Exposure to Methylmercury Does Not Change the Sexual Preference of C57BL Male Mice
Mercury is a powerful neurotoxin, teratogen, and endocrine disruptor in animals. Methylmercury (MeHg) is the most biologically available and toxic form of mercury [1 3] It has been reported that exposure to MeHg can change the sexual preference of white ibises [4,5] It is unclear whether MeHg can change the sexual preference in higher mammals. Here, we used C57BL male mice to examine the pairing behavior outcomes of exposure to different concentrations of MeHg. C57BL mice (male, 1 month old; female, 3 month old) were purchased from Sino British SIPPR/BK Laboratory Animals (Shanghai, China) All animal procedures were approved by the ethics committee of the Second Military Medical University and performed in compliance with the institutional guide for care and use of laboratory animals. First, sesame oil preference and fox urine avoidance experiments were performed in all male mice to test the olfactory function by using a T maze (n 50) [6] One horizontal arm of the T maze was empty, and the other was set 1.5 mL sesame oil or 1.5 mL fox urine. The mouse was put in the vertical arm. After it entered the horizontal arms, the door between the vertical and horizontal arms was closed. We recorded the time that the mouse spent in each horizontal arm for 5 min. The mice with normal olfactory function preferred the sesame oil versus air and preferred air versus fox urine (170 6.5 seconds to sesame oil vs. 130 6.5 seconds to air, 224 8.2 seconds to air vs. 76 8.2 seconds to fox urine, P 0.01, Figure 1A) The mice with olfactory dysfunction were discarded in the following experiments (n 2) The other 48 male C57BL mice were divided into three groups (control and two doses of MeHg exposure groups) The control group was feed with normal food fixed with vehicle (corn oil) (n 16) In MeHg groups, MeHg was fixed into the animal food with two different doses (1 and 10 parts per million (ppm) of food) and administrated consecutively for 3 months. The doses we set were environmentally relevant concentrations that mammals may be exposed in MeHg polluted regions [4] Mice were started the MeHg treatment from 1 month old rather than from nursing period because they were more tolerant at that time. We measured the body weight every month. There is no difference in the body weight between the control group and two MeHg groups (P 0.05, Figure 1B) After 3 months feed, the mating choice, bedding preference, and genital odor preference experiments were performed accordingly to examine the effect of MeHg exposure on the sexual preference [6] In the mating choice experiment, a sexually naive male C57BL mouse and an oestrous female C57BL mouse were placed into the home cage of the test male for 5 min. Mouse mouse mounting activities were recorded and analyzed using a Mouse and Rat Spontaneous Activity Video Analysis System (JLBehv LAG 4, Shanghai Jiliang Software Technology Co. Ltd. Shanghai, China) In each group, 100% of males preferred to mounting female targets. There is no difference between the control group and MeHg groups (P 0.05, Figure 2A) In the bedding preference experiment, a cage was divided into two sides and was covered with female or male bedding on each side. Bedding was not changed for 3 days from group housed adult C57BL mice. Male and female bedding were prevented from mixing by two plastic bars with a hole which the mice can freely cross. A wire netting separated the test mice from the bedding (the upper panel of Figure 2B) The test mouse was placed in the center, and we accounted the time that the mouse spent on the two side for 5 min. After each experiment, the cage was washed with alcohol and water to remove odor. All the three groups of males prefer female bedding (control group, 95.5 19.1 seconds to male vs. 205 19.1 seconds to female, P 0.01; 1 ppm group, 97.1 19.8 seconds to male vs. 203 19.8 seconds to female,
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