Interspecies comparison of selected pollution biomarkers in dreissenid spp. inhabiting pristine and moderately polluted sites.

Stress biomarkers, which can outline impacts of contaminants in aquatic biota at the biochemical level, are increasingly used as early warning tools in environmental monitoring. Reliable biomarker based assessment schemes, however, request appropriate knowledge of baseline levels of selected endpoints, and the potential influence of a range of natural influencing factors (both abiotic and biotic) as well. In this study, we examined the interspecies variability of various biomarkers (metallothioneins (MT), ethoxyresorufin-O-deethylase activity (EROD), lipid peroxidation (LPO), DNA strand breaks (DNA_sb), vitellogenin-like proteins (Vtg)) in Dreissena polymorpha and Dreissena bugensis inhabiting either pristine- or moderately impacted sites of Lake Balaton (Hungary). Levels of all biomarkers considered revealed low interspecies variability in the two dreissenid species at all sampling sites, with consistently higher (but statistically insignificant) values in Dreissena polymorpha. Levels of all biomarkers varied within the two investigated seasons, with significant influence of the reproduction cycle particularly on the levels of metallothioneins and vitellogenin-like proteins. Each biomarker considered was elevated by October, with significantly higher values in the mussels inhabiting harbours. Insignificant spatial and temporal variability in the general health indicators (condition index, total protein content) of dreissenids was observed, which, in parallel with evident rise in biomarker levels, apparently suggest that the anthropogenic impacts in harbours affect mussel fitness yet at sub organismal level. Our data might serve useful basis for future environmental monitoring surveys, especially in habitats where the progressive replacement of Dreissena polymorpha by Dreissena bugensis is taking place, as the interspecies variability in susceptibility to chemical stress of the two species is well comparable.

Seasonal and size-related variation of subcellular biomarkers in quagga mussels (Dreissena bugensis) inhabiting sites affected by moderate contamination with complex mixtures of pollutants.

The size-related differences in subcellular biomarker responses were assessed in Dreissena bugensis mussels inhabiting harbours moderately affected by pollution with complex mixtures of heavy metals and polycyclic aromatic hydrocarbons (PAHs). Adult D. bugensis samples were collected from three harbours of Lake Balaton (Hungary) characterized by moderate shipping activity, and as reference site, from a highly protected remote area of the lake. Biomarkers of exposure (metallothioneins (MTs), ethoxyresorufin-o-deethylase (EROD)), oxidative stress (lipid peroxidation (LPO), DNA strand breaks (DNAsb)) and possible endocrine disruption (vitellogenin-like proteins (VTG)) were analysed in whole-tissue homogenates of differently sized groups of mussels in relation to environmental parameters and priority pollutants (heavy metals and polycyclic aromatic hydrocarbons). Integrated biomarker response (IBR) indices were calculated for biomarker responses gained through in situ measurements to signalize critical sites and to better distinguish natural tendencies from biological effects of contaminants. Biomarker responses showed close positive correlation in case of MT, EROD, LPO, and DNAsb and negative correlation with VTG levels with mussel shell length in autumn, when higher levels of biomarkers appeared, possibly due to natural lifecycle changes of animals.

Neonicotinoid insecticides inhibit cholinergic neurotransmission in a molluscan (Lymnaea stagnalis) nervous system.

Neonicotinoids are highly potent and selective systemic insecticides, but their widespread use also has a growing impact on non-target animals and contaminates the environment, including surface waters. We tested the neonicotinoid insecticides commercially available in Hungary (acetamiprid, Mospilan; imidacloprid, Kohinor; thiamethoxam, Actara; clothianidin, Apacs; thiacloprid, Calypso) on cholinergic synapses that exist between the VD4 and RPeD1 neurons in the central nervous system of the pond snail Lymnaea stagnalis. In the concentration range used (0.01-1 mg/ml), neither chemical acted as an acetylcholine (ACh) agonist; instead, both displayed antagonist activity, inhibiting the cholinergic excitatory components of the VD4-RPeD1 connection. Thiacloprid (0.01 mg/ml) blocked almost 90% of excitatory postsynaptic potentials (EPSPs), while the less effective thiamethoxam (0.1 mg/ml) reduced the synaptic responses by about 15%. The ACh-evoked membrane responses of the RPeD1 neuron were similarly inhibited by the neonicotinoids, confirming that the same ACh receptor (AChR) target was involved. We conclude that neonicotinoids act on nicotinergic acetylcholine receptors (nAChRs) in the snail CNS. This has been established previously in the insect CNS; however, our data indicate differences in the background mechanism or the nAChR binding site in the snail. Here, we provide the first results concerning neonicotinoid-related toxic effects on the neuronal connections in the molluscan nervous system. Aquatic animals, including molluscs, are at direct risk while facing contaminated surface waters, and snails may provide a suitable model for further studies of the behavioral/neuronal consequences of intoxication by neonicotinoids.

Evaluation of multixenobiotic resistance in dreissenid mussels as a screening tool for toxicity in freshwater sediments.

The multixenobiotic defense mechanism (MXR) in aquatic organisms was recognized as a first-line defense system, and its potential use as an early biomarker of exposure to environmental stress has raised attention in the last two decades. To evaluate the relevance of this biomarker in the freshwater mussel Dreissena polymorpha, we studied its responsiveness within laboratory exposures to contaminants sequestered in freshwater sediments affected by moderate anthropogenic impact. The effectiveness of this biomarker was assessed by comparing the MXR-transporter activities determined in bivalves first with toxicity scores recorded with the D. rerio embryo developmental assay. Both bioassays were applied in the sediment contact test format. As a second evaluation approach, MXR activities determined in exposed mussels were compared with sediment-contamination data integrated into toxic units on the basis of acute toxicity to Daphnia magna. In D. polymorpha subjected to acute exposure with moderately polluted sediments, we detected limited (22-33 %) but statistically significant induction of MXR activity. Mean MXR activities significantly correlated with TU values computed for test sediments. MXR activities in mussels showed strong positive correlation with the metal load of sediments and proved to be unrelated to the contamination with polycyclic aromatic compounds. MXR activity in laboratory-exposed mussels showed low variability within treatments and thus reliably reflected even low contaminant differences between the negative reference and moderately polluted harbor sediments. The strong correlation found in this study between the MXR-transporter activity in exposed mussels and environmentally realistic sediment contamination underscores the fairly good sensitivity of this biomarker in laboratory testing conditions to signal the bioavailability of sediment bound contaminants, and it may also anticipate even the incidence of toxicity to biota.

Pharmacological studies confirm neurotoxic metabolite(s) produced by the bloom-forming Cylindrospermopsis raciborskii in Hungary.

A rapid cyanobacterial bloom of Cylindrospermopsis raciborskii (3.2 × 10(4) filaments/mL) was detected early November, 2012, in the Fancsika pond (East Hungary). The strong discoloration of water was accompanied by a substantial fish mortality (even dead cats were seen on the site), raising the possibility of some toxic metabolites in the water produced by the bloom-forming cyanobacteria (C. raciborskii). The potential neuronal targets of the toxic substances in the bloom sample were studied on identified neurons (RPas) in the central nervous system of Helix pomatia. The effects of the crude aqueous extracts of the Fancsika bloom sample (FBS) and the laboratory isolate of C. raciborskii from the pond (FLI) were compared with reference samples: C. raciborskii ACT 9505 (isolated in 1995 from Lake Balaton, Hungary), the cylindrospermopsin producer AQS, and the neurotoxin (anatoxin-a, homoanatoxin-a) producer Oscillatoria sp. (PCC 6506) strains. Electrophysiological tests showed that both FBS and FLI samples as well the ACT 9505 extracts modulate the acetylcholine receptors (AChRs) of the neurons, evoking ACh agonist effects, then inhibiting the ACh-evoked neuronal responses. Dose-response data suggested about the same range of toxicity of FBS and FLI samples (EC50 = 0.397 mg/mL and 0.917 mg/mL, respectively) and ACT 9505 extracts (EC50 = 0.734 mg/mL). The extract of the neurotoxin-producing PCC 6506 strain, however, proved to be the strongest inhibitor of the ACh responses on the same neurons (EC50 = 0.073 mg/mL). The presented results demonstrated an anatoxin-a-like cholinergic inhibitory effects of cyanobacterial extracts (both the environmental FBS sample, and the laboratory isolate, FLI) by some (yet unidentified) toxic components in the matrix of secondary metabolites. Previous pharmacological studies of cyanobacterial samples collected in other locations (Balaton, West Hungary) resulted in similar conclusions; therefore, we cannot exclude that this chemotype of C. raciborskii which produce anatoxin-a like neuroactive substances is more widely distributed in this region.

Neurotoxic effects evoked by cyanobacterial extracts suggest multiple receptors involved in electrophysiological responses of molluscan (CNS, heart) models.

The responses of the snail central neurons (Helix pomatia, Lymnaea stagnalis) and the isolated Helix heart were characterized evoked by cyanobacterial extracts (Cylindrospermopsis raciborskii ACT strains) isolated from Lake Balaton (Hungary). The nicotinergic acetylcholine (ACh) receptors in the CNS (both excitatory and inhibitory) were blocked by the extracts of ACT 9502 and ACT 9505 strains and the anatoxin- a (homoanatoxin-a) producing reference strain of Oscillatoria sp. (PCC 6506), similar to the inhibitory effects of the pure anatoxin-a. The enhancement of the ACh responses by the ACT 9504 extract suggests additional, probably acetylcholine esterase inhibitory mechanisms. On the isolated Helix heart the crude ACT 9505 and PCC 6506 extracts evoked frequency increase and transient twitch contraction, opposite to the ACh evoked heart relaxation. Anatoxin-a similarly contracted the heart but did not increase its contration frequency. These data suggest the involvement of some non-cholinergic mechanisms, acting very likely by direct modulation of the electrical or contractile system of the isolated heart. Diversity of the effects evoked by the cyanobacterial extracts in the CNS and heart suggest pharmacologically different neuroactive components among the secondary metabolites of the cyanobacteria acting on both (anatoxin-a like) cholinergic and (unidentified) non-cholinergic receptors.

Effects of rotenone and other mitochondrial complex I inhibitors on the brine shrimp Artemia.

(Artemia) nauplii was used to asses the toxicity of rotenone, MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), MP+ (1-methyl-4-phenylpyridinium) and the effect of L-DOPA co-treatment with rotenone. Rotenone had a dose dependent effect on mortality (LC50: 0.37 ± 0.04 µM mean ± S E, n = 24), while MPTP and MP+ proved to be toxic in millimolar range (LC50: 0.21 ± 0.09 mM and 0.20 ± 0.08 mM, respectively, n = 4). L-DOPA (50-200 µM) co-treatment increased the survival of the rotenone-treated animals (LC50: 0.51 ± 0.23 µM, 1.03 ± 0.66 µM, and 0.76 ± 0.52 µM, respectively). In the whole body tissue homogenates of Artemia, sublethal (up to 0.3 µM) concentrations of rotenone increased the glutathione S-transferase (GST) activity by up to 50 about percent (LC50: 53.3 ± 6.8 nM/min/mg protein, against 34.7 ± 3.6 nM/min/mg protein, n = 4). Nauplii treated in 100 mM L-DOPA and rotenone together showed further increase of GST activity all across the range of rotenone concentrations. These results on Artemia nauplii show similarities with other animal models, when complex I inhibitors were tested. Biochemical measurements suggest a protective role of L-DOPA by increasing the GST activity as part of the intracellular defences during toxin-evoked oxidative stress.

Humoral serotonin and dopamine modulate the feeding in the snail, Helix pomatia.

We investigated the effect of elevated levels of humoral 5HT and DA on the feeding latency of Helix pomatia, 1 day, 3 days and 10 days following satiation, by injecting monoamines into the haemocoel. HPLC assay of monoamines showed that both 5HT and DA are present in pmol/ml concentrations in the haemolymph of both starved and non-starved animals. Elevated levels of 5HT and DA were most effective at decreasing the feeding latency 10 days following satiation when DA decreased the feeding latency in a concentration dependent manner between 10(-7) and 10(-5) M whereas 5HT levels decreased the feeding latency only at 10(-6) M but increased it at 10(-5) M. Immunocytochemistry revealed that both 5HT3 and D1 receptor-like immuno-reactivity are present in cell bodies located in the same areas of the buccal ganglia. Our observations suggest that both humoral DA and 5HT mutually modulate the activity of the feeding CPG through neurons which have these receptors.

The terrestrial snail, Helix pomatia, adapts to environmental conditions by the modulation of central arousal.

The osmotic stimulation which is able to change the behavioral state of the animal are most effective during rainy weather while they are less effective during dry weather conditions. In isolated CNS preparations from aestivated animals the highest firing activity and serotonin sensitivity of the serotonergic (RPas) heart modulator neurons are recorded during rainy weather and the lowest parameters are observed in dry conditions. In aestivated animals the serotonin (5HT) content in both the CNS and the foot is higher than the dopamine (DA) content during rainy weather, while in dry weather the DA level is higher than the 5HT. The inactivation-reactivation process is accompanied by decreasing both the DA and 5HT levels in the CNS and increasing them in the peripheral organs. Our results suggest that aestivated animals adapt to (favorable and unfavorable) environmental conditions by modulating their central arousal state through changing the levels and distribution of monoamines (5HT, DA) in their body.

Neuronal background of activation of estivated snails, with special attention to the monoaminergic system: a biochemical, physiological, and neuroanatomical study.

Osmotic stimulation activates both estivated and inactivated specimens of Helix pomatia and increases their central arousal. High-pressure liquid chromatography has shown that, during activation, the level of both serotonin and dopamine decreases in the central nervous system (CNS) but increases in the foot and heart, organs that are involved in the eversion of the body. In isolated CNS from activated animals, the firing frequency of the heart-modulator serotonergic (RPas) neurons is significantly higher than that in the CNS of estivated or inactivated animals. These neurons innervate both the heart and the anterior aorta. In semi-intact preparations, distilled water (an osmotic stimulus) applied to the mantle collar increases their firing frequency, whereas tactile stimulation evokes their inhibition. Extracellularly applied monoamines mimic the effect of peripheral stimuli: serotonin (0.1-10 microM) increases the activity of the RPas neurons, whereas dopamine (0.1-10 microM) inhibits their activity. Tyrosine-hydroxylase immunocytochemistry and retrograde neurobiotin tracing have revealed similar bipolar receptor cells in the mantle collar and tail, organs that are exposed to environmental stimuli in estivated animals. Serotonin immunocytochemistry carried out on the same tissues does not visualize receptor cells but labels a dense network of fibers that appear to innervate neurobiotin-labeled receptor cells. The combination of neurobiotin-labeling of RPas neurons and immunolabeling suggests that RPas neurons receive direct dopaminergic inputs from receptor cells and serotonergic inputs from central serotonergic neurons, indicating that central serotonergic neurons are interconnected. Thus, the RPas neurons may belong to neuronal elements of the arousal system.

Thee effect of food intake on the central monoaminergic system in the snail, Lymnaea stagnalis.

We investigated the effect of food intake on the serotonin and dopamine levels of the CNS as well as on the spontaneous firing activity of the CGC in isolated preparations from starved, feeding and satiated animals. Furthermore we investigated the effects of 1 microM serotonin and/or dopamine and their mixture on the firing activity of the CGC. The HPLC assay of serotonin and dopamine showed that during food intake both the serotonin and dopamine levels of the CNS increased whereas in satiated animals their levels were not significantly more than the control levels. Recording from the CGC in isolated CNS preparation from starved, feeding or satiated animals showed that feeding increased the firing frequency of the CGC compared to the starved control. The application of 1 microM dopamine decreased the firing frequency whereas the application of 1 microM serotonin increased the firing frequency of the CGC. We conclude that during food intake the external and internal food stimuli increase the activity of the central monoaminergic system and also increase the levels of monoamines in the CNS. Furthermore, we also suggest that the increased dopamine and serotonin levels both affect the activity of the serotonergic neurons during the different phases of feeding.

Cadmium ions modulate GABA induced currents in molluscan neurons.

The effect of Cd2+, as one of the most widespread toxic environmental pollutants, was studied on gamma-aminobutyric acid (GABA) evoked responses of identified neurons in the central nervous system of the pond snail, LYmnaea stagnalis L. (Gastropoda). In the experiments, the modulation of the action of GABA both on neuronal activity (current clamp recording) and on the a GABA activated membrane Cl- current (voltage clamp studies) has been shown. It was found that: 1. GABA could evoked three different various types of response in GABA sensitive neurons: i) hyperpolarization with strong inhibition of ongoing spike activity, ii) short depolarization with an increase of spike the activity, iii) biphasic respone with a short excitation followed by a more prolonged long inhibition. 2. In low-Cl- solution the inhibitory action of GABA was reduced or eliminated, but the excitatory one was not or only moderately affected. 3. CdCl2 inhibited the GABA evoked hyperpolarization, but left intact or only slightly reduced the excitation evoked by GABA. 4. The inward Cl- current evoked by GABA at a -75 mV holding potential was slightly augmented in the presence of I micromol/l Cd2+, but was reduced or blocked at higher cadmium concentrations. The effect of Cd2+ was concentration and time dependent. 5. Parallel with reducing the GABA evoked current, cadmium increased both the time to peak and the half inactivation time of the current. 6. CdCl2 alone, in 50 micromol/l concentration, induced a 1-2 nA inward current. The blocking effect of cadmium on GABA activated inhibitory processes can be an important component of the neuro-toxic effects of this heavy metal ion.

In vivo and in vitro exposure to PCB 153 reduces long-term potentiation.

We examined the effects of gestational and lactational exposure to polychlorinated biphenyl (PCB) 153 (2,4,5,2',4',5'-hexaCB) on the magnitude of long-term potentiation (LTP) observed in the CA1 region of hippocampal brain slices prepared from rats at 30 days of age. We compared these actions to those observed when PCB 153 is dissolved in normal Krebs-Ringer solution and perfused on slices from control rats of the same age. In vivo exposure was at three dose levels (1. 25, 5, and 20 mg/kg/day) from gestational day 3 through weaning at postnatal day 21. Although responses to low-frequency stimulation of the Schaffer collateral pathway in exposed animals were not different from controls, significantly reduced LTP was induced after tetanic stimulation, even at the lowest dose studied. We observed a comparable depression of LTP when control slices were perfused with Krebs-Ringer that had been equilibrated with PCB 153 in a generator column. Neither in vivo nor in vitro exposure significantly altered the input-output curves obtained before tetanic stimulation, but both suppressed the increase in response observed in controls after tetanic stimulation. Because LTP is thought to be correlated with learning ability, these observations may provide at least a partial mechanism to explain the reduction of intelligence quotient observed in humans exposed to PCBs early in development.

Effect of inorganic and organic tin compounds on ACh- and voltage-activated Na currents.

1. Inorganic tin and organotin compounds, occurring in aquatic ecosystems, are toxic and can cause behavioral abnormalities in living organisms. To determine the possible neuronal basis of these actions, the effects of both forms of Sn were studied on identified neurones of the mollusk, Lymnaea stagnalis L. 2. SnCl2 caused a dose-dependent decrease in the acetylcholine (Ach)-induced inward current. The effective threshold concentration, measured by a two microelectrode voltage clamp technique, was 0.1 microM, and the maximal effect occurred at 5 microM SnCl2. The depression of the inward current was greater after a 10 min preapplication (20%) than after 3 min treatment (7%). 3. The next series of experiments compared the actions of inorganic or organic tin compounds. In whole cell clamp experiments both (CH3)2SnCl2 and (CH3)3SnCl, like inorganic Sn, decreased the amplitude of Ach-induced current. Increasing the duration of the preapplication time resulted in an increase in the effect, but the action was not reversible. SnCl2 treatment caused a concentration-dependent alteration (initial potentiation followed by depression) of the amplitude of I(Na(V)) over the whole voltage range and slightly shifted the I-V curves to the left. In contrast, trimethyl tin decreased the amplitude of I(Na(V)) only at high concentration (100 microM). The activation time course of I(Na) was increased (tau = 0.43 ms in control and 0.55 ms in Sn), but Sn did not alter the inactivation parameters (tau = 3.43 and 3.41 ms). 4. These results support earlier findings that agonist- and voltage-activated channels are direct targets of toxic metals. We conclude that tin in both inorganic and organic forms acts at neuronal membranes to modulate synaptic transmission through direct actions on agonist-activated ion channels, and suggest that these actions may be the basis of the altered behavior of animals in tin-polluted environments.

Aluminum enhances the voltage activated sodium currents in the neurons of the pond snail Lymnaea stagnalis L.

1. The effects of aluminum on voltage activated sodium currents (VASCs) were investigated by using the conventional two-electrode voltage clamp technique in Lymnaea stagnalis L. neurons. The peak amplitude, kinetics, and voltage-dependence of activation and inactivation of the sodium currents were studied in the presence of 5-500 microM AlCl3, at pH = 7.7. 2. There was a significant concentration-dependent increase in the peak amplitude of sodium currents after Al treatment, ED50 = 67 microM. The threshold concentration of the enhancement was 50 microM. The maximal peak increase of 143% was caused by a 500 microM aluminum. The action of aluminum on VASCs developed slowly, and it is not recovered by washing within 20 min. 3. There was little alteration of the voltage-dependence of the current. It was not a significant effect on the activation- and inactivation time constants of INa, but the steady-state inactivation curve shifted to negative direction on the voltage axis in the presence of Al. 4. The leak currents were not influenced by aluminum up to the highest concentration applied.

Facilitation of miniature GABAergic currents by ruthenium red in neonatal rat hippocampal neurons.

The whole cell configuration of the patch-clamp technique was used to study the modulation gamma-aminobutyric acid (GABA)-mediated postsynaptic currents by ruthenium red in CA3 hippocampal neurons in slices obtained from postnatal (P) days P6-P10 old rats. In the presence of kynurenic acid (1 mM), ruthenium red (100 microM) completely blocked stimulus-elicited GABA-mediated postsynaptic currents and reduced by 50% the amplitude of the spontaneous ones. Ruthenium red (100 microM) increased the frequency but not the amplitude of miniature GABAergic currents recorded in the presence of tetrodotoxin (1 microM) and kynurenic acid (1 mM), an effect that was prevented by heparin (100 microM). Ruthenium red did not modify the kinetics of miniature postsynaptic currents and the currents induced by exogenous application of GABA (10 microM) in the presence of tetrodotoxin, suggesting that its action was presynaptic in origin. The effects of ruthenium red on quantal GABA release was independent of external calcium. In a nominally Ca2+-free solution the potentiating effect induced by this polyvalent cation on miniature postsynaptic currents was still present. Intracellular calcium stores were not involved in ruthenium red action, because this polyvalent cation was able to facilitate miniature currents also in the presence of thapsigargin (10-20 microM). These results indicate that ruthenium red has a dual action on GABA release from GABAergic interneurons: it reduces the amplitude of spontaneous events and increases the frequency of miniature currents. The former effect is calcium-dependent, whereas the latter is calcium independent.

Postsynaptic induction of mossy fibre long term depression in developing rat hippocampus.

The whole cell configuration of the patch clamp technique was used to study the mechanisms of induction of long term depression (LTD) occurring at the mossy fibre-CA3 synapse between postnatal (P) day 6 and P13. In control conditions, when two pulses were delivered to the mossy fibres with an interval of 50 ms a potentiation of the EPSC evoked by the second pulse associated with a reduction in the number of failures was observed. Tetanization of the mossy fibres induced LTD of the responses to the first and second stimulus without affecting the paired pulse facilitation. Loading the postsynaptic cell with BAPTA prevented the induction of LTD but did not modify the paired pulse facilitation, suggesting that LTD induction occurs at the postsynaptic site.

HgCl2 potentiates GABA activated currents in Lymnaea stagnalis L. neurones.

Using the concentration clamp technique the effects of inorganic mercury (HgCl2) on gamma-aminobutyric acid (GABA) activated Cl- currents were studied on Lymnaea neurons. Hg2+ at concentrations of 0.01-1 microM caused a significant increase in the GABA induced chloride permeability of the cell membrane. With simultaneous application of GABA and 0.1 microM Hg2+ the amplitudes of the GABA activated inward Cl- currents were increased to a mean of 168%. The threshold concentration for potentiation was 0.01 microM Hg2+, while the GABA induced Cl- current was suppressed at concentrations higher than 100 microM. HgCl2 was also effective in decreasing the time-to-peak value and the time of inactivation of the GABA evoked Cl- current. In the presence of 0.1 microM mercury chloride the peak reached the maximum 1.6 times faster than without mercury. The effect was independent of the membrane potential, and mercury did not change the reversal potential of the GABA activated Cl- current. Application of mercury alone at concentrations effective to enhance GABA evoked Cl- permeability caused no or only a very small sustained inward current. However, higher mercury concentrations or repeated mercury application greatly increased the membrane permeability, and this effect could not be reversed by a 15 min wash. The results suggest, that low mercury concentrations act on GABA-ergic synapses at a postsynaptic site, and the potentiation of the ligand gated Cl- current may be an important factor in the mechanism of Hg2+ neurotoxicity.