Pyrethroids of the most potent class antagonize GABA action at the crayfish neuromuscular junction.

Deltamethrin and three insecticidal cyano analogs causing the Type II pyrethroid syndrome increased the input resistance of crayfish claw opener muscle fibers bathed in gamma-aminobutyric acid (GABA). In contrast, two non-toxic stereoisomers and three insecticidal pyrethroids causing the Type I syndrome were inactive. Known GABA antagonists including picrotoxinin (PTX) induced an effect similar to, although quicker than, that caused by the active pyrethroids. Two benzodiazepines reduced the potency of PTX and deltamethrin. Cyanophenoxybenzyl pyrethroids therefore appear to act on the GABA receptor-ionophore complex.