Heterogeneity of cortical and hippocampal 5-HT1A receptors: a reappraisal of homogenate binding with 8-[3H]hydroxydipropylaminotetralin.
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 low-affinity (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 microM 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.
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Département de Physiologie, Faculté de Médecine, Université de Montréal, Québec, Canada.
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D017371:8-Hydroxy-2-(di-n-propylamino)tetralin / Q000378:metabolism*; D000818:Animals; D001667:Binding, Competitive; D001679:Biogenic Amines / Q000378:metabolism*; D002065:Buspirone / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D010134:Fenclonine / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D009928:Organ Specificity; D010869:Pindolol / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011985:Receptors, Serotonin / Q000378:metabolism*; D012701:Serotonin / Q000494:pharmacology*; D012702:Serotonin Antagonists; D010133:p-Chloroamphetamine / Q000494:pharmacology
D001679:Biogenic Amines; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; D012701:Serotonin; D006165:Guanylyl Imidodiphosphate; D010133:p-Chloroamphetamine; D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D010869:Pindolol; D010134:Fenclonine; D002065:Buspirone