On the development of behavioral tolerance to organophosphates. IV: EEGand visual evoked responses.

Several earlier studies showed that, in contrast with DFP, repeated injections with soman did not lead to behavioral tolerance in rats. The reason for the difference between the effects of these two organophosphate cholinesterase inhibitors was not clear and a neurophysiological approach was undertaken. Four experiments (A, B, C and D) were carried out, each consisting of three groups of rats, SC injected with saline, DFP (600 micrograms/kg) or soman (60 micrograms/kg) respectively. In Experiment B and D the rats were trained to criterion in a two-way shuttlebox. Thereafter, the animals of Experiment B were fitted with suitable electrodes and two days later their EEGs and visual evoked responses (VERs) were recorded, 1 and 24 h after a single dose of the above-mentioned compounds. In Experiment D the trained animals were subsequently injected 3 times per week for 4 weeks with the same doses and their performance was tested 5 days per week, 1 and 24 h after injection. After those 4 weeks, when the DFP-treated animals had developed behavioral tolerance, electrodes were fitted and EEGs and VERs were recorded after two days, again 1 and 24 h after injection, as in Experiment B. The difference with Experiments A and C was that these animals were not trained. Otherwise, treatment schedules and recording procedures of Experiment A were identical to those of Experiments B and of Experiment C to those of Experiment D. In all cases the EEGs and VERs were recorded from animals slowly walking in a rotating hollow transparent wheel. The results show a similar pattern in all four experiments.(ABSTRACT TRUNCATED AT 250 WORDS)