[3H]spiroxatrine labels a serotonin1A-like site in the rat hippocampus

[3H]Spiroxatrine was examined as a potential ligand for the labeling of 5-HT1A sites in the rat hippocampus. Analysis of the binding of [3H]spiroxatrine in the absence and presence of varying concentrations of three monoamine neurotransmitters revealed that serotonin (5-HT) had high affinity (IC50 = 20.7 nM for the [3H]spiroxatrine binding sites, consistent with the labeling of 5-HT1 sites, while dopamine and norepinephrine had very low affinity (IC50 = 57600 nM and greater than 10(-4) M respectively). Saturation studies of the binding of [3H]spiroxatrine revealed a single population of sites with a Kd = 2.21 nM. Further pharmacologic characterization with the 5-HT1A ligands 8-hydroxy-2-(di-n-propylamino)tetralin, ipsapirone, and WB4101 and the butyrophenone compounds spiperone and haloperidol gave results that were consistent with [3H]spiroxatrine labeling 5-HT1A sites. This ligand produced stable, reproducible binding with a good ratio of specific to nonspecific binding. The binding of [3H]spiroxatrine was sensitive to GTP, suggesting that this ligand may act as an agonist. This was supported by the finding that spiroxatrine inhibits forskolin-stimulated adenylate cyclase activity (a proposed 5-HT1A receptor model) in the rat hippocampus. Since [3H]spiroxatrine is structurally distinct from other currently available radioligands for the 5-HT1A site, it should provide new information about the properties of this putative serotonergic receptor.