The neuronal nicotinic α7 acetylcholine receptor (α7 nAChR) has been proposed as a potential target for a wide range of neurodegenerative and psychiatric diseases, but also for poisoning by organophosphorus compounds (nerve agents and pesticides). This life threatening poisoning is caused by over-stimulation of cholinergic receptors due to inhibition of acetylcholinesterase (AChE). Bisquaternary pyridinium oximes are used as reactivators of the inhibited AChE. However, several organophosphates form AChE complexes which cannot be reactivated by oximes. Findings in rodent studies postulated a direct interaction of bisquaternary pyridinium oximes with nAChR, which could contribute to their therapeutic effect. Yet, these results could not be verified in receptor binding studies with human nAChR. Therefore this study investigated functional interactions of bisquaternary pyridinium oximes with neuronal human α7 nAChR.

Human α7 nAChR were expressed in stably transfected rat pituitary tumor cells. Whole-cell patch clamping under voltage-clamping conditions (-70 mV) was performed with planar electrodes in an 8-channel Patchliner system (Nanion Technologies GmbH). Cholinergic currents were activated over a period of 280 ms by 100 µM nicotine and modulated by 10 µM PNU 120596. The dose-response relationship of the bisquarterinary pyridinium oximes HI-6, obidoxime and MMB-4 was measured at concentrations between 0.01-100 µM.

Cholinergic induced inward sodium currents could be triggered concentration-dependent by nicotine (1-100 µM) and stopped within a few ms, because of receptor desensitisation. This desensitisation could be prevented by adding the positive allosteric modulator PNU 120596 (10 µM). Under such conditions, two oximes, HI 6 and MMB4, reduced the cation inward current of human α7 nAChR with an IC₅₀ of 83 ± 30 nM for HI-6 and 2.8 ± 2.3 µM for MMB-4. In contrast, nicotinic activation of the human α7 subtype was not affected by obidoxime in µM range. Without PNU 120596 only HI-6 and MMB-4 enhanced nicotinic inward current of the human α7 nAChR with an EC₅₀ of 2.9 ± 1.8 nM (HI-6) and 21.3 ± 15 µM (MMB-4).

These results demonstrated that HI-6 and MMB-4 interact with the human α7 nAChR. Similar to the urea analog PNU 120596, the bispyridinium-oximes HI 6 and MMB-4 did 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 investigated.