THE MODULATORY EFFECT OF SCHISTOFLRFamide ON HEART AND SKELETAL MUSCLE IN THE LOCUST SCHISTOCERCA GREGARIA

SchistoFLRFamide (PDVDHVFLRFamide) is one of the endogenous FMRFamide-like neuropeptides found in the nervous system of the locust Schistocerca gregaria (Robb et al. 1989; Robb and Evans, 1990). FMRFamide-like neuropeptides modulate the actions of a wide variety of both skeletal and visceral muscles in insects (Evans and Myers, 1986a; Schoofs et al. 1993b) and have been suggested to act both as circulatory hormones and as locally released neurotransmitters (see discussion in Robb and Evans, 1990). However, the behavioural context(s) in which such neuropeptides are released is at present unclear, although recent evidence suggests they may have a role in feeding behaviour (Elia et al. 1993). SchistoFLRFamide belongs to one of the subclasses of FMRFamide-like neuropeptides found in insects, which also includes leucomyosuppressin (pQDVDHVFLRFamide; Holman et al. 1986), ManducaFLRFamide (pQDVVHSFLRFamide; Kingan et al. 1990) and TDVDHVFLRFamide found in both Drosophila melanogaster (Nichols, 1992) and Neobellieria bullata (Fonagy et al. 1992). The sequence of SchistoFLRFamide has also been found in Locusta migratoria (Schoofs et al. 1993a). SchistoFLRFamide was originally isolated from the nervous system of Schistocerca gregaria using a combination of radioimmunoassay, HPLC and bioassay (Robb et al. 1989; Robb and Evans, 1990). Preliminary data indicate that it inhibits spontaneous contractions of the locust heart and has a complex dose-dependent effect upon contractions induced in the extensor tibiae muscle by stimulation of the slow excitatory motoneurone (Robb et al. 1989). In addition, SchistoFLRFamide has recently been shown to decrease the amplitude and frequency of myogenic contractions and to reduce basal tension in locust oviduct muscle (Peeff et al. 1993). The present paper provides a more detailed account of the modulatory effect of SchistoFLRFamide on heart and skeletal muscle in the locust Schistocerca gregaria. The effects of the synthetic peptide were measured on the semi-isolated locust heart (Cuthbert and Evans, 1989). The results are expressed as the percentage change in the amplitude of the heartbeat and its frequency with respect to measurements obtained immediately before the application of the peptide. Measurements of heart rate and contraction amplitude were averaged over a 1 min period, starting (a) 1 min before the beginning of the peptide pulse, (b) 1 min after the start of the peptide pulse, (c) 4 min after the start of the peptide pulse and (d) 1 min after the end of the peptide pulse. The bioactivity of synthetic SchistoFLRFamide was also assayed on another well-defined FMRFamide bioassay preparation from the locust. The effects were measured on the twitch tension evoked in the extensor tibiae muscle of the hindleg of the locust by stimulating the slow excitatory motoneurone to this muscle at 1 Hz (Evans and Myers, 1986b). The results are expressed as the maximal effects on the twitch amplitude.