Functional responses and subunit composition of presynaptic nicotinic receptor subtypes explored using the novel agonist 5-iodo-A-85380

The novel compound 5-iodo-A-85380 binds with higher affinity to alpha4beta2* nicotinic acetylcholine receptors (nAChR), compared with other nAChR subtypes (Mukhin et al., 2000). In the present study, we have confirmed that in competition binding assays for three major nAChR subtypes, 5-iodo-A-85380 is 850 and 27,000-fold more potent at rat brain alpha4beta2* binding sites than at alpha3beta4 and alpha subtypes, respectively. In functional assays, 5-iodo-A-85380 potently activated (EC50 12-35 nM) both alpha-CTx-MII-sensitive and -insensitive components of [H-3]dopamine release from rat striatal synaptosomes, corresponding to alpha6beta2* and alpha4beta2* nAChR, respectively. 5-Iodo-A-85380 was markedly less potent at eliciting [H-3]ACh release from rat interpeduncular nucleus synaptosomes, [3H]noradrenaline release from rat hippocampal slices, and Ca2+ increases in a cell line expressing rat alpha3beta4 nAChR (EC50 = 5, 3.2, 1.6 muM, respectively). As predicted by ligand binding studies, 5-iodo-A-85380 is a more discriminating agonist than the parent compound epibatidine. However, it is not specific for alpha4beta2* nAChR as it also potently activates alpha6beta2* nAChR.