A vasopressin-like peptide was isolated from the locust Locusta migratoria (Proux et al., 1987). Related peptides have been identified in some other insects species (e. g. Tribolium castaneum and Nasonia vitripennis), but not in Drosophila or other dipteran insects, nor in Bombyx mori or Apis mellifera (Stafflinger et al 2008). The invertebrate forms of vasopressin-like peptides have a well conserved sequence and have been designated inotocins (Stafflinger et al 2008) and are cyclic peptides, due to an internal cystein bridge (Li et al., 2008). The sequence of Tribolium inotocin is CLITNCPRGamide (Li et al., 2008). Inotocin receptors were identified from Tribolium and Nasonia and display strong similarities to inotocin GPCRs from the water flee Daphnia pulex and the snail Lymnaea stagnalis, as well as mouse receptors to oxytocin and vasopressin (Aikins et al., 2008; Stafflinger et al 2008).
Inotocin is expressed in only two neurons in the subesophageal ganglion of locusts and Triboleum with branches arborizing extensively in the brain and ventral nerve cord and vasopressin-like immunoreactivity is also seen in a similar pair of neurons in cockroaches and mantids (Aikins et al., 2008; Davis and Hilderbrand, 1992; Tyrer et al., 1993). The inotocin receptor of Tribolium is expressed mainly within the CNS, and not in renal tubules (Aikins et al., 2008; Stafflinger et al 2008).
In Triboleum inotocin indirectly triggers diuresis (Aikins et al., 2008). Since the receptor appears expressed mainly within the CNS and no branches of the inotocin neurons reside outside the CNS, it was hypothesized that inotocin regulates neurosecretory cells that produce a diuretic hormone.
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