A comparison of physostigmine and soman using taste aversion and nociception.

The effects of a reversible (physostigmine) and an irreversible (soman) acetylcholinesterase inhibitor were compared in terms of nociception using the rat tail flick (TF), and hot plate (HP) tests. The conditioned taste aversion (CTA) procedure was employed to evaluate the stimulus properties of these drugs. In all procedures physostigmine salicylate was administered at doses of 0.20, 0.32, 0.45, or 0.65 mg/kg, IM. Soman was administered at doses ranging from 40 to 80 microgram/kg, IM. Using either TF or HP, physostigmine and soman were evaluated at 40 min following injection. Both physostigmine and soman produced dose-related effects on each measure of nociception. The median effective dose of physostigmine was 0.27 mg/kg on TF and 0.55 mg/kg on HP. For soman, the median effective dose was 54 micrograms/kg on TF and 52 micrograms/kg on HP. In the CTA procedure, a novel, saccharin sweetened solution was paired with either physostigmine or soman. Three days later rats were offered a choice of saccharin or tap water. Both soman and physostigmine produced dose-related CTAs over the range of doses studied. Whereas the ED50 for soman was 59 micrograms/kg with significant effects at doses of 60 micrograms/kg or more, physostigmine produced effects at doses of 0.45 mg/kg or greater (ED50 = 0.05 mg/kg) with no additional changes in saccharin preference beyond 0.45 mg/kg. The data suggest that these experimental procedures, TF, HP, and conditioned taste aversion may be employed to evaluate the behavioral toxicity of physostigmine and soman.