Examining the role of Chloride Homeostasis and PGE2 signaling in the Neuroendocrine stress response to inflammation

Samuel A. Mestern, The University of Western Ontario

Abstract

The brain senses inflammatory signals and drives the release of glucocorticoids (GCs) — potent immunosuppressants — via the activation of the hypothalamic-pituitary-adrenal (HPA) axis. This inflammation-induced HPA axis activation is largely mediated by prostaglandin E2 (PGE2), acting on two subtypes of the PGE2 receptor, EP1 and EP3. Recently, our group revealed EP3 signaling mechanisms that excite HPA axis regulatory neurons. This thesis sought to tease out the remaining EP1 signaling mechanisms. Considering that the excitability of HPA axis regulatory neurons is constrained by GABA_A receptor-mediated synaptic inhibition that relies on low-level intracellular Cl^-. We hypothesized that PGE2-EP1 signaling impairs GABA_A receptor-mediated inhibition by increasing intracellular Cl^- levels. We used two electrophysiological approaches (perforated patch and whole-cell recordings) and showed that PGE2 depolarizes the reversal potential of GABA_A receptor currents (E_GABA), an indicator of intracellular Cl^- elevation. The effect of PGE2 was mimicked by EP1 agonist and prevented by EP1 antagonist. The depolarizing shift was slow to develop but became significant by 20 min post PGE2. Our results indicate that PGE2-EP1 coupling induces a slow depolarizing shift in E_GABA for the excitation of PVN-CRH neurons during inflammation.