Nicotinic acetylcholine receptors are members of the ligand-gated ion channel 
superfamily, that includes also gamma-amino-butiric-acid(A), glycine, and 
5-hydroxytryptamine(3) receptors. Functional nicotinic acetylcholine receptors 
result from the association of five subunits each contributing to the pore 
lining. The major neuronal nicotinic acetylcholine receptors are heterologous 
pentamers of alpha4beta2 subunits (brain), or alpha3beta4 subunits (autonomic 
ganglia). Another class of neuronal receptors that are found both in the central 
and peripheral nervous system is the homomeric alpha7 receptor. The muscle 
receptor subtypes comprise of alphabetadeltagamma (embryonal) or 
alphabetadeltaepsilon (adult) subunits. Although nicotinic acetylcholine 
receptors are not directly involved in the hypnotic component of anesthesia, it 
is possible that modulation of central nicotinic transmission by volatile agents 
contributes to analgesia. The main effect of anesthetic agents on nicotinic 
acetylcholine receptors is inhibitory. Volatile anesthetics and ketamine are the 
most potent inhibitors both at alpha4beta2 and alpha3beta4 receptors with 
clinically relevant IC(50) values. Neuronal nicotinic acetylcholine receptors 
are more sensitive to anesthetics than their muscle counterparts, with the 
exception of the alpha7 receptor. Several intravenous anesthetics such as 
barbiturates, etomidate, and propofol exert also an inhibitory effect on the 
nicotinic acetylcholine receptors, but only at concentrations higher than those 
necessary for anesthesia. Usual clinical concentrations of curare cause 
competitive inhibition of muscle nicotinic acetylcholine receptors while higher 
concentrations may induce open channel blockade. Neuronal nAChRs like 
alpha4beta2 and alpha3beta4 are inhibited by atracurium, a curare derivative, 
but at low concentrations the alpha4beta2 receptor is activated. Inhibition of 
sympathetic transmission by clinically relevant concentrations of some 
anesthetic agents is probably one of the factors involved in arterial 
hypotension during anesthesia.