A combined in ovo-in vitro system for studies of volatile compounds on brain development: differential effects of carbon tetrachloride on neurones and astrocytes

In Vitro Testing of Neurotoxicity

Many of the toxic compounds that are at large in the environment represent a risk to our neuronal functions. Chemicals may have a direct or indirect effect on the nervous system and they may interfere with general biochemical properties or specific neuronal structures and processes. In this review, a brief presentation of the major neurotoxicological targets is given, together with a discussion of some aspects of the use of different in vitro models for screening purposes and mechanistic studies. It is believed that in vitro methods offer special opportunities for the development of new neurotoxicological assays, and that this development will mainly involve cultured model systems. Therefore, a presentation of nerve and glia tissue culture methods is given, followed by an overview of how information on the action of mercury and mercurials, excitotoxins and acrylamide has been obtained through the use of cultured cell models. It is concluded that the developmental potential in cell neurotoxicology lies within the areas of separation and identification of cells representative for different structures in the nervous system, co-cultivation of different cell types, in vivo/in vitro (ex vivo) procedures, chemically defined media, metabolic competent cultures of human cells and improved physiological conditions for cultivation and exposure.

Biotransformation of carbon tetrachloride in cultured neurons and astrocytes

The ability of brain neuronal cells to metabolize carbon tetrachloride (CCl(4)) has been studied in an attempt to explain earlier observed toxic effects of CCl(4) on these cells. The expression of cytochrome P-450, the glutathione (GSH) content and the activity of glutathione-S-transferase (GST) were measured in cultured neurons and astrocytes from chick embryo cerebral hemispheres. The metabolism of CCl(4) in the neuron and astrocyte cultures was also assessed by determining the formation of: CCl(2) in membrane preparations of these cells. In the membrane fractions of neurons and astrocytes, no measurable levels of cytochrome P-450 were observed. Nevertheless, neurons as well as astrocytes had a capacity for the metabolism of CCl(4). The metabolic capacity of the neurons was significantly greater than that of the astrocytes. The neuron cultures had a higher initial content of GSH and a higher control activity of GST than had the astrocytes. Neither the GSH level nor GST activity were significantly affected in the neuron cultures after exposure to CCl(4). In astrocyte cultures 2 mm CCl(4) slightly depleted the GSH level and significantly induced GST activity. At 3 mm CCl(4), GSH was depleted by 30% and by more than 50% at 4 mm CCl(4). It can be concluded that the metabolic activation of CCl(4) was higher in neurons than in astrocytes. This can explain the earlier observation of CCl(4)-induced lipid peroxidation in cultured neurons. Moreover, neuron GSH was not able to protect these cells against CCl(4)-induced peroxidative damage. In the astrocytes, on the other hand, GSH and GST appeared to have a role in detoxification of CCl(4).

Effects of carbon tetrachloride on embryonic development studied in the post-implantation rat embryo culture system and in chick embryos in ovo

The effects of carbon tetrachloride on embryonic development were investigated in a mammalian and a non-mammalian system. In the former, whole-rat embryos, taken at 9.5 days of gestation, were exposed in vitro to different concentrations of CCl(4) (10, 100, 300, 600 and 1000 mug/ml) in rat serum with or without a rat liver microsomal activating system (S-9 mix). In the latter system, chick embryos in ovo were exposed to different concentrations of CCl(4) vapour (25, 35 and 75 ppm). When studied in the whole-rat embryo culture system without metabolic activation, concentrations of up to 300 mug CCl(4)/ml had no effect on the overall development. Concentrations -600 mug CCl(4)/ml affected the somite number, growth and morphology of the embryos, which can be interpreted as general toxicity. In the presence of S-9 mix, toxicity occurred at concentrations -300 mug/ml. In ovo exposure to CCl(4) showed that 25 ppm caused a decrease in the number of somites. At 35 ppm, CCl(4) induced further toxicity, as indicated by reduced somite number and growth and increased malformation rates. The results indicate that effects on morphogenic events appeared in both systems at concentration levels that also affected the overall development and that, independently of the choice of species or route of administration, CCl(4) has no potential to induce specific malformation patterns. The presence of a metabolic system in the rat embryo cultures approximately doubled the toxicity of CCl(4).

Effect of carbon tetrachloride on inositol 1,4,5-trisphosphate dependent and independent regulation of rat brain microsomal Ca2+ flux

Carbon tetrachloride (CCl4) is a highly toxic industrial solvent with pronounced effects on the liver and brain. CCl4 is enzymatically cleaved to produce free radicals which attack membrane components, including proteins. Earlier reports indicated that CCl4 affects Ca(2+)-regulated events in the brain. Hence, the present study was initiated to determine whether CCl4 affects inositol 1,4,5-trisphosphate (IP3) receptor binding, free-Ca2+ movements across the microsomal membrane and protein kinase C (PKC) activity in rat brain, since IP3, Ca2+ and PKC are known to be involved in signal transduction. [3H]IP3 binding, free-Ca2+ movements and 45Ca2+ uptake were determined using rat brain microsomes and PKC activity was determined in the cytosolic fraction. CCl4 in vitro decreased [3H]IP3 binding to microsomes. IP3 mediated Ca2+ release from microsomes was inhibited and also the reuptake of IP3-released Ca2+ into microsomes was decreased in the presence of CCl4. CCl4 at concentrations < 2 microM independently released Ca2+ from microsomes. Uptake of total Ca2+ into microsomes was inhibited by CCl4 as observed with 45Ca(2+)-uptake studies. CCl4 at 1 microM inhibited PKC activity by 50%. Thus, perturbations in the binding of IP3 to its receptor sites, changes in the Ca2+ flux across the microsomal membrane and modulation of PKC activity by CCl4 in vitro suggested that CCl4 may exert neurotoxicity by altering signal transduction pathways.

Effects of Carbon Tetrachloride on Perfused Cultures of Hepatic and Neuronal Cells

Cultured hepatocytes and hemisphere neurons from chick embryos and mouse neuroblastoma cells were exposed to carbon tetrachloride (CC14; 0, 1, 2, 3 and 4mM) for 1 hour, using a perfusion system developed for studying the effects of volatile substances. In the perfused cultures, three parameters were compared: lipid peroxidation, membrane integrity and cellular respiration. In addition, cytochrome C oxidase activity was determined after incubation of cell homogenates with CC14. A concentration-dependent increase in lipid peroxidation and membrane permeability was found in the neuroblastoma cells. The hepatocytes responded to a lesser extent with respect to membrane permeability and their lipid peroxidation did not differ from that of controls. The hepatocytes responded with a 35% decrease in respiration when exposed to 3mM CC14, and a 20% decrease in cytochrome C oxidase activity after treatment with 1.5mM CCl4. In the neuronal cells, much smaller decreases in respiration were found and their cytochrome C oxidase activity remained unaffected. These results are very similar to those obtained after incubation in a closed chamber system. However, the perfused cells were found to be less sensitive to CCl4than cells exposed under static conditions.

Combined effects of carbon tetrachloride and chlordecone on calmodulin activity in gerbil brain

The potentiation of carbon tetrachloride (CCl4) toxicity by chlordecone (CD) pretreatment in different animal models is well established. However, these studies have only dealt with hepatotoxicity. The present study was initiated to determine whether CD preexposure potentiates CCl4 neurotoxicity in gerbils. Gerbils were chosen for the reason that the metabolism of CD in gerbil is similar to that of humans. Gerbils (50-80 g), fed on diet without or with CD (10 ppm) for 15 d, were challenged with a single dose of CCl4 (15 microliters, ip). Ca(2+)-ATPase and calmodulin (CaM) activities were determined in gerbil brain P2 fraction and cytosol, respectively, at intervals of 0.5, 2, 6, 12, and 24 h after CCl4 administration. Ca(2+)-ATPase and CaM activities were decreased at 0.5 and 2 h in both CD-preexposed and CCl4-treated gerbils. However, CaM activity returned to normal levels after 6 h and Ca(2+)-ATPase activity showed 80% recovery after 2 h. In vitro experiments showed that CCl4 alone at 5 microM concentration inhibited Ca(2+)-ATPase activity up to 50%. Combination of CD (0.5 microM) and CCl4 (1 and 5 microM) on Ca(2+)-ATPase activity showed no additive effect in vitro. Interaction between CCl4 and CaM was studied in the presence and absence of CD by monitoring NPN fluorescence. The decrease in NPN fluorescence observed with CCl4 was not potentiated by CD preincubation. These data suggest that CD does not enhance CCl4-induced alterations of Ca(2+)-ATPase and CaM activities in gerbil brain.

Differential effect of carbon tetrachloride on the cell membranes of neurons and astrocytes

Primary cultures of neurons and astrocytes from chick embryos were used to study the effect of CCl4 on the plasma membranes. The cultures were exposed to 0, 1, 2, 3 or 4 mM CCl4 in a closed chamber system for 30 or 60 minutes. The effects of the exposure were examined by means of scanning electron microscopy and by measuring the degree of lipid peroxidation. In neuron cultures the presence of 1 mM CCl4 for 60 min caused holes in the plasma membranes and led to a swelling of the cell bodies. At 4 mM CCl4 the membranes were totally destroyed, leaving cytoskeletal elements visible. In astrocyte cultures, on the other hand, no effects up to 2 mM were observed. At 4 mM some cells had rounded up, but the membranes were still intact. These data correspond very well with the results that neurons, in contrast to astrocytes, show a concentration-dependent increase in lipid peroxidation. The results from this study may indicate that the mechanism of action of CCl4 is different in neurons and astrocytes.

A Comparative Study of the Effects of Carbon Tetrachloride on Neurons, Astrocytes and Hepatocytes

The effects of carbon tetrachloride (CC14) on lipid peroxidation and respiratory activity in central nervous system (CNS) cells (neurons and astrocytes) have been studied and compared with effects on hepatocytes.

Cell cultures of neurons, astrocytes and hepatocytes were prepared from 8-, 14- and 15-day-old chick embryos, respectively. After appropriate intervals, each type of culture was exposed to 0, 2, 3 and 4mM CC14 in a closed-chamber system for 30 or 60 minutes.

The neurons, in contrast to both the astrocytes and the hepatocytes, showed a concentration-dependent increase in lipid peroxidation. Pretreatment of astrocyte and hepatocyte cultures with phenobarbital did not increase the degree of lipid peroxidation in these cells. Also, according to protein content, the neuron cultures appeared to be the most sensitive. Respiratory activity was drastically inhibited (70%) in the hepatocyte cultures and slightly inhibited (30–35%) in the CNS cell cultures, after exposure for 60 minutes to 2mM CC14.