Abstract

Neuronal nicotinic receptors (nAChRs) have been implicated in pathology associated with neurological diseases and aberrant cognitive states such as addiction and schizophrenia. The design of subtype-specific cholinergic drugs is dependent on identification of key amino acids that play a significant role in determining subunit-specific agonist efficacy. 1,1-Dimethyl-4-phenylpiperazinium (DMPP) and a series of piperazium (PIP)-derived cholinergic agonists (1,1 dimethyl-4-acetylpiperizinium iodide, EthylPIP, PropylPIP, and ButylPIP) were used to identify a site (position 84) in homomeric neuronal nAChRs, which is a partial determinant of pharmacological specificity. In contrast to absolutely conserved amino acids within the nicotinic superfamily, the amino acid in position 84 can be polar or nonpolar. The addition of one methylene to PropylPIP to form ButylPIP eliminated channel activation of but not binding to the chick α7 homomeric nAChR (leucine in position 84). In rat α7 (glutamine in position 84), ButylPIP was an agonist. 1,1-Dimethyl-4-phenylpiperazinium, a structural analog of ButylPIP, activates the rat α7 but is a weak partial agonist of the chick α7. Mutation of the chick α7 (L84Q) restored activation by ButylPIP, and the corresponding mutation in rat α7 (Q84L) abolished activation by ButylPIP. These mutations had moderate effects on the apparent affinity for acetylcholine, increasing its affinity for chick α7 and decreasing it for rat α7. Thus, the amino acid in position 84 (a residue on the periphery of the highly conserved loop A of the cys-loop superfamily of receptors) can potentially be exploited to produce subtype-specific drugs and can provide insights into the structure of the binding domain.