5-HT(1A) receptor agonist-antagonist binding affinity difference as a measure of intrinsic activity in recombinant and native tissue systems

A novel ergot alkaloid as a 5-HT(1A) inhibitor produced by Dicyma sp

In the course of a search for small-molecule inhibitors of 5-hydroxytryptamine receptors we have identified a novel ergoline derivative (1) from the fungal culture of Dicyma sp. This compound has a pK(i) of 10.2 versus the 5-hydroxytryptamine(1A) receptor subtype. The structure was elucidated by extensive NMR spectroscopy and mass spectrometry.

Analysis of molecular determinants of affinity and relative efficacy of a series of R- and S-2-(dipropylamino)tetralins at the 5-HT1A serotonin receptor

1. Factors influencing agonist affinity and relative efficacy have been studied for the 5-HT(1A) serotonin receptor using membranes of CHO cells expressing the human form of the receptor and a series of R-and S-2-(dipropylamino)tetralins (nonhydroxylated and monohydroxylated (5-OH, 6-OH, 7-OH, 8-OH) species). 2. Ligand binding studies were used to determine dissociation constants for agonist binding to the 5-HT(1A) receptor: (a) K(i) values for agonists were determined in competition versus the binding of the agonist [(3)H]-8-OH DPAT. Competition data were all fitted best by a one-binding site model. (b) K(i) values for agonists were also determined in competition versus the binding of the antagonist [(3)H]-NAD-199. Competition data were all fitted best by a two-binding site model, and agonist affinities for the higher (K(h)) and lower affinity (K(l)) sites were determined. 3. The ability of the agonists to activate the 5-HT(1A) receptor was determined using stimulation of [(35)S]-GTPgammaS binding. Maximal effects of agonists (E(max)) and their potencies (EC(50)) were determined from concentration/response curves for stimulation of [(35)S]-GTPgammaS binding. 4. K(l)/K(h) determined from ligand binding assays correlated with the relative efficacy (relative E(max)) of agonists determined in [(35)S]-GTPgammaS binding assays. There was also a correlation between K(l)/K(h) and K(l)/EC(50) for agonists determined from ligand binding and [(35)S]-GTPgammaS binding assays. 5. Simulations of agonist binding and effect data were performed using the Ternary Complex Model in order to assess the use of K(l)/K(h) for predicting the relative efficacy of agonists.

5-HT(1A) receptor imaging in the human brain: effect of tryptophan depletion and infusion on [(18)F]MPPF binding

The 5-HT(1A) receptor has been implicated in a variety of physiological processes, psychiatric disorders, and neurodegenerative disorders. [(18)F]MPPF is a useful radioligand for quantitative imaging of 5-HT(1A) receptors in human subjects. Previous studies have shown that the binding of some radioligands is sensitive to changes in neurotransmitter concentration, whereas in other cases, binding is not affected. In the present study we investigated if [(18)F]MPPF binding to the 5-HT(1A) receptor is sensitive to changes in 5-HT. Changes in 5-HT levels were achieved by influencing its synthesis through tryptophan depletion, including a tryptophan-free amino acid drink 4.5 h prior to the PET scan and tryptophan infusion (10 mg/ml, 50 mg/kg, 30 min, starting 60 min prior to the PET scan). Binding of [(18)F]MPPF in the brain of six healthy, male volunteers was compared in these two conditions. Mean binding potentials in the medial temporal cortex, cortical regions, and raphe nucleus did not significantly differ between the two conditions. The results of the study show that, under the experimental conditions used, [(18)F]MPPF binding was not affected. It is hypothesized that the increases in 5-HT levels needed to produce a measurable effect on [(18)F]MPPF binding would be significantly greater than that possible with tryptophan manipulation. Therefore, in pathological conditions, where such large increases in 5-HT levels are not expected, [(18)F]MPPF seems a useful ligand to measure 5-HT(1A) receptor distribution without the interference of endogenous 5-HT.

Efficacy of antipsychotic agents at human 5-HT(1A) receptors determined by [3H]WAY100,635 binding affinity ratios: relationship to efficacy for G-protein activation

5-HT(1A) receptors are implicated in the aetiology of schizophrenia. Herein, the influence of 15 antipsychotics on the binding of the selective 'neutral' antagonist, [3H]WAY100,635 ([3H]N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)-cyclo-hexanecarboxamide), was examined at human 5-HT(1A) receptors expressed in Chinese Hamster Ovary cells. In competition binding experiments, 5-HT displayed biphasic isotherms which were shifted to the right in the presence of the G-protein uncoupling agent, GTPgammaS (100 microM). In analogy, the isotherms of ziprasidone, quetiapine and S16924 (((R-2-[1-[2-(2,3-dihydro-benzo[1,4]dioxin-5-yloxy)-ethyl]-pyrrolidin-3yl]-1-(4-fluoro-phenyl)-ethanone), were displaced to the right by GTPgammaS, consistent with agonist actions. Binding of several other antipsychotics, such as ocaperidone, olanzapine and risperidone, was little influenced by GTPgammaS. Isotherms of the neuroleptics, haloperidol, chlorpromazine and thioridazine were shifted to the left in the presence of GTPgammaS, suggesting inverse agonist properties. For most ligands, the magnitude of affinity changes induced by GTPgammaS (alteration in pK(i) values) correlated well with their previously determined efficacies in [35S]GTPgammaS binding studies [Eur. J. Pharmacol. 355 (1998) 245]. In contrast, the affinity of the 'atypical' antipsychotic agent, clozapine, which is a known partial agonist at 5-HT(1A) receptors, was less influenced by GTPgammaS. When the ratio of high-/low-affinity values was plotted against efficacy, hyperbolic isotherms were obtained, consistent with a modified ternary complex model which assumes that receptors can adopt active conformations in the absence of agonist. In conclusion, modulation of [3H]-WAY100,635 binding by GTPgammaS differentiated agonist vs. inverse agonist properties of antipsychotics at 5-HT(1A) receptors. These may contribute to differing profiles of antipsychotic activity.

Agonist properties of pindolol at h5-HT1A receptors coupled to mitogen-activated protein kinase

At h5-HT1A receptors, stably transfected into Chinese Hamster Ovary Cells (CHO-h5-HT1A), the selective 5-HT1A receptor agonist, (+)8-hydroxy-dipropyl-amino-tetralin, ((+)8-OH-DPAT), transiently activated mitogen-activated protein kinase (MAPK) with a pEC50 of 8.5. The arylalkylamine, (-)-pindolol, also behaved as an agonist with a maximal effect of 57% relative to (+)8-OH-DPAT (100%), and with a pEC50 of 7.2. The selective 5-HT(1A) receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo-hexane carboxamide (WAY100,635), blocked (+)8-OH-DPAT- and (-)-pindolol-induced MAPK activation with pK(B)s of 9.7 and 9.9, respectively, whereas the selective 5-HT(1B) receptor antagonist, 1'-Methyl-5-[2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)biphenyl-4-ylcarbonyl]-2,3,6,7-tetrahydro-5H-spiro[furo[2,3-f]indole-3,4'-piperidine] (SB224,289) was inactive. Pertussis toxin blocked the actions of (+)8-OH-DPAT and (-)-pindolol demonstrating implication of G(i)/G(o) proteins. Thus, stimulation of MAPK provides an intracellular marker and signal for expression of the agonist actions of (-)-pindolol at h5-HT1A receptors.

SB-272183, a selective 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptor antagonist in native tissue

A novel compound, SB-272183 (5-Chloro-2, 3-dihydro-6-[4-methylpiperazin-1-yl]-1[4-pyridin-4-yl]napth-1-ylaminocarbonyl]-1H-indole), has been shown to have high affinity for human 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors with pK(i) values of 8.0, 8.1 and 8.7 respectively and is at least 30 fold selective over a range of other receptors. [(35)S]-GTPgammaS binding studies showed that SB-272183 acts as a partial agonist at human recombinant 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors with intrinsic activities of 0.4, 0.4 and 0.8 respectively, compared to 5-HT. SB-272183 inhibited 5-HT-induced stimulation of [(35)S]-GTPgammaS binding at human 5-HT(1A) and 5-HT(1B) receptors to give pA(2) values of 8.2 and 8.5 respectively. However, from [(35)S]-GTPgammaS autoradiographic studies in rat and human dorsal raphe nucleus, SB-272183 did not display intrinsic activity up to 10 microM but did block 5-HT-induced stimulation of [(35)S]-GTPgammaS binding. From electrophysiological studies in rat raphe slices in vitro, SB-272183 did not effect cell firing rate up to 1 microM but was able to attenuate (+)8-OH-DPAT-induced inhibition of cell firing to give an apparent pK(b) of 7.1. SB-272183 potentiated electrically-stimulated [(3)H]-5-HT release from rat and guinea-pig cortical slices at 100 and 1000 nM, similar to results previously obtained with the 5-HT(1B) and 5-HT(1D) receptor antagonist, GR127935. Fast cyclic voltammetry studies in rat dorsal raphe nucleus showed that SB-272183 could block sumatriptan-induced inhibition of 5-HT efflux, with an apparent pK(b) of 7.2, but did not effect basal efflux up to 1 microM. These studies show that, in vitro, SB-272183 acts as an antagonist at native tissue 5-HT(1A), 5-HT(1B) and 5-HT(1D) receptors.

Differential modulation by GTPgammaS of agonist and inverse agonist binding to h5-HT(1A) receptors revealed by [3H]-WAY100,635

1. The interaction of serotonergic ligands at human (h) 5-HT(1A) receptors expressed in Chinese hamster ovary cells was examined with the selective 'neutral' 5-HT(1A) antagonist [(3)H]-WAY100,635. Its binding was saturable (K(D)=0.056 nM) with a B(max) (3.65 pmol mg(-1)) significantly higher than that of two other selective 5-HT(1A) radioligands: the partial agonist, [(3)H]-S15535 (2.77 pmol mg(-1)) and the agonist, [(3)H]-8-OH-DPAT (2.02 pmol mg(-1)). 2. The influence of GTPgammaS (100 microM) on the binding affinity of 15 serotonergic agonists, partial agonists, antagonists and inverse agonists was investigated in competition binding experiments with [(3)H]-WAY100,635. 3. Agonists, including 5-HT, 8-OH-DPAT and buspirone, displayed biphasic isotherms which shifted to the right in the presence of GTPgammaS. In contrast, isotherms of the inverse agonists, methiothepin, (+)butaclamol and spiperone, were shifted to the left in the presence of GTPgammaS. Unlabelled WAY100,635 was the only ligand that was unaffected by GTPgammaS, consistent with 'neutral' antagonist properties. 4. The magnitude of affinity changes induced by GTPgammaS for 13 ligands was highly correlated (r = 0.98) with their efficacy (positive and negative) previously determined by [(35)S]GTPgammaS binding. 5. In contrast, the napthylpiperazine derivative and high efficacy agonist, S14506, displayed only a modest GTPgammaS shift, in accordance with previous indications of 'atypical' binding properties of this ligand. A further full agonist, S14671, which is chemically closely-related to S14506, also displayed a minimal GTPgammaS shift, underpinning this observation. 6. In conclusion, [(3)H]-WAY100,635 constitutes a useful neutral antagonist radioligand for the characterization of drug actions at h5-HT(1A) receptors. GTPgammaS-induced affinity changes of agonist and inverse agonist competition isotherms generally correlate well with ligand efficacy, with the notable exception of two chemically-similar agents, S14506 and S14671, which are efficacious agonists, yet relatively insensitive to h5-HT(1A) receptor/G-protein coupling changes.