Differential inhibition of [H-3]-oxotremorine-M and [H-3]-quinuclinidyl benzilate binding to muscarinic receptors in rat brain membranes with acetylcholinesterase inhibitors

The potential interaction of acetylcholinesterase inhibitors with cholinerg ic receptors may play a significant role in the therapeutic and/or side-eff ects associated with this class of compound. In the present study, the capa city of acetylcholinesterase inhibitors to interact with muscarinic recepto rs was assessed by their ability to displace both [H-3]-oxotremorine-M and [H-3]-quinuclinidyl benzilate binding in rat brain membranes. The [H-3]-qui nuclinidyl benzilate/[H-3] -oxotremorine-M affinity ratios permitted predic tions to be made of either the antagonist or agonist properties of the diff erent compounds. A series of compounds, representative of the principal cla sses of acetylcholinesterase inhibitors, displaced [H-3]-oxotremorine-M bin ding with high-to-moderate potency (ambenonium > neostigmine=pyridostigmine =tacrine > physostigimine > edrophonium=galanthamine > desoxypeganine) wher eas only ambenonium and tacrine displaced [H-3]-quinuclinidyl benzilate bin ding. Inhibitors such as desoxypeganine, parathion and gramine demonstrated negligible inhibition of the binding of both radioligands. Scatchard plots constructed from the inhibition of [H-3]-oxotremorine-M binding in the abs ence and presence of different inhibitors showed an unaltered B-max and a r educed affinity constant, indicative of potential competitive or allosteric mechanisms. The capacity of acetylcholinesterase inhibitors, with the exce ption of tacrine and ambenonium, to displace bound [H-3]-oxotremorine-M in preference to [H-3]-quinuclinidyl benzilate predicts that the former compou nds could act as potential agonists at muscarinic receptors. Moreover, the rank order for potency in inhibiting acetylcholinesterase (ambenonium > neo stigmine=physostigmine=tacrine > pyridostigmine=edrophonium=galathamine > d esoxypeganine > parathion > gramine) indicated that the most effective inhi bitors of acetylcholinesterase also displaced [H-3]-oxotremorine-M to the g reatest extent. The capacity of these inhibitors to displace [H-3]-oxotremo rine-M binding preclude their utilisation for the prevention of acetylcholi ne catabolism in rat brain membranes, the latter being required to estimate the binding of acetylcholine to [H-3]-oxotremorine-M-labelled muscarinic r eceptors. However, fasciculin-2, a potent peptide inhibitor of acetylcholin esterase (IC50 24 nM), did prevent catabolism of acetylcholine in rat brain membranes with an atypical inhibition isotherm of [H-3]-oxotremorine-M bin ding, thus permitting an estimation of the "global affinity" of acetylcholi ne (K-i 85 nM) for [H-3] -oxotremorine-M-labelled muscarinic receptors in r at brain.