PK 11195 is a selective and specific ligand for the peripheral-type benzodiazepine binding site. Its potential for in vivo visualisation of lesioned human brain using positron emission tomography (PET) is currently being assessed. The present study examines the relationship between the temporal development of a local ischaemic lesion with its associated cell populations and the binding of [3H]PK 11195 in rat brain. Unilateral cortical infarcts were induced using the photosensitive dye Rose Bengal. At time intervals from 1 to 7 days after lesioning, the localisation of [3H]PK 11195 binding was determined using in vivo and in vitro autoradiography. Sections adjacent to those used for autoradiography were processed for immunohistochemistry using glial fibrillary acidic protein for astrocytes and ED-1 for macrophages. The results show that the binding of [3H]PK 11195 correlates in both time and spatial localisation with the appearance of macrophages around the lesion. Reactive astrocytes, although present, occupy a separate region in the tissue surrounding the lesion and lie outside the region defined by the [3H]PK 11195 binding. We conclude that the [3H]PK 11195 signal associated with this ischaemic lesion originates primarily from binding to macrophages and that [11C]PK 11195 could be used for imaging acute inflammatory response in human brain using PET.

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