A single exposure to an organophosphate insecticide disrupts developmental patterns of hippocampal cell proliferation and overall brain size in a migratory songbird.

Developmental exposure to neurotoxic insecticides, such as the organophosphate chlorpyrifos (CPF), can cause deleterious effects on the brain and behaviour of non-target organisms. Migratory birds are at an increased risk of exposure due to their extensive space use across seasons. However, there is limited understanding of how acute sublethal exposure to neurotoxic insecticides affects neural development that underpin fitness-related behaviours such as migration. Successful migration relies on spatial cognition, which is controlled in part by the hippocampus, a brain region on the dorsal surface of the avian brain. In this study, we exposed a migratory population of European starling (Sturnus vulgaris) nestlings to an acute sublethal dose of CPF (6 mg/kg-body weight) or a vehicle control (sunflower oil). Nestlings were dosed soon after hatching (post-hatch day [PHD] 1-3), blood sampled to measure cholinesterase (ChE) enzyme activity approximately 16 h after dosing (PHD 2-4), and brains were collected during the early (PHD 5-7), middle (PHD 11-13) and late (PHD 19-20) phases of development. Proliferating cells were stained with the exogenous marker 5-ethynyl-2'-deoxyuridine (EdU) and counted throughout the hippocampus using unbiased stereology to produce high-resolution data across groups. In addition, DAPI staining was used to measure the volumes of the hippocampus and the telencephalon. Individuals exposed to CPF had significantly decreased (approx. one half) plasma ChE activity, confirming the efficacy of our treatment. CPF exposure did not affect growth (body mass, tarsus, and wing chord), nor did it affect hippocampal volume across age classes. However, CPF exposure resulted in an abnormal change in hippocampal cell proliferation across development, with CPF-dosed birds showing a more pronounced decrease in cell proliferation from middle to late development whereas control birds showed a more pronounced decrease from early to middle-development, indicating an altered developmental trajectory of hippocampal cell proliferation. In addition, across age classes, CPF-exposed birds had a larger telencephalon than control birds, although the mechanism behind this unexpected result is unknown. Nonetheless, these results demonstrate that even a single dose of a neurotoxic insecticide during key developmental windows can disturb normal brain developmental processes, thus providing a basis for future investigation of possible behavioural and ecological consequences.