Acetylcholine-binding protein (AChBP) is a water-soluble protein released from molluscan glial cells and modulates ACh-mediated synaptic transmission. Acetylcholine-binding protein (AChBP) is a water-soluble homolog of the ligand-binding domain of nicotinic receptors and other members of the pharmaceutically important family of pentameric ligand-gated ion channels (LGICs), GABAA, GABAC, 5-HT3 serotonin, and glycine receptors. The crystal structure of AChBP from Lymnaea stagnalis has become an established model for the extracellular domain of the pentameric LGICs, and homology models have been generated to analyze receptor-ligand interactions. AChBP has pharmacological properties similar to the homomeric alpha7 subtype of nicotinic ACh receptors (nAChRs), with relatively weak affinity for ACh and a 10-fold higher affinity for nicotine.