Functional effects of the muscarinic receptor agonist, xanomeline, at 5‐HT 1 and 5‐HT 2 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‐HT 1 and 5‐HT 2 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 [ 3 H]‐Quinuclidinyl benzilate vs that of [ 3 H]‐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‐HT 1 and 5‐HT 2 receptor subtypes. 3 Carbachol and xanomeline stimulated basal [ 35 S]‐GTPγS 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 [ 35 S]‐GTPγS binding was inhibited by himbacine with an apparent pK b of 6.3, was not attenuated by pirenzepine up to 3 μ m and was inhibited by the selective 5‐HT 1A 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‐HT 1A receptors. Similar studies on human cloned receptors confirmed that xanomeline is an agonist at human cloned 5‐HT 1A and 5‐HT 1B receptors. 4 In studies using the fluorescent cytoplasmic Ca 2+ indicator FLUO‐3AM, xanomeline induced an increase in cytoplasmic Ca 2+ concentration in SH‐SY5Y cells expressing recombinant human 5‐HT 2C 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 Ca 2+ concentration in cells expressing h5‐HT 2A , h5‐HT 2B and h5‐HT 2C receptors with potencies similar to its affinity at these receptors. 5 These studies indicate that xanomeline is a potent agonist at 5‐HT 1A and 5‐HT 1B receptors and an antagonist at 5‐HT 2 receptor subtypes.British Journal of Pharmacology (1998) 125 , 1413–1420; doi: 10.1038/sj.bjp.0702201