Ketanserin and spiperone as templates for novel serotonin 5-HT(2A) antagonists

The structures of ketanserin (1) and spiperone (2) were examined in detail to determine the role of various substituent groups on 5-HT(2A) receptor affinity and selectivity. It was found that the presence of the quinazoline ring of ketanserin detracts from selectivity and that various ring-opened analogs displayed ketanserin-like affinity and up to 30-fold enhanced selectivity. The triazaspirodecanone portion of spiperone is a major determinant of its 5-HT affinity and selectivity. The conformational rigidity imposed by the ring, as well as the nature of the N(1)-substituent, are important factors in controlling binding at 5-HT(2A), 5-HT(2C), 5-HT(1A), and dopamine D2 receptors. Replacement of the N(1)-phenyl ring of spiperone with a methyl group (KML-010; 48) resulted in a compound that binds at 5-HT(2A) receptors with slightly lower affinity than spiperone, but that lacked affinity (Ki >10,000 nM) for 5-HT(2C) and 5-HT(1A) receptors and binds with 400-fold reduced affinity at D2 receptors.

Constitutive activity of G-protein coupled receptors: emphasis on serotonin receptors

Several lines of evidence indicate that G-protein coupled receptors (GPCR) may exist in a state that allows a tonic level of stimulation in vivo (constitutive activity). Several native forms of GPCR, when expressed in recombinant cell lines, display significant signal transduction stimulation in the absence of activating ligand. Many GPCR, including three serotonin receptors, display robust constitutive activation upon the mutation of a single amino acid, indicating mutations producing inappropriate constitutive activation may be etiological factors in diseases. If constitutive activity of GPCR is as common a phenomenon as some researchers suspect, this would suggest significant alterations in the classical model of ligand-receptor interactions. One of the most significant implications of constitutive activity for pharmacologists and medicinal chemists, is the possibility of developing drugs that lower the level of constitutive activity. Such compounds have been termed inverse agonists . These drugs, in theory, would have different physiological effects, and therefore possibly different therapeutic potential, than classical competitive receptor antagonists ( neutral antagonists ). Theoretical issues concerning constitutive activity in the GPCR family and some of the evidence supporting the existence of constitutive activity in the GPCR family is reviewed. Studies are presented demonstrating the procedures for producing and characterizing constitutive activated forms of serotonin receptors, including the demonstration of inverse agonist activity of drugs on these receptors.

Evaluation of isotryptamine derivatives at 5-HT(2) serotonin receptors

On the basis that meta-chlorophenylpiperazine (mCPP; 1) is a nonselective 5-HT(2C) agonist, that benz-fused tryptamines (e.g., 5) display enhanced 5-HT(2) affinity, and that certain isotryptamines 3 reportedly bind with enhanced affinity and selectivity at 5-HT(2C) receptors, we prepared and examined a series of isotryptamine-related analogues as potentially selective 5-HT(2C) agonists. None of the compounds displayed selectivity for 5-HT(2C) versus 5-HT(2A) receptors. Detailed re-examination of a compound previously reported to display 100-fold 5-HT(2C) selectivity [i.e., S(+)-5,6-difluoro-alpha-methylisotryptamine] revealed that its selectivity versus 5-HT(2A) receptors was, at best, only 10-fold.

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.

The binding of arylguanidines at 5-HT(3) serotonin receptors: a structure-affinity investigation

The 5-HT(3) receptor binding affinities of nine pairs of aryl-substituted arylguanidines and arylbiguanides were examined and the results suggest the likelihood that both classes of agents utilize common receptor binding features. The effects of structural modification were also examined using CoMFA. 1-(3,4,5-Trichlorophenyl)guanidine (5-HT(3) K(i)=0.7 nM) was identified as a very high-affinity arylguanidine. The structures of the high-affinity arylguanidines are inconsistent with current 5-HT(3) pharmacophore models.

Inverse agonist activity of atypical antipsychotic drugs at human 5-hydroxytryptamine2C receptors

Clozapine is the prototype atypical antipsychotic drug, producing little or no extrapyramidal side effects, while improving negative symptoms of psychosis. Clozapine's high affinity for serotonin receptors has been hypothesized to confer the unique antipsychotic properties of this drug. Recently, we demonstrated that both typical and atypical antipsychotic drugs are inverse agonists at constitutively active 5-hydroxytryptamine2A (5-HT(2A)) receptors. To determine whether inverse agonist activity at 5-HT(2C) receptors plays a role in antipsychotic efficacy, typical and atypical antipsychotic drugs were tested for inhibition of basal inositol phosphate production in mammalian cells expressing rat or human 5-HT(2C) receptors. Atypical antipsychotic drugs (sertindole, clozapine, olanzapine, ziprasidone, risperidone, zotepine, tiospirone, fluperlapine, tenilapine) displayed potent inverse agonist activity at rat and human 5-HT(2C) receptors. Typical antipsychotic drugs (chlorpromazine, loxapine, thioridazine, prochlorperazine, perphenazine, mesoridazine, trifluperidol, fluphenazine, spiperone, haloperidol, pimozide, penfluridol, thiothixene) were devoid of inverse agonist activity, with the exception of loxapine. We review the evidence that loxapine has unique properties characteristic of both atypical and typical antipsychotic drugs. Several typical antipsychotic drugs (chlorpromazine, thioridazine, spiperone, thiothixene) displayed neutral antagonist activity by reversing clozapine inverse agonism. These data suggest that 5-HT(2C) inverse agonist activity is associated with atypical antipsychotic drugs with moderate to high affinity for 5-HT(2C) receptors, and imply that effects of atypical antipsychotic drugs on the 5-HT(2C) receptor may play a role in their unique clinical properties. These data also imply that dysfunction of brain 5-HT(2C) receptor systems may be one of the factors involved in the etiology of psychosis.

Behavioural sensitization to cocaine is dissociated from changes in striatal NMDA receptor levels

Changes in glutamate transmission and alterations in glutamate receptor expression produced by the repeated administration of psychomotor stimulant drugs are considered an important neuroadaptation underlying the development and expression of behavioural and neurochemical sensitization to stimulant drugs. Two parallel experiments investigated the effects of repeated cocaine administration (five, once daily injections of 15 mg/kg, i.p.; 2 weeks withdrawal) on the expression of behavioural sensitization in response to a cocaine challenge (20 mg/kg, i.p.) and the changes in NMDA receptor binding in pooled tissue from the nucleus accumbens and the striatum. Compared with acute cocaine controls (n = 11), animals administered cocaine repeatedly displayed a sensitized stereotypic response to the cocaine challenge injection (n = 8). Despite this, no differences in either NMDA receptor density or affinity were observed between rats administered repeatedly with cocaine or saline, as indexed by [3H]MK-801 binding. The present findings call to question the rationale for NMDA receptor-based pharmacotherapies for the treatment of the enduring symptomatology of stimulant addiction.

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.

Binding of beta-carbolines and related agents at serotonin (5-HT(2) and 5-HT(1A)), dopamine (D(2)) and benzodiazepine receptors

A large series of beta-carbolines was examined for their ability to bind at [3H]agonist-labeled 5-HT(2A) serotonin receptors. Selected beta-carbolines were also examined at 5-HT(2C) serotonin receptors, 5-HT(1A) serotonin receptors, dopamine D(2) receptors, and benzodiazepine receptors. Indolealkylamines and phenylisopropylamines were also evaluated in some of these binding assays. The beta-carbolines were found to bind with modest affinity at 5-HT(2A) receptors, and affinity was highly dependent upon the presence of ring substituents and ring saturation. The beta-carbolines displayed little to no affinity for 5-HT(1A) serotonin receptors, dopamine D(2) receptors and, with the exception of beta-CCM, for benzodiazepine receptors. Examples of beta-carbolines, indolealkylamines (i.e. N,N-dimethyltryptamine analogs), and phenylisopropylamines have been previously shown to produce common stimulus effects in animals trained to discriminate the phenylisopropylamine hallucinogen DOM (i.e. 1-(2, 5-dimethoxy-4-methylphenyl)-2-aminopropane) from vehicle. Although the only common receptor population that might account for this action is 5-HT(2A), on the basis of a lack of enhanced affinity for agonist-labeled 5-HT(2A) receptors, as well as on their lack of agonist action in the PI hydrolysis assay, it is difficult to conclude that the beta-carbolines behave in a manner consistent with that of other classical hallucinogens.

Agonist high and low affinity state ratios predict drug intrinsic activity and a revised ternary complex mechanism at serotonin 5-HT(2A) and 5-HT(2C) receptors

The ternary complex model as applied to G-protein coupled receptors (GPCR) predicts that an agonist binds with low affinity (K(L)) to the free receptor (R), leading to an agonist/receptor/G-protein complex. This ternary complex displays high agonist affinity (K(H)), resulting in signal transduction. Classical dogma states that the ratio K(L)/K(H) predicts intrinsic activity of drugs: the higher the ratio the higher the intrinsic activity. This model was based on studies in which K(L) and K(H) were indirectly determined by computer analyses of antagonist radioligand binding data. In order to investigate the relationship of K(L), K(H), and intrinsic activity for agonists at 5-HT(2A) and 5-HT(2C) receptors, we utilized (3)H-agonist and (3)H-antagonist radioligands to directly determine K(H) and K(L). Comparisons of the log K(L)/K(H) ratios and intrinsic activities of drugs for stimulating intracellular phosphatidylinositol (PI) hydrolysis revealed a strong correlation for 5-HT(2A) (r(2) = 0.92) and 5-HT(2C) (r(2) = 0.96) receptors. The data were fit to computer simulations based on the original ternary complex model and the revised ternary complex model in which an activated state of the receptor (R*) exists in equilibrium with the resting state of the receptor (R). Data produced for both 5-HT(2A) and 5-HT(2C) receptors were better-fitted to a revised ternary complex model, rather than the classical ternary complex model. These data support a revised model for the molecular events coupling GPCR to activation of G-proteins and indicate that a strong correlation between the K(L)/K(H) ratio and intrinsic activity for agonist action at GPCR is consistent with the existence of R*.

Beta-blocker selectivity at cloned human beta 1- and beta 2-adrenergic receptors

The ratio between the affinities of beta-blockers for the beta2- and beta1-receptors is often used to predict the cardioselectivity and the potential consequences of blocking beta2-receptor-mediated effects of adrenergic receptor blockers. These ratios have been traditionally determined using various in vitro models of beta2 and beta1-receptor antagonist activity, including isolated organ preparations and radioligand binding in tissues from various species. The data from these studies, while useful, are complicated by the use of different preparations, techniques, and nonhuman models. Recombinant cell lines expressing human beta2 and beta1 receptors have been developed, allowing for the direct comparison of the affinities of the beta-blockers for the beta2 and beta1 receptors under identical conditions, and allowing a precise determination of the beta1-receptor selectivity of the beta-blockers. Bisoprolol, atenolol, propranolol, betaxolol, metoprolol, carvedilol, and ICI 118, 551 were compared for their beta-receptor selectivity using membranes prepared from recombinant cells selectively expressing human beta2 and beta1 receptors. Bisoprolol was found to have the highest selectivity for the beta1 receptor, displaying a beta2/beta1 ratio of 19 (a 19-fold higher affinity for the beta3 receptor than for the beta2 receptor). Atenolol, metoprolol, and betaxolol displayed lower, selectivity for the beta1 receptor, whereas propranolol and carvedilol displayed no significant beta-adrenergic selectivity. ICI 118,55 was selective for the beta2 receptor. The equilibrium dissociation constants of the beta-blockers for the beta1 and beta2 receptors were generally similar to previously reported values. The affinity ratios were also generally similar to previously reported values.

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.

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.

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.

Creation of a constitutively activated state of the 5-hydroxytryptamine2A receptor by site-directed mutagenesis: inverse agonist activity of antipsychotic drugs

Single amino acid mutations in the third intracellular loop, as well as other domains of G protein-coupled receptors, have been shown to confer drastic changes in receptor properties and have been postulated to be responsible for various disease states. To determine whether an amino acid mutation can confer dramatic alterations in the 5-hydroxytryptamine2A (5-HT2A) receptor, we mutated amino acid 322 to lysine (C322K), glutamate (C322E) or arginine (C322R). Transient expression of the mutant receptors revealed properties associated with constitutive activity. Radioligand binding studies revealed an increase in 5-HT affinity from 293 nM (native) to 86 nM (C322E), 25 nM (C322K) and 11 nM (C322R). 5-HT potency for stimulation of inositol phosphate production increased from 152 nM (native) to 61 nM (C322E) and 25 nM (C322K). Basal inositol phosphate levels in COS-7 cells expressing C322K and C322E mutant receptors were 8-fold and 4-fold higher, respectively, than cells expressing native 5-HT2A receptors. Basal levels of inositol phosphate stimulated by C322K receptors represented 48% of total inositol phosphate production stimulated by native receptors in the presence of 10 microM 5-HT. Antipsychotic drugs (chlorpromazine, clozapine, haloperidol, loxapine and risperidone) displayed inverse agonist activity by inhibiting C322K constitutive activation of phosphatidylinositol hydrolysis. These data indicate that amino acid 322 in the 5-HT2A receptor plays an important role in maintaining the inactive conformation and provide further evidence that amino acid mutations can produce profound alterations in G protein-coupled receptor activity.

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.

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.

Investigation of hallucinogenic and related beta-carbolines

Certain beta-carbolines are known to be hallucinogenic in humans, and several produce stimulus effects in animals similar to those of the classical hallucinogen 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM). Classical hallucinogens bind at 5-HT2 serotonin receptors and these receptors are thought to play a role in their mechanism of action. In the present study, we examined the binding of 15 beta-carbolines at rat 5-HT2A and 5-HT2C receptors. Affinities (Ki values) of the beta-carbolines ranged from about 100 nM to greater than 10,000 nM depending upon the degree of saturation of the pyridyl ring, and upon the presence and location of methoxy substituents in the benzenoid ring. In a further study, six rats were trained to discriminate the hallucinogenic beta-carboline harmaline (3.0 mg/kg, i.p.) from vehicle using a VI-15s schedule of reinforcement. This represents the first time a hallucinogenic beta-carboline has been used as a training drug in a drug discrimination study. Administration of DOM to the harmaline-trained animals resulted in 76% harmaline-appropriate responding at 1.25 mg/kg DOM and disruption of behavior at a higher dose. Taken together, the results of the present investigation demonstrate that: (a) certain beta-carbolines bind at 5-HT2 receptors; (b) that harmaline serves as a training drug at 3.0 mg/kg in drug discrimination studies with rats as subjects; and that (c) there is some similarity between the stimulus effects produced by harmaline and DOM.

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.

Activating mutations of the serotonin 5-HT2C receptor

Site-directed mutagenesis was performed to create a mutant serotonin 5-HT2C receptor that would mimic the active conformation of the native receptor. Structural alteration of receptor conformation was achieved by changing amino acid no. 312 from serine to phenylalanine (S312F) or lysine (S312K). After expression in COS-7 cells, the binding affinity of 5-HT for [3H]mesulergine-labeled 5-HT2C receptors increased from 203 nM (native) to 76 nM for S312F and 6.6 nM for S312K mutant receptors. 5-HT potency for stimulation of phosphatidylinositol (PI) hydrolysis increased from 70 nM (native) to 28 nM for S312F and 2.7 nM for S312K mutant receptors. The mutant receptors were constitutively active, stimulating PI hydrolysis in the absence of agonist. S312F and S312K mutations resulted in twofold and five-fold increases, respectively, in basal levels of PI hydrolysis. Mianserin and mesulergine displayed inverse agonist activity by decreasing basal levels of PI hydrolysis stimulated by S312K mutant receptors. [3H]5-HT and [3H]mesulergine labeled the same number of S312K mutant receptors and 5'-guanylylimidodiphosphate had no effect on [3H]5-HT binding. These results indicate that serine --> lysine mutation at amino acid no. 312 produces an agonist high-affinity state of the 5-HT2C receptor that spontaneously couples to G proteins and stimulates PI hydrolysis in the absence of agonist.

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.

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.

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.

Radioligand-binding study of noribogaine, a likely metabolite of ibogaine

Radioligand-binding studies were performed to ascertain the actions of noribogaine, a suspected metabolite of ibogaine, on opioid receptors. Consistent with previous results, ibogaine showed highest affinity for kappa opioid receptors (Ki = 3.77 +/- 0.81 microM), less affinity for mu receptors (Ki = 11.04 +/- 0.66 microM) and no affinity for delta receptors (Ki > 100 microM). Noribogaine showed a higher affinity than ibogaine for all of the opioid receptors: kappa Ki = 0.96 +/- 0.08 microM, mu Ki = 2.66 +/- 0.62 microM and delta Ki = 24.72 +/- 2.26 microM. These data suggest that noribogaine is active in vivo and that it may contribute to ibogaine's pharmacological effects.