The pharmacological profile of SK-951 ((-)4-amino-
N-[2-(1-azabicyclo[3.3.0]octan-5-yl) ethyl]-5-chloro-2, 3-dihydro-2-methylbenzo[
b]furan-7-carboxamide hemifumarate) was identified in relation to serotonin 5-HT
3 and 5-HT
4 receptors by the receptor binding assay and functional studies. The receptor binding assay showed that SK-951 bound to the 5-HT
3 receptor with a high affinity, to the 5-HT
4 receptor with relatively higher affinity and to the muscarinic M
2 receptor with a low affinity, but not to dopamine D
1 and D
2 and serotonin 5-HT
1 and 5-HT
2 and muscarinic M
1 and M
3 receptors. SK-951 caused relaxations of tunica muscularis mucosae preparations from rat esophagus which were precontracted with carbachol, and the effects were antagonized by GR113808, a selective 5-HT
4 antagonist. In the longitudinal muscle with myenteric plexus (LMMP) preparations from guinea pig ileum, SK-951 enhanced the electrically-stimulated contraction of preparations in which the 5-HT
1, 5-HT
2 and 5-HT
3 receptors were blocked, and it enhanced the electrically-stimulated release of [
3H]acetylcholine (ACh). These effects of SK-951 were antagonized by GR113808. SK-951 inhibited the 5-HT
3 receptor-mediated contractions. These results indicate that SK-951 possesses properties of an agonist for the 5-HT
4 receptor and an antagonist for the 5HT
3 receptor. Thus, SK-951 is a new and potent 5-HT
4-receptor agonist and causes contractions of guinea pig ileum mediated by enhancement of ACh release via the 5-HT
4 receptor.
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