UNIVERSIDAD COMPLUTENSE DE MADRID FACULTAD DE CIENCIAS FÍSICAS Departamento de Física Aplicada III (Electricidad y Electrónica) TESIS DOCTORAL Biophysical mechanisms and sources of extracellular potentials in the hippocampus Mecanismos biofísicos y fuentes de los potenciales extracelulares en el hipocampo MEMORIA PARA OPTAR AL GRADO DE DOCTOR PRESENTADA POR Antonio Fernández Ruiz Directores Miguel Sancho Ruiz Sagrario Muñoz San Martín Madrid, 2016 © Antonio Fernández Ruiz, 2015 UNIVERSIDAD COMPLUTENSE DE MADRID FACULTAD DE CIENCIAS FÍSICAS DEPARTAMENTO DE FÍSICA APLICADA III Biophysical mechanisms and sources of extracellular potentials in the hippocampus Mecanismos biofísicos y fuentes de los potenciales extracelulares en el hipocampo Memoria para optar al grado de Doctor presentada por: Antonio Fernández Ruiz Directores: Miguel Sancho Ruiz Sagrario Muñoz San Martín Madrid, 2015 2 A ti que te fuiste antes de tiempo pero nos dejaste lo mejor que tenemos: tu recuerdo, tu fuerza y tu alegría. Tu hijo. 3 Contents Acknowledgements ....................................................................................................................... 5 Abstract ......................................................................................................................................... 8 Resumen ....................................................................................................................................... 10 1. Introduction ............................................................................................................................ 12 1.1. Genesis and study of the macroscopic electrical activity in the brain ............................ 12 1.2. Anatomy and physiology of the hippocampal formation ............................................... 19 1.2.1. Anatomic and functional organization of the hippocampal formation ................ 19 1.2.2. Hippocampal LFP patterns .................................................................................. 22 1.3. Goals of the Thesis ......................................................................................................... 26 2. Methods ................................................................................................................................. 28 2.1. Experimental Procedures ................................................................................................ 28 2.2. LFP source decomposition ............................................................................................. 29 2.2.1. Independent Component Analysis ...................................................................... 30 2.2.2. Current Source Density Analysis ....................................................................... 34 2.3. Time-frequency analysis of LFPs .................................................................................. 36 2.4. Single unit analysis ........................................................................................................ 40 2.5. Modeling of LFPs with Finite Elements Method .......................................................... 42 3. Results ................................................................................................................................... 46 3.1. Current sources of hippocampal LFPs ........................................................................... 46 3.1.1. Experimental investigation .................................................................................. 47 3.1.1.1. CA1 ......................................................................................................... 47 3.1.1.2. Dentate Gyrus ......................................................................................... 51 3.1.1.3. Characterization of hippocampal LFP sources ........................................ 55 3.1.2. Finite Elements Method simulations of LFPs ..................................................... 57 3.2. Biophysical factors shaping the generation of LFPs in the Dentate Gyrus ................... 63 3.2.1. Influence of synaptic domains and cell morphology ........................................... 63 4 3.2.2. Influence of Dentate Gyrus cytoarchitecture ....................................................... 69 3.2.3. Influence of spatiotemporal dynamics of synaptic inputs .................................... 71 3.3. Theta-gamma cross-frequency coupling during a memory task ..................................... 77 3.3.1. Sources of gamma oscillations in CA1 ................................................................ 77 3.3.2. Coherence segregation of layer-specific gamma sources .................................... 81 3.3.3. Theta-phase coordination of gamma oscillations in CA1 .................................... 82 3.3.4. Variation of gamma oscillations along the CA1 transversal axis ........................ 85 3.3.5. Theta-gamma cross-frequency coupling during a memory task .......................... 90 4. Discussion .............................................................................................................................. 94 4.1. Methodological challenges in the study of brain oscillations ......................................... 94 4.2. A novel approach to the study of brain oscillations ....................................................... 97 4.3. Geometric and dynamical factors shape extracellular potentials in the Dentate Gyrus . 99 4.4. Theta-gamma dynamics reveal network computations during behaviour .................... 101 5. Conclusions .......................................................................................................................... 104 Appendix A: Publications list of the author ............................................................................... 107 Appendix B: Acronyms .........................,,.................................................................................. 108 References .................................................................................................................................. 109 5 Acknowledgements Since I was an undergrad student in Sevilla starting to discover what Science was really about and specially after finishing my studies and initiating the research career, many people help me to arrive where I am now. Starting from those early times in the School of Biology in the University of Sevilla and later in the Complutense of Madrid I have to remember a few of my professors who had the patient and energy to show me their research, let me be around and answer my thousand questions: Josefina Cano, Ángel Pastor, Sara Morcuende, Esperanza Rodriguez, Carmen Rúa and many others. Even if I was convinced of trying to become a scientist I was not yet sure that I will choose the Neurosciences. I learned to enjoy many different fields of Biology: Evolution, Genetics, Zoology thanks especially to amazing readings of great scientists (SJ Gould, Jacob and Monod, Darwin and so many others) and some enthusiastic professors. Sadly not all my professors or class mates were equally enthusiastic about science and research; however with some of them who truly were I shared many interesting and stimulating talks. Among those I remember specially some: Alvarito, Gonzalo, Miguel, Curro, Alberto, David. After finishing my Biology studies my interest towards the physical sciences had grown more and more, plus I felt the necessity of complement my formation with some basic formal knowledge about Physics and Maths. I would probably never develop such interest if it were not because the inspiring readings of some of the brightest minds of the past century: Heisenberg, Schrodinger, Einstein, etc. I really enjoyed those readings as well as learning about Quantum Mechanics, Particle Physics and Logic. However, I realized that was not the field where I could work my best, so despite those temptations I finally found my place in the Neuroscience field. During my Physics Master I met great and helpful professors (Miguel Sancho, Sagrario Muñoz, Tatiana Alieva, Fernando Sols and others) and mates from the Schools of Physics and Mathematics (Pilar, Pablo, Ricardo, David, Jaime, Jesus). And I also met a very special person. Although I had some previous research experience I really started doing science in the Cajal Institute. The years I spent there constitute in general a good experience. In the laboratory 6 of Óscar I learned many things: to love the Electrophysiology, to perform experiments, to plan and carry out my own research project, etc. I was more than lucky with some of my lab mates, Azahara and Gonzalo, with whom I shared the enthusiasm of discovery and the long hours that it usually takes. The time I share with them both inside and outside the lab was the best part of all what I spent in Madrid. I was also very lucky with some of the other students of the Institute for so many good moments and conversations, specially Maria, Jorge and Edu; and some of the researchers, Liset, Gonzalo. However, during those years I also got to know the worst face of academic life and the conflicts that arise when people are more driven for other motivations than the pursue of knowledge and the enjoyment of Science both as a career and as a lifestyle. After my stage in the Cajal I went to the laboratory of Gyuri Buzsáki in the NYU. That was the best of the lucks and a step forward into the real international academic world, beyond the idiosyncratic borders of Spanish scientific community. I am very grateful to the generosity, energy and enthusiasm of Gyuri, who embraced me and showed me how far I could go once I found the proper path. I am also grateful to many people who generously shared their data or codes with me and others that help me with my experiments: Erik, Tony, Kenji, Dion, Yuta, Gabrielle and others. I have to give special thanks to Tony Berényi for sharing his great data and advices and help me to do crazy silicon probe implantations both in the NYU and after that in his laboratory in the University of Szeged. Also to my hungarian mates Misi, Ancsa and Gábor. Finally I came back to accomplish my PhD at the Complutense. I have the biggest of the gratitude to Miguel and Sagrario who despite the complicate situation I brought to them had no hesitation to embrace me and help with all the possible things; personal, scientific and administrative. Without that it would have not been possible the present thesis. Although the funding climate and support for research was far from ideal in Spain in the last years, I was lucky of having during my university and research stages the economic support of different institutions: Universidad de Sevilla, the Xlab and Georg-August Universitát of Gőttingen, Universidad Complutense de Madrid, Ministerios de Educación y de Ciencia, CSIC y JAE, European Molecular Biology Organization (EMBO), Fundación La Caixa, Sociedad Española de Neurociencia (SENC), Federation of European Neuroscience (FENS) . 7 The PhD work is many times rewarding and enjoyable, but not always. In that times I found one my greatest shelters and sources of renew energy in the books. The words on some people with very similar concerns, worries or hopes than mines often help me during this time: Borges, Hesse, Dostoyevski, Wittgenstein, Cernuda, Machado, Asimov and many many others. Por último, a llegar hasta aquí me han ayudado mucho otras personas de fuera del ámbito académico, especialmente mi familia. A Azahara le tengo que agradecer doblemente. En lo personal, por estar siempre a mi lado de una punta a otra del mundo con su constante amor, comprensión y, algo nada fácil, paciencia. Y en lo profesional por ayudarme con los experimentos y análisis sin importar que fueran vacaciones o de madrugada. A mi hermano por no perder la confianza en mí ni desesperarse a pesar de mi difícil carácter olvidadizo y ensimismado, y por su constante apoyo. A mi padre por su inagotable interés acerca de todo lo que yo hago y por enseñarme el valor del esfuerzo, la constancia y la superación personal, desde niño hasta ahora. Y a mi madre por su infinito amor, alegría y optimismo. Por enseñarme a enfrentar del mejor modo cualquier adversidad, a no perder la fe en mí mismo ni desanimarme nunca y a que el futuro está siempre lleno de oportunidades para quien quiere buscarlas. Si no fuera por ellos tampoco estaría hoy aquí. 8 Abstract In last decades the study of the Nervous System function has moved gradually from the cellular and molecular levels to a more systemic perspective. Due to the recent developments in technologies to record and manipulate large populations of cells in behaving animals, the available volume of data has rapidly increased. Although this boost in the amount of data regarding the function of neural circuits in action opens new and exciting roads to understanding information processing in the brain, it also poses new challenges to their analysis and interpretation. One variable that can capture the activity of multiple neural populations in different brain areas with high spatial and temporal resolution is the local field potential (LFP). Despite the potential usefulness of LFPs to read out the computations performed by brain circuits during behavior, its study has been hampered by the difficulties in extracting meaningful information from it. In the present thesis, we aim to study the biophysical basis of the LFP as a way to reach a deeper understanding of it in terms of its underlying physiological mechanisms. We focused on the rodent hippocampus due to the important role that it plays in many cognitive functions (such as memory, learning or spatial navigation) and the advantages of its simplified layered structure for the study of LFPs. Taking advantage of high-density silicone probe recordings in behaving rats, we were able to map the spatiotemporal distribution of LFPs along the dorsoventral and transversal axes of the hippocampus. We implemented a novel method consisting in the combination of Independent Component Analysis (ICA) and Current Source Density (CSD) analysis to separate and identify the synaptic sources of hippocampal LFPs. In addition we built a tridimensional model of the rat dorsal hippocampus where the spatiotemporal characteristics of those sources were implemented and LFPs were simulated by means of Finite Elements Method. Those simulations reproduced the common hippocampal LFP patterns and laminar characteristics, thus enabling us to reach a better understanding of the generation of LFPs in the hippocampus and verifying the accuracy of the ICA+CSD decomposition of experimental data. The simulations also allowed us to test the influence of structural and dynamical factors such as tissue geometry, synaptic arrangement and input synchrony in the shape and propagation of LFPs. 9 To prove the relevance of the knowledge gained about the biophysical basis of LFPs in the hippocampus we addressed the issue of theta and gamma dynamics in the hippocampal- entorhinal circuit during different behavioral states. We found three sources of gamma oscillations in the CA1 region with different laminar distribution, spectral characteristics and theta-phase preference. Slow gamma (30-60 Hz), elicited by the CA3 input to the CA1 pyramidal cells, peak on the descending theta phase; while mid-frequency gamma (60-120 Hz), brought about by entorhinal layer 3 input to CA1, dominate on the peak of the theta cycle. A third source of very fast oscillations (100-180 Hz) peaked on the theta through and was of local origin. Those theta-coupled gamma oscillations were present in both exploration and REM sleep albeit with different strength. We showed that during different behavioral states and stages of a memory task, CA3 and entorhinal gamma inputs could compete or cooperate to modulate CA1 output and interregional communication. The results presented in this thesis constitute an advance in our understanding and interpretation of LFPs and brain oscillations. They highlight the importance of the employment of adequate experimental and analytical methods to interrogate the activity of brain circuits and point to the LFP as a useful although complex variable in this purpose.