Stephen R. Zukin

Specific Binding of [3H] phencyclidine in rat central nervous tissue: further characterization and technical considerations

The interaction of phencyclidine (PCP) with its specific receptor sites in the central nervous system has been further characterized. Kinetic association and dissociation rate constants of 2.9 X 10(6) M-1 and 4.8 X 10(-1) min-1 were determined, yielding a kinetic KD of 1.6 X 10(-7) M, in agreement with the KD previously determined at equilibrium. Permissible separation time of 13 s was calculated from the kinetic data, well above the actual separation time of less than 10 s in the rapid filtration assay. Presoaking of filters in 0.01% poly-L-lysine eliminated displacable [3H]PCP adsorption to filter material. Binding data obtained via centrifugation assays was identical to that obtained with the rapid filtration method. Stereospecificity of the PCP receptor was demonstrated by the finding that (+)-ketamine is four-fold more potent than (-)-ketamine in displacing specifically bound [3H]PCP. Several proteolytic enzymes including trypsin, papain and thermolysin potently inactivated PCP receptors. Detailed regional distribution studies showed highest density of PCP receptors in subicular cortex and hippocampus, intermediate levels in hypothalamus, striatum, frontal cortex and cerebellum, lower levels in brainstem and spinal cord, and negligible levels in corpus callosum, a white-matter control area. Benzomorphan opiates with PCP-like behavioral effects interact with the PCP receptor. These data support the pharmacological relevance of the PCP receptor site as demonstrated by the rapid filtration method.

An endogenous ligand of the brain σ/PCP receptor antagonizes NMDA-induced neurotransmitter release

The present study provides evidence for the presence of an endogenous ligand for the phencyclidine (PCP) receptor of mammalian brain. Partially purified bovine hippocampal extracts potently and dose dependently inhibit binding to PCP receptors of [3H]N-(1-[2-thienyl]-cyclohexyl)piperidine (TCP), a highly potent and specific ligand of PCP receptors. In addition to demonstrating PCP-like binding properties, the partially purified extract mimics biological actions of PCP upon neurotransmitter release. HPLC fractions active in the [3]TCP binding assay, by contrast to fractions inactive in the binding assay, potently elicited stimulation of spontaneous acetylcholine and dopamine efflux and inhibited NMDA-stimulated release of acetylcholine and dopamine. The transmitter release assay provides validation of a PCP-like physiological activity exerted by bovine hippocampal extracts partially purified by HPLC.