Identification, Localization in the Central Nervous System and Novel Myostimulatory Effect of Allatostatins in Tenebrio molitor Beetle

Allatostatins (ASTs) are pleiotropic insect neuropeptides that are potent myoinhibitors of muscle contractions. In this study, we identified and immunolocalized peptides from the MIP/AST and PISCF/AST families in the nervous system of a model beetle, Tenebrio molitor. Neurons containing MIPs were immunolocalized in the brains of adults and the ventral nerve cords of larvae, pupae and imagines of this species as well as in the retrocerebral complex. PISCFs were immunolocalized in the ventral nerve cord of all stages as well as the brain of the adult beetle. Faint signals were also observed in the corpus allatum but not in the corpus cardiacum. The results allowed us to deduce the sequences of three neuropeptides belonging to MIP/ASTs, Tenmo-MIP4—NWGQFGXWa, Tenmo-MIP5—SKWDNFRGSWa and Tenmo-MIP6—EPAWSNLKGIWa, and one peptide from the PISCF/AST family, QSRYXQCYFNPISCX. Furthermore, we showed for the first time myostimulatory action of endogenous MIP/ASTs. Tenmo-MIP5 caused dose-dependent stimulation of the contractile activity of the beetle oviduct muscles, showing a sigmoidal curve up to 81.20% at the 10⁻⁸ M concentration, and the EC₅₀ value for the myostimulatory effect of this peptide was 8.50 × 10⁻¹² M. This is the first report of myostimulatory action of an endogenous myoinhibitory peptide in insect muscles.