“TOXICOLOGICAL INVESTIGATIONS OF CADMIUM CHLORIDE AND MERCURY CHLORIDE FOLLOWING SIMULTANEOUS EXPOSURE IN ZEBRAFISH” 3064
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JAU, JUNAGADH
Abstract
The present study was carried out to investigate the toxicological interaction of
cadmium chloride (CdCl2, 1 mg/L) and mercury chloride (HgCl2, 30 µg/L) following
simultaneous exposure for 21 days in adult female zebrafish. In the present study, 224
adult female zebrafish (56 fish in each group) were randomly divided into four groups.
The zebrafish of control group were maintained in normal condition without any
treatment (R.O water with standard range of temperature and pH). The zebrafish of
second group were maintained in R.O. water containing cadmium chloride at the
strength of 1 ppm. The zebrafish of third group were maintained in R.O. water
containing mercury chloride at the strength of 30 µg/L of water. The zebrafish of fourth
group were exposed to both cadmium chloride and mercury chloride at above
mentioned strength.
Noticeable signs of toxicity were not observed except freezing and hiding
behavior in all toxicity groups. The exposure of cadmium chloride and mercury chloride
alone and in combination did not affect the physical parameters like body weight, body
width at middle and body length. Light-dark preference test and novel tank test were
also carried out for the assessment of changes in behavior of zebrafish at the day 7 and
day 21. The total time spent in light and total no. of entries in light by zebrafish were
significantly lower in all toxicity groups as compared to control group at the day 7.
Among all toxicity groups, Cd-exposed group significantly spent less time in the light
compartment with less number of entries in the light compartment as compared to that
observed in Hg and Cd + Hg-exposed zebrafish at the day 7. Total time spent in the
dark compartment and total number of entries in the dark compartment by zebrafish of
all toxicity groups were significantly higher and lower, respectively, as compared to
that of control group. The Cd-exposed zebrafish spent significantly more time in the
dark compartment with less number of entries in the dark compartment as compared to
Hg and Cd + Hg-exposed zebrafish at day 7. The latency time to white side was
significantly higher in all toxicity groups as compared to that of control group. Among
all groups, latency to white was found significantly higher in Hg-exposed group as
compared to Cd and Cd + Hg-exposed groups at day 7. Similar pattern of alterations in
behavioral parameters were observed in all toxicity groups at day 21. However, Cd and
Cd + Hg-exposed groups were at par with all behavioral parameters except latency to
white side at day 21. The latency to white side was non-significantly higher in all
toxicity groups as compared to that of control group at day 21. In context to the novel tank test, total time spent in lower zone by zebrafish of all toxicity groups were
significantly higher as compared to that of control group at day 7 and 21. The changes
in behavior clearly indicates anxiety in zebrafish after exposure to Cd, Hg and Cd + Hg.
In the brain, SOD activity was significantly lower in all toxicity groups as
compared to that of control group. The SOD activity in Hg-exposed group was found
lower as compared to Cd and Cd + Hg exposed groups. The CAT activity in brain of
zebrafish of all toxicity groups were non-significantly lower as compared to that of
control group. In Cd and Cd + Hg-exposed groups, GSH level was found significantly
lower as compared to that of control group. However, the GSH level in Hg-exposed
group was non-significantly lower as compared to control group. The alteration in
activity of SOD or CAT or GSH resulted in higher level of MDA in brain of zebrafish
in all toxicity groups. As compared to group exposed to Cd and Cd + Hg, Hg-exposed
group displayed significantly more lipid peroxidation in brain.
The SOD activity in intestine was significantly increased in all toxicity groups
as compared to that of control group. The Hg-exposed group exhibited significantly
higher SOD activity as compared to Cd and Cd + Hg-exposed groups. The CAT activity
in the intestine of zebrafish of all toxicity groups were non-significantly lower as
compared to that of control group. The GSH level in the intestine of zebrafish in all
toxicity groups were significantly lower as compared to that of control group. The
MDA level in the intestine of zebrafish in all toxicity groups were significantly higher
as compared to that of control group. Exposure to Hg resulted in significantly more
lipid peroxidation in intestine of zebrafish as compared to that produced by exposure to
Cd alone and Cd + Hg. In the ovary, the SOD and CAT activities were non-significantly
lower in all toxicity groups as compared to that of control group. The levels of GSH
and MDA were significantly lower and higher, respectively in Cd + Hg-exposed group
as compared to other groups. The GSH and MDA levels in Cd and Hg exposed groups
were non-significantly lower and higher, respectively, as compared to that of control
group.
The zebrafish exposed to Cd, Hg and Cd + Hg exhibited non-significantly
down-regulation of SOD, CAT and NrF2 mRNA expression level in the brain. In Cd +
Hg exposed group, NrF2 mRNA expression level in the brain was found to be
significantly down-regulated as compared to that observed in Cd and Hg-exposed
groups. In intestine, SOD and CAT mRNA expression levels were non-significantly
up-regulated. However, the NrF2 mRNA expression level was to be found non significantly down-regulated in all toxicity groups. The SOD mRNA expression level
in the ovary of zebrafish exposed to Hg was significantly up-regulated as compared to
that of Cd and Cd + Hg-exposed groups. The CAT and NrF2 mRNA expression levels
in the ovary were non-significantly lower in all toxicity groups. The exposure of
cadmium (1 mg/L), mercury (30 µg/L) and cadmium (1 mg/L) + mercury (30 µg/L) in
combination resulted in various histopathological changes in brain, intestine, ovary,
liver, kidney, gills and retinal layer of zebrafish.
In conclusion, cadmium chloride and mercury chloride alone and in
combination produced anxiety like behavior, oxidative stress mediated alterations in
brain, intestine and ovary of zebrafish. Both cadmium chloride and mercury chloride
alone and in combination affected the SOD, CAT and NrF2 mRNA expression in brain,
intestine and ovary of zebrafish. The both metals alone and in combination resulted in
significant histopathological changes in brain, intestine, ovary, liver, kidney, gills and
retinal layer. However, exposure to cadmium chloride for 21 days did not show the
toxicological interaction upon simultaneous exposure with mercury chloride in adult
female zebrafish.