Estrogen and G protein-coupled estrogen receptor influence the response to hypoxia and respiratory neuroplasticity.

Abstract

The importance of studying of sex as a biologic variable has garnered increasing attention, even provoking the National Institutes of Health to create a policy requiring researchers to analyze their results while using sex as a variable. This direction is a vast undertaking because there is a massive gap in the literature in the inclusion of both sexes, let alone analyzing results based on sex. Research needs to fill both gaps – to include sex as a biologic variable and to correct the historical underrepresentation of women, even via the use of female animals, in biomedical research. Specific to the study of neuroplasticity, it is clear that sex hormones are integral to the facilitation of plasticity, but there are many unknowns surrounding these mechanisms. This thesis focuses on one type of neuroplasticity: phrenic long-term facilitation (pLTF). pLTF is a type of spinal respiratory plasticity that is commonly elicited with shorts bursts of acute intermittent hypoxia (AIH). It is known that removing the gonads (GDX) in animals eliminates pLTF, and that it is restored via estrogen in both males and females. In GDX-females, pLTF can be restored with estrogen supplementation, and in GDX-males testosterone must be converted into estrogen via aromatase. Further, females also only exhibit pLTF during the proestrus phase of the estrus cycle (when circulating levels of estrogen are high), and the female mechanism for estrogen’s role is nongenomic. It is also known that sex hormones are important to the response to hypoxia as a stimulus. Studies in this area have provided diverse and often conflicting data. Part of the reason for the assortment of findings is due to the wide range in the variety of stimuli, physiological variables attended to, and measurement strategies. However, it has been established that men and women have different cardiorespiratory responses to hypoxia, and those differences largely increase after women go through menopause. This thesis investigated the roles of estrogen and a single estrogen receptor to both hypoxia as a stimulus and pLTF. It was found that in females, the loss of estrogen from GDX did not impact responses to hypoxia, but that aged females had a reduced magnitude in their reaction to hypoxia. Differences in metabolism were critical to the analysis of this finding. The role of G protein-coupled estrogen receptor (GPER) in the respiratory system was investigated. A colocalization experiment demonstrated that GPER is expressed equally in male and female phrenic motor neurons (a novel finding). Using injections of a GPER antagonist versus a placebo, it was found that GPER antagonism led to a decreased magnitude in the male response to hypoxia, but not the female response. Similar to the first study, differences in metabolism were central. Lastly, injections of GPER antagonist versus a placebo did not appear to impact male or female pLTF when AIH was administered. However, females that were injected with GPER antagonist but not stimulated with AIH also demonstrated pLTF. Much remains to explore to better understand this finding, and several mechanisms are proposed to explain this result.