Pharmacology & Pharmacy, 2011, 2, 375-385
doi:10.4236/pp.2011.24049 Published Online October 2011 (http://www.SciRP.org/journal/pp)
375
Neuroprotection by Melatonin on Mercury
Induced Toxicity in the Rat Brain
Mandava V. Rao, Anshita R. Purohit
Zoology Department, University School of Sciences, Gujarat University, Ahmedabad, India.
Received May 23rd, 2011; revised July 28th, 2011; accepted September 20th, 2011.
ABSTRACT
Free radicals are common outcome o f no rmal aerob ic cellu la r metabolism. In-built antioxid ant system of body plays its
decisive role in prevention of any loss due to free radicals. However, imbalanced defense mechanism of antioxidants
and overproduction or incorporation of free radicals from environment to living systems leads to serious damage. It
also attacks nervous system resulting in neural-degeneration. In order to evaluate the neurotoxic effect on the brain
parts of mercury in our study, oxidative stress indices of enzymatic and non enzymatic components were measured in
rats intoxicated with mercury (2 mg and 4 mg/kg body weight) for 60 days to adult rats. Along with gravimetry, tissue
burden was also recorded. Alterations in these indices were further supported by ultrastructural studies carried out in
the brain as indicated by myelin disintegration, cell organelle alterations and neuronal loss by mercury poisoning.
Treatment with the antioxidant melatonin (N-acetyl 5-methoxy tryptamine, 5 mg/kg) prevented mercury exerted toxicity
due to its antioxidant property. The pathological changes were also ameliorated in the brain region comparatively to
support biochemical profile of brain. Thus, melatonin produced neuroprotection ag ainst mercury poiso ni n g in rat s.
Keywords: Mercuric Chloride, Melatonin, Neuroprotection, Cerebral Hemisphere, Cerebellum, Medulla Oblongata,
Rats
1. Introduction
Inorganic mercury present in the environment is a well
established toxicant to human health [1,2]. Mercury is
released in the environment by human activity such as
mining, smelting, extensive industrial and agricultural
usage, combustion of fossil fuels and other industrial
release. It enters the body in variety of chemical forms
that are—elemental, inorganic and organic, exhibiting its
toxicology characters including neurotoxicity, nephro-
toxicity, reproductive toxicity and gastrointestinal toxic-
ity with ulceration and hemorrhage [3-7]. Various mecha-
nisms have been proposed to explain the biological toxic-
ity of mercuric chloride, including oxidative stress. Hg2+
reacts with thiol groups (–SH), thus depleting intracellu-
lar thiols, especially glutathione and causing oxidative
stress or predisposing cells to it [8]. Mercury is a major
neurotoxicant [9,10]; generates high levels of reactive
oxygen species (ROS) and oxidative stress, depletes glu-
tathione and thiols causing increased neurotoxicity from
interactions of ROS, glutamate, and dopamine [11]; kills
or inhibits production of brain tubulin cells [12-14]; in-
hibits production of neurotransmitters by inhibiting: cal-
cium-dependent neurotransmitter release [10], dihydro-
teridine reductase [14], nitric oxide synthase [15], block-
ing neurotransmitter amino acids [16], causes abnormal
migration of neurons in the cerebral cortex [17] and
hence continues to pose appreciable risk to human health
as evidenced by the tragic epidemics of mercury poison-
ing in Japan and Iraq [18]. Ingestion of mercury com-
pounds from sea food diet is associated with aberrant
central nervous system (CNS) functions [19-21]. Addi-
tionally, researchers reported that exposure to mercury
can cause immune, sensory, neurological, motor and be-
havioral dysfunctions similar to traits associated with
autism spectrum disorders (ASDs), and that these simi-
larities extend to neuroanatomy, neurotransmitters and
biochemistry. It also affects antioxidant system in the cell,
resulting in loss of membrane integrity and finally cellu-
lar necrosis [22]. Other antioxidants, including ascorbic
acid and vitamin E, have been reported to be depleted in
HgCl2 treated rats [23]. Basu et al. (2007) [24] demon-
strated the reduction in neurochemical enzymes like
cholinesterase (ChE) and monoamine oxidase (MAO) in
the river otters after mercury exposure. In vivo and in

Neuroprotection by Melatonin on Mercury Induced Toxicity in the Rat Brain 376
vitro experiments were also employed by Basu et al.
(2008) [25] to demonstrate inhibiting effects of mercury
on the two key muscarinic cholinergic receptor subtypes
(M1 and M2) in the two brain regions (occipital cortex
and brain stem) of captive mink. Many experiments sug-
gest that oxidative stress can be involved in cellular
damage and that it can be implicated in the toxicity of
many xenobiotics [20]. It is well known that cerebral
hemisphere (CH), cerebellum (C) and medulla oblongata
(MO) are responsible for regulating primary sensory
functions and motor coordination, balance and postural
stability as well as autonomous functions respectively.
Melatonin (N-acetyl-5-methoxy tryptamine) is an in-
doleamine secreted by the pineal gland which is located
in the dorsal surface of the hypothalamus and was proven
to be a free radical scavenger just over a decade ago [26].
The efficacy of melatonin in functioning to overcome
oxidative stress relates to its direct free radical scaveng-
ing actions [27,28], its ability to enhance the activities of
number of antioxidative enzymes [29-31], its stimulatory
actions on the synthesis of another important intracellular
antioxidant-glutathione [32], its efficacy to reduce elec-
tron leakage from the mitochondrial electron transport
chain [33] and its synergistic interactions with the other
antioxidants [34]. The interactions of melatonin mem-
brane-bound receptors are believed to mediate endocrine
and circadian rhythm effects of it. Through in vivo and in
vitro studies, it was known to exhibit a potent free radical
scavenging activity and also protects peroxidative dam-
age [35]. Effects of mercury on neurotoxicity and its
amelioration by melatonin are demonstrated earlier [36].
Hypothetically, melatonin plays a key role in interaction
with known mercury free radical generation [37,38]. The
present study was hence undertaken to evaluate the ef-
fects of mercury in relation to oxidative stress, its histo-
logical and ultrastuctural changes in relation to functions
of CH, C and MO. Further, role of melatonin on mercury
exerted toxicity of brain region was also evaluated; as
such study was less attempted.
2. Materials and Methods
2.1. Chemicals
Melatonin and mercuric chloride were purchased from
HiMedia (Mumbai). All the chemicals used in the ex-
periment were of the highest purity available.
2.2. Administration of the Dose
Adult Wistar rats (Rattus no rvegicus) weighing 250 - 300
gm were procured from Cadila Pharma under the Animal
Maintainance and Registeration No. 167/1999/CPCSEA
from the Ministry of Social Justice and Empowerment,
Govt. of India which was purely surplus outbreed stock.
The rationale of selection of male rats only is to avoid
confounder such as different metabolic rate of genders
(male and female), including sex hormone, lactation and
pregnancy, which are inherent to females. The animals
were housed under standard temperature (24˚C ± 1˚C),
operating on a 12 h dark/light condition. They were fed
on standard rodent chow and water ad libitum.
The experimental animals were divided into 5 groups
(n = 10). The LD50 of HgCl2 received per rat was 37
mg/kg body weight [39]. Group I served as control and
were provided with distilled water. Low dose of mercuric
chloride (2 mg/kg body weight) was given to group II by
the help of canula in the gastric gavage. High dose of
mercuric chloride (4 mg/kg body weight) was received
by Group III. Group IV was administered with melatonin
(5 mg/kg body weight) intraperitoneally while group V
received melatonin half an hour before the administration
of high dose of HgCl2. The reason for melatonin admini-
stration 25 - 30 min before HgCl2 introduction was the
rapidity of melatonin metabolism [40]. The doses were
repeated daily for 60 regular days.
2.3. Gravimetric Studies
Body and organ weights of the rats treated with mercury
were measured after and before necropsy respectively.
2.4. Tissue Collection
The animals were weighed and necropsy was performed
after autopsy. Brain was dissected, weighed and all the 3
parts of brains, viz. cerebral hemisphere (CH), cerebel-
lum (C) and medulla oblongata (MO) were carefully
separated, washed with ice-cold normal saline solution
and placed on ice. The samples were weighed and soni-
cated in the respective buffers as per the protocol and the
assays were performed within 24 h of the animal dissec-
tion and sample preparation.
2.5. Biochemical Parameters
The thiobarbituric acid reactive species (TBARS) levels
in cerebral hemisphere, cerebellum and medulla oblon-
gata of control and all treated animals were determined
by the method of Ohkawa et al. (1979) [41]. The estima-
tion of glutathione (GSH) and glutathione peroxidase
(GPx) in all groups of rats was carried out by the method
of Ellman (1959) [42] and Rotruck et al. (1973) [43] re-
spectively. Activity of glutathione reductase was esti-
mated by the method of Carlberg and Mannervik (1985)
[44]. Glutathione-S-transferase activity was measured in
the brain of control and treated group animals by the
modified method of Habig et al. (1974) [45] whereas,
protein carbonyl activity was done by the method of Le-
vine et al. (1994) [46]. Levels of hydrogen peroxide were
determined by the modified technique of Pick and Kei-

Neuroprotection by Melatonin on Mercury Induced Toxicity in the Rat Brain377
sari (1981) [47]. The method of Lowry et al., (1951) [48]
was employed for total protein estimation. Analysis of
total ascorbic acid was done by the method of Roe and
Kiether (1943) [49]. Mercury levels in the brain were
estimated on mercury analyzer (ECIL, Hyderabad) using
acid digestion method followed by cool vaporization of
the tissue sample.
2.6. Ultrastuctural Studies
CH, C and MO were fixed, dehydrated and cleared be-
fore embedding it for sectioning. Primary fixation was
done using 3% gluteraldehyde in 0.2 M Phosphate Buffer
(pH 7.4). Secondary fixation was carried out using 1%
Osmium tetroxide in the same buffer. Dehydration was
carried out using ascending alcohol grade i.e., 50, 70, 80,
90 and 100% alcohol after which clearing was done with
propylene oxide. The tissue was then embedded in Epoxy
resin. 60nm thin sections were taken on copper grid for
ultrastructural studies. The grid was stained with uranyl
acetate and lead citrate and observed under transmission
electron microscope (TEM).
2.7. Statistical Analysis
Results were expressed as Mean ± SEM. SPSS software
(version 16) was used for all statistical analysis. Individ-
ual groups mean were compared by student’s t-test. The
data was analysed using one way analysis of variance
(ANOVA) followed by Tukey’s honestly significant dif-
ference (HSD) post hoc for comparing among different
groups and significance was accepted when p < 0.05. The
percent amelioration was also calculated by using the
following formula [50].
% amelioration = (pro oxidant group – respective
antioxidant group)/(pro oxidant group – Control) × 100.
3. Results
In the present study, the chronic administration of HgCl2
to rats caused an imbalance in oxidative stress as moni-
tored by means of antioxidant indices, which is con-
fronted by transmission electron microscopic studies on
the brain.
3.1. Effect of MC on Gravimetric Data and
Mercury Levels
3.1.1. Body Weights
Treatment of MC to rats significantly (p < 0.05) reduced
the body weight. Besides weight loss, the animal showed
signs of toxication like yellowish fur and a cachectic ap-
pearance. High dose of MC resulted in further significant
reduction (p < 0.001) within the dose period of 60 days.
The amelioration obtained in the body weight after me-
latonin supplementation was 50% (Table 1).
3.1.2. Organ W ei gh ts
Total weight of brain, cerebral hemisphere (CH), cere-
bellum (C) and medulla oblongata (MO) were weighed
after sacrifice to nearest milligram by digital balance.
Mercury treatment reduced the organ weights of rats in a
dose dependent way, where high dose brought about a
statistically significant (p < 0.001) reduction in total
weight, CH, C and MO. Melatonin co-administration
gave 45.5%, 36.9%, 31.2% and 70% amelioration re-
spectively (Table 1).
3.1.3. Mercury Levels
Mercury retention was measured with the help of mer-
cury analyzer where a significant retention (p < 0.001) in
the mercury levels were noted in C and MO in treated
rats and CH had also a significant retention (p < 0.05) at
high dose only. Amelioration was 87.2%, 84.8% and
90.6% in CH, C and MO respectively (Table 2).
3.2. Effect of MC on Enzymatic and Non
Enzymatic Antioxidants
3.2.1. MDA, Catalase, Superoxide Dismutase and
Ascorbic Acid Levels
A significant (p < 0.001; p < 0.05) increase in levels of
MDA and catalase activity were observed in the MC
treated groups in CH, C and MO, as compared to the
control group. However, the amelioration obtained after
melatonin administration for MDA was 68.4%, 50.8%
and 58.7% respectively whereas, for catalase was 62.3%,
20% and 26.9% respectively. Treatment with MC brought
about a decline in SOD (p < 0.001) and TAA levels of CH,
C and MO by mercury feeding. The amelioration was
41%, 20.7% and 25.8% respectively for SOD and 15.2%,
14.1% and 22.3% respectively for total ascorbic acid
after MLT co-administration (Table 3).
3.2.2. GSH, G Px and GR
Treatment with MC induced a significant (p < 0.001) fall
in GSH levels in the brain compared with control giving
47.1%. 68.8% and 83.8% amelioration after melatonin
supplements to the rats. Similarly, Glutathione peroxi-
dase (GPx) activity declined in all brain regions. A sig-
nificant (p < 0.001) reduction in its values were noted in
cerebral hemisphere, cerebellum and medulla oblongata
in mercury treated rats. A significant (p < 0.05) reduction
of GPx activity in C and MO were recorded at combina-
tion level. The activity of GR recorded a highly signifi-
cant (p < 0.001) decline in CH, C and MO by MC treat-
ment for 60 days. With melatonin and mercury combina-
tion, a reduction (p < 0.01) was noticed in CH and C only
(Table 4).
3.2.3. GST, Protein Carbonyl, H2O2 and Total (–SH)
Glutathione-S-transferase (GST) showed increase (p <

Neuroprotection by Melatonin on Mercury Induced Toxicity in the Rat Brain 378
Table 1. Body and organ weights of control and experimental groups.
HgCl2
Parameters Control
LD HD
Melatonin Melatonin + HD Anova Amelioration (%)
Total body weights (gm) 425 ± 11.10 354 ± 4.00NS 279 ± 6.40* 347 ± 9.94NS 306 ± 4.00NS 51.86# 50.00
Total brain weights (mg) 2163.4 ± 14.6 2044.1 ± 10.7+1992 ± 3.93+ 2182 ± 6.65NS 2070 ± 13.7** 56.28# 45.50
Cerebral hemisphere (mg) 1332 ± 15.39 1258 ± 17.4* 1148 ± 17.9+ 1349 ± 17.48NS 1216 ± 15.32NS 24.60# 36.90
Cerebellum (mg) 480.0 ± 4.10 440.7 ± 9.92** 425.0 ± 3.544+475.2 ± 15.2NS 442.2 ± 8.26NS 16.54# 31.27
Medulla oblongata (mg) 441.0 ± 8.47 434.0 ± 10.00** 421.1 ± 9.40 +460.0 ± 11.70NS 435.0 ± 11.30NS 7.133# 70.00
Table 2. Mercury levels measure d in contro l and e x pe r i me ntal groups.
HgCl2
Parameters Organs Control
LD HD
Melatonin Melatonin + HD Anova Amelioration (%)
CH 0.30 ± 0.10 5.59 ± 0.05+ 9.67 ± 0.14+0.09 ± 0.14NS 2.72 ± 0.15NS 2.78# 87.20
C 0.27 ± 0.09 4.89 ± 0.04+ 8.73 ± 0.06+0.02 ± 0.20NS 2.57 ± 0.08NS 3.57# 84.84
Mercury Levels
(μg/gm tissue
weight)
MO 0.24 ± 0.12 4.09 ± 0.09+ 7.74 ± 0.08+0.00 ± 0.07NS 2.03 ± 0.17NS 3.75# 90.63
Values are Mean ± S.E., HgCl2 = Mercuric Chloride; LD = Low Dose; HD = High Dose; CH = Cerebral Hemisphere; C = Cerebellum; MO = Medulla Oblon-
gata; NS = Non Significant; *p < 0.05; **p < 0.01; +p < 0.001; significant analysis of variance at #p ≤ 0.05.
Table 3. Enzymes of antioxidant system in brain of control and treated animals.
HgCl2
Parameters Organs Control
LD HD
Melatonin Melatonin + HD Anova Amelioration (%)
CH 37.22 ± 3.54 58.11 ± 1.46+66.83 ± 5.60+45.55 ± 2.28NS 46.55 ± 2.63* 11.39# 68.49
C 40.76 ± 2.28 57.32 ± 2.00+71.77 ± 1.62+39.18 ± 8.20NS 56.00 ± 6.41NS 7.49# 50.85
LPO
(nanomoles of
MDA/100 mg
tissue weight) MO 37.5 ± 1.77 42.94 ± 0.63** 51.05 ± 1.66+38.87 ± 1.62NS 43.09 ± 1.04** 16.48# 58.74
CH 2.277 ± 0.07 1.499 ± 0.04+1.277 ± 0.08+1.99 ± 0.04NS 1.68 ± 0.07** 2.58# 41.00
C 1.733 ± 0.04 0.975 ± 0.01+0.873 ± 0.03+1.271 ± 0.05NS 1.051 ± 0.03* 4.19# 20.77
SOD
(units/mg protein)
MO 2.306 ± 0.09 1.145 ± 0.02+1.069 ± 0.04+2.054 ± 0.01NS 1.38 ± 0.15NS 1.77# 25.80
CH 18.37 ± 1.27 17.67 ± 3.7NS 14.25 ± 3.33*21.81 ± 4.41NS 16.82 ± 2.34NS 0.42# 62.37
C 30.5 ± 3.47 26.69 ± 0.9NS 24.17 ± 2.13*30.72 ± 4.6NS 25.44 ± 2.99NS 0.67# 20.06
Catalase
(µ moles H2O2
consumed/
min/mg protein) MO 34.94 ± 3.69 31.32 ± 2.73NS 25.16 ± 4.83*35.2 ± 3.78NS 27.8 ± 4.2NS 0.78# 26.99
CH 2.46 ± 0.65 2.22 ± 0.42NS1.72 ± 0.07NS2.75 ± 0.44NS 1.83 ± 0.21NS 0.91# 15.20
C 2.85 ± 0.68 2.60 ± 0.59NS1.51 ± 0.11NS 2.94 ± 0.14NS 1.70 ± 0.42NS 2.14# 14.17
TAA
(mg/gm tissue
weight)
MO 2.62 ± 0.31 2.14 ± 0.28NS 1.41 ± 0.12NS 2.66 ± 0.13NS 1.68 ± 0.28NS 4.47# 22.31
Values are Mean ± S.E., HgCl2 = Mercuric Chloride; LD = Low Dose; HD = High Dose; CH = Cerebral Hemisphere; C = Cerebellum and MO = Medulla
Oblongata; LPO = Lipid Peroxidation; SOD = Superoxide Dismutase; NS = Non Significant; *p < 0.05; **p < 0.01; +p < 0.001; significant analysis of variance
at #p ≤ 0.05.

Neuroprotection by Melatonin on Mercury Induced Toxicity in the Rat Brain379
Table 4. Non enzymatic and enzymatic antioxidant system in brain of experimental animals.
HgCl2
Parameters Organs Control
LD HD
Melatonin Melatonin + HD Anova Amelioration
(%)
CH 5.40 ± 0.11 3.62 ± 0.02+ 2.28 ± 0.08+ 5.54 ± 0.11NS 3.75 ± 0.05** 10.7# 47.11
C 5.54 ± 0.06 4.1 ± 0.04+ 2.52 ± 0.08+ 5.61 ± 0.08NS 4.60 ± 0.01** 16.81# 68.87
GSH
(μg/100 mg
tissue weight)
MO 4.22 ± 0.05 3.38 ± 0.07+ 1.38 ± 0.12+ 4.50 ± 0.03NS 3.76 ± 0.06** 13.89# 83.80
CH 0.071 ± 0.015 0.011 ± 0.006+0.006 ± 0.001+0.047 ± 0.002NS 0.016 ± 0.001NS 2.04# 15.38
C 0.063 ± 0.014 0.028 ± 0.002*0.015 ± 0.003** 0.050 ± 0.002NS0.022 ± 0.002* 2.75# 14.93
GPx
(GSH consumed/
min/mg protein)
MO 0.087 ± 0.004 0.053 ± 0.001+0.027 ± 0.005+0.082 ± 0.001NS0.042 ± 0.005* 9.51# 25.00
CH 54.95 ± 2.60 28.05 ± 0.24+25.49 ± 0.30+55.98 ± 2.90NS 28.89 ± 1.27** 3.31# 11.54
C 31.72 ± 0.33 19.30 ± 0.40+14.27 ± 0.41+30.28 ± 0.32NS 19.00 ± 0.39* 19.3# 27.10
GR
(moles NADPH
oxidized/min/mg
protein) MO 51.65 ± 2.42 32.80 ± 2.05+25.54 ± 0.05+51.73 ± 0.69NS 30.75 ± 0.69NS 0.07# 19.98
Values are Mean ± S.E., HgCl2 = Mercuric Chloride; LD = Low Dose; HD = High Dose; CH = Cerebral Hemisphere; C = Cerebellum and MO = Medulla
Oblongata; GSH = Glutathione-SH; GPx = Glutathione Peroxidase; GR = Glutathione Reductase; NS = Non Significant; *p < 0.05; **p < 0.01; +p < 0.001;
significant analysis of variance at #p ≤ 0.05.
0.001; p < 0.05) in pro-oxidant treated groups. Antioxi-
dant supplement to the intoxicated rat proved efficient in
ameliorating HgCl2 induced toxicity comparable to con-
trol values giving 54.2%, 64.1% and 40% amelioration.
Protein carbonyl levels were significantly (p < 0.005)
increased after the treatment. The percent amelioration
after melatonin administration was 81.8%, 43.7% and
33.7% respectively. Hydrogen peroxide (H2O2) levels
were found to be increased by MC administration. MLT
supplementation gave 67.9%, 78.4% and 67.2% amelio-
ration in CH, C and MO respectively. Total sulfhydryl
groups showed a significant decline (p < 0.01) in CH, C
and MO with mercury intoxication which, after the ad-
ministration of melatonin gave 65.2%, 20.4% and 43.9%
amelioration respectively (Table 5).
3.3. Effect of MC on Ultrastructure of Brain
The Transmission electron (TE) micrographs of rat brain
revealed that myelinated nerve (My) of control rat brain
showed internal (i) and external (e) mesa axons with mi-
tochondria (Mi) and vesicles inside it. Mitochondria out-
side the axon appear to be normal including occurrence
of occasional ribosomes (Figure 1). On the other hand,
electron micrograph of a treated rat brain indicated dis-
torted axon with discontinuous myelin sheath (*). A re-
duction in number of vesicles (Ve) and mitochondria in
the axon were noticed. The mitochondria (Mi) were also
swollen in the matrix (Fig u re 2). In the micrograph of rat
brain co-treated with melatonin, the mitochondria ap-
peared normal with dense vesicular components which
appeared to that of control rats (Figure 3). The nucleus
(N) of the control rat brain cell has continuous nuclear
membrane (NM) with diffused chromatin material inside
it (Figure 4). But the nucleus turned blebbed in the
treated rats and the nuclear membrane becomes discon-
tinuous and puffed (#) at irregular intervals with in-
creased intracellular space (S) (Figure 5). These changes
of the nucleus were not observed in the brain of mercury
treated rats co-administered with melatonin but the in-
tracellular spaces are still persistent (Figure 6).
4. Discussion
Environmental and occupational pollution leads to ac-
cumulation of heavy metals like mercury resulting into
serious health problems [51-53]. Organic and inorganic
mercury salts have been proved to be potent toxic agents
in animals including human. Chronic neurotoxicity is
reported at concentration > 35 µg/L, whereas acute toxic-
ity occurs at concentrations > 200 µg/L [14]. HgCl2 in-
gestion has been shown to reduce body and organ weights
in laboratory animals [54]. Similarly, in the present study,
a significant reduction in body and brain weights was
observed. Besides weight loss, the animal showed signs
of toxication like yellowish fur and a cachectic appear-
ance. Numerous studies indicate that mercuric ions in-
teract with glutathione (GSH) and other (–SH) groups in
the presence of hydrogen peroxide, leading to the gen-
eration of reactive oxygen species (ROS). These ROS
subsequently include lipid peroxidation measured by
thiobarbituric acid reaction for malondialdehyde in the

Neuroprotection by Melatonin on Mercury Induced Toxicity in the Rat Brain 380
Table 5. Parameters studied in brain for Oxidative stress and enzymatic antioxidant system.
HgCl2
Parameters Organs Control
LD HD
Melatonin Melatonin + HD Anova Amelioration
(%)
CH 0.0058 ± 0.0003 0.00594 ± 0.0002NS 0.00658 ± 0.0002*0.00592 ± 0.0002NS 0.00612 ± 0.0001NS 0.82# 54.28
C 0.0035 ± 0.0001 0.0044 ± 0.0001+0.00545 ± 0.0002+0.00352 ± 0.0002NS 0.0120 ± 0.0012NS 1.77# 64.10
GST (units/mg
protein)
MO 0.00256 ± 0.001 0.0027 ± 0.0003NS 0.00296 ± 0.0001*0.00260 ± 0.0001NS 0.00280 ± 0.0002NS 4.19# 40.00
CH 10.7 ± 0.80 11.7 ± 0.95NS 12.9 ± 0.97* 09.7 ± 0.50NS 11.1 ± 0.40NS 1.85# 81.80
C 09.3 ± 0.70 10.0 ± 0.29NS 10.9 ± 0.07* 09.4 ± 0.60NS 10.2 ± 0.13NS 6.02# 43.75
Protein
Carbonyl (nmol/
mg protein)
MO 10.2 ± 0.33 10.7 ± 0.30NS 11.1 ± 0.28* 10.0 ± 0.70NS 10.8 ± 0.30NS 1.05# 33.70
CH 69.6 ± 5.64 77.2 ± 5.31NS 80.2 ± 1.20NS 64.0 ± 5.02NS 73.0 ± 3.06NS 1.68# 67.90
C 55.2 ± 4.70 58.8 ± 1.74NS 75.6 ± 3.42NS 54.4 ± 3.48NS 59.6 ± 2.03NS 2.18# 78.43
H2O2
(µM of H2O2
formed/100 mg
tissue wt) MO 50.6 ± 5.29 66.2 ± 4.56NS 74.4 ± 3.61NS 48.2 ± 1.15NS 58.4 ± 4.16NS 4.27# 67.20
CH 2.10 ± 0.17 1.59 ± 0.09* 1.47 ± 0.05** 1.54 ± 0.06NS 1.06 ± 0.12** 7.12# 65.23
C 1.66 ± 0.12 1.54 ± 0.12NS 1.22 ± 0.11* 1.68 ± 0.1NS 1.31 ± 0.05NS 2.69# 20.45
Total (–SH)
(µg/100 mg
fresh tissue wt)
MO 1.97 ± 0.14 1.65 ± 0.04NS 1.42 ± 0.17* 2.01 ± 0.01NS 1.66 ± 0.077 1.75# 43.90
Values are Mean ± S.E., HgCl2 = Mercuric Chloride; LD = Low Dose; HD = High Dose; CH = Cerebral Hemisphere; C = Cerebellum and MO = Medulla
Oblongata; GST = Glutathione S-Transferase; H2O2 = Hydrogen Peroxide; Total (–SH) = Total Sulphydryl Groups. NS = Non Significant; *p < 0.05; **p < 0.01;
+p < 0.001; significant analysis of variance at #p ≤ 0.05.
1
Figure 1. Electron micrograph of control rat brain showing
normal myelinated nerve with external (e) and internal (i)
mesa axons as well as mitochondria (Mi) and vesicles (Ve)
present inside the axon (×6300).
brain. Lipid peroxidation is known to be one of the mo-
lecular mechanisms for cell injury in acute mercury poi-
soning and is associated with a decrease in cellular anti-
oxidants such as glutathione ascorbate, superoxide dis-
mutase (SOD) and catalase (CAT) [55,56]. Catalase ac-
2
Figure 2. Electron micrograph of treated rat brain showing
discontinuous myelin sheath (*) around the distorted axon
(×6300).
tivity was found to be decreased in this study. This en-
zyme is responsible for balancing the production of H2O2
and superoxide radicals. As CAT activity decreases, the
level of H2O2 is found to increase in the present data as
justified by reduced levels of this enzyme. The decreased
activity of SOD further indicated an increased superoxide

Neuroprotection by Melatonin on Mercury Induced Toxicity in the Rat Brain381
3
Figure 3. Electron micrograph of mercury and melatonin
treated rat where myelin sheath is still less continuous
(×6300).
4
Figure 4. Electron micrograph of control rat brain nucleus
(N) with nuclear membrane (NM) (×5000).
5
Figure 5. Electron micrograph of treated rat brain nucleus
with puffed nuclear membrane (#) at irregular intervals
and large intracellular spaces (S) (×5000).
6
Figure 6. Electron micrograph of mercury plus melatonin
rat brain nucleus revealing normal ultrastructural features
(×5000).
radical production and consequently higher hydroxyl
radical formation. Enhanced levels of hydroxyl radical
also initiate LPO levels to rise in the brain regions. Glu-
tathione is the most abundant low molecular weight thiol
containing compound in the living cells. Its reduced form
contributes to the stabilizing thiol groups of membrane
enzymes and by acting as a reducing agent for hydrogen
peroxide and free radicals, to protect the cells against
oxidative stress [57] and detoxification of ROS [58,59].
It also participates in the enzymatic reduction of mem-
brane hydroperoxy-phospholipids and prevents the for-
mation of secondary alkoxyl radicals when organic per-
oxides are homolyzed. In the present study, reduction in
GR and GPx followed by an increase in GST activity in
the brain regions reflected an increased free radical (ROS)
generation after mercury intoxication. Elevation in GST
activity indicates the occurrence of oxidative stress status
of the brain. Since GST is considered to be an oxidative
stress marker, it helps in detoxification of xenobiotics by
catalyzing the conjugation of electrophillic molecules
with GSH [60]. Alterations in these enzymes led finally
to the depletion of GSH levels resulting in oxidative
damage of the nerve tissue in the present study as sup-
ported by ultrastructural studies. Ascorbic acid is a pow-
erful reducing agent which helps in activating several
enzymes and is also known as an antioxidant for detoxi-
fying several toxic substances [61]. In mercury treated
rats, the concentration of ascorbate was reduced as it
helps to induce cellular GSH levels in stress condition
[37]. These results thus indicate an oxidative damage
caused by mercury treatment, the fact of which is proven
by increased malondialdehyde formation and subse-
quently compensated by utilization of ascorbic acid to
increase the GSH levels and to balance the LPO levels.

Neuroprotection by Melatonin on Mercury Induced Toxicity in the Rat Brain 382
As shown in our previous investigation [37], protein
levels were reduced by the ingestion of mercuric chloride.
Increased protein carbonyl is a marker for the oxidation
of proteins. The free metal ions bind to the cations bind-
ing locations on the protein and in the presence of H2O2
and O2, it can transform side chain amine groups on sev-
eral amino acids into carbonyls [62]. The tissue damage
is further correlated by its increased accumulation of the
toxicant in the brain regions indicating the development
of neurodegenerative disorder caused by mercury intoxi-
cation [63,64]. These effects were further reconfirmed by
the electron microscopic study of brain which showed
discontinuous myelin sheath around the axon including
swollen mitochondria, blebbed nucleus as well as nuclear
membrane changes with the treatment of mercuric chlo-
ride. Mercury treatment further depletes the mitochon-
drial enzymes, in turn, leading to severe mitochondrial
damage as documented in our earlier reports leading to
cellular metabolic insult in cells and tissue [65,66]. These
degenerative changes in the brain could again be corre-
lated with the reduction of total proteins and lipid levels
[37]. Accumulation of mercury compounds in different
parts of the central nervous system (CNS) (olfactory
bulbs, cerebral hemispheres, cerebellum, medulla oblon-
gata and spinal cord) in relation to the cytoarchitectural
changes in myelin sheath as well as in glycosidases lev-
els was reported in animals. Its accumulation also re-
sulted in degeneration and inhibition of enzymes [67] in
support of our data resulting in loss of brain functions.
Numerous in vitro and in vivo studies have demon-
strated the ability of melatonin to protect against free
radical destructions [29,37,51,68,69]. Supplementation of
melatonin to mercury intoxicated rats in this study was
thus effective in quenching oxidative stress as well as
structural changes exerted by mercury intoxication. It is
well known that melatonin and its subsequent metabo-
lites are powerful antioxidants in prevention of free radi-
cal production and quenching of these radicals by en-
hancing the defensive function during stress state to pro-
tect neural tissue structure and function as reported in
other tissue (Rao and Bhavana, 2008; Rao et al. 2010).
5. Conclusions
This study reveals that inorganic mercury alters CH
functions and structure followed by C and MO in the
brain by binding to the thiol group of proteins, in turn,
disturbing their various enzymatic and non enzymatic
components of defense system. Subsequently, it also
brought about ultrastructural changes in brain of treated
rats. Co-administration of melatonin exhibited definite
protective role against mercuric chloride induced brain
dysfunctions due to its inherent antioxidant properties
Thus, melatonin may be useful as a therapeutic agent to
human population exposed to heavy metals at work
places.
6. Acknowledgements
This work was supported by Research Grants from Gu-
jarat State Council on Science and Technology (Gujcost),
Gandhinagar, India to Prof. M. V. Rao. I would also like
to acknowledge Prof. M. N. Patel for his technical ad-
vices to statistically analyze the data.
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Abbreviations
MLT: Melatonin
HD: High Dose
LD: Low Dose
Mi: Mitochondria
My: Myelinated Nerve Fibre
NMy: Non Myelinated Nerve Fibre
N: Nucleus
NM: Nuclear Membrane
S: Intracellular Space