In vivo TSPO and cannabinoid receptor type 2 availability early in post-stroke neuroinflammation in rats: a positron emission tomography study

BACKGROUND: Upregulated levels of 18-kDa translocator proteins (TSPO) and type 2 endocannabinoid receptors (CB2) are considered to reflect different aspects of microglia-related neuroinflammatory responses in the brain. Relative to the increase in the TSPO expression that occurs slightly later during neuroinflammation in a proinflammatory fashion, CB2 activation is considered to relate to the neuroprotective responses that occurs predominantly at an early stage of brain disorders. These findings, however, were deduced from studies with different animal samples under different experimental settings. Here, we aimed to examined the differences in TSPO binding and CB2 availability at an early stage of stroke in the same animal using positron emission tomography (PET). METHODS: We used a total of eight Sprague-Dawley rats that underwent photothrombotic stroke surgery. The binding levels of a TSPO tracer [(11)C](R)PK11195 and a CB2 tracer [(11)C]NE40 were measured at 24 h after the surgery in the same animal using PET in combination with immunohistochemistry for CB2 and several other markers. A morphological inspection was also performed with X-ray computed tomography for small animals. RESULTS: The levels of [(11)C]NE40 binding potential (BP(ND)) were significantly higher in the cerebral cortical region on the lesion side than those on the non-lesion side, whereas no difference was found in the levels of [(11)C](R)PK11195 BP(ND) between hemispheres. The tracer influx index (R1) data were all reduced on the lesion side irrespective of tracers. This increase in [(11)C]NE40 BP(ND) was concomitant with an elevation in CB2 expression mainly within the microglia in the peri-infarct area, as shown by immunohistochemical examinations with Iba-1, CD11b/c+, and NG2+ staining. CONCLUSIONS: The present results provide in vivo evidence of different responses of microglia occurring in the acute state of stroke. The use of the CB2 tracer [(11)C]NE40 allows us to evaluate the roles played by the neuroprotective aspect of microglia in acute neuroinflammatory processes. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12974-017-0851-4) contains supplementary material, which is available to authorized users.