Characterization of [3H]CHIBA-1001 binding to α7 nicotinic acetylcholine receptors in the brain from rat, monkey, and human

Accumulating evidence suggests that the α7 subtype of nicotinic acetylcholine receptors (nAChRs) plays a role in the pathophysiology of neuropsychiatric diseases, including schizophrenia and Alzheimer's disease. Currently, there are no suitable small molecule radioligands for α7 nAChRs in the brain, although [125I]α-bungarotoxin has been widely used as a radioligand for α7 nAChRs. In the present study, we characterized a new radioligand, 4-[3H]methylphenyl 2,5-diazabicyclo[3.2.2]nonane-2-carboxylate ([3H]CHIBA-1001), a derivative of the selective α7 nAChR agonist SSR180711, in brain membranes from rat, monkey, and human. Scatchard analysis revealed an apparent equilibrium dissociation constant (Kd) of 193.4 nM in rat brain membranes at 4 °C, and the maximal number of binding sites (Bmax) was 346.2 fmol/mg protein. The order of drugs for the inhibition of [3H]CHIBA-1001 binding to rat brain membranes is SSR180711 > A-844606 > MG624 > epibatidine > DMAB > A-582941, suggesting a similarity of α7 nAChR pharmacological profiles. In contrast, α-bungarotoxin, MLA, and nicotine were found to be very weak. The distribution of [3H]CHIBA-1001 binding to crude membranes from dissected regions of rat, monkey, and human brain was different from that of [125I]α-bungarotoxin binding, suggesting that [3H]CHIBA-1001 binding sites may not be identical to [125I]α-bungarotoxin binding in the brain. In summary, [3H]CHIBA-1001 would be a useful radioligand for α7 nAChRs in the brains of rodents, non-human primates, and humans.