The muscular contractions of the midgut of the cockroach, Diploptera punctata: effects of the insect neuropeptides proctolin and leucomyosuppressin.

We have previously shown differential expression of leucomyosuppressin (LMS) mRNA in apparent endocrine cells in the anterior region of midguts of the cockroach Diploptera punctata, using in situ hybridization. In contrast, other FMRFamide-related peptides, as revealed by immunohistochemistry, have been found most abundantly in the posterior region in both apparent endocrine cells and nerve tracts. Here, we partially purified extracts of anterior and posterior cockroach midguts, using HPLC coupled with radioimmunoassay, and found, among multiple FMRFamide-like immunoreactive fractions, one fraction co-eluting with LMS in both regions. The presence of a co-eluting fraction in the posterior region, in the absence of LMS mRNA positive endocrine cells suggests that LMS might therefore be present in nerve tracts running along the length of the midgut. Using a circular muscle contraction assay from different portions of midgut, we determined the effects of LMS, proctolin and a variety of other midgut peptides on contractions of the midgut of Diploptera. Proctolin caused a sustained tonic contraction in the anterior midgut, the amplitude of which was dose-dependent. In contrast, LMS, and its relative SchistoFLRFamide, reduced the amplitude of these contractions. LMS and SchistoFLRFamide also inhibited spontaneous phasic contractions, which were elicited by proctolin application in only a few preparations. Other postulated midgut peptides did not induce or inhibit contractions, nor augment the proctolin-induced contractions. The C-terminal truncated sequences of LMS, HVFLRFamide and VFLRFamide, were sufficient to reduce the amplitude of the proctolin-induced contractions. This work illustrates a possible physiological role for LMS in Diploptera midguts, in the passage of food along the alimentary canal.