Determination of threshold adverse effect doses of percutaneous VX exposure in African green monkeys.

Percutaneous exposure to the chemical warfare nerve agent VX was evaluated in African green monkeys (n=9). Doses of VX (7.5-100 µg/kg) were applied to the skin for 60 min and residual agent was quantified (before decontamination) to estimate the absorbed dose. Monkeys were evaluated for the presence or absence of clinical signs of toxicity and blood was sampled periodically (30 min--12 weeks) following exposure to measure the degree of circulating acetylcholinesterase (AChE) inhibition. Monkeys were also evaluated for behavioral changes from VX exposure using a serial probe recognition (SPR) task. The lowest observable adverse effect level (LOAEL) for the production of major clinical signs was determined to be 42.22 µg/kg (absorbed dose estimate=17.36 µg/kg) and the LOAEL for AChE inhibition was 13.33 µg/kg (absorbed dose estimate=6.53 µg/kg). Behavioral performance was unaffected at doses that, while producing substantial AChE inhibition, did not produce clinical signs. VX represents a substantial threat as a contact hazard and these results complement previous studies using the percutaneous route of exposure with VX and extend the findings to a non-human primate species.

Evaluation of miosis, behavior and cholinesterase inhibition from low-level, whole-body vapor exposure to soman in African green monkeys (Chlorocebus sabeus).

BACKGROUND: Relatively little is known about the effects of very low-level exposures to nerve agents where few signs or symptoms are present. METHODS: African green monkeys (Chlorocebus sabeus) (n = 8) were exposed for 10 min, whole-body, to a single concentration of soman (0.028-0.891 mg/m³). RESULTS: EC50 values for miosis were determined to be 0.055 mg/m³ and 0.132 mg/m³ when defined as a 50 percent reduction in pupil area and diameter, respectively. In general, performance on a serial probe recognition task remained unchanged at lower concentrations, but responding was suppressed at the largest concentration tested. Soman produced concentration-dependent inhibition of acetylcholinesterase activity and, to a lesser extent, butyrylcholinesterase activity. CONCLUSIONS: These results characterize threshold soman exposure concentrations that produce miosis in the absence of other overt signs of toxicity and extend previous studies indicating that miosis is a valuable early indicator for the detection of soman vapor exposure.

Determination of miosis threshold from whole-body vapor exposure to sarin in African green monkeys.

We determined the threshold concentration of sarin vapor exposure producing miosis in African green monkeys (Chlorocebus aethiops). Monkeys (n=8) were exposed to a single concentration of sarin (0.069-0.701mg/m3) for 10min. Changes in pupil size were measured from photographs taken before and after the exposure. Sarin EC50 values for miosis were determined to be 0.166mg/m3 when miosis was defined as a 50% reduction in pupil area and 0.469mg/m3 when miosis was defined as a 50% reduction in pupil diameter. Monkeys were also evaluated for behavioral changes from sarin exposure using a serial probe recognition test and performance remained essentially unchanged for all monkeys. None of the concentrations of sarin produced specific clinical signs of toxicity other than miosis. Sarin was regenerated from blood sampled following exposure in a concentration-dependent fashion. Consistent with a predominant inhibition of acetylcholinesterase (AChE), more sarin was consistently found in RBC fractions than in plasma fractions. Further, elimination of regenerated sarin from RBC fractions was slower than from plasma fractions. Blood samples following exposure also showed concentration-dependent inhibition of AChE activity and, to a lesser extent, butyrylcholinesterase activity. At the largest exposure concentration, AChE inhibition was substantial, reducing activity to approximately 40% of baseline. The results characterize sarin exposure concentrations that produce miosis in a large primate species in the absence of other overt signs of toxicity. Further, these results extend previous studies indicating that miosis is a valid early indicator for the detection of sarin vapor exposure.

Evaluation of cognitive and biochemical effects of low-level exposure to sarin in rhesus and African green monkeys.

We investigated the potential of low-level exposures to the chemical warfare nerve agent, sarin, to produce adverse effects. Rhesus (Macaca mulatta) and African green monkeys (Chlorocebus acthiops) were trained on a serial probe recognition (SPR) task before IM administration of a low-level concentration (5.87 microg/kg or 2.93 microg/kg) of sarin. Blood was sampled before agent administration and at various times following administration. Sarin administration did not disrupt performance on the SPR task in either species. Major dependent measures characterizing performance (accuracy, number of completed trials per session, average choice response time) were largely unaffected on the day sarin was administered as well as on subsequent testing sessions occurring over several weeks following administration. Analyses of red blood cell (RBC) and plasma samples revealed that sarin administration produced a substantial degree of inhibition of circulating acetylcholinesterase (AChE) in RBC fractions and butyrylcholinesterase (BChE) in plasma fractions, which only slowly recovered. In this regard, AChE activity was inhibited to a greater extent than BChE activity. Blood samples were also evaluated for regenerated sarin, which was found in RBC and plasma fractions in both species and showed orderly elimination functions. More sarin was regenerated from RBC fractions than from plasma fractions. Elimination of regenerated sarin was much slower in RBC than plasma and exceeded the expected time of AChE aging, suggesting the presence of additional sarin binding sites. In general, effects were similar in both species. Taken together, our results show that while the concentrations of sarin administered were clearly biochemically active, they were below those that are required to produce a disruption of behavioral performance.

Effects of anticholinergics on serial-probe recognition accuracy of rhesus macaques (Macaca mulatta).

Potential deleterious behavioral effects of the anticholinergics biperiden and scopolamine were examined via the performance of rhesus monkeys on a serial-probe recognition (SPR) procedure. On each trial, six unique stimuli (list items) were presented sequentially followed by a choice phase. In the choice phase, two stimuli were presented, a standard or 'default' stimulus (a white rectangle) and a 'probe' stimulus that differed with each choice trial. Choosing the probe stimulus was considered correct if the probe matched one of the list items; otherwise, choosing the default stimulus was considered correct. Behavior was examined under a range of doses of biperiden (0.001-1.0 mg/kg) and scopolamine (0.0056-0.03 mg/kg). Scopolamine (0.01-0.03 mg/kg) and biperiden (0.3-1.0 mg/kg) reduced overall accuracy. At the highest dose, scopolamine, but not biperiden, reduced the number of trials completed per session. The results suggest that doses of scopolamine and biperiden necessary to prevent or eliminate organophosphate induced seizures may affect performance adversely. However, because the degree of impairment from biperiden was modest, further examination of this anticonvulsant may be warranted.