Elsevier

NeuroToxicology

Volume 25, Issue 3, March 2004, Pages 481-494
NeuroToxicology

Spine Density and Dendritic Branching Pattern of Hippocampal CA1 Pyramidal Neurons in Neonatal Rats Chronically Exposed to the Organophosphate Paraoxon

https://doi.org/10.1016/j.neuro.2004.01.007Get rights and content

Abstract

The organophosphate cholinesterase (ChE) inhibitor paraoxon is the oxidized active metabolite of parathion, a pesticide whose use in agriculture has been matter of increasing concern. The present work was aimed at reproducing a prolonged exposure to low concentrations of paraoxon and assessing possible damage to the hippocampus during the period of most significant cholinergic development. Male Wistar rats were given, from P8 to P20, subcutaneous daily injections of paraoxon (0.1, 0.15 and 0.2 mg/kg). The rate of body weight gain was reduced by all doses of paraoxon and brain ChE activity progressively decreased up to 60% by P21. Some deaths occurred in the beginning of the treatment, but the surviving animals showed neither convulsions nor overt signs of cholinergic hyperstimulation. Morphometric analysis of Lucifer Yellow-stained CA1 pyramidal neurons in coronal sections of the hippocampus showed that by P21 paraoxon caused a decrease in spine density on basal but not on secondary apical dendrites. The dendritic arborization and the pyramidal and granular cell body layers were not altered by paraoxon. ChE staining decreased in all hippocampal and dentate gyrus regions studied, whereas choline acetyltransferase (ChAT) and zinc-positive fibers remained as in control. In summary, chronic exposure to low paraoxon concentrations during the period of rapid brain development caused significant and selective decrease in basal dendritic spine density of the CA1 pyramidal neurons. Distinct modulation of the basal tree at the stratum oriens by the interplay of cholinergic afferent and GABAergic interneurons, as well as the remodeling process in response to a repetitive and rather mild paraoxon insult, may account for this selective susceptibility of basal dendritic spines. The hippocampal alterations described here occurred in the absence of toxic cholinergic signs and may affect brain development and cause functional deficits that could continue into adulthood.

Section snippets

INTRODUCTION

Paraoxon, a powerful organophosphate (OP) cholinesterase (ChE) inhibitor, is the oxidized metabolite of parathion, one of the most acutely toxic pesticides registered by the US Environmental Protection Agency (EPA) and used worldwide. Parathion is readily absorbed through the skin and mucosal membranes and, according to reports from World Health Organization it represents the number one cause of occupational and accidental intoxication and fatalities among the pesticides (FAO, 2003).

Most of the

Experimental Animals

Male Wistar rats were maintained in a 12 h light/dark cycle with access to food and water ad libitum. Paraoxon (90% purity, Sigma, St. Louis, MO, USA) was dissolved in peanut oil and dilutions were made such that 1 μl/g body weight was administered. Three doses of paraoxon (0.1, 0.15 or 0.2 mg/kg) were tested. Pups received daily, from P8 to P20, subcutaneous injections of paraoxon, and morphometric analyses were performed at P21. In one set of experiments, animals were exposed to paraoxon up to

Body Development and Mortality

Male pups were injected daily with 0.1, 0.15 or 0.2 mg/kg body weight paraoxon from P8 to P20, a time during which significant brain cholinergic development and synaptogenesis are known to occur. At P8, there was no significant difference in body weight between the control and treated groups (ANOVA, P>0.11), but the rate of animal growth was progressively reduced during the 13-day period of paraoxon treatment at all doses tested (Fig. 1). A trend of increased loss at higher doses could be seen,

DISCUSSION

The present study was aimed at analyzing the effects of daily exposure of newborn rats from P8 to P20 to low doses of paraoxon (0.1–0.2 mg/kg) on hippocampal cytoarchitecture and on morphometric characteristics of the CA1 pyramidal neurons that may be correlated with the main cholinergic markers, ChE and ChAT, during a period when the brain cholinergic system undergoes significant development. Although paraoxon produced a reduction in the rate of body weight gain, diminished the whole brain ChE

Acknowledgements

This work was supported by US Army Medical Research and Development Command contract DAMD-17-95-C-5063; USPHS Grant NS41671; FUJB/UFRJ, CNPq, FAPERJ, PRONEX/Brazil. We are especially grateful to Ms. Mabel Zelle for her expert technical assistance.

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      However, another study demonstrated that exposure to non-symptomatic low dose of OP compound was able to induce brain damage (Munirathinam and Bahr, 2004). In addition, chronic exposure to non-convulsive doses of paraoxon could lead to a significant reduction in dendritic spine density of CA1 pyramidal neurons in rats (Santos et al., 2004). Alterations in the antioxidant system and oxidative damages have been shown after exposure to OP compounds.

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      Marked histological damage to the rat brain was found following sarin-induced seizure activity (Chapman et al., 2006). A significant decrease in dendritic spines in the hippocampal pyramidal neurons has also been observed in rats that were intoxicated with non-convulsive doses of paraoxon (Santos et al., 2004) and convulsive dose of soman (Carpentier et al., 1991). There might be several different mechanisms for OP-induced brain damage.

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      In the literature, differential vulnerability of spines between basal and apical dendrites due to exogenous or endogenous factors has been reported although the mechanisms involved are not clear. For instance, Santos et al. (2004) reported that neonatal rats exposed repetitively to low doses of paroxon (an organophosphate-type cholinesterase inhibitor) lost dendritic spines selectively in basal dendrites with no changes in apical dendrites of CA1 pyramidal neurons. Moreover, normal aging also results in decreases of the spine density on basal but not apical dendrites in C57BL/6 mice (von Bohlem und Halbach et al., 2006).

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