Objectives Gadolinium (Gd) affects microglial polarization during remyelination. We previously reported that the suppression of proinflammatory microglia was neuroprotective in intracerebral haemorrhage (ICH). The objective of the present study was to investigate the effects of Gd on microglial polarization and neuronal injury after ICH. Methods Gadolinium was intraperitoneally administered to ICH mice prepared by an intrastriatal microinjection of collagenase type VII. The polarization of M1, 2a, b and c microglia was evaluated by real‐time PCR using the respective markers. Changes in representative mRNAs were also confirmed by immunological methods. Neuroprotective effects were evaluated by counting NeuN‐positive cells and a behavioural analysis. Key findings One day after ICH, the mRNA levels of proinflammatory M1 microglial markers, such as inducible nitric oxide synthase (iNOS), and anti‐inflammatory M2 microglial markers, such as arginase1 (M2a, c), Ym1 (M2a), and transforming growth factor‐β (M2c), increased, while those of chemokine CCL1 (M2b) only increased after 3 days. Gd decreased the levels of all M1 and M2 markers. Arginase1 and iNOS protein levels also increased, and Gd reduced them due to apoptotic cell death. Gadolinium attenuated oedema, neuron loss, neurological deficits and the mortality rate without affecting haematoma sizes. Conclusions Gadolinium induced M1 and M2 microglial apoptosis and exerted acute neuroprotective effects after ICH.

We investigated the effect of gadolinium trichloride (Gd) on microglial polarization and neuronal injury after intracerebral hemorrhage (ICH). An in vivo mouse ICH model was prepared by intrastriatal microinjection of collagenase type VII. One day after ICH, the mRNA level of proinflammatory M1 microglial markers, such as inducible nitric oxide synthase (iNOS), increased. Anti-inflammatory M2 microglial markers arginase1 (M2a, c), Ym1 (M2a), and transforming growth factor-beta (M2c) increased 1 day after ICH, and chemokine CCL1 (M2b) increased after 3 days. Gd administration decreased these M1 and M2 markers. Arginase1 and iNOS protein levels also increased 3 days after ICH, and Gd decreased them because of the decrease of cell number due to apoptosis. Next, we investigated whether Gd had an anti-inflammatory effect in an ICH model. Three days after ICH, brain edema was formed, and the number of NeuN-positive cells, which indicates neuronal nuclei, decreased in the peripheral region inside the hematoma. Gd improved the edema, neuron loss, and behavioral abnormality, without affecting the hematoma size. Furthermore, Gd improved the mortality rate by ICH. Overall, Gd evoked M1 and M2 microglial apoptosis and had an acute protective effect after ICH.

Kynurenine 3-monooxygenase (KMO) is an enzyme situated at the junction to kynurenic acid (KYNA), an antagonist of the N-methyl-D-aspartate (NMDA) receptor, or to quinolinic acid (QUIN), an agonist of it, in the kynurenine pathway. To reveal the role(s) of KMO after intracerebral hemorrhage, the effect of thrombin, a serine protease, on KMO expression was investigated using primary-cultured microglia. Thrombin increased the KMO mRNA level and the effect reached a peak at 6 h. The increased KMO mRNA was suppressed by the p38 MAPK inhibitor but not the ERK and JNK inhibitors. The KMO protein level was also increased by thrombin in a concentration-dependent manner and the increased protein tended to be suppressed by the p38 MAPK inhibitor. The ratio of QUIN/KYNA, indicating the intensity of NMDA stimulation, was increased in the conditioned medium and the increased QUIN/KYNA ratio was suppressed by Ro61-8048, a KMO inhibitor, in a concentration-dependent manner. The increase of QUIN/KYNA ratio by KMO in microglia may be involved in hemorrhagic brain injury.

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