Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic
lateral sclerosis, and multiple sclerosis are characterized by a chronic and selective
process of neuronal cell death. Although the causes of neurodegenerative diseases
remain still unknown, it is now a well-established idea that more factors, such as
genetic, endogenous, and environmental, are involved. Among environmental causes,
the accumulation of mercury, a heavy metal considered a toxic agent, was largely studied
as a probable factor involved in neurodegenerative disease course. Mercury exists
in three main forms: elemental mercury, inorganic mercury, and organic mercury (methylmercury
and ethylmercury). Sources of elemental mercury can be natural (volcanic emission)
or anthropogenic (coal-fired electric utilities, waste combustion, hazardous-waste
incinerators, and gold extraction). Moreover, mercury is still used as an antiseptic,
as a medical preservative, and as a fungicide. Dental amalgam can emit mercury vapor.
Mercury vapor, being highly volatile and lipid soluble, can cross the blood-brain
barrier and the lipid cell membranes and can be accumulated into the cells in its
inorganic forms. Also, methylmercury can pass through blood-brain and placental barriers,
causing serious damage in the central nervous system. This review describes the toxic
effects of mercury in cell cultures, in animal models, and in patients with neurodegenerative
diseases. In vitro experiments showed that mercury exposure was principally involved
in oxidative stress and apoptotic processes. Moreover, motor and cognitive impairment
and neural loss have been confirmed in various studies performed in animal models.
Finally, observational studies on patients with neurodegenerative diseases showed
discordant data about a possible mercury involvement.