Motor neuron uptake of low dose inorganic mercury

In animals, inorganic mercury can bypass the blood brain barrier and enter motor neurons. We sought to determine the lowest injected dose of mercury that could be detected in mouse motor neurons. Mice were injected intraperitoneally with mercuric chloride in doses from 0.05 to 2 micrograms/g body weight and studied between 5 days and 18 months after injection. After formalin fixation, 7 microns sections of cerebrum, cerebellum, brain stem, spinal cord and kidney were stained with silver nitrate autometallography. Five days after injection, mercury granules were detected at doses from 0.2 microgram/g upwards in the cell bodies of spinal and brain stem motor neurons, more granules being seen at the higher doses. Mercury granules were also seen in 5% of posterior root ganglion neurons. At doses from 0.05 microgram/g upwards mercury was detected 5 days later in renal tubule cells. Mercury was still present in motor neurons 6-11 months after injection, but by this time mercury had been cleared from the kidneys. Low doses of inorganic mercury are therefore selectively taken up and retained by motor neurons, making this neurotoxin a good candidate for a cause of sporadic motor neuron disease.