RS-056812-198: partial agonist on native and antagonist on cloned 5-HT3 receptors

Roles of 5-HT3 and 5-HT7 receptors in acute pruriceptive processing in mice

The roles of serotonin (5-HT) and/or noradrenaline in acute pruriceptive processing have been demonstrated using antidepressants, such as milnacipran, a serotonin and noradrenaline reuptake inhibitor, and mirtazapine, a noradrenergic and specific serotonergic antidepressant; however, the involvement of 5-HT in acute pruriceptive processing has not yet been elucidated in detail. Scratching events induced by chloroquine (CQ) were attenuated by the administration of milnacipran or mirtazapine, and these effects were reversed by a treatment with ondansetron, a 5-HT3 antagonist, or SB26970, a 5-HT7 antagonist. CQ-induced scratching events were also ameliorated by the intrathecal administration of 5-HT, SR572227A and RS56812 (5-HT3 agonists), and LP211 and LP44 (5-HT7 agonists), indicating the modulation of CQ-induced scratching events by 5-HT and noradrenaline. By contrast, histamine-induced scratching events were not markedly affected by the administration of 5-HT and 5-HT7 agonists, whereas 5-HT3 agonists exerted attenuating effects. Similarly, they were not clearly reversed by the administration of the 5-HT7 antagonist, unlike a 5-HT3 antagonist. Therefore, 5-HT is involved in the attenuating effects of milnacipran and mirtazapine on CQ- and histamine-induced scratching events, and 5-HT3 and 5-HT7 receptors play different roles in pruriceptive processing induced by histamine or CQ.

Effects of protostane-type triterpenoids on the 5-HT3A receptor-mediated ion current in Xenopus oocytes

Alisol derivatives are unique protostane-type triterpenoid compounds that are isolated from Alismatis rhizoma, which is a well-known traditional medicine in East Asia. In the present study, we investigated the effects of protostane-type triterpenoids (AA, Alisol A; AB, Alisol B; AB-ac, Alisol B 23-acetate; AC-ac, Alisol C 23-aceteate) on 5-HT-induced currents mediated by the human 5-HT(3)A receptor expressed in Xenopus laevis oocytes. Co-treatment with triterpenoids regulated the 5-HT-induced inward peak current in a concentration-dependent and reversible manner. In addition, regulation of I(5-HT) by triterpenoids occurred in a non-competitive manner. Taken together, these results indicate that triterpenoids may regulate the 5-HT(3)A receptors that are expressed in Xenopus oocytes. Furthermore, this regulation of the ligand-gated ion channel activity by triterpenoids may be one of the pharmacological actions of Alismatis rhizoma.

Effects of triterpenoids from Poria cocos Wolf on the serotonin type 3A receptor-mediated ion current in Xenopus oocytes

Poria cocos Wolf (P. cocos Wolf) is used to treat chronic gastritis, edema, nephrosis, gastric atony, acute gastroenteric catarrh, dizziness, emesis and vomiting. Triterpenoids are a class of natural compounds produced by P. cocos Wolf that contain acyclic 30-carbon precursors. In this study, we investigated the effect of triterpenoids (PA, Pachymic acid; DA, dehydroeburicoic acid; HA, 3beta-hydroxylanosta-7,9(11),24-trien-21-oic acid) on human 5-hydroxytryptamine 3A (5-HT(3A)) receptor channel activity, which is one of the ligand-gated ion channel families. The two-electrode voltage-clamp technique was used to examine the 5-HT3A mediated current. The inhibitory effect of triterpenoids on 5HT-induced inward current (I(5-HT)) occurred in a concentration dependent and reversible manner. Furthermore, the half-inhibitory concentrations (IC(50)) of PA, DA and HA were 3.2+/-0.2, 5.5+/-0.6 and 1.4+/-0.2 microM, respectively. This corresponded to an order of potency for the inhibition of I(5-HT) in oocytes expressing human 5-HT(3A) receptor of HA>PA>DA. Finally, inhibition of I(5HT) by triterpenoids occurred in a non-competitive manner, while inhibition by HA and PA showed more voltage-dependency. Taken together, these results indicate that triterpenoids may regulate the expressed 5-HT(3A) receptors in Xenopus oocytes. Furthermore, this regulation of the ligand-gated ion channel activity by triterpenoids may be one of the pharmacological actions of P. cocos Wolf.

Involvement of 5-HT receptors in the development and expression of methamphetamine-induced behavioral sensitization: 5-HT receptor channel and binding study

Methamphetamine (MAP) is one of the most commonly abused drugs in Asia, and previous studies suggest that serotonin 3 receptors (5-HT(3)) are involved in MAP-induced locomotion and reward. However, little is known about the role of 5-HT(3) receptors in MAP-induced behavioral sensitization. Here, we measured the effects of MDL 72222, a 5-HT(3) antagonist, and SR 57227 A, a 5-HT(3) agonist, on the development and expression of MAP-induced behavioral sensitization, and alternations of 5-HT(3) receptor binding labeled with the 5-HT(3)-selective antagonist, [(3)H]GR65630, in mice. In addition, we investigated the effects of MAP on 5-HT(3A) receptor channel activity in Xenopus laevis oocytes expressing 5-HT(3A) receptors. We found that MDL 72222 attenuated both the development and expression of behavioral sensitization to MAP (1.0 mg/kg, i.p.), and that this attenuating effect of MDL 72222 was reversed by pre-treatment with SR 57227 A. In oocytes expressing 5-HT(3A) receptor, MAP exhibited a dual modulation of 5-HT(3A) receptor channel activity, i.e. pre-treatment with a low dose of MAP (0.1 microm) enhanced 5-HT-induced inward peak current (I(5-HT)) but a high dose of MAP (100 microm) inhibited I(5-HT). The acute administration of MDL 72222 with MAP decreased [(3)H]GR65630 binding versus MAP alone in the mouse striatum. Our results suggest that MDL 72222 attenuates MAP-induced behavioral sensitization via 5-HT(3) receptors in the caudate putamen, and that 5-HT(3) receptor antagonists like MDL 72222 have potential as novel anti-psychotic agents for the treatment of MAP dependence and psychosis.

Promiscuous coassembly of serotonin 5-HT3 and nicotinic alpha4 receptor subunits into Ca(2+)-permeable ion channels

Serotonin (5-hydroxytryptamine) type 3 receptors (5-HT3R) and nicotinic acetylcholine receptors are structurally and functionally related proteins, yet distinct members of the family of ligand-gated ion channels. For most members of this family a diversity of heteromeric receptors is known at present. In contrast, known 5-HT3R subunits are all homologs of the same 5-HT3R-A subunit and form homopentameric receptors. Here we show, by heterologous expression followed by immunoprecipitation, that 5-HT3R and nicotinic acetylcholine receptor alpha4 subunits coassemble into a novel type of heteromeric ligand-gated ion channel, which is activated by 5-HT. The Ca2+ permeability of this heteromeric ion channel is enhanced as compared with that of the homomeric 5-HT3R channel. Heteromeric 5-HT3/alpha4 and homomeric 5-HT3Rs have similar pharmacological profiles, but distinct sensitivities to block by the antagonist d-tubocurarine. Coassembly of subunits beyond the boundaries of ligand-gated ion channel families may constitute an important mechanism contributing to the diverse properties and functions of native neurotransmitter receptors.

Pharmacological characterization of a rat 5-hydroxytryptamine type3 receptor subunit (r5-HT3A(b)) expressed in Xenopus laevis oocytes

The present study has utilized the two electrode voltage-clamp technique to examine the pharmacological profile of a splice variant of the rat orthologue of the 5-hydroxytryptamine type 3A subunit (5-HT3A(b)) heterologously expressed in Xenopus laevis oocytes. At negative holding potentials, bath applied 5-HT (300 nM - 10 microM) evoked a transient, concentration-dependent (EC50 = 1.1+/-0.1 microM), inward current. The response reversed in sign at a holding potential of -2.1+/-1.6 mV. The response to 5-HT was mimicked by the 5-HT3 receptor selective agonists 2-methyl-5-HT (EC50= 4.1+/-0.2 microM), 1-phenylbiguanide (EC50=3.0+/-0.1 microM), 3-chlorophenylbiguanide (EC50 = 140+/-10 nM), 3,5-dichlorophenylbiguanide (EC50 = 14.5+/-0.4 nM) and 2,5-dichlorophenylbiguanide (EC50 = 10.2+/-0.6 nM). With the exception of 2-methyl-5-HT, all of the agonists tested elicited maximal current responses comparable to those produced by a saturating concentration (10 microM) of 5-HT. Responses evoked by 5-HT at EC50 were blocked by the 5-HT3 receptor selective antagonist ondansetron (IC50=231+/-22 pM) and by the less selective agents (+)-tubocurarine (IC50=31.9+/-0.01 nM) and cocaine (IC50 = 2.1+/-0.2 microM). The data are discussed in the context of results previously obtained with the human and mouse orthologues of the 5-HT3A subunit. Overall, the study reinforces the conclusion that species differences detected for native 5-HT3 receptors extend to, and appear largely explained by, differences in the properties of homo-oligomeric receptors formed from 5-HT3A subunit orthologues.