The effect of chronic exposure of chinook salmon to benzo(a)pyrene and cortisol of CYP1A1 induction and susceptibility to a microsporidian parasite, Loma salmonae
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Wild populations of fish are faced with a multitude of stressors, which may
include human interaction, toxins, and disease. Benzo(a)pyrene (BaP), a known
carcinogen and immunotoxin, has been reported in the stomach contents of
juvenile chinook salmon, Oncorhynchus tshawytscha, in urban waterways. We
investigated the impact of chronic dietary exposure of environmentally relevant
levels of BaP on the immune system and cytochrome P4501A1 (CYP1A1)
expression in juvenile chinook salmon.
Two experiments were carried out in which juvenile fish were fed food
treated with ethanol (control diet), low or high concentrations of BaP, or cortisol.
In the first experiment we measured mitogen-stimulated proliferation of splenic
leukocytes using flow cytometry and a colorimetric assay using Alamar Blue[superscript TM]
Susceptibility to a microsporidian parasite, Loma salmonae, was evaluated in the
second experiment by quantification of xenomas in the gills. Hepatic CYP1A1
and plasma cortisol were measured in both experiments.
No significant trends were found in leukocyte mitogen activation or plasma
cortisol between treatments or days. However, western blot analysis of CYP1A1
concentration in liver revealed interesting patterns of induction: in cortisol fed
groups CYP1A1 was <20% of control on all days, groups fed low levels of BaP
were 250% of control values on days 8 and 21 then dropped below control
values on day 29, and groups fed high levels of BaP had less CYP1A1 than
controls on all days. Similar patterns of CYP1A1 levels were found in the
second experiment, and diseased control groups showed about a 55% decrease
in CYP1A1 concentration when compared with non-diseased control groups.
Susceptibility to L. salmonae was significantly higher in groups receiving cortisol.
Whereas there was no effect of the high BaP dose, the low BaP dose appeared
to increase disease susceptibility. This study supports concerns of stress and
toxin induced immune dysfunction in wild populations of fish.
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