This study monitors longitudinal resting-state brain activity and behavior change in a highly homogeneous white matter infarct rat model by using fcMRI (functional connectivity MRI) and rat behavior test. It aims to investigate the relationship between functional connectivity and structural connectivity and try to demonstrate measurements of fcMRI associate with behavioral impairment.

Surgery:

Twenty-three rats anesthetized with isoflurane were immobilized with a small animal stereotaxic frame. After making a small hole (2mm posterior, 3.1mm lateral to bregma) on the left side of the skull, a cannula were lowered to reach the PLIC (posterior limb of internal capsule, 7.3 mm ventral to bregma). An optical fiber was then lowered through the cannula. Rose Bengal dye was injected through the tail vein, followed by 1.5 minutes of light irradiation with a green laser to induce an ischemia lesion on the left PLIC.

Structural and functional MRI:

All the scans were performed using a 7.0 T MR Bruker scanner. Each animal was scanned for five times. Two of the five scan times which only includes T2WI images were at 4 hours and 1 day post lesion. These images were used to assess lesion size and location. The other scans were at 2 days before lesion, 4 days and 20 days post lesion. All of the three scans include rs-fMRI(resting state fMRI), DTI and T2WI sequence. During rs-fMRI scan rats were anesthetized with just one bolus of dexmedetomidine (0.05 mg/kg) and continuous delivery of 0.3% isoflurane. The rs-fMRI images were collected about 100 minutes after bolus of dexmedetomidine. Two to three rs-fMRI runs were collected until obvious head motion was observed.

Behavioral tests:

Behavioral tests including a modified neurological severity score (mNSS), forelimb use asymmetry test and a modified sticky tape removal test were used. These tests were performed prior to surgery and on days 4 and 20 after surgery.

Data Analysis:

fMRI data preprocessing included slice-timing correction, head motion correction, normalize, spatial smoothing, linear detrend and temporal filter. Rs-fMRI runs with head motion greater than 0.1mm in translation and 1 degree in rotation were discarded. Postprocessing steps were carried out in REST toolkit and SPM. For functional connectivity analysis, left S1FL (primary somatosensory cortex, forelimb region) area was chosen as the seed region. The ROI was defined by a fine segmented atlas. FWE and FDR correction were used as multiple comparison correction in one sample t-test and two sample t-test respectively. P<0.05 was considered significant in the group analysis.

High intensity in the PLIC area of the T2 images were showed in 18 of the 23 rats. The lesion density map shows the lesion mainly affect PLIC area and extend to part of caudate putamen (CPu), globus pallidus, thalamus and amygdala.

Compared to baseline, functional connectivity analysis showed the decreased inter-hemispheric connectivity between ipsilesional S1FL and contralesional primary somatosensory cortex(S1), secondary somatosensory cortex (S2), primary motor cortex, insular cortex on day 4 after lesion. Increased intra-hemispheric connectivity was observed between ipsilesional S1FL and S1, S2, CPu, thalamus, retrosplenial dysgranular cortex, visual cortex, auditory cortex, lateral geniculate, hippocampus on day 4 after lesion. Meanwhile, Increased inter-hemispheric connectivity was observed between ipsilesional S1FL and primary motor cortex, CPu, thalamus.

On post lesion day 20, only increased intra-hemispheric connectivity between ipsilesional S1FL and S1, retrosplenial dysgranular cortex, visual cortex, auditory cortex ,hippocampus, lateral geniculate was observed compared to baseline.

In this study, twenty-three rats underwent photothrombotic stroke lesioning in the PLIC with minimal affect to the nearby area and leave the sensorimotor areas intact. We found decreased inter-hemispheric connectivity between ipsilesional S1FL and contralesional hemisphere and increased intra-hemispheric connectivity in ipsilesional hemisphere. Since sensorimotor areas and other white matter were not affected by the lesion. This suggest that the input from peripheral nerve system and subcortical nuclei underlines the homologous inter-hemispheric functional connectivity.