Neurotoxicity and pattern of acetylcholinesterase inhibition in the brain regions of rat by bromophos and ethylbromophos

Developmental neurotoxicity of the organophosphorus insecticide chlorpyrifos: from clinical findings to preclinical models and potential mechanisms

Organophosphorus (OP) insecticides are pest-control agents heavily used worldwide. Unfortunately, they are also well known for the toxic effects that they can trigger in humans. Clinical manifestations of an acute exposure of humans to OP insecticides include a well-defined cholinergic crisis that develops as a result of the irreversible inhibition of acetylcholinesterase (AChE), the enzyme that hydrolyzes the neurotransmitter acetylcholine (ACh). Prolonged exposures to levels of OP insecticides that are insufficient to trigger signs of acute intoxication, which are hereafter referred to as subacute exposures, have also been associated with neurological deficits. In particular, epidemiological studies have reported statistically significant correlations between prenatal subacute exposures to OP insecticides, including chlorpyrifos, and neurological deficits that range from cognitive impairments to tremors in childhood. The primary objectives of this article are: (i) to address the short- and long-term neurological issues that have been associated with acute and subacute exposures of humans to OP insecticides, especially early in life (ii) to discuss the translational relevance of animal models of developmental exposure to OP insecticides, and (iii) to review mechanisms that are likely to contribute to the developmental neurotoxicity of OP insecticides. Most of the discussion will be focused on chlorpyrifos, the top-selling OP insecticide in the United States and throughout the world. These points are critical for the identification and development of safe and effective interventions to counter and/or prevent the neurotoxic effects of these chemicals in the developing brain. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Mechanism for the acute effects of organophosphate pesticides on the adult 5-HT system

The neurotransmitter serotonin (5-HT) is involved in mood disorder aetiology and it has been reported that (organophosphate) OP exposure affects 5-HT turnover. The aim of this study was to elucidate the mechanism underlying OP effects on the adult 5-HT system. First, acute in vivo administration of the OP diazinon (0, 1.3, 13 or 39 mg/kg i.p.) to male Hooded Lister rats inhibited the activity of the cholinergic enzyme acetylcholinesterase in blood and in the hippocampus, dorsal raphe nucleus (DRN), striatum and prefrontal cortex. Diazinon-induced cholinesterase inhibition was greatest in the DRN, the brain's major source of 5-HT neurones. Second, acute in vivo diazinon exposure (0 or 39 mg/kg i.p.) increased the basal firing rate of DRN neurones measured ex vivo in brain slices. The excitatory responses of DRN neurones to α₁-adrenoceptor or AMPA/kainate receptor activation were not affected by in vivo diazinon exposure but the inhibitory response to 5-HT was attenuated, indicating 5-HT_1A autoreceptor down-regulation. Finally, direct application of the diazinon metabolite diazinon oxon to naive rat brain slices increased the firing rate of DRN 5-HT neurones, as did chlorpyrifos-oxon, indicating the effect was not unique to diazinon. The oxon-induced augmentation of firing was blocked by the nicotinic acetylcholine receptor antagonist mecamylamine and the AMPA/kainate glutamate receptor antagonist DNQX. Together these data indicate that 1) acute OP exposure inhibits DRN cholinesterase, leading to acetylcholine accumulation, 2) the acetylcholine activates nicotinic receptors on 5-HT neurones and also on glutamatergic neurones, thus releasing glutamate and activating 5-HT neuronal AMPA/kainate receptors 3) the increase in 5-HT neuronal activity, and resulting 5-HT release, may lead to 5-HT_1A autoreceptor down-regulation. This mechanism may be involved in the reported increase in risk of developing anxiety and depression following occupational OP exposure.

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Organophosphate exposure inhibits dorsal raphe nucleus cholinesterase activity.

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Organophosphate oxon exposure activates 5-HT neurones in the dorsal raphe nucleus.

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Nicotinic and AMPA receptors mediate the oxon-induced activation of 5-HT neurones.

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Organophosphate exposure attenuates the response to 5-HT1A autoreceptor activation.

Low-level repeated exposure to diazinon and chlorpyrifos decrease anxiety-like behaviour in adult male rats as assessed by marble burying behaviour

Occupational exposure to organophosphate (OPs) pesticides is reported to increase in the risk of developing anxiety and depression. Preclinical studies using OP levels, which inhibit acetylcholinesterase activity, support the clinical observations, but little is known of the effects of exposure below this threshold. We examined the effects of low level OP exposure on behaviours and neurochemistry associated with affective disorders. Adult rats were administered either diazinon (1 mg/kg i.p.) which is present in sheep dip and flea collars, chlorpyrifos (1 mg/kg i.p.) which is present in crop sprays, or vehicle for 5 days. OP exposure did not affect acetylcholinesterase activity (blood, cerebellum, caudate putamen, hippocampus, prefrontal cortex), anhedonia-like behaviour (sucrose preference), working memory (novel object recognition), locomotor activity or anxiety-like behaviour in the open field arena. In contrast OP exposure attenuated marble burying behaviour, an ethological measure of anxiety. The diazinon-induced reduction in marble burying persisted after exposure cessation. In comparison to vehicle, dopamine levels were lowered by chlorpyrifos, but not diazinon. 5-HT levels and turnover were unaffected by OP exposure. However, 5-HT transporter expression was reduced by diazinon suggesting subtle changes in 5-HT transmission. These data indicate exposure to occupational and domestic OPs, below the threshold to inhibit acetylcholinesterase, can subtly alter behaviour and neurochemistry.

Brain regional heterogeneity and toxicological mechanisms of organophosphates and carbamates

The brain is a well-organized, yet highly complex, organ in the mammalian system. Most investigators use the whole brain, instead of a selected brain region(s), for biochemical analytes as toxicological endpoints. As a result, the obtained data is often of limited value, since their significance is compromised due to a reduced effect, and the investigators often arrive at an erroneous conclusion(s). By now, a plethora of knowledge reveals the brain regional variability for various biochemical/neurochemical determinants. This review describes the importance of brain regional heterogeneity in relation to cholinergic and noncholinergic determinants with particular reference to organophosphate (OP) and carbamate pesticides and OP nerve agents.

Monocrotophos induced oxidative damage associates with severe acetylcholinesterase inhibition in rat brain

BACKGROUND

Neurotoxicity of organophosphate pesticide poisoning, a lead cause of death in South Asia, has not been clearly elucidated. Organophosphates inhibit acetylcholinesterase and neurotoxicity is primarily a result of acetylcholine induced hyperactivation in different regions of the brain. Neurotoxicity also results from oxidative stress induced by acetylcholinesterase inhibition in the brain. Determining the severity of acetylcholinesterase inhibition that induces oxidative damage may help in developing strategies that protect the brain from organophosphate induced toxicity.

AIM

To determine the level of acetylcholinesterase inhibition that induces oxidative stress in the brain following organophosphate pesticide poisoning.

METHODS

Brains of rats subject to acute monocrotophos poisoning (0.8 LD(50) by gavage) were assessed for acetylcholinesterase activity, antioxidant response and oxidative damage 2.5 and 8h after poisoning and on recovery from poisoning 24h later after poisoning. Assessments were made in the cortex, striatum and hippocampus, cholinergic rich regions and cerebellum, targets of organophosphate pesticide poisoning. Analysis was in comparison to non poisoned controls.

RESULTS

High acetylcholinesterase activities were noted in striatum followed by hippocampus, cerebellum and cortex. Acute severe monocrotophos poisoning inhibited acetylcholinesterase 87% in striatum, 67% in hippocampus, 58% in cerebellum, 53% in cortex and increased glutathione levels significantly in all brain regions 2.5h after poisoning. Significant lipid peroxidation and antioxidant enzymes were induced 8h after poisoning, directly correlated to high acetylcholinesterase inhibition (>67%). Recovery from monocrotophos poisoning was associated with absence of lipid peroxidation in the brain although acetylcholinesterase inhibition persisted.

CONCLUSIONS

Neurotoxicity of monocrotophos poisoning is characterized by oxidative damage in regions of the brain that exhibit high acetylcholinesterase activity and severe acetylcholinesterase inhibition. Recovery from poisoning is associated with prolonged induction of antioxidants that protect against oxidative damage.

Physicochemical and Biological Data for the Development of Predictive Organophosphorus Pesticide QSARs and PBPK/PD Models for Human Risk Assessment

A search of the scientific literature was carried out for physiochemical and biological data [i.e., IC50, LD50, Kp (cm/h) for percutaneous absorption, skin/water and tissue/blood partition coefficients, inhibition ki values, and metabolic parameters such as Vmax and Km] on 31 organophosphorus pesticides (OPs) to support the development of predictive quantitative structure-activity relationship (QSAR) and physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) models for human risk assessment. Except for work on parathion, chlorpyrifos, and isofenphos, very few modeling data were found on the 31 OPs of interest. The available percutaneous absorption, partition coefficients and metabolic parameters were insufficient in number to develop predictive QSAR models. Metabolic kinetic parameters (Vmax, Km) varied according to enzyme source and the manner in which the enzymes were characterized. The metabolic activity of microsomes should be based on the kinetic activity of purified or cDNA-expressed cytochrome P450s (CYPs) and the specific content of each active CYP in tissue microsomes. Similar requirements are needed to assess the activity of tissue A- and B-esterases metabolizing OPs. A limited amount of acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and carboxylesterase (CaE) inhibition and recovery data were found in the literature on the 31 OPs. A program is needed to require the development of physicochemical and biological data to support risk assessment methodologies involving QSAR and PBPK/PD models.

Chronic, low-level exposure to the cholinesterase inhibitor DFP. I. Time course of neurochemical changes in the rat pontomesencephalic tegmentum

Rats were repeatedly administered with a low dose of diisopropylfluorosphosphate (DFP; 0.2 mg/kg/day, SC, for 9 or 21 days), an irreversible cholinesterase (ChE) inhibitor. Control rats received a daily injection of oil vehicle. Neurochemical changes occurring in the pontomesencephalic tegmentum (PMT), a brain stem region critically involved in behavioral state control, were evaluated at various times of treatment and after DFP withdrawal. First, enzyme assay revealed a profile of ChE inhibition in the whole PMT which looked like that observed in the striatum; both the inhibition and recovery proceeded more slowly than they did in the plasma. Second, quantitative histochemistry indicated that ChE activity in the mesopontine cholinergic nuclei and the pontine reticular formation progressively decreased across the first days of DFP exposure, to reach an asymptotic level of inhibition after 6 days (74-82% inhibition). The inhibition was less pronounced in the locus coeruleus (49%). Third, [3H]QNB autoradiography showed that muscarinic receptor density was unchanged in any of the PMT areas selected. These results are discussed regarding the question of regional variation in susceptibility to anti-ChE agents. To what extent behavioral state alterations occur concomitantly with ChE activity changes is assessed in the companion article.

In vitro sequestration of two organophosphorus homologs by the rat liver

Bromophos (Bp) and ethylbromophos (EBp) are two structurally homologous organophosphorus insecticides (OP) which show a 24-fold difference in their toxicity to the laboratory rat (LD50--2215 and 91 mg/kg b.w., respectively). The role of rat liver in the sequestration of the OP oxons was studied based on carboxylesterase (CaE) inhibition in vitro. Bromoxon (Bo) and ethylbromoxon (EBo) were greater inhibitors of rat hepatic CaE than brain acetylcholinesterase (AChE) with IC50 values at nanomolar and picomolar levels, respectively. The capacity of the liver to sequester OPs was determined by measuring AChE inhibition pre-incubated with or without liver homogenate. AChE inhibition by Bo decreased with increasing concentration of liver tissue, whereas it was unaffected in the case of EBo. The results imply that liver tissue contains binding sites, which sequester Bo thereby reducing the number of OP molecules available to inhibit AChE. Although CaE inhibition leads to sequestration, other binding sites in the liver may have a significant role in determining the toxicity of OPs. Differential sequestration of the OPs by hepatic tissue, therefore, could be important in understanding the role of differential saturation of the target molecules, which has a bearing on differential toxicity.