Specificity of phencyclidine-like drugs and benzomorphan opiates for two high affinity phencyclidine binding sites in guinea pig brain

Recently, the presence of two high affinity binding sites for phencyclidine were described in guinea pig brain, with one site coupled to the glutamate excitatory amino acid receptor, specifically activated by N-methyl-D-aspartate (NMDA) (site 1) and the other site associated with the dopamine (DA) reuptake carrier (site 2). Phencyclidine and its analogs, as well as the benzomorphan opiates, are known to interact with binding sites for phencyclidine. In this study, the equilibrium dissociation constants (Kd) of these compounds for the two binding sites for phencyclidine were determined. Phencyclidine and 1-[1-(2-thienyl)cyclohexyl]piperidine (TCP), an analog of PCP, were essentially non-selective between the two sites and also were the two drugs of the group observed to have the highest affinity for site 2. (+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cycloheptene-5,10-imine [(+)MK801] was the most selective agent for site 1, while none of the drugs tested showed selectivity for site 2. In humans, phencyclidine produces psychotomimetic effects, while (+)MK801 has been reported to produce minimal, if any, psychotomimetic effects, at doses sufficient to reduce seizures. These clinical observations, in conjunction with the present biochemical binding data, suggest that (+)MK801 may serve as a "marker" for site 1 and that the psychotomimetic effects of phencyclidine might be mediated by site 2.