Multi-parametric MRI assessment of melatonin regulating the polarization of microglia in rats after cerebral ischemia/reperfusion injury

Objectives Multi-parametric magnetic resonance imaging (MRI) can provide comprehensive and valuable information for precise diagnosis and treatment evaluation of a number of diseases. In this study, the neuroprotective effects of melatonin (Mel) on a rat model of cerebral ischemia/reperfusion injury (CIRI) were assessed by multi-parametric MRI combined with histopathological techniques for longitudinal monitoring of the lesion microenvironment.Methods Sixty Sprague Dawley (SD) rats were randomly divided into three groups: the Sham, CIRI and CIRI + Mel groups. At multiple time points after ischemia, MRI scanning was performed on a 7.0 Tesla MRI scanner. Multi-parametric MRI includes T2-weighted imaging (T2WI), diffusion weighted imaging (DWI), and chemical exchange saturation transfer (CEST)-MRI. CEST effects were calculated by the Lorentzian difference method, 3.5 ppm indicates amide protons of mobile proteins/peptide (Amide-CEST) and 2.0 ppm indicates amine protons (Guan-CEST). Multiple histopathological techniques were used to examine the histopathological changes and explore the therapeutic effects of Mel.Results T2WI and DWI-MRI could localize the infarct foci and areas in CIRI rats, which was further validated by staining, 2, 3, 5-triphenyl tetrazolium chloride (TTC) staining, hematoxylin and eosin (H&E) staining, and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labelling (TUNEL) staining. After Mel treatment, T2WI and DWI-MRI showed smaller infarct volume, and neurons displayed improved morphology with less apoptosis rates. Notably, Amide-CEST and Guan-CEST signal was decreased as early as 2 h after CIRI (all P < 0.01), reflecting the change of pH after ischemia. After Mel treatment, both Amide-CEST and Guan-CEST signal increased in ischemic cortex and striatum compared with control group (all P < 0.05). The immunofluorescence staining and western blotting analysis suggested the expression of M2 microglia was increased after Mel treatment; While, after Mel treatment the inflammatory factor interleukin-1 beta (IL-1 beta) was decreased compared with control CIRI rats.Conclusions Multi-parametric MRI was shown to be an effective method to monitor the brain damage in a rat model of CIRI and assessing the therapeutic effects of Mel treatment. Amide-CEST and Guan-CEST were especially sensitive to the changes in brain microenvironment during the early stages after-CIRI. Furthermore, the neuroprotective effect of Mel treatment is associated with its promotion of the microglia polarized to M2 type in CIRI rats.