Whole body exposure of marmoset monkeys to low levels of sarin: Lowest observable effect level in blood, and early effects on pupil size and EEG

Realistic scenarios for low-level exposure to nerve agents will often involve exposures over several hours to extremely low doses of agent. The purpose of this pilot study was to indicate for sarin: (i) the lowest observable effect level (LOEL), i.e., the C.t-value (t = 5 h) of exposure at which an internal dose (fluoride-regenerated sarin from blood BuChE) becomes measurable; (ii) The lowest observable adverse effect level (LOAEL) for miosis and EEG-abnormalities in marmosets. For that purpose a validated system was developed for exposing conscious animals to the lowest controllable concentrations of sarin (8-80 ppt: 0.05-0.5μg.m-3), and to increase concentrations (7-150 μg.m-3) and exposure times (t ≤ 5 h) until observable adverse effect levels become measurable. The ratio of pupil and iris diameters, measured on digital photographs taken on-line during exposure, was calculated as a measure for miosis. EEG-signals were recorded telemetrically; the averaged amounts of energy (μV2) per EEG-band of air-exposed animals were compared to the amounts of energy of the corresponding bands of sarin-exposed animals.
The LOEL was observed at Ct = 0.04 ± 0.01 mg.min.m-3. This level is several orders of magnitude lower than that based on classical measurement of depressed ChE activity. Pupil size was significantly (p < 0.05) decreased at a dose of 2.5 ± 0.8 mg.min.m-3. The earliest significant (p < 0.05) changes on the EEG-bands were observed at a dose of 0.2 mg.min.m-3. The latter LOAEL-value is an order of magnitude lower than that for miosis, and only 2-5 times higher than the LOEL. One year after the exposure to sarin vapor all marmosets still demonstrated significant (p < 0.05) EEG differences. In most marmosets the energy (μV2) per EEG-band was higher than that observed one year earlier, which might indicate that neurons had become more sensitive to excitation. Visual examination of the EEG-records revealed clear bursts of alpha frequencies (around 9 Hz), resembling sleep-spindles, which were more frequently present in exposed than in naive animals. These late changes in spindle oscillation might be the result of persistent changes in the cholinergic system due to exposure to sarin vapor one year before. In conclusion, LOEL levels, miosis, and EEG-abnormalities, may occur in humans during long term (5 h) exposure to levels of sarin which are not detectable by the currently fielded alarm systems.