SHRINKAGE OF MOTOR AXONS FOLLOWING SYSTEMIC EXPOSURE TO INORGANIC MERCURY

Systemically-administered inorganic mercury localizes to motor neurons , but it is not known if mercury injures these neurons. We therefore l ooked for signs of damage to the motor and sensory neurons of mice tha t had been exposed to inorganic mercury. Young adult mice were injecte d intraperitoneally with either 1 or 2 mu g/g of mercuric chloride and perfused 1 or 30 weeks later. The cellular distribution of mercury in the spinal cord was examined with silver nitrate autometallography. T he numbers and sizes of myelinated axons in the L5 anterior and poster ior roots were quantitated using an image analysis program. Mercury wa s found throughout the cytoplasm of motor neuron cell bodies after 1 w eek and in paranuclear aggregations after 30 weeks. Thirty weeks after exposure to either 1 or 2 mu g/g of mercury, fewer large myelinated a xons were seen in mercury-injected groups than in controls, though tot al numbers of myelinated axons did not differ between groups. A slight increase in numbers of small axons was seen in the posterior roots of mice exposed to 1 mu g/g of mercury. In conclusion, inorganic mercury remains within mouse neurons for prolonged periods and causes a reduc tion in the size of myelinated axons in the anterior root and to a les ser extent the posterior spinal root. Inorganic mercury within motor n eurons therefore appears to behave as a slowly-acting neurotoxin that shrinks motor axons.