Binding of β-carbolines and related agents at serotonin (5-HT₂ and 5-HT_1A), dopamine (D₂) and benzodiazepine receptors

Abstract

A large series of β-carbolines was examined for their ability to bind at [³H]agonist-labeled 5-HT_2A serotonin receptors. Selected β-carbolines were also examined at 5-HT_2C serotonin receptors, 5-HT_1A 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-HT_2A 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-HT_1A 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-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 β-carbolines behave in a manner consistent with that of other classical hallucinogens.

Introduction

Classical or arylalkylamine hallucinogens are divided into two broad chemical families: the indolealkylamines and the phenylalkylamines (reviewed: Glennon, 1998). The indolealkylamine category is comprised of (a) the ergolines (or lysergamides), such as derivatives of (+)-lysergic acid diethylamide (LSD), (b) the α-alkyltryptamines, such as 5-methoxy-α-methyltryptamine, and (c) the simple N-substituted tryptamines, such as N,N-dimethyltrptamine (DMT) (Fig. 1). The phenyl-alkylamine hallucinogens are divided into the (a) phenylethylamines, such as mescaline, and (b) the phenylisopropylamines, such as 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM). In tests of stimulus generalization with animals trained to discriminate DOM from vehicle, examples of agents from each category have been demonstrated to produce similar stimulus effects (reviewed: Glennon, 1996). In fact, this represents one of the criteria used for categorizing an agent as a classical hallucinogen (Glennon, 1998).

Although carefully controlled human studies are lacking, certain β-carbolines are considered to be hallucinogenic in humans (see Grella et al., 1998 for a recent overview). Like the above indolealkylamine hallucinogens, β-carbolines possess an indoleethylamine moiety embedded in their structures and, chemically, represent conformationally restricted analogs of the simpler N-substituted tryptamines. Certain β-carbolines, although not a major drug abuse problem in this country, have been used in plant-derived forms by South American natives for centuries (e.g. Deulofeu, 1967, Schultes, 1967, Rivier and Lindgren, 1972, Grob et al., 1996). Harmine, harmaline, and tetrahydroharmine are among the more common β-carbolines (Fig. 1). Harmine and harmaline have probably received the most attention and their hallucinogenic potencies are reported to be in the range of DMT (Pennes and Hoch, 1957, Naranjo, 1967, Naranjo, 1969, Naranjo, 1973). Harmaline and 6-methoxyharmalan have been shown to substitute for DOM in DOM-trained animals (Glennon et al., 1983).

Classical hallucinogens are thought to produce their behavioral effects, at least in part, via interaction with 5-HT₂ serotonin receptors in the brain (reviewed: Glennon, 1998). Although classical hallucinogens bind at multiple populations of 5-HT₂ receptors (i.e. 5-HT_2A and 5-HT_2C receptors), evidence is mounting that 5-HT_2A receptors are the primary targets of these agents (for further discussion see: Glennon, 1998, Nelson et al., 1999). We have investigated the binding of a limited series of β-carbolines and have shown that certain of these also bind at [³H]ketanserin-labeled 5-HT_2A receptors (Grella et al., 1998).

5-HT_2A receptors are thought to exist in two affinity states (i.e. a high affinity state and a low affinity state) (Battaglia et al., 1984). Radiolabeled antagonists, such as [³H]ketanserin, label both states of the receptor whereas radiolabeled agonists label the high affinity state. Binding of β-carbolines at the agonist-labeled high-affinity state of 5-HT_2A receptors has not been previously examined. Thus, it was of interest to determine the affinities of β-carbolines at these receptors. That is, if β-carbolines are 5-HT_2A agonists, they should possess a higher affinity at agonist-labeled sites than at antagonist-labeled sites, and such information would serve as a preliminary indicator of functional activity.

Certain β-carbolines produce effects in humans that are similar to more established hallucinogens such as LSD; nevertheless, the effects produced by these β-carbolines allow them to be distinguished from non-β-carboline hallucinogens (Pennes and Hoch, 1957, Naranjo, 1973). It is possible that hallucinogens might, in addition to their interaction at 5-HT₂ receptors, produce some of their effects via other mechanisms. Although phenylalkylamine hallucinogens have not been demonstrated to bind with significant affinity at any population of receptors other than 5-HT₂ receptors, indolealkylamines are known to bind with high affinity at 5-HT_1A receptors (Glennon, 1996). In fact, it has been suggested that 5-HT_1A receptors may play a prominent role in the actions of hallucinogenic indolealkylamines (e.g. Deliganis et al., 1991). In addition to high affinity for 5-HT_1A and 5-HT_2A receptors, LSD binds with high affinity at dopamine D₂ receptors and displays dopamine agonist character (e.g. Giacomelli et al., 1998). The binding of various phenylalkylamines at 5-HT_1A serotonin receptors or D₂ dopamine receptors has not been investigated. Nevertheless, there has been long-standing speculation that these receptor populations may play a role in mediating the behavioral effects of classical hallucinogens as a class. These issues have never been satisfactorily addressed.

Finally, certain nonhallucinogenic β-carbolines (e.g. β-CCM or methyl β-carboline-3-carboxylate) bind with high affinity at benzodiazepine (BZ) receptors; some possess anxiogenic character and others display anxiolytic properties (e.g. Hollinshead et al., 1990, Allen et al., 1992, Cox et al., 1998). Certain hallucinogenic agents are also known to produce anxiety (Strassman, 1984). Given that the β-carboline hallucinogens are closely related in structure to the β-carboline anxiogenic and anxiolytic agents, it was of interest to determine if they, too, bind at BZ receptors.

Thus, the primary purpose of the present investigation was to systematically examine the binding of a series of hallucinogen-related β-carbolines at (a) agonist-labeled 5-HT_2A serotonin receptors (b) 5-HT_2C serotonin receptors (c) 5-HT_1A serotonin receptors (d) D₂ dopamine receptors, and (e) benzodiazepine (BZ) receptors to determine whether or not they bind at these receptor populations and, where possible, to formulate structure–affinity relationships. A systematic binding study, such as that described herein, has not been previously reported for the β-carbolines; this is probably due to a lack of ready availability of many of the β-carbolines. Consequently, most of the necessary β-carbolines were synthesized expressly for the purpose of these investigations. A second goal of this work was to determine if 5-HT₂ serotonin receptors, 5-HT_1A serotonin receptors, or dopamine D₂ receptors better account for the common actions produced by indolealkylamines and phenylalkylamines. To this extent, radioligand binding data were also obtained for selected DMT analogs and phenylisopropylamines for purpose of comparison.