Adaptive response to neurotoxicity on locomotion behavior by severe metal exposure was investigated in Caenorhabditis elegans. Exposure to 2.5μM of metals induced a moderate but significant reduction of locomotion behavior and induction of hsp-16.2::gfp expression. After pre-exposure to 2.5μM of metals, the reduced locomotion behavior induced by subsequent 50 and 100μM of metal exposure were significantly prevented, and the induction of hsp-16.2::gfp expression caused by subsequent 50 and 100μM of metal exposure were significantly suppressed. In contrast, after pre-exposure to 50μM examined metals, the reduced locomotion behavior induced by subsequent 50 and 100μM metal exposure were further decreased, and the noticeable induction of hsp-16.2::gfp expression caused by subsequent severe metal exposure were further enhanced. Therefore, pre-treatment with mild metal exposure can activate the adaptive response to neurotoxicity on locomotion behavior induced by subsequent severe metal exposure in nematodes.

In this study, we investigated the locomotion behavior changes at different developmental stages in Caenorhabditis elegans exposed to metals for 4h. No obvious differences could be observed in young adults exposed to examined metals, and only exposure to 100μM of examined metals could significantly decrease the locomotion behaviors of L4 larvae. Incontrast, exposure to 50 and 100μM of examined metals induced noticeable repression of locomotion behaviors at L1-L3 larval stages, and a significant decrease of locomotion behaviors could be observed in L1 larvae exposed to Pb and Hg, and in L2 larvae exposed to Hgat the concentration of 2.5μM. More over, the L1-, L2-, and L3-larvae exposed to metals for 4h exhibited similar neurobehavioral toxicity manner to L4-larvae exposed to metals for 24h. Therefore, younger larvae showed more severe deficits in neurobehavioral phenotypes than L4 larvae and young adults in metal-expose dnematodes.

We examined the possible multiple defects induced by acute and prolonged exposure to high levels of manganese (Mn) solution by monitoring the endpoints of lifespan, development, reproduction, and stress response. Our data suggest that acute exposure (6 h) to Mn did not cause severe defects of life span, development, and reproduction, similarly, no significant defect could be found in animals exposed to a low concentration of Mn (2.5μmol/L) for 48 h. In contrast, prolonged exposure (48 h) to high Mn concentrations (75 and 200μmol/L) resulted in significant defects of life span, development, and reproduction, as well as the increase of the percentage of population with hsp-16.2::gfp expression indicating the obvious induction of stress responses in exposed animals. Moreover, prolonged exposure (48 h) to high concentrations (75 and 200 μmol/L) of Mn decreased the expression levels of antioxidant genes of sod-1, sod-2, sod-3, and sod-4 compared to control. Therefore, prolonged exposure to high concentrations of Mn will induce the severe defects of life span, development, and reproduction in nematodes possibly by affecting the stress response and expression of antioxidant genes in Caenorhabditis elegans.

Apart from the liver disruption, embryotoxicity and genotoxicity, microcystin (MC)-LR also could cause neurotoxicity. Nematode Caenorhabditis elegans was explored as a model to study the neurotoxicity. In the present study, we provided evidence to indicate the neurotoxicity on chemotaxis to NaCl and diacetyl, and thermotaxis from MC-LR exposure to C. elegans. As a result, higher concentrations of MC-LR caused significantly severe defects of chemotaxis to NaCl and diacetyl, and thermotaxis. The neurotoxicity on chemotaxis to NaCl and diacetyl, and thermotaxis from MC-LR exposure might be largely mediated by the damage on the corresponding sensory neurons (ASE, AWA, and AFD) and interneuron AIY. The expression levels of che-1 and odr-7 were significantly decreased (P<0.01) in animals exposed to MC-LR at concentrations lower than 10μg/L, whereas the expression levels of ttx-1 and ttx-3 could be significantly (P<0.01) lowered in animals even exposed to 1 μg/L of MC-LR. Moreover, both the chemotaxis to NaCl and diacetyl and the thermotaxis were more significantly reduced in MC-LR exposed mutants of che-1(p674), odr-7(ky4), ttx-1(p767), and ttx-3(ks5) than those in exposed wild-type N2 animals at the same concentrations.

Chemotaxis to water-soluble attractants is mainly controlled by ASE sensory neuron whose specification is regulated by che-1 in Caenorhabditis elegans. Our data suggested that exposure to high concentrations of metals, such as Pb, Cu, Ag, and Cr, would result in severe defects of chemotaxis to water-soluble attractants of NaCl, cAMP, and biotin. Moreover, the morphology of ASE neuron structures as observed by relative fluorescent intensities and relative size of fluorescent puncta of cell bodies, relative lengths of sensory endings in ASE neurons, and the expression patterns of che-1 were obviously altered in metal exposed animals when they meanwhile exhibited obvious chemotaxis defects to water-soluble attractants. In addition, the dendrite morphology could be noticeably changed in animals exposed to 150 μmol/L of Pb, Cu, and Ag. Furthermore, we observed significant decreases of chemotaxis to water-soluble attractants in Pb exposed che-1 mutant at concentrations more than 2.5 μmol/L, and in Cu, Ag, and Cr exposed che-1 mutant at concentrations more than 50 μmol/L. Therefore, impairment of the ASE neuron structures and functions may largely contribute to the appearance of chemotaxis defects to water-soluble attractants in metal exposed nematodes.

Few reports have examined the effects of metal exposure at larvae stages on reproduction in nematode Caenorhabditis elegans, an alternative model for chemical screening. In the present study, four metals (Pb, Hg, Cd, and Cr)were selected to investigate the effects that metal exposure has at different developmental stages (L1, L2, L3, L4, and young adult) on the reproduction of this test species. Larval nematodes were more sensitive to reproductive toxicity as demonstrated by altered brood size and generation time versus young adult nematodes. L1-larval nematodes showed the greatest susceptibility. Brood size was significantly correlated with concentrations of all examined metals, and generation time was correlated with concentrations of Pb and Hg. Hg exposure at the L1-larval stage induced the most severe reproductive toxicity among the examined metals; furthermore, the reproductive toxicities induced by 4 h metal exposure at the L1-larval stage were largely comparable to those from 1 d of metal exposure at the L4-larval stage. These results indicate that nematode reproduction is sensitive to adverse effects of several toxic metals at micromolar concentrations and specific developmental stages.

In this study, specific developmental stage for adults from day 1 to day 10 was selected to evaluate the chronic metal toxicity, because the population of dead nematodes and the accumulation of intestinal autofluorescence increased sharply after day 10. Chronic exposure to Cr, Pb, Cu, andHgcaused a significant elevation in fractions of dead animals after day 4, and resulted in a significant induction of hsp-16.2::gfp expression at all assayed metal concentrations. Moreover, chronic exposure to Ag, Cr, Pb, Cu, Hg, and Cd would induce a more severe stress response than exposure to Zn and Mn in intestine, and chronic exposure to Pb, Hg, Cr, Zn, and Mn would induce a more severe stress response than exposure to Ag, Cu and Cd in head neurons. Therefore, in determining the usefulness of animals in metal toxicity assessment, this study established a method using nematodes in testing the chronic metal toxicity.

Previous studies have revealed that metal exposure will cause severe deficits in perception behaviors. Herewe investigated the effects of metal (Hg, Cu, Ag, and Cr) exposure on thermotaxis to cultivation temperature in Caenorhabditis elegans. Our data suggest that exposure to higher concentrations of examined metals induced severe deficits in thermotaxis, and a significant reduction in thermotaxis could be even observed in nematodes exposed to 2.5μM of Hg. Moreover, exposure to higher concentrations of examined metals and 2.5μM of Hg induced significant decreases in relative intensities and relative sizes of fluorescent puncta of cell bodies in AFD thermosensory neurons. In addition, exposure to higher concentrations of examined metals resulted in a significant reduction in relative intensities and relative lengths of sensory endings in AFD neurons. Furthermore, the relative transcript levels of ttx-1, which functions in specifying the fate of AFD neuron,were significantly decreased in nematodes exposed to 2.5μMof Hg, and 50 and 100μM of examined metals. Thus, metal exposure at high concentrations will induce the severe deficits in thermotaxis to cultivation temperature possibly by altering the morphology or development of AFD neuron and damaging the molecular basis for function of AFD neuron in nematodes.

Among more than 75 variants of microcystin (MC), microcystin-LR (MC-LR) is one of the most common toxins. In this study, the feasibility of using Caenorhabditis elegans to evaluate MC-LR toxicity was studied. C. elegans was treated with MC-LR at different concentrations ranging from 0.1 to 80μg/L. The results showed that MC-LR could reduce lifespan, delay development, lengthen generation time, decrease brood size, suppress locomotion behavior, and decreases hsp-16-2-gfp expression. The endpoints of generation time, brood size, and percentage of the population expressing hsp-16-2-gfp were very sensitive to 1.0μg/L of MC-LR, and would be more useful for the evaluation of MC-LR toxicity. Furthermore, the tissue-specific hsp-16-2-gfp expressions were investigated in MC-LR-exposed animals, and the nervous system and intestine were primarily affected by MC-LR. Therefore, the generation time, brood size, and hsp-16-2-gfp expression in C. elegans can be explored to serve as valuable endpoints for evaluating the potential toxicity from MC-LR exposure.

The number of cell body or synapse made by Caenorhabditis elegans GABAergic neurons is constant during development. The neurotoxic effects of metal (Pb, Hg, Cu, Cd, Cr, and Mn) exposure on GABAergic motor neurons were investigated in C. elegans. Exposure to examined metals could not alter the position of GABA neurons, whereas exposure to high concentrations (75μM and 200μM) of metals caused noticeable axonal degeneration and neuronal loss in nerve cords, suggesting neurodegeneration will be induced by metal exposure to different degrees. In addition, exposure to Pb, Hg, Cu, and Cd at the low concentration (2.5μM) could also induce obviously neuronal loss. Moreover, exposure to high concentrations (75μM and/or 200μM) of most of examined metals significantly reduced the relative size and fluorescent intensities of AVL, RMEs, and RIS neurons. Therefore, the neurodegeneration and abnormal structuresmay be formed inGABAergicmotor neurons aftermetal exposure, and the endpoint of neuronal loss will be useful for the neurotoxicity assessment from trace metal exposure.