Analogues of the 5-HT1A serotonin antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine with reduced alpha 1-adrenergic affinity.

1-(2-Methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine (NAN-190; 1a) is a putative postsynaptic 5-HT1A serotonin antagonist. This high affinity ligand (Ki = 0.6 nM), although selective for 5-HT1A versus other 5-HT receptors, binds with nearly equal affinity at alpha 1-adrenergic receptors (Ki = 0.8 nm). Structure-affinity relationship studies were conducted in order to achieve an improved selectivity. Replacement of the phthalimide moiety by substituted benzamides led to retention of 5-HT1A affinity but to no improvement in selectivity, whereas replacement by alkyl amides proved beneficial, leading to an improvement in affinity and selectivity. Branching alpha to the amide carbonyl group and increased bulkiness of the alkyl moiety further improved 5-HT1A affinity and selectivity. 4-[4-(1-Adamantanecarboxamido)butyl]-1- (2-methoxyphenyl)piperazine (2j) was found to bind at 5-HT1A sites with high affinity (Ki = 0.4 nM) and with a 160-fold selectivity over alpha 1-adrenergic sites. Preliminary studies show that this agent retains antagonist activity as determined in a 5-HT1A-coupled adenylyl cyclase assay. Further functional studies are warranted to fully characterize this agent.

Binding of phenylalkylamine derivatives at 5-HT1C and 5-HT2 serotonin receptors: evidence for a lack of selectivity.

Certain phenylalkylamine derivatives have been considered to bind selectively at 5-HT2 serotonin receptors. It is now recognized that the most widely used derivatives, i.e., 1-(2,5-dimethoxy-4-X-phenyl)-2-aminopropanes where X = Me (DOM), Br (DOB), and I (DOI) (1-3, respectively) also bind at the more recently identified population of serotonin 5-HT1C receptors. The purpose of the present investigation was to determine whether simple phenylalkylamines bind selectively at one population of receptors over the other. An examination of 34 derivatives reveals (i) similar structure-affinity relationships and (ii) a significant correlation (r = greater than 0.9, n = 25) between 5-HT1C and 5-HT2 affinity. None of the compounds included in the present study displayed more than a 10-fold selectivity for one population of these receptors over the other; the results suggest that these compounds (including the widely used 5-HT2 agonists DOB and DOI) are 5-HT1C/5-HT2 agents.

Ketanserin analogues: the effect of structural modification on 5-HT2 serotonin receptor binding.

Ketanserin (1) is a fairly selective 5-HT2 antagonist that binds both at 5-HT2A and 5-HT2C receptors. A previous structure-affinity relationship study revealed that the structure of the piperidine-containing ketanserin molecule could be rather severely abbreviated with little effect on 5-HT2A affinity. The present investigation explores several inconsistencies identified in the earlier study and suggests that multiple modes of binding may be possible for ketanserin analogues. Perhaps the nature of the benzylic substituent is the most significant determinant of the manner in which these agents bind at 5-HT2A receptors, and it is possible that certain orientations may avail themselves of an auxiliary binding site. Depending upon the length of the piperidine N-alkyl chain, variation of the benzylic substituent from a carbonyl, to an alcohol, to a methylene group has a nonparallel influence on binding, and this may be further affected by the presence of a second ring nitrogen atom. The results of the present investigation provide evidence that although the structure of ketanserin can be abbreviated, and even modified by conversion of the piperidine ring to a piperazine, the resultant analogues may bind in more than one orientation at the receptors. A key structural feature that may play a prominent role in anchoring or orienting these compounds at 5-HT2A receptors is the benzylic carbonyl group.

Ketanserin analogues: structure-affinity relationships for 5-HT2 and 5-HT1C serotonin receptor binding.

Ketanserin is the prototypic 5-HT2 serotonin antagonist; although it has been an important tool for the study of serotonin pharmacology, it has had relatively little impact on drug design because remarkably little is known about its structure-affinity relationships. Furthermore, ketanserin also binds at 5-HT1C receptors and even less is known about the influence of its structural features on 5-HT1C receptor affinity. The present study reveals that the fluoro and carbonyl groups of the 4-fluorobenzoyl portion of ketanserin make small contributions to 5-HT2 binding and that the intact benzoylpiperidine moiety is an important feature. Ring-opening of the piperidine ring reduces affinity. Although the quinazoline-2,4-dione moiety also contributes to binding, it appears to play a smaller role and can be structurally simplified with retention of 5-HT2 affinity. N-(4-Phenylbutyl)-4-(4-fluorobenzoyl)piperidine (39), for example, binds with nearly the same affinity (Ki = 5.3 nM) as ketanserin (Ki = 3.5 nM). All of the compounds examined bind at 5-HT1C sites with lower affinity than ketanserin, and some of the simplified analogues bind with nearly 10 times the 5-HT2 versus 5-HT1C selectivity of ketanserin; however, none displays > 120-fold selectivity. Several of the compounds, such as the amide 32 and the urea 33 represent examples of new structural classes of 5-HT2 ligands.

Antagonism of 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane stimulus with a newly identified 5-HT2- versus 5-HT1C-selective antagonist.

DOM [i.e., 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane] is a 5-HT1C/2 serotonin agonist that exerts stimulus control of behavior in animals. In order to determine if the discriminative stimulus effect of DOM is 5-HT1C- or 5-HT2-mediated, it would be informative to conduct tests of stimulus antagonism with a 5-HT1C- or 5-HT2-selective antagonist. To date, no such agents exist. Although the neuroleptic agent spiperone binds at D2 dopamine receptors and 5-HT1A serotonin receptors, (a) it displays about a 1000-fold selectivity for 5-HT2 versus 5-HT1C sites and (b) it has been used as a "5-HT2-selective" antagonist. Because spiperone is a behaviorally disruptive agent, it is not suitable for use in drug-discrimination studies. Using the spiperone molecule as a starting point, a limited structure-affinity investigation was conducted in order to identify a suitable antagonist with high affinity and selectivity for 5-HT2 receptors, and yet an antagonist that might lack the disruptive actions of spiperone. Various modifications of the spiperone molecule were examined, but most resulted in decreased 5-HT2 affinity or in loss of selectivity. One compound, 8-[3-(4-fluorophenoxy)propyl]-1-phenyl-1,3,8-triazaspiro[4.5]de can-4-on e (26), was shown to bind at 5-HT2 sites with high affinity (Ki = 2 nM) and > 2,000-fold selectivity versus 5-HT1C sites. In tests of stimulus antagonism using rats trained to discriminate 1 mg/kg of DOM from saline vehicle, 26 behaved as a potent antagonist (ED50 = 0.003 mg/kg) and lacked the disruptive effects associated with spiperone. As such, (a) it would appear that the DOM stimulus is primarily a 5-HT2-mediated, and not 5-HT1C-mediated, phenomenon, and (b) compound 26 may find application in other pharmacologic investigations where spiperone may not be a suitable antagonist.

14 beta-[(p-nitrocinnamoyl)amino]morphinones, 14 beta-[(p-nitrocinnamoyl)amino]-7,8-dihydromorphinones, and their codeinone analogues: synthesis and receptor activity.

A series of 14 beta-[(nitrocinnamoyl)amino]codeinones and morphinones, some of which contain a 5 beta-methyl group, were prepared from 14 beta-aminocodeinones and 14 beta-[N-(cyclopropylmethyl)-amino]norcodeinones. The affinities of the target compounds for the mu, delta, and kappa opioid receptors were determined by radiolabeled binding experiments using bovine brain membranes. An analogous series of 7,8-dihydrocodeinones and morphinones was prepared and assayed in the same systems. The 3-methoxy derivatives 3 and 4 were more selective than the corresponding morphinones for the mu receptor. The 5 beta-methylcodeinones 25 and 27 had lower affinity at all receptors than the corresponding morphinones, but the 5 beta-methylmorphinones had affinities similar to the morphinones 5 and 6. A similar pattern was observed in the 7,8-dihydro series. Two compounds, 5 beta-methyl-14 beta-[(p-nitrocinnamoyl)amino]-7,8-dihydromorphinone, 20 (MET-CAMO), and N-(cyclopropylmethyl)-14 beta-[(p-nitrocinnamoyl)amino]-7,8-dihydronormorphinone, 22 (N-CPM-MET- CAMO), acted as nonequilibrium ligands in antinociception and membrane binding studies. In mice after icv administration, neither ligand showed any agonist activity but 8-24 h after administration both compounds acted as potent mu antagonists. A Scatchard plot of the effect of N-CPM-MET-CAMO on [3H]DAMGO ([3H]D-Ala2, (Me)-Phe4, Gly(ol)5] enkephalin) binding to bovine striatal membranes showed that there was a significant decrease in the Bmax value and a marginal effect on the Kd value suggesting that the number of binding sites was reduced. When taken together, these results support the view that 20 and 22 bind covalently to the mu receptor. On the other hand, when N-acetylcysteine and 22 were allowed to react in a buffered solution, 22 was recovered unchanged. Under these conditions no Michael reaction was observed.

Structure-activity relationships for the binding of arylpiperazines and arylbiguanides at 5-HT3 serotonin receptors.

Arylpiperazines are nonselective agents that bind at 5-HT3 serotonin receptors with moderate to high affinity, whereas 1-phenylbiguanide is a low-affinity but more selective 5-HT3 agonist. In an attempt to enhance the affinity of the latter agent, and working with the assumption that similarities might exist between the binding of the two types of agents, we formulated structure-activity relationships for the binding of the arylpiperazines and then incorporated those substituents, leading to high affinity for the arylpiperazines, into 1-phenylbiguanide. A subsequent investigation examined the structure-activity relationships of the arylbiguanides and identified arylguanidines as a novel class of 5-HT3 ligands. Although curious similarities exist between the structure-activity relationships of the arylpiperazines, arylbiguanides, and arylguanidines, it cannot be concluded that all three series of compounds are binding in the same manner. Furthermore, upon investigating pairs of compounds in the three series, the arylpiperazines behaved as 5-HT3 antagonists (von Bezold-Jarisch assay) whereas the arylbiguanides and arylguanidines acted as 5-HT3 agonists.

Influence of amine substituents on 5-HT2A versus 5-HT2C binding of phenylalkyl- and indolylalkylamines.

The effect of 15 different amine substituents on 5-HT2A and 5-HT2C serotonin receptor binding was investigated for two series of compounds (i.e., phenylalkylamine and indolylalkylamine derivatives). In general, amine substitution decreases receptor affinity; however, N-(4-bromobenzyl) substitution results in compounds that bind at 5-HT2A receptors with high affinity (Ki < 1 nM) and with > 100-fold selectivity. Although parallel structural modification in the two series result in parallel shifts in 5-HT2C binding, these same modifications alter 5-HT2A binding in a less consistent manner.

Spiperone: influence of spiro ring substituents on 5-HT2A serotonin receptor binding.

Spiperone (1) is a widely used pharmacological tool that acts as a potent dopamine D2, serotonin 5-HT1A, and serotonin 5-HT2A antagonist. Although spiperone also binds at 5-HT2C receptors, it is one of the very few agents that display some (ca. 1000-fold) binding selectivity for 5-HT2A versus 5-HT2C receptors and, hence, might serve as a useful template for the development of novel 5-HT2A antagonists if the impact of its various substituent groups on binding was known. In the present investigation we focused on the 1, 3,8-triazaspiro[4.5]decanone portion of spiperone and found that replacement of the N1-phenyl group with a methyl group only slightly decreased affinity for cloned rat 5-HT2A receptors. However, N1-methyl derivatives displayed significantly reduced affinity for 5-HT1A, 5-HT2C, and dopamine D2 receptors. Several representative examples were shown to behave as 5-HT2 antagonists. As such, N1-alkyl analogues of spiperone may afford entry into a novel series of 5-HT2A-selective antagonists.

Benzylimidazolines as h5-HT1B/1D serotonin receptor ligands: a structure-affinity investigation.

Benzylimidazolines may represent a class of 5-HT1D ligands that has yet to be exploited. On the basis of a previous report that the 2-(substituted-benzyl)imidazoline alpha-adrenergic agonist oxymetazoline (8) binds with high affinity at calf brain 5-HT1D receptors, we explored the structure-affinity relationships of a series of related derivatives. Each of the aromatic substituents was removed and then reinstated in a systematic manner to determine the influence of the individual substituents on binding. It was found that all of the aromatic substituents of 8 act in concert to impart high affinity. However, although the 3-hydroxy group could be removed without significantly reducing affinity for h5-HT1D (i.e., human 5-HT1Dalpha) receptors, this modification reduced h5-HT1B (i.e., human 5-HT1Dbeta) receptor affinity by nearly 50-fold. The 2, 6-dimethyl groups also contribute to binding but seem to play a greater role for h5-HT1B binding than h5-HT1D binding. With the appropriate structural modifications, several compounds were identified that display 20- to >100-fold selectivity for h5-HT1D versus h5-HT1B receptors. Preliminary functional data suggest that these compounds behave as agonists. Given that 5-HT1D agonists are currently being explored for their antimigraine action and that activation of h5-HT1B receptors might be associated with cardiovascular side effects, h5-HT1D-selective agents may offer a new lead for the development of therapeutically efficacious agents.

1-[4-(3-Phenylalkyl)phenyl]-2-aminopropanes as 5-HT(2A) partial agonists.

Phenylalkylamines such as 1-(4-bromo-2, 5-dimethoxyphenyl)-2-aminopropane (DOB; 1a) and its corresponding iodo derivative DOI (2) are commonly used 5-HT(2) serotonin agonists. Previous studies have established that the 2,5-dimethoxy substitution pattern found in these compounds is optimal for high affinity at 5-HT(2A) receptors and that substituents at the 4-position can modulate affinity over a wide range. We have previously shown, however, that when the 4-position is substituted with a 3-phenylpropyl substituent (i.e., 3), the compound binds with an affinity comparable to that of 1a but that it possesses 5-HT(2A) antagonist character. The present study examined the structure-affinity relationships of 3, and the results were very much unexpected. That is, the 2,5-dimethoxy substitution pattern of 3 is not required for high affinity. Either of the two methoxy groups can be removed without untoward effect on affinity, and relocation of the methoxy substituents actually enhances affinity by as much as an order of magnitude. None of the compounds displayed more than 20-fold selectivity for 5-HT(2A) over 5-HT(2C) receptors. In addition, several were demonstrated to act as 5-HT(2A) partial agonists. As such, the results of this study suggest that the structure-affinity relationships of phenylalkylamines as 5-HT(2A) ligands now be reinvestigated in greater detail.

1-[2-methoxy-5-(3-phenylpropyl)]-2-aminopropane unexpectedly shows 5-HT(2A) serotonin receptor affinity and antagonist character.

Certain phenylethylamines, such as 1-(4-bromo-2,5-dimethoxyphenyl)-2-aminopropane (DOB; 1a), are high-affinity 5-HT(2) agonists. Previous structure-affinity studies have concluded that both the 2,5-dimethoxy substitution pattern and the nature of substituents at the 4-position are important determinants of high affinity. We recently demonstrated that replacement of the bromo group of DOB with a 3-(phenyl)propyl substituent results in retention of affinity and that, counter to established structure-affinity relationships, the 2,5-dimethoxy substitution pattern is no longer a requirement for the binding. The present investigation extends these findings by examining a series of analogues, 3, lacking a 5-methoxy group. It was additionally found that shifting the phenylalkyl substituent from the 4- to the 5-position (e.g., 4i) also results in retention of affinity. For example, 1-(2-methoxy-5-(3-phenylpropyl)-2-aminopropane (6; the alpha-methyl derivative of 4i) binds at 5-HT(2A) receptors with high affinity (K(i) = 13 nM) and possesses 5-HT(2A) antagonist character. Thus, not only is the 2,5-dimethoxy substitution pattern not a requirement for the binding of certain phenylethylamines at 5-HT(2A) receptors, the presence of a 4-position substituent (previously thought to serve as a modulator of affinity of DOB-like agents) is also not required. Striking differences in the 5-HT(2A) binding requirements of the present compounds as compared to DOB-like agents suggest multiple substituent-dependent modes of binding.

Binding of O-alkyl derivatives of serotonin at human 5-HT1D beta receptors.

In humans, 5-HT1D serotonin receptors represent terminal autoreceptors, and there is some evidence that 5-HT1D ligands may be useful in the treatment of migraine. The most widely used 5-HT1D agonist is sumatriptan; however, this agent reportedly displays little selectivity for 5-HT1D versus 5-HT1A receptors. To identify novel serotonergic agents with enhanced 5-HT1D versus 5-HT1A selectivity, we attempted to take advantage of possible differences in the regions of bulk tolerance associated with the 5-position of the 5-HT binding sites for these two populations of receptors. Examination of a series of 5-(alkyloxy)tryptamine derivatives demonstrated that compounds with unbranched alkyl groups of up to eight carbon atoms bind with high affinity at human 5-HT1D beta receptors (Ki < 5 nM) but demonstrate less than 50-fold selectivity relative to 5-HT1A receptors. Alkyl groups longer than eight carbon atoms impart reduced affinity for 5-HT1A receptors whereas groups longer than nine carbon atoms lead to compounds with reduced affinity at 5-HT1D beta receptors. 5-(Nonyloxy)tryptamine (10) represents a compound with optimal 5-HT1D beta affinity (Ki = 1 nM) and selectivity (> 300-fold). Branching of the alkyl chain, to 5-[(7,7-dimethylheptyl)oxy]tryptamine (15), results in an agent with somewhat lower affinity (5-HT1D beta Ki = 2.3 nM) but with greater (i.e, 400-fold) 5-HT1D versus 5-HT1A selectivity. Replacement of the oxygen atom of 10 with a methylene group (i.e., 20), replacement of the O-proximate methylene with a carbonyl group (i.e., ester 26), or cyclization of the aminoethyl moiety to a carbazole (e.g., 34, 36) or beta-carboline (i.e., 37), result in reduced affinity and/or selectivity. None of the compounds examined displayed significant selectivity for 5-HT1D beta versus 5-HT1D alpha sites; nevertheless, compounds 10 (recently shown to have as a 5-HT1D agonist) and 15 represent the most 5-HT1D versus 5-HT1A selective agents reported to date.

2-(1-Naphthyloxy)ethylamines with enhanced affinity for human 5-HT1D beta (h5-HT1B) serotonin receptors.

Although the beta-adrenergic antagonist propranolol (1) binds at rodent 5-HT1B serotonin receptors, it displays low affinity (Ki > 10,000 nM) for its species homologue 5-HT1D beta (i.e., h5-HT1B) receptors. The structure of propranolol was systematically modified in an attempt to enhance its affinity for the latter population of receptors. Removal of the alkyl hydroxyl group, shortening of the O-alkyl chain from three to two methylene groups, and variation of the terminal amine substituent resulted in compounds, such as N-monomethyl-2-(1-naphthyloxy)-ethylamine (11; Ki = 26 nM), that display significantly higher h5-HT1B affinity than propranolol. Compound 11 was shown to bind equally well at human 5-HT1D alpha (h5-HT1D) receptors (Ki = 34 nM) and was further demonstrated to possess h5-HT1B agonist character in an adenylate cyclase assay. It would appear that such (aryloxy)alkylamines may represent a novel class of 5-HT1D receptor agonists.

Multiple serotonin receptor subtypes: molecular cloning and functional expression.

The field of serotonin (5HT) receptor pharmacology has been at least as dramatically altered by the advent of molecular neurobiology and recombinant DNA techniques as has any other neurotransmitter receptor field. The principal reason for this is the unforeseen multitude of genes expressing different types of 5HT receptors. Classic pharmacological studies as well as radioligand binding studies convinced workers in the field that there were multiple 5HT receptors. The extent of that multiplicity was not generally foreseen. At the time of this writing, no less than 13 5HT receptor genes have been cloned and functionally expressed. In addition, a fourteenth receptor has been well studied and will undoubtedly be cloned in the near future. These novel 5HT receptor clones are appearing at an astonishing rate. All 13 receptors have been cloned between the years 1987 to 1993, with more than half of the clones appearing in the literature in the last 18 months. Furthermore, there is no indication that all of the 5HT receptors present in the mammalian genome have been cloned. In fact, there are classic pharmacological data as well as radioligand binding data that implicate the existence of additional 5HT receptor subtypes not yet fully defined or cloned. Bringing order to this rapid outpouring of information at this time is a very difficult task. Not only is there a great multiplicity of receptor subtypes, each with a unique pharmacology and distribution, but there are species variants of 5HT receptors. In order to provide a concise and timely review, this article focuses on the strategies used to clone and express multiple 5HT receptors. Unique properties of the various receptors are emphasized.

Serotonin-dependent collagenase induction in rat myometrial smooth muscle cells: mediation by the 5-HT2 receptor.

Recent studies have shown that serotonin (5-hydroxytryptamine; 5-HT) is required for the induction of interstitial collagenase in cultured rat and human myometrial smooth muscle cells. The present study was performed to determine which serotonin receptor subtype mediates the induction of collagenase in these cells. [125I]DOI ((+/- )-1-(2,5-dimethoxy-4-[125I]iodophenyl)-2-aminopropane), a 5-HT2 receptor agonist radioligand, bound specifically to sites in myometrial cell membranes, and exhibited binding characteristics essentially identical to those observed with brain 5-HT2 receptors. Radioligands selective for other serotonin receptor subtypes (5-HT1 and 5-HT3) failed to yield detectable binding. Northern blot analysis demonstrated the presence of 5-HT2 mRNA in the uterine smooth muscle cell cultures, whereas transcripts for 5-HT1A and 5-HT1C receptors were not detectable. Moreover, RT-PCR indicated that 5-HT2 receptor mRNA is present in freshly isolated uterine tissue as well. Selective antagonists of the 5-HT2 receptor, ketanserin and spiperone, displayed concentration-dependent inhibition of serotonin-mediated collagenase induction in the myometrial cultures. These antagonists yielded IC50 values of 4.7 nM and 2.7 nM respectively, characteristic of values expected from a 5-HT2 receptor-mediated response. In addition, a number of selective 5-HT2 receptor agonists (quipazine, alpha-methyl-serotonin, DOI) mimicked the ability of serotonin to induce collagenase production, whereas compounds selective for 5-HT1 and 5-HT3 receptor subtypes had little effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin-mediated 5-HT2 receptor gene regulation in rat myometrial smooth muscle cells.

Serotonin (5-hydroxytryptamine; 5-HT) has recently been shown to induce collagenase production in myometrial smooth muscle cells (Jeffrey et al. (1991) J. Cell. Physiol. 146, 399-406) by activating transcription of the collagenase gene (Wilcox et al. (1992) J. Biol. Chem. 267, 20752-20757) following an interaction with the 5-HT2 receptor (Rydelek-Fitzgerald et al. (1993) Mol. Cell. Endocrinol. 91, 67-74). These studies were performed to investigate factors controlling the regulation of 5-HT2 receptors in these cells. Northern blot analysis indicates that serotonin increases levels of 5-HT2 receptor mRNA in cells by approximately 4-fold. Detectable increases in mRNA levels occur within 2 h after administration of serotonin with maximal levels occurring after 12 h. The 5-HT2 receptor antagonists, ketanserin and spiperone, inhibit the serotonin-mediated increase in receptor mRNA. Selective 5-HT2 receptor agonists ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI) and quipazine) mimic the effect of serotonin, whereas 5-HT1 and 5-HT3 receptor agonists ((+/-)-8-hydroxydipropylaminotetralin (8-OH-DPAT), 1-(3-chlorophenyl)piperazine dihydrochloride (mCPP), m-phenylbiguanide) have no effect. These data demonstrate that serotonin induces an increase in 5-HT2 receptor mRNA by interacting with the 5-HT2 receptor itself. Nuclear run-on analysis revealed that serotonin increases the initiation of 5-HT2 receptor mRNA synthesis. Moreover, the protein synthesis inhibitor, cycloheximide, prevents the induction of the mRNA for the receptor, demonstrating that serotonin-dependent increases in 5-HT2 receptor transcription require de novo protein synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Agonist activity of LSD and lisuride at cloned 5HT2A and 5HT2C receptors.

Evidence from studies with phenylisopropylamine hallucinogens indicates that the 5HT2A receptor is the likely target for the initiation of events leading to hallucinogenic activity associated with LSD and related drugs. Recently, lisuride (a purported non-hallucinogenic congener of LSD) was reported to be a potent antagonist at the 5HT2C receptor and an agonist at the 5HT2A receptor. LSD exhibited agonist activity at both receptors. These data were interpreted as indicating that the 5HT2C receptor might be the initiating site of action for hallucinogens. To test this hypothesis, recombinant cells expressing 5HT2A and 5HT2C receptors were used to determine the actions of LSD and lisuride. LSD and lisuride were potent partial agonists at 5HT2A receptors with EC50 values of 7.2 nM and 17 nM, respectively. Also, LSD and lisuride were partial agonists at 5HT2C receptors with EC50 values of 27 nM and 94 nM, respectively. We conclude that lisuride and LSD have similar actions at 5HT2A and 5HT2C receptors in recombinant cells. As agonist activity at brain 5HT2A receptors has been associated with hallucinogenic activity, these results indicate that lisuride may possess hallucinogenic activity, although the psychopharmacological effects of lisuride appear to be different from the hallucinogenic effects of LSD.

Creation of a constitutively activated state of the 5-HT2A receptor by site-directed mutagenesis: revelation of inverse agonist activity of antagonists.

Constitutively active GPCR have revealed novel properties of drugs that exhibit classical competitive antagonism at the native forms of GPCR. These drugs reverse basal levels of constitutive activity, indicating that they have inverse agonist activity. We were interested in determining if competitive antagonists of the native 5-HT2A receptor, in particular, antipsychotic drugs, exhibit inverse agonist activity at the constitutively active 5-HT2A receptor. All of the drugs tested reduced basal IP production of constitutively active 5-HT2A receptors, indicating that they all exhibited inverse agonist activity. Risperidone and ketanserin produced the greatest inhibition of basal IP production resulting in a reduction of basal activity in the C322K mutant receptor of 82% and 80%, respectively. Antipsychotic drugs display inverse agonist activity, indicating that stabilization of the inactive conformation of the 5-HT2A receptor may be a key component of their mechanism of action.

Pharmacological characterization of the constitutively activated state of the serotonin 5-HT2C receptor.

Previous studies from our laboratory have shown that the 5-HT2C serotonin receptor can be rendered constitutively active by changing amino acid 312 (third intracellular loop) from serine to lysine (S312K). In the present study, detailed radioligand binding analyses were performed to characterize the constitutively activated state of S312K mutant receptors. All agonists tested displayed high affinity for both [3H]5-HT and [3H]mesulergine binding to S312K receptors, but displayed low affinity for [3H]mesulergine binding to native 5-HT2C receptors. [3H]5-HT labeled the same total number of S312K binding sites as [3H]mesulergine. 5-HT2C antagonists inhibited S312K basal inositol phosphate production. These results suggest that S312K receptors mimic the active conformation of native 5-HT2C receptors and provide a good model system for evaluating drugs for inverse agonist activity. Also, S312K receptors may represent a new system for screening 5-HT2C agonist activity by comparing [3H]mesulergine binding to native and S312K mutant receptors.