The histaminergic system fulfills a major role in the maintenance of waking. Histaminergic neurons are located exclusively in the posterior hypothalamus from where they project to most areas of the central nervous system. The histamine H 3 receptors are autoreceptors damping histamine synthesis, the firing frequency of histamine neurons, and the release of histamine from axonal varicosities. It is noteworthy that this action also extends to heteroreceptors on the axons of most other neurotransmitter systems, allowing a powerful control over multiple homeostatic functions. The particular properties and locations of histamine H 3 receptors provide quite favorable attributes to make this a most promising target for pharmacological interventions of sleep and waking disorders associated with narcolepsy, Parkinson's disease, and other neuropsychiatric indications.
Sleep and WakingWakefulness and consciousness depend on perturbation of intrinsic cortical activity that is achieved through ascending activating systems taking a ventral route, the ascending reticular activating system of Moruzzi and Magoun, and a dorsal route whose main station is the hypothalamus. Both pathways send direct projections to the cortex and indirect ones through the thalamus, the door to perception of sensory input. Both waking and paradoxical sleep (REM) are conscious states and need activation by subcortical structures, although in different ways.Cholinergic neurons of the brainstem and the basal forebrain discharge tonically during both wakefulness and paradoxical sleep, can directly excite cortical neurons, and facilitate the thalamo-cortical transmission by inhibiting the thalamic reticular sleep-onset generator. Brainstem and basal forebrain cholinergic neurons are excited by glutamatergic, noradrenergic, and histaminergic neurons; they are particularly important in cortical electroencephalogram arousal, but do not seem to be essential for the waking state (Szymusiak and McGinty, 1986;Lin, 2000).Monoaminergic ascending projections containing catecholamines and serotonin are involved in sleep-waking regulation and the pathophysiology of major psychiatric disorders, schizophrenia, and depression, all of which include disturbance in sleep-waking. Inhibition of catecholamine synthesis decreases waking, and psychostimulants, such as amphetamine, increase waking through accumulation of catecholamines. A lesion of the serotonergic dorsal raphe causes insomnia. These aminergic systems mediate different behavioral expressions during waking, activate immediate early genes, and facilitate locomotion, perception, and cognition (reviewed in Jones, 2005).