Block of an insect central nervous system GABA receptor by cyclodiene and cyclohexane insecticides

The effects of a cyclodiene (endrin) and a cyclohexane (lindane) insecticide have been tested on gamma-aminobutyric acid (GABA) receptors in the central nervous system of the cockroach (Periplaneta americana), by using electrophysiological methods and an in vitro functional receptor assay. In electrophysiological experiments on an identified motor neuron (Df), endrin blocked the GABA response with a 50% inhibition concentration of 5.0 x 10(-7) M in a non-competitive manner. The actions of endrin were irreversible under the experimental conditions adopted. Increasing the intracellular chloride concentration reduced the effectiveness of endrin, whereas a change in the potassium concentration failed to influence the block by endrin of GABA responses. Lindane exhibited similar actions to endrin on insect GABA receptors, but was approximately an order of magnitude less effective. In a microsac preparation from cockroach nerve cords, endrin, at a concentration of 1.0 x 10(-5) M, completely blocked GABA-stimulated 36Cl- uptake, whereas the same concentration of lindane was less potent, only blocking about 40% of uptake under similar conditions. Neither insecticide had any effect on L-glutamate-activated chloride channels. The results demonstrate that endrin and lindane block functional insect neuronal GABA receptors.