Distribution of tachykinin‐related peptides in the brain of the tobacco budworm Heliothis virescens

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Invertebrate tachykinins, called tachykinin‐related peptides (TKRPs), are peptides related to vertebrate tachykinins and many insect species are reported to possess TKRPs (reviewed by, Nässel, 1999; Van Loy et al., 2010). The first insect TKRPs were identified in extracts of the brain, gnathal ganglion (the three fused subesophageal neuromers), and retrocerebral complex of the migratory locust, Locusta migratoria (Schoofs, Holman, Hayes, Kochansky, et al., 1990; Schoofs, Holman, Hayes, Nachman, & De Loof, 1990; Schoofs, Vanden Broeck, & De Loof, 1993). In total, five isoforms of locustatachykinins, that is, LomTK‐I–V, were identified in the locust (Schoofs, Holman, Hayes, Kochansky et al., 1990; Schoofs, Holman, Hayes, Nachman et al., 1990; Schoofs et al., 1993). Additional isoforms were subsequently reported in several other insect species (Table 1). The insect TKRPs are widely distributed in the central nervous system (CNS) and/or intestine (reviews, Nässel 1999; Van Loy et al., 2010). Exceptions are the sialokinins of the mosquito Aedes aegypti, being present in the salivary gland only (Champagne & Ribeiro, 1994). However, sialokinins differ from other insect TKRPs by showing the vertebrate‐ instead of the insect‐type tachykinin consensus motif (Van Loy et al., 2010).
In accordance with their widespread presence in insects, TKRPs play a variety of roles. A number of experiments have demonstrated a myotropic effect of these peptides. Thus, by acting on visceral muscle and glands the TKRPs were found to serve as hormones inducing contraction of the gut and oviduct (review Nässel, 1999; Siviter et al., 2000; Oeh, Antonicek, & Nauen, 2003; Kwok, Chung, Brugge, & Orchard, 2005). Furthermore, numerous findings of TKRPs in the CNS suggest important roles as neurotransmitters and/or neuromodulators (review, Nässel, 1999). These include a function in olfactory information processing (Gui et al., 2017; Ignell et al., 2009; Winther, Acebes, & Ferrús, 2006), contributing to aggressive arousal (Asahina et al., 2014), and effecting on locomotion (Kahsai, Martin, & Winther, 2010). Such findings are in agreement with anatomical studies showing strong tachykinin immunolabeling both in the antennal lobes and the central body, being involved in olfactory information processing and locomotor control, respectively, (Gui et al., 2017; Kahsai et al., 2010; Winther, Siviter, Isaac, Predel, & Nässel, 2003). The presence of TKRP‐immunoreactive neurons in the antennal lobes and central body was found in all insects studied, which suggests similar roles of TKRPs in olfactory information processing and locomotor control across the species (review, Nässel, 1999; Tables 2 and 3).
TKRP‐immunoreactive neurons were also detected in the gnathal ganglion and other brain regions, such as the optic lobes, mushroom bodies, superior and lateral protocerebrum, and tritocerebrum (review, Nässel, 1999; Tables 2 and 3). The distribution of TKRPs in the insect brain has been investigated in a variety of species (Table 3). In general, all investigated insect species show TKRP immunostaining in the brain. The number of TKRP‐immunoreactive neurons and neural arborizations within distinct brain regions vary considerably, however (review, Nässel, 1999; Tables 2 and 3). For instance, whereas the numbers of TKRP‐immunoreactive cells found in the locusts, L. migratoria and Schistocerca gregaria, are about 800 and 950, respectively, those of the flies, Calliphora vomitoria and Drosophila melanogaster, are about 150 (Lundquist, Clottens, Holman, Riehm, et al., 1994; Nässel, 1993; Vitzthum & Homberg, 1998; Winther et al., 2003). Furthermore, whereas the lamina, pedunculus, and mushroom body lobes of the cockroach, and the lamina, mushroom bodies, protocerebral bridge, ellipsoid body, and noduli of the fly showed no TKRP immunoreactivities, corresponding neuropils in the locust and some other species displayed such immunoreactivity (Lundquist, Clottens, Holman, Riehm, et al., 1994; Muren, Lundquist, & Nässel, 1995; Nässel, 1993; Winther et al., 2003; Table 3).
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