Neurotoxic esterase in peripheral nerve: assay, inhibition, and rate of resynthesis

Subchronic Neurotoxicity of Oil Formulations Containing Either Tricresyl Phosphate or Tri-Orthocresyl Phosphate

This study was conducted to determine the threshold concentration of tricresyl phosphate (TCP) in aviation engine oil able to cause delayed peripheral neuropathy in adult hens after repeated exposure. The study also evaluated the predictive value of endpoints usually used to measure acute peripheral neurotoxicity (neurotoxic esterase [NTE] inhibition, ataxia, and histopathologic changes), as measures of neurotoxicity in a subchronic study. Animals that received oil containing 3% TCP showed significant neurotoxicity that could not be accounted for by the small amount of TOCP present. Oil containing 1% TCP was without neurotoxic activity. There was an excellent correlation between percentage inhibition of NTE and development of neuropathy. An association was also seen for ataxia and neuropathology. Further study is needed to determine the phosphate ester isomers responsible for the significant neurotoxic potency demonstrated by the aviation engine oil containing 3% TCP.

Increased oxidative stress is associated with the development of organophosphate-induced delayed neuropathy

Organophosphate-induced delayed neuropathy (OPIDN) is a progressive neuropathic disorder that manifests in days to weeks following exposure to an acute dose of organophosphates. The precise mechanism involved in the development of OPIDN is not clear as it develops after many days of the cessation of cholinergic crisis. The present study has been designed to understand the role of oxidative stress in the development of OPIDN, wherein neuropathy was developed by the administration of acute dose of monocrotophos (MCP) or dichlorvos (2,2-dichlorovinyl dimethyl phosphate (DDVP)) to rats. Significant motor deficits in terms of reduced spontaneous locomotor activity and performance on narrow beam test were observed after 14 days of exposure to MCP or DDVP, which persisted even on day 28, suggesting the development of OPIDN. Rats with OPIDN also exhibited an increase in malondialdehyde levels along with a decrease in thiol content in cerebral cortex, cerebellum and brain stem. Concomitantly, the activities of antioxidant enzymes, superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase were reduced in the three brain regions. The biochemical and functional changes were associated with histological alterations in the brain regions studied. The results clearly indicate that the development of OPIDN is mediated in part through an increased oxidative stress and suggest that the strategies aimed at restoration of antioxidant capacity may be beneficial for the individuals with OPIDN-like symptoms.

Down-regulation of neuropathy target esterase by protein kinase C activation with PMA stimulation

Neuropathy target esterase (NTE) was originally identified as the primary target site of those organophosphorus compounds that induce delayed neuropathy in human and some animals. Here we examined the role of protein kinase C (PKC) in the regulation of the NTE activity in mammalian cells. Six-hour exposure of human neuroblastoma SK-N-SH cell to a PKC activator phorbol 12-myristate 13-acetate (PMA) decreased the activity of NTE, and this effect was blocked by the PKC inhibitor staurosporine. These results suggest that PKC down-regulates the activity of NTE. NTE protein levels were down-regulated by PMA-stimulation as detected by Western blot analysis using the NTE-specific antibody, which resulted from down-regulation of NTE mRNA level as verified by real-time reverse transcription polymerase chain reaction (RT-PCR). However, there were no changes in the activity or protein levels of stable expression of NTE esterase activity domain (NEST) in SK-N-SH cells and transient expression of full-length NTE construct in COS7 cells driven by cytomegalovirus (CMV) promoter rather than by the cell's own one, despite the absence or presence of PMA stimulation. Together, these findings suggest that stimulation with PMA reduces the expression of NTE mRNA levels but does not affect the exogenous promoter-driven NTE expression in mammalian cells.

Recovery of neuropathy target esterase activity after inhibition with mipafox and O-hexyl O-2,5-dichlorophenyl phosphoramidate in bovine chromaffin cell cultures

Neuropathy target esterase (NTE) is a membrane protein present in various tissues whose physiological function has been recently suggested to be the maintenance of phosphatidylcholine homeostasis. Inhibition and further modification of NTE by certain organophosphorus compounds (OPs) were related to the induction of the "organophosphorus induced delayed neuropathy". Bovine chromaffin cells were cultured at 75,000cells/well in 96-well plates and exposed to 25microM mipafox or 3microM O-hexyl O-2,5-dichlorophenyl phosphoramidate (HDCP) for 60min. Inhibitors were removed by washing cells three times with Krebs solution. Then NTE activity was assayed at 0, 24, 48 and 120h after exposure using the Biomek 1000 workstation. Immediately after mipafox treatment NTE activity represented 3% of the control (6.7+/-1.9mU/10(6) cells). At 24, 48 and 120h after removing inhibitor, recorded activities were 33%, 42% and 111% of their respective controls (5.7+/-3.1; 5.7+/-1.9; 5.4+/-0.0mU/10(6) cells, respectively). Treatment with HDCP also displayed a time-dependent pattern of NTE recovery. As NTE inhibited by phosphoramidates is not reactivated in homogenized tissues, these results confirm a time-dependent regeneration of NTE after inhibition by neuropathic OPs.

Testing for organophosphate-induced delayed polyneuropathy

Organophosphorous compounds may cause two distinct types of toxicity: acute cholinergic toxicity and organophosphate-induced delayed polyneuropathy (OPIDP). The ability of a compound to cause OPIDP is assessed as described by administering the compound to hens and screening the brain, spinal cord, and peripheral nerves for neuropathy target esterase activity to detect OPIDP and acetylcholinesterase activity to rule out the acute toxicity. This assay can also be used as part of a screen for protective agents.

Correlation of binding sites for diisopropyl phosphorofluoridate with cholinesterase and neuropathy target esterase in membrane and cytosol preparations from hen

To find new putative target(s) for organophosphorus induced delayed neurotoxicity (OPIDN), we investigated the biochemical and pharmacological characteristics of [3H] diisopropyl phosphorofluoridate (DFP) binding to membrane and cytosol preparations from the brain and spinal cord of hens. Specific [3H]DFP binding was determined by subtracting non-specific binding from total binding. The binding sites of [3H]DFP, an organophosphate that induces OPIDN, were found not only on membrane but also in cytosol. Reduction of subsequent ex vivo specific [3H]DFP binding by in vivo pretreatment with unlabeled DFP was found in cytosol, not membrane. The reduced binding lasted to the onset of OPIDN, especially in spinal cord. These results suggest that the specific DFP binding sites in cytosol, rather than on membrane, are the most important with regard to the initiation of OPIDN. Inhibitors of cholinesterase (ChE) and neuropathy target esterase (NTE) other than DFP did not affect specific [3H]DFP binding to either membranes or cytosol in vivo. Additionally, inhibition of the activities of these esterases by these compounds was not consistent with either the degree of inhibition of the [3H]DFP binding or a time-dependent manner of OPIDN. These results suggest that DFP binding site(s) involved in the initiation of OPIDN may be different from the active sites of ChE and NTE.

Localization of [3H]octylphosphonyl-labeled neuropathy target esterase by chicken nervous tissue autoradiography

Neuropathy target esterase (NTE) undergoes phosphorylation and aging as the initial steps in organophosphorus (OP)-induced delayed neuropathy (OPIDN). Localization of NTE is an important step in characterizing the mechanism of OPIDN. Earlier histochemical immunoreactivity or esterase assays localized NTE in areas of the brain and spinal cord rich in neuronal cell bodies and in the dorsal root ganglion. We use a more direct and quantitative autoradiographic approach of forming phosphorylated and aged [3H]octylphosphonyl-NTE on treatment with the highly potent [octyl-3H]octyl-4H-1,3,2-benzodioxaphosphorin 2-oxide to determine NTE as the labeling site resistant to the non-neuropathic paraoxon and sensitive to the neuropathic mipafox. NTE is observed in the cerebral cortical layer, some layers of the optic tectum, the gray matter of the spinal cord and the sensory neurons of the dorsal root ganglion to a higher extent than in adjacent areas.

An automatable microassay for phenyl valerate esterase activities sensitive to organophosphorus compounds

An automatable microassay method developed for phenyl valerate esterase (PVase) activity has been applied to determine the following activities in the soluble fraction of hen sciatic nerve: activity A (total PVase activity), activity B (paraoxon-resistant PVase activity), activity C (PVase activity resistant to 40 microM paraoxon and 250 microM mipafox) and neuropathy target esterase (NTE) activity (resistant to 40 microM paraoxon but sensitive to 250 microM mipafox), operationally defined as activity (B-C). This microassay is based on the technique described by Barril et al. (Toxicology. 1988. 49:107-114). The Automated Biomek 1000 Station was used, which guarantees both inter- and intra-assay reproducibility of the results, and shortens the total assay time. The technical problems involved when processing many samples were thus resolved and with same regards it can also apply manually and using a microplate reader. In the case of activity A, the sensitivity of the method allowed the detection of activity in 1 microgram of protein (0.15 mg fresh sciatic nerve tissue), and the response was linear for different concentrations of 0.15-1.7 mg fresh tissue. For B, C and NTE, sensitivity corresponded to 10 micrograms of protein (1.5 mg fresh tissue in the microassay), with a linear response in the range of 1.5-17 mg fresh tissue. The response was linear versus the time of enzyme-substrate reaction (30-150 min). As tissue concentration increased, the response became nonlinear at shorter time. The procedure may be used to measure other enzymatic activities that yield phenols and chlorophenols as reaction products.

Organophosphorus inhibition and heat inactivation kinetics of particulate and soluble forms of peripheral nerve neuropathy target esterase

Neuropathy target esterase (NTE) is the proposed target site for the mechanism of initiation of the so-called organophosphorus-induced delayed polyneuropathy (OPIDP). NTE is operationally defined in this article as the phenylvalerate esterase activity which is resistant to inhibition by 40 microM paraoxon and sensitive to 250 microM mipafox. Soluble (S-NTE) and particulate (P-NTE) forms of NTE had first been identified in hen sciatic nerve [E. Vilanova, J. Barril, V. Carrera, and M. C. Pellín (1990). J. Neurochem., 55, 1258-1265]. P-NTE and S-NTE showed different sensitivities to the inhibition by several organophosphorus compounds over a range of inhibitor concentrations for a 30 or 120 minute fixed inhibition time at 37 degrees C. S-NTE was less sensitive to the inhibition by O,O'-diisopropyl phosphorofluoridate (DFP), hexyl 2,5-dichlorophenyl phosphoramidate (H-DCP), and mipafox than P-NTE and brain NTE, while the opposite was true for O,S-dimethyl phosphoroamidothioate (methamidophos). For each of the four inhibitors assayed, S-NTE showed two components of different sensitivity according to the inhibition curves fitted with exponential models. However, the inhibition of P-NTE by mipafox, DFP, and HDCP did not show the presence of a considerable proportion of a second component. The kinetics of heat inactivation showed that P-NTE inactivated faster and to a greater extent than S-NTE. It is concluded that (1) sciatic nerve S-NTE is more different from brain NTE than P-NTE; (2) P-NTE and S-NTE have different sensitivities to the inhibition by the studied organophosphorous compounds; (3) the inhibition curves suggest that S-NTE has two different enzymatic components while these are not so evident for P-NTE.

Biochemical, neuropathological and behavioral studies in hens induced by acute exposure of tri-ortho-cresyl phosphate

A "hen model" or organophosphorus induced delayed neuropathy (OPIDN) has been developed using white leghorn exposed acutely to one of five dosages of tri-ortho-cresyl phosphate (TOCP), between 300 to 700 mg/Kg. Neuropathy target esterase was studied in brain and peripheral nerve 24 and 48 hrs following exposure. Behavioral symptoms abnormality was assessed from days 1 through 20 after exposure using a 7 point rating scale and neuropathological examination was conducted on sample collected from animals on days 0, 7, 14 and 21. Neuropathological abnormalities were indicated by damage scores between 0 (no damage) and 4 (gliosis of brain tissue, myelin loss, appearance of axonal foci etc and more than 55% degeneration of peripheral nerve fibres). TOCP (600 and 700 mg/Kg, orally) was able to inhibit NTE more than 75% in brain and peripheral nerves. TOCP at the same dosage was also capable of resulting maximal levels of neuropathological score at 4. After exposure to doses weight loss was observed abruptly in a greater extent at the beginning leading to a change in weight gain till the end of the experiment. Behavioral signs were also dose dependent. Symptoms (gain abnormality, ataxia, paresis) were noted on the early stage of experiment. Inhibition of NTE was 65% could not be reached in hens given TOCP without causing lethality and no significant ataxia or lesions developed in those birds. Behavioral signs were also observed to be late onset. These data indicate that more than 75% inhibition of peripheral nerve NTE after 24 hr exposure was predictive of severe behavioral abnormalities and pathology in the hen whereas less peripheral NTE inhibition was indicative of less severe behavioral abnormalities and a lower score for neuropathological damage.

In vivo inhibition by mipafox of soluble and particulate forms of organophosphorus neuropathy target esterase (NTE) in hen sciatic nerve

Neuropathy target esterase (NTE) is a protein suggested to be involved in the initiation mechanism of organophosphorus-induced delayed neuropathy (OPIDP). We previously described two different forms of NTE activity in hen sciatic nerve: a particulate form (P-NTE) representing 40-50% of total NTE activity in sciatic nerve, and a remaining soluble component (S-NTE). In brain tissue on the other hand, more than 90% of NTE activity was recovered as P-NTE. In this work we studied the in vivo inhibition of both NTE forms with different doses of mipafox and the results were compared with sensitivity to mipafox in vitro. The highest dose with no observable neuropathic effects (1.5 mg/kg mipafox p.o.) inhibited 33% P-NTE and 55% S-NTE activity. The difference between P-NTE and S-NTE activity was statistically significant (P < 0.001, n = 9). Higher doses (3 mg/kg) induced neuropathy and inhibited NTE more than 75%, but differences between P- and S-NTE were not significant (P > 0.5). The greater inhibition of S-NTE than P-NTE in vivo contrasts with the observation that S-NTE is less sensitive in vitro.

Soluble and particulate organophosphorus neuropathy target esterase in brain and sciatic nerve of the hen, cat, rat, and chick

Considerable evidence exists suggesting that the so-called neuropathy target esterase (NTE) is involved in the mechanisms responsible for organophosphorus-induced delayed polyneuropathy (OPIDP). Earlier studies in the adult hen, the habitually employed experimental model in OPIDP, have shown that most NTE activity in the brain is centered in particulate fractions, whereas approximately 50% of this activity in the sciatic nerve is encountered in soluble form, with the rest being particulate NTE. In the present work, we have studied the particulate and soluble fractional distribution of paraoxon-resistant phenylvalerate esterase activity (B activity), paraoxon- and mipafox-resistant phenylvalerate esterase activity (C activity), and NTE activity (B-C) according to ultracentrifugation criteria (100,000 g for 1 h). To this effect, two sensitive (adult hen and cat) and two scarcely sensitive (rat and chick) models were used. In all four experimental models, the distribution pattern was qualitatively similar: B activity and total NTE were much greater in brain (900-2,300 nmol/min/g of tissue) than in sciatic nerve (50-100 nmol/min/g of tissue). The proportion of soluble NTE in brain was very low (< 2%), whereas its presence in sciatic nerve was substantial (30-50%). The NTE/B ratio in brain was high for the particulate fraction (> 60%) and low in the soluble fraction (7-30%); in sciatic nerve the ratio was about 50% in both fractions.(ABSTRACT TRUNCATED AT 250 WORDS)

The search for the physiological functions of NTE; is NTE a receptor?

Neuropathy target esterase (NTE) was identified as the molecular target for organophosphate-induced delayed polyneuropathy several years ago but its physiological functions are still unknown. The mechanism which initiates neuropathy was thought to be a two step process: inhibition (phosphorylation) of NTE and aging of phosphorylated NTE. Depending on the occurrence of the second reaction (aging), inhibitors were ranked as neuropathic (forming an ageable NTE) and non-neuropathic (forming a non-ageable NTE). Non-neuropathic inhibitors protect from neuropathy if given before the neuropathic ones, because they occupy the catalytic centre of NTE. Thus the catalytic function of NTE seems irrelevant in maintaining the health of neurons. This paper reviews some new information concerning the interaction of NTE with its inhibitors as well as on a phenomenon called promotion of neuropathy. Some inhibitors which apparently form a non-ageable inhibited NTE were found to cause neuropathy, even though some of them must be given at very high doses. Moreover some 'non-neuropathic-protective' NTE inhibitors were found to exacerbate (promote) neuropathy when given after a neuropathic one. It is likely that the target for promotion is other than NTE. The hypothesis that NTE has some unknown receptorial functions where inhibitors act with different efficacy is discussed. NTE inhibitors have been ranked as full agonists (classic neuropathic inhibitors such as diisopropylfluorophosphate), partial agonists (protective or neuropathic, depending on the dose, such as methamidophos) and antagonists (protective, and neuropathic at the highest doses, such as phenylmethanesulfonyl fluoride). Age-related differences in the 'receptor' NTE might be responsible for the different sensitivities of juvenile and adult animals.

Biochemical properties and possible toxicological significance of various forms of NTE

NTE (neuropathy target esterase) is considered to be the target for organophosphorus-induced delayed polyneuropathy and is operationally measured by radiolabelling or by determining its esteratic activity as the paraoxon-resistant mipafox-sensitive phosphorylable site(s). From electrophoresis and density gradient centrifugation using radiolabelling techniques, several phosphorylable sites have been described in hen brain that are paraoxon-resistant mipafox-sensitive; however, only the majority electrophoresis band (155 kDa) shows properties related with the aging reaction. Kinetic criteria have also suggested two components of brain NTE (NTEA and NTEB). Most brain NTE is recovered in the particulate microsomal fraction and only about 1% in soluble fraction. In sciatic nerve about 50%/50% activity is recovered as soluble (S-NTE) or particulate (P-NTE) forms. A similar distribution were observed in hen, cat, rat and young chick. The fixed time inhibition curves show that P-NTE is more sensitive to mipafox, DFP and hexyl-DCP than S-NTE, while the reverse is true for methamidophos. P-NTE fits properly to one sensitive component while S-NTE fits better to two sensitive component models, except in the case of methamidophos. In vivo, significant differences in the inhibition of P- and S-NTE by mipafox were found only when using low non-neuropathic dosing. The possible significance of different NTE forms are discussed.

Delayed neurotoxic effect of sarin in mice after repeated inhalation exposure

Delayed neurotoxicity of sarin in mice after repeated inhalation exposure has been studied. Female mice exposed to atmospheric sarin (5 mg m-3 for 20 min) daily for 10 days developed muscular weakness of the limbs and slight ataxia on the 14th day after the start of the exposure. These changes were accompanied by significant inhibition of neurotoxic esterase (NTE) activity in the brain, spinal cord and platelets. Histopathology of the spinal cord of exposed animals showed focal axonal degeneration. These changes were comparatively less than in animals treated with the neurotoxic organophosphate, mipafox. Results from this study indicate that sarin may induce delayed neurotoxic effects in mice following repeated inhalation exposure.

Organophosphate polyneuropathy in chicks

Young animals are resistant to organophosphate-induced delayed neuropathy (OPIDP), although biochemical changes on Neuropathy Target Esterase (NTE) caused by neuropathic organophosphorus esters (OP) are similar to those observed in the sensitive hen. We report here that the resistance of chicks to single doses of neuropathic OPs is not absolute because ataxia was produced in 40-day-old chicks by 2,2-dichlorovinyl dibutyl phosphate (DBDCVP, 5.0 or 10.0 mg/kg s.c.) and by diisopropyl phosphorofluoridate (DFP, 2.0 mg/kg s.c.). However, the clinical picture was different from that usually seen in hens; spasticity and complete recovery being the main features. alpha-Tolyl sulphonyl fluoride (PMSF, 300 mg/kg s.c.) promoted both DBDCVP neuropathy (5.0 or 10.0 mg/kg s.c.) and non-neuropathic doses of DFP (1.5 mg/kg s.c.) or DBDCVP (1.0 mg/kg s.c.). The lowest promoting dose of PMSF given 24 hr after 1.5 mg/kg of DFP was 30 mg/kg. Higher doses had a more severe effect but no further increase of OPIDP severity was obtained with doses ranging from 90 to 300 mg/kg. PMSF (30 mg/kg) protected 40-day-old chicks from subsequent doses of neuropathic OPs even when a promoting dose of PMSF followed. At 60 days of age, chicks' resistance to OPIDP decreased because lower doses of neuropathic OPs became effective and, similarly to hens, PMSF did not fully protect from subsequent promotion. In 40-day-old chicks the threshold of NTE inhibition for OPIDP development was 95-97% (DBDCVP 5.0 mg/kg). When promotion followed initiation, the minimal effective inhibition of NTE for initiation by neuropathic OPs was about 90%. In 36-day-old chicks, PMSF (300 mg/kg) promoted OPIDP when given up to 5 days after DFP (1.5 mg/kg) when residual NTE inhibition in brain and sciatic nerve was about 40%. We conclude that chicks' resistance to OPIDP might reflect either a less effective initiation by phosphorylated NTE or a more efficient repair mechanism or both, and also that promotion is likely to involve a target other than NTE.

Clinical expression of organophosphate-induced delayed polyneuropathy in rats

Single doses of certain organophosphates (OP), such as dibutyl-2,2-dichlorovinyl phosphate (DBDCVP) cause organophosphate-induced delayed polyneuropathy (OPIDP) in hens. Clinical effects correlate with inhibition of neuropathy target esterase (NTE) which is considered the target for this toxicity. Pre-treatment with non-neuropathic NTE inhibitors, such as phenylmethanesulfonyl fluoride (PMSF), protects from OPIDP. However, when given after OPs, these compounds promote OPIDP. Chicks are relatively resistant to OPIDP despite high NTE inhibition. It has also always been reported that rats represent a species which is resistant to OPIDP and that they might develop morphological but not clinical signs of OPIDP. We report here that clinical OPIDP can be produced in 3.5- and 6-month-old rats by DBDCVP (5 mg/kg s.c.) and that it correlates with high (> 90%) NTE inhibition. When PMSF (120 mg/kg s.c. x 2) was given after DBDCVP, OPIDP was promoted. Pretreatment with PMSF protected from OPIDP. We conclude that resistance to OPIDP in the rat is age-related, as it is in the hen.

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.

Phenylmethanesulfonyl fluoride elicits and intensifies the clinical expression of neuropathic insults

It has been recently reported that phenylmethanesulfonyl fluoride (PMSF) when given to hens after a neuropathic organophosphate (OP) promotes organophosphate-induced delayed polyneuropathy (OPIDP). Chicks are resistant to OPIDP despite high inhibition/aging of neuropathy target esterase (NTE), the putative target of OPIDP initiation. However, when PMSF (300 mg/kg s.c.) is given to chicks after di-butyl 2,2-dichlorovinyl phosphate (DBDCVP, 1 or 5 mg/kg s.c.), OPIDP is promoted. Inhibition/aging of at least 30% of NTE was thought to be an essential prerequisite for promotion to be elicited in adult hens. However, we observed in hens that when NTE is maximally affected (greater than 90%) by phenyl N-methyl N-benzyl carbamate (40 mg/kg i.v.), a non-ageable inhibitor of NTE, and then PMSF is given (120 mg/kg/day s.c. x 3 days) clinical signs of neuropathy become evident. Methamidophos (50 mg/kg p.o. to hens), which produces in vivo a reactivatable form of inhibited NTE, was shown either to protect from or promote OPIDP caused by DBDCVP (0.45 mg/kg s.c.), depending on the sequence of dosing. Because very high doses of methamidophos cause OPIDP, we considered this effect to be a "self-promoted" OPIDP. We concluded that NTE inhibitors might have different intrinsic activities for producing OPIDP once NTE is affected. Aging might differentiate highly neuropathic OPs, like DBDCVP, from less neuropathic OPs, like methamidophos, or from the least neuropathic carbamates, which require promotion in order for neuropathy to be expressed.(ABSTRACT TRUNCATED AT 250 WORDS)

Anomalous biochemical responses in tests of the delayed neuropathic potential of methamidophos (O,S-dimethyl phosphorothioamidate), its resolved isomers and of some higher O-alkyl homologues

The interaction with neural neuropathy target esterase (NTE) and acetylcholinesterase (AChE) in vivo of methamidophos (O,S-dimethyl phosphorothioamidate), its resolved stereoisomers and five higher O-alkyl homologues has been examined along with the ability of these compounds to cause organophosphorus-induced delayed polyneuropathy (OPIDP) in adult hens. For the lower homologues AChE was more sensitive than NTE and it was impossible to achieve high inhibition of NTE in vivo without both prophylaxis and therapy against acute anticholinesterase effects; for the n-hexyl homologue high inhibition of NTE could be achieved without obvious anticholinesterase effects and spontaneous reactivation of inhibited AChE was seen as in vitro. The maximum tolerated dose of L(-) methamidophos or of the ethyl or iso-propyl homologues did not inhibit NTE more than 60%, and surviving birds did not develop OPIDP. The n-propyl, n-butyl and n-hexyl compounds caused typical OPIDP at doses causing a peak of 70-95% inhibition of NTE in brain, spinal cord and sciatic nerve soon after dosing. Racemic methamidophos caused unusually mild OPIDP associated with very high inhibition of NTE at doses estimated to be greater than 8 times the unprotected LD50 and the D-(+) isomer caused OPIDP at about 5-7 x LD50. Clinical effects correlated with histopathology in 19 out of 20 examined birds. In contrast to results of many previous studies with organophosphates and phosphonates, all these cases of OPIDP were associated with formation of inhibited NTE which could be reactivated ex vivo by treatment of autopsy tissue with KF solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of methamidophos with hen and human acetylcholinesterase and neuropathy target esterase

Methamidophos causes acute cholinergic toxicity in several species, including man, and organophosphate-induced delayed polyneuropathy which has been reported in man but not in the hen. Acetylcholinesterase (AChE) and neuropathy target esterase (NTE) are thought to be the molecular targets of acute and delayed toxicity, respectively. The rate constants of inhibition (ka) and reactivation (k + 3) of human and hen brain AChE and NTE by methamidophos resolved optical isomers are here reported. NTE inhibition was progressive and irreversible. Human and hen NTE ka (M-1.m-1) for D-(+) methamidophos was 88 and 59, respectively, and for L-(-) methamidophos 3.2 and 3.0, respectively. AChE spontaneously reactivates after inhibition. D-(+) methamidophos 10(-3).ka (M-1.m-1) for human and hen AChE was 0.24 and 0.13; 10(3).k+3 (m-1) was 0.83 and 0.69, respectively. L-(-) Methamidophos 10(-3).ka (M-1.m-1) for human and hen AChE was 5.7 and 2.8, whereas 10(3).k+3 (m-1) was 6.50 and 1.52, respectively. L-(-)-Inhibited AChE reactivated to about 60% for human and 30% for hen enzymes, respectively. D-(+)-Inhibited AChE reactivated to about 10-20% for both species. Maximal reactivation occurred within 4-6 h when a plateau was reached. The larger and faster reactivation of human AChE inhibited in vitro by L-(-) methamidophos suggests that a corresponding effect might be possible in vivo and therefore explain, in part, the relatively higher susceptibility of man to delayed polyneuropathy induced by racemic methamidophos which occurs, however, with doses always causing severe cholinergic toxicity.

Age sensitivity to organophosphate-induced delayed polyneuropathy. Biochemical and toxicological studies in developing chicks

Young animals are resistant to organophosphate-induced delayed polyneuropathy (OPIDP). The putative target protein in the nervous system for initiation of OPIDP in the adult hen is an enzyme called Neuropathy Target Esterase (NTE), which is dissected by selective inhibitors among nervous tissue esterases hydrolysing phenyl valerate (PV). We report here that the pool of PV-esterases sensitive to paraoxon was different in peripheral nerves of chicks as compared to that of hens while that of brain and spinal cord was not. NTE activity decreased with age in brain, spinal cord and peripheral nerve, but its sensitivity to several inhibitors remained unchanged. In the adult hen more than 70% inhibition of peripheral nerve NTE by neuropathic OPs is followed by deficit of retrograde axonal transport, axonal degeneration and paralysis. Similar NTE inhibition in 40-day-old or younger chicks however is not followed by changes in retrograde axonal transport nor by OPIDP. Chicks aged 60 to 80 days are only marginally sensitive to a single dose of DFP otherwise clearly neuropathic to hens. In vitro and in vivo phosphorylation by DFP and subsequent aging of brain NTE is similar both in chicks and in hens. The recovery of NTE activity monitored in vivo after inhibition by DFP is faster (half-life of about 3 days) in chick peripheral nerves as compared to chick brain, hen brain and hen peripheral nerve (half-life of about 5 days). It is concluded that the reduced sensitivity to OPIDP in chicks is not due to differences in OP-NTE interactions. The resistance might be explained by a more efficient repair mechanism, as suggested by the faster recovery of peripheral nerve NTE activity.

Promotion of organophosphate-induced delayed polyneuropathy by phenylmethanesulfonyl fluoride

Certain sulfonates, like phenylmethanesulfonyl fluoride (PMSF), carbamates, and phosphinates, when given prior to neuropathic doses of organophosphates such as diisopropyl phosphorofluoridate (DFP), protect hens from organophosphate-induced delayed polyneuropathy (OPIDP). Protection was related to inhibition of the putative target of OPIDP, which is called Neuropathy Target Esterase (NTE). NTE inhibition above 70-80% in the nervous system of hens followed by a molecular rearrangement called aging initiates OPIDP. PMSF and other protective chemicals inhibit NTE but OPIDP does not develop because aging cannot occur. DFP (1 mg/kg sc) inhibited NTE above 70-80% in peripheral nerve and caused OPIDP in hens. Lower doses (0.3 and 0.5 mg/kg sc) caused about 40-60% NTE inhibition and no or marginal OPIDP. Chlorpyrifos (90 mg/kg po) also caused OPIDP. When repeated (30 mg/kg sc daily for 9 days) or single (5-120 mg/kg sc) doses of PMSF were given after either DFP or chlorpyrifos, OPIDP developed in birds treated with nonneuropathic doses of DFP and was more severe in birds treated with chlorpyrifos or higher doses of DFP. PMSF increased NTE inhibition to greater than 90%. Promotion of OPIDP with a single dose of PMSF (120 mg/kg sc) was obtained in birds up to 11 days after a marginally neuropathic dose of DFP (0.5 mg/kg sc). Promotion was also obtained with phenyl N-methyl N-benzyl carbamate (40 mg/kg iv) but not with non-NTE inhibitors in vivo such as paraoxon or benzenesulfonyl fluoride when given at maximum tolerated doses. These results indicate that protection from OPIDP is only one effect of PMSF because promotion of OPIDP is also observed depending upon the sequence of dosing. Either effect is always related to the doses of PMSF, which inhibit NTE.

Chlorpyrifos-induced delayed polyneuropathy

Chlorpyrifos [0,0-diethyl 0-(3,5,6-trichloro-pyridyl) phosphorothioate] caused delayed polyneuropathy in man. Contrary to previous studies, we report here that it also causes delayed polyneuropathy in the hen, the animal model for this toxicity. The minimal neuropathic dose was 60-90 mg/kg p.o., corresponding to 4-6 times the estimated LD50. Consequently, pralidoxime (2-PAM) in conjunction with atropine was necessary to reverse acetylcholinesterase (AChE) inhibition and cholinergic toxicity in hens given high enough doses of chlorpyrifos to cause neuropathy. Chlorpyrifos was slowly absorbed after single oral doses and the threshold of inhibition (greater than 70%) of neuropathy target esterase (NTE), the putative target for delayed neuropathy, was reached within 5-6 days. High AChE inhibition (greater than 90%), however, was measured within hours after dosing because of the higher potency of chlorpyrifos to inhibit this enzyme. In vitro studies showed that chlorpyrifos-oxon, the active metabolite of chlorpyrifos, was 10-20 times more active against AChE than against NTE, confirming the clinical observation. No differences were seen between human and hen enzymes in this respect. Hen and human brain homogenates contain A-esterases which hydrolysed chlorpyrifos to about the same extent in both species. In conclusion, chlorpyrifos causes delayed polyneuropathy in the hen, as was reported in man. The reasons for previous negative data in the hen are probably due to the relatively lower doses which were used. Judging from in vitro studies with hen and human enzymes, there are no differences in the two species as far as their relative sensitivity to delayed polyneuropathy. It is likely that delayed polyneuropathy would develop in both species only after severe cholinergic toxicity requiring aggressive antidotal treatment.

Soluble and particulate forms of the organophosphorus neuropathy target esterase in hen sciatic nerve

Neuropathy target esterase (NTE) is the suggested "target" molecule involved in the initiation of organophosphorus-induced delayed polyneuropathy. Sciatic nerve NTE was separated into particulate (P-NTE) and soluble (S-NTE) fractions by ultracentrifugation at 100,000 g for 1 h in 0.32 M sucrose and compared with the corresponding brain extract. Total sciatic NTE activity was 80-100 nmol/min/g tissue from which 50-60% was recovered in the soluble supernatant fraction and the remaining 40-50% in the pellet fraction. About 90% of brain tissue activity (approximately 1,800 nmol/min/g tissue) was recovered as P-NTE. A similar distribution was obtained when more drastic centrifugation without sucrose was performed. P-NTE and S-NTE were distributed with the membrane and cytosolic markers assayed, respectively, glucose-6-phosphatase, Na+,K(+)-ATPase, 5'-nucleotidase, phospholipids, and lactate dehydrogenase. When the pH during the centrifugation was increased from 6.4 to 11, recovered P-NTE activity decreased from 1,750 to 118 nmol/min/g tissue for brain and from 31 to 12 nmol/min/g for sciatic nerve. However, S-NTE activity and total nonfractionated control activity were only slightly affected by the same pH treatment. The distribution pattern encountered may be better understood as representing two different proteins than an equilibrium between soluble and membrane-bound portions of a single protein, with P-NTE activity depending on a membrane factor from which it is separated through fractionation at high pH.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

In vivo and in vitro regional differential sensitivity of neuropathy target esterase to di-n-butyl-2,2-dichlorovinyl phosphate

Organophosphate-induced delayed polyneuropathy (OPIDP) is initiated by inhibition/aging of more than 70-75% of neuropathy target esterase (NTE). Di-n-butyl-2,2-dichlorovinyl phosphate (DBDCVP) (1 mg/kg s.c.) inhibited 96%, 86% and 83% of NTE in brain, spinal cord and peripheral nerve, respectively, and induced a typical central peripheral distal axonopathy in hens. A lower dose (0.45 mg/kg s.c.) caused 90%, 83% and 54% NTE inhibition in the same organs; by contrast, hens developed a spastic ataxia with axonal degeneration in spinal cord but not in peripheral nerve. With a dose of 0.2 mg/kg s.c., a suprathreshold inhibition of NTE was produced in brain (78%) but not in spinal cord (56%) and peripheral nerve (33%) and no morphological or clinical signs of neuropathy developed in hens. With doses up to 4.0 mg/kg s.c., acetylcholinesterase (AChE) inhibition was similar throughout the nervous system. In vitro time-course inhibition studies showed a different sensitivity to DBDCVP of NTE from peripheral nerve (ka = 5.4 x 10(6)) relative to that from spinal cord (ka = 13.9 x 10(6)) or brain (ka = 20.6 x 10(6)). In vitro I50s of DBDCVP for AChE were similar in brain, spinal cord and peripheral nerve (11-17 nM). These data support the hypothesis that the critical target for initiation of OPIDP is located in the nerve fiber, possibly in the axon and also suggest that peripheral nerve NTE has a different sensitivity to DBDCVP than the brain enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Prophylaxis and the mechanism for the initiation of organophosphorous compound-induced delayed neurotoxicity

Recent work concerned with the mechanism underlying the development of organophosphorous compound-induced delayed neurotoxicity (OPIDN) is reviewed. Topics covered include the prophylaxis of OPIDN by phenylmethylsulfonyl fluoride and other agents, neurotoxic esterase (NTE) as measured using either labelled di-isopropyl phosphorofluoridate or an esterase assay, and the relationship between NTE and the development of OPIDN. There is considerable evidence that NTE has the biochemical properties which should be expected for the initiation site for OPIDN. However, the in vitro assays as currently performed may not entirely reflect the behavior of organophosphorous compounds in vivo, or the assays may not be sensitive enough to identify the actual target. It is argued that prophylaxis is a distinguishing characteristic of OPIDN which is not necessarily related to NTE inhibition, although it does provide evidence that NTE is involved. It is concluded that the NTE hypothesis could be furthered by additional studies with peripheral nerve, more sensitive methods for the detection of potential binding sites, and the establishment of a physiological role for NTE which relates it to the neuropathy.

Biochemical and clinical tests of the delayed neuropathic potential of some O-alkyl O-dichlorophenyl phosphoramidate analogues of methamidophos (O,S-dimethyl phosphorothioamidate)

The interaction in vivo of four O-alkyl O-2,5-dichlorophenyl phosphoramidates with neural neuropathy target esterase (NTE) and acetylcholinesterase (AChE) and their ability to cause delayed polyneuropathy in hens has been examined. Previous studies in vitro (Vilanova, Johnson & Vicedo, Pestic. Biochem. Physiol., 28 (1987) 224) had led to the prediction that these compounds would not be neuropathic but, rather, would be prophylactic agents against organophosphorus-induced delayed polyneuropathy. In vivo the effects of these esters on the enzymes differ in 2 respects from effects in vitro: (i) Relative sensitivity of the enzymes was different: thus greater than 50% of brain NTE remained 24 h after an oral dose of 15 mg/kg of the n-hexyl ester while only 10-30% of AChE remained although NTE was the more sensitive enzyme in vitro; (ii) In no case could the inhibited NTE or AChE in autopsy samples from birds dosed with any of the 4 esters be reactivated by treatment with potassium fluoride in vitro: the inhibited enzymes produced by incubation of tissue with the esters in vitro had been reactivatable. Prophylaxis, with therapy in some cases, was required to prevent acute anticholinesterase poisoning when doses were sufficient to cause high inhibition of neural NTE. Inhibition in brain was typically 5-10% more than in spinal cord and 10-15% more than in sciatic nerve. Unambiguous signs of polyneuropathy (Grade 3 or more on an 8-point scale) were not seen in birds observed up to 3 weeks after doses which caused less than 70% inhibition of NTE in brain and spinal cord or less than 60% inhibition in sciatic nerve of pair-dosed birds assayed 24 h after dosing. Doses of 300, 10, 100 and 65 mg/kg, respectively, of the methyl, ethyl, n-butyl and n-hexyl esters caused greater than 70% inhibition of NTE in all 3 neural tissues and neuropathy in the majority of observed birds. Analysis of consolidated dose/response data from 36 assayed and 51 observed birds showed that effects of Grade 3 or more were produced in about 90% of birds when inhibition of NTE was greater than 90% in brain, greater than 85% in spinal cord or greater than 75% in sciatic nerve.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropathy target esterase: rates of turnover in vivo following covalent inhibition with phenyl di-n-pentylphosphinate

Phenyl di-n-pentylphosphinate is a reasonably stable easily synthesized inhibitor of neuropathy target esterase (NTE) with low anticholinesterase activity. Like phenylmethylsulphonyl fluoride it protects hens against neuropathic effects of compounds such as diisopropylphosphorofluoridate. At intervals up to 15 days after dosing hens (10 mg/kg s.c. to inhibit 90% NTE) assays were made of catalytically active and of phosphinylated NTE in autopsy tissue. The sum of these components was always within the range of catalytic activity in undosed controls. However, the half-life of reappearance of active NTE was 2.07 days +/- 0.13 (SD, n = 6) for brain and 3.62 days +/- 0.23 (SD, n = 6) for spinal cord--shorter than after dosing with phenylmethylsulphonyl fluoride. It is proposed that: (1) The physiological turnover mechanism cannot distinguish between catalytically active and di-n-pentylphosphinylated NTE although initiation of organophosphate-induced delayed polyneuropathy might involve recognition of aged di-alkyl-phosphorylated NTE as "foreign". (2) The short half-lives indicate a slow spontaneous dephosphinylation of inhibited NTE occurs in vivo as well as de novo synthesis. The difference in half-lives for brain and spinal cord NTE may be due to different rates of synthesis de novo or (more likely) to different rates of spontaneous reactivation of the inhibited NTE in the two tissues.

Blood copper in organophosphate-induced delayed polyneuropathy

Some organophosphorous esters cause a polyneuropathy which becomes clinically evident 2 weeks after a single dose. The pathogenesis involves modifications of a target protein, neuropathy target esterase, in the axons and a selective inhibition of retrograde axonal transport. It was suggested that copper metabolism might also be involved because of increased levels of plasma copper and ceruloplasmin in animals developing this polyneuropathy. Our results do not confirm this observation; treatment of hens with highly neuropathic single doses of two organophosphates (dihexyl-2,2-dichlorovinyl phosphate and mono-o-cresyl diphenyl phosphate) does not affect total and plasma free copper when measured several times during the development of polyneuropathy. We concluded that copper homeostasis is not affected and that copper changes are unlikely to be involved in the pathogenesis of this polyneuropathy.

Sciatic nerve neuropathy target esterase. Methods of assay, proximo-distal distribution and regeneration

Some organophosphorus compounds (OP) induce a delayed polyneuropathy (OPIDP) which is initiated by the phosphorylation of the so-called neuropathy target esterase (NTE). In this work some aspects of hen sciatic nerve NTE are studied. The assay method is reported and modifications are discussed and a combined method proposed. Proximo-distal distribution showed a significant difference from proximal (100 +/- 10%) to distal (69 +/- 9%) fragments, in accordance with reported data. The time course of in vivo regeneration after a single TOCP dose (200 mg/kg, post oral) showed some differences when compared with hen brain NTE. Sciatic nerve NTE showed a delay of 2-3 days before regeneration but then regenerated faster (74% activity at day 7) than brain NTE (50% activity at day 7). A slower rate of regeneration of distal than proximal segments has been suggested to explain higher sensitivity of distal segments [3], however in this work no significant differences were observed in the rate of regeneration when comparing proximal and distal fragments.

Low non-neuropathic tri-o-cresyl phosphate (TOCP) doses inhibit neuropathy target esterase near the neuropathic threshold in n-hexane pretreated hens

Simultaneous intoxication with hexacarbon solvents and organophosphorus compounds has been considered a possible critical factor in some occupational neuropathies and their interactions proved to cause potentiation effects in hens [1-3]. A high degree of inhibition of neuropathy target esterase (NTE) is needed to develop organophosphorus induced polyneuropathy (OPIDP). In this work, the inhibition of NTE, BuChE and AChE by TOCP on control and n-hexane pretreated (7-15 days, 300 mg/kg per day) hens is studied. Using a single TOCP dose of 200 mg/kg, n-hexane pretreated hens showed synergistic effects, but no significant differences were observed in the inhibition of cholinesterases and NTE in brain or spinal cord. With lower TOCP dose (20 mg/kg) statistically significant differences were observed, which were not drastic but could be important because they involved an increase of inhibition up to critical threshold values (from 40-50% to 60-70% inhibition). However, no clinical effects were observed in these animals. Possible mechanisms of neurotoxic interaction are discussed.

Progressive deficit of retrograde axonal transport is associated with the pathogenesis of di-n-butyl dichlorvos axonopathy

The induction of central-peripheral distal axonopathy in hens singly dosed with some organophosphorus (OP) compounds, such as di-n-butyl-2,2-dichlorovinyl phosphate (DBDCVP), requires greater than 80% organophosphorylation and subsequent intramolecular rearrangement ("aging") of a protein [neuropathy target esterase (NTE)] in the axon. Suprathreshold biochemical reaction, 24 h after dosing with DBDCVP (0.75-1.00 mg/kg s.c.), is shown to be associated with progressive decrement of retrograde axonal transport in sensory and motor fibers. The maximum transport deficit (about 70% reduction) is reached 7 days after DBDCVP, prior to the appearance of axonal degeneration and the onset of clinical signs of neuropathy (day 10-11). By contrast, phenylmethylsulfonyl fluoride (30 mg/kg s.c.), an agent that prevents the development of OP neuropathy by inhibiting NTE without the "aging" reaction, had no effect on axon transport, nerve fiber integrity, or clinical status and, when administered prior to a neurotoxic dose of DBDCVP (1.00 mg/kg s.c.), prevented DBDCVP effects. Paraoxon (0.2 mg/kg s.c.) neither inhibited NTE nor caused deficits in retrograde transport or neuropathy. Taken in concert, these studies demonstrate that induced deficits in retrograde transport are associated with the pathogenesis of OP-induced nerve-fiber degeneration and the threshold-initiating mechanism thereof.

Central-peripheral delayed neuropathy caused by diisopropyl phosphorofluoridate (DFP): segregation of peripheral nerve and spinal cord effects using biochemical, clinical, and morphological criteria

Systemic injection of diisopropyl phosphorofluoridate (DFP; 1 mg/kg, sc) causes delayed neuropathy in hens. This effect is associated with a high level of organophosphorylation of neuropathy target esterase (NTE) followed by an intramolecular rearrangement called "aging." Phenylmethanesulfonyl fluoride (PMSF) also attacks the active center of NTE but "aging" cannot occur. This compound does not cause neuropathy and protects against a subsequent challenge systemic dose of DFP. Intraarterial injection of DFP (0.185 mg/kg) into only one leg of hens caused a high NTE inhibition (greater than 80%) in the sciatic nerve of the injected leg, but not in other parts of the nervous system (37% average). A unilateral neuropathy with typical histopathological lesions developed in the injected leg. PMSF (0.55 mg/kg) injected into each sciatic artery caused 47% inhibition of sciatic nerve NTE but only 17-22% inhibition of NTE elsewhere; it did not produce clinical or histopathological lesions. When these hens were challenged with DFP (1 mg/kg, sc), high inhibition of residual-free NTE (greater than 85%) occurred throughout the nervous system and clinical signs of a syndrome different from the classical delayed neuropathy developed: this spinal cord type of ataxia was associated with histopathological lesions in the spinal cord but not in peripheral nerve. PMSF (1 mg/kg) injected into only one sciatic artery caused selective protective inhibition of sciatic nerve NTE of that leg. After systemic challenge by DFP, clinical effects expressed were a combination of spinal cord ataxia plus unilateral peripheral neuropathy. The challenge dose of DFP (1 mg/kg, sc) was insufficient to produce clear histopathological lesions in unprotected peripheral nerves although spinal lesions were found in these hens. Thus clinical evaluation of the peripheral nervous system by means of walking tests and a simple test of "leg retraction" reflexes was more sensitive and specific in diagnosis of peripheral neuropathy than was the histopathology.

Kinetics of substrate hydrolysis and inhibition by mipafox of paraoxon-preinhibited hen brain esterase activity

For the purpose of assessing the neurotoxic potential of organophosphorus compounds, it has been determined that paraoxon-preinhibited hen brain has both neurotoxicant (mipafox)-sensitive (neurotoxic esterase; NTE) and -insensitive esterase components. Several experiments designed to investigate the kinetic parameters governing the reaction of these esterases with two substrates and one organophosphorus inhibitor are presented. First, kinetic parameters for the hydrolysis of phenyl valerate and phenyl phenylacetate were measured. At 37 degrees C, the Km values of NTE for phenyl valerate and phenyl phenylacetate were found to be about 1.4 X 10(-3) and 1.6 X 10(-4) M respectively. At 25 degrees C, the Km of NTE for phenyl valerate was determined to be about 2.4 X 10(-3) M. Secondly, the kinetic constants of NTE for mipafox were measured at both 25 degrees C and 37 degrees C. With either phenyl valerate or phenyl phenylacetate as substrate, the Km at 37 degrees C was determined to be about 1.8 X 10(-4) M, and the phosphorylation constant (k2) was about 1.1 min-1. For phenyl valerate only, the Km at 25 degrees C was found to be about 6 X 10(-4) M, and the k2 was about 0.7 min-1. The data obtained at 25 degrees C were analysed by using a two-component model without formation of Michaelis complex, a two-component model with formation of Michaelis complex on the second component (NTE), or a three-component model without formation of Michaelis complex. The fact that the Michaelis model fit the data significantly better than either of the other two models indicates that the higher apparent Ki values that occur with low concentrations of mipafox are due to formation of Michaelis complex at high concentrations, rather than because of the presence of two NTE isoenzymes, as has been suggested by other investigators.

The relationship between neurological damage and neurotoxic esterase inhibition in rats acutely exposed to tri-ortho-cresyl phosphate

A rodent model of organophosphorus-induced delayed neuropathy (OPIDN) has been developed using Long-Evans adult male rats exposed to tri-ortho-cresyl phosphate (TOCP). In the present study an attempt was made to relate neurochemical with neuropathological changes in rats exposed to single dosages of TOCP ranging from 145 to 3480 mg/kg. The degree of neurotoxic esterase (NTE) inhibition, measured at 20 and 44 hr and at 14 days postexposure was correlated with the appearance of spinal cord pathology 14 days postexposure in a separate group of similarly dosed rats. Those dosages that inhibited mean NTE activity in spinal cord greater than or equal to 72% and brain greater than or equal to 66% of control values within 44 hr postexposure produced marked spinal cord pathology 14 days postexposure in greater than or equal to 90% of similarly dosed animals. In contrast, dosages of TOCP which inhibited mean NTE activity in the spinal cord less than or equal to 65% and in the brain less than or equal to 57% produced spinal cord pathology in less than or equal to 15% of the animals. These data indicate that NTE inhibition may be used as a biochemical predictor for TOCP-induced neurological damage in rats.

Axoplasmic transport and turnaround of neurotoxic esterase in hen sciatic nerve

We have recently found that there is a proximo-distal delay in the recovery of neurotoxic esterase (NTE) following inhibition along the sciatic nerve of the hen. To determine whether this delay could be due to a requirement for the transport of newly synthesized NTE from the cell body, we investigated the transport of NTE by measuring the rate of accumulation of activity at either one or two ligations. Although rapid turnaround of accumulated protein confounds calculation of the transport rate, it appeared that NTE is transported down the hen sciatic nerve at a rate close to 300 mm/day. Acetylcholinesterase (AChE) was found to be transported at a rate of about 500 mm/day, which is close to the expected rate of fast axoplasmic transport in the chicken. The relatively rapid turnaround of NTE compared with the retrograde transport rate precluded the estimation of a retrograde transport rate. A model is presented that accounts for turnaround as a result of exchange between mobile and stationary transport pools. Exchange of NTE between pools may account for the rapid turnaround of NTE described in this paper and for the proximo-distal delay in recovery as a dilution of newly synthesized NTE in the anterograde fast transport pool by inhibited protein as it travels down the nerve.

Intra-arterial injection of diisopropylfluorophosphate or phenylmethanesulphonyl fluoride produces unilateral neuropathy or protection, respectively, in hens

Hens injected in one sciatic artery with diisopropylfluorophosphate (DFP) (0.184 mg/kg) developed monolateral ataxia on the injected side 10-12 days later. The inhibition of neuropathy target esterase (NTE) was 85% in the sciatic nerve of the injected leg and less than 60% in the contralateral sciatic nerve, in spinal cord and in brain. Other hens injected in the wing vein with the same dose of DFP showed low inhibition of NTE in the nervous system and did not develop delayed neuropathy. Hens injected in one sciatic artery with phenylmethanesulphonyl fluoride (PMSF) (1 mg/kg) and 24 hr later with high subcutaneous dose of DFP (1.1 mg/kg) developed monolateral ataxia 10-12 days later on the side not injected with PMSF. The level of NTE inhibition after PMSF was greater than 40% in the sciatic nerve on the injected side compared with less than 20% in other parts of the nervous system. The same dose of PMSF injected in the wing vein produced low NTE inhibition in the nervous system and failed to protect the animals from the same high systemic dose of DFP. We conclude that both toxic and protective effects of NTE inhibitors for delayed neuropathy are better related to the level of NTE inhibition in the peripheral nerve on the site of injection than to NTE inhibition in other parts of the nervous system. Furthermore we suggest that NTE inhibition should also be measured in the peripheral nerve in the standard toxicity testing for organophosphate-induced delayed neurotoxicity.

Effects of soman on neuritic outgrowth and substrate utilization by explants of the rat superior cervical ganglion

The influence of the organophosphate, soman (pinacolyl methylphosphonofluoridate), on neuritic growth and substrate utilization by mammalian autonomic neural tissue was studied using explants of the rat superior cervical ganglion as a model. Soman produced a dose-dependent decrease in neuritic outgrowth from explants of this tissue maintained in a serum-free medium. Acetylcholinesterase in the explant as well as in culture media also decreased. In contrast, the effects of soman on explant metabolism were modest. Total protein and DNA content of the tissue was not affected. Only marginal changes in substrate utilization were detected; glucose use was unaltered, lactate production increased 20% with the highest soman tested. Soman increased the content of phosphocreatine in ganglion explants. This increase occurred in the absence of changes in the oxidation-reduction state of NAD calculated from pyruvate/lactate ratios. The results indicated that soman inhibited neuritic outgrowth from explants of the rat superior cervical ganglion in the absence of major effects on substrate utilization by this tissue.

The correlation between the recovery rate of neurotoxic esterase activity and sensitivity to organophosphorus-induced delayed neurotoxicity

Neurotoxic esterase (NTE) has been proposed to be the initiation site of organophosphorus compound-induced delayed neurotoxicity (OPIDN). There are two apparent problems associated with this hypothesis: NTE activity in the brain returns to nearly normal levels before the onset of the neuropathy, and NTE is present in and inhibited by organophosphorus compounds in young animals and other species which are relatively insensitive to the neurotoxic effects of these compounds. This paper presents data suggesting that differences in the recovery rates of NTE activity may account for some of these discrepancies. First, the onset of recovery of NTE activity following sc administration of 1.7 mg/kg of O,O-diisopropylphosphorofluoridate (DFP) in the hen sciatic nerve occurred several days later than in the brain. Furthermore, recovery was slower in distal than proximal parts of the nerve. This information indicates that NTE activity is depressed for a longer period at the site of the neuropathy than it would appear from the measurement of NTE activity in brain. Second, the rate of recovery of NTE activity was faster in the brains of chicks, of rats, and of hens treated with a daily po dose of 15 mg/kg cortisone acetate than it was in untreated hens. However, there was no significant increase in the NTE recovery rate in the peripheral nerves of the chicks or the cortisone-treated hens. Thus, it appears that although slower distal recovery could account for the greater sensitivity of longer axons to OPIDN, other factors are operating in chicks and cortisone-treated hens.

[Brain changes in parathion poisoning: observations in 42 cases]

A total of 42 cases were examined neuropathologically to determine possible toxic changes occurring in the brain after parathion intoxication. Sporadic anoxic alterations were observed in 7% of 41 cases in which the cause of death was acute intoxication. Nearly all cases (93%), however, showed marked hyperemia, often coupled with small reactionsless, periventricular hemorrhages which occurred in 40% of the cases. In one third of the cases (33%) there was moderate swelling of the oligodendroglia. Whereas histological evidence of edema was found in nearly one third of the cases (30%), comparison of the brain weights in these subjects with those of a large comparative collective showed definite pathologic brain weights in only five cases (12%) with 95% confidence limits. It is true that in 18 cases (42%) the brain weight was above the normal value if the confidence limit is ignored. Pronounced anoxic alterations were observed in only one case in which the individual survived the acute intoxication for 4 weeks after initial respiratory arrest. Predominantly toxic changes, however, could not be detected in any of the cases examined. This negative morphologic finding does not agree with the physiologic alterations reported by other investigators using animal models; they considered the cause of death in cases of parathion intoxication to be the result of toxic paralysis of the respiratory center. The literature was discussed.

Neurotoxic esterase. Identification of two isoenzymes in hen brain

Two phenyl valerate hydrolyzing carboxylesterases (EC 3.1.1.1) of hen brain were identified as neurotoxic esterases (NTEA and NTEB) by kinetic analysis of organophosphorus inhibition curves. The activities of both NTE isoenzymes with phenyl valerate (PV) as substrate and their inhibition rate constants were determined in six different animals. In-vivo-application of a single oral dose of 500 mg/kg triorthocresyl phosphate (TOCP) caused 86% inhibition of NTEA and 93% inhibition of NTEB within 24 h. Total NTE activity (NTEA plus NTEB) determined by kinetic analysis shows an excellent correlation (r = 0.989) to NTE activity simultaneously tested with a differential NTE assay. The excellent sensitivity (97%) and high specificity (79%) of the NTE differential test is demonstrated.