Transport Processes and Nervous System Function

Eunice Kennedy Shriver National Institute Of Child Health & Human Development

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Abstract

We have been investigating several different biophysical mechanisms associated with neuronal excitation that may be possible to measure and map using MRI. Having successfully constructed and tested an experimental system to interrogate organotypic cultured brain cortical slices using diffusion MRI, we showed promising preliminary results, relating changes in the measured apparent diffusion coefficient (ADC) map associated with various environmental changes and challenges to which these cultured tissues were subjected. These included oxygen and glucose deprivation, osmotic pressure challenges, temperature changes, etc. One hypothesis that emerged from these studies was that active water transport processes occurring at many different length scales (cell streaming, water flow across organelle and cell membranes, etc.) could be the basis of a new biophysically based fMRI method. This insight prompted the development of a theory to explain how microscopic fluid flows affect the measured diffusion weighted MRI (DWI) signal and possibly the ADC measured in tissues (i.e., pseudo-diffusion). We also developed an experimental test system, a modified RheoNMR instrument, in which well-characterized flow field distributions can be generated that result in a known and predictable amounts of pseudo-diffusion. The importance of these combined theoretical and experimental studies is that if such microscopic motions, like streaming, water flow across membranes, etc., manifest themselves as additional signal loss in DWIs, then we could use this information to infer distinct aspects of cell function and vitality, including features of excitability by judiciously analyzing MRI data. This idea represents a significant advance over the prior Intravoxel Incoherent Motion (IVIM) concept proposed by Le Bihan et al, which only considers the effect of random water motions caused by microcirculatory blood flow as contributing to observed pseudo-diffusion in vivo. We have continued to expand and amplify these studies. Former Visiting Fellow, and now Professor Ruiliang Bai, investigated possible relationships between neuronal excitation and different MRI contrasts, such as diffusion, and transverse and longitudinal relaxation. Among other things, Dr. Bai showed that diffusion imaging was sensitive to changes caused by stroke and epilepsy-like biological perturbations, while non-pathological changes in neuronal firing associated with normal activity could not be detected by diffusion fMRI. We continue to explore this exciting new area of research, probing ever smaller length and time scales and length scales Another area of interest has been in improving our measurement of relaxation/diffusion/exchange processes in living tissue, particularly taking advantage of advanced data compression techniques (such as compressed sensing) to obtain 1D and 2D relaxation spectra suitable for in vitro and in vivo studies. We have actively been developing advanced methods to migrate 2D relaxation spectroscopic imaging into viable pre-clinical and clinical methods which may be valuable in assessing water transport in different compartments within neurons or axons. We have also been involved in complementary studies to understand how induced electric and magnetic fields are distributed within the brain and how they could selectively affect different neuronal populations. We have performed detailed calculations using the finite element method (FEM) to predict the electric field and current density distributions induced in the brain during Transcranial Magnetic Stimulation (TMS). Previously, we found that both tissue heterogeneity and anisotropy of the electrical conductivity (i.e., the electrical conductivity tensor field) distort these induced fields, and even create excitatory or inhibitory hot spots in some brain regions that were previously not predicted. More recently, we developed realistic FEM models of cortical folds, containing gyri and sulci, showing that this more complicated cortical anatomy can also significantly affect the induced electric field distribution within the tissue, and the location and types of nerve cells that could be excited or depressed by such stimuli. More recently, we have been developing full 3D models of electric field deposition within the brain, obtained from 3D diffusion tensor MRI data. We are continuing to marry our macroscopic FEM models of TMS with microscopic models of neuronal excitability in the CNS in order to predict the locus of excitation in TMS and even the populations of neurons that are excited or depressed. This knowledge is important to have in addressing, for instance, the safety and basis of efficacy of TMS for the treatment of clinical depression--an application we helped pioneer in the early '90s with our colleagues Mark George (NIMH) and Eric Wassermann (NINDS). Despite its growing use and subsequent FDA approval for treating persistent clinical depression and migraines, it is still not known what action induced electromagnetic fields have in the brain in therapeutic TMS, and specifically which and what populations of neurons or axons TMS might trigger or depress when applied. Our research attempts to provide a biophysical basis for understanding the physiology of this and other clinical applications of TMS to help in part assess its safety and efficacy. More recent studies of ours have focused on the microscopic effects of these electric and magnetic fields on cells in the nervous system, moving from the macro to the microscale in our modeling activities. Moreover, we have not limited ourselves to TMS. Recently, we have also been applying these advanced FEM models to explain the physical basis for Direct Current Excitation (DCE) as well as other therapeutic uses of AC electric fields at different frequencies on the brain. A surprising offshoot of this TMS project has been the recent study of the possible anti-mitotic effect of applied electric fields and their therapeutic use in treating brain cancers, particularly Glioblastoma Multiforme (GBM). The electric fields used in this application are in the 100-300 kHz frequency range and have an amplitude of approximately 1 V/cm or greater. According to our calculations, these fields will not cause neural stimulation, but enter cells and may interfere with mitotic spindle formation, required for cell division, or interfere with cell membrane pinching, which occurs just before two daughter cells are formed from one parent cell. We proposed that an efficient alternative means to deliver electric fields to brain regions is by electromagnetic induction rather than using electrodes placed on the skin. This idea resulted in a patent application for devices that could be used to assess the effect of electric fields on tissue as well as therapeutic devices for treating various brain cancers. Although in a preliminary stage of development, our group continues to work on advancing this technology by developing technology that can deliver such induced fields to in vitro cell and tissue cultures. We believe that in addition to its possibly clinical applications, it may provide us with a means to perturb normally developing cells to help better understand different biophysical forces and flows at work during different phases of the cell cycle.