NCS-MPP YRIDYL)-P-ISOTHIOCYANOBENZAMIDO]-ETHYL-PIPERAZINE) - A HIGH-AFFINITY AND IRREVERSIBLE 5-HT1A RECEPTOR-LIGAND

A novel irreversible 5-HT1A receptor binding ligand, NCS-MPP (4-(2'-me thoxy-phenyl)-1 [2'-(N-2 ridyl)-p-isothiocyanobenzamido]-ethyl-piperaz ine), based on the new 5-HT1A receptor antagonist p-MPPI (4-(2'-methox y-phenyl)-1-[2'-(N-2 ''-pyridyl)-p-iodobenzamido]- was synthesized, an d its binding characteristics were evaluated using in vitro homogenate binding with rat hippocampal membranes. The K-i value of NCS-MPP was estimated to be 1.8 +/- 0.2 nM using analysis of concentration-depende nt inhibition for the binding of [I-125]p-MPPI to 5-HT1A receptors. No vaScreen of NCS-MPP showed low to moderate binding affinities to alpha -1, alpha-2-adrenergic and 5-HT2 receptors, with K-i values of 350, 42 0 and 103 nM, respectively. These data strongly suggest that the ligan d bound to 5-HT1A receptors b with high affinity and high selectivity. Irreversible inhibition of [I-125]p-MPPI binding by NCS-MPP following a 5 min incubation at room temperature was concentration dependent; t he inhibition increased to 50% at a concentration less than 10 nM, and became more pronounced (similar to 90%) at 400 nM. Under similar assa y conditions, NCS-MPP was significantly less efficient in irreversibly inhibiting agonist ligand [I-125]8-OH-PIPAT binding to 5-HT1A recepto rs at lower concentrations (<10 nM). After pretreatment of membranes w ith a low concentration of NCS-MPP (2 nM), there was an apparent loss of [I-125]p-MPPI binding sites, as expected, but no change in the bind ing affinity (K-d) was observed. However, the significant increase in K-d at a higher concentration of NCS-MPP (50 nM) indicated that there may be a secondary alkylation site, which may not be directly involved in p-MPPI binding to receptors; nevertheless, it would lead to an inc reased K-d value. The availability of an irreversible ligand, NCS-MPP, may provide a useful tool for studies of 5-HT1A receptors in the cent ral nervous system.