Inhibitory Actions of Tropeines on the α3 Glycine Receptor Function

Abstract

Glycine receptors (GlyRs) are chloride-permeable pentameric ligand-gated ion channels. The inhibitory activity of GlyRs is essential for many physiological processes, such as motor control and respiration. In addition, several pathological states, such as hyperekplexia, epilepsy, and chronic pain, are associated with abnormal glycinergic inhibition. Recent studies have pointed out that positive allosteric modulators targeting the GlyR α3 subunit (α3GlyR) displayed beneficial effects in chronic pain models. Interestingly, previous electrophysiological studies have shown that tropeines, which are a family of synthetic antagonists of the serotonin type 3 receptors (5-HT 3 Rs), potentiate the activity of GlyRs conformed by α1 subunits. However, despite its importance as a pharmacological target in chronic pain, it is currently unknown whether the α3GlyR function is modulated by tropeines. Using electrophysiological techniques and molecular docking simulations, here we show that tropeines are inhibitors of the α3GlyR function. Tropisetron, a prototypical tropeine, exerted concentration-dependent inhibitory effects on α3GlyRs at the low micromolar range. In addition, three other tropeines showed similar effects. Single-channel recordings show that tropisetron inhibition is associated with a decrease in the open probability of the ion channel. Molecular docking assays suggest that tropeines preferentially bind to an agonist-free, closed state of the ion channel. The tropeine binding occurs in a discrete pocket around the vicinity of the orthosteric site within the extracellular domain of α3GlyR. Thus, our results describe the pharmacological modulation of tropeines on α3GlyRs. These findings may contribute to the development of GlyR-selective tropeine derivatives for basic and/or clinical applications.

Glycine Receptors in Spinal Nociceptive Control—An Update

“…Critical for the interaction of propofol To highlight the relevance of inter-subunit binding sites, models showing the interface of two adjacent α3 subunits are also shown. The interaction of tropisetron with the orthosteric site of α3 GlyRs was modeled according to [44]. The AM-3607 binding site was reconstructed as described in [11].…”

“…The effects of tropeines have been mainly explored on α1 GlyRs, and less in α3 GlyRs. A recent report explored the modulation of homopentameric α3 GlyRs by tropeines [44]. Tropisetron did not potentiate α3 GlyRs, but rather caused concentration-dependent inhibition in the low micromolar range.…”

Drug Development in Channelopathies: Allosteric Modulation of Ligand-Gated and Voltage-Gated Ion Channels

“…All PAMs of GlyRs exhibit advantageous activities in the treatment of neuropathic pain. Very recently, Martín et al reported that tropisetron dose-dependently inhibited glycine-induced currents of α3 GlyRs, indicating that tropisetron is a NAM of the receptor . Molecular docking simulations predicted a tropisetron binding site next to the orthosteric sites in the ECD of α3 GlyRs.…”

“…Very recently, Marti ́n et al reported that tropisetron dose-dependently inhibited glycineinduced currents of α3 GlyRs, indicating that tropisetron is a NAM of the receptor. 82 Molecular docking simulations predicted a tropisetron binding site next to the orthosteric sites in the ECD of α3 GlyRs. Additionally, tropisetron acted as a PAM and NAM of α1 and α2 GlyRs, respectively.…”

Modulators of the Inhibitory Glycine Receptor

“…It potentiated α1 GlyR-mediated currents in HEK293 cells at femto- to nanomolar concentrations but was an inhibitor at micromolar concentrations. , α3 GlyRs in HEK293 cells were inhibited by micromolar concentrations of tropisetron, and no potentiation was observed. A binding site for tropisetron in the open conformation of the receptor protein was proposed by structure modeling. , 3α-(3′-Methoxy-benzoyloxy)­nortropane (MBN) was identified as modulator of glycine- and taurine-induced GlyRs currents in Xenopus laevis oocytes. Several substitutions around the agonist binding site of the α1 interface were investigated .…”