The GABA receptor Rdl mutation in Drosophila melanogaster influences the efficacy of a cholinesterase inhibiting insecticide

Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the insect central nervous system (CNS) where GABA-gated chloride channels (GABA_A receptors) are an important insecticide target. Rdl (resistance to dieldrin) is a point mutation in the M2 domain of the GABA_A receptor, which not only confers resistance to dieldrin, but other GABAergic insecticides like lindane and fipronil. In this study, the Rdl strain of D. melanogaster surprisingly displayed resistance to propoxur, an acetylcholinesterase (AChE) inhibitor. The oral LC₅₀ of propoxur in the susceptible strain was 16 ppm, but this value increased to 998 ppm in the resistant strain resulting in a resistance ratio of 63. Extracellular electrophysiology showed that propoxur induced a sharp decline in the nervous system firing rate of D. melanogaster larvae, and the half-maximal inhibition concentration (IC₅₀) was not significantly different between two strains. However, when the similar IC₅₀ was evaluated, the time for the CNS to reach the maximum nerve firing rate was significantly longer in the resistant strain (207 sec), when compared to the susceptible strain (114 sec). AChE kinetics indicated that the maximum reaction velocity (V_max) was also significantly different between the two strains, with a greater V_max in the Rdl strain (47.4 µM/mg protein), compared to the susceptible strain (36.9 µM/mg protein). These results indicated that the Rdl mutation at the GABA_A receptor influences the biochemistry and physiological effect of AChE.