Radioligand binding assay of M1-M5 muscarinic cholinergic receptor subtypes in human peripheral blood lymphocytes

Analysis of lymphocyte muscarinic cholinergic receptors using quantitative techniques such as radioligand binding assay is made difficult due to the l ow density of these sites and the lack of subtype-specific selectivity of m ost available muscarinic ligands. In this study, a combined kinetic and equ ilibrium labeling technique recently developed for brain tissue was used fo r labeling the five muscarinic cholinergic receptor subtypes in intact huma n peripheral blood lymphocytes. No specific muscarinic M1 receptor binding was detectable in human peripheral blood lymphocytes using [H-3]-pirenzepin e as a ligand. Labeling of M2-M5 muscarinic receptors using [3H]N-methyl-sc opolamine (NMS) by occluding various receptor subtypes with muscarinic anta gonist and mamba venom resulted in the labeling of M2-M5 receptors in brain as well as in human peripheral blood lymphocytes. The relative density of different receptor subtypes was M3 > M5 > M4 > M2. The development of a rep roducible technique for assaying muscarinic cholinergic receptor subtypes e xpressed by human peripheral blood lymphocytes may contribute to clarify th eir role in lymphocyte function. (C) 1999 Elsevier Science B.V. All rights reserved.