An atypical residue in the pore of Varroa destructor GABA-activated RDL receptors affects picrotoxin block and thymol modulation

Kerry L Price,Sarah C.R Lummis

doi:10.1016/j.ibmb.2014.10.002

Abstract

GABA-activated RDL receptors are the insect equivalent of mammalian GABAA receptors, and play a vital role in neurotransmission and insecticide action. Here we clone the pore lining M2 region of the Varroa mite RDL receptor and show that it has 4 atypical residues when compared to M2 regions of most other insects, including bees, which are the major host of Varroa mites. We create mutant Drosophila RDL receptors containing these substitutions and characterise their effects on function. Using two electrode voltage clamp electrophysiology we show that one substitution (T6′M) ablates picrotoxin inhibition and increases the potency of GABA. This mutation also alters the effect of thymol, which enhances both insect and mammalian GABA responses, and is widely used as a miticide. Thymol decreases the GABA EC50 of WT receptors, enhancing responses, but in T6′M-containing receptors it is inhibitory. The other 3 atypical residues have no major effects on either the GABA EC50, the picrotoxin potency or the effect of thymol. In conclusion we show that the RDL 6′ residue is important for channel block, activation and modulation, and understanding its function also has the potential to prove useful in the design of Varroa-specific insecticidal agents.

Highlights

The ectoparasitic mite, Varroa destructor, is the primary pest of the honeybee Apis mellifera
g-aminobutyric acid (GABA)-activated resistant to dieldrin (RDL) receptors are present in the nervous system of insects and likely play a major role in inhibitory responses, similar to the role of GABAA receptors in vertebrates
RDL receptors have pharmacological characteristics distinct to those of GABAA receptors, which mean they lend themselves to being a target for insecticidally active compounds, examples of which include cyclodienes and fipronil

Summary

Introduction

The ectoparasitic mite, Varroa destructor, is the primary pest of the honeybee Apis mellifera. It is found in large parts of the world and is spreading to others, is still absent from some areas such as Australia (Rosenkranz et al, 2010). It is a problem because it has significant effects on bees, weakening colonies by introducing viruses such as the deformed wing virus, and is considered a major causal factor in colony collapse disorder (CCD). The effect of Varroa is usually minimised using synthetic control agents, such as fluvalinate and coumaphos, as well as natural compounds including the essential oil thymol, but these too may have adverse effects on bee health (Waliwitiya et al, 2010; Frost et al, 2013; Zhu et al, 2014).

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