Zg. Gao et Cg. Liu, [H-3] TRICYCLOPINATE BINDING TO BRAIN MUSCARINIC ACETYLCHOLINE-RECEPTORS - A COMPARISON WITH [H-3] QUINUCLIDINYL BENZILATE, Pharmacological research, 33(4-5), 1996, pp. 283-289
The purpose of our study was to investigate the binding characteristic
s of a newly synthesized compound, tricyclopinate, at muscarinic acety
lcholine receptors from rat cerebral cortex. This was achieved through
the use of radiolabelled quinuclidinyl benzilate and radiolabelled tr
icyclopinate. Our data demonstrated that the saturation binding parame
ters of [H-3]tricyclopinate (K-d=0.10 nM, B-max=1056 fmol mg(-1)) were
almost identical to those of [H-3]quinuclidinyl benzilate (K-d=0.11 n
M, B-max=1022 fmol mg(-1)); both ligands fit a one site model of recep
tor-ligand interaction. Concentration-inhibition curves were used to d
etermine K-i values for four antimuscarinic compounds. The rank order
of potencies of the antagonists for displacement of the two ligands wa
s: tricyclopinate=quinuclidinyl benzilate>atropine> pirenzepine. The c
ompetition binding parameters of [H-3]tricyclopinate were similar to t
hose of [H-3]quinuclidinyl benzilate. The associate rate constants (K-
1) were 0.25 and 0.21 nM(-1) min(-1) for [H-3]tricyclopinate and [H-3]
quinuclidinyl benzilate, respectively. The dissociation of bound [H-3]
tricyclopinate from central muscarinic acetylcholine receptors was com
plete and was modified by the allosteric agent, gallamine. By comparis
on, only half of the bound [H-3]quinuclidinyl benzilate was dissociate
d from muscarinic acetylcholine receptors and the dissociation of boun
d [H-3]quinuclidinyl benzilate was not modified by gallamine. The diss
ociation rate constants (K--1) were 0.0325 and 0.0072 min(-1) for [H-3
]tricyclopinate and [H-3]quinuclidinyl benzilate, respectively. These
results showed that the two ligands have different binding characteris
tics to muscarinic acetylcholine receptors. [H-3]tricyclopinate should
be very useful for further study of central muscarinic acetylcholine
receptors; it might complement the use of [H-3]N-methylscopolamine and
[H-3]quinuclidinyl benzilate in the study of muscarinic acetylcholine
receptors. (C) 1996 The Italian Pharmacological Society.