Myo-inositol is a ubiquitous carbohydrate that is present in large amounts in brain tissue and is involved in neuronal signalling and osmoregulation. This sugar is an essential component of the inositol signalling system, which is a post-receptor second messenger signalling system found in many cells.Myo-inositol is the precursor of membrane inositol phospholipids, which are critically linked to a number of CNS receptor signalling systems, including muscarinic, serotonergic, adrenergic, metabotropic and histaminergic systems, and those linked to cholecystokinin, tachykinins, neurotensin, platelet activating factor and other transmitters.Upon stimulation of these receptors, a signal is transmitted through a guanosine triphosphate (GTP)-binding protein (Gq), which then activates the enzyme phospholipase C. This results in the release of a second messenger, inositol 1,4,5-trisphosphate (InsP3), from membrane inositol phospholipids. InsP3then causes the release of free intracellular calcium into the cytosol, activating a number of enzymes or receptors.Myo-inositol in the brain is derived from 3 sources: (i) receptor stimulation (a salvage pathway); (ii)de novosynthesis from glucose; and (iii) uptake of dietarymyo-inositol through plasma membranemyo-inositol transporters. Mostmyo-inositol is probably derived from the first 2 sources, which are controlled through the lithium-sensitive enzymemyo-inositol monophosphatase (IMPase). This enzyme acts uponmyo-inositol monophosphates, hydrolysing them to release freemyo-inositol. Recent biochemical, molecular and crystallographic studies have demonstrated that the overall metabolism of brain inositol is closely modulated by this enzyme. Lithium salts, which are commonly used in various psychiatric conditions, inhibit this enzyme, and this action has been implicated as a therapeutic mechanism of action of lithium.A change in the availability of CNS inositol may lead to altered brain cell signalling pathways and, eventually, to the development of a neuropsychiatric disorder. Recent evidence indicates thatmyo-inositol has psychoactive effects, with initial studies demonstrating effectiveness in the treatment of depression, panic disorder and obsessive-compulsive disorder. At present, the exact mechanism of these clinical effects is uncertain. The development of various inositol system-based drugs may lead to future psychoactive drugs designed to modulate a second messenger cascade of events rather than a receptor system, and will lead to further understanding of CNS disease from a post-receptor second messenger perspective.