Decreased anxiety in juvenile rats following exposure to low levels of chlorpyrifos during development

Abstract

Exposure to chlorpyrifos (CPF) during the late preweanling period in rats inhibits the endocannabinoid metabolizing enzymes fatty acid hydrolase (FAAH) and monoacylglycerol lipase (MAGL), resulting in accumulation of their respective substrates anandamide (AEA) and 2-arachidonylglycerol (2-AG). This occurs at 1.0mg/kg, but at a lower dosage (0.5mg/kg) only FAAH and AEA are affected with no measurable inhibition of either cholinesterase (ChE) or MAGL. The endocannabinoid system plays a vital role in nervous system development and may be an important developmental target for CPF. The endocannabinoid system plays an important role in the regulation of anxiety and, at higher dosages, developmental exposure to CPF alters anxiety-like behavior. However, it is not clear whether exposure to low dosages of CPF that do not inhibit ChE will cause any persistent effects on anxiety-like behavior. To determine if this occurs, 10-day old rat pups were exposed daily for 7 days to either corn oil or 0.5, 0.75, or 1.0mg/kg CPF by oral gavage. At 12h following the last CPF administration, 1.0mg/kg resulted in significant inhibition of FAAH, MAGL, and ChE, whereas 0.5 and 0.75mg/kg resulted in significant inhibition of only FAAH. AEA levels were significantly elevated in all three treatment groups as were palmitoylethanolamide and oleoylethanolamide, which are also substrates for FAAH. 2-AG levels were significantly elevated by 0.75 and 1.0mg/kg but not 0.5mg/kg. On day 25, the latency to emerge from a dark container into a highly illuminated novel open field was measured as an indicator of anxiety. All three CPF treatment groups spent significantly less time in the dark container prior to emerging as compared to the control group, suggesting a decreased level of anxiety. This demonstrates that repeated preweanling exposure to dosages of CPF that do not inhibit brain ChE can induce a decline in the level of anxiety that is detectable during the early postweanling period.

Keywords: Anxiety; Chlorpyrifos; Developmental; Endocannabinoid; Organophosphate insecticides.

Figure 1

Rates of weight gain of (A) female and (B) male rat pups exposed daily from postnatal day 10 through 16 to either corn oil (control), 0.5, 0.75, or 1.0 mg/kg chlorpyrifos (CPF). Values are expressed as weight ± SE (n =13–23).

Figure 2

Specific activity of (A) cholinesterase (ChE), (B) monoacylglycerol lipase (MAGL), and (C) fatty acid amide hydrolase (FAAH) in the forebrain of rat pups at 12 h after last administration following daily exposure from postnatal day 10 through 16 to either corn oil (control) or 0.5, 0.75, or 1.0 mg/kg chlorpyrifos (CPF). Values are expressed as nmole (ChE) or pmole (MAGL and FAAH) product min⁻¹ mg protein⁻¹ ± SE (n = 6–12). Percent inhibition for each treatment group as compared to its respective control is presented in the oval overlaying the corresponding bar. Bars indicated with an asterisk (*) are statistically significant (p ≤ 0.05) from control.

Figure 3

Levels of (A) arachidonoylethanolamide (AEA) and (B) 2-arachidonylglycerol (2-AG) in the forebrain of rat pups at 12 h after last administration following daily exposure from postnatal day 10 through 16 to either corn oil (control) or 0.5, 0.75, or 1.0 mg/kg chlorpyrifos (CPF). Values are expressed as either pmoles/ g tissue (AEA) or nmoles/g tissue (2-AG) ± SE (n = 8–14). Percent change from control levels is presented in the oval overlaying the corresponding bar. Bars indicated with an asterisk (*) are statistically significant (p ≤ 0.05) from the respective control.

Figure 4

Levels of oleoylethanolamide (OEA) in the forebrain of (A) female and (B) male rat pups at 12 h after last administration following daily exposure from postnatal day 10 through 16 to either corn oil (control) or 0.5, 0.75, or 1.0 mg/kg chlorpyrifos (CPF). Values are expressed as either pmoles/ g tissue ± SE (n = 4–7). Percent change from control levels is presented in the oval overlaying the corresponding bar. Bars indicated with an asterisk (*) are statistically significant (p ≤ 0.05) from the respective control.

Figure 5

Levels of palmitoylethanolamide (PEA) in the forebrain of (A) female and (B) male rat pups at 12 h after last administration following daily exposure from postnatal day 10 through 16 to either corn oil (control) or 0.5, 0.75, or 1.0 mg/kg chlorpyrifos (CPF). Values are expressed as either pmoles/ g tissue ± SE (n = 4–7). Percent change from control levels is presented in the oval overlaying the corresponding bar. Bars indicated with an asterisk (*) are statistically significant (p ≤ 0.05) from the respective control.

Figure 6

Time of emergence from a dark container into a novel aversive environment by (A) female and (B) male rats on postnatal day 25 following daily exposure from postnatal day 10 through 16 to either corn oil (control) or 0.5, 0.75, or 1.0 mg/kg chlorpyrifos (CPF). Values are expressed as time ± SE (n =13–23). Bars indicated with an asterisk (*) are statistically significant (p ≤ 0.05) from control.