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  • A relatively independent review of the history of the


    A relatively independent review of the history of the discovery of GABA (and L-glutamate) as neurotransmitters in the CNS can be found in ‘When and why amino acids?’ in the in 2010 by Kresimir Krnjevic , who spent a post-doc in Canberra in 1956–1958.
    Introduction The insect RDL GABA receptor is a major target of insecticides (Casida, 2009). Lindane and cyclodienes, such as dieldrin and α-endosulfan, and phenylpyrazole insecticides, such as fipronil, are noncompetitive antagonists (NCAs) against insect RDL GABA receptors (Fig. 1). NCAs cause hyperexcitation of the nervous system and the death of insect pests by inhibiting the GABA-induced influx of chloride ions into nerve cells. The insect GABA receptor subunit gene was cloned from dieldrin-resistant Drosophila melanogaster and designated as Rdl (resistant to dieldrin) (ffrench-Constant et al., 1991). The RDL GABA receptor consists of a large N-terminal extracellular domain, four membrane-spanning regions (M1–M4), three loops (M1–M2 loop, M2–M3 loop, and M3–M4 loop), and a small C-terminal extracellular domain (Fig. 2). Alternatively spliced exons (3a or 3b, 6c or 6d) were reported in D. melanogaster Rdl by ffrench-Constant and Rocheleau (1993). Alternative splicing produced four variants: ac-, ad-, bc-, and bd-types. The pharmacological features of the recombinant RDL GABA receptor subunit homomers are similar to those of the native insect GABA receptors (Buckingham et al., 2005). There are many prominent reviews concerning resistance to insecticides targeting RDL GABA receptors (ffrench-Constant et al., 2000, Ozoe, 2013, Feyereisen et al., 2015). This review focuses on the mutations in pest RDL GABA receptors that confer resistance to NCAs, especially the novel ACY-241 found in planthoppers. In Asia, there are three serious planthoppers damaging rice; the brown planthopper Nilaparvata lugens, the white backed planthopper Sogatella furcifera, and the small brown planthopper Laodelphax stratella, S. furcifera and L. stratella have developed fipronil resistance. It is important to understand the novel mechanisms of fipronil resistance in these planthoppers.
    Cyclodiene resistance Lindane and cyclodienes, such as dieldrin and α-endosulfan, represent the first generation of NCAs (Fig. 1). It has been reported that A2′S and A2′G mutations in M2 of the RDL GABA receptor (Fig. 2) confer resistance to lindane and cyclodienes in various species, such as D. melanogaster (ffrench-Constant et al., 1993a), D. simulans (Thompson et al., 1993b), Musca domestica (Thompson et al., 1993b, Gao et al., 2007), Haematobia irritans (Domingues et al. 2013), Tribolium castaneum (Thompson et al., 1993b, Miyazaki et al., 1995), Oulema oryzae (Nakao et al., 2012a), Hypothenemus hampei (Navarro et al., 2010), Myzus persicae (Anthony et al., 1998), Diabrotica virgifera virgifera (Wang et al., 2013), Plutella xylostella (Li et al., 2006), Periplaneta Americana (Thompson et al., 1993b), Blattella germanica (Hansen et al., 2005), Bemisia tabaci (Anthony et al., 1995), Ctenocephalides felis (Bass et al., 2004), Aedes aegypti (Thompson et al., 1993a), Anopheles gambiae, An. arabiensis (Du et al., 2005), An. acontius, An. barbirostris, An. farauti, An. nigerrimus, An. sundaicus, An. vagus (Asih et al., 2012), An. funestus (Wondji et al., 2011), An. albopictus, and Culex pipiens quinquefasciatus (Tantely et al., 2010). Hope et al. (2010) reported that the T6′L mutation (Fig. 2) is associated with dieldrin resistance in the cattle tick, Rhipicephalus (Boophilus) microplus. In this review, the index number for the M2 membrane-spanning region (Charnet et al., 1990) was used to aid in comparison of M2 mutations in GABA receptors from various species. According to this index number, arginine residue at the cytoplasmic end of M2 is numbered 0′ (Fig. 2). Fitness cost of cyclodiene resistance has been reported by some studies. In An. gambiae and An. Stephensi mosquitoes, homozygous resistant females were less responsive to oviposition stimuli and produced fewer eggs per unit of blood. And they flied less during the periods available for seeking hosts or oviposition sites (Rowland, 1991a, Rowland, 1991b). Compared with homozygote resistant males, heterozygote males showed a higher mating success in An. gambiae (Platt et al., 2015). Cyclodiene-resistant D. melanogaster showed temporary paralysis and were unable to locomote by exposure to 38°C (ffrench-Constant et al., 1993b). Although the fitness cost of resistance in the absence of insecticide selection appears to have reduced the population of cyclodiene-resistant pests, A2′S and A2′G mutations in M2 of the RDL GABA receptor can be observed at relatively high frequencies (ffrench-Constant et al., 2000). Pesticides including dieldrin and endosulfan have been banned for over 15 years in La Réunion, but high frequency of cyclodiene-resistant allele in Cx. pipiens were observed (Tantely et al., 2010). As the reason, a persistence of dieldrin and/or endosulfan in the soil or an illegal use of these pesticides was speculated. As other possible causes, effects of pesticides, such as, fipronil, lindane, and avermectin family, were considered (Tantely et al., 2010).