[H-3] 1-[2-(2-THIENYL)CYCLOHEXYL]PIPERIDINE LABELS 2 HIGH-AFFINITY BINDING-SITES IN HUMAN CORTEX - FURTHER EVIDENCE FOR PHENCYCLIDINE BINDING-SITES ASSOCIATED WITH THE BIOGENIC-AMINE REUPTAKE COMPLEX

Previous work demonstrated two high-affinity PCP binding sites in guinea pig brain labeled by [H-3]TCP (1-{1-[2-thienyl]cyclohexyl}piperidine): site 1 {N-methyl-D-aspartate [NMDA]-associated) and site 2 {dopamine-reuptake complex associated}. The present study examined brain membranes prepared from various species, including human, for the presence of site 2, defined as binding in the presence of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate ((+)-MK801) minus binding in the presence of 10-mu-M TCP (nonspecific binding). Studies were conducted in absence of sodium which was found to be inhibitory to [H-3]TCP binding. The results demonstrated detectable levels of site 2 in brain membranes of guinea pig, rabbit, pig, mouse, sheep, and human but not in the rat or chicken. Using human cortical membranes, site 2 was the predominant binding site. Detailed studies conducted with human cortical tissue showed that high-affinity dopamine {l-[2-[bis(4-fluorophenyl)-methoxy] ethyl]-4-(3-phenylpropyl)piperazine (GBR12909)}, [1,2]benzo(b)thiophenylcyclo-hexylpiperidine (BTCP), and serotonin (fluoxetine) uptake inhibitors produced a wash-resistant inhibition of [H-3]TCP binding to site 2, but not site 1. Preincubation of guinea pig brain membranes with BTCP was shown to produce an increase in the dissociation rate of [H-3]TCP from PCP site 2. Structure activity studies with various uptake inhibitors showed that GBR12909, benztropine, fluoxetine, and BTCP have higher affinity for site 2 than for site 1. (+)-MK801, ketamine, and tiletamine were very selective for site 1, whereas dexoxadrol and TCP were moderately selective for site 1. These results suggest that human cortex possesses high-affinity PCP binding sites associated with biogenic reuptake binding sites, and that guinea pig brain, but not rat brain, may be an appropriate animal model for studying PCP site 2 in human brain.