Submitted to: Experimental Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 13, 2003
Publication Date: May 1, 2003
Citation: Davis, N.T., Blackburn, M.B., Golubeva, E.G., Hildebrand, J.G. 2003. Localization of myoinhibitory peptide immunoreactivity in manduca sexta: indications that the peptide has a role in molting and ecdysis. Experimental Biology.
Interpretive Summary: Molting in insects is controlled by a hormone known as ecdysone, which is secreted by glands in the thorax. Prior to molting, ecdysone is rapidly secreted by these glands for a period of time, after which secretion abruptly ceases. A peptide (small protein) called MIP-I, originally discovered in the nervous system of the tobacco hornworm, has been shown previously to inhibit ecdysone release when injected into the silkworm. However, there was no evidence that MIP-I actually regulates ecdysone in normal insects. In this study the distribution of MIP-I in the nervous system of the tobacco hornworm was determined. The peptide was found throughout the nervous system, as well as a pair of glands near the rectum of the hornworm. It was found that a massive release of MIP-I occurred from these glands when blood levels of ecdysone were rapidly declining. These results strongly suggest that MIP-I is involved in the cessation of ecdysone secretion. This information will benefit scientists that study the developmental biology and endocrinology of insects. Ultimately, novel insecticides or growth regulators, which mimic the biological activity of MIP-I may be developed, which are safer to wildlife, livestock and humans.
It is known that for normal development to occur in larval Manduca sexta, ecdysteroid titers must drop following their sudden rise; others have shown that this decline may be due to myoinhibitory peptide-I (MIP), which has an inhibitory effect, in vitro, on the release of ecdysone by the prothoracic glands of Bombyx mori. Using an antiserum to MIP, we show neurohemal release of a MIP-like peptide from the epiproctodeal gland, which is located on the proctodeal nerve, just anterior to the rectum. These glands are also present in B. mori. In fourth-instar larvae of M. sexta, these glands show a distinct cycle of synthesis and massive release of their secretory product; the release occurs at the time of rapid decline in ecdysteroid titers. We believe that our results indicate that the function of the epiproctodeal glands is to release a MIP-like peptide, which may then act on the prothoracic glands to inhibit the release of ecdysone. In addition, we have found that MIP immunoreactivity is co-localized with that of crustacean cardioactive peptide (CCAP) in the interneurons designated 704; these peptides appear to be co-released at the time of ecdysis. It is known that CCAP can initiate the ecdysis motor program; our results suggest that MIP may have a similar function.