Electrophysiological investigation of the effect of structurally different bispyridinium non-oxime compounds on human α7-nicotinic acetylcholine receptor activity—An in vitro structure-activity analysis

Abstract Organophosphorus compounds, including nerve agents and pesticides, exert their toxicity through irreversible inhibition of acetylcholinesterase (AChE) resulting in an accumulation of acetylcholine and functional impairment of muscarinic and nicotinic acetylcholine receptors. Current therapy comprises oximes to reactivate AChE and atropine to antagonize effects induced by muscarinic acetylcholine receptors. Nicotinic malfunction leading to depression of the central and peripheral respiratory system is not directly treated calling for alternative therapeutic interventions. In the present study, we investigated the electrophysiological properties of the human nAChR subtype α7 (hα7-nAChR) and the functional effect of the 4- tert -butyl bispyridinium (BP) compound MB327 and of a series of novel substituted bispyridinium compounds on the receptors by an automated patch clamp technique. Activation of hα7-nAChRs was induced by nicotine and acetylcholine demonstrating rapid cationic influx up to 100 μM. Agonist-induced currents decayed within a few milliseconds revealing fast desensitization of the receptors. Application of higher agonist concentrations led to a decline of current amplitudes which seemed to be due to increasing receptor desensitization. When 100 μM of agonist was coapplied with low concentrations of the well characterized α7-specific positive allosteric modulator PNU-120596 (1 μM–10 μM), the maximum response and duration of nAChR activation were markedly augmented indicating an elongated mean open-time of receptors and prevention of receptor desensitization. However, co-application of increasing PNU-120596 concentrations (>10 μM) with agonist induced a decline of potentiated current responses. Although less pronounced than PNU-120596, six of the twenty tested substituted BP compounds, in particular those with a substituent at 3-position and 4-position at the pyridinium moieties, were found to potentiate current responses of hα7-nAChRs, most pronounced MB327.This effect was clearly depended on the presence of the agonist indicating a positive allosteric mechanism of these compounds. Besides potentiation at low concentrations, these compounds seem to interact at different binding sites on hα7-nAChRs since enhancement decreased at high concentrations. The residual fourteen BP compounds, possessing either an isopropyl-group or more than one group at the pyridinium moiety, antagonized nicotinic currents exhibiting IC 50 of low up to high micromolar concentrations (∼ 1 μM–300 μM).