Myotropic insect neuropeptides

Myotropic Insect Neuropeptide Families from the Cockroach Leucophaea maderae Structure—Activity Reiationships... 

In recent years, the improvement of appropriate techniques has facilitated the chemical identification of a number of insect neuropeptides as well as comparison with their respective counterparts in vertebrates. Among these are metabolic, myotropic, allatotropic, and allatostatic factors. 

Beginning with the initial success of Brown and Starratt (7,8) and continuing to the present, myotropic bioassays have played a major role in the isolation and structural characterization of insect neuropeptides. The success will likely continue since myotropic bioassays are sensitive, rapid, reliable, and reproducible characteristics desirable to the isolation specialist. Insect neuropeptide isolation projects utilizing a myotropic bioassay have been responsible for the discovery of four structurally unique peptide families and the initial demonstration of the existence of natural-analog series which may be quite common in insects. 

Structure-activity relationship studies for three myotropic families of insect neuropeptides first isolated from Leucophaea maderae are discussed. 

Kuniyoshi H., Nagasawa H., Ando T., Suzuki A., Nachman R. J. and Holman G. M. (1992b) Cross-reactivity between pheromone biosynthesis activating neuropeptide (PBAN) and myotropic pyrokinin insect peptides. Biosci. Biotech. Biochem. 56,167-168. 

The leucokinins (LK s) are a new class of insect myotropic neuropeptides isolated from head extracts of the cockroach L. maderae. These octapeptides all contain a similar core sequence of 5 amino acids that extend from position 4 through 8. This sequence Phe-X-Ser-Trp-Gly-NH2 seems to be required for hindgut stimulation. The initial response of the hindgut to the LK s was characterized by an increase in the frequency and/or amplitude of phasic contractions (50, ). At higher peptide concentrations, a tonic component was generally present. All of the LK s showed a response at 3 x 10 M that was 5-10% above the mean level of spontaneous activity. The maximum response for each of the peptides was recorded at a concentration 2.1 X 10" M. Thus, the intrinsic activities for the LK s are nearly equal because the dose-response curves have about the same asymptotic limits. A comparison of the dose concentrations that gave a half maximal response (ED q) for the 8 peptides is shown in Table I. 

The related achetakinins double the rate of fluid secretion by isolated Acheta Malpighian tubules (Coast, G. M., et al. J. Insect Phvsiol.. in press.)- In addition, some leucokinins stimulate fluid secretion and/or depolarize transepithelial membrane potentials in the malpighian tubules of the yellow fever mosquito, Aedes aegvpti. in a fashion similar to several uncharacterized peptides isolated from mosquito head extracts (23). Thus, the neuropeptide family of leucokinins, achetakinins, and homologs may function in the control of water and ion balance, as well as myotropic activity, in a number of insects. To determine the relationships of the C-terminal sequence of the leucokinins to myotropic activity, analogs were synthesized and tested on the isolated cockroach hindgut. 

An overwhelming number of fully characterized neuropeptides, especially in insects, have myotropic activity one way or the other these peptides regulate the contractile activity of visceral and/or skeletal muscles. One possible reason for this abundance of sequenced neuropeptides that regulate muscle activity may be the successful usage of a very simple bioassay technique [see 151]. 

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