus. Currently available and future drugs exert their therapeutic effects in the three major classes of sleep disorder (insomnia, hypersomnia, parasomnia) by modifying neurotransmission at distinct sites within the arousal-controlling neuronal network. This enables classification of therapeutic drugs for sleep disorders on the basis of their modes of action: drugs that interact with the GABAergic sleep-promoting system, drugs that interact with different wakefulness-promoting systems and drugs that modulate the level of arousal by mechanisms that do not initially involve the basic network (e.g. melatonin, adenosine). The development of novel therapeutic drugs for sleep disorders is based on the synthesis of molecular/cellular mechanisms and the sites of action within the arousal-controlling neuronal network.