In Vivo Inhibition of Acetylcholinesterase Activity in Functionally Different Tissues of the Freshwater Fish, Cyprinus carpio, Under Chlorpyrifos Exposure

The inhibitory effect of chlorpyrifos on acetylcholinesterase (AChE) activity in different concentrations and exposure periods was investigated in the fish, Cyprinus carpio. Sublethal concentrations 14% (0.0224 mg/l) and 7% (0.0112 mg/l) of the lethal concentration (0.160 mg/l) of chlorpyrifos were used in the present study. Carp were exposed to both toxicant concentrations for 1, 7, and 14 days and were allowed to recover in toxicant-free medium for 7 days after 14 days of exposure. AChE activity was determined spectrophotometrically using acetylthiocholine iodide as substrate in the tissues of brain, gill, liver, and muscle. The present study showed time and concentration dependent inhibition of AChE activity by chlorpyrifos in the tissues of the fish, C. carpio. The highest decrease in AChE activity was recorded in brain followed by muscle, gill, and liver on day 14 in both sublethal concentrations. AChE activity increased during the recovery period, but was still lower than the control group after both sublethal concentrations. Carp in toxic media exhibited erratic and darting swimming movements, hyperexcitability, and loss of equilibrium. These symptoms persisted even after the recovery period and'were due to inhibition of brain AChE activity. Depression of AChE activity suggested decreased cholinergic transmission and consequent accumulation of acetylcholine (ACh). Excess accumulation of ACh led to prolonged excitatory postsynaptic potentials resulting in repeated, uncontrolled firing of neurons and cessation of nerve impulses. This led to behavioural and morphological changes due to impaired neurophysiology of the fish. Greater elimination half-life and biotransformation of sequestered chlorpyrifos in the storage tissues may be attributed to the fact that AChE activity failed to reach control levels even after the recovery period.