Exploring the pharmacological properties of insect nicotinic acetylcholine receptors Steeve H. Thany 1 , Guy Lenaers 2 , Vale´ rie Raymond-Delpech 3 , David B. Sattelle 4 and Bruno Lapied 1 1 Laboratoire Re´ cepteurs et Canaux Ioniques Membranaires (RCIM), UPRES EA 2647/USC INRA, Universite´ d’Angers, UFR Sciences, 2 Boulevard Lavoisier, 49045 Angers cedex, France 2 INSERM U583, Institut des Neurosciences de Montpellier IFR76, Universite´ de Montpellier, BP 74103, 34091 Montpellier cedex, France 3 Centre de Recherches sur la Cognition Animale (CRCA), CNRS-UMR 5169, Universite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France 4 MRC Functional Genetics Unit, Department of Physiology, Anatomy and Genetics, University of Oxford, Le Gros Clark Building, South Parks Road, Oxford OX1 3QX, UK Insect nicotinic acetylcholine (nACh) receptors are molecular targets of insecticides such as neonicotinoids that are used to control disease-carrying insects and agricultural pests. To date, several insect nACh receptor subunits have been identified, indicating different nACh receptor subtypes and pharmacological profiles. Because of the difficulty in expressing functional insect nACh receptors in heterologous systems, new research tools are needed. Studies on insects resistant to the insecticide imidacloprid and on laboratory-generated hybrid and chimaeric nACh receptors in vitro have pro- vided information about the molecular basis of receptor diversity, neonicotinoid resistance and selectivity. Addi- tionally, recent results indicate that the sensitivity of insect nACh receptors to imidacloprid can be modulated by intracellular phosphorylation mechanisms, which offers a new approach to studying insect nACh receptor pharmacology. Molecular diversity of insect nicotinic acetylcholine receptor subunits Insect nicotinic acetylcholine (nACh) receptor subunits, like vertebrate nACh receptor subunits, consist of a large N-terminal extracellular domain involved in agonist bind- ing, followed by three transmembrane regions (TM1–TM3, with TM2 lining the channel), a large intracellular loop, a fourth transmembrane domain (TM4) and a C-terminal extracellular region (Figure 1). The presence of two vicinal cysteine residues, equivalent to Cys192 and Cys193 in the electric organ (Torpedo marmorata) a1 subunit, which are known to be involved in ACh binding, defines nACh recep- tor a subunits in insects, vertebrates and all other species whereas non-a subunits (b, g, d or e) lack this motif. From comparison of the insect and vertebrate nACh receptor subunit sequences, it is evident that six insect monophy- letic groups diverged from a common ancestor distinct from the one that led to the vertebrate subunits (e.g. a1, a2, a3, a4, a8 and b1 subunits). One such group includes all the b subunits except for the Drosophila Db2 and Db3 subunits. Additional subunits share a common ancestor with the vertebrate a7 and a8 subunits, which characteristically form a-bungarotoxin (a-Bgt; see Glossary)-sensitive nACh receptor subtypes, thereby defining a monophyletic group of a7-like subunits [1,2] (Figure 2). Consequently, as pre- viously shown [3,4], subunit combinations determine the distinct pharmacological properties of insect nACh recep- tors and the sensitivity to neonicotinoid insecticides of insect nACh receptors compared with their vertebrate counterparts. The pharmacological properties of insect nACh receptors have been investigated using several dif- ferent approaches, including vertebrate–insect hybrid receptors and mutation of residues believed to be involved in ligand binding. In the present review, we discuss emer- ging data on the pharmacological properties of insect nACh receptors and offer new tools for their study. Evidence for different nACh receptor subtypes The subunit composition of native insect nACh receptors remains unclear, largely because the heterologous expres- sion of functional insect nACh receptors has proved diffi- cult. Nevertheless, behavioural studies using different nicotinic agonists and antagonists [5,6] or Drosophila Da7 mutants [7] have established that the insect central Opinion TRENDS in Pharmacological Sciences Vol.28 No.1 Glossary a-Bungarotoxin (a-Bgt): toxin from snake venom. a-Bgt binding is considered to represent the distribution of a7-subunit-containing nACh receptors. DEG: degeneration of certain neurons. In Caenorhabditis elegans, there are 42 different nACh receptor subunits, including the deg-3 group. des-2 is another gene in this group. DES: degeneration suppressor. Mutations in the gene encoding this protein suppress the degeneration caused by deg-3. Drosophila Da7 mutant: excisions of P elements in the Da7 subunits lead to several alleles. nACh receptor subtype: a specific combination of identical (homomeric) or different (heteromeric) subunits that forms a pentameric nACh receptor. Corresponding author: Thany, S.H. (steeve.thany@univ-angers.fr). Available online 6 December 2006. www.sciencedirect.com 0165-6147/$ – see front matter ß 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tips.2006.11.006