Correlation between low/high affinity ratios for 5-HT(1A) receptors and intrinsic activity.

G protein-coupled receptors exist in G protein-coupled and -uncoupled forms that exhibit high and low affinity for agonists, respectively. Consequently, affinity differences of a compound for the high vs. the low affinity state of a receptor have been used to estimate its intrinsic activity at that receptor. We examined the affinity of a series of compounds for 5-hydroxytryptamine(1A) (5-HT(1A)) receptor sites labeled with 0.2 nM 3H-8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) (high affinity), or with 0.25 nM [3H]4-(2'-methoxy-)-phenyl-1-[2'-(N-2"-pyridyl)-p-fluorobenzamido] eth yl-piperazine ([3H]p-MPPF) in the presence of 100 microM guanylylimidodiphosphate (Gpp(NH)p) (low affinity) in rat hippocampal membranes. For a variety of 5-HT(1A) receptor ligands, the low/high affinity ratio (ranging from 110 for 5-HT to 0.12 for spiperone) was in good agreement with their reported intrinsic activity. Positive rank correlations were found between low/high affinity ratios and intrinsic activities (E(max) values) reported in the literature. The high efficacy 5-HT(1A) receptor agonists, 1[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxynaphtyl)piperaz ine (S-14506) and dihydroergotamine, however, had similar, high affinity for both G protein-coupled and -uncoupled forms of the receptor. The Hill coefficients for both compounds were markedly higher than 1.0, suggesting that positive cooperativity could be responsible for the unexpected results. The 5-HT(1A) receptor agonist activity of dihydroergotamine and S-14506, assessed by measuring the inhibition of forskolin-stimulated cAMP accumulation, was blocked completely by pertussis toxin, reinforcing the suggested involvement of an inhibitory G protein in their effects. Taken together, the results suggest that, although the low/high affinity ratio of a ligand for 5-HT(1A) receptors generally covaries with its intrinsic activity, dihydroergotamine and S-14506 may interact with 5-HT(1A) receptors in a manner different from that of other 5-HT(1A) receptor agonists. Their effects, however, appear to be G(i) protein-dependent.