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 (Ki values > 1 μ). 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 μ). 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 μ). In contrast, RTI‐4793‐14 had high affinity for PCP site 2 (38 nM), low affinity for PCP site 1 (> 36 μ), 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.