Gender Differences in the Uptake of Inorganic Mercury by Motor Neurons
ROGER PAMPHLETT, KENNETH B. R. EWAN, ROBERT MCQUILTY AND PATRICIA WALEY
Gender differences have been noted in the tissue distribution of mercury. We sought to determine if the uptake of low-dose inorganic mercury into motor neurons dilifers between male and female mice. Four male and four female mice were injected IP with 0.5 mg HgClZ/kg. In 50-pm sections of lumbar spinal cord stained with autometallography, six motor neuron cell bodies were selected for study. The volume , percentage of mercury granules in the cell bodies was estimated using a confocal microscope. Mercury granules occupied more perikaryal volume in motor neurons from female mice (mean 3.7%) than from male mice (mean 2.2%) (p 0.05). After the same dose. the amount of renal mercury measured by mass spectrometry was significantly less in six female than five male mice.
In conclusion, female mice take up more inorganic mercury into their motor neurons than do male mice. This may be related to a smaller deposition of mercury in the female kidney. leaving more circulating mercury available to be taken up by motor axons.
It has been suspected for some time that heavy metals are involved in the loss of motor neurons found in human motor neuron disease (9). This suspicion has been strengthened by the finding that systemically administered inorganic mercury is taken up by motor neurons of experimental animals (10,11), and that at very low doses mercuric chloride (HgCl2) is taken up selectively by spinal and brain stem motor neurons (16). While examining motor neurons from HgCl;-injected mice that had been stained for mercury (4). we noticed that mercury could be detected at lower injected doses in the motor neurons of female than of male mice. We wondered therefore if the uptake of mercury into motor neurons was more pronounced in female mice.
Although gender differences have been noted before in the uptake of mercury by the Whole brain (Table 1), it is not known whether uptake of mercury into different groups of neurons is gender sensitive, because quantitation of mercury in individual neurons has not been possible. A method for measuring silver-coated granules of mercury within motor neurons has, however, recently been described (5). We used this technique to see if a difference in the motor neuron up take of between male and female mice could be demonstrated quantitatively.
Effect of Gender on Kidney Mercury
The amount of mercury in the kidneys of male mice (mean The autornetallographic technique stains either mercury.
82.9 pig/g, SE i 3.0 p.g/g) differed significantly from that in fe- silver, or gold in cell bodies (4). The experimental animals difmale mice (mean 5141 pig/g, SE i 7.2 pg/g) (t 4.3,p 0.002) fered from controls only in that they had been exposed to in(Fig. 6). Variance in male and females was not significantly organic mercury, and sections from each group were predifferent. treated with potassium cyanide to eliminate any possible
contamination with silver. Gold is seen on autometallography only if the sections are radiated with ultraviolet light (4). Elemental analysis of autometallographic granules in the kidneys of rats exposed to mercury have shown mercury at the center of the granules (15). In our mice, therefore, the technique would have demonstrated mercury only.
The toxokinetics of inorganic mercury in mice depends on the size ofthe dose, the route of administration, and the strain and gender of the mouse (12). In this experiment all factors
other than gender were kept constant so that any differences in mercury uptake between males and females could be determined. Using a newly described technique for quantitating silver-coated mercury granules (5), we found that female mice took up 1.7 times more mercury into their motor neurons than did male mice.
Previous workers have shown gender differences in the uptake of mercury into the brain and kidney of adult rodents (6-8, 12—14,20,21) (Table 1). Although these studies have consisently shown more mercury in female brains, four reported more mercury in the male kidney and three more mercury in the female kidney. This variation illustrates the difficulty of comparing mercury uptake when studies differ in species, strain, form of mercury given. route of administration. the method of mercury measurement, and the chemical form of mercury measured. The only other studies that used HgCl; as the form of mercury administered had similar results to ours in that more mercury was found in the brains of female and in the kidneys of male mice (12,14). It has been suggested that the gender difference in tissue uptake of mercury can be explained by testosterone, because if male mice are castrated their distribution of mercury is similar to females except for brain levels. which remain the same as noncastrated males (7). Testosterone may play a part in renal uptake of mercury through its effects on glutathione and y~glutamyltranspeptidasc. which are involved in the transport and incorporation of mercury into the kidney (18,19).
FIG. 5. The motor neurons of female mice contained significantly more mercury than malc mice. Bars standard errors.
The relatively low uptake of mercury by the female kidney could leave more mercury free to enter other tissues. Inorganic mercury does not cross the blood~brain barrier easily (3), but mercury ions appear to be able to enter motor neurons by entering the distal axon at the neuromuscular junction, after which the ions are transported retrogradely to the cell body (1). If less mercury is taken up by the kidney. more circulating mercury would be available to enter muscle and hence be taken up by distal motor axons. However. low mercury cannot fully explain the increased brain mercury in females because female mice still have more brain mercury than castrated males (7).
Much interest has been raised by the finding that inorganic mercury in experimental animals is deposited primarily in tor neurons (1 1), because heavy metals have been implicated as zi possible cause of sporadic motor neuron disease in humans (9). However, in motor neuron disease slightly more males than females are affected. and the mean age of onset for males is 62 years compared to 68 years for females (Z). Although it is not known if motor neuron uptake of mercury differs in human adult males and females, if the rodent results are extrapolated to humans one would predict that motor neurons of human female would take up more mercury.
This disparity in the gender differences between the incidence of motor neuron disease and the motor neuron uptake of mercury is indirect evidence against inorganic mercury playing a part in the pathogenesis of motor neuron disease.