Abstract
Serotonin induces elevation of cyclic AMP levels in the dorsal abdominal integument of fifth-instar larvae and adults of Rhodnius prolixus. The effect in adults is dose-dependent (threshold 0.2 μM, maximum 2 μM serotonin), mimicked slightly (8–24%) by the aminergic agonists dl-octopamine, dl-synephrine, and l-norepinephrine, but not by dopamine, and inhibited by the vertebrate aminergic antagonists mianserin, methysergide and cyproheptidine, but not by phentolamine, propranolol or gramine. We suggest that cyclic AMP is a second messenger for serotonin's role in cuticular plasticization and that it might have other feeding-associated actions on the epidermis. Serotonin also appears to induce cyclic AMP elevation in adult anterior midgut (crop) and in Malpighian tubules from fifth-instar larvae and adults. These findings, when combined with an earlier report of increased haemolymph serotonin concentrations during feeding in fifth-instar larvae, suggest that serotonin may act directly on the Malpighian tubules during the normal course of diuresis in Rhodnius, perhaps synergizing diuretic hormone action. It may also increase fluid transport across the crop.
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