Binding Studies and Photoaffinity Labeling Identify Two Classes of Phencyclidine Receptors in Rat Brain

Binding and photoaffinity labeling experiments were employed in order to differentiate 1-(1-phenylcyclohexyl)piperidine (PCP) receptor sites in rat brain. Two classes of PCP receptors were characterized and localized: one class binds [³H]-N-[1-(2-thienyl)cyclohexyl] piperidine ([³H]TCP) with high affinity (K_d = 10–15 nM) and the other binds the ligand with a relatively low affinity (K_d = 80–100 nM). The two classes of sites have different patterns of distribution. Forebrain regions are characterized by high-affinity sites (hippocampus > frontal cortex > thalamus > olfactory bulb > hypothalamus), but some parts (e.g., hippocampus, hypothalamus) contain low-affinity sites as well. In the cerebellum only low-affinity sites were detected. Binding sites for [³H]PCP and for its photolabile analogue [³H]azido-PCP showed a regional distribution similar to that of the [³H]TCP sites. The neuroleptic drug haloperidol did not block binding to either the high- or the low-affinity [³H]TCP sites, whereas Ca²⁺ inhibited binding to both. Photoaffinity labeling of the PCP receptors with [³H]AZ-PCP indicated that five specifically labeled polypeptides of these receptors (M_r 90000, 62000, 49000, 40000, and 33 000) are unevenly distributed in the rat brain. Two of the stereoselectively labeled polypeptides (M_r90000 and 33 000) appear to be associated with the high- and low-affinity [³H]TCP-binding sites; the density of the M_r90000 polypeptide in various brain regions correlates well with the localization of the high-affinity sites, whereas the density of the M_r33 000 polypeptide correlates best with the distribution of the low-affinity sites. The results are compatible with the existence of two classes of PCP receptors in the rat brain, each having a distinct polypeptide that carries the ligand recognition site and has a selective localization in the brain.