A-186253, a specific antagonist of the α4β2 nAChRs: its properties and potential to study brain nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors expressed in the ventralposterolateral thalamic nucleus play an important role in anti-allodynic effects

BACKGROUND AND PURPOSE

Much interest is currently being focused on the anti-nociceptive effects mediated by nicotinic acetylcholine (nACh) receptors, including their location and mechanism of action. The purpose of this study was to elucidate these issues using 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5IA), a nACh receptor agonist, and [(125)I]5IA.

EXPERIMENTAL APPROACH

We partially ligated the sciatic nerve of Sprague-Dawley rat to induce neuropathic pain [Seltzer's partial sciatic nerve ligation (PSL) model]. We then examined the changes in nACh receptor density in the CNS using [(125)I]5IA autoradiography and the involvement of nACh receptors in anti-nociceptive effects in the region where changes occurred.

KEY RESULTS

Autoradiographic studies showed that the accumulation of [(125)I]5IA and the number of nACh receptors in the thalamus of PSL rats were increased about twofold compared with those in the sham-operated rats. No change was observed in other brain regions. Rats injected in the ventral posterolateral thalamic nucleus (VPL) with 5IA demonstrated a significant and dose-dependent anti-allodynic effect and this effect was completely antagonized by mecamylamine, injected with 5IA, into the VPL. The blockade of nACh receptors in the VPL by mecamylamine decreased by 70% the anti-allodynic effect of 5IA, given i.c.v. Moreover, mecamylamine given intra-VPL by itself, induced significant hyperalgesia.

CONCLUSIONS AND IMPLICATIONS

Our findings suggest that the nACh receptors expressed in the VPL play an important role in the anti-allodynic effects produced by exogenous and endogenous agonists.

Modeling differential binding of alpha4beta2 nicotinic acetylcholine receptor with agonists and antagonists

Three-dimensional structures of both the open- and closed-channel states of alpha4beta2 receptor have been modeled and used to study their binding with representative agonists and antagonists. The obtained binding structures and free energies consistently reveal that antagonists bind more favorably with the closed-channel state and agonists bind more favorably with the open-channel state. The computational insights have led us to propose a computational strategy and protocol predicting whether a receptor ligand is an agonist or antagonist. Using the computational protocol, one only needs to calculate the relative binding free energies for a ligand binding with the open- and closed-channel structures. The ligand is predicted to be an agonist if the binding free energy calculated for the ligand binding with the open-channel state is significantly lower than that for its binding with the closed-channel state. If the binding free energy of a ligand with the open-channel state is higher than that with the closed-channel, the ligand is predicted to be an antagonist. The binding free energies calculated for all of the ligands binding with their most favorable channel states of the receptor are all close to the corresponding experimentally derived binding free energies. The new computational insights obtained and novel computational strategy and protocol proposed in this study are expected to be valuable in structure-based rational design of novel agonists/antagonists of nAChRs as therapeutic agents.

Determinants of alpha-conotoxin BuIA selectivity on the nicotinic acetylcholine receptor beta subunit

Neuronal nicotinic acetylcholine receptors (nAChRs) are pentamers composed of alpha and beta subunits. Different molecular compositions of these subunits constitute various receptor subtypes that are implicated in the pathophysiology and/or treatment of several disease states but are difficult to distinguish pharmacologically. Alpha-conotoxins are a group of small, structurally defined peptides that may be used to molecularly dissect the nAChR-binding site. Heteromeric nAChRs generally contain either a beta2 or beta4 subunit in addition to an alpha subunit at the ligand-binding interface. Alpha-conotoxin BuIA kinetically distinguishes between beta2- and beta4-containing nAChRs, with long off times for the latter. Mutational studies were used to assess the influence of residues that line the putative acetylcholine-binding pocket but differ between beta2 and beta4 subunits. Residues Thr/Lys59, Val/Ile111, and Phe/Gln119 of the respective beta2 and beta4 subunits are critical to off-rate differences. Among these residues, Thr59 of nAChR beta2 may interfere with effective access to the binding site, whereas Lys59 may facilitate this binding.

2-Fluoro-3-(4-nitro-phenyl)deschloroepibatidine is a novel potent competitive antagonist of human neuronal alpha4beta2 nAChRs

A patch-clamp technique in a whole-cell configuration was used to examine the functional activity of recently developed 2-fluoro-3-(substituted phenyl)deschloroepibatidine analogs on two major subtypes of neuronal nicotinic acetylcholine receptors (nAChRs), alpha4beta2 and alpha3beta4, that predominate in the central and peripheral nervous systems, respectively. These epibatidine analogs have been shown previously to possess high binding affinity to alpha4beta2 but not to alpha7 nAChRs and to inhibit nicotine-induced analgesia in behavioral pain tests. The 2-fluoro-3-(4-nitro-phenyl)deschloroepibatidine (4-nitro-PFEB) exhibited the most pronounced antagonist activity among these analogs when tested electrophysiologically on alpha4beta2 nAChRs. It inhibited acetylcholine (ACh)-induced currents in a concentration-dependent manner with an IC(50) value of 0.1 microM and produced complete inhibition at approximately 1 microM concentration. 4-Nitro-PFEB at 0.1 microM concentration produced a 4-fold rightward shift in the ACh concentration-response curve without altering maximum ACh-induced response. This inhibitory effect of 4-nitro-PFEB was voltage- and use-independent and was partially reversible at its 1 microM concentration. The rise and decay kinetics of ACh-induced currents was not altered in the presence of 4-nitro-PFEB. In contrast to alpha4beta2 nAChRs, this compound did not affect alpha3beta4 nAChR-mediated currents at < or =1 microM (IC(50) approximately 63.9 microM). Overall, these functional data agree with previous binding and behavioral findings and suggest collectively that 4-nitro-PFEB is the most effective and selective antagonist of alpha4beta2 versus alpha3beta4 and alpha7 nAChRs among the tested analogs, acting on alpha4beta2 nAChR through a competitive mechanism with a potency 17-fold higher than that of dihydro-beta-erythroidine.

Functional properties of neuronal nicotinic acetylcholine receptors in the chick retina during development

Acetylcholine (ACh) has been recognized for a long time as a major neurotransmitter in the retina, however, little is known about the contribution of acetylcholine receptors in synaptic processing. Moreover, even less information is available concerning their role during development. To address this question further, we examined the physiological and pharmacological properties of neuronal nicotinic acetylcholine receptors (nAChRs) in retinal ganglion cells from embryonic (E) 12-18-day-old Leghorn chicks. Patch-clamp recordings in whole-cell configuration revealed that at E12 approximately 21% of the ganglion cells responded to acetylcholine pulses with inward currents. The number of responsive cells progressively increased to 57% at E15 to reach up to 15 positive cells out of 15 cells tested at E18. Acetylcholine-evoked responses could be subdivided, according to their time course, into fast and slowly desensitizing. Taking advantage of the selectivity of the frog toxin epibatidine (Epi), that preferentially activates heteromeric neuronal nicotinic acetylcholine receptors, we compared the currents evoked by this toxin vs. the effects of acetylcholine. A further characterization of the receptor diversity during development was to assess their sensitivity to the alpha-conotoxin MII (alpha-CTX-MII), which has been shown to preferentially block alpha6- and alpha3beta2-containing receptors. These data demonstrate that ganglion cells of the chick retina express multiple receptor subtypes that progressively develop as a function of retina maturation.