[125I]Iodomethyllycaconitine binds to alpha7 nicotinic acetylcholine receptors in monkey brain

We examined the binding of the novel nicotinic acetylcholine receptor (nAChR) ligand [125I]iodomethyllycaconitine (iodoMLA) in the brains of M. cynomologous (macaque) monkeys. [125I]iodoMLA bound throughout the brain with the greatest density in the thalamus and moderate intensity in the basal ganglia and cortical regions. The Kd and Bmax in whole brain tissue were similar whether 1 mM nicotine (Kd 33.25 +/- 15.17 nM, Bmax 5.80 +/- 1.06 fmol/mg) or 2 microM of the alpha7-selective antagonist alpha-bungarotoxin (Kd 46.12 +/- 18.45 nM, Bmax 6.30 +/- 1.06 fmol/mg) was used for nonspecific binding. The subtype-selectivity of this ligand was further studied with competition binding studies using nicotine, alpha-bungarotoxin and noniodinated MLA. Each ligand completely inhibited [125I]iodoMLA binding throughout the monkey brain, with Ki values of 2.23 +/- 0.85 microM for nicotine, 2.72 +/- 1.71 nM for alpha-bungarotoxin and 1.83 +/- 0.35 nM MLA in the caudate and 2.03 +/- 1.14 microM, 2.65 +/- 0.86 nM and 3.32 +/- 0.71 nM, respectively, in the putamen. The alpha3beta2/alpha6*-selective antagonist alpha-conotoxin MII failed to inhibit [125I]iodoMLA binding in any brain region. In monkeys with cognitive deficits resulting from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration, [125I]iodoMLA binding was significantly increased in the striatum, similar to results previously observed for [125I]alpha-bungarotoxin. These results suggest that, under the present experimental conditions, [125I]iodoMLA was selective for alpha7-containing nAChRs and did not bind to alpha6-containing nAChRs. This radioligand may be a useful tool for selectively imaging alpha7-containing nAChRs in vivo.