Differential profiles of cholinesterase inhibition and neurobehavioral effects in rats exposed to fenamiphos or profenofos

The relationship between cholinesterase (ChE) inhibition and neurobehavioral changes was examined using two ChE-inhibiting organophosphorus (OP) pesticides, fenamiphos and profenofos. Both pesticides produce considerable blood ChE inhibition, but relatively little brain inhibition up to almost lethal doses. Interestingly, pronounced neurobehavioral signs were produced by fenamiphos but not profenofos. After a single oral dose, both pesticides greatly inhibited blood ChE (87-98% inhibition), yet whole brain ChE was only inhibited by 9-14% at the highest doses. Fenamiphos produced dose-dependent effects on many behavioral measures. Despite the similar ChE inhibition profile, profenofos produced no observable changes in behavior. Treatment with anticholinergic drugs was used to evaluate the contribution of peripheral versus central ChE inhibition. Scopolamine (SCO) and methylscopolamine (MSC) were used as central/peripheral and peripheral-only cholinergic receptor blockers, respectively, in combination with fenamiphos. Neither drug altered the effects of fenamiphos on ChE inhibition. Some behavioral effects of fenamiphos were blocked or attenuated only by SCO, whereas other effects were blocked by both drugs. These data indicate that some of the pronounced neurobehavioral changes observed following fenamiphos dosing may be centrally mediated (blocked by SCO only), despite the small amount of inhibition of brain ChE. Other behavioral changes may be mediated more peripherally (blocked by both MSC and SCO). To test the hypothesis that regionally specific ChE inhibition may be responsible for these effects, the same dose of fenamiphos used in the previous studies was given and one half of the brain was dissected into regions. There was significant ChE inhibition in the pons and medulla, cerebellum, striatum, hippocampus, and half-brain but not in the rest-of-brain and frontal cortex; however, the magnitude of inhibition was relatively small across the regions measured. Thus, the centrally mediated neurobehavioral effects of fenamiphos could not be explained based on differential regional brain ChE inhibition alone. Despite the low level of brain ChE inhibition, some behavioral effects of fenamiphos were centrally mediated, and there was little regional specificity of ChE inhibition that could account for the behavioral changes observed.

Neurotoxicity produced by dibromoacetic acid in drinking water of rats

An evaluation of potential adverse human health effects of disinfection byproducts requires study of both cancer and noncancer endpoints; however, no studies have evaluated the neurotoxic potential of a common haloacetic acid, dibromoacetic acid (DBA). This study characterized the neurotoxicity of DBA during 6-month exposure in the drinking water of rats. Adolescent male and female Fischer 344 rats were administered DBA at 0, 0.2, 0.6, and 1.5 g/l. On a mg/kg/day basis, the consumed dosages decreased greatly over the exposure period, with average intakes of 0, 20, 72, and 161 mg/kg/day. Weight gain was depressed in the high-concentration group, and concentration-related diarrhea and hair loss were observed early in exposure. Testing with a functional observational battery and motor activity took place before dosing and at 1, 2, 4, and 6 months. DBA produced concentration-related neuromuscular toxicity (mid and high concentrations) characterized by limb weakness, mild gait abnormalities, and hypotonia, as well as sensorimotor depression (all concentrations), with decreased responses to a tail-pinch and click. Other signs of toxicity at the highest concentration included decreased activity and chest clasping. Neurotoxicity was evident as early as one month, but did not progress with continued exposure. The major neuropathological finding was degeneration of spinal cord nerve fibers (mid and high concentrations). Cellular vacuolization in spinal cord gray matter (mostly) and in white matter (occasionally) tracts was also observed. No treatment-related changes were seen in brain, eyes, peripheral nerves, or peripheral ganglia. The lowest-observable effect level for neurobehavioral changes was 20 mg/kg/day (produced by 0.2 g/l, lowest concentration tested), whereas this dosage was a no-effect level for neuropathological changes. These studies suggest that neurotoxicity should be considered in the overall hazard evaluation of haloacetic acids.

Neurobehavioral evaluations of mixtures of trichloroethylene, heptachlor, and di(2-ethylhexyl)phthalate in a full-factorial design

One approach to the toxicological evaluation of chemical mixtures is to construct full dose-response curves for each compound in the presence of a range of doses of each of the other compounds, i.e., a factorial design. This study was undertaken as part of an interdisciplinary project to evaluate a mixture of three environmental pollutants. A full-factorial design was undertaken to determine the neurobehavioral consequences of short-term repeated exposure to five dose levels each of three chemicals, in order to characterize potential two- and three-way interactions. Adult female F344 rats received (p.o.) for 10 days either one of five doses of trichloroethylene, di(2-ethylhexyl)phthalate, or heptachlor, or else one of all possible chemical combinations. Neurobehavioral evaluations were conducted using motor activity and an abbreviated functional observational battery. Response-surface analysis was applied to each of the endpoints. Hypotheses were tested based on the estimated model parameters; of primary interest was the overall test for interaction among the three chemicals. In addition, an abbreviated design was created by fitting only a subset of the data to the model. In general, significant overall interactions that deviated from response additivity were detected for most endpoints (11 of 14). All of the interactions on the neurobehavioral endpoints showed either antagonism, or else an interaction that could not be fully characterized. Often the results of the abbreviated dataset analysis were not the same as for the full-factorial design. This study was extremely intensive, in terms of the number of rats and time required for conduct of the study as well as the data analysis. These results underscore the need for more economical approaches to evaluate the toxic effects of mixtures of chemicals.

Neurobehavioral assessments of rats perinatally exposed to a commercial mixture of polychlorinated biphenyls

Because of behavioral deficits associated with gestational exposure to PCBs in children, we sought to quantify neurobehavioral effects of perinatal exposure to Aroclor 1254(R) (A1254), a commercial mixture of PCBs, in rats. Pregnant Long-Evans rats were fed A1254 at doses of 0, 1.0, or 6.0 mg/kg/day throughout gestation and nursing. The growth and behavior of their male and female offspring were assessed both during development and as adults, using a variety of behavioral tests that included a neurobehavioral screening battery (functional observational battery [FOB] and automated tests of locomotor activity), habituation of motor activity, acquisition of a visual discrimination, and performance of a visual signal-detection task. During the suckling period, A1254 at 6 mg/kg reduced survival and body weight gain of offspring of both sexes; however, locomotor activity was unaffected, and only small and transient changes in other measures were evident. In adulthood, perinatal exposure to A1254 did not affect habituation of locomotor activity, acquisition of the visual discrimination, or sustained attention. Rats performing the signal-detection task were challenged with cocaine (0, 1.25, 2.5, 5.0 mg/kg) and haloperidol (0, 0.003, 0.010, 0.030 mg/kg) to probe the integrity of dopaminergic systems in the central nervous system (CNS). A1254 did not alter the impairment of attention caused by haloperidol. Cocaine reduced false alarms more in controls than in rats exposed to A1254, but the effect was not clearly related to the dose of A1254. Perinatal exposure to this commercial PCB mixture had very little effect on these tests of behavior during development and in adulthood.

Dopamine transporter binding in the rat striatum is increased by gestational, perinatal, and adolescent exposure to heptachlor

Heptachlor is a persistent cyclodiene pesticide that affects GABAergic function. Recent reports indicate that heptachlor exposure also alters dopamine transporter (DAT) expression and function in adult mice. The aim of this study was to determine whether gestational, perinatal, and/or adolescent heptachlor exposure in rats altered dopamine-receptor and DAT binding. Adolescent exposure to dieldrin was included to evaluate the generality of the findings. Sprague-Dawley rats received doses (po) ranging from 0 to 8.4 mg/kg/day of heptachlor, or dieldrin, 3 mg/kg/day, during different developmental periods. There were dose-related decreases in maternal weight gain and pup survival, as well as delayed righting reflex, at heptachlor doses > or =3 mg/kg/day. There were no changes in striatal dopamine receptor-D1 ([(3)H]SCH-23390) and -D2 ([(3)H]spiperone) binding in preweanling pups exposed perinatally to heptachlor, and no differences in the response of adult rats to the motor activity-increasing effects of d-amphetamine. However, there were significant (27-64%) increases in striatal DAT binding of [(3)H]mazindol in preweanling rats exposed only gestationally. In rats exposed perinatally and/or during adolescence, there were also increases (34-65%) in striatal DAT binding at postnatal days (PND) 22, 43, and 128. Adolescent exposure to dieldrin also increased DAT binding. In other rats exposed perinatally and throughout adolescence, even the lowest dose of heptachlor 0.3 mg/kg/d increased DAT binding on PND 130. The DAT affinity for mazindol was unchanged in heptachlor-exposed striata. In vitro binding studies indicated that heptachlor (> or =10 microM) displaced mazindol binding. Thus, gestational, perinatal, and/or adolescent exposure to heptachlor produced an increase in DAT binding as early as PND 10, and this change persisted into adulthood.

The effects of perinatal tebuconazole exposure on adult neurological, immunological, and reproductive function in rats

Studies are under way to address concerns of potential persistent immunotoxic, reproductive, and neurotoxic effects of perinatal exposure to several pesticides. Tebuconazole, a triazole fungicide, was evaluated as part of this project. Sprague-Dawley dams were administered tebuconazole (0, 6, 20, or 60 mg/kg) by oral gavage daily from gestational day 14 to postnatal day (PND)7; the pups were then dosed daily at the same levels from PND7-42. Separate groups of rats were used for testing of immunological parameters, neurobehavioral testing using a screening battery of functional tests, and cognitive evaluations. Other groups of rats were evaluated for reproductive development and function, while yet others were sacrificed at the end of the dosing period for histological analyses of major organs systems, including neuropathological assessments. Pup viability and body weight were decreased in the highest dose group. There were no differences in the fertility indices in the exposed rats mated as adults. In the sheep RBC-immunized high-dose rats, spleen weights and cellularity were increased, and the ratio of cell types was altered compared to controls. There were, however, no biologically significant changes in the immune function of these rats. At necropsy on PND46 or 152, kidney, liver, and spleen weights were altered by tebuconazole treatment, but a dose-response relationship was not clear for most organs; only decreased kidney and increased liver weights were consistent in both sexes. Histological analyses were generally unremarkable outside of the brain. One month after the end of dosing, acquisition of learning the platform location in a water tank (i.e., Morris water maze) was impaired in the high-dose group; there were no differences in neuromuscular ability, motor activity, or swim speed to account for this finding. Furthermore, there was no effect on recall of the position during a free-swim trial. Neuropathological evaluations revealed pyknotic cells across hippocampal cell fields in animals of all tebuconazole treatment groups, with the highest incidence in the 20 and 60 mg/kg/day dose groups, coincident with cell loss within pyramidal cell layer of CA3-4 cell fields of the hippocampus and layer V of the neocortex. Thus, perinatal exposure to tebuconazole produced neurobehavioral deficits and neuropathology in rats, but did not alter immunological or reproductive function.

Neurobehavioral screening in rodents

Neurobehavioral evaluations are an important component of screening for neurotoxicological effects. A neurobehavioral test battery is chosen to assess a variety of endpoints, including autonomic, neuromuscular, sensory, and excitability functions. Such a battery includes observation of gait and posture, reactivity and arousal, gait and postural characteristics, involuntary/abnormal motor movements, clinical signs and manipulation of neurological reflexes and reactions and neuromuscular activity and postural reactions. Finally, the battery includes assessment of sensory responses.

Neurotoxicological outcomes of perinatal heptachlor exposure in the rat

The developing nervous system has been identified as a potential target of pesticide exposure. Heptachlor is a cyclodiene pesticide that was widely used for many years, and for which inadvertent exposure to children and fetuses took place in the early 1980s; yet little is known regarding the developmental neurotoxicity of it and other cyclodienes. The aim of this study was to determine whether perinatal heptachlor exposure results in persistent alterations in nervous system function. Pregnant Sprague-Dawley dams were dosed from gestational day (GD) 12 to postnatal day (PND) 7, whereupon the rat pups were dosed directly until PND 21 (group A) or PND 42 (group B). Dose levels were 0, 0.03, 0.3, or 3 mg/kg/day, po. There were no dose-related effects on maternal weight, litter size, or pup growth. GABA(A) receptor binding (using [(35)S] tert-butylbicyclophosphorothionate; TBPS) and GABA-stimulated Cl- flux were evaluated in control and high-dose brain tissues taken on PND 7, 21, and 43. The B(max) values for [(35)S]-TBPS binding in brainstem, but not cortex, were decreased in female rats across all ages tested. There were no such changes in male rats, nor were K(D) values altered in either tissue or gender. GABA-stimulated Cl- flux was decreased in female cortex synaptoneurosomes only on PND 21. The ontogeny of the righting response (PND 2-5) was delayed in the high-dose females. All subsequent testing took place a week to months after dosing ceased. The functional observational battery (FOB) showed treatment-related, but not necessarily dose-related, changes in different aspects of the rat's reactivity and activity levels. Group-A rats also showed altered within-session habituation of motor activity. There were no heptachlor-related differences in motor activity following challenge with a range of chlordiazepoxide doses. Cognitive assessments were conducted in both groups of rats. There were no statistically significant differences among treatment groups in a one-trial passive avoidance test, although there was a trend toward less learning. In group B, rats (both sexes), heptachlor altered spatial learning in the Morris water maze during two weeks of daily training (2 trials/day). On probe trials, heptachlor-treated rats did not show significant preference for the correct quadrant (all dose groups in males, high dose in females). These rats did not show alterations on subsequent working-memory training (where the platform position was relearned each day). Thus, perinatal exposure to heptachlor produced neurochemical and persistent neurobehavioral changes, including alterations in spatial learning and memory.

Comparing cognitive and screening tests for neurotoxicity. Effects of acute chlorpyrifos on visual signal detection and a neurobehavioral test battery in rats

It is often assumed that cognitive function is more sensitive to neurotoxic chemicals than are the unconditioned behaviors employed in neurobehavioral screens; however, direct comparisons of the sensitivity of these test methods are lacking. The present studies were conducted to compare the effects of the widely used cholinesterase-inhibiting insecticide, chlorpyrifos (O,O'-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothionate, CPF), on a visual signal detection task (SDT) with its effects on a neurobehavioral test battery. Adult male Long-Evans rats were trained to perform the SDT, dosed with CPF, and then assessed with both test instruments. Oral CPF (50 mg/kg) impaired signal detection for 8 days, and subcutaneous CPF (250 mg/kg) did so for 4 weeks. CPF (30 and 50 mg/kg po and 250 mg/kg sc) also lowered activity in the test battery for up to 18 days. Thus, CPF impaired attention and altered behavior in the test battery in the same dose ranges under two very different dosing scenarios.

The functional observational battery in adult and developing rats

Neurobehavioral screening methods, such as the functional observational battery (FOB), are now widely used to identify potential neurotoxicity of new and existing chemicals. These methods have been validated and a large database now exists for the effects of a wide range of chemicals. Since most of the observations recorded are subjective, the quality of the test data depends largely on the observer's ability to detect and describe changes in the animal's behavior and neurologic function. Efforts are underway to aid in the training of observers and to achieve consistency across laboratories in the use of these methods. With the increasing concern over potential neurotoxicological consequences of chemical exposure in the developing organism, there is growing interest in testing laboratory animals at very young ages. We present here an initial report of the development of an FOB suitable for young rats, using some modifications of the individual adult FOB test measures to make them age-appropriate. We have evaluated pre- and postweanling rats to determine the range of behaviors (as evaluated with the FOB) displayed at each age, develop appropriate scoring criteria, and collect control data to document the ontogeny of each of the endpoints in the FOB. This revised FOB protocol may be useful for assessing behavioral or neurological changes due to acute chemical exposure in young rats, or following gestational/lactational exposures typical of developmental neurotoxicity studies.

Dose-response and time-course of neurobehavioral changes following oral chlorpyrifos in rats of different ages

Young rats have been shown in several laboratories to be more sensitive to the neurotoxic effects of acute exposure to chlorpyrifos. To examine the neurobehavioral effects of chlorpyrifos as a function of age and dose, we conducted dose-response and time-course assessments in rats of three different ages (postnatal day, or PND, 17, 27, and adults). Doses were selected to span the effective dose range in each age group: PND17 - 4, 10, 20 mg/kg; PND27 - 10, 25, 50 mg/kg; adult - 10, 50, 100 mg/kg. Rats were tested at the time of peak effect on the day of dosing, and again at 1 and 3 days, and at 1 and 2 weeks after a single oral dose. There were age- and sex-related differences in the recovery of these behavioral effects; the adult males recovered from the behavioral effects more quickly than the other age groups, and the adult females showed the slowest recovery (up to at least 3 days). Although these doses had been shown previously to produce a similar degree of cholinesterase inhibition, the neurobehavioral alterations fell into the following three patterns of effect as a function of age. (1) Some endpoints (e.g., gait abnormalities, tremor) showed a dose-response curve that was shifted to the right in the older animals. Calculated ED50 values indicated that the PND17 rats were three- to five-fold more sensitive than the adults. (2) Some measures showed less effect in the youngest rats; for example, maximal motor activity decreases were half as great as with adults. (3) A few effects that were typically observed in adults, e.g., salivation, were not seen at all in the PND17 rats. Thus, differential responses on these neurobehavioral endpoints were observed as a function of age. These data suggest that, for some endpoints, young rats are more sensitive to a range of chlorpyrifos doses; however, the magnitude of age-related differences depends on the specific endpoint and time of assessment, as well as age and sex of the test subject.

Rapid neurobehavioral analysis of Pfiesteria piscicida effects in juvenile and adult rats

The estuarine dinoflagellate Pfiesteria piscicida is known to kill fish and has been associated with neurocognitive deficits in humans. We have developed a rat model to demonstrate that exposure to Pfiesteria causes significant learning impairments. This has been repeatedly seen as a choice accuracy impairment during radial-arm maze learning. Pfiesteria-induced effects were also seen in a locomotor activity test in the figure-8 apparatus. The current studies used the short-term radial-arm maze acquisition, the figure-8 activity test, and the functional observational battery (FOB) to assess Pfiesteria-induced neurobehavioral effects in adult and juvenile rats. In study 1, the neurobehavioral potency of three different Pfiesteria cultures (Pf 113, Pf 728, and Pf Vandermere) was assessed. Ninety-six (12 per group) adult female Sprague-Dawley rats were injected subcutaneously with a single dose of Pfiesteria taken from aquarium-cultured Pfiesteria (35,600 or 106,800 Pfiesteria cells per kilogram of rat body weight). One control group (N = 12) was injected with saline and one (N = 12) with aquarium water not containing Pfiesteria. All three of the Pfiesteria samples (p < 0.05) impaired choice accuracy over the first six sessions of training. At the time of the radial-arm maze choice accuracy impairment, no overt Pfiesteria-related effects were seen using an FOB, indicating that the Pfiesteria-induced choice accuracy deficit was not due to generalized debilitation. In the figure-8 apparatus, Pfiesteria treatment caused a significant decrease in mean locomotor activity. In study 2, the neurobehavioral effects of the Pf 728 sample type were assessed in juvenile rats. Twenty-four day-old male and female rats were injected with 35,600 or 106,800 Pf-728 Pfiesteria cells per kilogram of rat body weight. As with adult females, the juvenile rats showed a significant impairment in radial-arm maze choice accuracy. No changes in locomotor activity or the FOB were detected in the juvenile rats. Furthermore, there were no differences between male and female rats in the Pfiesteria-induced choice accuracy impairment. Pfiesteria effects on choice accuracy in the radial-arm maze in rats constitute a critical component of the model of Pfiesteria toxicity, because the hallmark of Pfiesteria toxicity in humans is cognitive dysfunction. Our finding that analysis of the first six sessions of radial-arm maze testing is sufficient for determining the effect means that this test will be useful as a rapid screen for identifying the critical neurotoxin(s) of Pfiesteria in future studies.

Comparison of the role of esterases in the differential age-related sensitivity to chlorpyrifos and methamidophos

More than 30 years ago, scientists recognized that, at a given dosage, the young rat was more sensitive than the adult to the toxicity of many organophosphorus, anticholinesterase pesticides. This observation went basically unexamined until recently. Renewed interest has emerged in scrutinizing this age-related sensitivity to pesticides, especially in light of the many new pesticides which are currently marketed. Our laboratory has been involved in comparing the age-related sensitivity of young and adult rats to chlorpyrifos [Dursban, Lorsban; O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphorothioate] and methamidophos (Monitor; O,S-dimethyl phosphoamidothioate). Using chlorpyrifos, there is marked age-related sensitivity: direct oral dosing of the preweanling rat (postnatal day 17; PND17) with chlorpyrifos causes a toxic response (defined either behaviorally or biochemically) at a approximately 5-fold lower dosage than in adults (oral, maximum tolerated dose for the PND17 is 20 mg/kg versus 100 mg/kg for the adult). Other studies have indicated that the rat detoxifies chlorpyrifos and its oxon by binding to carboxylesterases and hydrolysis by A-esterases. The young rat is deficient in both these detoxification enzymes, which may explain the increased sensitivity of the young to chlorpyrifos toxicity. The age-related pattern for methamidophos is distinctly different: the oral, maximum tolerated dose is the same (8 mg/kg) whether the animal is 17 days old or an adult. We present data which indicate that methamidophos is not detoxified appreciably either in vivo or in vitro by A-esterases or carboxylesterases. Therefore, we submit the following hypothesis: organophosphorus pesticides, like chlorpyrifos, which are detoxified via A-esterases or carboxylesterases are more likely to exhibit age-related differences in sensitivity than pesticides which are not detoxified via these routes.

Behavioral evaluation of the neurotoxicity produced by dichloroacetic acid in rats

Dichloroacetic acid (DCA) is commonly found in drinking water as a by-product of chlorination disinfection. It is a known neurotoxicant in rats, dogs, and humans. We have characterized DCA neurotoxicity in rats using a neurobehavioral screening battery under varying exposure durations (acute, subchronic, and chronic) and routes of administration (oral gavage and drinking water). Studies were conducted in both weanling and adult rats, and comparisons were made between Long-Evans and Fischer-344 rats. DCA produced neuromuscular toxicity comprised of limb weakness and deficits in gait and righting reflex; altered gait and decreased hindlimb grip strength were the earliest indicators of toxicity. Other effects included mild tremors, ocular abnormalities, and a unique chest-clasping response (seen in Fischer-344 rats only). Neurotoxicity was permanent (i.e., through 2 years) following a 6-month exposure to high dose levels, whereas the effects of intermediate dose levels with exposures of 3 months or less were slowly reversible. The severity, specificity, and recovery of neurological changes were route, duration, and strain dependent. Fischer-344 rats were more sensitive than Long-Evans rats, and weanling rats may be somewhat more sensitive than adults. Oral gavage produced significantly less toxicity compared to the same intake level received in drinking water. Neurotoxicity was progressive with continued exposure, and was observed at exposure levels as low as 16 mg/kg/day (lowest dose level tested) when administered via drinking water in subchronic studies. The data from these studies characterize the neurotoxicity produced by DCA, and show it to be more pronounced, persistent, and occurring at lower exposures than has been previously reported. Further research should take into account these marked route, age, and strain differences.

Gestational exposure to chlorpyrifos: dose response profiles for cholinesterase and carboxylesterase activity

This study investigates the in vivo dose response profiles of the target enzyme cholinesterase (ChE) and the detoxifying enzymes carboxylesterase (CaE) in the fetal and maternal compartments of pregnant rats dosed with chlorpyrifos [(O,O'-diethyl O-3,5,6-trichloro-2-pyridyl) phosphorothionate], a commonly used organophosphorus insecticide. Pregnant rats were dosed daily (po) with chlorpyrifos in corn oil (0, 3, 5, 7, or 10 mg/kg) on gestational days (GD) 14-18. Animals were sacrificed 5 h after the last chlorpyrifos dose (time of maximum brain cholinesterase inhibition) for analysis of ChE and CaE activity in maternal blood, liver, brain, placenta, and fetal liver and brain. The in vitro sensitivity (i.e., IC50, 30 min, 26 degrees C) of CaE also was determined by assaying the activity remaining after incubation with a range of chlorpyrifos-oxon concentrations. In vivo exposure to 10 mg/kg chlorpyrifos from GD14-18 caused overt maternal toxicity, with dose-related decreases in ChE activity more notable in maternal brain than fetal brain. Dose-related effects were also seen with chlorpyrifos-induced inhibition of fetal liver ChE and maternal brain CaE activities. Gestational exposure caused no inhibition of placental ChE or CaE, fetal brain CaE, or maternal blood CaE. ChE activities in the maternal blood and liver, as well as fetal and maternal liver CaE, however, were maximally inhibited by even the lowest dosage of chlorpyrifos. The in vitro sensitivity profiles of CaE to chlorpyrifos-oxon inhibition were valuable in predicting and verifying the in vivo CaE response profiles. Both the in vivo and in vitro findings indicated that fetal liver CaE inhibition was an extremely sensitive indicator of fetal chlorpyrifos exposure.

Comparison of aldicarb and methamidophos neurotoxicity at different ages in the rat: behavioral and biochemical parameters

Young organisms are often more sensitive to the toxic effects of pesticides, and this finding has spurred research on further characterization of this susceptibility. The neurotoxic effects of cholinesterase (ChE)-inhibiting pesticides are of particular concern for human health risk assessment due to the widespread exposure potential in children. This study evaluated age-related differences in susceptibility for a carbamate (aldicarb) and an organophosphorus pesticide (methamidophos). Comparisons were made between preweanling (Postnatal Day 17, PND17), postweanling (PND27), and adult (approximately PND70) male and female rats. All were acute studies using oral administration. Sensitivity was quantified by (1) determination of maximally-tolerated doses (MTDs); (2) measurement of brain and blood ChE inhibition; and (3) neurobehavioral evaluation using end points known to be sensitive indicators of exposure to anticholinesterases. MTD data showed that preweanling rats were twice as sensitive as adults to aldicarb, but there was no differential sensitivity to methamidophos. The dose-response data for brain ChE inhibition followed a similar pattern of age-related differences, and similar levels of inhibition were measured at the MTD regardless of age. Dose-response and time course studies of neurobehavioral end points indicated that differential effects due to age depend on the behavioral end point examined. Following aldicarb administration, the dose-response curves for a few end points overlapped; however, the young rats otherwise showed fewer signs of toxicity than did the adults despite similar levels of brain ChE inhibition. Motor activity assessment showed that aldicarb did not produce any activity depression in PND17 rats, whereas the data for the PND27 and adult rats overlapped. With methamidophos, the dose-response curves for most end points for preweanling and adult rats were quite similar. Aldicarb-induced ChE inhibition was readily reversible in all age groups, whereas with methamidophos, enzyme activity recovered more rapidly in the young. Most behavioral alterations had recovered by 24 h with either pesticide. The results of these studies indicate that (1) ChE-inhibiting pesticides are not all the same regarding relative sensitivity of the young; (2) age-related differences were reflected in both the MTDs and degree of ChE inhibition; and (3) age-related differences in neurobehavioral measures depended on the pesticide and on the end points examined.

Pfiesteria toxin and learning performance

Pfiesteria piscicida is an estuarine dinoflagellate involved with fish kills along the east coast of the United States. We previously documented a radial-arm maze learning deficit in rats exposed to Pfiesteria that may be related to cognitive deficits seen in humans after accidental Pfiesteria exposure. The current study elucidated important behavioral parameters of this deficit. There were six dose groups. Forty (10/group) adult female Sprague-Dawley rats were injected (s.c.) with a single dose of Pfiesteria taken from aquarium-cultured Pfiesteria (35,600, 106,800, or 320,400 Pfiesteria cells/kg of rat body weight or a cell-free filtrate of the 106,800 cells/kg dose). One control group (N = 10) was injected with saline and one (N = 10) with aquarium water not containing Pfiesteria. Half of the rats in each group were tested on an 8-arm radial maze in a standard test room, and the other half were tested on the radial maze in a sound-attenuating chamber. In the standard maze room, there was a significant effect of Pfiesteria (p < 0.05) impairing choice accuracy improvement over the first six sessions of training among rats administered 106,800, 320,400, and the 106,800 cells/kg filtered sample. In contrast, there was no indication of an effect of Pfiesteria when the rats were tested on the same configuration radial maze in the sound-attenuating chamber. After 18 sessions of training in one room, the rats were switched for six sessions of testing in the other room and finally were switched back to their original room for three sessions. There was a significant Pfiesteria-induced deficit when the rats were tested in the standard test room but not when they were tested in the sound-attenuating chamber. When the Pfiesteria-exposed rats were initially switched from the sound-attenuating chamber to the standard test room they performed significantly worse than controls, whereas Pfiesteria-treated rats switched from the standard test room to the sound-attenuating chamber did not perform differently from controls. These results suggest that the Pfiesteria-induced learning impairment may result from the negative impact of distracting stimuli. At the time of the learning impairment, no overt Pfiesteria-related effects were seen using a functional observational battery and no overall response latency effects were seen, indicating that the Pfiesteria-induced choice accuracy deficit was not due to generalized debilitation. In the initial use of the figure-8 maze in this line of research, the rats in the same Pfiesteria treatment groups that showed significant deficits in the radial-arm maze showed greater declines in activity rates in a 1-h figure-8 locomotor activity test. Both the 106,800 and 320,400 Pfiesteria cells/kg groups showed significantly greater linear trends of activity decline relative to tank water-treated controls. This reflected an initial slight hyperactivity in the Pfiesteria-treated animals followed by a decrease to control levels. Pfiesteria effects in the figure-8 maze and in early radial-arm maze training may be useful in a rapid screen for identifying the critical toxin(s) of Pfiesteria in future studies.

Age- and gender-related differences in sensitivity to chlorpyrifos in the rat reflect developmental profiles of esterase activities

Young rats are more sensitive than adults to a single oral dose of chlorpyrifos, an organophosphorus pesticide. A direct comparison of chlorpyrifos effects in young (postnatal day 17; PND17), adolescent (PND27), and adult (70 days) Long-Evans rats was conducted to determine quantitative and possibly qualitative differences in sensitivity in terms of behavioral changes and cholinesterase (ChE; total cholinesterase activity) inhibition at these three ages. Male and female rats were administered chlorpyrifos orally at one of two doses (PND17, 5 or 20 mg/kg; PND27, 20 or 50 mg/kg; adult, 20 or 80 mg/kg) and tested at either 3.5 or 6.5 h after dosing. Behavioral testing included observational evaluations and measurements of motor activity and was followed immediately by tissue collection for ChE determination in brain and blood. For both behavioral changes and ChE inhibition, peak effects occurred at 3.5 h in adult male and PND27 rats (both sexes) and at 6.5 h in adult female and PND17 rats (both sexes). Comparisons of the 20 mg/kg dose across ages showed generally less ChE inhibition and fewer behavioral effects with increasing age, except that the adult females were similar to the PND27 rats. The high dose used for each age group produced similar brain ChE inhibition (80-90%) and generally similar behavioral effects. Interestingly, a few end-points in the young rats were less affected than in adults at this level of ChE inhibition. The degree of ChE inhibition in the brain more closely paralleled the blood inhibition in the younger rats, compared to the adults. Carboxylesterase (CaE) and A-esterase are known to play an important role in the detoxification of organophosphates and may be partially responsible for these sensitivity differences. Liver and plasma CaE and A-esterase activities were measured in untreated male rats on PND1, 4, 7, 12, 17, and 21 and in adults of both sexes (82-92 days old). Preweanling rats had considerably less activity of both enzymes, and adult females had less liver CaE activity than males. These differences in detoxifying enzymes correlate with the age-related differences in behavioral and biochemical effects, as well as the gender differences seen in adult rats, and thus may be a major influence on the differential sensitivity to chlorpyrifos.

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.

Age- and gender-related differences in the time course of behavioral and biochemical effects produced by oral chlorpyrifos in rats

It is well known that young animals are generally more sensitive to lethal effects of cholinesterase-inhibiting pesticides, but there are sparse data comparing less-than-lethal effects. We compared the behavioral and biochemical toxicity of chlorpyrifos in young (postnatal Day 17; PND17) and adult (about 70 days old) rats. First, we established that the magnitude of the age-related differences decreased as the rat matures. Next, we evaluated the time course of a single oral dose of chlorpyrifos in adult and PND17 male and female rats. Behavioral changes were assessed using a functional observational battery (with age-appropriate modifications for pre-weanling rats) and an evaluation of motor activity. Cholinesterase (ChE) activity was measured in brain and peripheral tissues and muscarinic receptor binding assays were conducted on selected tissues. Rats received either vehicle (corn oil) or chlorpyrifos (adult dose: 80 mg/kg; PND17 dose: 15 mg/kg); these doses were equally effective in inhibiting ChE. The rats were tested, and tissues were then taken at 1, 2, 3.5, 6.5, 24, 72, 168, or 336 h after dosing. In adult rats, peak behavioral changes and ChE inhibition occurred in males at 3.5 h after dosing, while in females the onset of functional changes was sooner, the time course was more protracted and recovery was slower. In PND17 rats, maximal behavioral effects and ChE inhibition occurred at 6.5 h after dosing, and there were no gender-related differences. Behavioral changes showed partial to full recovery at 24 to 72 h, whereas ChE inhibition recovered markedly slower. Blood and brain ChE activity in young rats had nearly recovered by 1 week after dosing, whereas brain ChE in adults had not recovered at 2 weeks. Muscarinic-receptor binding assays revealed apparent down-regulation in some brain areas, mostly at 24 and 72 h. PND17 rats generally showed more receptor down-regulation than adults, whereas only adult female rats showed receptor changes in striatal tissue that persisted for 2 weeks. Thus, compared to adults (1) PND17 rats show similar behavioral changes and ChE inhibition although at a five-fold lower dose; (2) the onset of maximal effects is somewhat delayed in the young rats; (3) ChE activity tended to recover more quickly in the young rats; (4) young rats appear to have more extensive muscarinic receptor down-regulation, and (5) young rats show no gender-related differences.

The IPCS Collaborative Study on Neurobehavioral Screening. I. Background and genesis

Numerous events over several years culminated in recognition of the need to explicitly evaluate the nervous system as a potential target for environmental chemicals. Based on recommendations from several international expert panels, the International Programme on Chemical Safety (IPCS) sponsored the Collaborative Study on Neurobehavioral Screening Methods. A Steering Committee was created to oversee the project, develop the testing protocol, recruit participating laboratories and review and analyze the data. The protocol specified the tests, the chemicals (supplied from a common source) and the exposure conditions (acute and repeated dosing). Test methods were based upon existing practices in toxicological screening as well as recent advances in neurotoxicity screening. Chemicals were selected to produce different profiles of neurobehavioral effects. Considerable latitude was afforded the participating laboratories in the choice of several key variables (e.g., strain of rat, testing device for motor activity assessment) that could potentially affect the results of the experiments. The approach therefore provided a standardized yet flexible protocol for evaluating the reproducibility of neurobehavioral screening data in diverse laboratory settings.

The IPCS Collaborative Study on Neurobehavioral Screening Methods: IV. Control data. Steering Group

The goal of the International Programme on Chemical Safety (IPCS) Collaborative Study on Neurobehavioral Screening Methods was to determine the intra- and inter-laboratory reliability of a functional observational battery (FOB) and an automated assessment of motor activity in eight laboratories worldwide. The control data were crucial to the outcome of the studies in terms of sensitivity and reliability of the test measures, which in turn impact on the between-laboratory comparisons of chemical effects. In addition, analyses of control data can aid in determining endpoints that may require modification to improve their sensitivity and reliability. The control data from the eight laboratories were examined in terms of the following parameters: 1) control variability within studies for each laboratory; 2) within-laboratory replicability of control values across studies; 3) within-laboratory stability of control values over the course of testing for a given study; and 4) between-laboratory comparisons of parameters (1), (2), and (3). The analyses indicated considerable differences across endpoints, wherein some measures showed high variability and little replicability, while others were extremely reproducible. Generally, there were similar ranges of variability and replicability of control data across laboratories, although in some cases one or two laboratories were markedly different from the others. The physiological (weight, body temperature) and neuromuscular (grip strength, landing foot splay) endpoints exhibited the least variability, whereas the subjective assessments of reactivity varied the most. These data indicate a reasonable degree of comparability in the data generated in the participating laboratories.