Animal models in the neurotoxicology of 2,4-D.

2,4-D is a selective pre- and postemergence herbicide used for several crops. It is hazardous for the environment and risk for humans; therefore, several studies attempt to evaluate its effects and consequences of its use. The nervous system is supposedly a target for this herbicide, and this comprehensive review gathers the information about animal models that have been used for the study of the neurotoxicity of 2,4-D. The studies used several methods to evaluate the neurotoxicity of this herbicide, most of which used rodents, mainly rats, two used fish, and one used chicken eggs. The main behavioral effect observed concerned alterations in locomotor patterns and reduced motor activity. Biochemical analysis showed decreased levels of serotonin (5-HT) and increased levels of its metabolites and increased or decreased levels of DA and its metabolites depending on the brain area analyzed. Hypomyelination is also a possible effect of 2,4-D when the exposure occurs during the proliferation and development of the oligodendrocytes. The worst neuropathologic effects were observed in fish. Since most studies focused on the neurotoxicity of 2,4-D in rodents, the effect it may have on other species and groups of animals, especially with different physiology, is unclear and it should be researched.

Developmental exposure to glyphosate-based herbicide and depressive-like behavior in adult offspring: Implication of glutamate excitotoxicity and oxidative stress.

We have previously demonstrated that maternal exposure to glyphosate-based herbicide (GBH) leads to glutamate excitotoxicity in 15-day-old rat hippocampus. The present study was conducted in order to investigate the effects of subchronic exposure to GBH on some neurochemical and behavioral parameters in immature and adult offspring. Rats were exposed to 1% GBH in drinking water (corresponding to 0.36% of glyphosate) from gestational day 5 until postnatal day (PND)-15 or PND60. Results showed that GBH exposure during both prenatal and postnatal periods causes oxidative stress, affects cholinergic and glutamatergic neurotransmission in offspring hippocampus from immature and adult rats. The subchronic exposure to the pesticide decreased L-[14C]-glutamate uptake and increased 45Ca2+ influx in 60-day-old rat hippocampus, suggesting a persistent glutamate excitotoxicity from developmental period (PND15) to adulthood (PND60). Moreover, GBH exposure alters the serum levels of the astrocytic protein S100B. The effects of GBH exposure were associated with oxidative stress and depressive-like behavior in offspring on PND60, as demonstrated by the prolonged immobility time and decreased time of climbing observed in forced swimming test. The mechanisms underlying the GBH-induced neurotoxicity involve the NMDA receptor activation, impairment of cholinergic transmission, astrocyte dysfunction, ERK1/2 overactivation, decreased p65 NF-κB phosphorylation, which are associated with oxidative stress and glutamate excitotoxicity. These neurochemical events may contribute, at least in part, to the depressive-like behavior observed in adult offspring.

URB597 reduces biochemical, behavioral and morphological alterations in two neurotoxic models in rats.

BACKGROUND: URB597 is a compound largely linked to the inhibition of fatty acid amide hydrolase (FAAH), an enzyme responsible for the metabolic degradation of the endocannabinoid anandamide (AEA). Despite this pharmacological property accounts for its modulatory profile demonstrated in some neurotoxic paradigms, the possible protective properties of this agent have been poorly investigated, and deserve exploration in different neurotoxic models. In this study, we explored the effects of URB597 on oxidative damage to lipids and other major endpoints of toxicity in two neurotoxic models in vivo in rats (the first one produced by the mitochondrial neurotoxin 3-nitropropionic acid [3-NP], and the other generated by the striatal injection of the pro-oxidant toxin 6-hydroxidopamine [6-OHDA]) in order to provide further supporting evidence of its modulatory profile. METHODS: Male Wistar adult rats were treated for 5 or 7 consecutive days with URB597 (0.3mg/kg, i.p.) and simultaneously exposed to three injections of 3-NP (30mg/kg, i.p.) or a single intrastriatal infusion of 6-OHDA (0.02mg/2µl), respectively. Twenty four hours after all treatments were administered, lipid peroxidation was measured in the striatum of 3-NP-treated rats, and in the midbrain of 6-OHDA-treated rats. Motor skills and histological assessment in the striatum were also evaluated in 3-NP-treated rats 6 and 7days after the last drug administration, respectively; whereas apomorphine-induced circling behavior and tyrosine hydroxylase immunolocalization in the striatum and substantia nigra were investigated 21 and 22days after the last drug infusion, respectively. RESULTS: URB597 prevented the oxidative damage to lipids induced by 3-NP in the striatum, and this effect could account for the attenuation of motor deficits in this model. Attenuation of motor disturbances induced by URB597 in both models was associated with the morphological preservation of the striatum in the 3-NP model and the partial preservation of tyrosine hydroxylase in the 6-OHDA model in the SNpc and striatum. CONCLUSION: The modulatory actions exerted by URB597 in both toxic models support its potential against toxic conditions implying motor and neurochemical alterations linked to energy depletion, excitotoxicity and oxidative stress. Although most of these effects could be attributable to its action on FAAH and further AEA accumulation, in light of our present findings other properties are suggested.

Chronic exposure to organophosphate (OP) pesticides and neuropsychological functioning in farm workers: a review.

BACKGROUND: Previous studies have demonstrated that acute poisoning from exposure to organophosphate (OP) pesticides in agricultural workers causes adverse health effects. However, neuropsychological and cognitive effects of chronic occupational exposure to OP pesticides remain controversial. OBJECTIVE: To identify, evaluate, and systematize existing evidence regarding chronic exposure to OP pesticides and neuropsychological effects in farmworkers. METHODS: Using the PubMed search engine, a systematic review process was implemented and replicated according to the PRISMA statement. Eligibility criteria included workers over 18 years of age exposed to OP pesticides as well as assessment of neuropsychological and cognitive functioning. Search terms were in English and Spanish languages and included organophosphate and workers. RESULTS: Of the search results, 33 of 1,256 articles meet eligibility criteria. Twenty-four studies found an association between chronic occupational exposure to OP pesticides and low neuropsychological performance in workers. We classified nine of the studies to have study design limitations. Studies indicated occupational exposure to OP pesticides is linked to difficulties in executive functions, psychomotor speed, verbal, memory, attention, processing speed, visual-spatial functioning, and coordination. Nine studies find no relationship between OP pesticides exposure and neuropsychological performance. CONCLUSIONS: Overall, evidence suggests an association between chronic occupational exposure to OP pesticides and neuropsychological effects. However, there is no consensus about the specific cognitive skills affected.

Exposure to an organochlorine pesticide (chlordecone) and development of 18-month-old infants.

Chlordecone is a persistent organochlorine pesticide that was used in the French West Indies until the early 1990s for banana weevil borer control. Human exposure to this chemical in this area still occurs nowadays due to consumption of contaminated food. Although adverse effects on neurodevelopment, including tremors and memory deficits, have been documented in experimental studies conducted with rodents exposed during the gestational and neonatal periods, no study has been conducted yet to determine if chlordecone alters child development. This study examines the relation of gestational and postnatal exposure to chlordecone to infant development at 18 months of age in a birth-cohort of Guadeloupean children. In a prospective longitudinal study conducted in Guadeloupe (Timoun mother-child cohort study), exposure to chlordecone was measured at birth from an umbilical cord blood sample (n=141) and from a breast milk sample collected at 3 months postpartum (n=75). Toddlers were assessed using an adapted version of the Ages and Stages Questionnaire. Higher chlordecone concentrations in cord blood were associated with poorer fine motor scores. When analyses were conducted separately for boys and girls, this effect was only observed among boys. These results suggest that prenatal exposure to chlordecone is associated with specific impairments in fine motor function in boys, and add to the growing evidence that exposure to organochlorine pesticides early in life impairs child development.

Neurotoxic effects of intrathecal magnesium sulphate.

BACKGROUND AND OBJECTIVES: To assess the potential neurotoxic effects at the ultrastructural level of magnesium sulfate administered intrathecally as a single or multi-dose. METHODS: Our study was conducted with 24 Sprague-Dawley rats that weighed 250-300 g. After a 4-hour fast, the rats were given 10 mg.kg(-1) xylazine chloride intraperitoneal and then randomly allocated into three groups. Group I (n = 8) received 0.9% normal saline, Group II (n = 8) was given one intrathecal injection of 0.02 mL of 15% magnesium sulphate, and Group III (n = 8) was given 0.02 mL of 15% magnesium sulphate once a day for seven days. The injections were given within 0.40x50 mm from the lumbar area. After seven days, the animals were sacrificed under anesthesia with an aortic injection of 10% formaldehyde and their tissues were fixed. The medulla spinalis was then examined and histopathologically evaluated under an electron microscope. The Kruskal-Wallis test was used for statistical evaluation. A value of p < .05 was considered to be statistically significant. RESULTS: Significant neurodegeneration was detected in rats given single or repeated magnesium sulphate injections compared to the control group. The histopathological evaluation score of this group was also high. CONCLUSIONS: Based on electron microscopic examination, we found that intrathecal magnesium sulphate administration induced neurodegeneration.

Hippocampal neuronal loss, decreased GFAP immunoreactivity and cognitive impairment following experimental intoxication of rats with aluminum citrate.

Aluminum (Al) is a neurotoxic agent with deleterious actions on cognitive processes. Nevertheless, few studies have investigated the neuropathological effects underlying the Al-induced cognitive impairment. We have explored the effects of acute Al citrate intoxication on both hippocampal morphology and mnemonic processes in rodents. Adult male Wistar rats were intoxicated with a daily dose of Al citrate (320 mg/kg) during 4 days by gavage. Animals were perfused at 8 (G2), 17 (G3) and 31 days (G4) after intoxication. Control animals were treated with sodium citrate (G1). Animals were submitted to behavioral tests of open field and elevated T-maze. Immunohistochemistry was performed to label neurons (anti-NeuN) and astrocytes (anti-GFAP) in both CA1 and CA3 hippocampal regions. There was an increase in the locomotor activity in open field test for G2 in comparison to control group and other groups (ANOVA-Bonferroni, P<0.05). The elevated T-maze avoidance latency (AL) was higher in all intoxicated groups compared to control (P<0.05) in avoidance 1. These values remained elevated in avoidance 2 (P<0.05), but abruptly decreased in G2 and G3, but not in G1 and G4 animals in avoidance 3 (P<0.05). There were no significant differences for 1 and 2 escape latencies. There were intense neuronal loss and a progressive decrease in GFAP immunoreactivity in the hippocampus of intoxicated animals. The results suggest that Al citrate treatment induces deficits on learning and memory concomitant with neuronal loss and astrocyte impairment in the hippocampus of intoxicated rats.

Neuropharmacological analysis of caffeic acid in rats.

The aim of the present study was to investigate the effect of intraperitoneal administration of caffeic acid (0.5, 1, 2, 4 or 8 mg/kg) on elevated plus-maze and open field tasks in rats and its possible neuroprotection/neurotoxicity using the comet assay. Caffeic acid at 1 mg/kg increased the number of entries and the time spent in the open arms on plus-maze, suggesting an anxiolytic-like effect when used in lower doses without affecting locomotion and exploration on the open field. Furthermore, a protective effect against hydrogen peroxide-induced oxidative damage on brain tissue was observed through the treatment with caffeic acid at 1 and 8 mg/kg. However, in the highest dose, caffeic acid induced DNA damage in brain tissue.

Anticonvulsant and behavioural effects of the denatured venom of the social wasp Polybia occidentalis (Polistinae, Vespidae).

Several investigations demonstrate that neurotoxins isolated from venoms of spiders and wasps may exert specific and selective activity on structures of the mammalian CNS. In the present work we examine the neurological effects of the low molecular weight compounds of the denatured venom of the neotropical social wasp Polybia occidentalis in freely moving rats. Central administration of denatured venom decreased the duration of exploratory, elevation and grooming behaviours on the open field. Moreover, denatured venom inhibited convulsing action of bicuculline (ED50 57 microg/microl), picrotoxin (ED50 75 microg/microl) and kainic acid (ED50 44 microg/microl), although it was ineffective against pentylenetetrazole-induced seizures. Despite of its inhibitory activity, toxic effects on motor performance examined in the rotarod test were not found, not even in extremely high doses. Also, denatured venom moderately reduced the spontaneous locomotor activity at anticonvulsant doses. These findings may indicate that the denatured venom has anticonvulsant activity with scarce propensity to cause neurological side-effects. Further studies are necessary to isolate the active compound and establish its mechanism of action.

Maternal milk as methylmercury source for suckling mice: neurotoxic effects involved with the cerebellar glutamatergic system.

Methylmercury (MeHg) is a highly neurotoxic compound and several studies have reported intoxication signs in children whose mothers were exposed to this environmental toxicant. Although it is well established that the in utero exposure to MeHg causes neurological deficits in animals and humans, there is no evidence of the exclusive contribution of lactational exposure to MeHg as a possible cause of neurotoxicity in the offspring. In this study, we investigated the exclusive contribution of MeHg exposure through maternal milk on biochemical parameters related to the glutamatergic homeostasis (glutamate uptake by slices) and to the oxidative stress (total and nonprotein sulfhydryl groups, nonprotein hydroperoxides, glutathione peroxidase and catalase activities) in the cerebellum of suckling mice (Swiss albino). The same parameters were also evaluated in the cerebellum of mothers. Our results showed, for the first time, that lactational exposure to MeHg caused a high percent of inhibition (50%) on glutamate uptake by cerebellar slices in pups. Contrarily, this effect was not observed in mothers, which were submitted to a direct oral exposure to MeHg (15 mg/l in drinking water). In addition, behavioral/functional changes were observed in the weaning mice exposed to MeHg. It was observed an increase in the levels of nonprotein hydroperoxides in cerebellum, and this increase was negatively correlated to the glutamate uptake by cerebellar slices. This study indicates that (1) the exposure of lactating mice to MeHg causes inhibition of the glutamate uptake by cerebellar slices in the offspring; (2) this inhibitory effect seems to be related to increased levels of hydroperoxide.

Establishing stable test performance in tests from the Behavioral Assessment and Research System (BARS).

Research to identify adverse effects in humans chronically exposed to neurotoxic substances in the workplace or environment typically assesses people at one point in time in a cross-sectional study. The most widely used strategy employs performance measures taken from a single point in time and compares these with either performance of a control group or established normative data. However, multiple comparison points of the same people on the same test allow the dissection of acute--from chronic--exposure effects, among other important questions. When performance measures are used from multiple points in time, within-subject deviations are examined. For either research design, the goal is to minimize the effects of practice and to obtain stable performance on a test. Demographic variables such as age, education, and cultural background or ethnicity influence performance on neurobehavioral tests. These variables may also influence the development of stable performance. Different populations may have different learning curves so that stable performance on a test is achieved with different amounts of practice. This is especially important when making comparisons across groups that may not have equivalent backgrounds. The performance of three groups, English-speaking adults, Spanish-speaking adolescents, and Spanish-speaking migrant adolescents, was examined. Each group completed a battery of neurobehavioral tests from the Behavioral Assessment and Research System (BARS) during four sessions. Repeated measures ANOVAs were used to investigate performance across time. Tests measuring motor performance produced stable performance from the first session. More complex tasks that involved attention and memory showed a practice effect across sessions.

Identification of functional domains affected by developmental exposure to methylmercury: Faroe islands and related studies.

The Faroe Islands study is a prospective study designed to assess the neurological and behavioral consequences of in utero exposure to methylmercury (meHg). Maternal exposure to meHg was through consumption of fish and intermittent higher-level exposure through pilot whale meat, while consumption of pilot whale blubber resulted in maternal exposure to PCBs. Analysis of the neurobehavioral domains affected revealed impairment in attention, memory, and auditory processing, impairment in primary auditory function, and to a lesser extent motor impairment. For four of the eight endpoints affected by meHg exposure atp < .10, impairment was also correlated (p < .10) with in utero PCB exposure as measured by cord tissue PCB levels. Further analyses provide evidence for an independent effect of PCBs and meHg on these endpoints. Cross-sectional studies in a smaller number of children in the Amazon and Madeira by the same group of investigators, in which average meHg maternal hair levels were about twice as high those in the Faroe Islands, identified auditory, visual, and/or motor deficits, with little or no evidence of deficits in attention or memory. However, the results of the cross-sectional studies must be interpreted with caution, due to limited statistical power as well as a lack of opportunity to correlate effects to in utero exposure.