Abstract
The effects of the organophosphate insecticide dichlorvos were studied on the cercal-afferent giant-interneurone synapses in the sixth abdominal ganglion of the cockroach Periplaneta americana, using the single-fibre oil-gap method. When compared to eserine, dichlorvos showed a dose-dependent anticholinesterase effect. Both eserine (10−7 M) and dichlorvos (2 × 10−8 M) increased the amplitude and duration of the composite excitatory postsynaptic potential and acetylcholine potential, an effect corresponding to the well-known anticholinesterase one. However, during the prolonged action of these compounds, a subsequent decrease of the composite excitatory postsynaptic potential amplitude occurred, whereas the amplitude and duration of the acetylcholine potential continued to increase. The secondary effect (a decrease of composite excitatory postsynaptic potential amplitude) might be attributed to a direct action of the non-hydrolysed acetylcholine remaining in the synaptic cleft on presynaptic acetylcholine muscarinic receptors, reducing the acetylcholine release by a negative feedback mechanism or to a direct action of dichlorvos on presynaptic and postsynaptic acetylcholine receptors.
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