Neuropeptides Related to Tachykinins and Leucokinins in the Developing Nervous System of Insects

Abstract: In insects more than 300 neuropeptides have been identified and these have been implicated in a vast range of functional roles. These peptides have been grouped into families based on amino acid sequence similarities. This thesis focuses on insect neuropeptides of two families, the leucokinin-like peptides (LKs) and tachykinin-related peptides (TRPs). The distribution of LKs and TRPs has been studied in developing and adult blowflies, moths and dragonflies. In the blowfly Calliphora vomitoria two isoforms of TRPs have been identified (CavTK-I and -II). It was shown here by a combination of high performance liquid chromatography, radioimmunoassay and immunoenzyme assay that these two isoforms are present in a ratio of about 1:1 in different portions of the central nervous system and intestine. One or two additional isoforms of TRPs may be present in the larval nervous system, but not in adults. These are yet to be isolated and sequenced. Immunocytochemistry was used to localize TRPs in neurons of the developing nervous system of the blowfly. Several TRP-containing neurons could be identified throughout postembryonic development, suggesting that larval neurons are remodeled and utilized by the adult fly. The distribution of TRPs was also studied during postembryonic development of the moth Spodoptera litura and in the adult nervous system and intestine of the dragonfly Aeshna grandis. In the moth the distribution of TRPs in the brain is similar to that in the blowfly, but is different in the ventral nerve cord. Several neuron types could be identified throughout metamorphosis. A novel finding was that in the moth brain there are TRP-containing neurosecretory cells with release sites in the corpora cardiaca. In the dragonfly ventral nerve cord the distribution of TRPs was different from that of other insects: immunoreactive cell bodies were only detected in the terminal abdominal ganglion. It can be suggested that the TRPs in the studied insects are mainly used as neuromodulators by a variety of interneurons and as regulators of midgut function or as neurohormones released from the midgut endocrine cells. Leucokinin-like immunoreactivity was detected in 2 pairs of identified neurons in each of the abdominal ganglia of the blowfly from first instar larvae to adults. These neurons are lateral neurosecretory cells with axon terminals in peripherally located neurohemal areas. A similar distribution of leucokinin-like immunoreactivity was seen during postembryonic development of the moth Spodoptera litura. In three species of dragonflies similar neurosecretory cells were detected, but instead of 2 pairs per abdominal ganglion up to 10 pairs per ganglion were seen. In conclusion it appears as if the TRPs have variable distributions in interneurons of the different studied insects, especially in the ventral nerve cord. The LKs on the other hand appear distributed in a phylogenetically preserved fashion in neurosecretory cells of abdominal ganglia. Both peptides are likely to have multiple functions both as neuromodulators and as neurohormones.