Visualization of live, mammalian neurons during Kainate-infusion using magnetic resonance microscopy

Abstract

Since its first description and development in the late 20th century, diffusion magnetic resonance imaging (dMRI) has proven useful in describing the microstructural details of biological tissues. Signal generated from the protons of water molecules undergoing Brownian motion produces contrast based on the varied diffusivity of tissue types. Images employing diffusion contrast were first used to describe the diffusion characteristics of tissues, later used to describe the fiber orientations of white matter through tractography, and most recently proposed as a functional contrast method capable of delineating neuronal firing in the active brain. Thanks to the molecular origins of its signal source, diffusion contrast is inherently useful at describing features of the microenvironment; however, limitations in achievable resolution in magnetic resonance imaging (MRI) scans precluded direct visualization of tissue microstructure for decades following MRI’s inception as an imaging modality. Even after advancements in MRI hardware had permitted the visualization of mammalian cells, these specialized systems could only accommodate fixed specimens that prohibited the observation and characterization of physiological processes. The goal of the current study was to visualize cellular structure and investigate the subcellular origins of the functional diffusion contrast mechanism (DfMRI) in living, mammalian tissue explants. Using a combination of ultra-high field spectrometers, micro radio frequency (RF) coils, and an MRI-compatible superfusion device, we are able to report the first live, mammalian cells—α-motor neurons—visualized with magnetic resonance microscopy (MRM). We are also able to report changes in the apparent diffusion of the stratum oriens within the hippocampus—a layer comprised primarily of pyramidal cell axons and basal dendrites—and the spinal cord’s ventral horn following exposure to kainate.

Anesthesia-related brain microstructure modulations detected by diffusion MRI

“…When allowing for three compartments our simulations show that MD and MK can decrease with an ICS volume fraction decrease (corresponding to cell shrinkage or ECS increase). In our simulations, we assign compartmental diffusivity values in agreement with those from previous studies 27,28 and the relation between ICS/ECS diffusivity (ICS faster than ECS) agrees with microscopy work exploring compartmental diffusivities in fixed and perfused brain samples 34,35 . Our simulations are nevertheless exploratory and serve only to elucidate if a cellular volume fraction change in sleep/anesthesia can be consistent with our DKI observations.…”

Metabolic activity diffusion imaging (MADI): I. Metabolic, cytometric modeling and simulations

“…In our results, the probabilities of encountering ( ρ , V) and permeating (k io ) cell membranes seem sufficient to describe tissue water diffusion well: no empirical trans‐membrane “viscosity” disparities (different Ds) are necessary. Consistent with this, water molecules near and in the larger neuronal cell body (soma) ( ⟨ V ⟩ , ~1 pL 83 ) exhibit greater diffusivity than water molecules in surrounding, cell membrane‐rich, highly dendritic neuropil regions 84 . (Possible implications of this for brain ρ and V values are given in 8 …”

“…Consistent with this, water molecules near and in the larger neuronal cell body (soma) (⟨V⟩, $1 pL 83 ) exhibit greater diffusivity than water molecules in surrounding, cell membrane-rich, highly dendritic neuropil regions. 84 (Possible implications of this for brain ρ and V values are given in. 8 ) Membrane "concentration" and permeability 85 appear to be diffusion-limiting.…”

On the Origins of Diffusion MRI Signal Changes in Stroke

“…Moreover, if similar effects were to be applied externally, this would approximately double the volume of restricted water and change his conclusions significantly. Aside from that, MR microscopy studies of frog ova ( 6 ), Aplysia californica neurons ( 7 ), fixed animal ( 10 ), and human ( 11 ) neurons, and most recently live rat neurons ( 13 ) using MR microscopy methods, do not support this mechanism based upon membrane-bound water. If water diffusion was restricted at the cell membrane to the extent needed to affect the observed clinical signal changes seen in stroke, then DW images of these cells should include a relatively thick bright rim around the cell membranes where the restriction allegedly occurs.…”