Differential inhibition of [3H]-oxotremorine-M and [3H]-quinuclinidyl benzilate binding to muscarinic receptors in rat brain membranes with acetylcholinesterase inhibitors.
The potential interaction of acetylcholinesterase inhibitors with cholinergic receptors may play a significant role in the therapeutic and/or side-effects associated with this class of compound. In the present study, the capacity of acetylcholinesterase inhibitors to interact with muscarinic receptors was assessed by their ability to displace both [3H]-oxotremorine-M and [3H]-quinuclinidyl benzilate binding in rat brain membranes. The [3H]-quinuclinidyl benzilate/[3H]-oxotremorine-M affinity ratios permitted predictions to be made of either the antagonist or agonist properties of the different compounds. A series of compounds, representative of the principal classes of acetylcholinesterase inhibitors, displaced [3H]-oxotremorine-M binding with high-to-moderate potency (ambenonium>neostigmine=pyridostigmine=tacrine>physostigmine> edrophonium=galanthamine>desoxypeganine) whereas only ambenonium and tacrine displaced [3H]-quinuclinidyl benzilate binding. Inhibitors such as desoxypeganine, parathion and gramine demonstrated negligible inhibition of the binding of both radioligands. Scatchard plots constructed from the inhibition of [3H]-oxotremorine-M binding in the absence and presence of different inhibitors showed an unaltered Bmax and a reduced affinity constant, indicative of potential competitive or allosteric mechanisms. The capacity of acetylcholinesterase inhibitors, with the exception of tacrine and ambenonium, to displace bound [3H]-oxotremorine-M in preference to [3H]quinuclinidyl benzilate predicts that the former compounds could act as potential agonists at muscarinic receptors. Moreover, the rank order for potency in inhibiting acetylcholinesterase (ambenonium>neostigmine=physostigmine =tacrine>pyridostigmine=edrophonium=galanthamine >desoxypeganine>parathion>gramine) indicated that the most effective inhibitors of acetylcholinesterase also displaced [3H]-oxotremorine-M to the greatest extent. The capacity of these inhibitors to displace [3H]-oxotremorine-M binding preclude their utilisation for the prevention of acetylcholine catabolism in rat brain membranes, the latter being required to estimate the binding of acetylcholine to [3H]-oxotremorine-M-labelled muscarinic receptors. However, fasciculin-2, a potent peptide inhibitor of acetylcholinesterase (IC50 24 nM), did prevent catabolism of acetylcholine in rat brain membranes with an atypical inhibition isotherm of [3H]-oxotremorine-M binding, thus permitting an estimation of the "global affinity" of acetylcholine (Ki 85 nM) for [3H]-oxotremorine-M-labelled muscarinic receptors in rat brain.
Lockhart
B
B
Institut de Recherches Servier, Division of Cerebral Pathology, Croissy-sur-Seine, France. lockhart@netgrs.com
Closier
M
M
Howard
K
K
Steward
C
C
Lestage
P
P
D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D002800:Cholinesterase Inhibitors / Q000494:pharmacology*; D004546:Elapid Venoms / Q000494:pharmacology; D004593:Electrophorus; D018721:Muscarinic Agonists / Q000378:metabolism*; D010095:Oxotremorine / Q000031:analogs & derivatives* / Q000378:metabolism*; D011813:Quinuclidinyl Benzilate / Q000378:metabolism*; D051381:Rats; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism*
D002800:Cholinesterase Inhibitors; D004546:Elapid Venoms; D018721:Muscarinic Agonists; D011976:Receptors, Muscarinic; D010095:Oxotremorine; C042743:oxotremorine M; D011813:Quinuclidinyl Benzilate; C044937:fasciculin