The mode of action and neurotoxic properties of the nitromethylene heterocycle insecticides

Abstract

The mode of action of the nitromethylene heterocycle group of insecticides was investigated in the American cockroach, Periplaneta americana. In vivo symptomology was characterized by a definite sequence of events, starting with abdominal quivering and followed, in order, by wing flexing, uncontrollable preening, leg tremors, violent whole body shaking, prostration, and death. Neurophysiological experiments showed that the nitromethylene heterocycles have dramatic effects on nerve impulse transmission at the cholinergic cercal nerve-giant fiber synapses located within the cockroach sixth abdominal ganglion. The effects were biphasic and were characterized by an initial increase in the frequency of spontaneous giant fiber discharges, followed by the development of a complete block to nerve impulse propagation. The nitromethylene heterocycles did not inhibit insect cholinesterase. Neuropharmacological experiments showed that the specific site of action was postsynaptic, and probably involved an agonistic effect on postsynaptic acetylcholine receptors.