Proctolin affects gut functions in lepidopteran larvae

We examined if the pentapeptide proctolin, a well-investigated myotropic neutrotrasmitter/neuromodulator for which a possible role in insect control has been suggested, affected the gastrointestinal function in lepidopteran larvae. We verified in the model species Bombyx mori (Lepidoptera, Bombicidae) if the visceral muscles of the larval gut expressed receptors for the peptide, by monitoring the spontaneous contractions of the hindgut in the absence or in the presence of proctolin. The frequency of the peristaltic contractions increased only at a very high concentration of the peptide, suggesting that proctolin could effectively bind with low affinity to non-specific receptors. The outcome on larval fitness of increasing doses of proctolin orally administered to fourth instar larvae were investigated by recording the standard nutritional indices. A significant decrease of the Efficiency of Conversion of Ingested and Digested food to body substance with a concomitant reduction of the Relative Growth Rate (RGR) was observed at the lowest dose tested (0.18 mg/g larva). The amount of peptide administered appeared critical, because a 10-fold higher dose of proctolin abolished the negative effect on RGR by a stimulation of the feeding behaviour, possibly as a result of the activation of compensatory mechanisms. A similar decrease of larval RGR with a low concentration of proctolin was observed also in the polyphagous pest Spodoptera littoralis (Lepidoptera, Noctuidae).