Small-animal PET imaging of the type 1 and type 2 cannabinoid receptors in a photothrombotic stroke model

Purpose

Recent ex vivo and pharmacological evidence suggests involvement of the endocannabinoid system in the pathophysiology of stroke, but conflicting roles for type 1 and 2 cannabinoid receptors (CB₁ and CB₂) have been suggested. The purpose of this study was to evaluate CB₁ and CB₂ receptor binding over time in vivo in a rat photothrombotic stroke model using PET.

Methods

CB₁ and CB₂ microPET imaging was performed at regular time-points up to 2?weeks after stroke using [¹⁸F]MK-9470 and [¹¹C]NE40. Stroke size was measured using MRI at 9.4?T. Ex vivo validation was performed via immunostaining for CB₁ and CB₂. Immunofluorescent double stainings were also performed with markers for astrocytes (GFAP) and macrophages/microglia (CD68).

Results

[¹⁸F]MK-9470 PET showed a strong increase in CB₁ binding 24?h and 72?h after stroke in the cortex surrounding the lesion, extending to the insular cortex 24?h after surgery. These alterations were consistently confirmed by CB₁ immunohistochemical staining. [¹¹C]NE40 did not show any significant differences between stroke and sham-operated animals, although staining for CB₂ revealed minor immunoreactivity at 1 and 2?weeks after stroke in this model. Both CB ₁ ⁺ and CB ₂ ⁺ cells showed minor immunoreactivity for CD68.

Conclusion

Time-dependent and regionally strongly increased CB₁, but not CB₂, binding are early consequences of photothrombotic stroke. Pharmacological interventions should primarily aim at CB₁ signalling as the role of CB₂ seems minor in the acute and subacute phases of stroke.