 PK11195 1-2-chlorophenyl-N-methyl-N-11-methylpropyl-3 isoquinolinecarboxamide is a highly specific ligand for the peripheral benzodiazepine-binding site, which is particularly abundant on cells of the mononuclear macrophage line Myers et al., 1991. In normal human brain, the peripheral-type benzodiazepine receptor ligand PK11195 exhibits low to minimal binding primarily associated with the choroid plexus, ependymal linings and glial cells. However, following neuronal damage, the cells involved in the ensuing gliosis, microglia, show a marked increase in expression of these sites Stephenson et al., 1995;Conway et al., 1998. 