a b s t r a c tOrganic mercury (Hg) species exert their toxicity primarily in the central nervous system. The food relevant Hg species methylmercury (MeHg) has been frequently studied regarding its neurotoxic effects in vitro and in vivo. Neurotoxicity of thiomersal, which is used as a preservative in medical preparations, is to date less characterised. Due to dealkylation of organic Hg or oxidation of elemental Hg, inorganic Hg is present in the brain albeit these species are not able to readily cross the blood brain barrier. This study compared for the first time toxic effects of organic MeHg chloride (MeHgCl) and thiomersal as well as inorganic mercury chloride (HgCl 2 ) in differentiated human neurons (LUHMES) and human astrocytes (CCF-STTG1). The three Hg species differ in their degree and mechanism of toxicity in those two types of brain cells. Generally, neurons are more susceptible to Hg species induced cytotoxicity as compared to astrocytes. This might be due to the massive cellular mercury uptake in the differentiated neurons. The organic compounds exerted stronger cytotoxic effects as compared to inorganic HgCl 2 . In contrast to HgCl 2 exposure, organic Hg compounds seem to induce the apoptotic cascade in neurons following low-level exposure. No indicators for apoptosis were identified for both inorganic and organic mercury species in astrocytes. Our studies clearly demonstrate species-specific toxic mechanisms. A mixed exposure towards all Hg species in the brain can be assumed. Thus, prospectively coexposure studies as well as cocultures of neurons and astrocytes could provide additional information in the investigation of Hg induced neurotoxicity.

Mercury (Hg) is an environmental contaminant. Whereas within terrestrial food sources it is mostly found as inorganic Hg, in fish and seafood it is largely present in form of methylmercury. Because of its antibacterial/antifungal properties the organic Hg compound thiomersal is used as a preservative in medical preparations. However, exposure to organic Hg promotes primarily neurological effects. The tolerable weekly intake (TWI) of 1.6 mg/kg body weight for methylmercury has been recently reevaluated by the EFSA in 2012. Based on epidemiological studies a new TWI of 1.3 mg/kg body weight has been established EFSA Panel on Contaminants in the Food Chain, 2012. Up to date the transfer of Hg compounds into the brain and the mechanisms of Hg species induced neurotoxicity are not clearly understood. Here we apply an in vitro model of the blood-cerebrospinalfluid (CSF) barrier to identify the transfer mechanisms of different Hg species in the brain and to characterize their effects on the barrier properties. In first studies effects of mercury chloride (HgCl 2 ), methylmercury (MeHgCl) and thiomersal (THI) on the barrier integrity have been analyzed. Quantitative analysis of the total Hg amount in aliquots of both, blood and brain side of the in vitro barrier system, will give information about the Hg-species dependent transfer properties. The respective method to quantify Hg via ICP-MS/MS has already been established. Our studies indicate that the barrier system is significantly more sensitive towards organic Hg species as compared to inorganic compounds. These results are in agreement with our studies concerning the cytotoxicity of the Hg compounds in the barrier building porcine epithelial cells of the Plexus choroideus.In order to identify potential target cells of Hg mediated neurotoxicity, cytotoxic effects of the Hg species have been performed in different cell lines. Using the neutral red uptake assay based on the lysosomal integrity, differentiated human neurons (LUHMES) seem to be more sensitive towards organic and inorganic Hg species compared to human astrocytes. In both cell lines inorganic Hg shows a lower cytotoxicity compared to the organic species.