[3H]tricyclopinate binding to brain muscarinic acetylcholine receptors: a comparison with [3H]quinuclidinyl benzilate.

The purpose of our study was to investigate the binding characteristics of a newly synthesized compound, tricyclopinate, at muscarinic acetylcholine receptors from rat cerebral cortex. This was achieved through the use of radiolabelled quinuclidinyl benzilate and radiolabelled tricyclopinate. Our data demonstrated that the saturation binding parameters of [3H]tricyclopinate (Kd = 0.10 nM, Bmax = 1056 fmol mg-1) were almost identical to those of [3H]quinuclidinyl benzilate (Kd = 0.11 nM, Bmax = 1022 fmol mg-1); both ligands fit a one site model of receptor-ligand interaction. Concentration-inhibition curves were used to determine Ki values for four antimuscarinic compounds. The rank order of potencies of the antagonists for displacement of the two ligands was: tricyclopinate = quinuclidinyl benzilate > atropine > pirenzepine. The competition binding parameters of [3H]tricyclopinate were similar to those of [3H]quinuclidinyl benzilate. The associate rate constants (Ki) were 0.25 and 0.21 nM-1 min-1 for [3H]tricyclopinate and [3H]quinuclidinyl benzilate, respectively. The dissociation of bound [3H]tricyclopinate from central muscarinic acetylcholine receptors was complete and was modified by the allosteric agent, gallamine. By comparison, only half of the bound [3H]quinuclidinyl benzilate was dissociated from muscarinic acetylcholine receptors and the dissociation of bound [3H]quinuclidinyl benzilate was not modified by gallamine. The dissociation rate constants (K-i) were 0.0325 and 0.0072 min-1 for [3H]tricyclopinate and [3H]quinuclidinyl benzilate, respectively. These results showed that the two ligands have different binding characteristics to muscarinic acetylcholine receptors. [3H]tricyclopinate should be very useful for further study of central muscarinic acetylcholine receptors; it might complement the use of [3H]N-methylscopolamine and [3H]quinuclidinyl benzilate in the study of muscarinic acetylcholine receptors.