Synthesis of heteroaromatic tropeines and heterogeneous binding to glycine receptors

Heteroaromatic carboxylic esters of (nor) tropine were synthesized. Tropine esters displaced [(3)H]strychnine binding to glycine receptors of rat spinal cord with low Hill slopes. Two-site displacement resulted in nanomolar IC(50,1) and micromolar IC(50,2) values, and IC(50,2)/IC(50,1) ratios up to 615 depending on the heteroaromatic rings and N-methyl substitution. Nortropeines displayed high affinity and low heterogeneity. IC(50,1) and IC(50,2) values of tropeines did not correlate suggesting different binding modes/sites. Glycine potentiated only the nanomolar displacement reflecting positive allosteric interactions and potentiation of ionophore function. Affinities of three (nor) tropeines were different for glycine receptors but identical for 5-HT(3) receptors. (C) 2009 Elsevier Ltd. All rights reserved.