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

Abstract

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 n m. 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.