Neurochemical Biomarkers to Assess Mercury's Health Impacts in Birds.

Abstract

Mercury (Hg) is a neurotoxicant that can be associated with changes in neurochemical receptors and enzymes. The hypothesis tested here is that Hg induces changes in neurochemical biomarkers involved in glutamate and γ-aminobutyric acid (GABA) neurotransmission that may warn of clinical and behavioral impacts in birds. This project examined associations between Hg and neurochemistry in birds exposed in the field, and between Hg and neurochemistry, neuropathology, and neurobehavior in birds exposed developmentally in the laboratory. It also evaluates postmortem conditions that impact biomarker stability. In field studies, herring gulls and bald eagles displayed dissimilar Hg exposures, with brain total Hg (THg) levels reaching 2.0 μg/g dry weight in gulls and 34 μg/g in eagles. Gulls showed no associations between the NMDA or cholinergic receptors and THg, while eagles showed a positive association between glutamine synthetase (GS) and THg/inorganic Hg (IHg), negative association between the NMDA receptor and THg/IHg, and negative relationship between glutamic acid decarboxylase (GAD) and IHg. In the laboratory, white leghorn chickens exposed to Hg via egg injection showed no consistent associations between biomarkers (NMDA, GABA receptors, GS, GAD) and Hg in whole brain or discrete brain regions at embryonic days 10, 14, and 19 or posthatch days 1 and 7, despite brain THg levels reaching 21.6 μg/g dry weight. Neurobehavior (righting, balance, startle) and neuropathology did not relate to Hg. Japanese quail embryos showed no neurochemical changes, but thick-billed murre embryos displayed a negative association between THg and GAD and a positive association between THg and the NMDA receptor. Zebra finches exposed via maternal deposition displayed no neurochemical changes. Postmortem stability of biomarkers under environmentally relevant conditions was evaluated in chicken embryos in the laboratory. The NMDA receptor and GAD were stable under most conditions, but the GABA receptor and GS displayed reduced binding and activity. This work demonstrates that dietary Hg relates to neurochemistry in some wild birds, but there may but. developmental exposure may not impact neurological outcomes. This work helps identify Hg-associated neurochemical changes, provides information about effects of developmental exposure, and increases knowledge of impacts across several avian species, exposures, and timepoints.