Effects on open-field behavior of mice exposed to multiple doses of methyl mercury

Coordination Deficits Induced in Young Adult Mice Treated with Methylmercury

Male and female C57BL/6J mice starting at postnatal (P) day 34 were exposed orally to five divided doses totaling 1.0 or 5.0 mg/kg of methylmercury (MeHg; given as methylmercuric chloride) or sterile deionized water in moistened rodent chow. After a 5-day waiting period, control and MeHg-treated mice were subjected to a standard battery of behavior tests for balance and motor coordination. Latency to falling on the accelerating rota-rod was significantly decreased in 5.0 mg/kg MeHg-exposed mice when compared to control mice. In the open field, horizontal exploration with respect to total distance traveled during the first 5 min on the first test day was significantly reduced in 1.0 mg/kg MeHg-exposed mice when compared to control mice. Rearing activity was not affected by MeHg treatment. In the footprint analysis, angle of foot placement measured in 1.0 mg/kg MeHg-treated mice was significantly greater compared to control mice. Base stance and stride length were unaffected by MeHg treatment. On the vertical pole test, 10 mice from each treatment group fell off the pole during the time the pole was shifted from a horizontal position to a vertical position, whereas none of the control mice fell. These results indicate that short-term, low to moderate doses of MeHg in young adult mice can be detrimental to motor coordination and balance.

Methylmercuric chloride induces activation of neuronal stress circuitry and alters exploratory behavior in the mouse

Methylmercury (MeHg) is a well-known neurotoxicant, responsible for neurological and cognitive alterations. However, there is very little information available on the effects of MeHg administration on activation of murine neuronal pathways involved in the stress response, and whether this is altered as a function of repeated exposure to MeHg. Moreover, interactions between MeHg and other psychogenic and inflammatory stressors have yet to be fully determined. Acute i.p. exposure of male C57BL/6J mice to MeHg (2-8 mg/kg) dose-dependently attenuated exploratory behavior in the open field in the presence and absence of a novel object. In addition, increased numbers of c-Fos immunoreactive cells appeared in response to acute i.p. and i.c.v. MeHg within thalamic (anterior paraventricular nucleus of the thalamus (PVA)/posterior paraventricular nucleus of the thalamus (PV)), hypothalamic (paraventricular nucleus of the hypothalamus (PVN)), central amygdaloid nucleus (CeC), septal and hippocampal (dentate gyrus) nuclei, medial bed nucleus (BSTm) and the locus coeruleus (Lc). The increase in c-Fos positive cells in response to acute i.p. and i.c.v. MeHg did not appear to be influenced further by open field exposure. Repeated administration of MeHg led to an attenuation of most parameters of open field behavior altered by acute MeHg. However, increased c-Fos was significant in the CeC, Dg, supracapsular bed nucleus (BSTs), and Lc. Moreover, open field exposure after repeated treatments resulted in significant c-Fos responses in similar areas. Interestingly, 3 days after the final repeated MeHg dose (2 or 4 mg/kg) c-Fos increases to an immunogenic stressor (LPS) were not affected by MeHg pretreatment. These results demonstrate that systemic exposure to acute and repeated MeHg serves to activate the brain's stress circuitry, and furthermore appears to engage normal neuronal habituation processes.

Neurobehavioral changes in mice chronically exposed to methylmercury during fetal and early postnatal development

Pregnant C57BL/6 mice were chronically treated with 0, 4, 6, or 8 ppm of methylmercury chloride (MeHg) in drinking water during fetal and early postnatal development. Four behavioral functions were analyzed in female and male offspring between the age of 6 and 12 weeks: motor coordination learning on the rotarod; training to spatial alternation in the standard T maze followed by a working memory test with delays; spontaneous locomotion and rearings in the open field; reference and working memory assessment in the modified T maze [Behav. Neurosci. 102 (1988) 635]. Chronic perinatal treatment with MeHg resulted in moderate brain levels of mercury near birth which rapidly decreased during nursing. MeHg exerted an effect on the performance of females, but not of males, on two of the four measurements. All treated females exhibited less locomotion than control mice when the open field was new, but not in the following four sessions when the environment was becoming increasingly familiar. Working memory was impaired in females treated with 6 and 8 ppm of MeHg in the modified T maze, but not on the test with delays in the standard T maze. Taken together, these results show that chronic exposure to MeHg during fetal and postnatal development had sex-dependent effects on horizontal exploration and on working memory in the modified T maze, and no effects on motor coordination learning and reference memory.

Effects of in utero or suckling exposure to cerium (citrate) on the postnatal development of the mouse

Gravid female mice received either a single subcutaneous dose of cerium citrate (80 mg Ce/kg) or an equivalent (in citrate) dose of sodium citrate on d 7 or 12 of gestation or on d 2 postpartum. To separate effects of prenatal and postnatal exposure, a cross-fostering design was employed. The weight and gross activity of the neonates were assessed on d 8 or 13 postpartum. Open-field behavioral parameters, accelerating rotarod performance, and passive avoidance learning were assessed on d 60-65 postpartum. Maternal offspring retrieval latency was measured on d 3 postpartum. Analyses revealed that neonatal weight was reduced both in offspring exposed to Ce in utero and in the offspring of mothers receiving Ce during lactation/suckling. Ce also appeared to affect maternal/offspring interaction: pups exposed prenatally to Ce were retrieved in less time than control pups. Except for an increased frequency of rearings in the open field of adult offspring exposed to Ce in utero, Ce exposure had no apparent effect on behavioral parameters, either in neonatal or adult offspring.