Mercury is among the ten most dangerous chemicals for public health, and is a priority concern for the 128 signatory countries of the Minamata Convention. Mercury emissions to the atmosphere increased 20% between 2010 and 2015, with South America, Sub-Saharan Africa and Southeast Asia as the main contributors. Approximately 80% of the total mercury emissions in South America is from the Amazon, where the presence of the metal is ubiquitous and highly dynamic. The presence of this metal is likely increasing, with global consequences, due to events of the last two years including extensive biomass burning and deforestation, as well as mining activities and the construction of large-scale projects, such as dams. Here we present a concise profile of this mobilization, highlighting the human exposure to this metal in areas without mining history. Mercury reaches the food chain in its most toxic form, methylmercury, intoxicating human populations through the intake of contaminated fish. Amazonian populations present levels over 6 ppm of hair mercury and, according to the 175:250:5:1 ratio for methylmercury intake : mercury hair : mercury brain : mercury blood, consume 2-6 times the internationally recognized reference doses. This exposure is alarmingly higher than that of other populations worldwide. A possible biphasic behavior of the mercury-related phenomena, with consequences that may not be observed in populations with lower levels, is hypothesized, supporting the need of improving our knowledge of this type of chronic exposure. It is urgent that we address this serious public health problem in the Amazon, especially considering that human exposure may be increasing in the near future. All actions in this region carry the potential to have global repercussions.

Microglia originate from yolk sac-primitive macrophages and auto-proliferate into adulthood without replacement by bone marrow-derived circulating cells. In inflammation, stroke, aging, or infection, microglia have been shown to contribute to brain pathology in both deleterious and beneficial ways, which have been studied extensively. However, less is known about their role in the healthy adult brain. Astrocytes and oligodendrocytes are widely accepted to strongly contribute to the maintenance of brain homeostasis and to modulate neuronal function. On the other hand, contribution of microglia to cognition and behavior is only beginning to be understood. The ability to probe their function has become possible using microglial depletion assays and conditional mutants. Studies have shown that the absence of microglia results in cognitive and learning deficits in rodents during development, but this effect is less pronounced in adults. However, evidence suggests that microglia play a role in cognition and learning in adulthood and, at a cellular level, may modulate adult neurogenesis. This review presents the case for repositioning microglia as key contributors to the maintenance of homeostasis and cognitive processes in the healthy adult brain, in addition to their classical role as sentinels coordinating the neuroinflammatory response to tissue damage and disease.

Human exposure to mercury is a major public health concern, causing neurological outcomes such as motor and visual impairment and learning disabilities. Currently, human exposure in the Amazon is among the highest in the world. A recent systematic review (doi:10.1016/j.jtemb.2018.12.001), however, highlighted the lack of high-quality studies on mercury-associated neurotoxicity. There is, therefore, a need to improve research and much to still learn about how exposure correlates with disease. In this review, we discuss studies evaluating the associations between neurological disturbances and mercury body burden in Amazonian populations, to generate recommendations for future studies. A systematic search was performed during July 2020, in Pubmed/Medline, SCOPUS and SCIELO databases with the terms (mercury*) and (Amazon*). Four inclusion criteria were used: original article (1), with Amazonian populations (2), quantifying exposure (mercury levels) (3), and evaluating neurological outcomes (4). The extracted data included characteristics (as year or origin of authorship) and details of the research (as locations and type of participants or mercury levels and neurological assessments). Thirty-four studies, most concentrated within three main river basins (Tapajós, Tocantins, and Madeira) and related to environmental exposure, were found. Mercury body burden was two to ten times higher than recommended and main neurological findings were cognitive, vision, motor, somatosensory and emotional deficits. Important insights are described that support novel approaches to researching mercury exposure and intoxication, as well as prevention and intervention strategies. As a signatory country to the Minamata Convention, Brazil has the opportunity to play a central role in improving human health and leading the research on mercury intoxication.