New photoaffinity labels for rat brain muscarinic acetylcholine receptors

Localization of the ligand binding site on muscarinic acetylcholine receptors is one of the new fields of interest opened by the recent determination of their primary structures. Owing to their interesting photochemical properties, aryldiazonium salts may be considered as appropriate tools for "tagging" the agonist/antagonist binding domain and to get precise identification and positioning of covalently labelled residues along the primary sequence of these receptors. A series of aryldiazonium derivatives and some of their azido-analogs were synthesized and their reversible muscarinic binding component was assessed through competition experiments involving either the whole population of receptor sites [( 3H]QNB assay) or the super high affinity of their agonist binding sites [( 3H]OXO-M assay). Three compounds fulfilled the criteria for efficient photolabels, allowing substantial and irreversible occupation of the receptor sites to be obtained. Interestingly, the two diazonium derivatives which were selected have been previously described as potent photoprobes of the peripheral nicotinic receptor and of acetylcholinesterase, though displaying lower binding affinities for these acetylcholine binding proteins than for the muscarinic receptors. These findings, together with the all-to-none photolabelling efficiency observed for a quinuclidine derivative, substituted either by an azido or a diazonium group, are discussed. Finally, the apparent lack of binding selectivity of these new photo-affinity probes towards muscarinic receptor affinity states or subtypes should allow comparative studies of the acetylcholine binding site on different muscarinic receptor proteins, obtained either through purification procedures or expression of separate gene products.