Developmental exposure of rats to chlorpyrifos leads to behavioral alterations in adulthood, involving serotonergic mechanisms and resembling animal models of depression

Abstract

Developmental exposure to chlorpyrifos (CPF) causes persistent changes in serotonergic (5HT) systems. We administered 1 mg/kg/day CPF to rats on postnatal days 1-4, a regimen below the threshold for systemic toxicity. When tested in adulthood, CPF-exposed animals showed abnormalities in behavioral tests that involve 5HT mechanisms. In the elevated plus maze, males treated with CPF spent more time in the open arms, an effect seen with 5HT deficiencies in animal models of depression. Similarly, in an anhedonia test, the CPF-exposed group showed a decreased preference for chocolate milk versus water. Developmental CPF exposure also has lasting effects on cognitive function. We replicated our earlier finding that developmental CPF exposure ablates the normal sex differences in 16-arm radial maze learning and memory: during acquisition training, control male rats typically perform more accurately than do control females, but CPF treatment eliminated this normal sex difference. Females exposed to CPF showed a reduction in working and reference memory errors down to the rate of control males. Conversely, CPF-exposed males exhibited an increase in working and reference memory errors. After radial-arm acquisition training, we assessed the role of 5HT by challenging the animals with the 5HT2 receptor antagonist ketanserin. Ketanserin did not affect performance in controls but elicited dose-dependent increases in working and reference memory errors in the CPF group, indicating an abnormal dependence on 5HT systems. Our results indicate that neonatal CPF exposures, classically thought to be subtoxic, produce lasting changes in 5HT-related behaviors that resemble animal models of depression.

Figure 1. Effects of neonatal CPF exposure on performance in the elevated plus maze, presented as the percentage of time spent in the open arms. Data represent means and SEs. ANOVA indicated a main effect of CPF treatment (p < 0.0005) and an interaction of treatment × sex (p < 0.06). Separate analyses were conducted for males and females, showing a significant treatment effect only in males (asterisk). Control males and females also differ from each other significantly (p < 0.008).

Figure 2. Effects of neonatal CPF exposure on the consumption of chocolate milk compared with water. Data represent means and SEs of the ratio of chocolate milk consumption to water during a 2-hr period. ANOVA indicates a main effect of CPF (p < 0.02) without an interaction of treatment × sex, so lower-order evaluations were not performed for males and females.

Figure 3. Effects of neonatal CPF exposure on working memory errors in the 16-arm radial maze in (A) males and (B) females. Values shown are means and SEs across groupings of three sessions. ANOVA indicated a significant treatment × sex interaction (p < 0.01), with the females showing a main treatment effect of CPF (p < 0.02).

Figure 4. Effects of neonatal CPF exposure on reference memory errors (means and SEs) in the 16-arm radial maze in (A) males and (B) females. Values shown are means and SEs across groupings of three sessions. ANOVA indicated a significant treatment × sex interaction (p < 0.004). Both groups showed treatment effects, with males displaying an increase in errors (p < 0.03) and females a decrease (p < 0.06).

Figure 5. Effects of the ketanserin challenge in the 16-arm radial maze showing effects on error rates for working memory (A) and reference memory (B) at three different ketanserin doses (0.5, 1, and 2 mg/kg). Data represent means and SEs, with values for males and females combined because of the absence of an interaction of treatment × sex. In (A), ANOVA indicated a main effect of CPF treatment (p < 0.0001) and an interaction of CPF × ketanserin dose (p < 0.0001). In (B), both terms were also significant (p < 0.0001 and p < 0.002). Because of the interaction of the two treatments, separate analyses were then conducted for the effects of ketanserin in the control and CPF-treated groups. For both measures, ketanserin had no significant effect in the controls, whereas it elicited significant increases in errors in the CPF group (p < 0.0001 for A; p < 0.002 for B). : *Ketanserin doses that elicited significant increases in errors.