5HI (5-hydroxyindole) enhances the affinity of α7 nAChR agonists but not antagonists for this receptor (Grantham et al., 2004, SFN Mtg Abstr. 956.13). In principle, this radioligand binding assay could be used to rapidly identify agonists within a large library of compounds. We measured 5HI enhancement of the binding affinities of several benzylidene-anabaseines whose apparent efficacies had been measured by electrophysiological methods (Papke et al., 2004; Stokes et al., 2004). Anabaseine (>95% ACh current) was a full agonist, 3-(2,4-Dihydroxybenzylidene)-anabaseine (67% ACh) and (3-(2,4-Dimethoxybenzylidene)-anabaseine, DMXBA) (47% ACh) were strong partial agonists, and 3-(4-Thiomethoxybenzylidene)-anabaseine (15% ACh) was a very weak partial agonist. The ability of each compound to displace 125I-BTX binding was determined in the presence or absence of 2 mM 5HI. At least 10 different agonist concentrations in quadruplicate were used to generate each Prism software-fitted curve. Enhancement of unlabelled ...