EFFECTS OF DEVELOPMENTAL METHYLMERCURY EXPOSURE OR LIFETIME LEAD-EXPOSURE ON VIBRATION SENSITIVITY FUNCTION IN MONKEYS

Paresthesias are a frequently reported symptom of methylmercury exposu re in adults, while peripheral neuropathy has long been known to resul t from high-level lead exposure. Despite this, no objective determinat ion of somatosensory function has been performed in either humans or a nimals for either toxicant. Somatosensory function was assessed follow ing developmental methylmercury exposure or lifetime lead exposure in monkeys (Macaca fascicularis) by determination of sensitivity to vibra tion applied to the fingertip. Vibration thresholds were determined ov er a number of frequencies, from 25 to 250 Hz, by means of a behaviora l procedure. The underside of the tip of the monkey's middle finger wa s 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 frequencie s tested, the amplitude of the vibration was manipulated systematicall y to determine the monkey's threshold of detection. Four of five monke ys dosed with methylmercury from birth to 7 years of age, with blood m ercury levels during dosing of 0.8-1.1 mu g/g, exhibited elevated thre sholds when tested at 18 years of age. Two monkeys dosed in utero thro ugh 4 years of age, with blood mercury levels of 0.35 mu g/g during do sing, were impaired when tested at 15 years of age, while two monkeys with histories of blood mercury levels of 0.70 were relatively unimpai red. Lifetime exposure to lead resulting in stable blood lead levels o f 20-25 mu g/dl resulted in elevated vibration sensitivity only at the highest frequency in all four individuals tested. However, only two o f six individuals with high lead levels (60-130 mu g/dl) exhibited imp airment. These results represent severe impairment in methylmercury-ex posed monkeys Il years after cessation of dosing and extend previous r esearch documenting visual and auditory impairment in these monkeys. W hile effects of lifetime lead exposure on vibration sensitivity observ ed in the present study are suggestive, there is clearly a need for fu rther research.