Defining the Mechanisms That Mediate Sexual Differentiation of the Developing Hippocampus

Abstract

Studying the processes by which male and female brains develop differently is not only a rich source for understanding the contrasting mechanisms of brain development that enable an organism to respond appropriately as an adult to intrinsic and extrinsic factors, it is also important for understanding the etiology of the numerous neurodevelopmental disorders that exhibit a sex bias in prevalence or presentation. The hippocampus is an area of the brain responsible for context-dependent memory and regulation of the stress axis, and as such is implicated in many sex-biased neurodevelopmental disorders. There are two striking sex differences in the hippocampus of neonatal rats which may fundamentally shape the circuitry of this region of the brain differently between males and females. First, roughly twice as many proliferating cells are present in the hippocampus during the first week of life in males, compared to females, and second, the timing of the developmental shift in which GABA signaling switches from depolarizing to hyperpolarizing occurs later in males. This thesis sought to determine the mechanisms that promote the sex difference in depolarizing GABA in the neonatal hippocampus of rats, and whether there is a causal relationship between depolarizing GABA and cell genesis in this context. One set of experiments tested the role of the neurotrophin BDNF. Analyses of Bdnf gene expression patterns revealed a baseline sex difference that mirrored the sex difference in cell proliferation. However, Bdnf content in response to steroid hormone signaling in the neonatal hippocampus showed subregion-specific expression patterns that did not correlate with cell proliferation, indicating cell-type specificity of BDNF function in the developing hippocampus. A second set of experiments found female-biased expression in the neonatal hippocampus of several microRNAs known to regulate cell proliferation and neurogenesis. One of these microRNAs, mir124, was tested for its potential role in regulating cell proliferation and the depolarizing response to GABA, using a combination of in vitro and in vivo approaches. Functional studies also tested the role of miR124 in regulating the expression of NKCC1, a key chloride channel involved in regulating depolarizing GABA and proliferation.