Allosteric modulation of alpha4beta2 nicotinic acetylcholine receptors by HEPES

Abstract

A number of new positive allosteric modulators (PAMs) have been reported that enhance responses of neuronal alpha7 and alpha4beta2 nicotinic acetylcholine receptor subtypes to orthosteric ligands. PAMs represent promising new leads for the development of therapeutic agents for disorders involving alterations in nicotinic neurotransmission including Autism, Alzheimer's and Parkinson's disease. During our recent studies of alpha4beta2 PAMs, we identified a novel effect of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES). The effects of HEPES were evaluated in a phosphate buffered recording solution using two-electrode voltage clamp techniques and alpha4beta2 and alpha7 nicotinic acetylcholine receptor subtypes expressed in Xenopus laevis oocytes. Acetylcholine induced responses of high-sensitivity alpha4beta2 receptors were potentiated 190% by co-exposure to HEPES. Responses were inhibited at higher concentrations (bell-shaped concentration/response curve). Coincidentally, at concentrations of HEPES typically used in oocyte recording (5–10 mM), the potentiating effects of HEPES are matched by its inhibitory effects, thus producing no net effect. Mutagenesis results suggest HEPES potentiates the high-sensitivity stoichiometry of the alpha4beta2 receptors through action at the beta2+/beta2− interface and is dependent on residue beta2D218. HEPES did not potentiate low-sensitivity alpha4beta2 receptors and did not produce any observable effect on acetylcholine induced responses on alpha7 nicotinic acetylcholine receptors.

Targeting α4β2 Nicotinic Acetylcholine Receptors in Central Nervous System Disorders: Perspectives on Positive Allosteric Modulation as a Therapeutic Approach

“…Zn 2+ is also believed to exert its LS stoichiometry potentiating effect through the same a4/a4 interface, whereas Zn 2+ binding in the a4(-)/b2 (+) interfaces has an inhibitory effect at both stoichiometries [91]. In contrast, based on lack of effect at the LS stoichiometry, and mutagenesis studies, HEPES is suggested to bind to the b2/b2 interface in the HS receptor [79]. Galantamine and 17b-estradiol have also been proposed to bind to an extracellular allosteric binding site ( [93] and [68], respectively).…”

“…PAMs such as NS9283 1 [15], 17b-estradiol [68] and galantamine [76] are mainly characterized by left-shifting the agonist concentration-response relationship for the a4b2 nAChR, thus making the agonist more potent at the receptor without affecting the maximal response evoked by the agonist through the receptor. In contrast, other PAMs have been reported to significantly increase the efficacy of the agonist at the receptors, including dFBr [73], NS206 [82] and HEPES at HS receptors [79], with dFBr also exhibiting a minor effect on ACh potency [72,]. PAMs with marked effects on both potency and efficacy are LY-2087101 [80], the NS9283 analogue 5k [84] and Zn 2+ on LS receptors [91].…”

“…So far, binding sites/regions have been identified for NS9283 and NS206 [82], Zn 2+ [91], galantamine [93], 17b-estradiol [68], LY-2087101 [94] and HEPES [79]. As a result of its binding site, a given PAM may display selective action at one stoichiometry or differential effects at the two stoichiometries.…”

“…Another key issue within the field of a4b2 PAMs that still remains to be elucidated is the respective impact of selectively potentiating ACh at either HS or LS receptors. As detailed above there are several arguments to how modulation of HS and LS receptors may be coupled to volume and wired transmission, respectively, but the insight into this matter is currently limited by the availability of only two PAMs that display stoichiometry-selective potentiating effects [79,82].…”

“…[67]), several of the PAMs display a bell-shaped concentration-response relationship at the receptor. This includes HEPES at HS receptors [79], dFBr [74], galantamine [76], Zn 2+ [91] and atropine, scopolamine and physostigmine [70].…”