Priority Existing Chemical

k.3Effects in animals

In different strains of rats, single oral exposures to Mn chloride by gavage have resulted in LD50 values of 275-804 mg/kg bw/day. Similar single exposures to Mn sulphate and Mn acetate have resulted in LD50 values of 782 and 1082 mg/kg bw/day respectively.

In non-lethal doses, decreased activity, alertness, muscle tone, touch response and respiration have been recorded in mice dosed with 58 mg Mn chloride/kg bw by oral gavage.

Little information is available regarding dermal toxicity, irritation and sensitisation properties of inorganic Mn compounds, possibly due to low dermal absorption. A single murine local lymph node assay study reported no cell proliferation with Mn salts inferring little sensitisation potential.

As in acute studies, neurological and pulmonary effects appear also to be a consequence of repeated Mn exposure. In contrast to acute bolus gavage administration, inorganic Mn appears to be more tolerated when administered over a longer term in feeding studies. The apparent differences in survival for bolus gavage versus feeding could be explained by species differences and/or the inability of clearance mechanisms to adequately handle high acute loads versus similar loads spread over time (ATSDR 2000).

Mice showed increased susceptibility to infection when exposed to Mn dioxide via inhalation for up to 4 days. Mild lung inflammation is reported in rhesus monkeys exposed to atmospheric Mn dioxide for 10 months.

A spectrum of neurological effects has been recorded in animals following short-term or chronic Mn exposure. Decreased motor activity or increased activity and aggression have been observed in rodents in food or drinking water studies. Although no evidence of neurological effects was observed in repeat dose studies in monkeys exposed to 20-40 mg Mn chloride/m³, movement tremors with increased Mn in the globus pallidus and substantia nigra have been reported in monkeys following IV administration of 5-40 mg Mn/kg bw (as Mn chloride). In rodents, significant alterations in pup retrieval and open field behaviour are also reported with short-term exposure to inorganic Mn compounds.

In addition to activity and behavioural signs, repeat dose studies also report changes in brain histochemistry, brain enzyme function and neurotransmitter levels in rats and mice following Mn at oral doses ranging from 1 to 2270 mg Mn/kg bw over 14-364 days. Decreased levels of dopamine in the caudate and globus pallidus regions of the brain are reported in rhesus monkeys exposed via inhalation for up to 2 years to 30 mg Mn/m³ (as Mn dioxide). Neurochemical changes are also reported in neonate rats at Mn levels similar to or slightly higher than dietary levels suggesting a particular susceptibility of the young to Mn.

To further investigate the impact of Mn for susceptible subpopulations, bioaccumulation and neurotoxicity of Mn have been investigated also in animal models of chronic liver disease (Salehi et al., 2001). Male rats with portacaval anastomosis were exposed via inhalation to 3050 μg/m³ Mn phosphate for 4 weeks. Mn levels in blood, lung, cerebellum, frontal cortex and globus pallidus were significantly elevated compared to unexposed portacaval shunted rats. Neuronal cell losses from the frontal cortex, caudate putamen and globus pallidus was also significantly higher in Mn exposed rats. These results show Mn bioaccumulation and neurotoxicity following intranasal and respiratory tract (inhalation) exposure in animals with compromised Mn clearance.

The exacerbating effects of Mn exposure on neurological dysfunction have also been investigated in animal models of pre-parkinsonism (Witholt, Gwiazda and Smith 2000). Female rats in which a pre-parkinsonism state was induced by intrastriatal injections of 6-hydroxydopamine received IP injections of 4.8 mg Mn/kg bw (as MnCl₂) thrice weekly for 5 weeks. In contrast to control pre-parkinsonism rats in which no abnormalities were detected in neurobehavioural tests, rats also receiving Mn showed significant impairment of neurobehaviour in 8 of 10 neurofunctional tests. These results suggest that chronic Mn exposure may increase the risk of neurobehavioural impairment in pre-parkinsonism subpopulations.

Animal studies also show effects of Mn exposure on reproduction and development. Studies in rabbits, rats and mice report degenerative changes in the testes. In rabbits, a single dose of 160 mg Mn/kg bw (as Mn dioxide) resulted in slow degeneration of seminiferous tubules over 1-8 months with a loss of spermatogenesis leading to infertility.

In female rats, slight decreases in pregnancy rates were observed with Mn exposure via diet. In mice, exposure to 85 mg Mn/m³ via inhalation for 16 weeks prior to and 17 days after conception was reported to decrease pup weight and activity. In a similar fashion to studies showing more efficient absorption of Mn following parental compared to oral administration, other developmental studies suggest that parenteral administration may cause greater toxicity when compared to administration via gavage. In rats, IV injection of 1.1 mg Mn chloride/kg bw on gestation days 6-17 induced mild skeletal malformations in foetuses. No effects were observed at 0.28 mg Mn/kg bw.

A recent fertility study was conducted by Elbetieha et al. (2001) in mice given Mn chloride in drinking water at 1-8 g/L for 12 weeks. Males showed decreased fertility at the highest dose but not at lower doses. Females showed no reductions in fertility at any dose but numbers of implantations and viable foetuses were significantly reduced at the highest dose. Females also showed significantly increased ovarian weights at 4 and 8 g/L and increased uterine weights at all doses. Although results are mixed, data indicate that Mn has the capacity to induce reproductive and developmental effects in animals.

The carcinogenic potential of Mn has also been examined in rodent studies but only limited oral exposure data are available and results are equivocal. Tumours of the lungs (following IP injection), pancreatic, pituitary and thyroid gland follicular cell adenomas are reported in separate studies in rats and mice but incidences are often not dose responsive, only marginally increased compared to internal controls and often within the bounds of historical controls. These data based on rodent studies do not provide sufficient evidence for carcinogenic properties of Mn.

In reverse mutation assays in bacteria and fungi, at least some forms of inorganic Mn are reported to have mutagenic potential. Mixed findings are also reported in in vitro clastogenicity studies using mammalian and plant cell cultures. Similar inconsistencies are also reported in in vivo mammalian studies. From these data, no firm conclusions can be drawn regarding the genotoxic properties of Mn.