ToxSci Advance Access originally published online on July 31, 2007
Toxicological Sciences 2007 100(1):118-127; doi:10.1093/toxsci/kfm195
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Altered Muscarinic Acetylcholine Receptor Subtype Binding in Neonatal Rat Brain following Exposure to Chlorpyrifos or Methyl Parathion
Center for Environmental Health Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi State, Mississippi 39762
1 To whom correspondence should be addressed at Center for Environmental Health Sciences, College of Veterinary Medicine, PO Box 6100, Mississippi State, MS 39762. Fax: (662) 325-1031. E-mail: rlcarr{at}cvm.msstate.edu.
Received May 24, 2007; accepted July 19, 2007
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ABSTRACT |
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The neurodevelopmental effects of two organophosphorus (OP) insecticides, chlorpyrifos (CPS) and methyl parathion (MPS), on cholinesterase (ChE) activity and muscarinic acetylcholine receptor (mAChR) binding were investigated in neonatal rat brain. Animals were orally gavaged using an incremental dosing regimen from postnatal day 1 (PND1) until PND8 with a low, medium, and high dosage for both CPS and MPS. On PND4 and PND8, ChE activity was measured in whole brain while the total and subtype densities of mAChRs were measured in three brain sections: area anterior to optic chiasma (anterior forebrain), area from the optic chiasma to the medulla/pons (posterior forebrain); and the medulla/pons excluding the cerebellum. The ligands 3H-pirenzepine, 3H-AF-DX 384, 3H-4-DAMP, and 3H-QNB were used to measure the maximal binding of the M1, M2/M4, and M3 subtypes and total mAChR receptors, respectively. In the anterior and the posterior forebrain, the levels of all mAChRs nearly doubled from PND4 to PND8, while in the medulla/pons, M1- and M3-subtype mAChR densities were low and did not increase and M2/M4 subtype and total mAChR slightly increased from PND4 to PND8. Reduction of ChE activity and mAChR binding by CPS or MPS was more evident in rats at PND8 than at PND4. With respect to mAChR binding, the greatest effects were observed in the medulla/pons and the least effects were observed in the posterior region of the forebrain. These results demonstrate that OPs exert adverse effects on rat central nervous system development through the cholinergic system in an age- and region-dependent manner.
Key Words: organophosphate; chlorpyrifos; methyl parathion; cholinesterase; muscarinic acetylcholine receptor.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Organophosphorus (OP) compounds are frequently used insecticides due to their high insecticidal activity, low environmental persistence, moderate acute toxicity, and low cost. Chlorpyrifos (CPS) and methyl parathion (MPS) have been two widely used OP insecticides in the United States. CPS is a moderately toxic Class II insecticide with a rat oral LD50 of 135–163 mg/kg (Meister, 1989
). MPS is a Class I insecticide with a rat oral LD50 of 14–24 mg/kg (Gaines, 1960). CPS and MPS both exert their toxicity primarily through the inhibition of acetylcholinesterase (AChE) with the subsequent accumulation of the neurotransmitter acetylcholine.
The cholinergic system, including the acetylcholine receptors, AChE and choline acetyltransferase, plays an important role in brain function and early development (Hohmann and Berger-Sweeney, 1998; Lauder and Schambra, 1999). The persistent overstimulation of the muscarinic acetylcholine receptors (mAChRs), which occurs during a repeated OP exposure, frequently results in a decrease in receptor number, which is a physiological response to the exposure, as well as changes in other components (Costa et al., 1982; Hoskins and Ho, 1992; Russell and Overstreet, 1987). It is possible that alteration of these cholinergic components during brain development could perturb normal development and neurological function possibly resulting in long-term neurochemical and behavioral deficits.
Human beings are at risk of exposure to OP insecticides from agriculture, industry, and household uses. The Center for Disease Control and Prevention (CDC) recently released the Third National Report on Human Exposure to Environmental Chemicals and documents that 93% of U.S. residents, who were examined between 1999 and 2000, had detectable levels of the CPS metabolite 3,5,6-trichloro-2-pyridinol in their bodies (CDC, 2005). In this report, children who were between 6 and 11 years old had higher levels (3.48 µg/g creatinine) of 3,5,6-trichloro-2-pyridinol in their urine than did adults (1.49 µg/g creatinine) between the ages of 20 and 59. Even though CPS and MPS are banned for household use in the United States (U.S. EPA, 2002), they are still widely used in agriculture. Several epidemiological studies have shown an association between OP insecticide exposure and neurological alterations (Kamel and Hoppin, 2004). Children and infants are at greater risk to the toxic effects of OP chemicals for a variety of reasons, including their relative higher exposure to these compounds, immaturely developed hepatic detoxication enzymes, and incompletely formed neurological pathways (Eskenazi et al., 1999; Faustman et al., 2000; Liu et al., 1999). Animal models have been a useful tool in studying the age-dependent neurotoxicity of low-dose OP insecticide exposure in human infants as compared to adults (Vidair, 2004). It has previously been shown that neonatal rats are more sensitive to the acute toxic effects of OP insecticides, including CPS and MPS, than adult rats (Atterberry et al., 1997; Pope et al., 1991; Zheng et al., 2000). Even though OP-exposed young rats tend to recover AChE activity more quickly than adult rats, young rats appear to exhibit more extensive reductions in mAChR levels following OP exposure (Moser and Padilla, 1998).
Previously, our laboratories have shown that repeated postnatal oral exposure of rat pups to CPS or MPS with high dosages from postnatal day 1 (PND1) to 21 resulted in ChE inhibition and decreased surface and total mAChR densities in whole brain (Richardson and Chambers, 2005; Tang et al., 1999, 2003). In addition, when pregnant rats were orally dosed with CPS (at 7 mg/kg) between gestational days (GD) 6 and 20, ChE activities and the levels of the surface, total, and M1/M3-subtype mAChR receptors, but not M2/M4 subtype, were reduced at PND1 and 3 (Richardson and Chambers, 2004). However, no effects on ChE activity or mAChR binding were observed by PND6. In other laboratories, when rat dams were injected sc with CPS (1 or 5 mg/kg) during GD 9–12, the M2/M4 subtype of mAChR in adult rats was decreased in the cerebral cortex, corpus striatum, and hippocampus on PND60 but not on PND30 (Qiao et al., 2004). Persistent cholinergic synaptic dysfunction and cognitive performance deficits have also been reported in rats following postnatal sc exposures to CPS (Levin et al., 2001; Slotkin et al., 2001). Comparing age-sensitivity effects of CPS and MPS on mAChR, Pope and Liu (1997) reported that the total receptor binding in the adult rat brain was more extensively reduced by CPS treatment (sc), but were less reduced by MPS treatment as compared to that in neonatal brain. Mechanisms that have been invoked to account for this discrepancy include the faster rates of bioactivation of CPS compared with MPS to the corresponding oxons (which may or may not be true), differences in rates of corresponding oxon detoxication, and differences in rates of AChE reactivation following inhibition.
The present study was designed to compare the dose-response effects of CPS and MPS exposure on the developing brain with respect to the following three aspects. First, to determine any differences in the response of the cholinergic system after the exposure to CPS or to MPS, we have determined ChE activity and mAChR levels on PND4 or PND8. Second, to identify which region is most vulnerable to the adverse effects of CPS or MPS, we quantified mAChR levels in the anterior forebrain, which contains the cerebral cortex and corpus striatum, in the posterior forebrain, which contains the hippocampus, and in the medulla/pons. Third, to find out which mAChR subtypes are more susceptible to the OP-induced effects, we chose four radioligands to measure the different receptor subtype densities. 3H-Pirenzepine was used to measure the maximal binding (Bmax) of the M1-subtype mAChRs (Watson et al., 1982). 3H-AF-DX 384 was used to specifically label M2/M4-subtype receptors in rat brain (Castoldi et al., 1991). 3H-4-DAMP binds two subpopulation of muscarinic sites: one class is high-affinity, low capacity sites which are primarily M3 subtype and another class is lower affinity, high capacity sites which are primarily M1-subtype receptors (Araujo et al., 1991). In this study, a low concentration of 3H-4-DAMP was used to identify the M3-subtype mAChRs. 3H-QNB was used to measure the Bmax of total mAChR binding.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Chemicals.
Analytical grade CPS and MPS were generously supplied by Dr Howard Chambers (Department of Entomology and Plant Pathology, Mississippi State University) (purity > 98%). 3H-5,11-dihydro-11-(((2-(2-((dipropylamino)methyl)-1-piperidinyl)ethyl)amino)carbonyl)-6H-pyrido(2,3b)(1,4)-benzodiazepin-6-one methansulfonate (AF-DX 384, 120 Ci/mmol), 3H-4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, 80 Ci/mmol), 3H-pirenzepine (86 Ci/mmol), and 3H-quinuclidinyl benzilate (QNB, 42 Ci/mmol) were purchased from PerkinElmer Life Sciences, Inc. (Boston, MA). All other chemicals were obtained from Sigma Chemical Co. (St Louis, MO).
Experimental design.
Adult male and female Sprague-Dawley rats from Charles River Laboratories (Wilmington, MA) were used as breeders. All animals were kept in a temperature-controlled room at 22 ± 2°C with a 12:12-h light/dark cycle in an Association for Assessment and Accreditation of Laboratory Animal Care-accredited facility. Rats were given standard laboratory chow and tap water ad libitum. All procedures were previously approved by the Mississippi State University Animal Care and Use Committee.
Dosing regimen.
The day of birth was designated PND0. Individual pups of each sex were assigned to each treatment within each litter. The size of the litter was adjusted as much as possible in order to obtain litters of the same size (8–10 pups) and even distribution of male and female pups within each litter. Using an incremental dosing regimen, CPS and MPS were dissolved in corn oil and administered to pups by daily oral gavage at a volume of 0.5 ml/kg body weight beginning at PND1 until PND4 or PND8. Control pups received corn oil daily from PND1 to PND4 or PND8. The treatment groups were CPS low (1.0 mg/kg from PND1 to 8); CPS medium (1.0 mg/kg from PND1 to 5 and 2.0 mg/kg from PND6 to 8); CPS high (1.5 mg/kg from PND1 to 5 and 3.0 mg/kg from 1 to 8); MPS low (0.2 mg/kg from PND1 to 8); MPS medium (0.2 mg/kg from PND1 to 5 and 0.4 mg/kg from PND6 to 8); and MPS high (0.3 mg/kg from PND1 to 5 and 0.6 mg/kg from PND6 to 8. Rat pups were sacrificed 4 h after the last dosage at PND4 or PND8.
This incremental dosing regiment was designed to avoid OP-induced mortality and overt signs of toxicity while the inhibition of AChE activity could be detected in the young animals. The regimen is similar to previously published studies (Richardson and Chambers, 2005; Tang et al., 2003) although lower dosages were used here.
Tissue preparation.
For the ChE assay, whole brains without cerebellum and medulla/pons were homogenized in Tris-HCl (0.05M, pH 7.4) buffer at a concentration of 40 mg/ml. For the mAChR-binding assay, the rat brain tissue was separated into three sections: area anterior to optic chiasma, area from the optic chiasma to the medulla/pons and cerebellum, and the medulla/pons. Membrane preparation and binding assays were conducted by using the modified method of Araujo et al. (1991). Briefly, the brain tissue was homogenized in Tris-HCl buffer (pH 7.4, which contained 36.3mM Tris-HCl, 13.7mM Tris base, 120mM NaCl, 5mM KCl, 2mM CaCl2, 1mM MgCl2) and centrifuged at 48,000 x g at 4°C for 10 min. The pellet was resuspended, homogenized, and centrifuged under the same condition as above. The final pellet was homogenized in Tris-HCl buffer for the receptor-binding assay.
ChE assay.
The ChE activity of the brain samples was determined using a continuous spectrophotometric assay (modification of Ellman et al., 1961) with acetylthiocholine as the substrate and 5,5'-dithio-bisnitrobenzoic acid as the chromagen (Chambers et al., 1988).
mAChR-binding assay.
Membrane preparations (about 50 µg protein) from the three sections were incubated with 3H-pirenzepine (20nM), 3H-AF-DX 384 (10nM), 3H-4-DAMP (1nM), or 3H-QNB (1nM) to measure the Bmax of ligand to the following mAChR subtypes: M1, M2/M4, M3, or total mAChR, respectively. The reaction mixtures were incubated in 96-well microplate (200 µl per well) at 37°C for 60 min. The binding of 3H ligands to the membrane fraction was determined by harvesting the labeled membranes by vacuum filtration onto GF/B filters (Packard Unifilter-96, Meriden, CT). Scintillation cocktail was added to each well and the filter plate was counted in a Packard Microplate Scintillation Counter. The specific binding was calculated by total binding minus nonspecific binding (determined when membrane fractions are incubated with each individual 3H-labeled ligand in the presence of 10µM atropine).
Protein assay.
Protein concentrations were measured by the method of Lowry et al. (1951) with bovine serum albumin as the standard.
Statistical analysis.
Statistical analysis was performed by ANOVA using the Mixed procedure (Littell et al., 1996) followed by separation of means using least significant difference. An overall ANOVA for each ligand was performed to identify differences in the main effects (age, brain region, sex, and treatment) and all possible interactions. This analysis included litter and sex x treatment x litter as random effects. Significant main effects identified included age, brain region, and treatment but not sex. No interactions were significant with the exception of an age x region interaction. Therefore, for subsequent analysis to identify treatment effects, analysis for each ligand in each brain region was performed separately for each age and since there were no significant sex or sex-related interactions, data from males and females were pooled for statistical analysis. An additional analysis was performed on control values to determine age differences for binding of each ligand in each brain region. The criterion for significance was set at p 
0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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No signs of overt functional toxicity of OP treatment were observed in the rats. As described in the methods, the OPs were administered using an incremental dosing paradigm where the low-dosage group received a constant dosage throughout treatment and the medium dosage group received the same dosage as the low group from PND1 to PND5. Thus, data from the low and medium dosages are identical and are presented on the PND4 figures as a low dosage.
With respect to body weight gain, a significant reduction was observed with the high dosage of CPS on both PND4 and PND8, while no significant decrease in body weight gain was observed with MPS at either dosage (Fig. 1A). The retardation of body weight gain with the high dosage of CPS was expected as it has been reported previously (Betancourt and Carr, 2004). With respect to whole-brain ChE activity, control ChE activity increased between PND4 and PND8 as expected. On PND4, all dosages of both CPS and MPS significantly inhibited ChE activity with CPS yielding greater inhibition than MPS (Fig. 1B). While ChE activity appeared to be inhibited in a dose-related manner with CPS on both PND4 and PND8, a dose-related decrease was not evident with MPS at either age.
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= 0.05) are indicated with an asterisk (*). Percent decrease from control at each age for each statistically significant value is presented in the oval overlaying the corresponding bar.In control animals, both the anterior forebrain and posterior forebrain had comparable levels of binding sites for all three receptor subtypes on PND4 and these binding sites increased significantly with age (Table 1). Total receptor binding was also similar between these two regions and increased significantly with age. However, in the medulla/pons, the M1 and M3 receptor levels were much lower than those in the other regions and did not increase with age. The M2/M4-subtype receptors appeared to constitute the majority of mAChR-binding sites in the medulla/pons and exhibited an age-dependent increase in binding as did total receptor binding (Table 1).
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In the anterior forebrain, there were no significant differences in the M1-subtype levels between control and OP-treated rats on PND4 (Fig. 2A) but by PND8 a significant decrease was observed with the high dosages of both CPS and MPS (Fig. 3A). On PND4, there were no significant effects of OP treatment on the M2/M4-subtype levels (Fig. 2B) but by PND8, the high dosages of both CPS and MPS and the medium dosage of MPS significantly reduced the levels of the M2/M4 subtype (Fig. 3B). The levels of the M3 subtype were significantly reduced on PND4 with the high dosage of MPS (Fig. 2C), whereas on PND8 only the high dosage of CPS reduced the M3-subtype levels although both the medium and high dosages of MPS demonstrated a trend toward a significant decrease (Fig. 3C). With respect to total mAChR levels, a significant decrease was observed on PND4 with the high dosages of both CPS and MPS (Fig. 2D). By PND8, all dosages except the low dosage of CPS significantly reduced total mAChR levels (Fig. 3D) although the low dosage of CPS demonstrated a trend toward a significant decrease.
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In the posterior forebrain, there were no significant effects of OP treatment on M1-subtype levels on either PND4 (Fig. 4A) or PND8 (Fig. 5A) although the high dosages of both CPS and MPS appeared to decrease binding at both ages. The M2/M4-subtype levels were not significantly reduced by treatment on either PND4 (Fig. 4B) or PND8 (Fig. 5B) although a trend toward a significant decrease was observed with the high dosage of MPS on PND4 and with the medium dosage of CPS and the high dosages of CPS and MPS on PND8 (Fig. 5B). The M3 subtype was not significantly affected by either CPS or MPS on PND4 (Fig. 4C), but by PND8, the high dosages of both CPS and MPS significantly reduced M3-subtype levels (Fig. 5C). With respect to total mAChR levels, the high dosages of both CPS and MPS reduced total mAChR levels on PND4 (Fig. 4D), while on PND8 only the high dosage of CPS significantly reduced total mAChR levels (Fig. 5D) although the medium dosage of CPS demonstrated a trend toward a significant decrease.
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In the medulla/pons, no significant effects of CPS or MPS in M1-subtype levels were observed on PND4 (Fig. 6A), but by PND8 the high dosages of both CPS and MPS had significantly reduced M1-subtype levels (Fig. 7A). The M2/M4-subtype levels were not affected by OP exposure on PND4 (Fig. 6B), but were significantly reduced by all dosages of both compounds on PND8 (Fig. 7B). On PND4, the high dosages of both CPS and MPS significantly reduced M3-subtype levels (Fig. 6C). By PND8, all three dosages of CPS and the high dosage of MPS reduced M3-subtype levels (Fig. 7C) and the low and medium dosages of MPS demonstrated trends toward a significant decrease as well. With respect to total mAChR levels, no effects were observed on PND4 (Fig. 6D), but by PND8 all three dosages of MPS and the medium and high dosages of CPS had significantly reduced total mAChR levels (Fig. 7D).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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Previous studies in our laboratories have demonstrated that repeated postnatal oral exposures to CPS or MPS caused persistent ChE inhibition and transient decreases of total mAChR densities in various brain preparations including whole brain excluding cerebellum (Tang et al., 1999
, 2003), whole brain excluding cerebellum and medulla/pons (Richardson and Chambers, 2005), and in the forebrain (similar to that used in this study) (Betancourt and Carr, 2004). Our data agree with that of Betancourt and Carr (2004) where CPS decreased total mAChR in the anterior forebrain on PND4. While only nonstatistically significant reductions of total mAChR were observed on PND6 in rats exposed to either CPS (Richardson and Chambers, 2005; Tang et al., 1999) or MPS (Tang et al., 2003), we observed significant decreases in total mAChR in both the anterior and posterior forebrain regions on both PND4 and PND8. However, the current experimental design, regions analyzed, statistical analysis, and dosages administered were different from the previous two studies and these could be factors in the discrepancies between previous and present studies.
The route of administration and vehicle used may be important factors in the magnitude of the adverse effects caused by OPs in the developing brain. Song et al. (1997) observed that when rats were injected sc from PND1 to PND4 with 1 mg/kg CPS dissolved in DMSO, a 10% reduction was observed on PND5 in the level of the M1 subtype but not the M2 subtype in the forebrain. In contrast, we did not observe any significant changes in either M1 or M2 subtype in any brain region studied using a similar dosage of CPS dissolved in corn oil and administered orally. The use of DMSO as a vehicle would result in different pharmacokinetics than when using corn oil as a vehicle. DMSO would allow CPS to rapidly enter the blood stream, whereas corn oil would result in a much slower rate of entry into the blood stream. However, regardless of route or vehicle, both studies report similar ChE inhibition, but the time when ChE activity was measured was different. Song et al. (1997) measured ChE activity 24 h following the last administration, whereas we measured ChE activity 4 h after administration. In neonatal animals, ChE activity recovers rapidly following OP exposure and substantial recovery can occur in 24 h (Betancourt and Carr, 2004). The combination of this rapid recovery along with the use of DMSO as a vehicle suggests that the ChE inhibition detected 24 h following the last exposure might have underestimated the impact of the CPS exposure on ChE activity.
The effects of MPS and CPS were compared because these two compounds exhibit differential pharmacokinetic and pharmacodynamic behavior that may influence the neurodevelopmental effects elicited by these compounds. It has been demonstrated in vitro using brain membrane preparations that chlorpyrifos-oxon (CPO) is a more potent inhibitor of AChE activity than methyl paraoxon (MPO), which is in direct contrast to the acutely lethal effects caused by both parent compounds since MPS is more acutely toxic than CPS (Chambers and Chambers, 1991; Gaines, 1960). The reactivation rate for the phosphorylated ChE by CPO and MPO has been estimated to be approximately 2.5 days and 2 h, respectively, which may suggest that CPS-induced ChE inhibition would be more persistent (Chambers, 1992). However, the aging rate of dimethyl-phosphorylated ChE is faster than diethyl-phosphorylated ChE which may suggest that a larger amount of permanently inhibited ChE would be present following MPS exposure than CPS exposure (Wilson et al., 1992). While the ChE inhibition data obtained with these compounds, especially the high dosages, appear to agree with the in vitro potency of the two OPs studied, the faster reactivation of MPO inhibited ChE could have contributed to the greater inhibition of ChE observed in the CPS-treated animals as compared to the MPS-treated animals. Since we measured ChE at 4 h after exposure, substantial reactivation of the MPO inhibited ChE would have occurred and undoubtedly some aging of ChE would have occurred as well. It could be that the lack of a dose-response pattern of ChE inhibition following MPS exposure could be due to significant reactivation of ChE such that the majority of inhibited ChE remaining is aged. However, the basis for the lack of a dose-response pattern of ChE inhibition following MPS exposure is not clear. However, ChE potency may only partially explain the toxicological differences between these two compounds and other parameters may play roles as well. For example, it has been suggested that in addition to systemic toxicity induced by ChE inhibition, neurodevelopmental toxicity, such as deficits in neuritic outgrowth and subsequent abnormal cholinergic innervation, could contribute to differential adverse outcomes of different OPs (Slotkin et al., 2006).
This study demonstrates that whole-brain ChE inhibition by CPS treatment was greater than by MPS treatment at both PND4 and PND8. However, CPS and MPS did not exert substantially different effects on mAChR levels and neither compound produced any significant dose-response effects. However, on PND8, CPS appeared to produce some visual dose-dependent effects in total binding while MPS did not. This is similar to the ChE data. However, the basis for the lack of a dose response of mAChR levels in our study is not clear. This could be a reflection of a lower responsiveness of the mAChR at this early age to a low-dose OP exposure. In our experience, dosages greater than 1.5 mg/kg CPS and 0.3 mg/kg MPS (the high dosages used here) induce significant mortality in 1- to 4-day-old neonatal rats. Thus, if it were possible to administer higher dosages, a dose- response pattern may be clearer.
With respect to subtype, on PND8, in the medulla/pons, the M2/M4 subtype was the most sensitive subtype to OP exposure, followed in order by the total receptors, M3-subtype, and M1-subtype receptors. The explanation might be that in medulla/pons, the M2/M4 subtype was the majority of mAChR, which made it easier to detect changes in receptor numbers. In the anterior forebrain, total mAChR levels had the biggest decrease on PND8, followed by M2/M4-, M1- and M3-subtype receptors. The greater effects on total mAChR levels may be due to the additive effects of the subtypes. Muscarinic AChR levels in the posterior forebrain were the least sensitive to OP exposure. It was found that only total mAChR levels were reduced on PND4 and PND8, while the M3 subtype had decreased by PND8. In summary, it was evident that CPS and MPS could exert neurotoxicity in developing brains by decreasing the levels of cholinergic system components. However, it must also be considered that the decrease of mAChR levels could be due to an OP-induced developmental delay. Such a delay during the time of development of many other neurotransmitter systems could have the potential to cause additional neurological alterations, such as impairment of cognitive function in weanling rats as observed by Jett et al. (2001).
In conclusion, the basis for the effects of OP insecticides in developmental neurotoxicity is still unclear. Possible mechanisms may include OPs indirectly affecting mAChR levels by inhibiting AChE activity or, alternatively, by directly inducing downregulation of mAChR (Liu et al., 2002). Previous studies have reported that CPS interferes with the M2/M4 mAChR–mediated cAMP-signaling pathway in rat cortex, both in vivo and in vitro (Ward and Mundy, 1996; Zhang et al., 2002) and alteration of this cascade might negatively affect cell differentiation (Song et al., 1997). OPs can also interact with nicotinic acetylcholine receptors, which may lead to negative effects (Katz et al., 1997). However, besides interacting with and inducing persistent deficiencies in cholinergic synaptic components (Slotkin et al., 2001), OP exposure can induce adverse effects in other areas such as targeting glial cells (Zurich et al., 2004), disturbing serotonergic systems (Aldridge et al., 2003, 2005), and modifying neurotrophin levels (Betancourt and Carr, 2004). The mechanisms by which OP exposure results in these altered parameters are not fully understood and require further investigation.
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FUNDING |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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National Institute of Health (R01 ES 10386); the Mississippi Agriculture and Forestry Experiment Station (MAFES) under MAFES project MISV-701030; the college of veterinary medicine, Mississippi State University.
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ACKNOWLEDGMENTS |
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The authors wish to thank Dr Howard Chambers for preparing CPS and MPS, Mr Shane Bennett for his expertise in animal handling, and Dr Sumalee Givaruangsawat for her statistical expertise. This paper is MAFES publication no. J-11141 and Center for Environmental Health Sciences publication no. 117.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONFUNDINGREFERENCES |
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