Functional characterization of muscarinic autoreceptors in rat and human neocortex

Electrically evoked overflow of [³H]acetylcholine in slices of rat neocortex and of human neocortex (freshly obtained during neurosurgical treatment of epilepsy or deep-seated tumors) was used to functionally characterize the muscarinic receptor subtype, which mediates autoinhibition of acetylcholine release in these tissues. In the rat neocortex, the following pKB values [CI₉₅] were calculated from the shifts to the right of the concentration-response curves of the full agonist oxotremorine in presence of subtype preferring muscarinic receptor antagonists: tripitramine: 9.1 [8.8, 9.4], tripinamide: 8.6 [8.5, 8.7], AQ-RA 741 (11-[[4-[4-(diethylamino)butyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido(2,3-b)[1,4]benzodiazepine-6-one): 8.2 [8.0, 8.4], himbacine: 8.0 [7.9, 8.1], 4-diphenylacetoxy-N-methylpiperidine methobromide: 8.0 [7.8, 8.1], methoctramine: 7.5 [7.4, 7.6], AF-DX 116 (11[[2-[(diethyl-amino)methyl]-1-piperidinyl] acetyl] 5,11-dihydro-6H-pyrido(2,3-b)[1,4]benzodiazepine-6-one): 7.1 [7.0, 7.3], hexahydro-sila-difenidol: 6.8 [6.7, 6.9], pirenzepine: 6.6 [6.4, 6.7], and 3,6a,11,14-tetrahydro-9-methoxy-2-methyl-12H-isoquino[1,2-b]pyrrolo[3,2-f] [1,3]benzoxazine-1-carboxylic acid ethyl ester (PD 102807): 6.0 [5.8, 6.2]. In the human neocortex the following values were found: tripitramine: 9.4 [9.3, 9.6], tripinamide: 9.0 [8.9, 9.2], AF-DX 116: 6.7 [6.4, 6.9], hexahydro-sila-difenidol: 6.6 [6.2, 6.9], and PD 102807: 6.5 [6.3, 6.6]. In correlation plots, these pKB values correspond best to published binding data on native or recombinant M₂ receptors but not to those on M₁, M₃, M₄, and M₅ receptors, suggesting that muscarinic autoreceptors of both the rat and human neocortex belong to the M₂ subtype. This observation lends further support to the development of M₂ receptor selective brain penetrating antagonists for application in Alzheimer's disease.