Competitive Potentiation of Acetylcholine Effects on Neuronal Nicotinic Receptors by Acetylcholinesterase‐Inhibiting Drugs

Abstract

Abstract:The effects of the acetylcholinesterase inhibitors physostigmine and tacrine on ␣4␤2 and ␣4␤4 subtypes of neuronal nicotinic acetylcholine (ACh) receptors, expressed in Xenopus laevis oocytes, have been investigated. In voltage-clamp experiments low concentrations of physostigmine and tacrine potentiate ion currents induced by low concentrations of ACh, whereas at high concentrations they inhibit ACh-induced ion currents. These dual effects result in bell-shaped concentrationeffect curves. Physostigmine and tacrine, by themselves, do not act as nicotinic receptor agonists. The larger potentiation is observed with 10 M physostigmine on ␣4␤4 nicotinic receptors and amounts to 70% at 1 M ACh. The mechanism underlying the effects of physostigmine on ␣4␤4 ACh receptors has been investigated in detail. Potentiation of ACh-induced ion current by low concentrations of physostigmine is surmounted at elevated concentrations of ACh, indicating that this is a competitive effect. Conversely, inhibition of ACh-induced ion current by high concentrations of physostigmine is not surmounted at high concentrations of ACh, and this effect appears mainly due to noncompetitive, voltage-dependent ion channel block. Radioligand binding experiments demonstrating displacement of the nicotinic receptor agonist 125 I-epibatidine from its recognition sites on ␣4␤4 ACh receptors by physostigmine confirm that physostigmine is a competitive ligand at these receptors. A twosite equilibrium receptor occupation model, combined with noncompetitive ion channel block, accounts for the dual effects of physostigmine and tacrine on ACh-induced ion currents. It is concluded that these acetylcholinesterase-inhibiting drugs interact with the ACh recognition sites and are coagonists of ACh on ␣4-containing nicotinic ACh receptors. Key Words: Neuronal nicotinic acetylcholine receptor-Acetylcholinesterase inhibitorTacrine -Physostigmine -Two-site receptor occupation model-Xenopus oocyte.

Functional Characterization of AT-1001, an α3β4 Nicotinic Acetylcholine Receptor Ligand, at Human α3β4 and α4β2 nAChR

“…Similar effects were previously reported for cholinergic drugs at α4β2 nAChRs. 23,26 The AT-1001 concentration-inhibition curve at α3β4 nAChR ( Figure 3B) therefore has a complex shape that is better fitted by a two-site equation (Equation 1) that was initially developed to take into account the bell-shaped inhibition curve observed with some antagonists at the α3β4 nAChR. 22 This equation was subsequently elaborated further to take into account the possibility that the test compound might also act as an open-channel blocker.…”

Activation and Block of the Adult Muscle-Type Nicotinic Receptor by Physostigmine: Single-Channel Studies

“…Together, the whole-cell and single-channel findings are suggestive of low potency and/or low efficacy of physostigmine on the nAChR. Physostigmine-mediated potentiation of currents elicited by low doses of ACh has been shown for neuronal (Zwart et al, 2000) as well as muscle-type nicotinic receptors (Svobodova et al, 2006).…”

“…The plant-derived APLs physostigmine and galantamine were originally identified as acetylcholinesterase inhibitors, leading to their use in symptomatic treatment of neurological disorders involving memory deficits (Davis et al, 1978;van Dyck et al, 2000). Later work showed that the drugs also act on the nAChR where they potentiate currents elicited by low concentrations of nicotinic agonists; cause direct activation of the channel; and, at higher doses, block channel activity (Albuquerque et al, 1984;Okonjo et al, 1991;Zwart et al, 2000).…”

“…In contrast, no whole-cell currents in response to up to 1 mM physostigmine were detected in oocytes expressing rat neuronal ␣4␤2 or ␣4␤4 receptors (Zwart et al, 2000), or COS cells expressing mouse muscle embryonic (␣␤␥␦) receptors (Svobodova et al, 2006). Single-channel patch clamp has shown that channel openings in the presence of physostigmine have a conductance identical to that for ACh, but, in contrast to channel activity elicited by high concentrations of ACh, physostigmine-induced openings are typically not condensed into single-channel clusters (Shaw et al, 1985;Wachtel, 1993;Cooper et al, 1996).…”

“…In macroscopic recordings, coapplication of physostigmine with ACh results in accelerated apparent desensitization and a reduction of peak response (Storch et al, 1995;Zwart et al, 2000;Svobodova et al, 2006). In singlechannel recordings, high concentrations of physostigmine produce a reduction in the mean open duration and an emergence of a novel 7-to 8-ms closed time component (Shaw et al, 1985;Wachtel, 1993).…”