Histopathological effects of experimental exposure to lead on nervous system in albino female rats

Article history: Received October 30, 2019 Accepted December 1, 2019 Available online September 7, 2020 Lead toxicity is a common health issue. Lead (Pb) is harmful to vital organs of body particularly the nervous system. This study aimed to estimate the effects of lead on the cerebellum, cerebrum and spinal cord in rat model, focusing on histopathological changes. 24 female mature albino rats of 200-300g randomly divided into 2 groups, the first is the control, and the second group were treated with lead acetate at dose 30mg/kg B.W. for 30 days. Microscopic examination revealed degeneration and necrosis of Purkinje cells and molecular cells and decrease in the number of granular cells and molecular cells also observed. Some Purkinje cells lost axons and shrunken and some areas showed depletion of Purkinje cells. Congestion of blood vessels with perivascular cuffing of mononuclear inflammatory cells, hemorrhage, neurophagia, glial nodules were observed in the brain parenchyma. Demyelination reported in white matter, with microglial proliferation around vertebral canal of spinal cord. This study referred to the increased risk of central nervous system damage due to the exposure to lead.


Introduction
Lead is a common environmental and factory-made (heavy metal) contaminant, which cause problem of body health. It is used in many industrial applications such as lead -acid batteries, cosmetics, hair coloring dyes, printing dyes and leaded crystal ware (1,2).
Lead has a noxious effect on organs and tissues especially the brain and peripheral nerves (3). Lead poisoning is a medical condition also known as saturnism or plumbism, it is occurred via oral pathway or less widely via the respiratory pathway or skin (inorganic lead). It affects to the CNS, PNS, kidney, liver, bone marrow, bone, gastrointestinal tract, blood vessels, reproductive and endocrine systems, which appear as acute, subacute or chronic forms (4).
According to (5) exposure to lead is inevitable, and it happens through many ways including contaminated air, water and food. Lead enters the blood circulation and parenchymal tissues and finally to the bones. Lead has a direct effect on enzymes containing sulfhydryl groups, the thiol component of RBCs, antioxidant protection, and mitochondria in many cells, which is observed in the medical condition. Beside to the cerebellar hemorrhage and edema as well as capillary injury, lead is also immune suppressant, nephrotoxic, teratogenic, gametotoxic and noxious to the hematopoietic organs (6,7).
Lead is known to be one of a number of neurotoxins that interfere with and reduced the signaling of ion calcium in nerve processes, also affects many different regions of brain tissue including the cerebral cortex, cerebellum and hippocampus. The damage of blood capillary vessels in the brain cause bleeding and brain swelling. The mechanism by which the nervous signs of encephalopathy and the lesions of peripheral nerve degeneration which appears to be related to the degenerative changes and apoptosis seen in the nervous tissue (8). The work of this study aimed to inspect the sequel of exposure to lead acetate on central nervous tissues in rat model, focusing on histopathological changes.

Materials and methods
The present study was authenticated by scientific committee in the Pathology Department, College of Veterinary Medicine, University of Mosul, Iraq. 24 Female mature albino rats Rattus rattus, of 2.5 months old and body weight of 200-300g, were separated into two groups. The control group was treated only by physiological salt solution throughout the experiment. The second group was treated with lead acetate orally by intragastric gavage at a dose of 30 mg /kg B.W. for 30 days (1).
After the complete of the experiment, rats euthanized by chloroform, after that the brain and spinal cord were collected, and reserved in neutral buffered formalin at 10% concentration for the preparation of tissue sections. They were impregnated and blocked in paraffin wax, sectioned into5 micron thickness sections, then stained with hematoxylin and eosin stains (9). These sections were then examined for histopathological changes.

Cerebellum
Microscopic examination of cerebellum sections of control animals showed normal architecture, which composed of outer molecular layer, inner granular layer cells and in between the single layer of Purkinje cells. Rats treated with lead acetate showed histopathological changes include degeneration and necrosis of Purkinje cells and molecular cells, decrease in the amount of granular cells, Purkinje cells and molecular cells also observed Some Purkinje cells lose axons and shrunken and some areas showed depletion of Purkinje cells, congestion of blood vessels with hemorrhage in molecular layer. Additionally, perivascular cuffing of mononuclear inflammatory cells in the leptomeninges of cerebellum also observed ( Figure 1).

Cerebrum
The Microscopic examination of cerebrum sections showed vasogenic edema, expanded perivascular spaces, and perineuronal edema. Degeneration and necrosis of cortical neurons, with multifocal areas of encephalomalacia. Congestion of blood vessels with perivascular cuffing of mononuclear inflammatory, Additionally, hemorrhage, neurophagia, focal gliosis was observed in the brain paranchyma. Thickening of meninges with congestion of blood vessels and infiltration of inflammatory cells. Also Alzheimer type II astrocytes were observed which characterized by double nuclei which surround by clear space (Figure 2).

Spinal cord
The microscopic examination of spinal cord sections showed histological alteration characterized by degenerative changes and necrotic lesions in motor neuronal cells of gray matter, and losing their dendrites, with pyknosis of nucleus, and infiltration of microglial cells. Additionally, hemorrhage also observed in the white mater and gray matter. Demyelination showed in white matter, with microglial infiltration around vertebral canal of spinal cord (Figure 3).

Discussion
Lead toxicity is a metal poisoning triggered by the accumulation of lead in the tissues (10). It is a potent neurotoxic agent, affecting nervous system (2,3). The brain tissue is vulnerable to the effects of lead injury (11,12). Lead is capable to pass through the endothelium of the blood brain barrier because it can replace for ions of the calcium and be taken up by Ca +2 -ATPase pumps, thus interfering with synapse formation. This study was conducted to assess the histopathological effects of long used of lead acetate on the central nervous tissues (Cerebrum, cerebellum and spinal cord). Histopathological changes result from exposed to lead showed cerebrum tissue degeneration and necrosis of cortical neurons with multifocal area of encephalomalacia, as well as congestion of blood vessels with perivascular cuffing of mononuclear inflammatory cells. Additionally, hemorrhage, neurophagia and glial nodules were observed. These finding are agreement with those reported by (11,13,14) which they exposed mice, rats and guinea pigs to lead and observed Vascular lesions additionally to encephalpathic injury of lead mediated at the neuronal level.
Other researchers have established that lead has toxic effects on the brain of rats, inducing vascular damage in additions to parenchymal necrotic lesions and vacuolation in the hypothalamus (8,15).
Histopathological examination of cerebellum and spinal cord revealed vasogenic edema, thickening of meninges with degeneration and necrosis of neurons as well as congestion of blood vessels and infiltration of inflammatory cells. Moreover, observed of Alzheimer type II astrocytes. All these finding were in agreement with other studies that described loss of neurons in different layers of cerebellum cortex, as well as degenerative and necrosis of Purkinje cells, with widening of perineuronal space (2,16).
These histopathological changes in the cerebrum, cerebellum and Spinal cord could be related to oxidative stress, and the imbalance between the production of reactive oxygen species ROS and the capability of antioxidant mechanisms to deactivate them. Many of these reactive oxygen mediators lead to cellular organelles injury and reproduction of another toxic molecule.
Free radicals can trigger nuclear kappa B factor (NF-κB); a redox-sensitive transcription factor that trigger the genes of inflammation and later reproduced of multiple inflammatory chemical mediators (17,18).
Several workers estimated the possible effects of free radicals species in the inducing of lead toxicity (18)(19)(20) which cause rapid lipid peroxidation and depletion the activity of antioxidants enzymes, for example the glutathione peroxidase and superoxide dismutase of brain tissue (21,22). In addition to elevated reactive oxygen species, the levels of nitric oxide were found to be increased in brain tissue (23). The degenerative and necrotic changes recorded in the neurons cerebrum, cerebellum and Spinal cord represent ischemic changes resulting from damage of blood vessels. Moreover, lead accumulates caused damages to the mitochondria. Biosynthesis, a function of neuronal mitochondrial activity, is affected by lead, with disruptive effects on the synaptic transmission in the brain. Lead also killing cells of brain by apoptosis (4,24).

Conclusions
This study referred to the increased risk of central nervous system damage due to the exposure to lead.