Neuropeptides from five different
neuropeptide families [
Manduca sexta allatostatin (Manse-AS), and Manse-AS deletion analogue(5-15), M. sexta
allatotropin (Manse-AT),
leucomyosuppressin,
perisulfakinin, and myoinhibitory
peptide I (MIP I)] were assayed for their ability to affect the development and food consumption of penultimate and last larval instars of two lepidopteran species, L. oleracea and S. littoralis.
Injections of Manse-AS deletion analogue(5-15), Manse-AT,
perisulfakinin, and MIP I had no observable effects on development, food consumption, or mortality compared to controls. Single
injections of Manse-AS significantly reduced the
weight gain and increased mortality of L. oleracea and S. littoralis larvae compared to controls. By contrast, feeding Manse-AS to L. oleracea had no such effects. These differences were probably due to the degradation of the
peptide by digestive
enzymes in the foregut of L. oleracea. In studies in vitro,
perisulfakinin, and MIP I had no effect on the spontaneous foregut contractions of L. oleracea larvae.
Leucomyosuppressin, however, had myoinhibitory effects on the foregut. Single
injections of
leucomyosuppressin significantly reduced the
weight gain and food consumption of L. oleracea and S. littoralis larvae and increased mortality. These data suggest that the deleterious effects observed in vivo were due to the myoinhibition by Manse-AS and
leucomyosuppressin of the normal peristaltic movements of the gut either by the intact
peptide or by its cleavage products resulting from degradation in the haemolymph.