Early postnatal parathion exposure in rats causes sex-selective cognitive impairment and neurotransmitter defects which emerge in aging

Abstract

Developmental exposure of rats to the organophosphate (OP) pesticides leads to altered neurobehavioral function in juvenile and young adult stages. The current study was conducted to determine whether effects of neonatal parathion exposure on cognitive performance persist in older adult and aged rats, and the relationship of behavioral changes to underlying cholinergic and serotonergic mechanisms. We administered parathion to rat pups on postnatal days 1-4, at doses spanning the threshold for the initial signs of systemic toxicity and for barely detectable cholinesterase inhibition (0.1 or 0.2 mg/kg/day). Beginning at 14 months of age and continuing until 19 months, the rats were trained in the 16-arm radial maze. Controls showed the normal sex difference in this spatial learning and memory task, with the males committing significantly fewer working memory errors than females. Neonatal parathion exposure eliminated the sex difference primarily by causing impairment in males. In association with the effects on cognitive performance, neonatal parathion exposure elicited widespread abnormalities in indices of serotonergic (5HT) and cholinergic synaptic function, characterized by upregulation of 5HT(2) receptors and the 5HT transporter, deficits in choline acetyltransferase activity and nicotinic cholinergic receptors, and increases in hemicholinium-3 binding to the presynaptic choline transporter. Within-animal correlations between behavior and neurochemistry indicated a specific correlation between working memory performance and hippocampal hemicholinium-3 binding; parathion exposure eliminated this relationship. Like the behavioral effects, males showed greater effects of parathion on neurochemical parameters. This study demonstrates the sex-selective, long-term behavioral alterations caused by otherwise nontoxic neonatal exposure to parathion, with effects increasingly expressed with aging.

Figure 1

Effects of neonatal PRT exposure on radial-arm maze accuracy averaged over all phases of testing, shown as mean ± SE of total working + reference memory errors. Control rats showed the normal, significant (p<0.05) sex difference for spatial memory tasks, with males having fewer errors. In males, PRT caused significant increases in errors, erasing the sex differences.

Figure 2

Effects of neonatal PRT exposure on working memory performance during acquisition (A) and subsequent retests (B), shown as mean ± SE. Averaged over the 18 sessions of acquisition training, controls showed a significant (p<0.05) sex difference, with males committing fewer errors than females. In males, PRT caused significant increases in errors, erasing the sex differences. PRT effects present during acquisition training (A) persisted into the first retest period but disappeared with additional training in the second retest (B).

Figure 3

Effects of neonatal PRT exposure on reference memory performance during acquisition (A) and subsequent retests (B), shown as mean ± SE. Averaged over the 18 sessions of acquisition training, controls showed a significant (p<0.05) sex difference, with males committing fewer errors than females. In males, parathion caused significant increases in errors, erasing the sex differences. Parathion effects present during acquisition training (A) disappeared with additional training in the retest periods (B).

Figure 4

Effects of neonatal PRT exposure on 5HT synaptic markers. Data represent means ± SE, presented as the percent change from control values (Table 1). The result of the global ANOVA is shown at the top of the panel, with lower-order tests shown within the panel for each measure. Because there were no interactions of treatment × region, individual tests for each region were not conducted and only main effects are shown. Abbreviations: f/p cx, frontal/parietal cortex; mb, midbrain; NS, not significant.

Figure 5

Effects of neonatal PRT exposure on brain ChAT activity. Data represent means ± SE, presented as the percent change from control values (see Table 1). The significant results of the multivariate ANOVA are shown at the top of the panel, with lower-order tests for each region shown below the panel. Where there was no interaction of treatment × sex, only main effects are shown; in the brainstem, which did show a treatment × sex interaction, asterisks denote individual values that differ from the corresponding control. Abbreviations: f/p cx, frontal/parietal cortex; t/o cx, temporal/occipital cortex; hp, hippocampus; str, striatum; mb, midbrain; bs, brainstem.

Figure 6

Effects of neonatal parathion exposure on (A) HC3 binding to the presynaptic, high-affinity choline transporter, and (B) nAChR binding. Data represent means ± SE, presented as the percent change from control values (Table 1). The results of multivariate ANOVA are shown at the top of each panel; because of the treatment × sex interactions, values were subdivided into the individual treatments for males and females before performing lower-order tests, shown below the panel. Because there were no interactions of treatment × region, individual tests for each region were not conducted and only main effects are shown. Abbreviations: f/p cx, frontal/parietal cortex; t/o cx, temporal/occipital cortex; hp, hippocampus; str, striatum; mb, midbrain; bs, brainstem; NS, not significant.

Figure 7

Correlation between hippocampal HC3 binding and mean working memory errors for sessions 13–18: (A) all treatment groups, (B) controls, (C) parathion 0.1 mg/kg and (D) parathion 0.2 mg/kg).