Methodological approaches to primate behavioral toxicological testing.

The monkey may often be the best model with which to characterize low-level effects of neurotoxicants, including those produced by developmental exposure. The nervous system of the monkey is very similar to the human and, like humans, monkeys have a long period of gestation, infancy, and sexual immaturity during which the nervous system continues to develop. The sensory systems of monkeys are very similar to humans, and intellectual capabilities may be tested that are not possible using other common laboratory species. The effects of low-level developmental exposure to neurotoxicants have been studied in the cynomolgus monkey using operant conditioning techniques to detect subtle defects. Intermittent schedules, a standard tool in behavioral pharmacology, have proved sensitive indicators of toxicity produced by developmental lead exposure. Such intellectual functions as learning, memory, adaptability, and distractibility have been explored by techniques including discrimination reversal, matching to sample, and delayed alternation. Such techniques have revealed impairment produced by lead similar to that observed in lead-exposed children. Visual deficits produced by methylmercury have been revealed by psychophysical techniques, in the absence of any obvious signs of toxicity.