Effect of Bispiridinium Compounds on Human Nicotinic α7 Receptor Currents

Abstract

Nicotinic acetylcholine receptors (nAChRs) are important targets in drug discovery due to their high physiological importance in pathogenesis and poisoning. Life threatening poisoning by organophosphorus compounds results in over-stimulation of cholinergic receptors by blocking acetylcholinesterase (AChE). Antidotal therapy is based on reactivation of inhibited AChE by oximes, i.e. obidoxime. A direct interaction of bispyridinium oximes with nicotinic receptors was postulated in rodent studies, but could not be verified in human tissues. Therefore this study investigated interactions of bispyridinium oximes with neuronal human nicotinic α7 receptor.Human α7 nAChR were expressed in stably transfected rat pituitary tumor cells (GH4C1). Whole-cell patch clamping was performed with planar electrodes in an 8-channel Patchliner system (Nanion Technologies GmbH, Munich, Germany). Cholinergic currents were activated by nicotine and modulated by PNU 120596. The dose-response relationship of the bispyridinium oximes obidoxime and HI 6 was measured in concentrations between 0.01-100 μM.Nicotinic induced inward currents stopped within a few ms, because of receptor desensitisation. This desensitisation could be prevented by adding positive allosteric modulator PNU 120596. Under such conditions, HI 6 reduced the cation inward current of human nicotinic α7 receptors with an IC50 of 83 ± 30 nM. In contrast, nicotinic activation of the human α7 subtype was not affected by obidoxime in μM range. Without PNU 120596 both HI 6 and obidoxime enhanced nicotinic inward current of the human α7 receptor.These results demonstrated that HI 6 and obidoxime interact with the human nicotinic α7 receptor. As PNU 120596 the bispyridinium-oximes can not only increase the maximal amplitude and potency of nicotinic evoked α7 nAChR currents, but also suppress desensitization. This suggests an allosteric effect of these substances. The therapeutic consequences in cases of poisoning by organophosphorus compounds has yet to be tested.