Miotic Tolerance to Sarin Vapor Exposure: Role of the Sympathetic and Parasympathetic Nervous Systems

Pupil size change in agricultural workers exposed to pesticides

CLINICAL RELEVANCE

Pupil size evaluation using clinical examination may be important for detecting and monitoring individuals at risk of neurotoxic effects from chemical exposure, as it may enable early intervention and the implementation of preventive measures.

BACKGROUND

This work aimed to investigate the association between pesticide exposure and pupil size. Pupil size is regulated by muscarinic and nicotinic receptors, and it is well-established that common pesticide chemicals disrupt this regulation.

METHODS

Twenty agricultural workers exposed to pesticides, and twenty participants not exposed, underwent visual screening, and pupil size evaluation under mesopic and photopic conditions. Additionally, signs of neurotoxicity and pesticide exposure in both groups were evaluated using the modified version of the neurotoxic symptoms questionnaire (Q16) and measuring cholinesterase (AChE) levels in blood, respectively.

RESULTS

Agricultural workers exposed to pesticides had a score indicating medium-high level of neurotoxicity (49.85 (SD ± 8.94)) which was significantly higher (t (36) = 7.659, p ≤ 0.0001) than non-exposed participants who had low levels of neurotoxicity (27.25 SD ± 8.86). There was a significant difference in pupil size (mm) under mesopic (t (19) 4.42 p = 0.003) and scotopic (t (19) 4.63, p = 0.0002) conditions between the two groups. Additionally, there was a significant difference in AChE blood levels (t (19) 2.94 p = 0.008) between exposed and non-exposed participants, indicating that exposed workers had low levels of this enzyme (average exposed group 3381 U/L (SD ± 1306)) compared to the non-exposed group (average non-exposed group 4765 U/L (SD ± 1300)). A significant negative correlation between AChE levels, years of exposure, and pupil size was found. The latter finding importantly showed that smaller pupils are associated with the accumulation of acetylcholine or a decrease in the activity of the enzyme AChE.

CONCLUSION

Pupil size of agricultural workers exposed to pesticides can be abnormal and is associated with neurotoxicity as indicated by symptomatology and cholinesterase levels. Evaluation of pupil size may be useful for clinically detecting neurotoxicity.

Eyeing up the Future of the Pupillary Light Reflex in Neurodiagnostics

The pupillary light reflex (PLR) describes the constriction and subsequent dilation of the pupil in response to light as a result of the antagonistic actions of the iris sphincter and dilator muscles. Since these muscles are innervated by the parasympathetic and sympathetic nervous systems, respectively, different parameters of the PLR can be used as indicators for either sympathetic or parasympathetic modulation. Thus, the PLR provides an important metric of autonomic nervous system function that has been exploited for a wide range of clinical applications. Measurement of the PLR using dynamic pupillometry is now an established quantitative, non-invasive tool in assessment of traumatic head injuries. This review examines the more recent application of dynamic pupillometry as a diagnostic tool for a wide range of clinical conditions, varying from neurodegenerative disease to exposure to toxic chemicals, as well as its potential in the non-invasive diagnosis of infectious disease.

Development of a microinstillation model of inhalation exposure to assess lung injury following exposure to toxic chemicals and nerve agents in Guinea pigs

Respiratory disturbances due to chemical warfare nerve agents (CWNAs) are the starting point of mass casualty and the primary cause of death by these weapons of terror and mass destruction. However, very few studies have been implemented to assess respiratory toxicity and exacerbation induced by CWNAs, especially methylphosphonothioic acid S-(2-(bis(1-methylethyl)amino)ethyl)O-ethyl ester (VX). In this study, we developed a microinstillation technique of inhalation exposure to assess lung injury following exposure to CWNAs and toxic chemicals. Guinea pigs were gently intubated by placing a microcatheter into the trachea 1.5 to 2.0 cm centrally above the bifurcation. This location is crucial to deliver aerosolized agents uniformly to the lung's lobes. The placement of the tube is calculated by measuring the distance from the upper front teeth to the tracheal bifurcation, which is typically 8.5 cm for guinea pigs of equivalent size and a weight range of 250 g to 300 g. The catheter is capable of withstanding 100 psi pressure; the terminus has five peripheral holes to pump air that aerosolizes the nerve agent that is delivered in the central hole. The microcatheter is regulated by a central control system to deliver the aerosolized agent in a volume lower than the tidal volume of the guinea pigs. The average particle size of the nerve agent delivered was 1.48 +/- 0.07 micrometer. The microinstillation technology has been validated by exposing the animals to Coomassie brilliant blue, which showed a uniform distribution of the dye in different lung lobes. In addition, the concentration of the dye in the lungs correlated with the dose/time of exposure. Furthermore, histopathological analysis confirmed the absence of barotraumas following micoinstillation. This novel technique delivers the agent safely, requires less amount of agent, avoids exposure to skin, pelt, and eye, and circumvents the concern of deposition of the particles in the nasal and palette due to the switching of breathing from nasal to oronasal in whole-body dynamic chamber or nose only exposure. Currently, we are using this inhalation exposure technique to investigate lung injuries and respiratory disturbances following direct exposure to VX.

Alterations in autonomic function in the guinea pig eye following exposure to dichlorvos vapor

The present study investigated the effect of the organophosphate, dichlorvos (DDVP), on ocular function and cholinesterase activity in guinea pigs, using a single-animal-head-only vapor exposure system. All animals exhibited signs of mild organophosphate poisoning (e.g., salivation, chewing, lacrimation, urination, defecation, and rhinorrhea) after the 20-min exposure, regardless of the DDVP exposure concentration (e.g., 35 mg/m(3), 55 mg/m(3), and 75 mg/m(3)). Pupil constriction or miosis was the most pronounced effect seen after vapor exposure. The postexposure pupil size for the 35 mg/m(3) group was 45.8 +/- 3.68% of the preexposure baseline measurement. Postexposure pupil size in the 55- (38 +/- 1.36%) and 75 mg/m(3) (38.1 +/- 1.72%) groups was significantly less than both the preexposure baseline level and the 35 mg/m(3) group. All groups exhibited enhanced an pupillary response to light after DDVP exposure. The enhanced light response remained even after recovery from miosis (approximately 1 h after exposure). Measurement of cholinesterase activity revealed that even though pupil size had recovered, acetyl- and butyrylcholinesterase remained significantly inhibited in the blood.

Development of Miotic Cross-Tolerance Between Pyridostigmine and Sarin Vapor

The organophosphorous nerve agent sarin (GB) and the carbamate pyridostigmine bromide (PB) both inhibit acetylcholinesterase (AChE), leading to overstimulation of muscarinic receptors. Both GB and PB produce miosis through stimulation of ocular muscarinic receptors. This study investigated 2 hypotheses: (1) that the miotic response to PB would decrease following repeated injections; and (2) that repeated administration of PB would result in tolerance to the miotic effect of GB vapor. Rats were injected intramuscularly with saline, 0.04 mg/kg, 0.5 mg/kg, or 1.4 mg/kg of PB twice daily for 8 consecutive days. After day 3, animals injected with 1.4 mg/kg PB developed miotic tolerance. Twenty-four (24) h following the final PB injection, the rats were exposed to GB vapor (4.0 mg/m(3)). A similar magnitude of miosis was observed in all groups after GB exposure. However, the rate of recovery of pupil size in animals pretreated with 0.5 and 1.4 mg/kg PB was significantly increased. Twenty (20) h following exposure to GB vapor, the pupils of animals pretreated with 1.4 mg/kg PB had recovered to 77% +/- 4% of their pre-exposure baseline, whereas the saline-injected controls had recovered to only 52% +/- 2% of their pre-exposure baseline. The increased rate of recovery does not appear to be a result of protection of pupillary muscarinic receptors by the higher doses of PB, as there was no longer PB present in the animal at the time of GB exposure. These results demonstrate the development of tolerance to the miotic effect of PB following repeated exposures, and also suggest that cross-tolerance between PB and GB occurs. However, because the magnitude of the response was not reduced, the PB pretreatment and its associated miotic cross-tolerance does not appear to diminish the effectiveness of miosis as a biomarker of acute exposure to nerve agent vapor.