Behavioural anomalies were evidenced right
from the day of exposure to sublethal concentrations
of chlorpyrifos and are due to inhibition of brain
AChE. Inhibition of AChE activity results in the
accumulation of ACh and signs of cholinergic
toxicity. Chawanrat et al. (2007) reported that
inhibition of brain AChE activity is an early process
of sublethal exposure to chlorpyrifos in hybrid catfish
and hence support the above observed behavioural
changes in the exposed fish. Overall impairments in
fish behavioural responses and morphological
deformities even under recovery periods may be due
to inhibition of brain and musculature AChE activity
by chlorpyrifos-oxon via biotransformation of
sequestered chlorpyrifos in the storage organs.
Chlorpyrifos (CPF) inhibits AChE due to the effects
of their active oxygen analog chlorpyrifos-oxon
(CPF-oxon) (Timchalk et al., 2002). Sequestered
chlorpyrifos might have been biotransformed to their
active oxygen analog chlorpyrifos-oxon via a
desulfuration reaction initiated by cytochrome P450
(CYP) (Amitai et al., 1998; Poet et al., 2003),
dearylation reaction utilizing the same enzymes and
A-esterase (Poet et al., 2003). Furthermore,
physiological reactions, such as activation of
biotransformation enzyme systems in the presence of
xenobiotic substances enable the organisms to survive
in subacute exposures. This may be the reason for
insignificant mortality observed during this study.