Excitotoxicity in the brain

Neurochemical targets and behavioral effects of organohalogen compounds: an update

Organohalogen compounds (OHCs) have been used and still are used extensively as pesticides, flame retardants, hydraulic fluids, and in other industrial applications. These compounds are stable, most often lipophilic, and may therefore easily biomagnify. Today these compounds are found distributed both in human tissue, including breast milk, and in wildlife animals. In the late 1960s and early 1970s, high levels of the polychlorinated biphenyls (PCBs) and the pesticide dichlorodiphenyl trichloroethane (DDT) were detected in the environment. In the 1970s it was discovered that PCBs and some chlorinated pesticides, such as lindane, have neurotoxic potentials after both acute and chronic exposure. Although the use of PCBs, DDT, and other halogenated pesticides has been reduced, and environmental levels of these compounds are slowly diminishing, other halogenated compounds with potential of toxic effects are being found in the environment. These include the brominated flame retardants, chlorinated paraffins (PCAs), and perfluorinated compounds, whose levels are increasing. It is now established that several OHCs have neurobehavioral effects, indicating adverse effects on the central nervous system (CNS). For instance, several reports have shown that OHCs alter neurotransmitter functions in CNS and Ca2+ homeostatic processes, induce protein kinase C (PKC) and phospholipase A2 (PLA2) mobilization, and induce oxidative stress. In this review we summarize the findings of the neurobehavioral and neurochemical effects of some of the major OHCs with our main focus on the PCBs. Further, we try to elucidate, on the basis of available literature, the possible implications of these findings on human health.

Polychlorinated biphenyl mixtures (Aroclors) induce apoptosis via Bcl-2, Bax and caspase-3 proteins in neuronal cell cultures

Polychlorinated biphenyls (PCBs) are a group of persistent and widely dispersed environmental pollutants, some of which may be neurotoxic. In the present study, we have investigated the effect of PCB commercial mixtures (Aroclors) on neuronal cell cultures by assessing cell viability and apoptotic cell death. We have combined morphological and biochemical techniques to establish the relevance of apoptosis in neuronal cell death induced by Aroclors. Treatment with both Aroclor 1248 and Aroclor 1260 caused the loss of cell viability and accelerated apoptosis both in a concentration- and time-dependent manner. However, the extent of apoptosis resulted greater for Aroclor 1248 than for Aroclor 1260. This is correlated with the loss of cell viability since Aroclor 1248 is more cytotoxic. The apoptosis induced by Aroclors involves the increase of caspase-3 activity. To correlate the caspase-3 activity with respect to changes in protein processing, caspase-3 precursor protein (procaspase-3) was evaluated by Western blot analysis. Also, Bcl-2 and Bax protein were assessed in order to elucidate the cell death machinery induced in cortical neuronal cell cultures by Aroclor 1248. The results indicate that the increase in Aroclor-induced apoptosis correlates with a reduction in the expression of antiapoptotic Bcl-2 and an increase in the expression of proapoptotic Bax. These results suggest that, with our experimental conditions, Aroclors induce apoptosis in primary cultures of cortical neurons via proteins of the Bcl-2 and caspase families.

The contributions of excitotoxicity, glutathione depletion and DNA repair in chemically induced injury to neurones: exemplified with toxic effects on cerebellar granule cells

Six chemicals, 2-halopropionic acids, thiophene, methylhalides, methylmercury, methylazoxymethanol (MAM) and trichlorfon (Fig. 1), that cause selective necrosis to the cerebellum, in particular to cerebellar granule cells, have been reviewed. The basis for the selective toxicity to these neurones is not fully understood, but mechanisms known to contribute to the neuronal cell death are discussed. All six compounds decrease cerebral glutathione (GSH), due to conjugation with the xenobiotic, thereby reducing cellular antioxidant status and making the cells more vulnerable to reactive oxygen species. 2-Halopropionic acids and methylmercury appear to also act via an excitotoxic mechanism leading to elevated intracellular Ca2+, increased reactive oxygen species and ultimately impaired mitochondrial function. In contrast, the methylhalides, trichlorfon and MAM all methylate DNA and inhibit O6-guanine-DNA methyltransferase (OGMT), an important DNA repair enzyme. We propose that a combination of reduced antioxidant status plus excitotoxicity or DNA damage is required to cause cerebellar neuronal cell death with these chemicals. The small size of cerebellar granule cells, the unique subunit composition of their N-methyl-d-aspartate (NMDA) receptors, their low DNA repair ability, low levels of calcium-binding proteins and vulnerability during postnatal brain development and distribution of glutathione and its conjugating and metabolizing enzymes are all important factors in determining the sensitivity of cerebellar granule cells to toxic compounds.

The effect of brominated flame retardants on neurotransmitter uptake into rat brain synaptosomes and vesicles

The environmental levels of brominated flame retardants (BFRs) are increasing, but little is known about their toxic effects. In this paper, we show that some of the most important BFRs in commercial use today, have a neurotoxicological potential. Hexabromocyclododecane (HBCD) and tetrabromobisphenol-A (TBBPA) inhibit plasma membrane uptake of the neurotransmitters dopamine, glutamate and gamma-amino-n-butyric acid (GABA) at a concentration level similar to what previously found for polychlorinated biphenyls (PCBs) and even for ecstasy. The IC(50) value for HBCD on dopamine uptake was 4 microM, and the IC(50) values for TBBPA were 9, 6 and 16 microM for dopamine, glutamate and GABA, respectively. HBCD also inhibited glutamate uptake at low concentrations, but never achieved more than 50% inhibition. The inhibition was primarily due to their effect on the membrane potential, measured by the membrane potential marker tetraphenylphosphonium bromide (TPP(+)). Other brominated flame retardants such as octaBDE and decaBDE did not have any effects on uptake. TBBPA, HBCD and even the pentabrominated diphenylether mixture (pentaBDE, DE-71, Great Lakes) also inhibited the vesicular uptake of dopamine with an IC(50) value of 3, 3 and 8 microM, respectively. The neurotoxicological consequences of these findings for environmental contaminants such as BFRs and PCBs are discussed.

Relationship between lipophilicity of C6-10 hydrocarbon solvents and their ROS-inducing potency in rat cerebellar granule cells

We have studied the effects of aliphatic, alicyclic, and aromatic C6-10 solvents on the formation of reactive oxygen species (ROS) in rat cerebellar granule cell cultures. ROS formation was assessed by monitoring oxidation of 2',7'-dichlorofluorescin (DCFH) to the fluorescent compound 2',7'-dichlorofluorescein (DCF). We found that aromatic solvents with C > 7, and aliphatic and alicyclic solvents with C > or = 7 induce ROS formation in rat cerebellar granule cells in vitro. The response increased with increasing solvent concentration. The potency of the compounds within each homologous group seemed to be correlated to their octanol water partition-coefficients. The aromatic solvents were generally less efficient in inducing ROS formation than the aliphatic and the alicyclic compounds.

The effect of polychlorinated biphenyls on the high affinity uptake of the neurotransmitters, dopamine, serotonin, glutamate and GABA, into rat brain synaptosomes

Studies have shown that polychlorinated biphenyls (PCB) may affect cognitive functions both in human and also in experimental animals. We have investigated whether this effect could be caused by an inhibition of the uptake of selected neurotransmitters into rat brain synaptosomes. Ortho-chlorinated biphenyls were found to inhibit transmitter transport into synaptosomes from rat brain. In contrast, several nonortho-chlorinated biphenyls did not inhibit uptake. The uptake of dopamine, glutamate, GABA and serotonin was inhibited by the PCB mixtures, Aroclor 1242 and 1254. Under identical condition, the uptake of dopamine was inhibited more efficient than that of glutamate. The inhibition of neurotransmitter uptake was found to be dependent on the chlorination patterns of the PCB congeners, (i) ortho-chlorinated PCBs with four to five chlorine substituents (with the exception of 2,2',6,6'-TeCB) were the most effective inhibitors; (ii) hexa- or heptachlorinated PCBs were poor inhibitors or partial inhibitors (e.g. 2,2',4,4',5,5'-HCB) of glutamate and GABA uptake. Kinetic studies indicated that Aroclor 1242 inhibited dopamine uptake mainly competitively. The uptake of glutamate and GABA was inhibited in either a mixed competitive or in a non-competitive way, respectively. The neurotoxic consequences of the effect of different PCBs on neurotransmitter uptake on the uptake into synaptosomes are discussed.

Cerebellum as a target for toxic substances

The Purkinje cells and the granule cells are the most important targets in cerebellum for toxic substances. The Purkinje cells are among the largest neuron in the brain and are very sensitive to ischaemia, bilirubin, ethanol and diphenylhydantoin. The granule cells are small and seem to be sensitive to loss of intracellular glutathione. Granule cells are sensitive to methyl halides, thiophene, methyl mercury, 2-chloropropionic acid and trichlorfon. The Purkinje cells appear in the rat brain on pre-natal day 14-16, whereas the granule cells appear post-natally. Both cells are sensitive to excitotoxic chemicals and also to an effect on DNA or its repair mechanisms.