3H]methylcarbamylcholine, a new radioligand for studying brain nicotinic receptors

Abstract

A new radioligand, [ 3H]methylcarbamylcholine, has been developed for the study of the nicotinic cholinergic and nicotine-like binding sites in rat brain membranes. A Scatchard analysis with the radioligand yielded a K d of 1.1 × 10 −9M and a B max of 4.0 × 10 −14 moles/mg protein which compares with a lower affinity site for (−)-[ 3H]nicotine having a K d of 3 × 10 −9M and a B max of 2 × 10 −14 moles/mg. Comparable values for the K d were obtained from a Hill plot and from calculations based on rate constants for association and dissociation. A comparison of the binding affinities of various nicotine analogues, nicotinic cholinergic agents, and other neurotropic agents revealed a close similarity between the two radioligands, with the exception that quaternization of nicotine or carbamate esters increased affinity by at least an order of magnitude with [ 3H]methylcarbamylcholine and resulted in a comparable decrease in affinity with [ 3H]nicotine as the ligand. The binding of [ 3H]methylcarbamylcholine, like [ 3H]nicotine, was not displaceable by muscarinic cholinergic antagonists. It was concluded that, although [ 3H]methylcarbamylcholine and [ 3H]nicotine bind to a common receptor in brain, the functional and chemical characteristics of the receptor(s) differ in some respects from peripheral nicotinic cholinergic receptors.