Pharmacological Characterization of the Allosteric Modulator Desformylflustrabromine and Its Interaction with α4β2 Neuronal Nicotinic Acetylcholine Receptor Orthosteric Ligands

Abstract

Neuronal nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop superfamily of ligand-gated ion channels. nAChRs are involved in modulating nicotinic-based signal transmission in the central nervous system and are implicated in a range of disorders. Desformylflustrabromine (dFBr) is a positive allosteric modulator that potentiates ␣4␤2 nAChRs. It has been reported that dFBr is selective for the ␣4␤2 receptor relative to other common nAChR subtypes (Neurosci Lett 373: 144 -149, 2005). Coapplication of dFBr with acetylcholine (ACh) produces a bell-shaped dose-response curve with a peak potentiation of more than 265% (Bioorg Med Chem Lett 17:4855-4860, 2007) at dFBr concentrations Ͻ10 M and inhibition of responses at concentrations Ͼ10 M. The potentiation and inhibition components of dFBr-modulated responses were examined by using two-electrode voltage clamp and human ␣4␤2 nAChRs expressed in Xenopus laevis oocytes. Currents to both partial and full agonists were potentiated by dFBr. Responses to low-efficacy agonists were potentiated significantly more than responses to high-efficacy agonists. Antagonist pIC 50 values were unaffected by coapplication of dFBr. In addition to its potentiating effects, dFBr was able to induce current spikes when applied to desensitized receptors, suggestive of a shift in equilibrium from the desensitized to open conformation. In contrast to potentiation, inhibition of ACh responses by dFBr depends on membrane potential and is probably the result of open-channel block by dFBr and ACh. Our data indicate distinct mechanisms for the potentiation and inhibition components of dFBr action. dFBr could prove useful for therapeutic enhancement of responses at ␣4␤2-containing synapses.

Desformylflustrabromine Modulatesα4β2 Neuronal Nicotinic Acetylcholine Receptor High- and Low-Sensitivity Isoforms at Allosteric Clefts Containing theβ2 Subunit

“…265% without altering ACh potency. ACh-induced currents were found to be inhibited, but not potentiated, on other common nAChR subtypes (Sala et al, 2005;Kim et al, 2007;Weltzin and Schulte, 2010). At high concentrations of dFBr (.…”

“…At high concentrations of dFBr (. 10 mM), a4b2 receptors are inhibited by a mechanism that appears to involve dFBr openchannel block (Weltzin and Schulte, 2010). The mechanism of dFBr potentiation has been proposed to involve alteration of the equilibrium between open and desensitized receptor conformations, although a single channel analysis has not been reported (Sala et al, 2005;Weltzin and Schulte, 2010).…”

A Novel α2/α4 Subtype-selective Positive Allosteric Modulator of Nicotinic Acetylcholine Receptors Acting from the C-tail of an α Subunit

“…EC 50 values for Br-PBTC ranged from 0.261 to 0.660 M (Table 1), equal to the most potent nAChR PAMs (6, 7). At Ͼ3 M, Br-PBTC inhibited its own potentiation effect, perhaps because it behaved as an open channel blocker like some other nAChR PAMs and ACh itself (26). Br-PBTC did not alter activation by ACh of ␣3␤2 or ␣3␤4 nAChRs (Fig.…”

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

“…For instance, the increase in agonist efficacy observed in the presence of dFBr is likely explained by the ability of this ligand to disrupt the desensitization of the a4b2 nAChR [73]. Accordingly, the lack of effect on agonist efficacy observed for NS9283 may be explained by its lack of effect on desensitization [14].…”

“…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].…”