Associate Editor: C.N. PopeFunctional consequences of repeated organophosphate exposure: Potential non-cholinergic mechanisms
Introduction
The purpose of this review is to: 1) describe the variety of means by which individuals come in contact with organophosphates (OPs), 2) provide an overview of the various toxicological symptoms, particularly the neurobehavioral symptoms associated with repeated exposures to OPs and 3) discuss recent evidence to support the argument that the canonical (cholinesterase-based) mechanism of OP toxicity cannot alone explain the wide-variety of adverse consequences of OP exposure that have been described. For the second and third objectives, acute OP toxicity will be briefly discussed; however, the neurobehavioral symptoms emphasized in this review (and the proposed mechanisms thereof) will primarily apply to those observed in the absence of overt signs of acute toxicity. Diverse semantics have been used for this form of toxicity (subacute, subtoxic, subclinical, subthreshold, etc.), thus it is important to reiterate that in this review, protracted neurobehavioral symptoms occurring in the absence of antecedent signs of acute toxicity, but not necessarily in the absence of cholinesterase inhibition, will be emphasized. It is also important to note that cholinesterase inhibition can occur in the absence of acutely toxic effects, but not vice versa; thus these two descriptors may represent a gradation of direct effects. The non-cholinesterase targets discussed in this review could be affected in both the case of acute-high level exposure as well as repeated lower level exposure (however, in the acute setting the non-cholinesterase related physiologic effects might be difficult to distinguish). Moreover, while several non-cholinesterase-based mechanisms of OP toxicity will be briefly discussed (e.g., OP target proteins, oxidative stress, neuroinflammation), the primary focus here will be on more recently introduced (potential) targets of OPs (e.g., axonal transport, neurotrophins and mitochondrial dynamics).
The generic term “organophosphate” or “OP” is used for a wide variety of chemicals that are derived from phosphoric, phosphonic and phosphinic acids (see Fig. 1). The French chemists, Jean Louis Lassaigne and Philip de Clermount are credited with the synthesis of the first OPs in the nineteenth century, while the initial development of OPs as insecticides and chemical warfare agents early in the twentieth century is primarily attributed to the German chemist Gerhard Schrader (Gallo and Lawryk, 1991, Tucker, 2006). Since these early years literally hundreds of OP-based compounds have been synthesized and they are found in insecticides (e.g., malathion, parathion, diazinon, chlorpyrifos), chemical warfare (“nerve”) agents (e.g., soman, sarin, tabun, VX), some ophthalmic agents (e.g., echothiophate, isoflurophate), antihelmintics (e.g., trichlorfon), herbicides (e.g., tribufos, merphos), as well as solvents, plasticizers, and extreme pressure additives for lubricants (Katz & Brooks, 2010). The widespread use of OPs (especially as insecticides, see below) has been an environmental health concern for many years and there are a number of reports suggesting that OPs might be associated with an increased risk of a variety of chronic illnesses including respiratory (e.g., chronic obstructive respiratory disease), metabolic (e.g., obesity, diabetes) and neurologic (e.g., Alzheimer's Disease Parkinson's disease) disorders (Hancock et al., 2008, Chakraborty et al., 2009, Hayden et al., 2010, Slotkin, 2011).
Pesticides (including OPs) have multiple applications in agricultural, industrial, and household settings and as a result they are used extensively worldwide. Their value in optimizing agricultural productivity, the control of deadly vector-borne illnesses (e.g., malaria, yellow fever, viral encephalitis, typhus, etc.), and “nuisance” pests (e.g., flies, roaches, ants, mosquitoes, etc.) is clear (reviewed by Cooper & Dobson, 2007). As consequence of their widespread use, however, pesticides (and their residues) are now among the most ubiquitous synthetic chemicals in our environment. Accordingly, inherent dangers to the public health exist since no pesticide is innocuous and all carry significant toxicological risks. The use of OP-based pesticides (specifically) is now considered a worldwide health problem since they are the most commonly used and the most often associated with toxicity to humans (see reviews, Buckley et al., 2004, Rohlman et al., 2011). In the United States (US) alone, the most recent estimates available (from 2007) indicate that approximately 33 million pounds of OP-based pesticides are applied annually (US EPA, 2011). Internationally, OP-pesticide poisonings are among the most common modes of poisoning-related fatalities (i.e., both intentional and unintentional), a phenomenon that has been attributed to ease of access to OPs and the relatively low level of regulations governing their use especially in developing countries (see reviews, Buckley et al., 2004, Dharmani and Jaga, 2005). In the US there are multiple regulations and safety training requirements under the purview of the Environmental Protection Agency (EPA) for the handling and transport of OPs. However, recent studies suggest that acute pesticide poisoning particularly in the agricultural industry (including poisonings with OP-based pesticides) continues to be a significant problem (Calvert et al., 2008) and moreover, pesticide poisonings are most likely underreported. It has been suggested that disproportionate numbers of agricultural workers are deterred from seeking health care in the US due to a number of factors including concerns related to immigration status, the lack of health insurance, unfamiliarity with (or the inability to qualify for) workers' compensation benefits, and the fear of job loss if they miss time from work to seek health care. In addition, other sources of the underreporting of pesticide poisonings include misdiagnosis by health care workers as well as their lack of awareness that they are required to report such incidents (i.e., when they are properly diagnosed) to public health officials (see Calvert et al., 2008).
While the risk of OP exposure as a result of extensive pesticide use is considerably higher for most people, the threat from intentional poisonings by rogue governments and terrorist organizations is an ongoing concern. It is now relatively well documented that in the 1980s the Iraqi military attacked Iranian military soldiers (Majnoon Island) and Kurdish civilians (Halabja) with OP-based nerve agents producing casualties estimated to be as high as “tens of thousands” (Macilwain, 1993, Barnaby, 1998, O'Leary, 2002, Hawrami and Ibrahim, 2004). Moreover, international news reports provide an almost daily reminder of the escalating terrorist activities throughout the world and it is clear that the use of toxic chemicals is a major goal of such groups. The Tokyo Sarin attack in March of 1995 revealed the danger of even a limited chemical attack given that 12 people were killed and over 5000 others required emergency medical evaluation and/or treatment (Suzuki et al., 1995, Nagao et al., 1997). This incident also clearly indicated that terrorist groups have the desire to use nerve agents on civilian populations and that they are capable of both acquiring and deploying them. Since the Tokyo attacks an increasing terrorist threat level can be surmised from a number of factors such as: the growth of militant religious groups with increasingly sophisticated and international capabilities, the increasing global availability of highly technical information regarding chemical (and biological) weapons on the internet, and the clear evidence of terrorist's interest in such weapons (Cronin, 2003). Based on several factors, the odds for a chemical attack by terrorists may be actually higher than biological or nuclear attacks due to the more widespread availability of raw materials for making chemical weapons. These materials include large stockpiles of military-grade chemical weapons that remain undestroyed or unaccounted for in multiple countries around the world. The Organization for the Prohibition of Chemical Weapons (OPCW) estimates that (as of September 30, 2010) 44,131, or 61.99%, of the world's declared stockpile of 71,194 metric tons of chemical agent have been verifiably destroyed (OPCW, 2011). However, this leaves nearly 30,000 metric tons undestroyed, and these numbers do not include the stockpiles of non-member states (e.g., Syria, North Korea) that have neither signed nor acceded to the Chemical Weapons Convention.
As opposed to the use of nerve agents, an equally significant and perhaps more likely domestic terrorist scenario would be the use of industrial or agricultural chemicals as weapons (Burklow et al., 2003). Industrial chemicals have (in fact) been used by terrorists as improvised explosives, incendiaries, and poisons in several incidents (Hughart, 1999). Notwithstanding the potential access of terrorists to nerve agent stockpiles in foreign countries (discussed above), in the US, the use of these weapons is limited by the security surrounding government chemical agent stockpiles and binary chemical agent storage, as well as the controlled access to precursor chemicals. While improvised chemical agents may be less toxic than weaponized (military) agents, they have rapid, highly visible impacts on human health and they can be dispersed by smoke, gas clouds, or food and medicine distribution networks (Hughart, 1999). Of the variety of chemicals that could be used as domestic chemical weapons, OPs certainly must rank near the top given their wide availability on hundreds of insecticide products.
Section snippets
Acute and chronic behavioral effects of organophosphates
The acute toxicity of OPs in humans has been associated with a host of central nervous system, cardiovascular, respiratory, gastrointestinal, sensory, and motor manifestations which are frequently life threatening (see reviews, Bardin et al., 1994, Collombet, 2011). Several studies also document long-term neuropsychiatric sequelae in subjects who have experienced acute OP toxicity. These include deficits in signal detection and information processing, sustained attention, memory, sequencing and
Organophosphate targets other than cholinesterase
OPs are believed to manifest their acute biological actions primarily through inhibiting the various forms of cholinesterase, the degradative enzyme for the neurotransmitter acetylcholine. Toxicity to the target organism is then mediated through elevation of synaptic levels of acetylcholine in tissues innervated by cholinergic neurons, and subsequent overstimulation of postsynaptic cells (reviewed by Ecobichon, 1991). While the inhibition of cholinesterase enzymes undoubtedly plays a key role
Study limitations and experimental strategies
The various studies discussed above provide insight as to how OPs (via non-cholinesterase targets) might affect a number of neuronal processes that could result in long term functional deficits including cognitive impairment. However, there are a number of questions that remain to be answered. For example, a considerable amount of the work related to OP effects on non-cholinesterase targets has been conducted in vitro and as a result it is currently unclear if the reported effects would occur
Potential therapeutic strategies
Given the wide variety of behavioral symptoms and neuropathological abnormalities that have been reported in individuals previously exposed to OPs, as well as the large list of potential OP targets, the design of rational therapeutic strategies is challenging. The text below (while speculative in some cases) is provided to highlight a few potential areas of interest and to stimulate discussion. The strategies discussed could potentially apply to the protracted neurological effects associated
Conclusion
Substantial evidence now suggests that the canonical (cholinesterase-based) mechanism of OP toxicity cannot alone account for the wide-variety of adverse consequences of OP exposure that have been described, particularly the long-term neuropsychiatric symptoms. OP interactions with proteins involved in fundamental neuronal processes such as axonal transport, neurotrophin support, and mitochondrial function (both oxidation-related processes as well as those that affect their morphology and
Conflict of interest statement
The author declares that there are no conflicts of interest.
Acknowledgments
The authors would like to thank Ms. Ashley Davis for her administrative assistance in preparing this article. This author's laboratory is currently supported by the National Institute of Environmental Health Sciences (ES012241), the National Institute on Aging (AG029617), and the National Institute on Drug Abuse (DA029127).
References (117)
- et al.
Psychiatric disorders among Egyptian pesticide applicators and formulators
Environmental Research
(1997) - et al.
Neonatal chlorpyrifos exposure targets multiple proteins governing the hepatic adenylyl cyclase signaling cascade: implications for neurotoxicity
Brain Research. Developmental Brain Research
(2000, May 11) - et al.
Diethylphosphorylation of rat cardiac M2 muscarinic receptor by chlorpyrifos oxon in vitro
Toxicology Letters
(2001, Feb 3) - et al.
Role of mitofusin 2 mutations in the physiopathology of Charcot–Marie–Tooth disease type 2A
Experimental Neurology
(2009, Aug) - et al.
Serine hydrolase targets of organophosphorus toxicants
Chemico-Biological Interactions
(2005) - et al.
Reaction of a specific tyrosine residue of papain with diisopropylfluorophosphate
Journal of Biological Chemistry
(1969, Aug 10) - et al.
Tissue-specific effects of chlorpyrifos on carboxylesterase and cholinesterase activity in adult rats: an in vitro and in vivo comparison
Fundamental and Applied Toxicology
(1997, Aug) - et al.
Mitochondrial trafficking and morphology in healthy and injured neurons
Progress in Neurobiology
(2006, Dec) - et al.
Acrylamide-induced neuropathy and impairment of axonal transport of proteins. II. Abnormal accumulations of smooth endoplasmic reticulum as sites of focal retention of fast transported pro
Brain Research
(1981) Nerve agent intoxication: recent neuropathophysiological findings and subsequent impact on medical management prospects
Toxicology and Applied Pharmacology
(2011, Sep 15)
The benefits of pesticides to mankind and the environment
Crop Protection
Current issues in organophosphate toxicology
Clinica Chimica Acta
Toxicity due to organophosphorus compounds: what about chronic exposure?
Transactions of the Royal Society of Tropical Medicine and Hygiene
Chlorpyrifos, chlorpyrifos-oxon, and diisopropylfluorophosphate inhibit kinesin-dependent microtubule motility
Toxicology and Applied Pharmacology
Sabeluzole, a memory-enhancing molecule, increases fast axonal transport in neuronal cell cultures
Experimental Neurology
Retrograde neurotrophin signaling: Trk-ing along the axon
Current Opinion in Neurobiology
Covalent binding of the organophosphorus agent FP-biotin to tyrosine in eight proteins that have no active site serine
Chemico-Biological Interactions
Mass spectral characterization of organophosphate-labeled lysine in peptides
Analytical Biochemistry
Nanoimages show disruption of tubulin polymerization by chlorpyrifos oxon: implications for neurotoxicity
Toxicology and Applied Pharmacology
Mass spectrometry identifies multiple organophosphorylated sites on tubulin
Toxicology and Applied Pharmacology
Mass spectrometry identifies covalent binding of soman, sarin, chlorpyrifos oxon, diisopropyl fluorophosphate, and FP-biotin to tyrosines on tubulin: a potential mechanism of long term toxicity by organophosphorus agents
Chemico-Biological Interactions
Repeated nicotine exposure in rats: effects on memory function, cholinergic markers and nerve growth factor
Neuroscience
Gulf War Illness: lessons from medically unexplained symptoms
Clinical Psychology Review
Impaired mitochondrial energy metabolism and neuronal apoptotic cell death after chronic dichlorvos (OP) exposure in rat brain
Neurotoxicology
Persistent behavioral consequences of neonatal chlorpyrifos exposure in rats
Developmental Brain Research
Early postnatal parathion exposure in rats causes sex-selective cognitive impairment and neurotransmitter defects which emerge in aging
Behavioural Brain Research
Repeated exposures to low-level chlorpyrifos results in impairments in sustained attention and increased impulsivity in rats
Neurotoxicology and Teratology
Definitive evidence for the acute sarin poisoning diagnosis in the Toyko subway
Toxicology and Applied Pharmacology
Possible involvement of a neurotrophic factor during the early stages of organophosphate-induced delayed neurotoxicity
Toxicology Letters
Microtubule-associated targets in chlorpyrifos oxon hippocampal neurotoxicity
Neuroscience
Cannabinoid CB1 receptor as a target for chlorpyrifos oxon and other organophosphorus pesticides
Toxicology Letters
Fatty acid amide hydrolase inhibition by neurotoxic organophosphorus pesticides
Toxicology and Applied Pharmacology
The potential for toxic effects of chronic, low-dose exposure to organophosphates
Toxicology Letters
Molecular mechanisms regulating the retrograde axonal transport of neurotrophins
Brain Research. Brain Research Reviews
Correlating neurobehavioral performance with biomarkers of organophosphorous pesticide exposure
Neurotoxicology
Chronic central nervous system effects of acute organophosphate pesticide intoxication. The Pesticide Health Effects Study Group
Lancet
Cholinergic systems in brain development and disruption by neurotoxicants: nicotine, environmental tobacco smoke, organophosphates
Toxicology and Applied Pharmacology
Does early-life exposure to organophosphate insecticides lead to prediabetes and obesity?
Reproductive Toxicology
Cellular mechanisms for developmental toxicity of chlorpyrifos: targeting the adenylyl cyclase signaling cascade
Toxicology and Applied Pharmacology
Acrylamide-induced neuropathy and impairment of axonal transport of proteins. I. Multifocal retention of fast transported proteins at the periphery of axons as revealed by light microscope
Brain Research
Mechanisms of organophosphorus ester-induced delayed neurotoxicity: Type I and Type II
Annual Review of Pharmacology and Toxicology
Manganese enhanced magnetic resonance imaging (MEMRI) for studying axonal transport deficits induced by organophosphates (OP)
Organophosphate and carbamate poisoning
Archives of Internal Medicine
The nuclear arsenals and nuclear disarmament
Medicine, Conflict, and Survival
DoD, CIA release Khamisiyah modeling data
GulfNEWS
The effect of chlorpyrifos and chlorpyrifos-oxon on brain cholinesterase, muscarinic receptor binding, and neurotrophin levels in rats following early postnatal exposure
Toxicological Sciences
Review of health consequences from high-, intermediate- and low-level exposure to organophosphorus nerve agents
Journal of Applied Toxicology
Overcoming apathy in research on organophosphate poisoning
BMJ
Industrial chemicals: terrorist weapons of opportunity
Pediatric Annals
Behavioral and neurochemical changes in rats dosed repeatedly with diisopropylfluorophosphate
Journal of Pharmacology and Experimental Therapeutics
Cited by (194)
Pesticides and tremor: An overview of association, mechanisms and confounders
2023, Environmental ResearchPlanarians as a model to study neurotoxic agents
2023, Advances in Neurotoxicology