Ribeiro_2022_Neurotoxicol__

Organophosphate (OP) chemicals include commonly used pesticides and chemical warfare agents, and mechanistically they are potent inhibitors of the cholinesterase (ChE) enzyme. Epidemiological studies report long-term neuropsychiatric issues, including depression and cognitive impairments in OP-exposed individuals. Chlorpyrifos (CPF) is one of the most widely used pesticides worldwide. Multiple laboratory studies have reported on either the long-term behavioral effect of an acute high-dose CPF (30-250mg/kg) or studied sub-chronic behavioral effects, particularly the motor and cognitive effects of repeated low-dose CPF. However, studies are lacking on chronic mood and depression-related morbidities following repeated CPF doses that would mimic occupationally relevant OP exposures. In this study, adult male rats were injected with CPF (1, 3, 5, or 10mg/kg/d, s.c.) for 21 consecutive days. Dependent on the CPF dose, ChE activity was inhibited approximately 60-80% in the blood and about 20-50% in the hippocampus at 2-days after the end of CPF exposures. Following a 12-week washout period, a complete recovery of ChE activity was noted. However, CPF-treated rats exhibited a dose-dependent increase in signs related to anhedonia (sucrose preference test), anxiety (open-field and elevated plus-maze), and despair (forced swim test) at this stage. To the best of our knowledge, this could be the first laboratory study that demonstrates a cause-effect relationship between occupational-like CPF exposures in adult rats and the development of long-term depression-related outcomes and could provide an experimental system to study molecular mechanisms underlying environmental OP exposures and the elevated risk for chronic behavioral deficits.