Recent Development of Radioligands for Imaging α7 Nicotinic Acetylcholine Receptors in the Brain

The α7 nicotinic acetylcholine receptors (nAChRs), ligand-gated Ca2+ channels composed of homopentamers of α7 subunits, represent the most abundant with α4β2 nAChRs in the brain. Several lines of evidence suggest that α7 nAChRs play a role in the physiology of neuropsychiatric diseases such as schizophrenia, Alzheimer's disease, anxiety, depression, and drug addiction; hence, α7 nAChRs seem to be attractive therapeutic targets for these diseases. Several researchers have attempted to develop radioligands that can be used to selectively and quantitatively examine the distribution of α7 nAChRs in the human brain with positron emission tomography (PET) or single photon emission tomography (SPECT). Although efforts are underway, very low density of α7 nAChR and scarcity of very high affinity ligands hamper the development of α7 subtype-selective radioligands for in vivo imaging. In this article, we review the recent topics on the development of PET/SPECT ligands for in vivo imaging of α7 nAChRs in the brain.