Comparative neurotoxicity study of mercury-based inorganic compounds including Ayurvedic medicines Rasasindura and Kajjali in zebrafish model

The acute neurotoxicity of inorganic mercury in Mactra chinensis philippi

Inorganic mercury (IHg) is hazardous to marine organisms especially resulting in neurotoxicity, bivalves are sensitive to pollutants as "ocean sentinel", but data on the neurotoxicity of IHg in bivalves are sparse. So we chosed M. chinensis philippi with typical neural structures in bivalves to investigate the neurotoxicity of IHg, which could be helpful to understand the specificity of neural regulation and the response characteristics of bivalves. After acute exposed to IHg (HgCl2) for 24 h, the metabolites of ganglion tissues in M. chinensis philippi were evaluated using 1H-nuclear magnetic resonance based metabolomics; Ca2+, neurotransmitters (nitric oxide, glutamate, acetylcholine) and related enzymes (calcineurin, nitric oxide synthase and acetylcholinesterase) were measured using biochemical detection. Compared to the control group, the levels of the nitric oxide (81.04 ± 12.84 μmol/g prot) and acetylcholine (30.93 ± 12.57 μg/mg prot) in M. chinensis philippi of IHg-treated were decreased, while glutamate (2.11 ± 0.61 mmol/L) increased significantly; the activity of nitric oxide synthase (679.34 ± 135.33 U/mg prot) was increased, while acetylcholinesterase (1.39 ± 0.44 U/mg prot) decreased significantly, and the activity of calcineurin (0.52 ± 0.02 U/mg prot) had a statistically insignificant increasing tendency. The concentration of Ca2+ (0.92 ± 0.46 mmol/g prot) in the IHg-treated group was significantly higher than that in the control group. OPLS-DA was performed to reveal the difference in metabolites between the control and IHg-challenged groups, the metabolites of glucose, glutamine, inosine, succinate, glutamate, homarine, and alanine were sensitive to IHg, subsequently metabolic pathways that were affected including glucose metabolism, glutamine metabolism, nucleotide metabolism, Krebs cycle, amino acid metabolism and osmotic regulation. In our study, IHg interfered with metabolites in M. chinensis philippi, thus the corresponding metabolic pathways were changed, which influenced the neurotransmitters subsequently. Furthermore, Ca2+overload affected the synthesis or degradation of the neurotransmitters, and then the altered neurotransmitters involved in changes in metabolic pathways again. Overall, we hypothesized that the neurotoxic effects of IHg on bivalve were in close contact with metabolism, neurotransmitters, related enzymes and Ca2+, which could be effective neurotoxic biomarkers for marine environmental quality assessment, and also provide effective data for the study of the regulatory mechanism of the nervous system in response to IHg in bivalves.

Temperature- and chemical-induced neurotoxicity in zebrafish

Throughout their lives, humans encounter a plethora of substances capable of inducing neurotoxic effects, including drugs, heavy metals and pesticides. Neurotoxicity manifests when exposure to these chemicals disrupts the normal functioning of the nervous system, and some neurotoxic agents have been linked to neurodegenerative pathologies such as Parkinson's and Alzheimer's disease. The growing concern surrounding the neurotoxic impacts of both naturally occurring and man-made toxic substances necessitates the identification of animal models for rapid testing across a wide spectrum of substances and concentrations, and the utilization of tools capable of detecting nervous system alterations spanning from the molecular level up to the behavioural one. Zebrafish (Danio rerio) is gaining prominence in the field of neuroscience due to its versatility. The possibility of analysing all developmental stages (embryo, larva and adult), applying the most common "omics" approaches (transcriptomics, proteomics, lipidomics, etc.) and conducting a wide range of behavioural tests makes zebrafish an excellent model for neurotoxicity studies. This review delves into the main experimental approaches adopted and the main markers analysed in neurotoxicity studies in zebrafish, showing that neurotoxic phenomena can be triggered not only by exposure to chemical substances but also by fluctuations in temperature. The findings presented here serve as a valuable resource for the study of neurotoxicity in zebrafish and define new scenarios in ecotoxicology suggesting that alterations in temperature can synergistically compound the neurotoxic effects of chemical substances, intensifying their detrimental impact on fish populations.

Sub-chronic oral toxicity evaluation of herbo-metallic formulation Arshakuthar rasa in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Arshakuthar rasa (AR) is a mercury based Ayurvedic herbo-metallic formulation. The concerns are being raised about the probable toxicity of mercury after prolonged use of AR. Hence, there is need for a long-term repeated in vivo toxicity study. The study will provide data with scientific evidence to enable safe use of the drug. Moreover, lack of toxicity study with AR incited us to perform sub-chronic study on rats.

AIM OF THE STUDY

The aim of the study is to generate data by performing a sub-chronic study to assess the toxicity of AR after its prolonged oral intake.

MATERIALS AND METHODS

The female and male rats were administered with 30 (low), 300 (medium) and 600 mg/kg BW/day (high) dose of AR for 90 consecutive days. The body weight, feed consumption and water intake were monitored weekly. On 91^st day, blood was collected from retro-orbital plexus of rats and then sacrificed to harvest the vital organs for biochemical, haematological, histopathological, genotoxicity along with the expression study of oxidative stress related genes and the biodistribution of elements in the blood.

RESULTS

Significant alterations in serum biochemical parameters were observed at the medium and high doses. The histopathological changes were in corroboration with biochemical changes at high dose in liver. There was no detectable level of mercury in blood, less to moderate biochemical changes, no haematological changes, moderate regulation of stress-related genes, and low genotoxicity. These results indicated that AR can be considered as moderately toxic above 600 mg/kg BW and mildly toxic at 300 mg/kg BW.

CONCLUSIONS

It may be interpreted that AR may not induce grave toxic response in human after long-duration of oral administration at therapeutic doses.

Characterization of neurobehavioral pattern in a zebrafish 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced model: A 96-hour behavioral study

Parkinson’s disease (PD) is the most common brain motor disorder, characterized by a substantial loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). Motor impairments, such as dyskinesia, bradykinesia, and resting tremors, are the hallmarks of PD. Despite ongoing research, the exact PD pathogenesis remains elusive due to the disease intricacy and difficulty in conducting human studies. Zebrafish (Danio rerio) has emerged as an ideal model for researching PD pathophysiology. Even though 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been used to induce PD in zebrafish, behavioural findings are frequently limited to a single time point (24 hours post-injection). In this sense, we aim to demonstrate the effects of MPTP on zebrafish swimming behaviour at multiple time points. We administered a single dosage of MPTP (200μg/g bw) via intraperitoneal injection (i/p) and assessed the locomotor activity and swimming pattern at 0h, 24h, and 96h post-injection through an open field test. Analysis of the behaviour revealed significant reductions in swimming velocity (cm/s) and distance travelled (cm), concurrent with an increase in freezing maintenance (duration and bouts) in zebrafish injected with MPTP. In addition, the MPTP-injected zebrafish exhibited complex swimming patterns, as measured by the turn angle, meander, and angular velocity, and showed abnormal swimming phenotypes, including freezing, looping, and erratic movement. To conclude, MPTP administration into adult zebrafish induced hypolocomotion and elicited motor incoordination. Plus, the effects of MPTP were observable 24 hours after the injection and still detectable 96 hours later. These findings contribute to the understanding of MPTP effects on adult zebrafish, particularly in terms of swimming behaviours, and may pave the way for a better understanding of the establishment of PD animal models in the future.

The effect of ocean warming on accumulation and cellular responsiveness to cobalt in Mytilus galloprovincialis

Cobalt (Co) is among the hazardous substances identified in aquatic environments. Industrialization and population growth have also contributed to climate change, namely in what concerns ocean temperature rise. The aim of the present study was to evaluate the influence of temperature rise on the impacts caused by Co on Mytilus galloprovincialis. To this end, mussels were exposed for 28 days to 17 °C and 21 °C, without and with 200 μg L^-1 of Co. Results showed no significant differences in Co bioaccumulation by the organisms between temperatures. A significant interaction between temperature and Co contamination was observed in terms of oxidative damage, detoxification capacity, and neurotoxicity, with a synergistic effect particularly evident in terms of biotransformation enzymes' activity. The obtained results point out that population survival and distribution may be limited in the long term, highlighting the need for future research on the combined effects of both stressors.

Mapping the distribution of mercury (II) chloride in zebrafish organs by benchtop micro-energy dispersive X-ray fluorescence: A proof of concept

BACKGROUND

Mercury (Hg) is a globally ubiquitous pollutant and one of the most dangerous metal contaminants, which presents a high risk of bioaccumulation in living organisms. In this study, we mapped the distribution of Hg and other trace elements in zebrafish (Danio rerio), which were exposed to mercury (II) chloride in order to assess its toxicity, bioaccumulation and distribution in fish organs.

METHODS

Adult zebrafish were exposed for 7 days to different concentrations of mercury (II) chloride and the elemental distribution was obtained through the micro-energy dispersive X-ray fluorescence technique (μ-EDXRF).

RESULTS

The results showed that Hg levels, measured in fish tissues, were indicative of bioaccumulation within some of its organs (e.g. visceral mass, gills), and that the physiological processes of accumulation were highly dose-dependent. In addition, the results showed higher concentrations of Hg in the gills. Moreover, other trace elements (e.g. Fe, Cu and Zn) levels were not altered after fish exposure to mercury(II) chloride.

CONCLUSION

The μ-EDXRF results were assessed along with the determination of some oxidative stress biomarkers (e.g. antioxidant enzymes) to understand the effects behind the Hg bioaccumulation and toxicity. These results suggest that the metabolic changes in zebrafish due to the exposure to Hg are consistent with oxidative stress.

Explaining Ayurvedic preparation of Rasasindura, its toxicological effects on NIH3T3 cell line and zebrafish larvae

Rasasindura is a mercury-based medicinal formulation that contains HgS (>99%). Although cinnabar ore was a well-known mineral in the past, the Ayurvedic practitioner adopted a critical and tedious procedure for the preparation of Rasasindura. Therefore, it is essential to understand the Ayurvedic process in the perspective of material science. Further, a toxicity study is also required as mercury is the main component in Rasasindura. Here, in the present study, we characterized Rasasindura and one of its intermediates (Kajjali) to understand the physicochemical changes that occur in the Ayurvedic process. Furthermore, we have assessed the toxicity of Kajjali and Rasasindura in NIH3T3 cell lines and zebrafish larvae. XRD analysis of Rasasindura confirms it as a highly pure α-HgS with size ranges from nano to micron sizes (starting from ∼80 nm). Whereas, Kajjali is a β-HgS having lower size ranges (starting from ∼30 nm). Rasasindura did not show significant cytotoxicity on NIH3T3 cell line up to 75 ppm, whereas for Kajjali, cytotoxicity was observed above 20 ppm. The higher toxicity of Kajjali is due to higher penetration of particles into the cells. However, in zebrafish larvae, even at too high concentrations (1000 ppm), both Rasasindura and Kajjali did not show any toxicity or morphological changes. This study concludes that Rasasindura is not toxic up to a reasonable concentration. Further, these two drugs did not contain toxic organic mercuric compound; otherwise, it could have been lethal to the zebrafish larvae.

Physico-chemical characterization of kajjali, black sulphide of mercury, with respect to the role of sulfur in its formation and structure

BACKGROUND

Kajjali is used as a base for Ayurvedic herbo-mineral medicines. It is a combination of mercury with sulfur in varying proportions. The ratio of sulfur (S) added to mercury (Hg) directly relates to the therapeutic efficacy of the compound.

OBJECTIVE

To analyze the physico-chemical characteristics of samaguna gandhaka kajjali (Hg: S = 1:1) and shadaguna gandhaka kajjali (Hg: S = 1:6).

MATERIALS AND METHODS

X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), Fourier transmission infrared spectroscopy, thermo-gravimetry analysis, and atomic absorption spectroscopy were applied to characterize each type of kajjali.

RESULTS

It was found that the particle size of the formed kajjali compound increases with a decrease in the mercury to sulfur ratio. The presence of excess sulfur does not change the surface oxidation states as revealed by the XPS analysis. No trace of mercury has been found in both samaguna gandhaka kajjali (SGK-1) and shadguna gandhaka kajjali (SGK-6), indicating a complete Hg reaction with S.

CONCLUSION

Kajjali simulates nanomaterial of the modern era and possesses therapeutic efficacy as mentioned in classical Ayurveda texts. Complete trituration of mercury and sulfur combination ends up with this kajjali formation incorporating the potency of nanotherapeutics.

Adaptive mechanisms induced by sparingly soluble mercury sulfide (HgS) in zebrafish: Behavioural and proteomics analysis

Mercury (Hg) causes great health concerns due to its extreme neurotoxicity. However, here we show that pretreatment of sparingly soluble mercury compound (HgS) could induce adaptive mechanisms in zebrafish, which can resist the neurotoxic effects of mercury chloride (HgCl₂). In this study, zebrafish were treated with HgS (in the form of 99% HgS arising from traditional Ayurvedic medicine Rasasindura (RS), chosen for its particle and crystallite sizes). This work was prompted by the traditional use of this form of HgS to treat nervous and immune-related diseases. Our investigation on zebrafish behaviour showed that RS pretreated fish group (RS-HG) was less severely affected by HgCl₂ exposure, as compared to the RS non-treated (VC-HG) group. Further, biochemical tests showed that RS pretreatment prevents alteration of reactive oxygen species (ROS), acetylcholinesterase (AChE), and cortisol as compared to the VC-HG group. Proteomics and bioinformatics studies of zebrafish brain tissues suggested that Rasasindura (RS-HG group) protects alteration of various protein expression related to KEGG pathways, including citrate cycle (TCA cycle) and glutathione metabolism that are directly or indirectly linked to the oxidative stress, against HgCl₂ induced neurotoxicity. We found that adaptive mechanisms were initiated by the initiation of response to stress (enrichment of GO:0006950 pathway), due to the accumulation of a small amount of ionic Hg (60 ± 10 ng/g) after 15 days of RS treatment. These adaptive mechanisms avoid further adverse neurotoxicity of HgCl₂. Thus, HgS (RS) pretreatment can induce protective effects in zebrafish.

Physicochemical Variation in Nanogold-Based Ayurved Medicine Suvarna Bhasma Produced by Various Manufacturers Lead to Different In Vivo Bioaccumulation Profiles

Suvarna Bhasma (SB) is a gold particle-based medicine that is used in Ayurved to treat tuberculosis, arthritis and nervous diseases. Traditionally, the Ayurved preparation processes of SB do exist, but they are all long, tedious and involve several steps. Due to this, there is a possibility of bypassing the necessary Ayurved processes or non-adherence to all steps or use of synthetic gold particles. Our aim is to characterize 5 commercial SB preparations from 5 different manufacturers. A comparative physicochemical, pharmacokinetic (PK) and bioaccumulation study was carried out on all the 5 SB preparations. The general appearance such as color and texture of these 5 samples were different from each other. The size, shape and gold concentration (from 32-98 wt%) varied among all the 5 SBs. The accumulation of ionic gold in zebrafish and gold concentration profiles in rat blood were found to be significantly different for all the 5 SBs. Non-compartmental PK model obtained from the concentration-time profile showed significant differences in various PK parameters such as peak concentration (C_max), half-life (t_1/2) and terminal elimination slope (λ_z) for all the 5 SB preparations. SB-B showed the highest C_max (8.55 μg/L), whereas SB-D showed the lowest C_max (4.66 μg/L). The dissolution of ionic gold from SBs in zebrafish tissue after the oral dose had a 5.5-fold difference between the highest and lowest ionic gold concentrations. All the 5 samples showed distinct physicochemical and biological properties. Based on characteristic microscopic morphology, it was found that 2 preparations among them were suspected of being manufactured by non-adherence to the mentioned Ayurved references.

Ayurvedic processing of α-HgS gives novel physicochemistry and distinct toxicokinetics in zebrafish

Rasasindura (RS) is an Ayurvedic medicine, which contains ∼99% α-HgS. It is used as a rejuvenating agent and commonly used to treat diseases such as syphilis, insomnia, high fever, and nervous disorders. Cinnabar ore (α-HgS) is a well-known mineral, which is readily available. Despite it, Ayurvedic practitioners adopted an involved and tedious procedure for the preparation of RS. In this study, three samples, one was Ayurvedic (RS), the second one was the commercial (HGS), and the third one was cinnabar ore (CN), were physiochemically examined. Zebrafish model was employed for toxicity study with an oral dose of 100 mg/kg/day for the three samples for 10 days. We found that RS conferred novel physicochemical properties, which were not seen in HGS and CN. Significantly, the average crystallite size of RS was lowest (26 nm) as compared to HGS (31 nm) and CN (34 nm), and the rate of increase of crystallite size with temperature was lowest in RS. RS did not show any significant behavioral toxicity in zebrafish, which was seen with the HGS-and CN-treated zebrafish. HGS-and CN-treated zebrafish showed a significantly high (∗∗∗p < 0.001) decrease (77 ± 7.6% and 51 ± 6.5%, respectively) of glutathione (GSH) levels in the brain, however, for RS-treated zebrafish, the change of GSH was insignificant (26 ± 2.5%, p > 0.05). Interestingly, HGS significantly altered the γ-aminobutyric acid (GABA) in brain tissue. Therefore, among all three samples, RS exhibited the lowest toxicity, which can be credited to the distinct toxicokinetics by these samples.

Physicochemical characterization of Suvarna Bhasma, its toxicity profiling in rat and behavioural assessment in zebrafish model

ETHNOPHARMACOLOGICAL RELEVANCE

Suvarna Bhasma is a gold-based Ayurved medicine that has a wide range of therapeutic indications like tuberculosis, diabetes mellitus, rheumatoid arthritis and nervous diseases. Suvarna Bhasma is also used in Suvarnaprashana, an Ayurved advocated therapy being practised to improve immunity in children.

AIM OF THE STUDY

To augment traditional understanding, here we present an evidence-based study on Suvarna Bhasma regarding its physicochemical properties, toxicity and efficacy.

MATERIALS AND METHODS

Suvarna Bhasma was characterised by physicochemical characterization techniques such as scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and atomic emission spectroscopy (ICP-AES). Toxicity of Suvarna Bhasma was studied in Holtzman rats with daily oral dose from 3 mg/kg (therapeutic dose, TD) up to 30 mg/kg (10 TD) body weight for 90 days. Behavioural study, such as motor and geotactic behaviour were examined in zebrafish model to find out any sign of neurotoxicity or behavioural changes due to Suvarna Bhasma administration.

RESULTS

Suvarna Bhasma has two types of gold particles, large ones (~60 μm) having irregular shapes, and nano-sized spherical particles (starting from ~10 nm), the latter coated with Fe, Si, O, P and Na. XRD study revealed that all the peaks of Suvarna Bhasma match well with pure gold (face centred cube) with crystallites size 45 ± 2.8 nm. In rat studies, some change in biochemical parameters such as urea, creatinine and alanine aminotransferase (ALT) was observed mainly at the higher therapeutic dose; however, those parameters were within the normal range. There were no significant macroscopic as well as microscopic treatment-related alteration observed, in any of the organs and tissues evaluated. In zebrafish behavioural study, the motor parameters of Suvarna Bhasma treated fish showed normal behaviour analogous to the vehicle control group. Interestingly, the geotactic behaviour showed anxiolytic effects of Suvarna Bhasma as evidenced by the time spent in the upper zone, and average swimming height. The anxiolytic effects persisted for more than 30 days after withdrawing the Suvarna Bhasma treatment.

CONCLUSIONS

Suvarna Bhasma contained spherical gold nanoparticles. It was nontoxic in rat model at the does tested. Suvarna Bhasma has anxiolytic effects in zebrafish behavioural model.

A multidimensional concept for mercury neuronal and sensory toxicity in fish - From toxicokinetics and biochemistry to morphometry and behavior

BACKGROUND

Neuronal and sensory toxicity of mercury (Hg) compounds has been largely investigated in humans/mammals with a focus on public health, while research in fish is less prolific and dispersed by different species. Well-established premises for mammals have been governing fish research, but some contradictory findings suggest that knowledge translation between these animal groups needs prudence [e.g. the relative higher neurotoxicity of methylmercury (MeHg) vs. inorganic Hg (iHg)]. Biochemical/physiological differences between the groups (e.g. higher brain regeneration in fish) may determine distinct patterns. This review undertakes the challenge of identifying sensitive cellular targets, Hg-driven biochemical/physiological vulnerabilities in fish, while discriminating specificities for Hg forms.

SCOPE OF REVIEW

A functional neuroanatomical perspective was conceived, comprising: (i) Hg occurrence in the aquatic environment; (ii) toxicokinetics on central nervous system (CNS)/sensory organs; (iii) effects on neurotransmission; (iv) biochemical/physiological effects on CNS/sensory organs; (v) morpho-structural changes on CNS/sensory organs; (vi) behavioral effects. The literature was also analyzed to generate a multidimensional conceptualization translated into a Rubik's Cube where key factors/processes were proposed.

MAJOR CONCLUSIONS

Hg neurosensory toxicity was unequivocally demonstrated. Some correspondence with toxicity mechanisms described for mammals (mainly at biochemical level) was identified. Although the research has been dispersed by numerous fish species, 29 key factors/processes were pinpointed.

GENERAL SIGNIFICANCE

Future trends were identified and translated into 25 factors/processes to be addressed. Unveiling the neurosensory toxicity of Hg in fish has a major motivation of protecting ichtyopopulations and ecosystems, but can also provide fundamental knowledge to the field of human neurodevelopment.