Functional Imaging of The Brain

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The overall goal of this work is to develop anatomical, functional, and molecular magnetic resonance imaging (MRI) techniques that allow non-invasive assessment of brain function and apply these tools to study plasticity and learning in the rodent brain. MRI techniques are having a broad impact on understanding brain. Anatomical based MRI has been very useful for separating gray and white matter and detecting numerous brain disorders. Functional MRI techniques enable detection of regions of the brain that are active during a task. Molecular MRI is an emerging area, whose major goal is to image a large variety of processes in tissues. The goal of this project is to translate MRI developments in all these areas to study system level changes that occur in the rodent brain during plasticity and learning. Aim 1: Progress has been slow in this aim because a fellow has gotten distracted by a new project and a second fellow has just recently joined just prior to COVID. Over the past few years, we have completed studies in the rodent brain that acquired very high temporal and spatial resolution functional MRI (fMRI) to monitor changes in hemodynamics as a surrogate marker of electrical activity during forepaw stimulation. We have demonstrated that fMRI from single venuoles can be detected with BOLD fMRI and that single arterioles from deep cortex can be effectively imaged using blood volume based MRI techniques. In related work we have demonstrated that initial BOLD response coincides with the neural input to the cortex. This has led to the idea that at high spatial resolution MRI can get laminar specific information. This past year there have been a number of studies from a number of different labs that indicate these ideas will transfer to human fMRI. We have begun studies to measure the onset distribution through the cortex of arteriole volume and to determine if we can measure the rate of back propagation of arteriole dilation from its origin through the cortical column. This is critical parameter to help interpret laminar specific fMRI. Aim 2: Over the past several years we have demonstrated that manganese (Mn) chloride enables MRI contrast that defines neural architecture, can monitor activity, can be used to trace neural connections and can be used to monitor neurodegeneration at a cytoarchitectural level. Much work using Manganese Enhanced MRI (MEMRI) has resulted in increasing our determination to understand mechanisms better. A study has been completed and submitted for publication that uses a hippocampal slice preparation to study mechanisms of Mn transport. A second study is close to completion that uses isolated pancreatic beta cells in addition to brain slices to study the synaptic mechanisms underlying the MRI properties of manganese. This work, in collaboration with Richard Leapman, has been able to accomplish very high resolution localization of Mn to synaptic vesicles in neurons and beta cells in frozen tissue helping to validate the model which had been hypothesized that Mn is released at synapses. We have begun a new project determine the cell distribution of Mn in brain and the transport systems responsible for this distribution. This will combine near cellular high resolution MRI (35-50 microns) with advanced histological tools to understand the cellular basis of MEMRI. Aim 3: Over the past few years we established a rodent model that uses peripheral denervation to study brain plasticity in response to the injury. Over the past couple of years we have shown that denervation of the infraorbital nerve leads to large increases in barrel cortex responses along the spared whisker pathway as well as large ipsilateral cortical activity consistent with our previous work in the forepaw and hindpaw. fMRI and manganese enhanced MRI predicted a strengthening of thalamo-cortical input along the spared pathway which was verified in slice electrophysiology studies in collaboration with John Isaac. Prior to this it was widely believed that the thalamo-cortical input was not capable of strengthening after the critical period but we have shown plasticity that mimics developmental plasticity can be reactivated. Two major questions are: Are more layer 4 stellate neurons firing to the same stimulation?; and, is the relative distribution of S1 output to S2 and M1 altered. We are addressing these questions with fluorescent Calcium imaging. We have published two major papers detailing cellular mechanism for takeover by the good whiskers of the denervated whiskers S1 barrel cortex via the corpus callosum input. This input can undergo LTP in the adult and the callosal inputs are strengthened on to layer 5 pyramidal neurons. This strengthening is so large that this synapse can no longer undergo LTP. We have completed a second paper that shows that this plasticity is very different depending on which area of the brain that the layer 5 neuron sends outputs. This is compelling evidence that this plasticity may have specific functional consequences. Armed with the synaptic changes occurring, we have begun to do head fixed whisker behavior tasks to address the issue of the behavioral consequences of this plasticity We will move these behavioral models into the MRI so that whole brain activity patterns can be measured. Aim 4: Progress in this aim continues to be slow due to lack of a specific person to carry out the work. We have re-established our ability to do fear condition experiments with a new behavioral set-up which was built to be consistent with the new NIMH Behavioral Core behavior enabling easy transfer of skills. We are validating that earlier Mn tracing experiments that indicated plasticity at novel synapses during fear conditioning are reproducible. If so we will take the approach as Aim 3 to determine the synaptic basis for the changes using slice electrophysiology.