Inhibition of digestive enzyme release by neuropeptides in larvae of Opisina arenosella (Lepidoptera: Cryptophasidae).

Leucokinins are a group of structurally related neuropeptides stimulating gut motility and fluid secretion by Malpighian tubule in insects. For studying effect of neuropeptides on digestive enzyme release, empty midgut tubes of larvae of Opisina arenosella ligated at both ends with hair were incubated with Leucokinins (LK I-VIII), LK analogues and Leucopyrokinin (LPK) in a bioassay apparatus at 37 degrees C for 30 min. The lumen contents were subsequently analyzed for digestive enzyme levels. The neuropeptides LK III, FFSWG amide, 122 A[1] WP-2, LPK and 434 [phi2] WP-1 inhibited the release of digestive enzymes, protease and amylase while LK VIII, unique in having tyrosine residue, stimulated protease release. The minimum sequence of amino acids at the C-terminal required for activity of LK peptides was found to be FXSWGamide (X=Asn, His, Ser, or Trp). The N-terminal pyroglutamate residue and proline at the C-terminal may contribute to the inhibitory effect of LPK on digestive enzyme release. The present study reveals for the first time an inhibitory effect for leucokinins and pyrokinin on the release of digestive enzymes from the insect midgut.