Tachykinin-related peptides (TRPs) have been identified in a number of invertebrates belonging to the mollusca, euchiurdea, crustacea and insecta. The TRPs are named after their sequence similarities and possible common evolutionary relationship to the vertebrate tachykinins. In insects the TRPs exist in several closely related molecular forms in each species, so called isoforms. In the fruitfly Drosophila melanogatser the TRP preprohormone gene (Dtk) was identified and characterized. The Dtk precursor was found to be expressed in endocrine cells of the midgut and in neurons of the central nervous system (CNS). The distribution of TRPs was studied in the brains of the cockroach Leucophaea maderae and in D. melanogaster. Their widespread distribution suggest a central neuromodulatory role in sensory processing, higher motor control and in the control of hormone release in both species. We mapped the expression of Dtk by in situ hybridization and immunocytochemistry during postembryonic development of the CNS of D. melanogaster. Several neurons could be identified throughout the postembryonic development suggesting that TRPs are expressed continuously in these neurons and may have functions during development.

We were able to induce release of TRPs in vitro from midguts of the locust Locusta migratoria and L. maderae. We also detected TRPs in the hemolymph of L. migratoria, in vivo, thus, establishing that the endocrine cells of the locust midgut are the likely source of circulating TRPs. The TRP content in the locust midgut decreased during starvation. These findings suggest that TRPs are released, as hormones or in a paracrine fashion from the midgut, and that they may have a role in some aspect of feeding of L. migratoria.

In L. maderae nine TRP isoforms (LemTRPs) have previously been isolated. We studied the distribution of the LemTRPs in order to reveal possible cell-specific isoform distribution. In the midgut of L. maderae we found that two of these isoforms are cell-specifically distributed. However, in the cockroach brain the isoforms studied appears to be colocalized. The LemTRPs displayed similar potencies in the isolated cockroach hindgut contraction bioassay. Using the same assay we found it likely that the LemTRPs act on the hindgut through a specific TRP receptor.