Benchmark IV axl90

Neurotoxin

Neurotoxin (from Ancient Greek: νευρών neuron “sinew” and τοξικόν toxikon “toxin”) is a classification given to an extensive category of exogenous chemical neurological insults (Spencer 2000) which can adversely affect function in both developing and mature nervous tissue (Olney 2002), though the term can also be used to classify endogenous compounds which when abnormally concentrated can also prove neurologically toxic (Spencer 2000). Though neurotoxins may commonly be viewed as neurologically destructive, they have significant applications in studying the nervous system (Kiernan 2005). Common examples of neurotoxins include lead (Lidsky 2003), ethanol (Heaton 2000), phencyclidine (PCP), ketamine, glutamate (Choi 1987), nitric oxide (NO) (Zhang 1994), botulinum toxin (Rosales 1996), tetanus toxin (Simpson 1986), and tetrodotoxin (Kiernan 2005). Neurotoxin activity is often characterized by the ability to inhibit neuron control over ion concentrations across the cell membrane (Kiernan 2005), or inter-neuron communication across a synapse (Arnon 2001). Local pathology of neurotoxin exposure often includes neuron excitotoxicity or apoptosis (Dikranian 2001), but can also include glial cell damage (Deng 2003). Macroscopic manifestations of neurotoxin exposure can include widespread central nervous system damage such as mental retardation|retardation (Olney 2002), persistent memory impairments (Jevtovic-Todorovic 2003), epilepsy, and dementia (Nadler 1978). Additionally, neurotoxin-mediated peripheral nervous system damage such as neuropathy or myopathy is common. Support has been shown for a number of treatments aimed at attenuating neurotoxin-mediated injury, such as antioxidant (Heaton 2000), antitoxin (Thyagarajan 2009) and ethanol (Takadera 1990) administration.

Background

Illustration of typical Multipolar neuron.

Exposure to neurotoxins in society is not new as civilizations have been exposed to neurologically destructive compounds for thousands of years. One notable example is possible significant lead exposure during the Roman Empire resulting from the development of extensive plumbing networks (Hodge 2002). In part, neurotoxins have been part of human history because of the fragile and susceptible nature of the nervous system, making it highly prone to disruption. The nervous tissue found in the brain, spinal cord, and periphery comprises an extraordinarily complex biological system that largely defines many of the unique traits of individuals. However, as with any highly complex system, even small perturbations to its environment can lead to significant functional disruptions. Properties leading to the susceptibility of nervous tissue include a high surface area of neurons, a high lipid content which retains lipophilic toxins, high blood flow to the brain inducing increased effective toxin exposure, and the persistence of neurons leading to compounding of damages (Dobbs 2009). As such, the nervous system has a number of mechanisms designed to protect it from internal, and external insults. The blood brain barrier (BBB) is one critical example of protection which prevents toxins and other adverse compounds from reaching the brain (Widmaier 2008). As the brain requires nutrient entry and waste removal, it is perfused by blood flow, however blood can carrier a number of ingested toxins which would induce significant neuron death if ever coming into contact with the nervous tissue. Thus, protective cells termed astrocytes surround the capillaries in the skull and absorb nutrients from the blood and subsequently transporting them to the neurons, effectively isolating the brain from a number of potential chemical insults (Widmaier 2008).

Astrocytes surrounding capillaries in the Brain to form the Blood Brain Barrier.

This barrier creates a tight hydrophobic layer around the capillaries in the brain, inhibiting the transport of a large or hydrophilic compounds. In addition to the BBB the choroid plexus provides a layer of protection against toxin absorption in the brain. The choroid plexusus are vascularized layers of tissue found in the third, fourth, and lateral ventricles of the brain, which through the function of their ependymal cells, are responsible for the synthesis of cerebrospinal fluid (CSF) (Martini 2009). Importantly, through selective passage of ions and nutrients while trapping heavy metals such as lead, the choroid plexuses maintain a strictly regulated environment which contains the brain and spinal cord (Widmaier 2008, Martini 2009).

Choroid Plexus.

By being hydrophobic and small or inhibiting astrocyte function, some compounds including certain neurotoxins are able to penetrate into the brain and induce significant damage. In modern times, scientists and physicians have been presented with the challenge of identifying and treating neurotoxins, which has resulting in a growing interest in both neurotoxicology research and clinical studies (Costa 2011). Though clinical neurotoxicology is largely a burgeoning field, extensive inroads have been made in identification and classification of many environmental neurotoxins, such that there are roughly between 750 and 1000 known potentially neurotoxic compounds (Dobbs 2009). Due to the critical importance of finding neurotoxins in common environments, specific protocols have been developed by the EPA for testing and determining neurotoxic effects of compounds (USEPA 1998). Additionally, in-vitro systems have increased in use as they provide significant improvements over the more common in-vivo systems of the past, such as tractable, uniform environments, and the elimination of contaminating effects of systemic metabolism (Costa 2011). However, in-vitro systems have presented problems as it has been difficult to properly replicate the complexities of the nervous system, such as the interactions between supporting astrocytes and neurons in creating the BBB (Harry 1998). To even further complicate the process of determining neurotoxins when testing in-vitro, neurotoxicity and cytotoxicity may be difficult to distinguish, as exposing neurons directly to compounds may not be possible in-vivo as it is in-vitro. Additionally, the response of cells to chemicals may not accurately convey a distinction between neurotoxins and cytotoxins, as symptoms like oxidative stress or skeletal modifications may occur in response to either (Gartlon 2006). Recently in an effort to address this complication, neurite outgrowths (either axonal or dendritic) in response to applied compounds have been proposed as a more accurate distinction between true neurotoxins and cytotoxins in an in-vitro testing environment. However, due to the significant inaccuracies associated with this process, it has been slow in gaining widespread support (Mundy 2008). Additionally, biochemical mechanisms have become more widely used in neurotoxin testing, such that compounds can be screened for sufficiency to induce cell mechanism interference, such as the inhibition of acetylcholinesterase capacity of organophosphates (includes DDT and sarin gas) (Lotti 2005). Though methods of determining neurotoxicity still require significant development, the identification of deleterious compounds and and toxin exposure symptoms has undergone significant improvement.

Applications in Neuroscience

Though diverse in chemical properties and functions, neurotoxins are bound by the common similarity of acting by some mechanism leading to either the disruption or destruction of necessary components within the nervous system. However, neurotoxins by their very design have shown highly useful in the field of neuroscience. As the nervous system in most organisms is both highly complex and necessary for survival, it has naturally become a significant target for predators or in self defense. As venomous organisms often use their neurotoxins to subdue a predator or prey very rapidly, toxins have evolved to become highly specific to their target channels such that toxin does not readily of the target (Adams 2003). As such neurotoxins provide an effective means by which certain elements of the nervous system may be accurately and efficiently targeted. An early example of neurotoxin based targeting used radiolabeled tetrodotoxin to assay sodium channels and obtain precise measurements about their concentration along nerve membranes (Adams 2003). Likewise through isolation of certain channel activities, neurotoxins have provided the ability to improve the original Hodgkin-Huxley model of the neuron in which it was theorized that single generic sodium and potassium channels could account for most nervous tissue function (Adams 2003). From this basic understanding, the use of common compounds such as tetrodotoxin, tetraethylammonium, and bungarotoxins have lead to a much deeper understanding of the distinct ways in which individual neurons may behave.

Mechanisms of Activity

As neurotoxins are compounds which adversely affect the nervous system, a number of mechanisms through which they function will be through the inhibition of neuron cellular processes. These inhibited processes can range from membrane depolarization mechanisms to inter-neuron communication. By inhibiting the ability for neurons to perform their expected intracellular functions, or pass a signal to a neighboring cell, neurotoxins can induce systemic nervous system arrest as in the case of Botulinum toxin (Arnon 2001), or even nervous tissue death (Brocardo 2011). The time required for the onset of symptoms upon neurotoxin exposure can vary between different toxins, being on the order of hours for Botulinum toxin (Thyagarajan 2009) and years for lead (Lewendon 2001).

Neurotoxin Classification	Neurotoxin
Na Channel Inhibitors	Tetrodotoxin (Kiernan 2005)
K Channel Inhibitors	Tetraethylammonium (Haghdoost-Yazdi 2011)
Cl Channel Inhibitors	Chlorotoxin (DeBin 1993), Curare (Koller 1988)
Ca Channel Inhibitors	Conotoxin (McClesky 1987)
Vesicular Communication Inhibitors	Botulinum Toxin (Garcia-Rodriguez 2011), Tetanus Toxin (Williamson 1996), Bungarotoxin (Dutrere 2006)
Blood Brain Barrier Inhibitors	Aluminum (Banks 1988), Mercury (Aschner 1990)
Cytoskeleton Interference	Arsenic (DeFuria 2006), Ammonia (Konopacka 2009)
Ca-Mediated Cytotoxicity	Lead (Bressler 1999)
NMDA Inhibitors	Ethanol (Lovinger 1989)
Endogenous Neurotoxin Sources	Nitric Oxide (Garthwaite 1988), Glutamate (Choi 1990)

Sodium Channel Inhibitors

Tetrodotoxin

The Puffer Fish; a well known Tetrodoxin producer.

Tetrodotoxin (TTX) is a poison produced by organisms belonging to the Tetraodontidae order, which includes the puffer fish, ocean sunfish, and porcupine fish (Chowdhury 2007). Within the puffer fish, which is a common delicacy especially in Japan, TTX is found in the liver, gonads, ovaries, intestines, and skin (Ahasan 2004, Kiernan 2005). TTX can be fatal if consumed, and has become common form of poisoning in many countries (How 2003). Common symptoms of TTX consumption include paraesthesia, (often restricted to the mouth and limbs) muscle weakness, nausea, and vomiting (Chowdhury 2007) and often manifest within 30 minutes of ingestion (Lau 1995). The primary mechanism by which TTX is toxic is through the inhibition of sodium channel function, which reduces the functional capacity of neuron communication. This inhibition largely affects a susceptible subset of sodium channels known as TTX-sensitive, (TTX-s) which also happens to be largely responsible for the sodium current that drives the depolarization phase of neuron action potentials (Kiernan 2005).

Inhibited signaling response resulting from neuron exposure to tetrodotoxin.

TTX-resistant (TTX-r) is another form of sodium channel which has limited sensitivity to TTX, and is largely found in small diameter axons such as those found in nociception neurons (Kiernan 2005). When significant levels of TTX is ingested, it will bind sodium channels on neurons and reduce their membrane permeability to sodium. This results in an increased effective threshold of required excitatory signals in order to induce an action potential in a postsynaptic neuron (Kiernan 2005). The effect of this increased signaling threshold is a reduced excitability of postsynaptic neurons, and subsequent loss of motor and sensory function which can result in paralysis and death. Though assisted ventilation may increase the chance of survival after TTX exposure, there is currently no antitoxin. However, the use of the acetylcholinesterase inhibitor Neostigmine, or the acetylcholine antagonist Atropine can increase sympathetic nerve activity enough to improve survival after TTX exposure (Chowdhury 2007).

Potassium Channel Inhibitors

Tetraethylammonium

Tetraethylammonium (TEA) is a compound that like a number of neurotoxins was first identified through its damaging effects to the nervous system and shown to have the capacity of inhibiting the function of motor nerves and thus the contraction of the musculature in a manner similar to that of curare (Standfield 1983). Additionally, through chronic TEA administration, muscular atrophy would be induced (Standfield 1983). It was later determined that TEA functions in-vivo primarily through its ability to inhibit both the potassium channels responsible for the delayed rectifier seen in an action potential and some population of calcium-dependent potassium channels (Haghdoost-Yazdi 2011). It is this capability to inhibit potassium flux in neurons that has made TEA one of the most important tools in neuroscience. It has been hypothesized that the ability for TEA to inhibit potassium channels is derived from its similar space-filling structure (Standfield 1983). What makes TEA very useful for neuroscientists, is its specific ability to eliminate potassium channel activity, thereby allowing the study of neuron response contributions of other ion channels such as voltage gated sodium channels (Roed 1989). In addition to its many uses in neuroscience research, TEA has been shown to perform as an effective treatment of Parkinson’s Disease through its ability to limit the progression of the disease (Haghdoost-Yasdi 2011).

Chlorine Channel Inhibitors

Chlorotoxin

Chlorotoxin (Cltx) is a the active compound found in scorpion venom, and is primarily toxic because of its ability to inhibit the conductance of chlorine channels (DeBin 1993). Ingestion of lethal volumes Cltx results in paralysis through this ion channel disruption. Similar to botulinum toxin, Cltx has been shown to possess significant therapeutic value. Evidence has been shown that Cltx can inhibit the ability for gliomas to infiltrate healthy nervous tissue in the brain, significantly reducing the potential invasive harm caused by tumors (Deshane 2003, Soroceanu 1998).

Curare

The term curare is often one of ambiguity, as it has been used to describe a number of poisons which at the time of naming were understood differently than in modern times, where the characterization has meant poisons used by South American tribes on arrows or darts, though has matured to specify a specific categorization of poisons which act on the neuromuscular junction to inhibit signaling and thus induce muscle relaxation (Bisset 1992). The neurotoxin category contains a number of distinct poisons, though all were originally purified from plants originated in South America (Bisset 1992). The effect with which ingested curare poison is usually associated is muscle paralysis and resultant death (Schlesinger 1946). Curare notably functions to inhibit GABA-induced chlorine channels, resulting in reduced ability for chlorine to flux (Koller 1988). As such, the neurotoxin is capable of significantly reducing neuromuscular junction signaling, an effect which has resulted in its use by anesthesiologists to produce muscular relaxation (Griffith 1942).

Calcium Channel Inhibitors

Conotoxin

This is produced by the marine cone snail (McClesky 1987).

Vesicular Communication Inhibitors

Botulinum Toxin

Mechanism of Botulinum Toxin neurotoxicity.

Botulinum Toxin (BTX) is group of neurotoxins consisting of seven distinct compounds, referred to as BTX-A,B,C,D,E,F,G which are produced by the bacterium Clostridium Botulinum, and lead to muscular paralysis (Brin 1998). A notably unique feature of BTX is its relatively common therapeutic use in treating dystonia and spasticity disorders (Brin 1998) as well as in inducing muscular atrophy (Rosales 1996) despite being the most poisonous substance known (Thyagarajan 2009). BTX functions peripherally to inhibit acetylcholine (ACh) release at the neuromuscular junction through degradation of the SNARE proteins required for ACh vesicle-membrane fusion (Garcia-Rodriguez 2011). As the toxin is highly biologically active, an estimated dose of 1μg/kg body weight is sufficient to induce an insufficient tidal volume and resultant death by asphyxiation (Arnon 2001). Due to its high toxicity, BTX antitoxins have been an active area of research. It has been shown that capsaicin (active compound responsible for heat in chili peppers) can bind the TRPV1 receptor expressed on cholinergic neurons and inhibit the toxic effects of BTX (Thyagarajan 2009).

Tetanus Toxin

Tetanus neurotoxin (TeNT) is a compound that functionally reduces inhibitory transmissions in the nervous system resulting in muscular tetany.

Clostridium Tetani Bacteria.

TeNT is similar to BTX, and is in fact highly similar in structure and origin, both belonging to the same category of clostridial neurotoxins (Simpson 1986). Like BTX, TeNT inhibits inter-neuron communication by means of vesicular neurotransmitter (NT) release (Williamson 1996). One notable difference between the two compounds is that while BTX inhibits muscular contractions, TeNT induces them. Though both toxins inhibit vesicle release at neuron synapses, the reason for this different manifestation is that BTX functions mainly in the peripheral nervous system (PNS) while TeNT is largely active in the central nervous system (CNS) (Montecucco 1986). This is a result of TeNT migration to through motor neurons to the inhibitory neurons of the spinal cord after entering through endocytosis (Pirazzini 2011). This results in a loss of function in inhibitory neurons within the CNS resulting in systemic muscular contractions. Similar to the prognosis of a lethal dose of BTX, TeNT leads to paralysis and subsequent suffocation (Pirazzini 2011).

Bungarotoxin

Bungarotoxin is a compound with known interaction with nicotinic acetylcholine receptors (nAChRs) which constitute a family of ion channels, whose activity is triggered by neurotransmitter binding (Tsetlin 2003). Bungarotoxin is produced in a number of different forms, though one of the commonly used forms is the long chain alpha form, α-bungarotoxin which is isolated from the banded krait snake (Dutrere 2006). Though extremely toxic if ingested, α-bungarotoxin has shown extensive usefulness in neuroscience as it is particularly adept at isolating nAChRs due to its high affinity to the receptors (Dutrere 2006). As there are multiple forms of bungarotoxin, there are different forms of nAChRs to which they will bind, and α-bungarotoxin is particularly specific for α7-nAChR (Liu 2008). This α7-nAChR functions to allow calcium ion influx into cells, and thus when blocked by ingested bungarotoxin will produce damaging effects, as ACh signaling will be inhibited (Liu 2008). Likewise, the use of α-bungarotoxin can be very useful in neuroscience if it is desirable to block calcium flux in order to isolate effects of other channels. Additionally, different forms of bungarotoxin may be useful for studying inhibited nAChRs and their resultant calcium ion flow in different systems of the body. For example α-bungarotoxin is specific for nAChRs found in the musculature and κ-bungarotoxin is specific for nAChRs found in neurons (Hue 2007).

Blood Brain Barrier Inhibitors

Aluminum

Neurotoxic behavior of aluminum is known to occur upon entry into the circulatory system, where it can migrate to the brain and inhibit some of the crucial functions of the blood brain barrier (BBB) (Banks 1988). A loss of function in the BBB can produce significant damage to the neurons in the CNS, as the barrier protecting the brain from other toxins found in the blood will no longer be capable of such action. Though the metal is known to be neurotoxic, effects are usually restricted to patients incapable of removing excess ions from the blood, such as those experiencing renal failure (King 1981). Patients experiencing aluminum toxicity can exhibit symptoms such as impaired learning and reduced motor coordination (Rabe 1982). Additionally, systemic aluminum levels are known to increase with age, and have been shown to correlate with Alzheimer’s Disease, implicating it as a neurotoxic causative compound of the disease (Walton 2006).

Mercury

Mercury is capable of inducing CNS damage by migrating into the brain by crossing the BBB (Aschner 1990). Mercury exists in a number of different compounds, though methylmercury (MeHg) is the only significantly neurotoxic form (Aschner 1990). MeHg is usually acquired through consumption of seafood, as it tends to concentrate in organisms high on the food chain (Chan 2011). It is known that the mercuric ion inhibits amino acid (AA) and glutamate (Glu) transport, potentially leading to excitotoxic effects (Brookes 1988).

Cytoskeleton Interference

Arsenic

Arsenic is a neurotoxin commonly found concentrated in areas exposed to agricultural runoff, mining, and smelting sites (Martinez-Finley 2011). One of the effects of arsenic ingestion during periods characterized by a developing nervous system is the inhibition of neurite growth (Liu 2009) which can occur both in PNS and the CNS (Vahidnia 2007). This neurite growth inhibition can often lead to defects in neural migration, and significant morphological changes of neurons during development (Rocha 2011), and neural tube defects are often seen in neonates (Brender 2005). As a metabolite of arsenic, arsenite is formed after ingestion of arsenic and has shown significant toxicity to neurons within about 24 hours of exposure. The mechanism of this cytotoxicity functions through arsenite induced increases in intracellular calcium ion levels within neurons, which may subsequently reduce mitochondrial transmembrane potential which activates caspases, triggering cell death (Rocha 2011). Another known function of arsenite is its destructive nature towards the cytoskeleton through inhibition of neurofilament transport (DeFuria 2006). This is particularly destructive as neurofilaments are used in basic cell structure and support. However, lithium administion has shown promise in restoring some of the lost neurofilament motility (DeFuria 2007). Additionally, similar to other neurotoxin treatments, the administration of certain antioxidants has shown some promise in reducing neurotoxicity of ingested arsenic (Rocha 2011).

Ammonia

Ammonia toxicity is often seen through two routes administration, either through consumption or through endogenous ailments such as hepatic failure (Matsuoka 1991, Buzanska 2000).

An Astrocyte, a cell notable for maintaining the blood brain barrier.

One notable case in which ammonia toxicity is common is in response to cirrhosis of the liver which results in hepatic encephalopathy, and can result in cerebral edema (Haussinger 2006). This cerebral edema can be the result of nervous cell remodeling. Resulting from increased concentrations, ammonia activity in-vivo has been shown to induce swelling of astrocytes in the brain through a produced increase in cGMP synthesis within the cells which leads to PKG-mediated cytoskeletal modifications (Konopacka 2009). The resultant effect of this toxicity can be reduced brain energy metabolism, and function. Importantly, the toxic effects of ammonia on astrocyte remodling can be reduced through administration of L-carnitine (Matsuoka 1991). Additionally, this astrocyte remodeling appears to be mediated through ammonia-induced mitochondrial permeability transition. This mitochondrial transition is a direct result of glutamine activity; a compound which forms from ammonia in-vivo (Norenberg 2004). Importantly, it has been shown that administration of antioxidants or glutaminase inhibitor can reduce this mitochondrial transition, and potentially also astrocyte remodling (Norenberg 2004).

Calcium-Mediated Cytotoxicity

Lead

A lead pipe, one of the more common sources of ingested lead.

Lead is a potent neurotoxin whose toxicity has been at least recognized for thousands of years (Lidskey 2003). Though neurotoxic effects for lead are found in both adults and young children, the developing brain is particularly susceptible to lead-induced harm, effects which can include apoptosis and excitotoxicity (Lidskey 2003). An underlying mechanism by which lead is able to cause harm is its ability to be transported by calcium ATPase pumps across the BBB, allowing for direct contact with the fragile cells within the central nervous system (Bradbury 1993). Neurotoxicity results from this ability for lead to act in a similar manner to calcium ions, as concentrated lead will lead to cellular uptake of calcium which disrupts cellular homeostasis and induces apoptosis (Bressler 1999). It is this intracellular calcium increase that activates protein kinase C (PKC), which manifests as learning deficits in children as a result of early lead exposure (Bressler 1999). In addition to inducing apoptosis, lead inhibits interneuron signaling through the disruption of calcium-mediated neurotransmitter release (Lasley 1999).

NMDA Inhibitors

Ethanol

Chronic ethanol ingestion has been shown to induce reorganization of cellular membrane constituents, favoring a bilayer marked by increased membrane concentrations of cholesterol and saturated fat (Leonard 1986). This is important as neurotransmitter transport can be impaired through vesicular transport inhibition, resulting in diminished neural network function. One significant example of reduced inter-neuron communication is the ability for ethanol to inhibit NMDA receptors in the hippocampus resulting in reduced LTP and memory acquisition (Lovinger 1989). However, with chronic ethanol intake, the susceptibility of these NMDA receptors to induce LTP increases in the mesolimbic dopamine neurons increases in an inositol 1,4,5-triphosphate (IP3) dependent manner (Bernier 2011). This reorganization may lead to neuronal cytotoxicity both through hyperactivation of postsynaptic neurons, and through induced addiction to continuous ethanol consumption. In addition to the neurotoxic effects of ethanol in mature organisms, chronic ingestion is capable of inducing severe developmental defects. Evidence was first shown in 1973 of a connection between chronic ethanol intake by mothers and defects in their offspring (Jones 1973).

Example of individual with Fetal Alcohol Syndrome.

This work was responsible for creating the classification of fetal alcohol syndrome; a disease characterized by common morphogenesis aberrations such as defects in craniofacial formation, limb development, and cardiovascular formation. The magnitude of ethanol neurotoxicity in fetuses leading to fetal alcohol syndrome has been shown to be dependent on antioxidant levels in the brain such as vitamin E (Mitchell 1999). As the fetal brain is relatively fragile and susceptible to induced stresses, severe deleterious effects of alcohol exposure can be seen in important areas such as the hippocampus and cerebellum. The severity of these effects is directly dependent upon the amount and frequency of ethanol consumption by the mother, and the stage in development of the fetus (Gil-Mohapel 2010). It is known that ethanol exposure results in reduced antioxidant levels, mitochondrial dysfunction (Chu 2007), and subsequent neuronal death, seemingly as a result of increased generation of reactive oxidative species (ROS) (Brocardo 2011) This is a plausible mechanism, as there is a reduced presence in the fetal brain of antioxidant enzymes such as catalase and peroxidase (Bergamini 2004). In support of this mechanism, administration of high levels of dietary vitamin E results in reduced or eliminated ethanol-induced neurotoxic effects in fetuses (Heaton 2000).

One example is the mechanism of ethanol neurotoxicity through its inhibition of the NMDA glutamate receptor. NMDA has been shown to play an important role in long-term potentiation (LTP) and consequently memory formation (Davis 1992). However, it has been shown that ethanol directly reduces intracellular Ca2+ accumulation through inhibited NMDA receptor activity, and thus reduced capacity for the occurrence of LTP (Takadera 1990).

Endogenous Neurotoxin Sources

Unlike most common sources of neurotoxins which are acquired by the body through ingestion, endogenous neurotoxins originate and exert their effects both in-vivo. Additionally, though most venoms and exogenous neurotoxins will rarely possess useful in-vivo capabilities, endogenous neurotoxins are commonly used by the body in useful and healthy ways, such as nitric oxide which is used in cell communication (Iadecola 1998). It is often only when these endogenous compound become highly concentrated that they lead to dangerous effects (Choi 1987).

Nitric Oxide

Though nitric oxide (NO) is commonly used by the nervous system in inter-neuron communication and signaling, it can be active in mechanisms leading to ischemia in the Cerebrum (Iadecola 1998). The neurotoxicity of NO is based on its importance in glutamate excitotoxicity, as NO is generated in a calcium-dependent manner in response to glutamate mediated NMDA activation, which occurs at an elevated rate in glutamate excitotoxicity (Garthwaite 1988). Though NO facilitates increased blood flow to potentially ischemic regions of the brain, it is also capable of increasing oxidative stress (Beckman 1990), inducing DNA damage and apoptosis (Bonfoco 1995). Thus an increased presence of NO in an ischemic area of the CNS can produce significantly toxic effects.

Glutamate

Glutamate, like nitric oxide is an endogenously produced compound used by neurons to perform normally, being present in small concentrations throughout the gray matter of the CNS (Choi 1987). One of the most notable uses of endogenous glutamate is its functionality as an excitatory neurotransmitter (Choi 1990). However, when concentrated, glutamate becomes toxic to surrounding neurons. This toxicity can be both a result of direct lethality of glutamate on neurons and a result of induced calcium flux into neurons leading to swelling and necrosis (Choi 1990). Support has been shown for these mechanisms playing significant roles in diseases and complications such as Huntington’s disease, epilepsy, and stroke (Choi 1987).

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External Links

Environmental Protection Agency at United States Environmental Protection Agency
Alcohol and Alcoholism at Oxford Medical Journals
Neurotoxicology at Elsevier Journals
Neurotoxin Institute at Neurotoxin Institute
Neurotoxins at Toxipedia