Autonomic control of cardiac output in teleost fishes

Vertebrates live in many environments, and the evolution of systems for maintaining a constant internal body state has been critical for their survival. The circulatory system provides oxygen and nutrients to keep body tissues alive as cardiovascular demands change and the blood volume pumped by the heart needs adjustment. This is precisely and rapidly controlled by neurons arranged in circuits in the autonomic nervous system. Yet our knowledge of how these circuits operate in any vertebrate is incomplete. Here I propose to investigate how such control works in the hearts of bony fishes, representing the ancestral prototype for cardiac function in vertebrates. The main factor in how much blood the heart pumps is how fast it beats. Cardiac pacemaker cells set heart rate by generating the electrical signal for contraction. The pacemaker rate is under neural control, but in fishes neither the detailed location of pacemaker cells, nor the circuits controlling heart rate, is well understood. In this study I will use the zebrafish, a model vertebrate for investigating a wide range of organ function, genetics and embryological development, to study 3 aspects of the heart. 1) What is the site and the rate-setting properties of pacemaker cells and how are these cells controlled? I have developed a new preparation of the zebrafish heart that beats regularly after isolation in a dish. With this I have now located the pacemaker zone, and will establish the properties of cells there that generate the heart rhythm. I will determine which components of the nervous system target pacemaker cells, and how these components are organized to influence heart rate. The results of this part of the study will show the operating principles involved in nervous regulation of the heart. 2) How does the control of the heart arise during development? A key component in understanding the operating principles of the heart is how the nervous system and heart become integrated during development. I will study this by probing the anatomy and function of the nervous system within the hearts of zebrafish eggs and larvae as the heart matures to meet ever-increasing requirements for delivering oxygen and nutrients. 3) Will the findings in zebrafish from the above studies apply more broadly across bony fishes? In studying biological systems in any one species, it is possible that the findings might be limited to just that species, or may apply only to a small group of related animals. To test whether the results of the studies described above may be general among fishes, I will repeat these studies in the Japanese medaka, representative of a group of fishes that evolved much later than zebrafish. Thus, beyond the immediate outcome of the proposed work, my results will have a major impact on our understanding of how the autonomic nervous system acts on the vertebrate heart to control its function.