Methamidophos : An anticholinesterase without significant effects on postsynaptic receptors or transmitter release

Methamidophos (0,5-dimethyl phosphoroamidothiolate, Tamaron R ), an organophosphate (OP) anticholinesterase of limited toxicity, is widely used as an insecticide and acaricide. To provide additional insight into the molecular basis of its action, we have used electrophysiological and biochemical techniques to study the effects of methamidophos on the neuromuscular junction of rat and frog and on the central nervous system of rat. Methamidophos has a relatively weak inhibitory action on cholinesterases in rat diaphragm muscle, brain and hippocampal homogenates, with IC 50 values on the order of 20 - 30 μM. An even weaker anticholinesterase activity was found in frog muscle homogenates, with the IC 50 being above 300 μM. As further evidence of anticholinesterase activity, methamidophos (1-100 μM) was able to reverse the blockade by d-tubocurarine (0.5 - 0.7 μM) of neuromuscular transmission in rat phrenic nervehemidiaphragm preparations. Inhibition of cholinesterase activity by methamidophos was long lasting, which is consistent with the formation by the agent of a covalent bond with the enzyme's active serine residue. The action was also slowly reversible, which suggests spontaneous reactivation of the enzyme. Electrophysiological studies at the rat neuromuscular junction showed that, due to its anticholinesterase activity, methamidophos increased the amplitude and prolonged the decay phase of nerve-evoked and spontaneous miniature endplate potentials. In contrast to other OP compounds, e.g. paraoxon (Rocha et al., 1996a), methamidophos did not affect neurotransmitter release, nor did it interact directly with the muscle nicotinic acetylcholine receptor. Moreover, in contrast to paraoxon, methamidophos did not affect the whole-cell currents induced by application of acetylcholine, glutamate or γ-aminobutyric acid recorded to cultured hippocampal neurons. Based on these data, methamidophos appears to have a selective effect on cholinesterase.