Pharmacological characterisation of 5-hydroxytryptamine-induced contractile effects in the isolated gut of the lepidopteran caterpillar Spodoptera frugiperda.

The indolealkylamine 5-hydroxytryptamine (5-HT, 0.1nM-1&mgr;M) caused dose-dependent increases in the number of contractions observed in guts isolated from the caterpillar Spodoptera frugiperda. Of the 5-HT analogues tested for agonist action, 2-methyl-5-HT (0.1-10&mgr;M) was a full agonist with reduced potency while alpha-methyl-5-HT (0.1-100&mgr;M), 5-carboxamidotryptamine (0.1-100&mgr;M), 5-methoxytryptamine (5-MeOT) (10nM-10&mgr;M), and tryptamine (1-100&mgr;M) were partial agonists. Incubation of isolated guts with proven mammalian 5-HT receptor antagonists showed that cyproheptadine (10nM-1&mgr;M), MDL 72222 (1-10&mgr;M), tropisetron (1-10&mgr;M) and 5-benzoyloxygramine (1-10&mgr;M) were potent non-competitive antagonists of 5-HT-induced tissue contraction. In comparison, ketanserin (0.1-1&mgr;M) was a competitive antagonist. The mammalian selective serotonin reuptake inhibitors, clomipramine (10nM-10&mgr;M) and fluoxetine (10nM-10&mgr;M) also caused non-competitive inhibition of 5-HT-induced contraction while fluvoxamine (10nM-10&mgr;M) was a weak competitive antagonist. Low doses of clomipramine (0.1&mgr;M) caused potentiation of 5-HT-induced gut contraction thereby suggesting the presence of 5-HT reuptake systems in this tissue. The contractile effects of 5-HT were inhibited by verapamil, Li(+) and H7 and potentiated by theophylline thereby indicating that L-type Ca(2+) channels, phosphatidylinositol second messengers and cAMP, respectively, are involved in 5-HT-induced tissue contraction. The 5-HT receptors mediating contractility in the gut of S. frugiperda have properties in common with mammalian 5-HT(2) and Drosophila 5-HT(dro2A/2B) receptors. In addition, these data suggest that the tissue also contains receptors that are similar to mammalian 5-ht(6) and 5-HT(7) as well as Drosophila(dro1) receptors. However, the primary amino acid sequence of these lepidopteran 5-HT receptors will have to be elucidated before full comparisons can be made.