pH-dependent modulation of agonist interactions with [3H]-ketanserin-labelled S2 serotonin receptors

Pharmacological and behavioral characterization of the 5-HT2A receptor in C57BL/6N mice

RATIONALE

The serotonin (5-HT) 2A receptor is implicated in numerous psychiatric disorders, making it an important, clinically relevant target. Despite the availability of transgenic mouse lines, the native mouse 5-HT(2A) receptor is not well-characterized.

OBJECTIVES

The goals of the current study were to determine 5-HT(2A) and 5-HT(2C) receptor densities in mouse cortex, establish a pharmacological profile of the mouse 5-HT(2A) receptor, and determine the effects of chronic drug treatment on 5-HT(2A) receptor density and 5-HT(2A) receptor-mediated behavior.

METHODS

Receptor densities were determined in cortex and frontal cortex via saturation binding assays using [(3)H]ketanserin or [(3)H]mesulergine. A pharmacological profile was established by displacing [(3)H]ketanserin binding with several ligands. Chronic treatment with 5-HT(2A/2C) receptor agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI), 5-HT(2A) receptor antagonist, MDL 11939, or vehicle was followed by 5-HT(2A) receptor density determination. Head twitch responses (HTRs) were counted on select days.

RESULTS

Mice had high 5-HT(2A), but low 5-HT(2C) receptor densities. Ligand binding affinities for mouse 5-HT(2A) receptors correlated with rat, but not rabbit or human, affinities. Chronically DOI-treated mice displayed reduced HTRs and 5-HT(2A) receptor density compared to saline-treated mice. Receptor density was unchanged following chronic treatment with MDL 11939.

CONCLUSIONS

The current study provides some basic information about mouse 5-HT(2A) and 5-HT(2C) receptors and provides comparisons to rats, rabbits, and humans. The current chronic agonist treatment study demonstrated an important similarity between the 5-HT(2A) receptor in mice, rats, and rabbits, while antagonist treatment revealed an interesting difference from previous studies in rabbits.

DR4004, a putative 5-HT(7) receptor antagonist, also has functional activity at the dopamine D2 receptor

The tetrahydrobenzindole, 2a-(4-(4-phenyl-1,2,3,6-tetrahydropyridyl)butyl)-2a,3,4,5-tetrahydrobenzo[cd]indol-2(1H)-one (DR4004) has been described as a highly selective antagonist for the 5-hydroxytryptamine(7) (5-HT(7)) receptor [J. Med. Chem. 42 (1999) 533]. Consistent with original data, DR4004 bound to rat hypothalamic membranes with an affinity of 7.3+/-0.2 (pK(i)+/-S.E.M.) for the 5-HT(7) receptor. However, competition binding studies showed that DR4004 had poor receptor selectivity with the following affinity profile; dopamine D2 receptor, alpha(1)-adrenoceptor > or =5-HT(7) receptor>histamine H(1) receptor, alpha(2)-adrenoceptor>dopamine D1 receptor>beta-adrenoceptor, muscarinic and 5-HT(2A/C) receptors. In conscious rats DR4004 (1, 5 or 10 mg/kg i.p.) produced a dose-dependent hyperglycaemia and hypothermia, but the former was reduced by the dopamine D2 receptor antagonist raclopride. Another 5-HT(7) receptor antagonist, (R)-3-(2-(2-(4-methylpiperidin-1-yl)-ethyl)pyrrolidine-1-sulfonyl)phenol (SB-269970) produced hypothermia but no hyperglycaemia. This study confirms that DR4004 has high affinity for the 5-HT(7) receptor but suggests that dopamine D2 receptor activity contributes to some of the in vivo effects.

Characteristics of agonist displacement of [3H]ketanserin binding to platelet 5-HT2A receptors: implications for psychiatric research

Brain 5-HT(2A) receptors exist in two agonist affinity states as a function of their coupling to Gq protein. This has not yet been shown in platelets. We examined [3H]ketanserin's saturable binding to platelet 5-HT2A receptors and characteristics of agonist displacement curves of [3H]ketanserin binding in healthy control subjects. [3H]ketanserin saturation curves showed a trend for a two-site model, reflecting two independent binding sites. At low [3H]ketanserin concentrations, agonist displacement curves were flat and best fit a two-site model, indicating the existence of two agonist affinity states. Guanylyl 5'-imidotriphosphate [Gpp(NH)p] induced a significant rightward shift in agonist displacement curves to fit a one-site model. Platelet membrane 5-HT2A receptors exist in two agonist affinity states that are regulated by Gq protein. Platelet 5-HT2A receptors provide an accessible model for examining possible dysregulation in the agonist affinity or coupling efficiency to the phosphoinositide system in psychiatric disorders and their modulation by psychotropic medications.

Inhibition of ligand binding to thromboxane A2/prostaglandin H2 receptors by diethylpyrocarbonate. Protection by receptor ligands and reversal by hydroxylamine

The potential of histidines to modulate the binding of agonists and antagonists to human platelet thromboxane A2 (TXA2) receptors was investigated. TXA2 receptors were purified from crude platelet membranes via affinity and wheat germ lectin chromatography. Radioligand binding studies were conducted using the TXA2, mimetic [125I]BOP (I-BOP (I-BOP = [1S-(1 alpha,2 beta(5Z),3 alpha(1E, 3R*),4 alpha)]-7-[3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)7-oxabicyclo- [2.2.1]heptan-2-yl]-5-heptenoic acid) and the TXA2 receptor antagonist [125I]SAP (I-SAP = 7-[(1R,2S,3S,5R)-6,6-dimethyl-3-(4-iodobenzene- sulfonylamino)-bicyclo-[3.1.1]hept-2-yl]-(5Z)-heptenoic acid). The histidine modifying reagent diethyl-pyrocarbonate (DEPC) produced a concentration (30-100 microM) dependent inhibition of binding of both [125I]BOP and [125I]SAP. DEPC treatment significantly (P < 0.05, N = 6) decreased the affinity of the receptor for [125I]SAP (Kd = 2.4 +/- 0.4 and 5.4 +/- 0.4 nM, control and DEPC, respectively) without significantly decreasing the Bmax. The effects of DEPC were reversed by hydroxylamine. The inhibition of [125I]BOP and [125I]SAP binding produced by DEPC was reduced significantly by prior incubation of the purified receptors with the TXA2 receptor agonist U-46619 or the TXA2 receptor antagonist SQ 29548. The results strongly support the notion that one or more histidines reside in a domain that can modulate ligand binding to the TXA2 receptor.

Molecular structural basis of ligand selectivity for 5-HT2 versus 5-HT1C cortical receptors

A molecular structural criterion of ligand selectivity for the 5-HT2 versus 5-HT1C receptor was hypothesized on the basis of radioligand binding data. Despite the large number of compounds which have been tested at both receptors, analysis of published data led to the identification of only five agents which are greater than 10-fold selective for the 5-HT2 versus the 5-HT1C receptor. Comparison of the two-dimensional structures revealed that, although these five compounds represent three distinct structural classes, they share a common structural feature located in the region hypothesized to be involved in receptor binding: a carbonyl or carboxyl oxygen interposed spatially between an aromatic ring and nitrogen atom. This structural feature was used to predict the relative selectivity of compounds that had not previously been analyzed at both the 5-HT2 and 5-HT1C receptors. All six drugs tested which contain the identified reactive carbonyl or carboxyl group were found to be selective for the 5-HT2 versus the 5-HT1C receptor with selectivity ratios ranging from 26 to 380. By contrast, three agents which are structurally similar but do not contain the reactive carbonyl or carboxyl group displayed equally high affinity for both receptor binding sites. Since the physiological roles of the 5-HT2 and 5-HT1C receptor are markedly different, it would be of potential clinical and scientific value to utilize this molecular structural feature to further identify chemical compounds which would selectively interact with only one of the two receptors.

Modulation of 5-hydroxytryptamine1A receptor density by nonhydrolyzable GTP analogues

Co-incubation of rat cortical membranes with 10(-4) M GTP results in a competitive inhibition of 5-hydroxytryptamine1A (5-HT1A) receptor binding sites labeled by [3H]8-hydroxy-2-(di-n-propylamino)tetralin [( 3H]8-OH-DPAT). Preincubation of cortical membranes with 10(-4) M GTP does not significantly change either KD or Bmax values, indicating that the effect of GTP is reversible. By contrast, GTP gamma S and 5'-guanylylimidodiphosphate (GppNHp) are nonhydrolyzable analogues of GTP which lengthen the time course of guanine nucleotide activation of guanine nucleotide binding proteins (G proteins) and thereby alter G protein-receptor interactions. These nonhydrolyzable GTP analogues were used to characterize the effects of persistent alterations in G proteins on [3H]8-OH-DPAT binding to 5-HT1A receptors. Co-incubation of rat cortical membranes with either 10(-4) M GTP gamma S or GppNHp results in a decrease in both the affinity and apparent density of 5-HT1A binding sites. Co-incubation with the nonhydrolyzable nucleotides reduces the affinity of [3H]8-OH-DPAT binding by 65-70% and lowers the density of the binding site by 53-61%. Similarly, preincubation of membranes with a 10(-4) M concentration of either GTP gamma S or GppNHp significantly increases the KD value and reduces the Bmax value of [3H]8-OH-DPAT binding. These results indicate that GTP gamma S and GppNHp induce persistent changes in 5-HT1A receptor-G protein interactions that are reflected as a decrease in the density of binding sites labeled by [3H]8-OH-DPAT.

Biochemical and pharmacological characterization of CGS 12066B, a selective serotonin-1B agonist

CGS 12066B is a novel pyrroloquinoxaline with selectivity for the serotonin-1B (5HT1B) recognition site as assessed by binding, biochemical and electrophysiological studies. The compound had an IC50 value of 51 nM at the 5HT1B recognition site as determined using the binding of [3H]5HT in the presence of 1 microM spiperone. At the 5HT1A receptor the compound had an IC50 value of 876 nM, providing a 5HT1A/5HT1B ratio of 17 in contrast to the putative 5HT1B selective agent trifluoromethylphenylpiperazine (TFMPP) which had a corresponding ratio of 3.6. The compound had minimal affinity for alpha 1-, alpha 2- and beta-adrenoceptors and for dopamine D-1 and D-2 receptors. CGS 12066B, in contrast to TFMPP, which was inactive, was found to inhibit dorsal raphe cell firing with an ED50 value of 358 nmol/kg i.v. The corresponding values for the 5HT1A selective agonists 8-OH-DPAT and ipsapirone were 1.3 and 33 nmol/kg. CGS 12066B was also effective in decreasing rat brain 5-HTP concentrations and inhibiting in vitro 5HT release. The data obtained indicate that CGS 12066B is a reasonably active 5HT1B site agonist, which due to its selectivity as compared to compounds such as TFMPP, will be a useful tool for evaluating the physiological role of such receptors in the mammalian CNS.

Typical and atypical antipsychotic occupancy of D2 and S2 receptors: an autoradiographic analysis in rat brain

In thin sections of rat brain, [3H]spiperone binds to D2 sites in the basal ganglia (caudate-putamen, nucleus accumbens, olfactory tubercle) and S2 sites in the claustrum and motor cortex. The in vitro displacement of [3H]spiperone from these regions was quantified autoradiographically with the "atypical" neuroleptics clozapine and thioridazine, which ameliorate psychosis, a "typical" neuroleptic, haloperidol, which also induces extrapyramidal side effects, or with metoclopramide, which induces extrapyramidal side effects but is an ineffective antipsychotic. Whereas metoclopramide was equipotent at D2 sites, haloperidol was less potent and clozapine and thioridazine more potent by 2- to 3-fold at competing for D2 sites in the nucleus accumbens or olfactory tubercle than in the caudate-putamen. As measured autoradiographically or with tissue homogenates, clozapine, thioridazine, and five other atypical neuroleptics were 4- to 800-times more potent at competing for S2 sites in the frontal cortex than for D2 sites in the basal ganglia. A preference of atypical antipsychotics for D2 receptors in the nucleus accumbens and olfactory tubercle and for the S2 receptor may explain the relative lack of extrapyramidal side effects produced by these compounds.

Dependence of the antagonism at human platelet 5-HT2 receptors by ketanserin on the reaction pH

The potency for the inhibition of 5-hydroxytryptamine (5-HT)-induced human platelet aggregation by ketanserin, pipamperone, spiperone, cyproheptadine and BW 501 in vitro decreased as the reaction pH increased progressively from 7.4 to 8.6, the largest shift (X 1125) in IC50 value (pH 7.4: 4 X 10(-9) M; pH 8.6: 4.5 X 10(-6) M) being found for ketanserin. With such an alkaline pH-shift, the fraction of the ionized form of the drugs decreased, reduction of the inhibitory capacity and of the ionized fraction being strongly correlated. Ketanserin (40 mg orally, - 15 h) in human volunteers, completely inhibited the 5-HT-induced platelet aggregation measured ex vivo, when tested at a reaction pH of 7.4; without gassing with CO2 5% -O2 95%, the plasma pH became alkaline and the inhibitory potency of the drug was strongly reduced (-60%). This study demonstrates the importance of the reaction pH for the aggregation of human platelets induced by 5-HT and its inhibition by ketanserin.

Antagonism of L-5-hydroxytryptophan-induced head twitching in rats by lisuride: a mixed 5-hydroxytryptamine agonist-antagonist?

Lisuride antagonized L-5-hydroxytryptophan (5-HTP)-induced head twitches at doses lower than those sufficient to induce the serotonin (5-HT) syndrome. Among several other 5-HT agonists tested, only LSD and 1-(m-trifluoromethylphenyl)-piperazine (TFMPP) shared this paradoxical profile. Assessment of various dopamine (DA) agonists revealed a lack of correlation between DA-mediated stereotyped behavior (indicative of postsynaptic DA agonism) and blockade of 5-HTP-induced head twitches. Lisuride displaced specific ligand binding from putative S1a, S1b and S2 receptors at nanomolar concentrations, and other drugs that blocked 5-HTP-induced head twitches also displaced binding at S2 sites. It is proposed that lisuride may have agonist properties at S1a receptors mediating the 5-HT syndrome but antagonist properties at S2 receptors mediating 5-HTP-induced head twitching.

Effects of different anticoagulants on human platelet size distribution and serotonin (5-HT) induced shape change and uptake kinetics

The effect of collecting blood with disodium ethylene diamine tetraacetate (EDTA), citrate (NAC) or acid sodium citrate-dextrose (ACD) as anticoagulants on platelet count and size distribution was investigated. No difference between the three preparations regarding platelet count was found in whole blood. Preparation of platelet-rich plasma (PRP) significantly reduced the platelet count in NAC-PRP (p less than 0.01) to a value of 288 X 10(9)/l compared to those of 365 X 10(9)/l and 368 X 10(9)/l in EDTA and ACD blood respectively. A significant shift in the platelet size in EDTA-PRP towards larger cell volumes was observed. There were no differences in the size distribution pattern between NAC-PRP and ACD-PRP in spite of the differences in platelet count. Platelet 5-HT uptake kinetics in EDTA-PRP showed a 50 per cent reduction in both Km and Vmax compared to that in ACD-PRP. The study shows that the receptor mediating 5-HT induced shape change has a direct opposite pH dependence than that of the 5-HT carrier. Interference of receptor-mediated responses in 5-HT uptake studies in human platelets is clearly minimized at a lowered pH. The finding is probably of importance in disorders associated with platelet hyperaggregability.