PARAOXON - CHOLINESTERASE-INDEPENDENT STIMULATION OF TRANSMITTER RELEASE AND SELECTIVE BLOCK OF LIGAND-GATED ION CHANNELS IN CULTURED HIPPOCAMPAL-NEURONS

Paraoxon (O,O-diethyl O-p-nitrophenyl phosphate) is the neurotoxic met abolite of the insecticide parathion (O,O-diethyl O-p-nitrophenyl phos phorothioate). The effects of organophosphorus compounds on peripheral synapses have been attributed to inhibition of cholinesterase and to direct actions on muscarinic and nicotinic receptors, but less is know n about the actions of organophosphorus compounds, including paraoxon, in the central nervous system. We investigated initially the effects of paraoxon on spontaneous transmitter release by recording miniature postsynaptic currents (MPSCs) from cultured rat hippocampal neurons us ing the whole-cell mode of the patch-clamp technique. Paraoxon (0.3 mu M) in the presence of tetrodotoxin (0.3 mu M) and atropine (1 mu M) c aused a significant increase in the frequency of gamma-aminobutyric ac id- and glutamate-mediated MPSCs, but did not change the peak amplitud es or decay-time constants of these MPSCs. In contrast, application of nicotinic agonists or antagonists did not change the MPSC frequency. The presynaptic effect of paraoxon shown here was not mediated by acti ons on muscarinic or nicotinic receptors, or by elevated acetylcholine levels secondary to inhibition of cholinesterase. In addition, agonis ts were applied to assess the postsynaptic effects of paraoxon on exci tatory and inhibitory amino acid receptors. Paraoxon (30 mu M-1 mM) bl ocked the ion channels of glycine, gamma-aminobutyric acid(A), N-methy l-D-aspartic acid and nicotinic acetylcholine receptors, but not the i on channels of kainate- and lpha-amino-3-hydroxy-5-methyl-4-isoxazolep ropionic acid-type glutamate receptors. The combined effects of paraox on on spontaneous transmitter release and on the functions of several ligand-gated receptors may constitute mechanisms relevant to the neuro toxicity of paraoxon.