RTI-4793-14, a new ligand with high affinity and selectivity for the (+)- MK801-insensitive [³H]1-[1-(2-thienyl)cyclohexyl]piperidine binding site (PCP site 2) of guinea pig brain

[³H]TCP, an analog of the dissociative anesthetic phencyclidine (PCP), binds with high affinity to two sites in guinea pig brain membranes, one that is MK-801 sensitive and one that is not. The MK-801-sensitive site (PCP site 1) is associated with NMDA receptors, whereas the MK-801-insensitive site (PCP site 2) may be associated with biogenic amine transporters (BAT). Although several 'BAT ligands' are known that bind selectively to PCP site 2 and not to PCP site 1 (such as indatraline), these compounds have low affinity for site 2 (K(i) values > 1 μM). Here we demonstrate that the novel pyrrole RTI-4793-14 is a selective, high affinity ligand for PCP site 2. We determined the IC₅₀ values of RTI-4793-14 and several reference compounds [PCP, (+)-MK801 and indatraline] for PCP site 1 (assayed with [³H](+)- MK801), PCP site 2 (assayed with [³H]TCP in the presence of 500 nM (+)- MK801) and a variety of BAT-related measures ([³H]CFT binding to the DA transporter, [³H]nisoxetine binding to the norepinephrine transporter, [³H]dopamine uptake, [³H]serotonin uptake). In addition, we determined the ability of RTI-4793-14 to block NMDA responses in cultured hippocampal neurons under voltage clamp. (+)-MK801 had high affinity for PCP site 1 (4.6 nM) and potently inhibited NMDA-induced responses, but was much less potent in the BAT-related measures (IC₅₀s > 10 μM). PCP had high affinity at PCP site 1 (IC₅₀ = 92 nM) and PCP site 2 (IC₅₀ = 117 nM), and was moderately potent in all BAT-related measures except [³H]nisoxetine binding. Indatraline was potent in BAT-related measures (IC₅₀s, 2 to 5 nM), but weak in other measures (IC₅₀s > 1 μM). In contrast, RTI-4793-14 had high affinity for PCP site 2 (38 nM), low affinity for PCP site 1 (> 36 μM), moderate IC₅₀s for all BAT-related measures, and negligible activity at NMDA receptors. Viewed collectively, these data indicate that RTI-4793-14 binds with high affinity and selectivity to PCP site 2 and provide further support for an association between PCP site 2 and the BATs.