Entry of low doses of mercury vapor into the nervous system

Inorganic mercury remains within neurons indefinitely and has been implicated in some human neurodegenerative diseases. We were interested in finding the lowest dose of mercury vapor that resulted in mercury deposition in neurons. Female BALB/c mice were exposed to 25 micrograms mercury/m3 for 2-20 hr or 500 micrograms mercury/m3 for 5-240 min. To see if female mouse neurons were more susceptible to mercury vapor than male neurons, male and female BALB/c mice were exposed to 50 micrograms mercury/m3 for 4-24 hr. Mice were perfused with formalin 1-30 weeks after exposure and paraffin sections of brain, spinal cord and kidney were stained for mercury with silver nitrate autometallography. On light microscopy, spinal motor neurons contained mercury granules after 12 hr exposure to 25 micrograms mercury/m3 or after 30 min exposure to 500 micrograms mercury/m3. Mercury remained in motor neurons 30 weeks after exposure. In female mice, mercury was seen in motor neurons at half the exposure times of male mice. In conclusion, low doses of mercury vapor, well within WHO guidelines for safe human occupational exposure, enter and remain within motor neurons of mice.