The L-Type Calcium Channel Blocker Nimodipine Mitigates Cytoskeletal Proteins Phosphorylation in Dichlorvos-Induced Delayed Neurotoxicity in Rats

Implications of organophosphate pesticides on brain cells and their contribution toward progression of Alzheimer's disease

The most widespread neurodegenerative disorder, Alzheimer's disease (AD) is marked by severe behavioral abnormalities, cognitive and functional impairments. It is inextricably linked with the deposition of amyloid β (Aβ) plaques and tau protein in the brain. Loss of white matter, neurons, synapses, and reactive microgliosis are also frequently observed in patients of AD. Although the causative mechanisms behind the neuropathological alterations in AD are not fully understood, they are likely influenced by hereditary and environmental factors. The etiology and pathogenesis of AD are significantly influenced by the cells of the central nervous system, namely, glial cells and neurons, which are directly engaged in the transmission of electrical signals and the processing of information. Emerging evidence suggests that exposure to organophosphate pesticides (OPPs) can trigger inflammatory responses in glial cells, leading to various cascades of events that contribute to neuroinflammation, neuronal damage, and ultimately, AD pathogenesis. Furthermore, there are striking similarities between the biomarkers associated with AD and OPPs, including neuroinflammation, oxidative stress, dysregulation of microRNA, and accumulation of toxic protein aggregates, such as amyloid β. These shared markers suggest a potential mechanistic link between OPP exposure and AD pathology. In this review, we attempt to address the role of OPPs on altered cell physiology of the brain cells leading to neuroinflammation, mitochondrial dysfunction, and oxidative stress linked with AD pathogenesis.

Systematic Review of Calcium Channels and Intracellular Calcium Signaling: Relevance to Pesticide Neurotoxicity

Pesticides of different chemical classes exert their toxic effects on the nervous system by acting on the different regulatory mechanisms of calcium (Ca2+) homeostasis. Pesticides have been shown to alter Ca2+ homeostasis, mainly by increasing its intracellular concentration above physiological levels. The pesticide-induced Ca2+ overload occurs through two main mechanisms: the entry of Ca2+ from the extracellular medium through the different types of Ca2+ channels present in the plasma membrane or its release into the cytoplasm from intracellular stocks, mainly from the endoplasmic reticulum. It has also been observed that intracellular increases in the Ca2+ concentrations are maintained over time, because pesticides inhibit the enzymes involved in reducing its levels. Thus, the alteration of Ca2+ levels can lead to the activation of various signaling pathways that generate oxidative stress, neuroinflammation and, finally, neuronal death. In this review, we also discuss some proposed strategies to counteract the detrimental effects of pesticides on Ca2+ homeostasis.

L- and T-type calcium channel blockers protect against the inhibitory effects of mipafox on neurite outgrowth and plasticity-related proteins in SH-SY5Y cells

Some organophosphorus compounds (OP), including the pesticide mipafox, produce late onset distal axonal degeneration, known as organophosphorus-induced delayed neuropathy (OPIDN). The underlying mechanism involves irreversible inhibition of neuropathy target esterase (NTE) activity, elevated intracellular calcium levels, increased activity of calcium-activated proteases and impaired neuritogenesis. Voltage-gated calcium channels (VGCC) appear to play a role in several neurologic disorders, including OPIDN. Therefore, this study aimed to examine and compare the neuroprotective effects of T-type (amiloride) and L-type (nimodipine) VGCC blockers induced by the inhibitory actions of mipafox on neurite outgrowth and axonal proteins of retinoic-acid-stimulated SH-SY5Y human neuroblastoma cells, a neuronal model widely employed to determine the neurotoxicity attributed to OP. Both nimodipine and amiloride significantly blocked augmentation of intracellular calcium levels and activity of calpains, as well as decreased neurite length, number of differentiated cells, and lowered concentrations of growth-associated protein 43 (GAP-43) and synapsin induced by mipafox. Only nimodipine inhibited reduction of synaptophysin levels produced by mipafox. These findings demonstrate a role for calcium and VGCC in the impairment of neuronal plasticity mediated by mipafox. Data also demonstrated the neuroprotective potential of T-type and L-type VGCC blockers to inhibit OP-mediated actions, which may be beneficial to counteract cases of pesticide poisoning.

Specific ion channels contribute to key elements of pathology during secondary degeneration following neurotrauma

BACKGROUND

Following partial injury to the central nervous system, cells beyond the initial injury site undergo secondary degeneration, exacerbating loss of neurons, compact myelin and function. Changes in Ca²⁺ flux are associated with metabolic and structural changes, but it is not yet clear how flux through specific ion channels contributes to the various pathologies. Here, partial optic nerve transection in adult female rats was used to model secondary degeneration. Treatment with combinations of three ion channel inhibitors was used as a tool to investigate which elements of oxidative and structural damage related to long term functional outcomes. The inhibitors employed were the voltage gated Ca²⁺ channel inhibitor Lomerizine (Lom), the Ca²⁺ permeable AMPA receptor inhibitor YM872 and the P2X₇ receptor inhibitor oxATP.

RESULTS

Following partial optic nerve transection, hyper-phosphorylation of Tau and acetylated tubulin immunoreactivity were increased, and Nogo-A immunoreactivity was decreased, indicating that axonal changes occurred acutely. All combinations of ion channel inhibitors reduced hyper-phosphorylation of Tau and increased Nogo-A immunoreactivity at day 3 after injury. However, only Lom/oxATP or all three inhibitors in combination significantly reduced acetylated tubulin immunoreactivity. Most combinations of ion channel inhibitors were effective in restoring the lengths of the paranode and the paranodal gap, indicative of the length of the node of Ranvier, following injury. However, only all three inhibitors in combination restored to normal Ankyrin G length at the node of Ranvier. Similarly, HNE immunoreactivity and loss of oligodendrocyte precursor cells were only limited by treatment with all three ion channel inhibitors in combination.

CONCLUSIONS

Data indicate that inhibiting any of a range of ion channels preserves certain elements of axon and node structure and limits some oxidative damage following injury, whereas ionic flux through all three channels must be inhibited to prevent lipid peroxidation and preserve Ankyrin G distribution and OPCs.

DEET potentiates the development and persistence of anticholinesterase dependent chronic pain signs in a rat model of Gulf War Illness pain

Exposure to DEET (N,N-diethyl-meta-toluamide) may have influenced the pattern of symptoms observed in soldiers with GWI (Gulf War Illness; Haley and Kurt, 1997). We examined how the addition of DEET (400mg/kg; 50% topical) to an exposure protocol of permethrin (2.6mg/kg; topical), chlorpyrifos (CP; 120mg/kg), and pyridostigmine bromide (PB;13mg/kg) altered the emergence and pattern of pain signs in an animal model of GWI pain (Nutter et al., 2015). Rats underwent behavioral testing before, during and after a 4week exposure: 1) hindlimb pressure withdrawal threshold; 2) ambulation (movement distance and rate); and 3) resting duration. Additional studies were conducted to assess the influence of acute DEET (10-100μM) on muscle and vascular nociceptor K_v7, K_DR, Na_v1.8 and Na_v1.9. We report that a 50% concentration of DEET enhanced the development and persistence of pain-signs. Rats exposed to all 4 compounds exhibited ambulation deficits that appeared 5-12weeks post-exposure and persisted through weeks 21-24. Rats exposed to only three agents (CP or PB excluded), did not fully develop ambulation deficits. When PB was excluded, rats also developed rest duration pain signs, in addition to ambulation deficits. There was no evidence that physiological doses of DEET acutely modified nociceptor K_v7, K_DR, Na_v1.8 or Na_v1.9 activities. Nevertheless, DEET augmented protocols decreased the conductance of K_v7 expressed in vascular nociceptors harvested from chronically exposed rats. We concluded that DEET enhanced the development and persistence of pain behaviors, but the anticholinesterases CP and PB played a determinant role.

In vitro study of the neuropathic potential of the organophosphorus compounds fenamiphos and profenofos: Comparison with mipafox and paraoxon

Organophosphorus-induced delayed neuropathy (OPIDN) is a central-peripheral distal axonopathy that develops 8-14 days after poisoning by a neuropathic organophosphorus compound (OP). Several OPs that caused OPIDN were withdrawn from the agricultural market due to induction of serious delayed effects. Therefore, the development of in vitro screenings able to differentiate neuropathic from non-neuropathic OPs is of crucial importance. Thus, the aim of this study was to evaluate the differences in the neurotoxic effects of mipafox (neuropathic OP) and paraoxon (non-neuropathic OP) in SH-SY5Y human neuroblastoma cells, using the inhibition and aging of neuropathy target esterase (NTE), inhibition of acetylcholinesterase (AChE), activation of calpain, neurite outgrowth, cytotoxicity and intracellular calcium as indicators. Additionally, the potential of fenamiphos and profenofos to cause acute and/or delayed effects was also evaluated. Mipafox had the lowest IC50 and induced the highest percentage of aging of NTE among the OPs evaluated. Only mipafox was able to cause calpain activation after 24 h of incubation. Concentrations of mipafox and fenamiphos which inhibited at least 70% of NTE were also able to reduce neurite outgrowth. Cytotoxicity was higher in non-neuropathic than in neuropathic OPs while the intracellular calcium levels were higher in neuropathic than in non-neuropathic OPs. In conclusion, the SH-SY5Y cellular model was selective to differentiate neuropathic from non-neuropathic OPs; fenamiphos, but not profenofos presented results compatible with the induction of OPIDN.

Biochemical, histopathological and clinical evaluation of delayed effects caused by methamidophos isoforms and TOCP in hens: ameliorative effects using control of calcium homeostasis

This work evaluated the potential of the isoforms of methamidophos to cause organophosphorus-induced delayed neuropathy (OPIDN) in hens. In addition to inhibition of neuropathy target esterase (NTE) and acetylcholinesterase (AChE), calpain activation, spinal cord lesions and clinical signs were assessed. The isoforms (+)-, (±)- and (-)-methamidophos were administered at 50mg/kg orally; tri-ortho-cresyl phosphate (TOCP) was administered (500mg/kg, po) as positive control for delayed neuropathy. The TOCP hens showed greater than 80% and approximately 20% inhibition of NTE and AChE in hen brain, respectively. Among the isoforms of methamidophos, only the (+)-methamidophos was capable of inhibiting NTE activity (approximately 60%) with statistically significant difference compared to the control group. Calpain activity in brain increased by 40% in TOCP hens compared to the control group when measured 24h after dosing and remained high (18% over control) 21 days after dosing. Hens that received (+)-methamidophos had calpain activity 12% greater than controls. The histopathological findings and clinical signs corroborated the biochemical results that indicated the potential of the (+)-methamidophos to be the isoform responsible for OPIDN induction. Protection against OPIDN was examined using a treatment of 2 doses of nimodipine (1mg/kg, i.m.) and one dose of calcium gluconate (5mg/kg, i.v.). The treatment decreased the effect of OPIDN-inducing TOCP and (+)-methamidophos on calpain activity, spinal cord lesions and clinical signs.

Mechanisms for consideration for intervention in the development of organophosphorus-induced delayed neuropathy

Organophosphorus-induced delayed neuropathy (OPIDN) is a neurodegenerative disorder characterised by ataxia progressing to paralysis with concomitant central and peripheral distal axonopathy. Symptoms of OPIDN in people include tingling of the hands and feet. This tingling is followed by sensory loss, progressive muscle weakness and flaccidity of the distal skeletal muscles of the lower and upper extremities and ataxia, which appear about 8-14 days after exposure. Some organophosphorus compounds (OPs) that are still used in worldwide agriculture have potential to induce OPIDN, including methamidophos, trichlorfon, dichlorvos and chorpyrifos. This review summarizes experimental attempts to prevent and/or treat OPIDN and the different mechanisms involved in each approach. The initial mechanism associated with development of OPIDN is phosphorylation and inhibition of neuropathy target esterase (NTE). The phosphorylated enzyme undergoes a second reaction known as "aging" that results in the loss of one of the "R" groups bound to the phosphorus of the OP. A second mechanism involved in OPIDN is an imbalance in calcium homeostasis. This can lead to the activation of calcium-activated neutral protease and increases in calcium/calmodulin-dependent protein kinases. These events contribute to aberrant phosphorylation of cytoskeletal proteins and protein digestion in the terminal axon that can proceed similarly to Wallerian-type degeneration. Several experimental studies demonstrated alleviation of the signs and symptoms of OPIDN by restoring calcium balance. Other studies have used preadministration of NTE inhibitors, such as carbamates, thiocarbamates, sulfonyl fluorides and phosphinate to prevent OPIDN. Progress is being made, but there is yet no single specific treatment available for use in clinical practice to prevent or alleviate the severe effects of OPIDN.

Antibodies to neural proteins in organophosphorus-induced delayed neuropathy (OPIDN) and its amelioration

The development of OPIDN and the efficacy of experimental intervention using the calcium-channel blocker verapamil were used as a model to test the serial time-measurements of serum autoantibodies against neuronal cytoskeletal proteins [e.g., neurofilament triplet (NF)] and glial proteins [myelin-basic protein (MBP) and glial fibrillary-acidic protein (GFAP)] as biomarkers of neurotoxicity and its amelioration. Ten White Leghorn hens (>7 months, 1.2-1.8 kg) were administered phenyl-saligenin phosphate (PSP; 2.5 mg/kg; im), a dose reported to induce a 70% decrease in neurotoxic esterase (NTE) activity. Five of the hens were administered verapamil (7 mg/kg; im) for 4 days starting one day before PSP administration. Serum was isolated from blood collected by serial brachial venepuncture before PSP (day 0) administration and on days 3, 7 and 21 after PSP administration, each hen acting as its own control. Serum antibodies (IgG) to NF-L, NF-M, NF-H, MBP, and GFAP were assayed using an ELISA. There were no detectable levels of antibodies on days 0 and 3. IgG against all neural proteins were detected on days 7 and 21, with titer levels being significantly (p< or =0.05) higher in sera of hens receiving PSP only. Anti-NF-L titers were highest compared to those against NF-M, NF-H or MBP at 21 days. Titers of anti-NF-L and anti-MBP significantly (p< or =0.01) correlated with clinical scores at days 7 and 21. Detection of anti-NF and anti-MBP antibodies confirms the neuroaxonal degeneration accompanied by myelin loss reported in this model of OPIDN and the amelioration of neuropathy using verapamil. The detection of anti-GFAP antibodies suggests CNS involvement in OPIDN, since astrocytes are only found therein. This study demonstrates that detection of neuroantibodies can be used as biomarkers of neuropathy development and to monitor the amelioration resulting from therapeutic intervention. Together with biomarkers of exposure neuroantibodies can be used to monitor neuropathogenesis due to environmental or occupational exposures.

Organophosphorus insecticides chlorpyrifos and diazinon and oxidative stress in neuronal cells in a genetic model of glutathione deficiency

Over the past several years evidence has been accumulating from in vivo animal studies, observations in humans, and in vitro studies, that organophosphorus (OP) insecticides may induce oxidative stress. Such effects may contribute to some of the toxic manifestations of OPs, particularly upon chronic or developmental exposures. The aim of this study was to investigate the role of oxidative stress in the neurotoxicity of two commonly used OPs, chlorpyrifos (CPF) and diazinon (DZ), their oxygen analogs (CPO and DZO), and their "inactive" metabolites (TCP and IMP), in neuronal cells from a genetic model of glutathione deficiency. Cerebellar granule neurons from wild type mice (Gclm +/+) and mice lacking the modifier subunit of glutamate cysteine ligase (Gclm -/-), the first and limiting step in the synthesis of glutathione (GSH), were utilized. The latter display very low levels of GSH and are more susceptible to the toxicity of agents that increase oxidative stress. CPO and DZO were the most cytotoxic compounds, followed by CPF and DZ, while TCP and IMP displayed lower toxicity. Toxicity was significantly higher (10- to 25-fold) in neurons from Gclm (-/-) mice, and was antagonized by various antioxidants. Depletion of GSH from Gclm (+/+) neurons significantly increased their sensitivity to OP toxicity. OPs increased intracellular levels of reactive oxygen species and lipid peroxidation and in both cases the effects were greater in neurons from Gclm (-/-) mice. OPs did not alter intracellular levels of GSH, but significantly increased those of oxidized glutathione (GSSG). Cytotoxicity was not antagonized by cholinergic antagonists, but was decreased by the calcium chelator BAPTA-AM. These studies indicate that cytotoxicity of OPs involves generation of reactive oxygen species and is modulated by intracellular GSH, and suggest that it may involve disturbances in intracellular homeostasis of calcium.