Inverse agonism and constitutive activity as functional correlates of serotonin h5-HT(1B) receptor/G-protein stoichiometry.
This study evaluated the influence of receptor/G-protein (R:G) stoichiometry on constitutive activity and the efficacy of agonists, partial agonists, and inverse agonists at human (h) 5-hydroxytryphamine 1B (5-HT(1B)) receptors. Two Chinese hamster ovary cell lines were used; they expressed 8.5 versus 0.4 pmol h5-HT(1B) receptors/mg (determined by [(3)H]GR125,743 saturation analysis) and 3.0 versus 1.5 pmol receptor-activated G-proteins/mg [determined by guanosine-5'-O-(3-[(35)S]thio)-triphosphate ([(35)S]GTPgammaS) isotopic dilution], respectively. Thus, they displayed R:G ratios of approximately 3.0 (RGhigh) and approximately 0.3 (RGlow), respectively. In competition-binding experiments, the agonists, 5-HT and sumatriptan, displayed fewer high-affinity (HA)-binding sites and the partial agonists, BMS181, 101 and L775,606, displayed decreased affinity in RGhigh versus RGlow membranes. In contrast, the inverse agonists, SB224,289 and, to a lesser extent, methiothepin, showed increased affinity. In G-protein activation experiments, both basal and 5-HT-activated [(35)S]GTPgammaS binding were higher in RGhigh than in RGlow membranes. Constitutive activity (determined by inhibition of basal [(35)S]GTPgammaS binding with GTPgammaS in the absence of receptor ligands) was more pronounced in RGhigh versus RGlow membranes, as revealed by the >5-fold greater proportion of HA sites. Correspondingly, the negative efficacy of inverse agonists was strikingly augmented, inasmuch as they suppressed approximately two-thirds of HA [(35)S]GTPgammaS binding in RGhigh membranes, but only approximately one-third in RGlow membranes. Furthermore, the efficacy of partial agonists was greater at RGhigh versus RGlow membranes, as estimated by their ability to enhance [(35)S]GTPgammaS binding. In conclusion, an increase in R:G ratios at h5-HT(1B) receptors was associated with an increase in relative efficacy of partial agonists and, most notably, an increase in both constitutive G-protein activation and negative efficacy of inverse agonists.
Newman-Tancredi
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A
Department of Psychopharmacology, Institut de Recherches Servier, Croissy-sur-Seine (Paris), France. newman_tancredi@hotmail.com
Audinot
V
V
Moreira
C
C
Verrièle
L
L
Millan
M J
MJ
D000818:Animals; D001667:Binding, Competitive; D016466:CHO Cells; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D006224:Cricetinae; D004305:Dose-Response Relationship, Drug; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism*; D008024:Ligands; D044302:Receptor, Serotonin, 5-HT1B; D011985:Receptors, Serotonin / Q000378:metabolism*; D012702:Serotonin Antagonists / Q000494:pharmacology*; D013462:Sulfur Radioisotopes; D014316:Tritium
D008024:Ligands; D044302:Receptor, Serotonin, 5-HT1B; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; D013462:Sulfur Radioisotopes; D014316:Tritium; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D019204:GTP-Binding Proteins