Muscarinic M2-selective ligands also recognize M4receptors in the rat brain: Evidence from combined in situ hybridization and receptor autoradiography

Abstract

We have used autoradiographic techniques to examine the characteristics and distribution of the binding of reported selective M₂ muscarinic ligands and compared them with the distribution of cells expressing mRNAs for the different subtypes of muscarinic receptors. Our results suggest that the M₂ ligands used in the present study ([³H]OXO‐M, [³H]AF‐DX 384, AF‐DX 116, methoctramine) also recognize M₄ receptors present in regions such as the striatum and olfactory tubercle. This is supported by (1) relative abundances of the different transcripts, with m2 mRNA being very scarce and m4 mRNA very abundant in these regions; (2) comparison of the pharmacological characteristics of M₂‐ligand binding sites in brain areas selected by their exclusive expression of M₂ receptors versus areas enriched in M₄ receptors. An important conclusion of these studies is that none of the muscarinic radioligands available at the present time appears to label specifically a single muscarinic receptor subtype population. Areas are suggested where autoradiographic techniques can be helpful in elucidating the subtype selectivity of existing and new ligands. © Wiley‐Liss, Inc.