Imaging Brain Inflammation with [¹¹C]PK11195 by PET and Induction of the Peripheral-Type Benzodiazepine Receptor after Transient Focal Ischemia in Rats

Abstract

[ 11 C]PK11195 is used in positron emission tomography (PET) studies for imaging brain inflammation in vivo as it binds to the peripheral-type benzodiazepine receptor (PBR) expressed by reactive glia and macrophages. However, features of the cellular reaction required to induce a positive [ 11 C]PK11195 signal are not well characterized. We performed [ 11 C]PK11195 PET and autoradiography in rats after transient focal cerebral ischemia. We determined [ 3 H]PK11195 binding and PBR expression in brain tissue and examined the lesion with several markers. [ 11 C]PK11195 standard uptake value increased at day 4 and grew further at day 7 within the ischemic core. Accordingly, ex vivo [ 3 H]PK11195 binding increased at day 4, and increases further at day 7. The PET signal also augmented in peripheral regions, but to a lesser extent than in the core. Binding in the region surrounding infarction was supported by [ 11 C]PK11195 autoradiography at day 7 showing that the radioactive signal extended beyond the infarcted core. Enhanced binding was preceded by increases in PBR mRNA expression in the ipsilateral hemisphere, and a 18-kDa band corresponding to PBR protein was detected. Peripheraltype benzodiazepine receptor immunohistochemistry showed subsets of ameboid microglia/macrophages within the infarcted core showing a distinctive strong PBR expression from day 4. These cells were often located surrounding microhemorrhages. Reactive astrocytes forming a rim surrounding infarction at day 7 also showed some PBR immunostaining. These results show cellular heterogeneity in the level of PBR expression, supporting that PBR is not a simple marker of inflammation, and that the extent of [ 11 C]PK11195 binding depends on intrinsic features of the inflammatory cells.

Methotrexate reduces hippocampal blood vessel density and activates microglia in rats but does not elevate central cytokine release

“…Since microglia can be activated by elevated cytokine levels indicative of neuroinflammatory processes which are associated with cognitive impairment as well [3,40] animals were also scanned for the uptake of [ 11 C]PK11195. Increased binding of this marker for peripheral benzodiazepine receptors in the brain is regarded as an indication of neuroinflammation [5,29]. However, the PET study revealed no difference in tracer uptake between control animals and animals treated with MTX.…”

“…Furthermore, hippocampal sections were immunohistochemically stained with an antibody visualizing ionized calcium binding adapter molecule 1 (IBA-1) which is upregulated in activated microglia [9,17]. Next to this immunohistochemical approach a ligand for peripheral benzodiazepine receptors, [ 11 C]PK11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide) was also administered and used as a tracer for PET [5,29]. In normal brain tissue binding of PK11195 is minimal, whereas in areas with activated microglia, in vivo binding is significantly increased [29].…”

“…Next to this immunohistochemical approach a ligand for peripheral benzodiazepine receptors, [ 11 C]PK11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide) was also administered and used as a tracer for PET [5,29]. In normal brain tissue binding of PK11195 is minimal, whereas in areas with activated microglia, in vivo binding is significantly increased [29]. Inflammatory processes in the periphery and in the brain were also studied by measuring cytokine levels in plasma and homogenized hippocampal tissue.…”

Assessment of Neuroinflammation in Transferred EAE Via a Translocator Protein Ligand

“…One of the first in vivo MRI studies on tEAE in rats (Morrissey et al, 1996) showed that MRI changes were observed well before the onset of major cellular infiltration and before the onset of clinical signs that made possible to assess quantitatively the breach of the blood brain barrier (BBB) and to distinguish in vivo between two components of the early phase of the lesion -inflammatory infiltrates and vasogenic oedema. Increased binding of several TSPO ligands has been reported after brain injury, including focal (Myers et al, 1991a;Myers et al, 1991b) and global (Stephenson et al, 1995) cerebral ischemia in the rat where heterogeneous level of expression of TSPO in different cells is seen in the core of infarction as early as 4 days after ischemia (Rojas et al, 2007).…”

PET imaging in ischemic cerebrovascular disease: current status and future directions

“…Several radioligands have been developed to image the activation of microglia in experimental models and in various diseases of the CNS [56] . Early studies in ischemia models using infl ammatory cells [57,58] . In permanent ischemia induced by microspheres injection into the MCA of rats, no increase in 3 H-PK 11195-binding was found in the infarct core 7 days after the attack, but the permanent MCA ischemia caused increased tracer-binding in the normoperfused peri-infarct zone, which was co-localized with increased glucose metabolism and accumulated microglia and macrophages.…”