Impact of manganese on the hippocampus metabolism in the context of MEMRI: a proton HRMAS MRS study

Abstract

Manganese enhanced MRI (MEMRI) offers many possibilities such as tract tracing of neuronal pathways and functional imaging in vivo. This technique necessitates a direct or indirect acute injection of MnCl₂ in the brain. Unfortunately, local concentrations of Mn²⁺ and its impact on metabolism after a single injection remain largely unknown. In this study, we combined in vivo MEMRI and ex vivo Proton High Resolution Magic Angle Spinning MRS (¹H HRMAS MRS) to investigate the delayed impact of Mn²⁺ on rat hippocampal metabolism. MEMRI images were acquired 24 h after MnCl₂ injection in the dentate gyrus of the rat hippocampus at two different Mn²⁺ doses: low (8 nmol; n = 16) and high (500 nmol; n = 10). The low Mn²⁺ dose had almost no impact on hippocampal metabolism while the high dose altered the amplitude of several metabolites (up to +54% for Glu and −71% for Asp). Moreover, at this high dose, the paramagnetic properties of Mn²⁺ led to a broadening of the resonances of several organic acids (lactate, glutamate, N-acetyl aspartate etc.), suggesting a chelation of Mn²⁺ and an impact on Mn²⁺ relaxivity. Metabolite amplitudes were well correlated with the Mn²⁺ concentrations measured with an MRI T₁-map (glutamate: R² = 0.8, p = 0.02; phosphoethanolamine: R² = 0.9, p = 0.0004; γ-aminobutyrate: R² = 0.7, p = 0.005 and phosphocholine: R² = 0.6, p = 0.04). To conclude, HRMAS is well suited to investigate Mn²⁺ impact on the metabolism. The low Mn²⁺ dose (8 nmol) usually used in the MEMRI experiment does not impact the hippocampal metabolism. The chelation of Mn²⁺ and its impact on relaxivity suggests an over-estimation of the Mn²⁺ concentration when measured through a T₁ map.