Neuronal nicotinic receptors in the human brain.

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of ligand gated ion channels which are widely distributed in the human brain. Multiple subtypes of these receptors exist, each with individual pharmacological and functional profiles. They mediate the effects of nicotine, a widely used drug of abuse, are involved in a number of physiological and behavioural processes and are additionally implicated in a number of pathological conditions such as Alzheimer's disease, Parkinson's disease and schizophrenia. The nAChRs have a pentameric structure composed of five membrane spanning subunits, of which nine different types have thus far been identified and cloned. The multiple subunits identified provide the basis for the heterogeneity of structure and function observed in the nAChR subtypes and are responsible for the individual characteristics of each. A substantial amount of information on human nAChR structure and function has come from studies on neuroblastoma cell lines which naturally express nAChRs and from recombinant nAChRs expressed in Xenopus oocytes. In vitro brain nAChR distribution can be mapped with a number of appropriate agonist and antagonist radioligands and subunit distribution may be mapped by in situ hybridization using subunit specific mRNA probes. Receptor distribution in the living human brain can be studied with noninvasive imaging techniques such as PET and SPECT, with a significant reduction in nAChRs in the brains of Alzheimer's patients having been identified with [11C] nicotine in PET studies. Despite the significant body of knowledge now accumulated about nAChRs, much remains to be elucidated. This review will attempt to describe the current knowledge on the nAChR subtypes in the human brain, their functional roles and neuropathological involvement.