Degeneration patterns in the chicken central nervous system induced by ingestion of the organophosphorus delayed neurotoxin tri-ortho-tolyl phosphate. A silver impregnation study

Involvement of autophagy in tri-ortho-cresyl phosphate- induced delayed neuropathy in hens

Autophagy is a highly conserved cellular self-degradative process that plays a housekeeping role in removing aggregated proteins and damaged organelles. Our recent work has found that tri-ortho-cresyl phosphate (TOCP), a neuropathic organophosphate (OP), decreased the level of beclin 1 (a key molecule in the process of autophagy) in hen nerve tissues (Song et al., 2012). However, the role of autophagy in the pathogenesis of organophosphorus ester-induced delayed neuropathy (OPIDN) remains unclear. Here, we investigated whether dysfunctional autophagy was associated with the initiation and development of TOCP-induced delayed neuropathy. Adult hens were given a single dose of 750mg/kg TOCP (p.o.) and sacrificed on days 1, 5, 10, and 21 after dosing, respectively. The formation of autophagosomes in spinal cord motor neurons was observed by transmission electron microscopy, the level of autophagy-related proteins in hen spinal cords and tibial nerves was determined by Western blot analysis. The results demonstrated that the number of autophagosomes was markedly increased in the myelinated and unmyelinated axons of hen spinal cords after TOCP exposure. In the meantime, the level of two molecular markers for autophagy, microtubule-associated protein light chain-3 (LC3) and p62/SQSTM1 in hen nerve tissues was significantly decreased and increased, respectively. Furthermore, a marked reduction in autophagy-regulated proteins including ULK 1, AMBRA 1, ATG 5, ATG 7, ATG 12 and VPS34 expression was also observed. Our results suggested that the administration of TOCP resulted in a significant inhibition of autophagy activity in neurons, which might be associated with the pathogenesis of OPIDN.

Delayed neurotoxicity of diisopropylfluorophosphate (DFP): autoradiographic localization of high-affinity [(3)H]DFP binding sites in the chicken spinal cord

The delayed neurotoxic organophosphate, diisopropylfluorophosphate (DFP) binds with high affinity to membrane-bound proteins from chicken nerve tissues. The autoradiographic distribution of [(3)H]DFP binding sites in spinal cord sections of chicken showed higher concentrations of binding sites in gray matter than in white matter. In the cervical region, fairly high densities of [(3)H]DFP binding sites were found in laminae X and to a lesser extent, in the ventral horn gray matter. To identify the membrane-associated DFP-binding proteins, detergent-solubilized membranes were labeled widi 5-10nM [(3)H]DFP (10pmol/mg protein) for 70 min at 37°C. Gel-exclusion chromatography of the [(3)H]DFP-radiolabeled membranes indicated at least two major radioactive proteins with apparent molecular weights of 150-670 kDa and 40-129 kDa. Although we could not identify the high affinity DFP binding proteins, the autoradiographic experiments clearly demonstrated that the DFP binding proteins localized on gray matter of chicken spinal cord.

Reduction of retrograde axonal transport associated-proteins motor proteins, dynein and dynactin in the spinal cord and cerebral cortex of hens by tri-ortho-cresyl phosphate (TOCP)

Tri-ortho-cresyl phosphate (TOCP) can cause a type of neurotoxicity known as organophosphate-induced delayed neuropathy (OPIDN). The characteristic axonal swelling containing aggregations of neurofilaments, microtubules, and multivesicular vesicles is consistent with a disturbance of axonal transport. We hypothesized that there existed a disturbance of molecular motor in the pathogenesis of OPIDN. In the present study, adult hens were treated with a dosage of 750 mg/kg TOCP by gavage, or pretreated 24h earlier with phenylmethanesulfonyl fluoride (PMSF) and subsequently with TOCP, then sacrificed on the time-points of 0, 1, 5, 10, and 21 days after dosing of TOCP, respectively. The level of kinesin-1, dynein, and dynactin in spinal cords and cerebral cortexes of hens was determined. Immunoblotting analysis showed a progressive decline of dynein and dynactin in spinal cords after dosing TOCP. Furthermore, a significant reduction in dynactin and dynein was observed in cerebral cortexes at several time-points post dosing TOCP. In contrast, no significant changes of kinesin-1 were observed throughout the period of experiment. When given before TOCP administration, PMSF could inhibit TOCP-induced motor protein disruption, while it protected hens against the delayed neuropathy. In conclusion, the reduction of the motor proteins, dynein and dynactin, might be associated with the disruption of retrograde neuronal axonal transport in OPIDN.

Phenylmethylsulfonyl fluoride protects against the degradation of neurofilaments in tri-ortho-cresyl phosphate (TOCP) induced delayed neuropathy

Tri-ortho-cresyl phosphate (TOCP) is an organophosphorus ester, which can cause a type of neurotoxicity known as organophosphate-induced delayed neuropathy (OPIDN). Our recent study has shown that the enhanced degradation of neurofilament (NF) in peripheral nerve of hens is an early event of TOCP-induced OPIDN (Song et al., 2009). The main objective of this investigation is to study the effect of TOCP administration on NF content and NF degradation when OPIDN is blocked by pretreatment with phenylmethylsulfonyl fluoride (PMSF). The hens were pretreated 24h earlier with PMSF and subsequently treated with a single dosage of 750 mg/kg TOCP, then sacrificed on the corresponding time points of 0, 1, 5, 10, and 21 days after dosing TOCP, respectively. The tibial nerves were dissected, homogenized, and centrifuged at 100,000 x g. The level of NF triplet protein in both pellet and supernatant fractions of tibial nerves was determined. Western blotting analysis showed a significant increase of three NF subunits in hens treated with PMSF and TOCP compared with the control. These changes were observed within 24h of PMSF administration and then followed by an obvious recovery. Furthermore, accompanied with the increase of NF content, a significant decline in NF-L degradation rate was observed in both fractions of tibial nerves. Taken together, these results demonstrated the pretreatment with PMSF could inhibit TOCP-induced NF degradation while it protected hens against the development of OPIDN, which suggested the inhibition of NF-associated protease in peripheral nerves might be an underlying protective mechanism of PMSF against OPIDN.

Neuropathological studies of rats following multiple exposure to tri-ortho-tolyl phosphate, chlorpyrifos and stress

Adult male Long-Evans rats were exposed to 2 neurotoxic organophosphates in a setting of chronic stress, over a 63-day period. The organophosphates were tri-ortho-tolyl phosphate (TOTP) administered in 14 gavage doses of 75, 150 or 300 mg/kg, and chlorpyrifos, given in two 60 mg/kg subcutaneous exposures. Corticosterone was added to the drinking water at 400 microg/ml, to model aspects of chronic stress. These compounds/dosages were administered individually and in combination, with appropriate controls, giving rise to 16 experimental groups. The major neuropathologic change was the presence of axonal degeneration progressing to myelinated fiber degeneration, mainly in distal regions of selected fiber tracts and peripheral nerve, seen in animals sacrificed on experimental day 63. The cervical spinal cord and medullary levels of the sensory gracile fasciculus were most prominently affected. This axonopathy/fiber degeneration was TOTP dose-related at the 300 and 150 mg/kg levels. There was association of this lesion with inhibition of the enzyme neurotoxic esterase in hippocampal tissue from TOTP-treated rats. Such an association categorizes this disease process as organophosphate ester-induced delayed neuropathy. Neither chlorpyrifos nor corticosterone appeared to contribute to the neuropathic events or the enzyme inhibition. A cohort of rats was maintained on the corticosterone dosing, but without additional exposure to TOTP or chlorpyrifos, for an additional 27 days. When these rats were examined on day 90, the nerve fiber degeneration had progressed in all experimental groups administered the 300 mg/kg dose of TOTP (lower doses were not studied at the 90-day interval), although hippocampal neurotoxic esterase had returned to control values.

Organophosphorus compound-induced delayed neurotoxicity in white leghorn hens assessed by Fluoro-Jade

Certain organophosphorus (OP) compounds can induce a delayed neuropathy, termed OPIDN, that involves central and peripheral nervous system axons, terminals, and perikarya. Historically, OPIDN has been characterized by staining neural sections with silver or hematoxylin and eosin (H and E). This study utilized a novel staining method, Fluoro-Jade, for evaluating the distribution and extent of OPIDN in the central nervous system of hens. Results were then compared to synoptically sectioned and stained H and E preparations. White Leghorn hens were injected with phenyl saligenin phosphate (PSP, 2.5 mg/kg, intramuscular [im]), triphenyl phosphite (TPPi, 500 mg/kg, subcutaneous [sc]), or dimethyl sulfoxide vehicle (DMSO, 0.5 ml/kg, im or sc) and evaluated clinically for signs of neurological dysfunction associated with OPIDN. Hens were sacrificed 7, 14, and 21 days post dosing. Brains and spinal cords were removed immediately following sacrifice, fixed in formalin, and embedded in paraffin. Microtome-cut sections (7 micro m) were then stained with Fluoro-Jade (0.001%, w/v) or H&E. Staining with Fluoro-Jade revealed time-dependent degeneration of nerve fibers and terminals (with PSP and TPPi), or cell bodies (with TPPi) in lamina VII, spinocerebellar, and medial pontine-spinal tracts of the lumbar spinal cord, in white matter and mossy fibers of foliae I-V and IX of the cerebellum, and in medullary, pontine, and midbrain nuclei and paleostriatal fibers surrounding the optic tract. TPPi-induced degeneration was more extensive than that induced by PSP and affected additional cerebellar folia, medullary, pontine, midbrain, and forebrain nuclei and fiber tracts. H&E-stained sections revealed fewer sites of neurodegeneration when compared to Fluoro-Jade. These results demonstrate that Fluoro-Jade is a sensitive method for staining neural tissue affected by OPIDN.

Effects of Prednisolone and Complex of Vitamin B 1 , B 2 , B 6 and B 12 on Organophosphorus Compound‐Induced Delayed Neurotoxicity

Protective effects of prednisolone as a synthetic adrenal cortical hormone and complex of vitamin B(1), B(2), B(6) and B (12) on organophosphorus compound-induced delayed neurotoxicity (OPIDN) caused by leptophos and tri-o-cresyl phosphate (TOCP) as organophosphates (OPs) were examined. Nine groups of hens (six for each) were used. Eight groups received intravenous injection of 30 mg/kg of leptophos or 40 mg/kg of TOCP (four groups in each). Among them, three groups which received leptophos were given (p.o.) predonisolone (2 mg/body), vitamin B complex (25 mg/body) or both 3 h after OPs injection and then every day for 15 d (one group for each); the same treatment was performed on three groups which received TOCP. The remaining one group served as controls. It was observed that delayed neuropathy induced by OPs could not be resisted completely by the treatment with prednisolone or vitamin B complex, but clinical signs of OPIDN and pathological changes in hens that received these two protective agents after OPs were less severe than those in hens that received only OPs. Of these groups, the improvement in clinical signs was best shown in hens that received the both two protective agents. In addition, improvement in clinical signs among the hens that did not deteriorate to paralysis was observed. In particular, those which developed mild ataxia recovered well. It is indicated that combining administration of prednisolone and vitamin B complex early before clinical signs are manifest is effective in alleviating neuropathy. It is also suggested that recovery or good prognosis will be expected, as long as progression of the clinical signs is prevented before paralysis develops in delayed neuropathy.

Nerve terminals as the primary site of acrylamide action: a hypothesis

Acrylamide (ACR) is considered to be prototypical among chemicals that cause a central-peripheral distal axonopathy. Multifocal neurofilamentous swellings and eventual degeneration of distal axon regions in the CNS and PNS have been traditionally considered the hallmark morphological features of this axonopathy. However, ACR has also been shown to produce early nerve terminal degeneration of somatosensory, somatomotor and autonomic nerve fibers under a variety of dosing conditions. Recent research from our laboratory has demonstrated that terminal degeneration precedes axonopathy during low-dose subchronic induction of neurotoxicity and occurs in the absence of axonopathy during higher-dose subacute intoxication. This relationship suggests that nerve terminal degeneration, and not axonopathy, is the primary or most important pathophysiologic lesion produced by ACR. In this hypothesis paper, we review evidence suggesting that nerve terminal degeneration is the hallmark lesion of ACR neurotoxicity, and we propose that this effect is mediated by the direct actions of ACR at nerve terminal sites. ACR is an electrophile and, therefore, sulfhydryl groups on presynaptic proteins represent rational molecular targets. Several presynaptic thiol-containing proteins (e.g. SNAP-25, NSF) are critically involved in formation of SNARE (soluble N-ethylmaleimide (NEM)-sensitive fusion protein receptor) complexes that mediate membrane fusion processes such as exocytosis and turnover of plasmalemmal proteins and other constituents. We hypothesize that ACR adduction of SNARE proteins disrupts assembly of fusion core complexes and thereby interferes with neurotransmission and presynaptic membrane turnover. General retardation of membrane turnover and accumulation of unincorporated materials could result in nerve terminal swelling and degeneration. A similar mechanism involving the long-term consequences of defective SNARE-based turnover of Na+/K(+)-ATPase and other axolemmal constituents might explain subchronic induction of axon degeneration. The ACR literature occupies a prominent position in neurotoxicology and has significantly influenced development of mechanistic hypotheses and classification schemes for neurotoxicants. Our proposal suggests a reevaluation of current classification schemes and mechanistic hypotheses that regard ACR axonopathy as a primary lesion.

Application of silver degeneration stains for neurotoxicity testing

Silver staining procedures have been used in numerous ways to render a variety of physical and biological features visible. In biological tissue, histologic protocols use silver to visualize diverse structures or features, such as reticulin, melanin, fungi, chromosome bands, nucleolar organizing regions, and different features in the nervous system. A comparison of the specific steps in these protocols indicates that the silver is "directed" to stain any given feature by the type of fixation, the pretreatment ("mordanting"), the composition of the silver-containing solution(s), and the form of development (reduction). Since the mechanisms of staining have not been understood historically (nor are they now), each method was developed by trial and error. Keystone methods such as those of Bodian and Bielschowsky exploit the nervous system's affinity for silver (argyrophilia). The beginning of a new era in brain research came with the recognition that distinct silver-impregnated morphologic changes occurring in damaged axons could be used for tracing axon pathways in experimental animals with specifically placed lesions. Improvements in staining methods used to selectively impregnate the disintegrating axons but to leave normal axons unstained were achieved by Nauta and Gygax (early workers with these procedures) and spawned a host of method variations known as the "Nauta" methods. Of these, the Fink-Heimer and de Olmos cupric-silver methods were able to unambiguously demonstrate disintegrating synaptic terminals, thereby allowing complete tracing of axon pathways. The late 1970s and 1980s witnessed innovative applications of these techniques. The silver methods once used to trace axon pathways became indicators of the extreme endpoint of neurotoxicity: disintegrative degeneration of neurons induced by neurotoxic chemicals that were administered systemically. The hallmark of neurotoxic substances is the selectivity with which each destroys specific populations or subpopulations of neurons. The high contrast and sensitivity of the silver degeneration stains greatly facilitate the screening process to detect these affected populations, especially when there is no basis for knowing where in the brain to look for damage. More recently, in addition to expanded use in screening for neurotoxic effects, the silver degeneration stains are being used to chart the neuron populations undergoing programmed cell death in the developing brain. Other newly developed silver methods have been refined to show nondisintegrative degeneration, such as the plaques,and tangles of Alzheimer's disease.

Mechanisms of toxic injury in the peripheral nervous system: neuropathologic considerations

The anatomical distribution and organization of the peripheral nervous system as well as its frequent ability to reflect neurotoxic injury make it useful for the study of nerve fiber and ganglionic lesions. Contemporary neuropathologic techniques provide sections with excellent light-microscopic resolution for use in making such assessments. The histopathologist examining such peripheral nerve samples may see several patterns of neurotoxic injury. Most common are axonopathies, conditions in which axonal alterations are noted; these axonopathies often progress toward the Wallerian-like degeneration of affected fibers. These are usually more severe in distal regions of the neurite, and they affect both peripheral and central fibers. Examples of such distal axonopathies are organophosphorous ester-induced delayed neuropathy, hexacarbon neuropathy, and p-bromophenylacetylurea intoxication. These axonopathies may have varying pathologic features and sometimes have incompletely understood toxic mechanisms. In such neuropathies with fiber degeneration, peripheral nerve axons may regenerate, which can complicate pathologic interpretation of neurotoxicity. On occasion neurotoxins elicit more severe injury in proximal regions of the fiber (not included in this review). Axonal pathology is also a feature of the neuronopathies, toxic states in which the primary injuries are found in neuronal cell bodies. This is exemplified by pyridoxine neurotoxicity, where there is sublethal or lethal damage to larger cytons in the sensory ganglia, with failure of such neurons to maintain their axons. Lastly, one may encounter myelinopathies, conditions in which the toxic effect is on the myelin-forming cell or sheath. An example of this is tellurium intoxication, where demyelination noted in young animals is coincident with toxin-induced interference of cholesterol synthesis by Schwann cells. In this paper, the above-noted examples of toxic neuropathy are discussed, with emphasis on mechanistic and morphologic considerations.

Optimal conduct of the neuropathology evaluation of organophosphorus induced delayed neuropathy in hens

Susceptibility of various areas of the nervous system to TOCP (triorthocresyl phosphate) induced delayed neuropathy was assessed in groups of seven hens respectively, intoxicated with a single oral does of 500 or 1000 mg/kg body weight. 18 hens were used as negative controls. About 3 weeks after the treatment the hens were submitted to fixation by whole body perfusion and their nervous system processed either to paraffin sections stained with Bodian's silver stain and luxol counterstain, or to semi-thin plastic sections stained with toluidine blue. The examined areas were the cerebellum, the spinal cord at upper cervical, thoracic and lumbar level, the sciatic nerve, and the posterior tibial nerve. The extent of nerve fiber degeneration was assessed independently by two pathologists using a semiquantitative scoring system. The most susceptible areas were the cerebellum and the tibial nerve, followed by the upper cervical spinal cord. Within the cerebellum the nerve fibers in the rostral lobules, especially IV and Va, were affected. Whereas the resolution of plastic section was superior to that of paraffin sections in the cerebellum (mid-longitudinal level) and the spinal cord (transverse level), in the peripheral nerves the lesions were best recognized in the longitudinal, paraffin sections. There was very good agreement between both pathologists with respect to detection and grading of lesions in the most susceptible areas, but poor agreement in the areas of low susceptibility, indicating the danger of false results when lesions are not very distinct. In the susceptible areas the lesions induced with 500 mg/kg were sufficiently prominent, indicating that this dose-level is acceptable as positive control. In the hen nervous system, examination of the most susceptible areas, especially the rostral cerebellar lobules, appears to be suitable for detection of any kind of organophosphorus induced, delayed neuropathy.

The pathogenesis of organophosphate polyneuropathy

This review discusses the facts regarding organophosphate-induced delayed polyneuropathy (OPIDP) as they are related to its pathogenesis rather than being a comprehensive review of all available data. Neuropathy target esterase (NTE) is considered to be the molecular target for OPIDP which is affected by several esterase inhibitors. Such inhibitors are ranked according to their toxicological effects as follows: 1. Phosphates, phosphoroamidates, and phosphonates cause OPIDP when high amounts of NTE are inhibited. In most cases 70 to 80% inhibition is enough, whereas in others much more is required. 2. Phosphinates, carbamates, and sulfonyl halides cause either protection from or promotion of OPIDP when given before or after a neuropathic OP, respectively. Both effects are related to doses that inhibit NTE. Neuropathy is also caused by the combined treatment with a carbamate and a sulfonyl fluoride. The potency of a given NTE inhibitor to cause OPIDP is related to the chemistry of the residue left attached to NTE, in addition to its affinity for the enzyme. The capability of inhibited NTE to undergo the aging process distinguishes inhibitors with high from those with negligible or very low potency to cause OPIDP. Therefore, protection from neuropathic doses of effective OPs is obtained when NTE is mostly inhibited with nonageable inhibitors. Promotion of OPIDP is likely to involve another site besides NTE because it might occur when almost all NTE is affected. Promotion affects either progression or expression of OPIDP after the initial biochemical lesion on NTE. Since only NTE inhibitors have been proven to be promoters, it is possible that this site is made available after the initiation of OPIDP and that it may have biochemical properties indistinguishable from those of NTE of naïve birds. Age-related resistance to OPIDP also seems to be related to either progression or expression of OPIDP and/or to the different physiology of NTE at a given age. Previously reported resistance of rats to clinical OPIDP seems also to be age-dependent. The physiological function(s) of NTE is unknown, but some practical gains have been obtained from its identification, including OPIDP risk assessment and biomonitoring.

Selective axonal and terminal degeneration in the chicken brainstem and cerebellum following exposure to bis(1-methylethyl)phosphorofluoridate (DFP)

Utilizing a variation of the Fink-Heimer method, we examined the extent and location of axonal and terminal degeneration within the chicken cervical spinal cord, brainstem and cerebellum resulting from a single subcutaneous dose of bis(1-methylethyl)phosphorofluoridate (DFP). The effects of DFP on the activities of whole-brain neuropathy target esterase (NTE) and cholinesterase (ChE) were also assessed as were the development and severity of clinical signs characteristic of organophosphorus-induced delayed neuropathy (OPIDN). Both whole brain NTE and ChE activities were maximally inhibited during the first 24 h post-exposure, showing gradual recovery over a period of 3 weeks. OPIDN clinical signs were not observed at 7 days post-DFP but progressed to severe ataxia by day 14 and paralysis by day 21. There was a relative absence of degeneration at 7 days, a dramatic increase in degeneration density at 14 days, and high density degeneration at both 21 and 28 days. Cervical spinal and medullary tracts containing axonal degeneration included the fasciculus gracilis, dorsal and ventral spinocerebellar tracts, spinal lemniscus, and the intramedullary portions of the glossopharyngeal and vagus nerves. Brainstem nuclei containing terminal degeneration included the lateral cervical, gracile-cuneate, external cuneate, and inferior olivary nuclei, the nucleus tractus solitarius, and the lateral and paragigantocellular lateral reticular nuclei. Mossy fiber degeneration was also present in cerebellar folia I-Vb. These results show that exposure to DFP causes axonal and terminal degeneration in ascending spinal tracts, brainstem nuclei and cerebellar folia associated with the transmission of somatic and visceral sensory information.

Protease activity in brain, nerve, and muscle of hens given neuropathy-inducing organophosphates and a calcium channel blocker

Activity of calcium-activated neutral protease (CANP or calpain), an enzyme responsible for degradation of axonal and muscle cytoskeletal elements, was determined in brain, sciatic nerve, and gastrocnemius muscle of hens given tri-ortho-tolyl phosphate (TOTP, 360 mg/kg po) or active congener phenyl saligenin phosphate (PSP, 2.5 mg/kg im) with and without a calcium channel blocker which ameliorated clinical signs of organophosphate-induced delayed neuropathy (nifedipine 1 mg/kg/day x 5). Calcium channel blocker administration was initiated 1 day prior to administration of organophosphate (OP). OP administration caused an increase in CANP activity in brain within 4 days and in sciatic nerve and gastrocnemius muscle within 2 days of administration. This increase did not occur if nifedipine was administered to PSP-treated hens. Total sciatic nerve calcium concentrations were also increased by PSP, but not until OP-treated hens were no longer being administered calcium blockers. This indicates that calcium channel blockers may contribute to amelioration of organophosphate-induced delayed neuropathy by attenuation of calcium-mediated disruption of axonal and muscle cytoskeletal homeostasis.