Heterogeneity of Cortical and Hippocampal 5‐HT_1A 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.

5-Hydroxytryptamine1Areceptor binding activity of bisbenzyltetrahydroisoquinoline alkaloids fromPopowia odoardi

“…These values were similar to those of Carli et al (1997) and Nénonéné et al (1996). The inhibition constant (K i ) for the competitive reference ligand (5-HT) was 3.25 (K d(H) ) and 3940 (K d(L) ) nM, and the values were similar to those of Nénonéné et al (1994). In the time-course experiment, association of [ 3 H] 8-OH-DPAT was completed in less than 90 min, clearly demonstrating that the 90-min incubation time was adequate ( Figure 1B).…”

[18F]F13640, a 5-HT1A Receptor Radiopharmaceutical Sensitive to Brain Serotonin Fluctuations

“…The development of such a radiotracer faces many challenges related to both its own kinetic properties and to the characteristics of the serotonin system (Paterson et al, 2010). For instance, the in vivo density of 5-HT 1A receptors in the high affinity (i.e., G-protein-coupled) state is considered to be low, as suggested by in vitro findings, which may explain the lack of sensitivity of antagonist radiotracers to changes in 5-HT levels: serotonin would compete with a very small portion of the total binding pool of such tracers (Nénonéné et al, 1994;Udo de Haes et al, 2005). This was also suggested by in vivo PET studies where agonists only show occupancies of 10-20% of the [ 18 F]antagonist-labeled sites (Bantick et al, 2004).…”

Radiosynthesis and Preclinical Evaluation of 18F-F13714 as a Fluorinated 5-HT1A Receptor Agonist Radioligand for PET Neuroimaging

“…Whereas 5-HT 1A antagonists bind to the total pool of receptors, 5-HT 1A agonists bind preferentially to the high-affinity state of the receptor (9)(10)(11). Therefore, differences between the magnitude of binding of antagonist and agonist radiotracers in a given central nervous system area may give an indication about the proportion of G-protein-coupled 5-HT 1A receptors, possibly providing us with a more accurate representation of the population of functional receptor (12).…”