Guanine nucleotides modulate cortical S2 serotonin receptors

Stable expression of constitutively activated mutant h5HT6 and h5HT7 serotonin receptors: inverse agonist activity of antipsychotic drugs

RATIONALE

In order to determine the possible relationship between antipsychotic drug properties and inverse agonist activity at h5HT6 and h5HT7 receptors, constitutively activated forms of these receptors were created by site-specific mutagenesis. Typical and atypical antipsychotic drugs were assayed for their potencies as inverse agonists at these mutated receptors.

OBJECTIVES

Stable cell lines expressing constitutively activated forms of the h5HT6 and h5HT7 receptors were created. Typical and atypical antipsychotic drugs demonstrating high to moderate affinities for the h5HT6 and h5HT7 receptors were assayed for their potencies in reversing the agonist-independent activity (inverse agonist activity).

RESULTS

The E322R h5HT6 mutant and the S267K h5HT7 mutant displayed sufficiently robust agonist-independent activity when expressed in stable cell lines to allow the detailed, concentration-dependent, investigation of the inverse agonist activity of typical and atypical antipsychotic drugs. All the drugs tested displayed inverse agonist activity at both the activated h5HT6 and h5HT7 receptors. Native forms of these receptors did not display any constitutive activity. Interestingly, atypical antipsychotic drugs displayed potent inverse agonist activity, relative to typical antipsychotic drugs, at the h5HT7 receptor. LSD displayed neutral antagonist properties at the mutant h5HT7 receptor.

CONCLUSIONS

Site-specific mutations in the third intracellular loop of the G(s)-coupled h5HT6 and h5HT7 receptors produce constitutive activation. Antipsychotic drugs display inverse agonist activity at the activated receptors. The inverse agonist mechanism-of-action of the atypical antipsychotic drugs at the h5HT7 receptors may be different from the typical antipsychotic drugs as these drugs displayed far higher potencies as inverse agonists at the h5HT7 receptor.

Alcohol-withdrawn animals have a prolonged increase in dopamine D2high receptors, reversed by general anesthesia: relation to relapse?

The biochemical basis for alcohol addiction and relapse is not known. Although ethanol promotes the release of dopamine like other drugs of abuse, many unknown factors remain to be investigated concerning the biochemical abnormalities which persist after ethanol drinking and which contribute to alcohol relapse. Although ethanol withdrawal is associated with enhanced sensitivity to dopamine in animals and humans, only minor changes in the striatal density of dopamine D2 receptors have been found in humans, and animals show a small reduction in striatal D2 receptors. But how can dopamine-related functions be increased in ethanol withdrawal in the face of an unchanged or reduced density of dopamine D2 receptors? Considering that ethanol sensitizes rats to amphetamine, and that the high-affinity state of D2, or D2High, is markedly increased in striata from amphetamine-sensitized rats, we measured the density of D2High in striata from rats withdrawn from ethanol. These sites were elevated by 360% (7.2 pmol/g) for at least 8 days after stopping ethanol and returned to normal levels of 2 pmol/g after 2 weeks of ethanol withdrawal. In addition, 1 h of deep general anesthesia given 5 days into withdrawal resulted in a normal level of D2High within 24 h. Because the D2High states are the functional form of D2, their elevated density in ethanol withdrawal may be related to ethanol relapse in humans. General anesthesia may alleviate aspects of alcohol or amphetamine abuse or psychosis associated with elevated D2High.

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.

Receptor pharmacology of MDMA and related hallucinogens

The data presented herein appear to strongly implicate the brain 5HT2 receptor as the site-of-action of the hallucinogenic PIAs and LSD. If so, this discovery represents a major step in understanding the molecular pharmacology of hallucinogenic drugs. Using radioactive hallucinogenic drugs, detailed properties of brain 5HT2 receptors indicating the interaction of 5HT2 receptors with GTP-binding proteins have been revealed. Autoradiographic studies have revealed an extensive cortical distribution of brain 5HT2 receptors; these studies have also suggested that the PIAs may be 5HT1C agonists. Radiolabeling studies in conjunction with drug discrimination studies indicate that MDMA is apparently "amphetamine-like" and not "LSD-like" while MDA is apparently both "LSD-like" and "amphetamine-like." However, MDMA does appear to possess the potential to act as a 5HT2 agonist at high dosages.

[125I]-2-(2,5-dimethoxy-4-iodophenyl)aminoethane ([125I]-2C-I) as a label for the 5-HT2 receptor in rat frontal cortex

Recent studies of 5-HT2 receptor binding have involved the use of radiolabeled agonists. This report describes the use of [125I]-2-(2,5-dimethoxy-4-iodophenyl)aminoethane ([125I]-2C-I) as a label for low-density 5-HT2 agonist binding sites. A nonhydrolyzable analog of GTP, GppNHp, was found to inhibit the high affinity binding of [125I]-2C-I. 5-HT and several 5-HT2 agonists and antagonists displayed high affinity for this site. In addition, a significant decrease in the Bmax value, but not the KD for [125I]-2C-I was observed at 37 degrees C as compared to that observed at 24 degrees C. Several structure-activity relationships were investigated for displacement of [125I]-2C-I, and the results are consistent with the importance of this receptor in the mechanism of action of hallucinogens. This study demonstrates the utility of [125I]-2C-I as a novel radioligand and provides further data that the 5-HT2 receptor is significantly linked to hallucinogenic activity for several compounds.

Comparison of [125I]iodolysergic acid diethylamide binding in human frontal cortex and platelet tissue

The human platelet contains a functional 5-hydroxytryptamine (5-HT) receptor that appears to resemble the 5-HT2 subtype. In this study, we have used the iodinated derivative [125I]iodolysergic acid diethylamide ([125I]iodoLSD) in an attempt to label 5-HT receptors in human platelet and frontal cortex membranes under identical assay conditions to compare the sites labelled in these two tissues. In human frontal cortex, [125I]iodoLSD labelled a single high-affinity site (KD = 0.35 +/- 0.02 nM). Displacement of specific [125I]iodoLSD binding indicated a typical 5-HT2 receptor inhibition profile, which demonstrated a significant linear correlation (r = 0.97, p less than 0.001, n = 17) with that observed using [3H]ketanserin. However, [125I]iodoLSD (Bmax = 136 +/- 7 fmol/mg of protein) labelled significantly fewer sites than [3H]ketanserin (Bmax = 258 +/- 19 fmol/mg of protein) (p less than 0.001, n = 6). In human platelet membranes, [125I]iodoLSD labelled a single site with affinity (KD = 0.37 +/- 0.03 nM) similar to that in frontal cortex. The inhibition profile in the platelet showed significant correlation with that in frontal cortex (r = 0.96, p less than 0.001, n = 16). We conclude that the site labelled by [125I]iodoLSD in human platelet membranes is biochemically similar to that in frontal cortex and most closely resembles the 5-HT2 receptor subtype, although the discrepancy in binding capacities of [125I]iodoLSD and [3H]ketanserin raises a question about the absolute nature of this receptor.

Ethanol does not affect serotonin receptor binding in rodent brain

The effects of ethanol on serotonin (5-hydroxytryptamine, 5-HT) receptor binding in rat and mouse brain were determined under in vitro conditions and in mouse brain following seven days of ethanol ingestion. 5-HT1A receptor characteristics were measured utilizing the agonist [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([ 3H]DPAT), and 5HT2 receptor-binding studies utilized the antagonist [3H]ketanserin. At the highest concentration of ethanol tested in vitro (680 mM), there was only 25% inhibition of [3H]DPAT binding in rat and mouse brain and 14% inhibition of [3H]ketanserin binding in rat brain. Effects of an anesthetic concentration of ethanol (100 mM) on agonist binding in the presence and absence of the guanine nucleotide GTP were also evaluated in vitro in mouse brain. In no case did ethanol (100 mM) significantly affect 5-HT1A or 5-HT2 receptor-binding characteristics. When 5-HT receptor characteristics were measured after mice consumed ethanol for seven days, there was no change in either 5-HT1A or 5-HT2 receptor-binding properties in any of the brain areas examined.

Radioligand binding evidence implicates the brain 5-HT2 receptor as a site of action for LSD and phenylisopropylamine hallucinogens

Alterations in brain serotonergic function have been implicated in the mechanism of action of LSD, mescaline, and other similarly acting hallucinogenic drugs of abuse such as STP (2,5-dimethoxyphenylisopropylamine; DOM). In order to test the hypothesis that the mechanism of action of LSD and phenylisopropylamine hallucinogens is through stimulation of a specific brain serotonin receptor sub-type, the affinities of these compounds for radiolabelled 5-HT2, 5-HT1A, 5-HT1B, and 5-HT1C receptors have been determined using recently developed in vitro radioligand binding methodologies. The 5-HT2 receptor was labelled with the agonist/hallucinogen radioligand 3H-DOB (4-bromo-2,5-dimethoxyphenylisopropylamine). The 5-HT1A, 5-HT1B, and 5-HT1C receptors were labelled with 3H-OH-DPAT, 3H-5-HT, and 3H-mesulergine, respectively. In general, the phenylisopropylamines displayed 10-100 fold higher affinities for the 5-HT2 receptor than for the 5-HT1C receptor and 100-1000 fold higher affinities for the 5-HT2 receptor than for the 5-HT1A or 5-HT1B receptor. There was a strong correlation between hallucinogenic potencies and 5-HT2 receptor affinities of the phenylisopropylamines (r = 0.90); the correlation coefficients for the 5-HT1A, 5-HT1B, and 5-HT1C were 0.73, 0.85, and 0.78, respectively. Because there is no evidence that 5-HT1A-selective or 5-HT1B-selective agonists are hallucinogenic and because the phenylisopropylamines are potent hallucinogens, a 5-HT2 receptor interaction is implicated and supports our previous suggestions to this effect. A secondary role for 5-HT1C receptors cannot be discounted at this time.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding to the serotonin 5-HT2 receptor by the enantiomers of 125I-DOI

Preliminary receptor binding experiments with the stereoisomers of 125I-DOI, a new putative 5-HT2 agonist ligand, were conducted. The results indicated specific binding for both enantiomers, with a two-site model giving the best fit to the binding data. R-125I-DOI showed a high affinity dissociation constant of 1.26 nanoMolar, while the less active S-enantiomer had a two-fold lower affinity. Competition experiments with several 5-HT2 agonists also indicated a two site model, with high affinity inhibition constants less than 10 nanoMolar. These results show a stereoselective effect of ligand binding to the 5-HT2 receptor. The use of this ligand to investigate agonist-receptor interactions could be instrumental in the elucidation of the role of serotonergic systems in the mechanism of action of hallucinogens.

Selectivity of serotonergic drugs for multiple brain serotonin receptors. Role of [3H]-4-bromo-2,5-dimethoxyphenylisopropylamine ([3H]DOB), a 5-HT2 agonist radioligand

The affinities of putative serotonin receptor agonists and antagonists for 5-HT1A, 5-HT1B, 5-HT1C, and 5-HT2 receptors were assayed using radioligand binding assays. The 5-HT1 sites were labeled with the agonist radioligands [3H]-8-hydroxy-2-(di-n-propylamino)-tetralin [3H]-8-OH-DPAT, [3H]-5-HT, and [3H]mesulergine. The 5-HT2 receptor was labeled with the antagonist radioligand [3H]ketanserin or the agonist radioligand [3H]-4-bromo-2,5-dimethoxyphenylisopropylamine ([3H]DOB). The apparent 5-HT1 receptor selectivity of agonist compounds was found to be 50- to 100-fold higher when the 5-HT2 receptor affinity was determined using the antagonist radioligand [3H]ketanserin than when the agonist radioligand [3H]DOB was used. Quipazine, a putative specific 5-HT2 agonist, appeared to be only 3-fold more potent at 5-HT2 than at 5-HT1A receptors when [3H]ketanserin was used as the 5-HT2 radioligand. When [3H]DOB was used as the 5-HT2 radioligand, quipazine was determined to be 100-fold more potent at 5-HT2 receptors than at 5-HT1A receptors. 1-(3-trifluoromethylphenyl)piperazine (TFMPP), a putative specific 5-HT1B receptor agonist was apparently 10-fold more potent at 5-HT1B receptors than at 5-HT2 receptors when [3H]ketanserin was used as the 5-HT2 radioligand. When [3H]DOB was used as the 5-HT2 radioligand, TFMPP was found to be equipotent at 5-HT1B and 5-HT2 receptors. Using the 5-HT2 antagonist radioligand [3H]ketanserin, a similar pattern of underestimating 5-HT2 receptor selectivity and/or overestimating 5-HT1A or 5-HT1B receptor selectivity was observed for a series of serotonin receptor agonists. Antagonist receptor selectivity was not affected significantly by the nature of the 5-HT2 receptor assay used. These data indicate that, by using an antagonist radioligand to label 5-HT2 receptors and agonist radioligands to label 5-HT1 receptors, the 5-HT1 receptor selectivity may be overestimated. This may be an especially severe problem in serotonin drug development as drugs that interact potently with 5-HT2 receptors have been reported to be psychoactive and/or hallucinogenic.