Effects of developmental methylmercury exposure or lifetime lead exposure on vibration sensitivity function in monkeys.

Paresthesias are a frequently reported symptom of methylmercury exposure in adults, while peripheral neuropathy has long been known to result from high-level lead exposure. Despite this, no objective determination of somatosensory function has been performed in either humans or animals for either toxicant. Somatosensory function was assessed following developmental methylmercury exposure or lifetime lead exposure in monkeys (Macaca fascicularis) by determination of sensitivity to vibration applied to the fingertip. Vibration thresholds were determined over a number of frequencies, from 25 to 250 Hz, by means of a behavioral procedure. The underside of the tip of the monkey's middle finger was precisely positioned over a blunt probe attached to a vibrator. The monkey signaled detection of the vibration by breaking contact with a stainless steel bar with the free hand. At each of the five frequencies tested, the amplitude of the vibration was manipulated systematically to determine the monkey's threshold of detection. Four of five monkeys dosed with methylmercury from birth to 7 years of age, with blood mercury levels during dosing of 0.8-1.1 micrograms/g, exhibited elevated thresholds when tested at 18 years of age. Two monkeys dosed in utero through 4 years of age, with blood mercury levels of 0.35 micrograms/g during dosing, were impaired when tested at 15 years of age, while two monkeys with histories of blood mercury levels of 0.70 were relatively unimpaired. Lifetime exposure to lead resulting in stable blood lead levels of 20-25 micrograms/dl resulted in elevated vibration sensitivity only at the highest frequency in all four individuals tested. However, only two of six individuals with high lead levels (60-130 micrograms/dl) exhibited impairment. These results represent severe impairment in methylmercury-exposed monkeys 11 years after cessation of dosing and extend previous research documenting visual and auditory impairment in these monkeys. While effects of lifetime lead exposure on vibration sensitivity observed in the present study are suggestive, there is clearly a need for further research.