| In vitro: |
| J Biol Chem . 2007 Jun 1;282(22):16016-35. | | Mechanisms for picrotoxinin and picrotin blocks of alpha2 homomeric glycine receptors[Pubmed: 17405877] | | Contrary to its effect on the gamma-aminobutyric acid type A and C receptors, picrotoxin antagonism of the alpha1 homomeric glycine receptors (GlyRs) has been shown to be non-use-dependent and nonselective between the picrotoxin components picrotoxinin and picrotin. Picrotoxin antagonism of the embryonic alpha2 homomeric GlyR is known to be use-dependent and reflects a channel-blocking mechanism, but the selectivity of picrotoxin antagonism of the embryonic alpha2 homomeric GlyRs between picrotoxinin and picrotin is unknown. Hence, we used the patch clamp recording technique in the outside-out configuration to investigate, at the single channel level, the mechanism of picrotin- and picrotoxinin-induced inhibition of currents, which were evoked by the activation of alpha2 homomeric GlyRs stably transfected into Chinese hamster ovary cells. Although both picrotoxinin and picrotin inhibited glycine-evoked outside-out currents, picrotin had a 30 times higher IC50 than picrotoxinin. Picrotin-evoked inhibition displayed voltage dependence, whereas picrotoxinin did not. Picrotoxinin and picrotin decreased the mean open time of the channel in a concentration-dependent manner, indicating that these picrotoxin components can bind to the receptor in its open state. When picrotin and glycine were co-applied, a large rebound current was observed at the end of the application. This rebound current was considerably smaller when picrotoxinin and glycine were co-applied. Both picrotin and picrotoxinin were unable to bind to the unbound conformation of the receptor, but both could be trapped at their binding site when the channel closed during glycine dissociation. Our data indicate that picrotoxinin and picrotin are not equivalent in blocking alpha2 homomeric GlyR. | | Vis Neurosci . Jul-Aug 2007;24(4):513-21 | | Glycine receptor subunit composition alters the action of GABA antagonists[Pubmed: 17659095] | | GABA receptor antagonists produce an unexpectedly significant inhibition of native glycine receptors in retina and in alpha1 or alpha2 homomeric glycine receptors (GlyRs) expressed in HEK 293 cells. In this study we evaluate this phenomenon in heteromeric glycine receptors, formed by mixing alpha1, alpha2, and beta subunits. Picrotoxinin, picrotin, SR95531, and bicuculline are all more effective antagonists at GlyRs containing alpha2 subunits than alpha1 subunits. Inclusion of beta subunits reduces the inhibitory potency of picrotoxinin and picrotin but increases the potency of SR95531 and bicuculline. As a result of these two factors, bicuculline is particularly poor at discriminating GABA and glycine receptors. Picrotin, which has been reported to be inactive at GABA receptors, blocks glycine currents in retina and in HEK293 cells, suggesting its utility as a selective glycine antagonist. However, picrotin is a more potent inhibitor of GABA than glycine in retinal neurons. We also tested if GABA and glycine receptor subunits can combine to form functional receptors. If GABAAR gamma2S subunits are co-expressed with GlyR alpha subunits, the mixed receptor is glycine-sensitive and GABA-insensitive. But the mixed receptor exhibits a non-competitive picrotoxinin inhibition that is not observed in the homomeric GlyRs. This suggests that glycine and GABA subunits can co-assemble to form functional glycine receptors. | | Mol Pharmacol . 2000 Jul;58(1):11-7. | | Subunit-specific action of an anticonvulsant thiobutyrolactone on recombinant glycine receptors involves a residue in the M2 membrane-spanning region[Pubmed: 10860922] | | | Neuroreport. 2012 Dec 5;23(17):1017-1020. | | Gating effects on picrotin block of glycine receptors[Pubmed: 23079787] | | Picrotoxin is a pore blocker that can differentiate ligand-gated inhibitory chloride channels. Even within one receptor type, such as the glycine receptor, picrotoxin block differs between subunits. The effect of subunit gating properties on block of the inhibitory glycine receptor (GlyR) was explored using heteromeric α subunit expression in voltage-clamped HEK293 cells. The α2 GlyR is more sensitive to picrotin block than the α1 GlyR, and this difference was used to explore whether mutations that interfered with gating of the α2 subunit would also interfere with picrotin block. Two mutations were used: one that decreased the glycine sensitivity of α2 by almost two log units and the other that was unresponsive to glycine. In both cases, the sensitivity to picrotin was essentially unaltered. The results indicated that α2 subunits can determine the picrotin sensitivity of α1α2-heteromeric receptors and that direct gating of the α2 subunit is not required for this picrotin inhibition. | | Neuropharmacology. Feb-Mar 2011;60(2-3):488-495. | | Ginkgolide B and bilobalide block the pore of the 5-HT₃receptor at a location that overlaps the picrotoxin binding site[Pubmed: 21059362] | | Extracts from the Ginkgo biloba tree are widely used as herbal medicines, and include bilobalide (BB) and ginkgolides A and B (GA and GB). Here we examine their effects on human 5-HT(3)A and 5-HT(3)AB receptors, and compare these to the effects of the structurally related compounds picrotin (PTN) and picrotoxinin (PXN), the two components of picrotoxin (PTX), a known channel blocker of 5-HT(3), nACh and GABA(A) receptors. The compounds inhibited 5-HT-induced responses of 5-HT(3) receptors expressed in Xenopus oocytes, with IC(50) values of 470 μM (BB), 730 μM (GB), 470 μM (PTN), 11 μM (PXN) and >1mM (GA) in 5-HT(3)A receptors, and 3.1mM (BB), 3.9 mM (GB), 2.7 mM (PTN), 62 μM (PXN) and >1mM (GA) in 5-HT(3)AB receptors. Radioligand binding on receptors expressed in HEK 293 cells showed none of the compounds displaced the specific 5-HT(3) receptor antagonist [(3)H]granisetron, confirming that they do not act at the agonist binding site. Inhibition by GB at 5-HT(3)A receptors is weakly use-dependent, and recovery is activity dependent, indicating channel block. To further probe their site of action at 5-HT(3)A receptors, BB and GB were applied alone or in combination with PXN, and the results fitted to a mathematical model; the data revealed partially overlapping sites of action. We conclude that BB and GB block the channel of the 5-HT(3)A receptor. Thus these compounds have comparable, although less potent, behaviour than at some other Cys-loop receptors, demonstrating their actions are conserved across the family. |
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