Exploring the behavioral, neurological, and physiological effects of methylmercury in a model songbird

Abstract

Methylmercury is a widespread environmental stressor known to disrupt reproductive and neural function of organisms even with exposure at sublethal levels. Previous studies of its effects on physiology and behavior have focused mainly on organisms tied to aquatic environments and rodents as models of human exposure. Importantly, more recent studies have shown that mercury also bioaccumulates in terrestrial food webs through emerging insects. As such, the overarching goal of this dissertation was to discern the effects of sublethal levels of mercury in songbirds, an understudied group exposed to the neurotoxicant. Using the zebra finch, Taeniopygia castanotis, as a model songbird, I aimed to better understand how lifelong exposure to methylmercury impacts spatial cognition and neural processes in the hippocampus, a region of the brain related to learning and memory (chapters 2 and 3). A second aim of this dissertation was to understand how chronic exposure to methylmercury across the lifespan and only during adulthood in zebra finches affects songbird physiology, specifically female reproductive physiology and endocrinology (chapters 4 and 5). Regarding the first aim, I found that lifelong, but not developmental- or adult-only exposure, resulted in impaired spatial learning abilities and memory. Particularly, I found that while finches exposed to methylmercury their entire lives took longer to pass a spatial learning task and were more likely to return to unrewarding food locations compared to controls, they displayed neither reduced hippocampus-to-telencephalon volumes nor reduced densities of neurons in the hippocampus, a region of the brain associated with spatial cognition. Methylmercury-exposed birds, however, did surprisingly display increased expression of doublecortin, a protein expressed in immature neurons, in an area of the telencephalon that displays neurogenesis, implying migration of these neurons to the hippocampus is hindered while neural survival possibly is not. In exploring the effects of adult methylmercury exposure on female reproductive physiology, I found no significant differences in ovarian tissue morphology, DNA damage, or estrogen production. Regarding my second aim, I found that histologically, adult exposure appears to cause differences in ovarian follicle morphology, but this was not statistically supported by the data I was able to generate to this point. After 4 months of methylmercury exposure in adult female finches, I observed no increased in DNA damage in the ovaries of exposed finches, but I did observe a decrease in circulating estradiol on average compared to controls when accounting for measurement of ovarian tissue. Comparing this to lifelong exposure, I found no difference in circulating estradiol in zebra finches exposed to organic mercury their entire lives compared to controls, though in these exposed birds, concentration of plasma mercury negatively correlated with estradiol concentration. That control and exposed finches have comparable neural densities but differences in spatial learning abilities indicate neural function in the hippocampus and other areas of the brain associated with spatial cognition is impaired. Based on the increase of immuno-labeling of doublecortin in a neuro-generative area of the telencephalon, I propose this impairment is occurring in the microtubules of exposed finches. Because the treatment used here is comparable to that of studies that have seen decreased reproductive success in zebra finches, methylmercury must be causing reproductive effects by other mechanisms than those explored in this dissertation. The work presented in this dissertation highlights the need for further research on the effects of sublethal methylmercury exposure in songbirds, an important group with both conservation concern and the ability to assist in bioindication of contaminants in their environments.