Labelling of “Peripheral‐Type” Benzodiazepine Binding Sites in the Rat Brain By Using [³H]PK 11195, an Isoquinoline Carboxamide Derivative: Kinetic Studies and Autoradiographic Localization

Abstract

PK 11195 [1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide] is a new ligand for the "peripheral-type" benzodiazepine binding sites, chemically unrelated to benzodiazepines. It displaces with a very high potency (IC50 congruent to 10(-9) M) [3H]-RO5-4864 (a benzodiazepine which specifically labels the peripheral-type sites) from its binding sites. [3H]PK 11195 binds to a membrane fraction from rat brain cortex and rat olfactory bulb in a saturable and reversible manner with a very high affinity (KD = 10(-9) M). The number of maximal binding sites was ten times greater in the olfactory bulb than in the brain cortex. The order of potency of several compounds as displacers at 25 degrees C (PK 11195 greater than RO5-4864 greater than diazepam greater than dipyridamole greater than clonazepam) demonstrates that [3H]PK 11195 binds to the peripheral-type benzodiazepine binding sites. The KD value for the [3H]PK 11195 binding is not affected by temperature changes, whereas RO5-4864 and diazepam affinities decrease with increasing temperatures. Autoradiographic images of [3H]PK 11195 binding to rat brain sections show that binding sites are mainly localized in the olfactory bulb, median eminence, choroid plexus, and ependyma. This ligand could be a useful tool to elucidate the physiological and pharmacological relevance of these binding sites.

[3H]‐idazoxan binding to rabbit cerebral cortex recognises multiple imidazoline I2‐type receptors: pharmacological characterization and relationship to monoamine oxidase

“…We suggest therefore that the [3H]-idazoxan binding site which has a high affinity for clorgyline is absent from rabbit cerebral cortex. Also the human placenta, which contains only MAO type A (Weyler & Salach, 1985) has a high density of 12 sites (Diamant et al, 1992 (Benavides et al, 1983).…”

Neuroinflammation Extends Brain Tissue at Risk to Vital Peri-Infarct Tissue: A Double Tracer [11C]PK11195- and [18F]FDG-PET Study

“…Different cell types involved in the neuroinflammatory response, namely activated microglia (Stephenson et al, 1995;Ji et al, 2008) and bloodborne cells of the monocyte-macrophage lineage (Zavala and Lenfant, 1987;Stephenson et al, 1995), express peripheral benzodiazepine receptors (PBRs). This expression pattern renders them detectable by the isoquinoline carboxamide PK11195 that selectively binds to PBRs (Benavides et al, 1983). In addition, reactive--but not resting--astrocytes demarcating the ischemic lesion from vital brain tissue have recently been shown to express PBRs (Chen et al, 2004;Rojas et al, 2007).…”

Brain and whole-body imaging in nonhuman primates of [11C]PBR28, a promising PET radioligand for peripheral benzodiazepine receptors

“…PBRs are transmembrane proteins located primarily in mitochondria and may play important roles in cholesterol transport, hormone synthesis, and immunomodulation (Papadopoulos et al, 2006). PBRs have lower concentrations in healthy brain than in peripheral organs and are primarily located in glial cells, with highest densities in olfactory bulb, choroid plexus, and the ependymal lining of the ventricles (Anholt et al, 1984;Benavides et al, 1983;Cymerman et al, 1986). PBR expression is markedly increased in activated microglia and reactive astrocytes in the central nervous system (Banati, 2002(Banati, , , 2003 and may be a useful biomarker of neuroinflammation.…”