Evaluation of the amount of the manganese entry in the neurons for activity-induced manganese-enhanced MRI

The activity-induced manganese-enhanced MRI (AIM-MRI) is one of the candidate methods for measuring the neuronal activity noninvasively in vivo. This method has the potential to measure the history of the neuronal activity, since the Mn²⁺ can enter the active neuron through voltage-dependent Ca²⁺ channels. However, it has not been clear that the relation between the neuronal activity and the accumulation of Mn²⁺ in the cell. At first, we confirmed that the longitudinal relaxation time of H⁺ (T₁) measured by means of MRI apparatus was related to the [Mn²⁺] using the pseudo intracellular phantoms with various concentration of Mn²⁺. The inverse of T₁(1/T₁) was proportional to [Mn²⁺], thus [Mn²⁺] can be measured by MRI quantitatively. Next, we investigated the relation between the neuronal activity and the accumulation of Mn²⁺ in the striatal GABAergic neurons using the method of the Mn²⁺ quenching of Fura-2 LR, a family of fura fluorescent Ca²⁺ indicators. The amount of the Ca²⁺ influx was correlated to the neuronal activity evoked by tetanic stimulations with various frequencies. In the same slice preparation, the amounts of the fluorescence quench induced by the tetanic stimulations under the condition of 50 μM MnCl₂ administration were recorded. As a result, the amount of the quench was proportional to the amount of the Ca²⁺ influx. These results suggested that a cell, which has higher activity, accumulated larger amount of Mn²⁺ in the neuron. Thus, our results supported that AIM-MRI can measure the history of the neuronal activities in the whole brain noninvasively in vivo.