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To utilize MRI methods for the visualization of brain regions that are involved in signal processing after a specific stimulus, either an endogenous or an injected exogenous contrast agent has to be employed. Neuronal activity has been shown to correlate with changes in blood flow and oxygen consumption leading to local changes in the concentration of deoxyhemoglobin, an endogenous contrast agent. Although functional MRI (fMRI) measurements based on the changed level of deoxygenated hemoglobin (BOLD effect) are frequently employed, there are certain limitations in using this method for small rodents. Thus, the measurement of BOLD fMRI signals in rats or mice requires the immobilization and, therefore, the anesthetization of the animal. To temporally decouple the stimulation from the MRI measurement, we use manganese-based contrast agents as an alternative activation marker. These manganese compounds are known to cross the blood-brain barrier and to enter neurons via low voltage-dependent calcium channels. Therefore, the uptake is controlled by changes in blood flow as well as alterations in synaptic activity. After specific stimulation, which can take place outside the MRI scanner, areas with a high manganese concentration will show an enhanced intensity in a T1-weighted MRI. In contrast to BOLD effect, the “staining” by manganese remains for several days after stimulation. Currently, the imaging based on manganese contrast agents is used to characterize the basal neuronal activity of knockout vs. wild-type mice.
Fig. 1: MR images of a mouse brain before (left), 1h (middle) and 24h (right) after injection of manganese chloride solution. Certain areas of the brain are only distinguishable with manganese, such as the hippocampus (upper row) or the olfactory bulb (lower row). Intensity variations are caused by internal image scaling.
General comment to Figures 2 and 3:
MR images of untreated mice (upper left) and mice injected subcutaneously with manganese chloride solution (upper right). For comparison the corresponding part of the Paxinos mouse brain atlas (lower left, Academic Press 2001) and the corresponding sections stained with manganese autometalography (lower right) are shown. Manganese uptake leads to an increase in signal intensity in the T1-weigthed MR image and an increase in staining intensity in auto-metalography.
Fig. 2: Pattern of manganese uptake in brain slices at the level of the amygdaloid complex. The regional differences in signal intensities are almost identical with both methods. Manganese uptake is high in the hippocampal formation, especially in the CA1 region (arrow head) and the mossy fiber layer below. Strong uptake can also be seen in the central nucleus of the amygdaloid complex (arrow). Note that the relative signal intensity in white matter (e.g. corpus callosum) is lower in the manganese-treated mouse.
Fig. 3: Pattern of manganese uptake at the level of the interpeduncular nucleus (arrow). Both methods show a high uptake in this nucleus.