Functional Imaging of The Brain

National Institute Of Neurological Disorders And Stroke

Investigators

Linked publications & trials

Abstract

Progress in this project was hampered by COVID due to the fact that animal numbers and staff time was severely limited. 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 the brain. Anatomical based MRI has been very useful for distinguishing 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: 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 venules 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 laminar information can be obtained with human fMRI. The basis for the laminar information is not clear, while our work has shown that onset of fMRI has laminar information, human work has used steady state changes. These are more dominated by fMRI offset times which are slow. Therefore we are determining the laminar differences in fMRI offset times which will allow us to model the steady-state laminar changes. We are using our ability to detect single arterioles to ask about onset propagation (and offset propagation) of volume changes to determine if there is laminar information at the level of specific arterioles. 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 published 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 release vesicles in 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 thalamocortical 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 thalamocortical input was not capable of strengthening after the critical period but we have shown plasticity that mimics developmental plasticity can be reactivated. Preliminary single cell expression data indicates that there may be two populations of stellate cells, those that can reactivate plasticity in this model and those that do not reactivate plasticity. 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. A second paper has been published 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 nearly completed the development of a head fixed whisker behavior task 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 and use genetic silencing to determine the impact of the takeover of the denervated whisker cortex by the unaffected whisker cortex. Aim 4: An interesting discovery has led to us to deviate from the original goals of this aim. A few years ago we discovered that cortical precursor cells can be grown in mature, brain tissue when implanted into the CSF. These tissues project to the host brain and the host brain projects to the tissue. We are completing an exciting study that characterizes the extent of host brain innervation and the effects of age on the interactions between host and brain-like tissues growing in the CSF space. We have also obtained exciting results that show these brain-like tissues can functionally couple to the olfactory system or the motor system of the host. The relevance of this functional coupling on behavior and the mechanism for how the host sends long distance projections into these tissues will be studied over the next couple of years.