Małgorzata Dukat

Binding of β-carbolines and related agents at serotonin (5-HT2 and 5-HT1A), dopamine (D2) and benzodiazepine receptors

A large series of β-carbolines was examined for their ability to bind at [3H]agonist-labeled 5-HT2A serotonin receptors. Selected β-carbolines were also examined at 5-HT2C serotonin receptors, 5-HT1A serotonin receptors, dopamine D2 receptors, and benzodiazepine receptors. Indolealkylamines and phenylisopropylamines were also evaluated in some of these binding assays. The β-carbolines were found to bind with modest affinity at 5-HT2A receptors, and affinity was highly dependent upon the presence of ring substituents and ring saturation. The β-carbolines displayed little to no affinity for 5-HT1A serotonin receptors, dopamine D2 receptors and, with the exception of β-CCM, for benzodiazepine receptors. Examples of β-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-HT2A, on the basis of a lack of enhanced affinity for agonist-labeled 5-HT2A receptors, as well as on their lack of agonist action in the PI hydrolysis assay, it is difficult to conclude that the β-carbolines behave in a manner consistent with that of other classical hallucinogens.

N1-(Benzenesulfonyl)tryptamines as novel 5-HT6 antagonists

N-Benzenesulfonyl-5-methoxy-N,N-dimethyltryptamine (BS/5-OMe DMT; 5) was shown to bind at human 5-HT6 serotonin receptors with high affinity (Ki = 2.3 nM) relative to serotonin (Ki = 78 nM). Structural variation failed to result in significantly enhanced affinity. BS/5-OMe DMT acts as an antagonist of 5-HT-stimulated adenylate cyclase (pA2 = 8.88 nM) and may represent the first member of a novel class of 5-HT6 antagonists.

Central nicotinic receptor ligands and pharmacophores

Multiple populations of pentameric nicotinic acetylcholinergic (nACh) receptors exist and several may be classified as central or neuronal. Neuronal nACh receptors, however, are primarily of the alpha 4 beta 2 and alpha 7 types, and these have been the focus of most recent investigations aimed at the development of novel agents and identification of pharmacophores. Selectivity data are limited. Furthermore, because several populations of nACh receptors might indirectly influence a given functional effect, it is difficult to discuss structure-activity relationships (SAR) in terms of differential SAR, or to formulate SAR on the basis of functional studies. For the most part, studies are limited to the formulation of structure-affinity relationships (SAFIR) for the binding of agents at nACh receptors, and for these the alpha 4 beta 2 population has been the most extensively investigated. SAFIR and newer agents are reviewed here with reference to earlier studies. Novel agents now have been identified that bind with up to 30 times higher affinity than nicotine and these are providing new insight into the understanding of nACh receptors.

Serotonin receptors and their ligands: A lack of selective agents

Four major families of serotonin (5-hydroxytryptamine; 5-HT) receptors have been identified: 5-HT1, 5-HT2, 5-HT3 and 5-HT4. At this time, there is a general consensus that the 5-HT1 family can be further subdivided into 5-HT1A, 5-HT1B, 5-HT1C, 5-HT1D, and 5-HT1P subpopulations. In addition, there are several other populations of less well-defined 5-HT receptors. The purpose of this presentation is to discuss 5-HT receptor nomenclature and the agents that are commonly used to investigate each receptor population in as much as it will serve to provide background for the remainder of the symposium. There is presently available an abundance of serotonergic agents; however, these agents are only semiselective, and none can be considered truly selective for a particular population of 5-HT receptors. As useful as these agents have been for the identification and characterization of 5-HT receptors, there remains a need for the development of new, more selective ligands.

Investigation of hallucinogenic and related β-carbolines

Abstract Certain β-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 β-carbolines at rat 5-HT2A and 5-HT2C receptors. Affinities (Ki values) of the β-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 β-carboline harmaline (3.0 mg/kg, i.p.) from vehicle using a VI-15s schedule of reinforcement. This represents the first time a hallucinogenic β-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 β-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.

Pyrrolidine-modified and 6-substituted analogs of nicotine: A structure-affinity investigation

Summary Because the structural requirements for the binding of nicotine to central nicotine receptors remain largely uninvestigated, we undertook a systematic investigation of pyrrolidine ring-opened analogs. This led to a subsequent investigation of related conformationally restricted derivatives of these analogs. The results are reported relative to the binding of several well-known and widely used nicotine receptor ligands. Although none of the ring-opened analogs binds with higher affinity than (−)nicotine (Ki = 2.3 nM), 3-(N-methyl-N-ethylaminomethyl)pyridine (12a; Ki = 28 nM) binds with significant affinity. A conformationally restricted analog of 12a, N-methyl [2,7]naphthyridine 30b (Ki = 18 nM), binds with similar affinity. 6-Substitution of 12a and racemic nicotine seems to be tolerated when the substituent is halogen or methyl. In functional studies (hypolocomotion and antinociception in mice; stimulus generalization in nicotine-trained rats) 30b retains nicotine-like properties. Several of the 6-substituted compounds were 2 to 20 times more potent than (±)nicotine. Although the intact pyrrolidine ring of nicotine appears important for optimal affinity, its presence is not an absolute requirement for activity, and 6-position substitution of the pyridine nucleus can influence both binding and functional activity.

Ketanserin and Spiperone as Templates for Novel Serotonin 5-HT2A 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.

Binding of Serotonin and N1-Benzenesulfonyltryptamine-Related Analogs at Human 5-HT6 Serotonin Receptors: Receptor Modeling Studies

A population of 100 graphics models of the human 5-HT6 serotonin receptor was constructed based on the structure of bovine rhodopsin. The endogenous tryptamine-based agonist serotonin (5-HT; 1) and the benzenesulfonyl-containing tryptamine-derived 5-HT6 receptor antagonist MS-245 (4a) were automatically docked with each of the 100 receptor models using a genetic algorithm approach. Similar studies were conducted with the more selective 5-HT6 receptor agonist EMDT (5) and optical isomers of EMDT-related analog 8, as well as with optical isomers of MS-245 (4a)-related and benzenesulfonyl-containing pyrrolidine 6 and aminotetralin 7. Although associated with the same general aromatic/hydrophobic binding cluster, 5-HT (1) and MS-245 (4a) were found to preferentially bind with distinct receptor conformations, and did so with different binding orientations (i.e., poses). A 5-HT pose/model was found to be common to EMDT (5) and its analogs, whereas that identified for MS-245 (4a) was found common to benzenesulfonyl-containing compounds. Specific amino acid residues were identified that can participate in binding, and evaluation of a sulfenamide analog of MS-245 indicates for the first time that the presence of the sulfonyl oxygen atoms enhances receptor affinity. The results indicate that the presence or absence of an N1-benzenesulfonyl group is a major determinant of the manner in which tryptamine-related agents bind at 5-HT6 serotonin receptors.

Musings on α4β2 Nicotinic Acetylcholine (nACh) Receptor Pharmacophore Models

Several pharmacophore models were previously formulated to account for the actions of nicotinic acetylcholinergic (nACh) agents. Most of these models were developed without express consideration of specific radioligand binding data because such data were not available at the time the models were described. In this review, the ability of these models to account for the binding of nicotinic agents at alpha4beta2 nACh receptors (or rat brain receptors for which alpha4beta2 receptors are the major component) is assessed. It seems that none of the early models can adequately explain the binding of these agents as a group. Furthermore, different series of nicotinic agents behave differently depending upon the nature of terminal amine substituents and the spacer that separates the amine from the pyridine ring. A region of bulk tolerance has been identified that accommodates substituents on some nicotinic ligands, but not the same substituents at seemingly corresponding locations of others. The concept of multiple modes of binding has been previously raised and, clearly, cannot yet be discarded. Nevertheless, new vector models seemingly provide a better picture of nACh receptor binding and account for many of the shortcomings associated with the earlier models.

Arylguanidine and arylbiguanide binding at 5-HT3 serotonin receptors: a QSAR study.

For a series of monosubstituted arylguanidines, 5-HT3 receptor affinity was found generally related to the electron withdrawing nature of the substituent at the aryl 3-position and the lipophilicity of the 4-position substituent. A broader examination of 35 arylguanidines and arylbiguanides revealed that affinity could be described by molecular polarizability, a Chi index term (8chiP), and the sum of all (-Cl) E-State values (SsCl) in the molecule.