Orcokinin, as the name suggests, was first isolated as a myostimulatory peptide from the crayfish Orconectes limosus (Stangier et al., 1992). Later it was identified in the cockroach Blattella germanica (Pascual et al., 2004) and has thereafter been detected for instance in the genomes of Bombyx, Anopheles and Apis (see Roller et al., 2008) and in Drosophila (Liu et al. 2006). In studied insect species the orcokinin precursor gene gives rise to at least two splice forms of the transcript, each producing distinct peptides (Yamanaka et al 2011; Chen et al., 2015). For instance in Drosophila the two splice forms of CG13565 give rise to one peptide each, orcokinin A and B (Chen et al. 2015). These non-amidated peptides have the sequences NFDEIDKASASFSILNQLV and GLDSIGGGHLI. In other insects multiple peptides are produced from each transcript, like in Locusta migratoria, where orcokinin A gives rise to 8, and orcokinin B to 6 predicted peptides (Hou et al, 2015). So far there is no identified orcokinin receptor in insects or crustaceans.
The peptide products of orcokinin A seems to be predominantly distributed in neurons of the CNS, whereas those of the B form can be found in both CNS and the midgut (Yamanaka et al., 2011; Sterkel et al., 2012; Chen et al., 2015). A detailed description of orcokinin A and B distribution has been provided for Drosophila (Chen et al., 2015), and the general distribution of orcokinins is described in several insect species (Hofer et al., 2005). In Drosophila, orcokinin A is found in a small number of neurons in the brain, including the pars intercerebralis, posterior brain, accessory medulla of the optic lobe (in neurons associated with, but distinct from, the clock neurons) (Chen et al., 2015). In the abdominal ganglia orcokinin A is present in segmental abdominal neurons, adjacent to neurons producing leucokinin (see kinins). Orcokinin B was detected in a single neuron in the abdominal ganglion, and in enteroendocrine cells of the anterior midgut (Chen et al., 2015).
A myostimulatory action of orcokinin has not been detected in insects so far (Pascual et al., 2004), but a role in central circuits of the brain is suggested from the peptide distribution in numerous interneurons of locusts and cockroaches (Hofer et al., 2005). More specifically a role of orcokinin in the circadian closk has been proposed for the cockroach Leucophaea maderae (Hofer and Homberg, 2006). In Bombyx orcokinin acts as neuronal prothoracicotropic factors stimulating ecdysteroird production in prothoracic glands in Bombyx mori (Yamanaka et al., 2011). Orcokinins also seem to play roles in gut function.
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