Methylmercury egg injections: part 2--pathology, neurochemistry, and behavior in the avian embryo and hatchling

Methylmercury (MeHg) is a toxic metal that has been frequently linked to neurochemical alterations, brain lesions, neurobehavioral changes, and reproductive impairments in wild and captive birds. Much less is known about the effects of MeHg on the developing avian brain and resulting effects on hatchling behavior. The objective of this work was to use air cell injection studies to investigate the effect of in ovo MeHg exposure on brain pathology and four neurochemical biomarkers (N-methyl-d-aspartate (NMDA) receptor, γ-aminobutyric acid (GABA) receptor, glutamine synthetase (GS) and glutamic acid decarboxylase (GAD)) that have previously been studied in wild birds, and on hatchling righting response, balance, and startle response. In a series of six studies, we exposed white leghorn chicken and Japanese quail embryos to methylmercury chloride (MeHgCl) (range: 0-6.4μg/g egg) via egg injection on embryonic day (ED) 0 and measured receptor levels and enzyme activity at different stages of embryonic (days 11, 14, and 19 in chicken; day 15 in quail) and hatchling (day 1 and day 7) development, and in whole brain or discrete brain regions (cerebrum, cerebellum, optic lobe). We assessed neurobehaviors on post hatch (PH) days 1 and 7. Despite accumulating relatively high levels of Hg in the brain, embryos and hatchlings did not consistently display neurochemical changes consistent with those seen in wild birds and laboratory mammals. Hatchlings also did not demonstrate behavioral alterations. Pathology did not indicate a difference in occurrence and types of lesions between control and dosed birds. These findings suggest that in ovo MeHg exposure alone may not be responsible for neurological impacts in bird. This work draws attention to factors, such as age and species, that may influence responses to MeHg in birds.