title: Salmon lice (Lepeophtheirus salmonis) showing varying emamectin benzoate susceptibilities differ in neuronal acetylcholine receptor and GABA-gated chloride channel mRNA expression.
abstract: BACKGROUND
  Caligid copepods, also called sea lice, are fish ectoparasites, some species of which cause significant problems in the mariculture of salmon, where the annual cost of infection is in excess of €300 million globally. At present, caligid control on farms is mainly achieved using medicinal treatments. However, the continued use of a restricted number of medicine actives potentially favours the development of drug resistance. Here, we report transcriptional changes in a laboratory strain of the caligid Lepeophtheirus salmonis (Krøyer, 1837) that is moderately (~7-fold) resistant to the avermectin compound emamectin benzoate (EMB), a component of the anti-salmon louse agent SLICE® (Merck Animal Health).
  
  
  RESULTS
  Suppression subtractive hybridisation (SSH) was used to enrich transcripts differentially expressed between EMB-resistant (PT) and drug-susceptible (S) laboratory strains of L. salmonis. SSH libraries were subjected to 454 sequencing. Further L. salmonis transcript sequences were available as expressed sequence tags (EST) from GenBank. Contiguous sequences were generated from both SSH and EST sequences and annotated. Transcriptional responses in PT and S salmon lice were investigated using custom 15 K oligonucleotide microarrays designed using the above sequence resources. In the absence of EMB exposure, 359 targets differed in transcript abundance between the two strains, these genes being enriched for functions such as calcium ion binding, chitin metabolism and muscle structure. γ-aminobutyric acid (GABA)-gated chloride channel (GABA-Cl) and neuronal acetylcholine receptor (nAChR) subunits showed significantly lower transcript levels in PT lice compared to S lice. Using RT-qPCR, the decrease in mRNA levels was estimated at ~1.4-fold for GABA-Cl and ~2.8-fold for nAChR. Salmon lice from the PT strain showed few transcriptional responses following acute exposure (1 or 3 h) to 200 μg L-1 of EMB, a drug concentration tolerated by PT lice, but toxic for S lice.
  
  
  CONCLUSIONS
  Avermectins are believed to exert their toxicity to invertebrates through interaction with glutamate-gated and GABA-gated chloride channels. Further potential drug targets include other Cys-loop ion channels such as nAChR. The present study demonstrates decreased transcript abundances of GABA-Cl and nAChR subunits in EMB-resistant salmon lice, suggesting their involvement in avermectin toxicity in caligids.
authors.forename: Stephen N
authors.initials: SN
authors.lastname: Carmichael
authors.forename: James E
authors.initials: JE
authors.lastname: Bron
authors.forename: John B
authors.initials: JB
authors.lastname: Taggart
authors.forename: Jacqueline H
authors.initials: JH
authors.lastname: Ireland
authors.forename: Michaël
authors.initials: M
authors.lastname: Bekaert
authors.forename: Stewart Tg
authors.initials: ST
authors.lastname: Burgess
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Skuce
authors.forename: Alasdair J
authors.initials: AJ
authors.lastname: Nisbet
authors.forename: Karim
authors.initials: K
authors.lastname: Gharbi
authors.forename: Armin
authors.initials: A
authors.lastname: Sturm
mesh_terms: D000818:Animals; D000977:Antiparasitic Agents / Q000494:pharmacology*; D033342:Copepoda / Q000187:drug effects* / Q000235:genetics; D004351:Drug Resistance / Q000235:genetics; D020224:Expressed Sequence Tags; D005393:Fish Diseases / Q000469:parasitology; D005786:Gene Expression Regulation / Q000187:drug effects; D007559:Ivermectin / Q000031:analogs & derivatives* / Q000494:pharmacology; D008297:Male; D009693:Nucleic Acid Hybridization; D020411:Oligonucleotide Array Sequence Analysis; D012333:RNA, Messenger / Q000235:genetics; D011963:Receptors, GABA-A / Q000235:genetics*; D011978:Receptors, Nicotinic / Q000235:genetics*; D059467:Transcriptome
chemical_list: D000977:Antiparasitic Agents; D012333:RNA, Messenger; D011963:Receptors, GABA-A; D011978:Receptors, Nicotinic; D007559:Ivermectin; C108024:emamectin benzoate