Heterogeneity of Cortical and Hippocampal 5‐HT1A Receptors: A Reappraisal of Homogenate Binding with 8‐[3H]Hydroxydipropylaminotetralin

Abstract: The selective serotonin (5‐HT) agonist 8‐hydroxydipropylaminotetralin (8‐OH‐DPAT) has been extensively used to characterize the physiological, biochemical, and behavioral features of the 5‐HT1A receptor. A further characterization of this receptor subtype was conducted with membrane preparations from rat cerebral cortex and hippocampus. The saturation binding isotherms of [3H]8‐ OH‐DPAT (free ligand from 200 pM to 160 nM) revealed high‐affinity 5‐HT1A receptors (KH= 0.7–0.8 nM) and lowaffinity (KL= 22–36 nM) binding sites. The kinetics of [3H]8‐OH‐DPAT binding were examined at two ligand concentrations, i.e., 1 and 10 nM, and in each case revealed two dissociation rate constants supporting the existence of high‐ and low‐affinity binding sites. When the high‐affinity sites were labeled with a 1 nM concentration of [3H]8‐ OH‐DPAT, the competition curves of agonist and antagonist drugs were best fit to a two‐site model, indicating the presence of two different 5‐HT1A binding sites or, alternatively, two affinity states, tentatively designated as 5‐HT1AHIGH and 5‐HT1ALOW. However, the low correlation between the affinities of various drugs for these sites indicates the existence of different and independent binding sites. To determine whether 5‐HT1A sites are modulated by 5′‐guanylylimidodiphosphate, inhibition experiments with 5‐HT were performed in the presence or in the absence of 100 μM 5′‐guanylylimidodiphosphate. The binding of 1 nM [3H]8‐OH‐DPAT to the 5‐HT1AHIGH site was dramatically (80%) reduced by 5′‐guanylylimidodiphosphate; in contrast, the low‐affinity site, or 5‐HT1ALOW, was seemingly insensitive to the guanine nucleotide. The findings suggest that the high‐affinity 5‐HT1AHIGH site corresponds to the classic 5‐HT1A receptor, whereas the novel 5‐HT1ALOW binding site, labeled by 1 nM [3H]8‐OH‐DPAT and having a micromolar affinity for 5‐HT, may not belong to the G protein family of receptors. To further investigate the relationship of 5‐HT1A sites and the 5‐HT innervation, rats were treated with p‐chlorophenylalanine or with the neurotoxin p‐chloroamphetamine. The inhibition of 5‐HT synthesis by p‐chlorophenylalanine did not alter either of the two 5‐HT1A sites, but deafferentation by p‐chloroamphetamine caused a loss of the low‐affinity [3H]8‐OH‐ DPAT binding sites, indicating‐that these novel binding sites may be located presynaptically on 5‐HT fibers and/or nerve terminals.