Associations between early life exposure to chlorpyrifos and behavioural and physiological stress response in Japanese quail

Chlorpyrifos (CPF) is an organophosphate insecticide that primarily exerts toxicity through acetylcholinesterase inhibition. Additional mechanisms, including disruption of the hypothalamic-pituitary-adrenal (HPA) axis, remain poorly understood. We examined the effects of early-life CPF exposure in Japanese quail (Coturnix japonica), with emphasis on HPA axis-related endpoints and toxicity thresholds. In a range-finding pilot study, unincubated quail eggs were injected in the air cell with solvent control (DMSO) or 20 µg/g CPF, and hatching success was monitored. Based on these results, a definitive study was conducted with five treatment groups: DMSO, 0.1, 1, and 10 µg/g CPF, and a corticosterone positive control (CORT; 0.12 ng/g). Forty eggs per group were incubated at 37.5 °C until hatching. Chicks were assessed for physiological endpoints (survival, growth, plasma corticosterone levels) and behavioural endpoints (tonic immobility, neophobia, and novel environment exploration) until 8 days post-hatch. Hatching success in the pilot and the definitive studies suggested an approximate LD50 between 10 and 20 µg/g CPF. Sublethal effects included an increase in body weight of 9% per day at 0.1 µg/g CPF compared to control, and reduced tonic immobility and activity at 1 and 10 µg/g CPF, respectively. No other endpoints were affected up to the highest tested concentration of CPF or in response to the CORT positive control. Overall, we observed post-hatch effects at concentrations below those previously reported to affect pre-hatch growth (40 µg/g CPF) and transcriptomic responses (4 µg/g CPF). Although our study showed little evidence for an effect of CPF on the HPA axis, our findings refine CPF toxicity thresholds and improve understanding of its mechanisms of action during early avian development.