The properties of muscarinic receptors in mammalian cerebral cortex.

1 The reaction of tritiated propylbenzilylcholine mustard ([(3)H]-PrBCM; N-2'-chloroethyl-N-[2'',3''-(3)H(2)] -propyl-2-aminoethylbenzilate) with homogenates of mammalian brain has been studied.2 The uptake can be divided into an atropine-sensitive component of fixed capacity (380 pmol/g protein in the rat) and an atropine-insensitive part.3 The atropine-sensitive portion is identified as muscarinic receptor by its insensitivity to nicotinic antagonists and anticholinesterases and its sensitivity to a range of muscarinic antagonists.4 The uptake of [(3)H]-PrBCM is also inhibited by muscarinic agonists and there is reasonable quantitative agreement between the affinities of agonists estimated in this way and in intact tissues by physiological responses.5 The fraction of [(3)H]-PrBCM uptake inhibited by muscarinic antagonists and agonists is the same.6 The amount of receptor found in six mammalian species was inversely related to the size of the brain, but the rates of alkylation and the sensitivity to atropine were not dissimilar.