Characterization of a Novel Serotonin Receptor Subtype (5‐HTls) in Rat CNS: Interaction with a GTP Binding Protein

Three pharmacologically distinct high‐affinity [3H]serotonin ([3H]5‐HT) binding sites were identified in spinal cord synaptosomes. [3H]5‐HT competition studies using selective 5‐HT1A receptor ligands indicated that ∼25% of high‐affinity synaptosomal [3H]5‐HT binding was inhibited by 5‐HT1A‐selective compounds, an estimate consistent with [3H](±)‐8‐hydroxy‐2‐(di‐n‐propylamino)tetralin([3H]8‐OH‐DPAT) saturation experiments in which 5‐HT1A receptors were directly labeled. [3H]5‐HT competition studies using high‐affinity 5‐HT1B compounds performed in the presence of 100 nM 8‐OH‐DPAT (to block 5‐HT1A receptors) indicated that∼26% of all specific, high‐affinity [3H]5‐HT binding to spinal cord synaptosomes was to 5‐HT1B receptors. [3H]5‐HT competition studies performed in the presence of 100 nM 8‐OH‐DPAT and 10 nM RU 24969 (to block 5‐HT1A and 5‐HT1B receptors, respectively) indicated that the remaining 49% of [3H]5‐HT binding did not possess the phar‐macologic profile previous reported for 5‐HT1c, 5‐HT1D, 5‐ HT1E, 5‐HT2, or 5‐HT3 receptors. This residual 49% of [3H]5‐ HT binding to spinal cord synaptosomes observed in the presence of 100 nM 8‐OH‐DPAT and 10 nM RU 24969 (subsequently referred to as “5‐HTls”) displayed high affinity and saturability (KD= 4.7 nM) in association/dissociation and saturation experiments. Addition of 300 μM GTP or the nonhydrolyzable form of GTP, 5′‐guanylylimidodiphosphate, inhibited [3H]5‐HT binding to 5‐HT1S receptors in saturation experiments by 35 and 57%, respectively, whereas ATP was without effect. [3H]5‐HT competition studies identified several 5‐HT agonists and antagonists with a high or moderately high affinity for 5‐HT1s receptors. Although 5‐HT1s receptors were the predominant 5‐HT1 receptor subtype present in spinal cord, no significant density of 5‐HT1s receptors was found in brainstem or frontal cortex.