Abstract
To quantify the effects of methylmercury (MeHg) on amacrine and on ON-bipolar cells in the retina, experiments were performed in MeHg-exposed groups of adult trahiras (Hoplias malabaricus) at two dose levels (2 and 6 microg/g, ip). The retinas of test and control groups were processed by mouse anti-parvalbumin and rabbit anti-alphaprotein kinase C (alphaPKC) immunocytochemistry. Morphology and soma location in the inner nuclear layer were used to identify immunoreactive parvalbumin (PV-IR) and alphaPKC (alphaPKC-IR) in wholemount preparations. Cell density, topography and isodensity maps were estimated using confocal images. PV-IR was detected in amacrine cells in the inner nuclear layer and in displaced amacrine cells from the ganglion cell layer, and alphaPKC-IR was detected in ON-bipolar cells. The MeHg-treated group (6 microg/g) showed significant reduction of the ON-bipolar alphaPKC-IR cell density (mean density = 1306 +/- 393 cells/mm2) compared to control (1886 +/- 892 cells/mm2; P < 0.001). The mean densities found for amacrine PV-IR cells in MeHg-treated retinas were 1040 +/- 56 cells/mm2 (2 microg/g) and 845 +/- 82 cells/mm2 (6 microg/g), also lower than control (1312 +/- 31 cells/mm2; P < 0.05), differently from the data observed in displaced PV-IR amacrine cells. These results show that MeHg changed the PV-IR amacrine cell density in a dose-dependent way, and reduced the density of alphaKC-IR bipolar cells at the dose of 6 microg/g. Further studies are needed to identify the physiological impact of these findings on visual function.
Highlights
Mercury intoxicationMercury can have several chemical forms (Hgo, Hg+, Hg2+, and organic combinations such as methylmercury) with different characteristics such as volatility, water solubility and facility to cross the lipid membrane barrier
Fish differ in the amount of methylmercury they accumulate according to their foraging characteristics
Sections were obtained from the retina of the control group to identify the cell types that were stained by antibodies against PV and αprotein kinase C (αPKC)
Summary
Mercury can have several chemical forms (Hgo, Hg+, Hg2+, and organic combinations such as methylmercury) with different characteristics such as volatility, water solubility and facility to cross the lipid membrane barrier. High mercury concentrations (112.4 to 2250 mg/ kg) have been found by dos Santos et al [10] in predatory fish (dourada, pescada, surubim, and tucunaré) from regions of current intense gold-mining activities, in the Tapajós River Basin (State of Pará). Some studies have revealed scotopic and photopic visual deficits after methylmercury exposure [22,23], but there is no evidence of the effects of methylmercury on specific retinal cell types using immunocytochemistry Bipolar cells receive their input from photoreceptors at the outer plexiform layer and send their output to amacrine and ganglion cells in the inner plexiform layer. Antibodies against parvalbumin (PV) and protein kinase C (αPKC) were used as markers for amacrine and ON-bipolar cells, respectively
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