Pharmacological characterisation of human 5-HT2 receptor subtypes

Prompted by conflicting literature, this study compared the pharmacology of human 5-hydroxytryptamine2 (5-HT2) receptors expressed in SH-SY5Y cells using a fluorometric imaging plate reader (FLIPR) based Ca2+ assay. 5-Hydroxytryptamine (5-HT) increased intracellular calcium concentration ([Ca2+]i) at 5-HT2A, 5-HT2B and 5-HT2C receptors (pEC(50)=7.73+/-0.03, 8.86+/-0.04 and 7.99+/-0.04, respectively) and these responses were inhibited by mesulergine (pKB=7.42+/-0.06, 8.77+/-0.10 and 9.52+/-0.11). A range of selective agonists and antagonists displayed the expected pharmacology at each receptor subtype. Sodium butyrate pretreatment increased receptor expression in SH-SY5Y/5-HT2B (15-fold) and SH-SY5Y/5-HT2C cells (7-fold) and increased agonist potencies and relative efficacies. In contrast, sodium butyrate pretreatment of SH-SY5Y/5-HT(2A) cells did not affect receptor expression. The present study provides a direct comparison of agonist and antagonist pharmacology at 5-HT(2) receptor subtypes in a homogenous system and confirms that agonist potency and efficacy varies with the level of receptor expression.

Phenyl benzenesulfonamides are novel and selective 5-HT6 antagonists: identification of N-(2,5-dibromo-3-fluorophenyl)-4-methoxy-3-piperazin-1-ylbenzenesulfonamide (SB-357134)

Substituted N-phenyl-4-methoxy-3-piperazin-1-ylbenzenesulfonamides and conformationally restricted analogues have been identified as high affinity and selective 5-HT6 antagonists. Compounds from this series had a range of pharmacokinetic profiles in rat and in general there was a correlation between clearance and CNS penetration. Based on its overall biological profile 2 (SB-357134) was selected for further pre-clinical evaluation.

Characterization of SB-271046: a potent, selective and orally active 5-HT(6) receptor antagonist

SB-271046, potently displaced [(3)H]-LSD and [(125)I]-SB-258585 from human 5-HT(6) receptors recombinantly expressed in HeLa cells in vitro (pK(i) 8.92 and 9.09 respectively). SB-271046 also displaced [(125)I]-SB-258585 from human caudate putamen and rat and pig striatum membranes (pK(i) 8.81, 9.02 and 8.55 respectively). SB-271046 was over 200 fold selective for the 5-HT(6) receptor vs. 55 other receptors, binding sites and ion channels. In functional studies on human 5-HT(6) receptors SB-271046 competitively antagonized 5-HT-induced stimulation of adenylyl cyclase activity with a pA(2) of 8.71. SB-271046 produced an increase in seizure threshold over a wide-dose range in the rat maximal electroshock seizure threshold (MEST) test, with a minimum effective dose of < or =0.1 mg kg(-1) p.o. and maximum effect at 4 h post-dose. The level of anticonvulsant activity achieved correlated well with the blood concentrations of SB-271046 (EC(50) of 0.16 microM) and brain concentrations of 0.01-0.04 microM at C(max). These data, together with the observed anticonvulsant activity of other selective 5-HT(6) receptor antagonists, SB-258510 (10 mg kg(-1), 2-6 h pre-test) and Ro 04-6790 (1-30 mg kg(-1), 1 h pre-test), in the rat MEST test, suggest that the anticonvulsant properties of SB-271046 are likely to be mediated by 5-HT(6) receptors. Overall, these studies demonstrate that SB-271046 is a potent and selective 5-HT(6) receptor antagonist and is orally active in the rat MEST test. SB-271046 represents a valuable tool for evaluating the in vivo central function of 5-HT(6) receptors.

Functional effects of the muscarinic receptor agonist, xanomeline, at 5-HT1 and 5-HT2 receptors

Xanomeline [3(3-hexyloxy-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-me thylpyridine] has been reported to act as a functionally selective muscarinic partial agonist with potential use in the treatment of Alzheimer's disease. This study examined the functional activity of xanomeline at 5-HT1 and 5-HT2 receptors in native tissue and/or human cloned receptors. Xanomeline had affinity for muscarinic receptors in rat cortical membranes where the ratio of the displacement affinity of [3H]-Quinuclidinyl benzilate vs that of [3H]-Oxotremorine-M was 16, indicative of partial agonist activity. Radioligand binding studies on human cloned receptors confirmed that xanomeline had substantial affinity for M1, M2, M3, M4, M5 receptors and also for 5-HT1 and 5-HT2 receptor subtypes. Carbachol and xanomeline stimulated basal [35S]-GTPgammaS binding in rat cortical membranes with micromolar affinity. The response to carbachol was attenuated by himbacine and pirenzepine with pA2 of 8.2, 6.9 respectively consistent with the response being mediated, predominantly, via M2 and M4 receptors. Xanomeline-induced stimulation of [35S]-GTPgammaS binding was inhibited by himbacine with an apparent pKb of 6.3, was not attenuated by pirenzepine up to 3 microM and was inhibited by the selective 5-HT1A antagonist WAY100635 with an apparent pKb of 9.4. These data suggest the agonist effect of xanomeline in this tissue is, in part, via 5-HT1A receptors. Similar studies on human cloned receptors confirmed that xanomeline is an agonist at human cloned 5-HT1A and 5-HT1B receptors. In studies using the fluorescent cytoplasmic Ca2+ indicator FLUO-3AM, xanomeline induced an increase in cytoplasmic Ca2+ concentration in SH-SY5Y cells expressing recombinant human 5-HT2C receptors. Atropine antagonized this response, consistent with mediation via endogenously-expressed muscarinic receptors. In the presence of atropine, xanomeline antagonized 5-HT-induced cytoplasmic changes in Ca2+ concentration in cells expressing h5-HT2A, h5-HT2B and h5-HT2c receptors with potencies similar to its affinity at these receptors. These studies indicate that xanomeline is a potent agonist at 5-HT1A and 5-HT1B receptors and an antagonist at 5-HT2 receptor subtypes.

In vitro and in vivo profile of SB 206553, a potent 5-HT2C/5-HT2B receptor antagonist with anxiolytic-like properties

1. SB 206553 (5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2 ,3-f]indole) displays a high affinity (pK1 7.9) for the cloned human 5-HT2C receptor expressed in HEK 293 cells and the 5-HT2B receptor (pA2 8.9) as measured in the rat stomach fundus preparation. SB 206553 has low affinity for cloned human 5-HT2A receptors expressed in HEK 293 cells (pK1 5.8) and (pK1 < 6) for a wide variety of other neurotransmitter receptors. 2. SB 206553 appears to be a surmountable antagonist of 5-HT-stimulated phosphoinositide hydrolysis in HEK 293 cells expressing the human 5-HT2C receptor (pKB 9.0). 3. The compound potently (ID50 5.5 mg kg-1, p.o., 0.27 mg kg-1, i.v.) inhibited the hypolocomotor response to m-chlorophenylpiperazine (mCPP), a putative model of 5-HT2C/5-HT2B receptor function in vivo. 4. At similar doses (2-20 mg kg-1, p.o.) SB 206553 increased total interaction scores in a rat social interaction test and increased punished responding in a rat Geller-Seifter conflict test. These effects are consistent with the possession of anxiolytic properties. 5. SB 206553 also increased suppressed responding in a marmoset conflict model of anxiety at somewhat higher doses (15 and 20 mg kg-1, p.o.) but also reduced unsuppressed responding. 6. These results suggest that SB 206553 is a potent mixed 5-HT2C/5-HT2B receptor antagonist with selectivity over the 5-HT2A and all other sites studied and possesses anxiolytic-like properties.