Functional studies of insecticides and acaricides acting on ligand-gated ion channels

Iqbal, Sofia (2019) Functional studies of insecticides and acaricides acting on ligand-gated ion channels. PhD thesis, University of Nottingham.

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Arthropod pests such as ticks and fleas are important ectoparasites of livestock and companion animals that cause economic losses, spread disease and pose a public health risk. They are most commonly controlled using chemical pesticides, the majority of which have a neurotoxic mode of action. Two important targets within the arthropod nervous system are the GABA receptor (GABAR) and the nicotinic acetylcholine receptor (nAChR). They are targeted by a diverse range of chemical classes, although prolonged use of certain compounds has resulted in the development of resistance. This continues to be an ever-increasing problem for effective pest control and is often caused by simple point mutations within the target that disrupt the high selectivity of the target-ligand interaction at the binding site.

This thesis describes investigations into the mode of action of compounds acting on these targets, their selectivity and the molecular basis of resistance. Bioassay screening of the most common species of flea, Ctenocephalides felis, highlighted cross-resistance and decreased efficacy of ethiprole in dieldrin-resistant fleas. This was attributed to a known point mutation (A301S) within the GABAR channel, and was further confirmed by two-electrode voltage clamp electrophysiology of Xenopus laevis oocytes expressing the A301S modified GABA receptor. The possible role of a second, novel, amino acid change (M354I), linked to fipronil resistance was also investigated. Interestingly, it was found that rather than conferring resistance, this second substitution instead restored susceptibility to ethiprole and fipronil insensitive A301S modified receptors. This was further verified by expression studies in vivo, using transgenic strains of the model organism Drosophila melanogaster.

An electrophysiological analysis of the cattle tick, Rhipicephalus microplus, nicotinic acetylcholine receptor (α6 subunit) was also carried out to investigate residues thought to be involved in the selective binding of spinosad to these homomeric receptors. Three residues previously identified from a modelling study, F265L, G324A and C329S, were introduced into the R. microplus α6 subunit cDNAs and expressed in Xenopus oocytes to form functional acetylcholine-responsive receptors. All three substitutions resulted in a reduced channel efficacy of spinosad, confirming that these residues do play a key role in spinosad binding.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Williamson, Martin
Field, Lin
Davies, Emyr
Turberg, Andreas
Mellor, Ian
Keywords: GABA receptor; Nicotinic acetylcholine receptor; Insecticide resistance; Spinosad
Subjects: Q Science > QL Zoology
Faculties/Schools: UK Campuses > Faculty of Science > School of Biosciences
Item ID: 56879
Depositing User: Iqbal, Sofia
Date Deposited: 20 Aug 2019 10:21
Last Modified: 17 Jul 2021 04:30

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