Identification of the nicotinic and muscarinic acetylcholine receptors in subcellular fractions of mouse brain

Binding of radioactive cholinergic ligands was assayed by equilibrium dialysis to mitochondrial and synaptosomal preparations from mouse brain. Whereas ³H-atropine bound to two sites. ³H-nicotine. ³H-pilocarpine and ³H-acetylcholine (ACh) (after inhibition of cholinesterases in the tissue preparation with pyridostigmine) bound reversibly to single sites with high affinities (7·2, 8·1 and 23 nm. respectively). The ³H-nicotine binding sites (3·1 pmol/g brain) were inhibited by nicotinic but not muscarinic drugs, and the reverse effect was on the ³H-pilocarpine binding sites (3·7 pmol/g brain). The ³H-ACh binding sites were inhibited by both nicotinic and muscarinic drugs and α-bungarotoxin. and equalled approximately the total concentration of the nicotinic and muscarinic sites.It is suggested that the ACh binding sites are on the ACh receptor (AChR), which is found at concentrations similar to acetylcholinesterase in the mouse brain. These AChR sites are divided almost equally between muscarinic and nicotinic types. The data also suggest that most of the binding sites for ³H-atropine or ¹²⁵I-α-bungarotoxin in the mouse brain are not on AChR.