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

1 Xanomeline [3(3-hexyloxy-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine] 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. 2 Xanomeline had affinity for muscarinic receptors in rat cortical membranes where the ratio of the displacement affinity of [H-3]-Quinuclidinyl benzilate vs that of [H-3]-Oxotremorine-M was 16, indicative of partial agonist activity. Radioligand binding studies on human cloned receptors confirmed that xanomeline had substantial affinity for M-1, M-2, M-3, M-4, M-5 receptors and also for 5-HT1 and 5-HT2 receptor subtypes. 3 Carbachol and xanomeline stimulated basal [S-35]-GTP gamma S binding in rat cortical membranes with micromolar affinity. The response to carbachol was attenuated by himbacine and pirenzepine with pA(2) of 8.2, 6.9 respectively consistent with the response being mediated, predominantly, via M-2 and M-4 receptors. Xanomeline-induced stimulation of [S-35]-GTP gamma S binding was inhibited by himbacine with an apparent pK(b) of 6.3, was not attenuated by pirenzepine up to 3 mu M and was inhibited by the selective 5-HT1A antagonist WAY100635 with an apparent pK(b) 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. 4 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-MT-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. 5 These studies indicate that xanomeline is a potent agonist at 5-HT1A and 5-HT1B receptors and an antagonist at 5-HT2 receptor subtypes.