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Manganese-enhanced magnetic resonance imaging (MEMRI) is a powerful tool for  in vivo  non-invasive whole-brain mapping of neuronal activity. Mn 2+  enters active neurons via voltage-gated calcium channels and increases local contrast in T 1 -weighted images. Given the property of Mn 2+  of axonal transport, this technique can also be used for tract tracing after local administration of the contrast agent. However, MEMRI is still not widely employed in basic research due to the lack of a complete description of the Mn 2+  dynamics in the brain. Here, we sought to investigate how the activity state of neurons modulates interneuronal Mn 2+  transport. To this end, we injected mice with low dose MnCl 2  2. (i.p., 20 mg/kg; repeatedly for 8 days) followed by two MEMRI scans at an interval of 1 week without further MnCl 2  injections. We assessed changes in T 1  contrast intensity before (scan 1) and after (scan 2) partial sensory deprivation (unilateral whisker trimming), while keeping the animals in a sensory enriched environment. After correcting for the general decay in Mn 2+  content, whole brain analysis revealed a single cluster with higher signal in scan 1 compared to scan 2: the left barrel cortex corresponding to the right untrimmed whiskers. In the inverse contrast (scan 2 > scan 1), a number of brain structures, including many efferents of the left barrel cortex were observed. These results suggest that continuous neuronal activity elicited by ongoing sensory stimulation accelerates Mn 2+  transport from the uptake site to its projection terminals, while the blockage of sensory-input and the resulting decrease in neuronal activity attenuates Mn 2+  transport. The description of this critical property of Mn 2+  dynamics in the brain allows a better understanding of MEMRI functional mechanisms, which will lead to more carefully designed experiments and clearer interpretation of the results.

In Vivo Visualization of Active Polysynaptic Circuits With Longitudinal Manganese-Enhanced MRI (MEMRI)