Abstract
Early development of the sympathetic nervous system is critical for proper function of both sympathetic nerve fibers and their peripheral target organs, including the heart. This development process relies on a series of coordinated events that establish the fundamental characteristics of both sympathetic nerves and target cardiac tissue, such that dysregulation in this early developmental period can lead to disease in adulthood. Sympathetic neurons and cardiomyocytes express and respond to factors that influence their growth and co-maturation, and understanding these interactions can provide insight that can be used to develop therapies for pathologies with developmental origins. One way in which we can investigate the function and dysfunction of the sympathetic-cardiac circuit is by studying the development of the Spontaneously Hypertensive Rat (SHR), a widely used animal model for primary hypertension. Studies of the SHR have revealed increased sympathetic drive four weeks before the establishment of hypertension, along with hyperinnervation of several target tissues into adulthood. Loss of sympathetic innervation in young animals attenuates hypertension and leads to changes in cardiac tissue morphology. While this highlights both the developmental influence the sympathetic nervous system has in this model of hypertension and importance of proper sympathetic innervation in cardiac function, the cellular and molecular basis of aberrant sympathetic growth in this system is not well understood. In this thesis I examine early postnatal sympathetic innervation in the SHR and their normotensive counterpart the Wistar Kyoto rat (WKY), characterizing altered growth of these fibers in cardiac tissue. I define a new early timepoint, at two days of age, by which cardiac innervation is increased in vivo, and I use in vitro co-culture of dissociated sympathetic neurons with target cardiomyocytes and ganglion-derived satellite glia to show that sympathetic innervation is predominantly regulated by target contact. I identify that these differential growth effects in the SHR are dependent on intrinsic properties of both neurons and cardiomyocytes, such that mixing cells from SHR and WKY strains abolishes the observed differences. I go on to identify a role for cardiomyocyte cell surface factor and membrane-bound ligand ephrinB1 in the maturation of sympathetic innervation on cardiomyocytes. I demonstrate that ephrinB1 influences local arborization of sympathetic fibers as well as the hypertrophic growth of cardiomyocytes, but that it does not act alone in driving fiber growth. Together, the work presented here demonstrates that early sympathetic innervation is regulated by target contact with developing cardiomyocytes, that reciprocal signaling plays a major part in the proper development of the sympathetic-cardiac circuit, and that these characteristics are altered in the SHR compared to the WKY.