Selective labelling of M1-receptors in autonomic ganglia with 3H-pirenzepine.

Using the classical muscarinic antagonist 3H-N-methyl-scopolamine as radioligand and unlabelled pirenzepine (PZ) as displacing agent, a heterogeneous muscarinic receptor population consisting of about 70% M1-receptors and 30% M2-receptor, can be demonstrated in crude membranes of calf sympathetic ganglia. In the same preparation only low and variable specific binding is detectable when 3H-PZ is used as marker ligand. The situation clearly improves when synaptosomes are prepared from the ganglia. Then binding of 3H-PZ can be measured with a favourable specific to non specific binding ratio and with features compatible with the selective labelling of ganglionic M1-receptor sites. It is concluded that the 3H-PZ binding assay in synaptosomes of autonomic ganglia is a novel method for the characterization of the peripheral M1-receptor.