Associate Editor: C.N. Pope
Functional consequences of repeated organophosphate exposure: Potential non-cholinergic mechanisms

https://doi.org/10.1016/j.pharmthera.2012.03.001Get rights and content

Abstract

The class of chemicals known as the “organophosphates” (OPs) comprises many of the most common agricultural and commercial pesticides that are used worldwide as well as the highly toxic chemical warfare agents. The mechanism of the acute toxicity of OPs in both target and non-target organisms is primarily attributed to inhibitory actions on various forms of cholinesterase leading to excessive peripheral and central cholinergic activity. However, there is now substantial evidence that this canonical (cholinesterase-based) mechanism cannot alone account for the wide-variety of adverse consequences of OP exposure that have been described, especially those associated with repeated exposures to levels that produce no overt signs of acute toxicity. This type of exposure has been associated with prolonged impairments in attention, memory, and other domains of cognition, as well as chronic illnesses where these symptoms are manifested (e.g., Gulf War Illness, Alzheimer's disease). Due to their highly reactive nature, it is not surprising that OPs might alter the function of a number of enzymes and proteins (in addition to cholinesterase). However, the wide variety of long-term neuropsychiatric symptoms that have been associated with OPs suggests that some basic or fundamental neuronal process was adversely affected during the exposure period. The purpose of this review is to discuss several non-cholinesterase targets of OPs that might affect such fundamental processes and includes cytoskeletal and motor proteins involved in axonal transport, neurotrophins and their receptors, and mitochondria (especially their morphology and movement in axons). Potential therapeutic implications of these OP interactions are also discussed.

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).

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