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 α4β2 ∗ 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 α4β2 ∗ binding sites than at α3β4 and α7 subtypes, respectively. In functional assays, 5-iodo-A-85380 potently activated (EC 50 12–35 nM) both α-CTx-MII-sensitive and -insensitive components of [ 3H]dopamine release from rat striatal synaptosomes, corresponding to α6β2 ∗ and α4β2 ∗ nAChR, respectively. 5-Iodo-A-85380 was markedly less potent at eliciting [ 3H]ACh release from rat interpeduncular nucleus synaptosomes, [ 3H]noradrenaline release from rat hippocampal slices, and Ca 2+ increases in a cell line expressing rat α3β4 nAChR (EC 50=5, 3.2, 1.6 μM, 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 α4β2 ∗ nAChR as it also potently activates α6β2 ∗ nAChR.