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

: 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× 10−9M) [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= 10−9M). 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°C (PK 11195 > RO5–4864 > diazepam > dipyridamole > 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.