Pharmacological properties of GR38032F, a novel antagonist at 5‐HT3 receptors

1 This paper describes the pharmacology of the novel 5‐hydroxytryptamine3 (5‐HT3) receptor antagonist GR38032F. 2 On the isolated vagus nerve and superior cervical ganglion of the rat, R,S‐GR38032F behaved as a reversible competitive antagonist of 5‐HT‐induced depolarization with pKB values of 8.61 ± 0.08 (n = 19) and 8.13 ± 0.07 (n = 16), respectively. The resolved R‐ and S‐isomers of GR38032F were approximately equipotent as 5‐HT antagonists on the rat vagus nerve: the pKB values were 8.95 ± 0.05 (n = 16) and 8.63 ± 0.08 (n = 17), respectively. R,S‐GR38032F was also an effective antagonist of 5‐HT on the rabbit isolated vagus nerve: in this case the pKB value was 9.40 ±0.14 (n = 4). 3 On the rabbit isolated heart, low concentrations of R,S‐GR38032F (3 × −1011 −1 × 10‐9 m) antagonized the positive chronotropic effect of 5‐HT and 2‐methyl‐5‐hydroxytryptamine (2‐methyl‐5‐HT). However, the effects of the compound did not appear consistent with simple reversible competition. 4 On the longitudinal smooth muscle of the guinea‐pig ileum, R,S‐GR38032F caused concentration‐dependent parallel rightward displacement of the 2‐methyl‐5‐HT concentration‐contraction response curve; in contrast, a portion of the response to 5‐HT appeared resistant to R,S‐GR38032F. pKB values estimated from the effects of the compound against 2‐methyl‐5‐HT or the inhibitable portion of the response to 5‐HT were 7.31 ± 0.06 (n = 8) and 7.33 ± 0.13 (n = 8), respectively. Against 2‐methyl‐5‐HT, R‐GR38032F seemed more potent (pKB 7.20 ± 0.10; n = 6) than S‐GR38032F (pKB 6.30 ± 0.05; n = 6). 5 R,S‐GR38032F is highly selective for 5‐HT3 receptors, and at concentrations of 3 × 10−6‐3 × 10−5m, had negligible agonist or antagonist activity on other 5‐HT or non‐5‐HT receptor‐containing tissues on which it was tested. 6 The potency and duration of action of R,S‐GR38032F in blocking 5‐HT3 receptors in vivo were assessed by measuring its ability to antagonize the bradycardic response to 5‐HT or 2‐methyl‐5‐HT administered intravenously (i.v.) to anaesthetized animals. For i.v. administration to the rat, the ED50 for R,S‐GR38032F against 2‐methyl‐5‐HT (100 μg kg−1) was 0.4 (95% confidence limits 0.18–0.87) μg kg−1 (n = 10); the corresponding value for oral administration to this species was 7.0 (3.0–22.0) μg kg−1 (n = 8–10 per dose level). R,S‐GR38032F was similarly effective in the anaesthetized cat. 7 The present results are discussed with reference to the postulated existence of subtypes of the 5‐HT3 receptor.