Neurotoxic and teratogenic effects of an organophosphorus insecticide (phenyl phosphonothioic acid-O-ethyl-O-[4-nitrophenyl] ester) on mallard development

Influence of lysophospholipid hydrolysis by the catalytic domain of neuropathy target esterase on the fluidity of bilayer lipid membranes

Neuropathy target esterase (NTE) is an integral membrane protein localized in the endoplasmic reticulum in neurons. Irreversible inhibition of NTE by certain organophosphorus compounds produces a paralysis known as organophosphorus compound-induced delayed neuropathy. In vitro, NTE has phospholipase/lysophospholipase activity that hydrolyses exogenously added single-chain lysophospholipids in preference to dual-chain phospholipids, and NTE mutations have been associated with motor neuron disease. NTE's physiological role is not well understood, although recent studies suggest that it may control the cytotoxic accumulation of lysophospholipids in membranes. We used the NTE catalytic domain (NEST) to hydrolyze palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (p-lysoPC) to palmitic acid in bilayer membranes comprising 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and the fluorophore 1-oleoyl-2-[12-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (NBD-PC). Translational diffusion coefficients (D(L)) in supported bilayer membranes were measured by fluorescence recovery after pattern photobleaching (FRAPP). The average D(L) for DOPC/p-lysoPC membranes without NEST was 2.44 microm(2)s(-1)+/-0.09; the D(L) for DOPC/p-lysoPC membranes containing NEST and diisopropylphosphorofluoridate, an inhibitor, was nearly identical at 2.45+/-0.08. By contrast, the D(L) for membranes comprising NEST, DOPC, and p-lysoPC was 2.28+/-0.07, significantly different from the system with inhibited NEST, due to NEST hydrolysis. Likewise, a system without NEST containing the amount of palmitic acid that would have been produced by NEST hydrolysis of p-lysoPC was identical at 2.26+/-0.06. These results indicate that NTE's catalytic activity can alter membrane fluidity.

Inhibition of acetylcholinesterase by metabolites of copper pyrithione (CuPT) and its possible involvement in vertebral deformity of a CuPT-exposed marine teleostean fish

In a previous study, we demonstrated that exposure to an antifouling biocide, copper pyrithione (CuPT), early during life induced vertebral deformity in the larvae of a marine fish, the mummichog (Fundulus heteroclitus). Skeletal deformities may be caused by inhibition by of acetylcholiensterase (AChE) activity, and to elucidate the mechanism underlying the CuPT-associated vertebral deformity, we first examined whether CuPT, zinc pyrithione (ZnPT), and their degradation products could inhibit AChE activity in the fish. Two of the degradation products, 2,2'-dipyridyldisulfide [(PS)(2)] and 2,2'-dithiobispyridine-N-oxide [(PT)(2)], but neither CuPT nor ZnPT, exhibited prominent AChE-inhibiting activity. Secondly, thin-layer chromatography revealed that mummichog hepatic microsomes metabolized CuPT to produce (PS)(2) in a microsome-dependent manner. The AChE inhibition induced in CuPT-exposed fish is likely due to (PS)(2) that was produced through metabolism of acquired CuPT. (PS)(2) may cause therefore skeletal deformity in CuPT-exposed fish by means of its neuromuscular blocking properties, through a mechanism similar to that proposed for animals exposed to organophosphorous pesticides.

Effects of Phos-Chek G75-F and Silv-Ex on red-winged blackbird (Agelaius phoeniceus) embryos

Effects of field application levels of wildfire control chemicals, Phos-Chek G75-F (PC) and Silv-Ex (SE), were examined on red-winged blackbird (Agelaius phoeniceus) embryos. Embryos were more sensitive to PC and SE when eggs were immersed for 10s at an early developmental stage (days 3-5 of incubation) than at a later stage (days 6-9 of incubation). The LC(50) (concentration causing 50% mortality) for early stage embryos exposed to PC was 213.3g/L (slope=1.6; 95% confidence interval [CI]=129.1-326.1). The no observed effect concentration (NOEC) was below 135g PC/L, which caused a significant increase in embryonic mortality and represents the lowest field coverage level of 1gal/100feet(2). The LC(50) for early stage embryos exposed to SE was 19.8g/L (slope=1.5; 95% CI=11.7-52.2). Significant mortality was observed at 10g SE/L and marginal at 7.5g SE/L with an apparent NOEC around 5g SE/L. Neither chemical resulted in apparent developmental malformations.

Ranking terrestrial vertebrate species for utility in biomonitoring and vulnerability to environmental contaminants

The measurement of contaminant tissue concentrations or exposure-related effects in biota has been used extensively to monitor pollution and environmental health. Terrestrial vertebrates have historically been an important group of species in such evaluations, not only because many are excellent sentinels of environmental contamination, but also because they are valued natural resources in their own right that may be adversely affected by toxicant exposure. Selection of appropriate vertebrates for biomonitoring studies frequently relies on expert opinion, although a few rigorous schemes are in use for predicting vulnerability of birds to the adverse effects of petroleum crude oil. A Utility Index that ranks terrestrial vertebrate species as potential sentinels of contaminants in a region, and a Vulnerability Index that assesses the threat of specific groups of contaminants to these species, have been developed to assist decision makers in risk assessments of persistent organic pollutants, cholinesterase-inhibiting pesticides, petroleum crude oil, mercury, and lead shot. Twenty-five terrestrial vertebrate species commonly found in Atlantic Coast estuarine habitat (Rattner et al. 2001a) were ranked for their utility as biomonitors of contamination and their vulnerability to pollutants in this region. No single species, taxa, or class of vertebrates was found to be an ideal sentinel for all groups of contaminants. Although birds have overwhelmingly been used to monitor contaminants compared to other terrestrial vertebrate classes, the nonmigratory nature and dietary habits of the snapping turtle and mink consistently resulted in ranking these species as excellent sentinels as well. Vulnerability of Atlantic Coast populations of these species varied considerably among groups of contaminants. Usually a particular species was found to be at high risk to only one or two groups of contaminants, although a noteworthy exception is the bald eagle, which is highly vulnerable to all five of the contaminant groups examined. This index could be further enhanced by generation of additional comparative toxicity data to facilitate interspecific extrapolations. The Utility and Vulnerability Indices have application to many types of habitats in addition to estuaries and are of value to natural resource and risk managers that routinely conduct local, regional, or national environmental quality assessments.

Gestational exposure to chlorpyrifos: apparent protection of the fetus?

Previous studies have shown that, in general, young, postnatal animals are more sensitive than adults to the toxic effects of anticholinesterase (antiChE) pesticides. Paradoxically, often fetal brain cholinesterase (ChE) is less inhibited than maternal brain after gestational exposure to an antiChE, presumably due to placental and fetal detoxification of the antiChE. The present investigation was designed to study selected toxicokinetic and toxicodynamic factors surrounding the toxicity of chlorpyrifos (CPF; [O,O'-diethyl O-3,5,6-trichloro-2-pyridyl] phosphorothionate) in pregnant rats dosed repeatedly or singly during late gestation. Dams were dosed daily (po) with CPF in corn oil (0 or 7 mg/kg) on gestational days (GD) 14 to 18. Animals were euthanized at 2 to 120 h after the last dose and tissues were collected for enzyme analysis. Using this dosing regimen, we found that (1) the time of maximal ChE inhibition was the same (i.e., 5-10 h after dosing) for both maternal and fetal brain, (2) the degree of fetal brain ChE inhibition was 4.7 times less than maternal brain inhibition, and (3) the detoxification potential (i.e., carboxylesterase and chlorpyrifos-oxonase) of the fetal tissues was very low compared to the maternal tissues. A separate group of experiments showed that if pregnant dams received only one oral dose of 7 or 10 mg/kg CPF on GD18, the degree of ChE inhibition in the fetal brain was comparable to the maternal brain ChE inhibition. Taking into consideration the net increase (more than fourfold) in fetal brain ChE activity from GD14 to 18 in control animals, and the fact that maternal brain ChE was inhibited more than fetal brain ChE only in a repeated-dosing regimen, we conclude that the fetus is not genuinely protected from the toxic effects of a given dose of CPF. We propose that fetal brain ChE is simply able to recover more fully between each dose as compared to maternal brain ChE, giving the illusion that the fetal compartment is less affected than the maternal compartment.

Development of esterase activities in the chicken before and after hatching

The embryonic chick has long been a model for developmental biology and has often been recommended as a model system in developmental toxicology. More recently, several investigators have shown that the chick embryo also provides a good model for identifying the neurotoxic effects of environmental pollutants, especially cholinesterase-inhibiting pesticides. Although numerous studies detail the structural development of chick embryos, few describe embryonic levels of enzyme synthesis and their changes during development. In this study, the development of esterase activity in chick embryos was measured from day 9 of incubation until 46 days after hatching. Brain acetylcholinesterase (AChE) activity was detected on day 9 of incubation at a concentration of 0.364 mumoles/min/g tissue. An increase between AChE activity and age of the embryos was observed. In the liver, the nonspecific cholinesterases (ChE) and carboxylesterase activities during incubation were not different from activities after the chicks had hatched. Plasma ChE and carboxylesterase activities did not change with age after hatching. Brain neuropathy target esterase (NTE) activity was not detected on day 9 of incubation and was extremely low (6.12 nmoles/15 min/mg protein) the next day, but increased rapidly with increasing age. This study demonstrates that chick embryos have developed esterase activities in the brain and liver by day 10 of incubation and again confirms that the insensitivity of chick embryos and young chicks to organophosphorus ester-induced delayed neurotoxicity is not due to absence of NTE. In addition, the results provide baseline data for evaluating the response of embryonic and immature chicks to neurotoxicants and teratogens.

Developmental toxicity of desbromoleptophos in chicks: enzyme inhibition, malformations and functional deficits

The relationship among inhibition of acetylcholinesterase (AChE), inhibition of neuropathy target enzyme (NTE), and developmental toxicity of the organophosphorus ester desbromoleptophos (DBL) was evaluated in chicks exposed on day 3 or day 15 of incubation or 10 days posthatching. DBL induced prolonged inhibition of AChE and NTE when administered either early or late in incubation, structural malformations if administered before organogenesis, posthatching paresis if administered after organogenesis, and delayed deficits of gait if administered after hatching. The posthatching paresis and abnormal gait are not determined solely by either AChE inhibition of NTE inhibition, since they occur in the absence of the latter and are not invariably seen in the presence of the former (Toxicology 49: 253-261; 1988).

Morphology and embryological interpretation of a congenital occipito-atlanto-axial malformation in a dog

A clinical, radiological, and morphological study of a congenital occipito-atlanto-axial malformation in a 13-week-old male Saint Bernard dog that became suddenly tetraplegic at 8 weeks is described. The dog was recumbent, had generalized muscle atrophy, but was alert and responsive. Pain was elicited when the head-neck junction and the cervical vertebrae were palpated, and a bony abnormality was palpated at the occiput and atlas. Clinical signs of upper motor neuron and general proprioceptive deficits in all four limbs were compatible with a focal lesion in the cervical spinal cord. Plain radiographs of the head and neck revealed malformation of the occipital bones, atlas, and axis, unilateral atlanto-occipital fusion, and atlanto-axial subluxation. At necropsy the right half of the atlantal neural arch was fused to the right exoccipital bone. On the axis, the dens was small, malformed, and deviated to the left; the transverse processes were enlarged; and the spinous process was small with a cleft caudally. The spinal cord was severely compressed at the level of the atlanto-axial articulation, and histological examination revealed extensive loss of neuronal cell bodies, axons, myelin, and the central canal. Reactive astrogliosis was also extensive. After a discussion of normal and abnormal development of the vertebral column and its joints, it was concluded that a failure of normal joint development at about 30 days of gestation in the dog could lead to congenital occipito-atlanto-axial malformation.

Embryotoxic and teratogenic effects of selenium in the diet of mallards

Mallards (Anas platyrhynchos) were fed a control diet, diets containing 1, 5, 10, or 25 ppm Se as sodium selenite, or a diet containing 10 ppm Se as seleno-DL-methionine in the first of two experiments. Selenium at 10 ppm as selenomethionine or 25 ppm as sodium selenite caused a 40-44% decrease in the total number of eggs that hatched compared to controls. Selenium at 25 ppm (sodium selenite) resulted in a 19% decrease in mean embryonic weight at 18 d of incubation, accompanied by a 6% decrease in crown-rump length. Ten parts per million Se as selenomethionine was more teratogenic than sodium selenite at 25 ppm. Selenomethionine (10 ppm Se) resulted in an incidence of 13.1% malformations that were often multiple, whereas sodium selenite (10 and 25 ppm Se) resulted in 3.6 and 4.2% malformations. The teratogenicity of selenomethionine was confirmed in a second experiment in which mallards received 1, 2, 4, 8, or 16 ppm Se as selenomethionine, resulting in 0.9, 0.5, 1.4, 6.8, and 67.9% malformations, respectively. These malformations included hydrocephaly, microphthalmia, lower bill defects, and foot defects with ectrodactyly. Both forms of selenium increased the incidence of edema and stunted embryonic growth. Selenomethionine (10 ppm Se) resulted in a significant increase of approximately 40% in plasma glutathione peroxidase activity and a 70% increase in sorbitol dehydrogenase activity (indicative of hepatotoxicity) in hatchlings. Sodium selenite (25 ppm Se) resulted in fourfold elevation in plasma uric acid concentration, indicative of renal alteration. Selenomethionine accumulated much better in eggs than did sodium selenite. These findings indicate that selenomethionine is considerably more teratogenic and generally more embryotoxic than sodium selenite, probably due to higher uptake of selenomethionine.

Effects of fenthion, fenitrothion and desbromoleptophos on gait, acetylcholinesterase, and neurotoxic esterase in young chicks after in ovo exposure

Previous studies have demonstrated that gait is affected in chicks exposed to organophosphorus esters (OPs) that induce delayed neurotoxicity (OPIDN) in adult hens. To investigate the developmental relationship between such functional deficits and OPIDN, chicks were exposed to 3 OPs with different OPIDN potential. Desbromoleptophos (DBL) induces OPIDN in adult hens; fenthion (FEN) has uncertain OPIDN potential; fenitrothion (FTR) does not induce OPIDN. Chicks were treated by injection into the egg on day 15 of incubation, after the presumed period of OP-induced structural teratogenesis. AChE and neurotoxic esterase (NTE) were assayed during incubation and in parallel with post-hatching evaluations of gait. DBL, 125 mg/kg in ovo, caused paralysis in 70% of chicks after hatching. The gait of surviving chicks was affected for at least 6 weeks and marked by toes curling under. NTE was inhibited until 10 days post-hatching and AChE until hatching. FEN did not inhibit NTE significantly, but AChE was significantly inhibited until hatching. Chicks exposed as embryos to FEN were hyperactive and aggressive. Gait was still affected 6 weeks after treatment with 3 mg/kg FEN. FTR at 125 mg/kg inhibited AChE until day 10 post-hatching, but neither inhibited NTE nor affected gait. The growth of OP-exposed chicks was not significantly decreased, so the decreased length and increased width of the stride could not be ascribed to stunted growth. We conclude that OPs cause irreversible effects on gait that are not related to their defined neurotoxic effects, since altered gait (1) occurs below the age of sensitivity to OPIDN, (2) is seen in the absence of NTE inhibition and (3) does not invariably accompany AChE inhibition.

Delayed neuropathy in adult Peking ducks induced by some organophosphorus esters

Trio-o-cresyl phosphate (TOCP), leptophos [O-methyl O-(4-bromo-2,5,-dichlorophenyl) phenylphosphonothioate] and cyanofenphos [O-ethyl O-(4-cyanophenyl) phenyl-phosphonothioate] were used to determine whether adult peking ducks would exhibit neurotoxicity after exposure to such chemicals. Clinical, histopathological, and specific biochemical tests were used to detect the neurologic dysfunctions that were induced by these neurotoxic agents. Ducks were orally treated with TOCP or leptophos at 100 or 10 mg/kg X d for 30 d, respectively. Another group of ducks received cyanofenphos at 4 mg/kg X d for 10 d. All the TOCP- and leptophos-treated ducks developed clinical signs of delayed neuropathy, as manifested by ataxia and paralysis. Two of the cyanofenphos-treated ducks died from cholinergic effect during the course of dosing. Surviving ducks of this group completely recovered from the cholinergic effect 2 or 3 d after finishing the dosing regimen. However, they developed signs of delayed neurotoxicity 10-17 d later. Surviving ducks of all groups were sacrificed for biochemical and/or histopathologic tests 1 d after the last treatment or when they became paralyzed. Histopathologic examinations indicated that degenerative lesions of axons consistent with the type occurring in delayed neurotoxicity were seen in all TOCP-, leptophos-, or cyanofenphos-treated ducks and were specially evident in sections of spinal cord. Biochemically, it was found that duck brain neurotoxic esterase (NTE) activity was inhibited in vivo to less than 15% of control levels as measured 24 h after the last treatment with TOCP, leptophos, or cyanofenphos. These results indicate that adult peking ducks could be used to screen organophosphorus compounds for delayed toxic neuropathy.