In vivo Effects of the Anatoxin-a on Striatal Dopamine Release

In Vitro Mutagenic and Genotoxic Assessment of Anatoxin-a Alone and in Combination with Cylindrospermopsin

Anatoxin-a (ATX-a) is a cyanobacterial toxin whose occurrence has been reported worldwide and has attracted increasing scientific interest due to its toxicity. Moreover, in nature, ATX-a usually appears together with other cyanotoxins, such as cylindrospermopsin (CYN), so possible interaction phenomena could happen and should be considered for risk assessment purposes. For this reason, the aim of this work was to explore the potential mutagenicity and genotoxicity of pure ATX-a and an ATX-a/CYN mixture using a battery of in vitro assays, including the bacterial reverse-mutation assay in Salmonella typhimurium (OECD 471) and the micronucleus test (MN) (OECD 487) on L5178Y Tk+/- cells. The results showed that ATX-a was not mutagenic either alone or in combination with CYN under the conditions tested. Nevertheless, genotoxic effects were observed for both ATX-a and its mixture with CYN following the in vitro MN assay. The genotoxicity exhibited by ATX-a was only observed in the absence of S9 mix, whereas in the cyanotoxin mixture the concentration-dependent genotoxicity of ATX-a/CYN in vitro was observed only in the presence of S9. Thus, the toxicity induced by cyanotoxin mixtures may vary from that produced by toxins alone, and consequently more studies are necessary in order to perform more realistic risk assessments.

Toxic Effects Produced by Anatoxin-a under Laboratory Conditions: A Review

The presence of cyanotoxins and its bioaccumulation in the food chain is an increasingly common problem worldwide. Despite the toxic effects produced by Anatoxin-a (ATX-a), this neurotoxin has been less studied compared to microcystins (MCs) and cylindrospermopsin (CYN). Studies conducted under laboratory conditions are of particular interest because these provide information which are directly related to the effects produced by the toxin. Currently, the World Health Organization (WHO) considers the ATX-a toxicological database inadequate to support the publication of a formal guideline reference value. Therefore, the aim of the present work is to compile all of the in vitro and in vivo toxicological studies performed so far and to identify potential data gaps. Results show that the number of reports is increasing in recent years. However, more in vitro studies are needed, mainly in standardized neuronal cell lines. Regarding in vivo studies, very few of them reflect conditions occurring in nature and further studies with longer periods of oral exposure would be of interest. Moreover, additional toxicological aspects of great interest such as mutagenicity, genotoxicity, immunotoxicity and alteration of hormonal balance need to be studied in depth.

Plant-cyanobacteria interactions: Beneficial and harmful effects of cyanobacterial bioactive compounds on soil-plant systems and subsequent risk to animal and human health

Plant-cyanobacteria interactions occur in different ways and at many different levels, both beneficial and harmful. Plant-cyanobacteria interactions, as a beneficial symbiosis, have long been demonstrated in rice-growing areas (Poaceae) where the most efficient nitrogen-fixing cyanobacteria are present in paddies. Moreover, cyanobacteria may in turn produce and/or secrete numerous bioactive compounds that have plant growth-promoting abilities or that may make the plant more resistant to abiotic or biotic stress. In recent years, there has been a growing worldwide interest in the use of cyanobacterial biomass as biofertilizers to replace chemical fertilizers, in part to overcome increasing organic-farming demands. However, the potential presence of harmful cyanotoxins has delayed the use of such cyanobacterial biomass, which can be found in large quantities in freshwater ecosystems around the world. In this review, we describe the existing evidence for the positive benefit of plant-cyanobacteria interactions and discuss the use of cyanobacterial biomass as biofertilizers and its growing worldwide interest. Although mass cyanobacterial blooms and scums are a current and emerging threat to the degradation of ecosystems and to animal and human health, they may serve as a source of numerous bioactive compounds with multiple positive effects that could be of use as an alternative to chemical fertilizers in the context of sustainable development.

The Toxicity Testing of Cyanobacterial Toxins In Vivo and In Vitro by Mouse Bioassay: A Review

Different biological methods based on bioactivity are available to detect cyanotoxins, including neurotoxicity, immunological interactions, hepatotoxicity, cytotoxicity, and enzymatic activity. The mouse bioassay is the first test employed in laboratory cultures, cell extracts, and water bloom materials to detect toxins. It is also used as a traditional method to estimate the LD50. Concerning the ease of access and low cost, it is the most common method for this purpose. In this method, a sample is injected intraperitoneally into adult mice, and accordingly, they are assayed and monitored for about 24 hours for toxic symptoms. The toxin can be detected using this method from minutes to a few hours; its type, e.g., hepatotoxin, neurotoxin, etc., can also be determined. However, this method is nonspecific, fails to detect low amounts, and cannot distinguish between homologues. Although the mouse bioassay is gradually replaced with new chemical and immunological methods, it is still the main technique to detect the bioactivity and efficacy of cyanotoxins using LD50 determined based on the survival time of animals exposed to the toxin. In addition, some countries oppose animal use in toxicity studies. However, high cost, ethical considerations, low-sensitivity, non-specificity, and prolonged processes persuade researchers to employ chemical and functional analysis techniques. The qualitative and quantitative analyses, as well as high specificity and sensitivity, are among the advantages of cytotoxicity tests to investigate cyanotoxins. The present study aimed at reviewing the results obtained from in-vitro and in-vivo investigations of the mouse bioassay to detect cyanotoxins, including microcystins, cylindrospermopsin, saxitoxins, etc.

Cyanotoxins and the Nervous System

Cyanobacteria are capable of producing a wide range of bioactive compounds with many considered to be toxins. Although there are a number of toxicological outcomes with respect to cyanobacterial exposure, this review aims to examine those which affect the central nervous system (CNS) or have neurotoxicological properties. Such exposures can be acute or chronic, and we detail issues concerning CNS entry, detection and remediation. Exposure can occur through a variety of media but, increasingly, exposure through air via inhalation may have greater significance and requires further investigation. Even though cyanobacterial toxins have traditionally been classified based on their primary mode of toxicity, increasing evidence suggests that some also possess neurotoxic properties and include known cyanotoxins and unknown compounds. Furthermore, chronic long-term exposure to these compounds is increasingly being identified as adversely affecting human health.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Complex Control of Striatal Neurotransmission by Nicotinic Acetylcholine Receptors via Excitatory Inputs onto Medium Spiny Neurons

The prevalence of nicotine dependence is higher than that for any other substance abuse disorder; still, the underlying mechanisms are not fully established. To this end, we studied acute effects by nicotine on neurotransmission in the dorsolateral striatum, a key brain region with respect to the formation of habits. Electrophysiological recordings in acutely isolated brain slices from rodent showed that nicotine (10 nm to 10 μm) produced an LTD of evoked field potentials. Current-clamp recordings revealed no significant effect by nicotine on membrane voltage or action potential frequency, indicating that the effect by nicotine is primarily synaptic. Nicotine did not modulate sIPSCs, or the connectivity between fast-spiking interneurons and medium spiny neurons, as assessed by whole-cell recordings combined with optogenetics. However, the frequency of sEPSCs was significantly depressed by nicotine. The effect by nicotine was mimicked by agonists targeting α7- or α4-containing nAChRs and blocked in slices pretreated with a mixture of antagonists targeting these receptor subtypes. Nicotine-induced LTD was furthermore inhibited by dopamine D2 receptor antagonist and occluded by D2 receptor agonist. In addition, modulation of cholinergic neurotransmission suppressed the responding to nicotine, which might reflect upon the postulated role for nAChRs as a presynaptic filter to differentially govern dopamine release depending on neuronal activity. Nicotine-induced suppression of excitatory inputs onto medium spiny neurons may promote nicotine-induced locomotor stimulation and putatively initiate neuroadaptations that could contribute to the transition toward compulsive drug taking.SIGNIFICANCE STATEMENT To decrease smoking, prevalence factors that may contribute to the development of nicotine addiction need to be identified. The data presented here show that nicotine suppresses striatal neurotransmission by selectively reducing the frequency of excitatory inputs to medium spiny neurons (MSNs) while rendering excitability, inhibitory neurotransmission, and fast-spiking interneuron-MSN connectivity unaltered. In addition, we show that the effect displayed by nicotine outlasts the presence of the drug, which could be fundamental for the addictive properties of nicotine. Considering the inhibitory tone displayed by MSNs on dopaminergic cell bodies and local terminals, nicotine-induced long-lasting depression of striatal output could play a role in behavioral transformations associated with nicotine use, and putatively elicit neuroadaptations underlying compulsive drug-seeking habits.

Estrogenic activity of cylindrospermopsin and anatoxin-a and their oxidative products by FeIII-B*/H2O2

The cyanotoxins released into waters during cyanobacterial blooms can pose serious hazards to humans and animals. Apart from their toxicological mechanisms, cyanotoxins have been shown to be involved in estrogenic activity by in vivo and in vitro assays; however, there is limited information on the change in estrogenicity of cyanotoxins following chemical oxidation. In this study, the estrogenic activity of cylindrospermopsin (CYL) and anatoxin-a (ANA) at concentrations ranging from 2.4 × 10^-7 M to 2.4 × 10^-12 M (CYL) and 7.1 × 10^-6 M to 7.1 × 10^-11 M (ANA), and after treatment by the Fe^III-B*/H₂O₂ catalyst system, was investigated by the yeast estrogen screen (YES) assay. The results indicate that CYL and ANA acted as agonists in the YES assay (CYL logEC₅₀ = -8.901; ANA logEC₅₀ = -6.789), their binding affinity to estrogen receptors is associated with their intrinsic properties, including ring structures and toxicant properties. CYL and ANA were shown to simulate endocrine disrupting chemicals (EDCs) to modulate the 17β-estradiol-induced estrogenic activity, resulting in non-monotonic dose responses. The treated CYL showed a significantly altered estrogenicity compared to the untreated CYL (T₍₂₎ = 8.168, p ≤ .05), while the estrogenicity of the treated ANA was not significantly different to the untreated ANA (T₍₂₎ = 1.295, p > .05). Intermediate products generated from CYL and ANA oxidized by Fe^III-B*/H₂O₂ were identified using Q-Exactive Tandem Mass Spectrometry (LC-MS/MS). Treatment with Fe^III-B*/H₂O₂ yielded open-ring by-products which likely resulted in CYL's reduced binding affinity to estrogen receptors. The insignificant change in the estrogenicity of treated ANA was possibly a result of its multiple ring structure products, which were likely able to bind to estrogen receptors. The comparisons for the estrogenicity of these cyanotoxins before and after Fe^III-B*/H₂O₂ treatment suggest that the reductions in estrogenicity achieved by oxidation were dependent on the levels of cyanotoxins removed, as well as the estrogenicity of the degradation products. This is the first study on the change in the estrogenicity of CYL and ANA upon oxidation by Fe^III-B*/H₂O₂, a high activity catalyst system.

Risk to human health associated with the environmental occurrence of cyanobacterial neurotoxic alkaloids anatoxins and saxitoxins

Cyanobacteria are ubiquitous photosynthetic micro-organisms forming blooms and scums in surface water; among them some species can produce cyanotoxins giving rise to some concern for human health and animal life. To date, more than 65 cyanobacterial neurotoxins have been described, of which the most studied are the groups of anatoxins and saxitoxins (STXs), comprising many different variants. In freshwaters, the hepatotoxic microcystins represent the most frequently detected cyanotoxin: on this basis, it could appear that neurotoxins are less relevant, but the low frequency of detection may partially reflect an a priori choice of target analytes, the low method sensitivity and the lack of certified standards. Cyanobacterial neurotoxins target cholinergic synapses or voltage-gated ion channels, blocking skeletal and respiratory muscles, thus leading to death by respiratory failure. This review reports and analyzes the available literature data on environmental occurrence of cyanobacterial neurotoxic alkaloids, namely anatoxins and STXs, their biosynthesis, toxicology and epidemiology, derivation of guidance values and action limits. These data are used as the basis to assess the risk posed to human health, identify critical exposure scenarios and highlight the major data gaps and research needs.

Toxicology and detection methods of the alkaloid neurotoxin produced by cyanobacteria, anatoxin-a

Freshwater resources are under stress due to naturally occurring conditions and human impacts. One of the consequences is the proliferation of cyanobacteria, microphytoplankton organisms that are capable to produce toxins called cyanotoxins. Anatoxin-a is one of the main cyanotoxins. It is a very potent neurotoxin that was already responsible for some animal fatalities. In this review we endeavor to divulgate much of the internationally published information about toxicology, occurrence and detection methods of anatoxin-a. Cyanobacteria generalities, anatoxin-a occurrence and production as well as anatoxin-a toxicology and its methods of detection are the aspects focused in this review. Remediation of anatoxin-a occurrence will be addressed with a public health perspective. Final remarks call the attention for some important gaps in the knowledge about this neurotoxin and its implication to public health. Alterations of aquatic ecosystems caused by anatoxin-a is also addressed. Although anatoxin-a is not the more frequent cyanotoxin worldwide, it has to be regarded as a health risk that can be fatal to terrestrial and aquatic organisms because of its high toxicity.

Mediation of glutamatergic receptors and nitric oxide on striatal dopamine release evoked by anatoxin-a. An in vivo microdialysis study

In this work, the involvement of ionotropic glutamatergic receptors and nitric oxide on striatal dopamine release induced by anatoxin-a was investigated in conscious and freely-moving rats. To study the participation of glutamatergic receptors, the effects of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine (MK-801) and d(-)-2-amino-5-phosphonopentanoic acid (APV), were examined. The perfusion of 3.5 mM anatoxin-a increased the extracellular dopamine levels to 701% relative to the basal. When CNQX was administered with 3.5 mM anatoxin-a, the increase of dopamine levels was 29% smaller than that observed with anatoxin-a alone. When MK-801 and APV were administered, the effect of anatoxin-a was attenuated 26% and 25% respectively in terms of that observed with anatoxin-a alone. And with CNQX plus MK-801, the effect of anatoxin-a was 53% inhibited in terms of the effect of anatoxin-a alone. These results suggest that the striatal dopamine release induced by anatoxin-a is partly mediated by activation of both ionotropic glutamatergic receptors. Since the neuronal form of nitric oxide synthase (nNOS) produces nitric oxide (NO) primarily in response to activation of NMDA receptors, it was tested if NO could play any role in the effect of anatoxin-a. Treatment with NOS inhibitors, L-nitro-arginine methyl ester (L-NAME) and d(-)-2-amino-5-phosphonopentanoic acid (7-NI), induced decreased anatoxin-a effects of 22% and 26% respectively. In conclusion, the present in vivo results demonstrate that anatoxin-a induced an indirect activation of ionotropic glutamatergic receptors (NMDA and AMPA/kainite receptors), which stimulate striatal dopamine release. On the other hand, activation of NMDA receptors may elicit NO increased levels enhancing dopamine release.

In vivo neurochemical characterization of Anatoxin-a evoked dopamine release from striatum

Anatoxin-a (AnTx) is a natural neurotoxin, which acts as a potent and stereoselective agonist at the nicotinic acetylcholine receptors. The in vivo actions of the AnTx on dopamine (DA) release are scarcely characterized. The aim of this study was to determine the neurochemical bases for AnTx-induced striatal DA release, using the brain microdialysis technique, in freely moving rats. Local application of AnTx (3.5 mM) through the microdialysis probe produced an increase in striatal DA levels (701 +/- 51% with respect to basal values). The effect of infusion of AnTx in Ca(2+)-free Ringer medium, in Na(+)-free Ringer medium and with TTX in the medium, was inhibited. Also, reserpine pre-treatment blocked the action of AnTx on striatal DA levels. To investigate the involvement of the DA transporter, the effects of AnTx were observed in the presence of nomifensine. The coadministration of AnTx and nomifensine evoked an additive effect on striatal DA levels. The latter results show that the DA release is not mediated by a decreased DA uptake. Taken as a whole, these results suggest that the effects of AnTx are predominantly mediated by an exocytotic mechanism, Ca(2+)-, Na(+)- and TTX-dependent, and not by a mechanism mediated by the DA transporter.