In vitro pharmacology of ambroxol: Potential serotonergic sites of action.

AIMS: Ambroxol is a muco-active agent with multiple, clinically relevant effects in the airway. Despite its widespread use and well documented clinical efficacy, there are few data on its mechanism of action and receptor pharmacology beyond sodium channel blockade and inhibition of guanylate cyclase. Accordingly, in vitro studies were conducted to determine its overall receptor pharmacology and possible sites of action. MATERIALS AND METHODS: In vitro radioligand binding/enzyme inhibition studies were conducted at 62 receptors, ion channels and enzymes using standard techniques. Additional in vitro studies were conducted to establish the potency of ambroxol at selected sites. KEY FINDINGS: These studies indicate that ambroxol has affinity for the 5-HT3 serotonin receptor, as well as affinity for the 5-HT serotonin transporter (SERT), with IC50 values of 17,600 nM and 19,500 nM respectively. In vitro functional studies in isolated guinea pig colon indicate that ambroxol is a 5-HT3 serotonin receptor antagonist with an IC50 value of 36,000 nM. SIGNIFICANCE: Together, these studies indicate that ambroxol may exert its beneficial properties via antagonism of the 5-HT3 serotonin receptor and/or inhibition of serotonin uptake (5-HT transport: SERT), in addition to its reported effects at the sodium channel and guanylate cyclase.

A structure-activity relationship study of benzylic modifications of 4-[1-(1-naphthyl)ethyl]-1H-imidazoles on alpha 1- and alpha 2-adrenergic receptors.

The naphthalene analog of medetomidine (1), 4-[1-(1-naphthyl)ethyl]-1H- imidazole (2), is a highly potent, selective alpha 2-adrenoceptor agonist. We have initiated a structure-activity relationship study of the replacement of the methyl group on the carbon bridge between the naphthalene and imidazole rings of 2 with a hydrogen, hydroxy, methoxy, carbonyl, or trifluoromethyl group and compared their biological activities with medetomidine 1 and the optical isomers of 2. Analogs of 2 were antagonists of alpha 2A-adrenoceptor-mediated human platelet aggregation and agonists on alpha 1- and alpha 2-adrenoceptors in guinea pig ileum. The rank order and potencies of these analogs on platelets (alpha 2A-subtype) and guinea pig ileum (alpha 1-subtype) were nearly the same, whereas racemic and S-(+)-2, desmethyl, and hydroxy analogs were potent agonists on alpha 2-adrenoceptors in guinea pig ileum. With the exception of the desmethyl analog 5, none of the other analogs were as potent as the parent drug 2 on alpha 2A- (human platelets), alpha 1- (guinea pig ileum), or alpha 2- (guinea pig ileum) adrenergic receptor systems. As with analog 2, the desmethyl- and methoxy-substituted analogs retained a greater alpha 2/alpha 1-selectivity in both functional (agonist activity) and biochemical (receptor displacement) studies. Receptor binding studies indicate that S-(+)-2 possessed greater affinity than the R-(-)-isomer on both alpha 1- and alpha 2-adrenoceptors in rat brain. In addition, R-(-)-2 did not show agonist activity in alpha 2-adrenoceptors of guinea pig ileum and was 10-fold more potent than S-(+)-2 as an antagonist of alpha 2A-adrenoceptors in human platelets. Thus, the nature of the substituent and the chirality at the carbon bridge between the naphthalene and imidazole rings play an important role in maintaining potent alpha 2-adrenoceptor activity and high alpha 2/alpha 1-selectivity within the 4-substituted imidazole class.

Resolution and adrenergic activities of the optical isomers of 4-[1-(1-naphthyl)ethyl]-1H-imidazole.

Recently we synthesized a naphthalene analog of medetomidine, 4-[1-(1-naphthyl)ethyl]-1H-imidazole hydrochloride (1), and found it to be highly potent in adrenergic systems. The separation of optical isomers of this naphthalene analog was achieved by using the isomers of tartaric acid. The optical purities of the isomers were determined by HPLC using a chiral column. Using X-ray analysis the (+)-isomer was determined to have the S absolute configuration. It has been reported that the (+)-isomer of medetomidine (2) is the most potent enantiomer on alpha 2-adrenergic receptors. There were both qualitative and quantitative differences in biological activities of the optical isomers of 1 in alpha 1- and alpha 2-adrenergic receptor systems of guinea pig ileum and human platelets. (+)-(S)-1, but not (-)-(R)-1 was a selective agonist of alpha 2-mediated responses in ileum whereas (-)-(R)-1 was more potent than (+)-(S)-1 as an inhibitor of alpha 2-mediated platelet aggregation.

A structure-affinity study of the binding of 4-substituted analogues of 1-(2,5-dimethoxyphenyl)-2-aminopropane at 5-HT2 serotonin receptors.

With [3H]ketanserin as the radioligand, structure-affinity relationships (SAFIRs) for binding at central 5-HT2 serotonin receptors (rat frontal cortex) were examined for a series of 27 4-substituted 1-(2,5-dimethoxyphenyl)-2-aminopropane derivatives (2,5-DMAs). The affinity (Ki values) ranged over a span of several orders of magnitude. It appears that the lipophilic character of the 4-position substituent plays a major role in determining the affinity of these agents for 5-HT2 receptors, 2,5-DMAs with polar 4-substituents (e.g. OH, NH2, COOH) display a very low affinity (Ki greater than 25,000 nM) for these receptors, whereas those with lipophilic functions display a significantly higher affinity. The results of these studies prompted us to synthesize and evaluate examples of newer lipophilic derivatives and several of these (e.g. n-hexyl, n-octyl) bind with very high (Ki values = 2.5 and 3 nM, respectively) affinities at central 5-HT2 sites. Although, 2,5-DMAs are generally considered to be 5-HT2 agonists, preliminary studies with isolated rat thoracic aorta suggest that some of the more lipophilic derivatives (e.g. the n-hexyl and n-octyl derivatives) are 5-HT2 antagonists.

D2 dopamine receptors modulate calcium channel currents and catecholamine secretion in bovine adrenal chromaffin cells.

Although dopamine is known to be present in sympathetic ganglia, its role and mode of action as a peripheral neurotransmitter are still poorly understood. Dopaminergic agonists have been shown to inhibit adrenal catecholamine release and calcium uptake. However, the specific dopamine receptor subtype mediating these effects and the receptor transduction mechanism remain unknown. We now provide evidence demonstrating 1) that slowly inactivating, voltage-gated calcium channels serve as a target site for dopaminergic modulation of chromaffin cell function and 2) that it is the D2 receptor subtype which mediates dopaminergic inhibitory effects on catecholamine secretion, 45Ca uptake and voltage-gated calcium currents. Whole cell patch clamp electrophysiological techniques were used to monitor directly voltage-gated Ca++ channels. The D2 agonist apomorphine but not the D1 agonist SKF 38393 reduced reversibly a slowly inactivating, voltage-gated calcium current in cultured chromaffin cells and this effect was blocked by the D2 receptor antagonist haloperidol. The presence of D2 but not D1 dopamine receptors on chromaffin cell membranes was demonstrated by radioligand binding methods, using the specific D1 and D2 receptor radioligands, [3H]SCH23390 and [3H]N-methylspiperone, respectively. Nicotine- and KCl (60 mM)-evoked catecholamine secretion and 45Ca uptake were inhibited by the D2 agonist, apomorphine, but not by the D1 agonist, SKF 38393. These inhibitory effects were prevented by the D2 antagonist, sulpiride, but not by the D1 antagonist, SCH 23390. D2 dopamine receptors appear to function as inhibitory modulators of adrenal catecholamine secretion with a mode of action involving inhibition of calcium channel currents.

Formulary-control procedures in a staff-model health maintenance organization.

Efforts to promote cost-effective drug prescribing in a health maintenance organization (HMO) through (1) revision of the formulary, (2) education, and (3) drug-use evaluations (DUEs) are described. New programs and procedures to promote more cost-effective prescribing were implemented in 1987 at a staff-model HMO with seven health-care centers and 40,000 members. Support to the pharmacy and therapeutics committee in developing formulary recommendations was enhanced, the formulary was revised and the number of listed drugs reduced by half, the focus of pharmacists was changed from purchasing and inventory control to education of physicians about cost-effective prescribing, and the most expensive drug categories were identified and educational efforts instituted. During the next two years substantial changes in the prescribing of oral contraceptives, nonsteroidal anti-inflammatory drugs, antiulcer agents, and anti-infectives, but not antihypertensive drugs, were identified through DUEs. Evaluation of cost savings was limited by the lack of an automated, integrated patient database, and it was difficult to assess the effect of changes in patient population. Revision of the formulary coordinated with enhanced educational efforts and DUEs resulted in more cost-effective prescribing in an HMO without placing severe restrictions on physicians or patients.

Identification and characterization of B2 bradykinin receptors in sheep nasal turbinate membranes.

Because bradykinin (BK) has been implicated as a mediator of upper respiratory tract symptomatology, specific 3H-BK binding was investigated in membrane homogenates prepared from sheep nasal turbinate tissue in order to identify and characterize the BK receptor subtype(s) present. 3H-BK saturation and Scatchard analyses revealed a single, high affinity, saturable site (KD of 0.098 nM) with a density of 0.44 pmol/g wet weight tissue. Competition experiments using B1 and B2 receptor agents revealed a B2-BK receptor pharmacology; the B2 agents BK, Lys-BK, NPC-567, [D-Phe7]-BK and [Thi,5,8 D-Phe7]-BK displayed nM affinity while the B1 agents [des-Arg9]-BK and [Leu,8 des-Arg9]-BK competed in the uM range. The absolute and rank order of affinities in this tissue paralleled that found in the guinea pig ileum. No specific binding was found using the putative B1 receptor radioligand 3H-[des-Arg9]-BK. Specific B2-BK receptor binding was not effected by the addition of non-hydrolyzable guanine or adenine nucleotides. These data confirm the presence of B2-BK receptors in this tissue and provide support for a role of BK in nasal function.

Mu opiate receptors are selectively labelled by [3H]carfentanil in human and rat brain.

[11C]Carfentanil is a potent opioid agonist currently in use as a specific PET (position emission tomography) scan radioligand for brain mu opioid receptors. In order to investigate the receptor interactions of carfentanil in detail [3H]carfentanil was used as a radioligand for labelling receptors in rat and human brain tissue homogenates. [3H]Carfentanil was found to bind saturably and with high affinity (KD = 0.08 +/- 0.01 nM) to membranes prepared from human cortical (Bmax = 42 +/- 3 fmol/mg) and thalamic (Bmax = 84 +/- 3 fmol/mg) tissues and rat cortex (Bmax = 82 +/- 4 fmol/mg) and diencephalon (Bmax = 105 +/- 5 fmol/mg). Association (1.23 +/- 0.19 X 10(10) Mol-1 X min-1 and dissociation rate (0.19 +/- 0.03 min-1) constants were determined in human cortical tissues; results from studies in rat cortical, and rat diencephalon tissue homogenates produced similar kinetic rate constants. Competition studies with a variety of drugs indicated that [3H]carfentanil interacts primarily with mu opioid receptors in the four tissues studied; the affinities of a series of non-radioactive opioid ligands were essentially identical in the four tissues (correlation coefficients = 0.88-0.93). Naloxone, morphine, DAGO [( D-Ala2-MePhe4-Gly-ol5]enkephalin), DADL [( D-Ala2-D-Leu5]enkephalin) and EKC (ehtylketazocine) potently displaced specific [3H]carfentanil binding with nM potency while the kappa agonist U-69593, the sigma agonists (+)-SKF 10047, (+)-3-PPP [3-hydroxyphenyl)-N-propylpiperidine) and haloperidol and PCP (phencyclidine) were less potent displacing agents. The higher affinities of DAGO and morphine versus DADL for the [3H]carfentanil binding sites indicates that delta opioid receptors are not being labelled.(ABSTRACT TRUNCATED AT 250 WORDS)

A role for histamine and H2-receptors in opioid antinociception.

To investigate the role of brain H2-receptors in opioid antinociceptive mechanisms, the effects of several antagonists of histamine H2-receptors were determined on morphine (MOR)-induced antinociception, opioid-mediated footshock-induced antinociception (FSIA) and on other opiate effects in rats. Zolantidine dimaleate (ZOL), the first brain-penetrating H2 antagonist (0.03-1.6 mumol/kg s.c.) caused a dose-related inhibition of MOR antinociception in both the tail-flick and hot-plate tests, with no effect on base-line responding. ZOL also inhibited opioid FISA with a similar potency. MOR-induced locomotor activity was also reduced by ZOL, but no effect was seen on MOR-induced hyperthermia, catalepsy or lethality. ZOL (10(-5) M) was inactive at mu, delta or kappa opioid receptors and showed at least 35-fold higher affinity at the H2-receptor than at receptors for serotonin, dopamine, norepinephrine or acetylcholine in brain. To clarify further the role of H2-receptors in ZOL's antiopiate activity, the potencies of seven structural congeners of ZOL were determined on the H2-receptor and on MOR antinociception. Over 3 orders of magnitude, the rank order of potencies of the compounds for inhibiting MOR antinociception was highly correlated with their potencies as H2 antagonists. Cimetidine, unlike other H2 antagonists, potentiated MOR antinociception, potentiated opioid FSIA and increased brain MOR levels, actions that are not likely to be due to blockade of H2-receptors. These findings strongly suggest that stimulation of opioid receptors leads to antinociception by mechanisms that include activation of brain H2-receptors.

N-(phthalimidoalkyl) derivatives of serotonergic agents: a common interaction at 5-HT1A serotonin binding sites?

Several classes of agents are known to bind at central 5-HT1A serotonin sites In order to challenge the hypothesis that these agents bind in a relatively similar manner (i.e., share common aryl and terminal amine sites), we prepared N-(phthalimidobutyl) derivatives of examples of several such agents. With regard to arylpiperazines, we had previously shown that introduction of this functionality at the terminal amine is tolerated by the receptor and normally results in a significant (greater than 10-fold) enhancement in affinity. The results of the present study show that this bulky functionality is also tolerated by the receptor when incorporated into examples of all other major classes of 5-HT1A agents (e.g., 2-aminotetralin, phenylalklamine, indolylalkylamine, and (aryloxy)alkylamine derivatives). The length of the alkyl chain that separates the terminal amine from the phthalimido group is of major importance, and a four-carbon chain appears optimal. Alteration of the length of this chain can have a significant influence on affinity; decreasing the chain length from four to three carbon atoms can reduce affinity by an order of magnitude, and further shortening can have an even more pronounced effect.

Design and synthesis of propranolol analogues as serotonergic agents.

Serotonin (5-HT) binds with nearly identical affinity at the various central 5-HT binding sites. Few agents bind with selectivity for 5-HT1A sites. The beta-adrenergic antagonist propranolol binds stereoselectively both at 5-HT1A and 5-HT1B sites (with a several-fold selectivity for the latter) and, whereas it is a 5-HT1A antagonist, it appears to be a 5-HT1B agonist. As such, it could serve as a lead compound for the development of new 5-HT1A and 5-HT1B agents. The purpose of the present study was to modify the structure of propranolol in such a manner so as to reduce its affinity for 5-HT1B and beta-adrenergic sites while, at the same time, retaining its affinity for 5-HT1A sites. Removal of the side-chain hydroxyl group of propranolol, and conversion of its secondary amine to a tertiary amine, reduced affinity for 5-HT1B and beta-adrenergic sites. In addition, shortening the side chain by one carbon atom resulted in compounds with affinity for hippocampal 5-HT1A sites comparable to that of racemic propranolol, but with a 30- to 500-fold lower affinity for 5-HT1B sites and a greater than 1000-fold lower affinity for beta-adrenergic sites. The results of these preliminary studies attest to the utility of this approach for the development of novel serotonergic agents.

Hallucinogenic drug interactions at human brain 5-HT2 receptors: implications for treating LSD-induced hallucinogenesis.

It has been shown that the hallucinogenic potencies of LSD, the phenylisopropylamines, such as DOB (4-bromo-2,5-dimethoxyphenylisopropylamine) and DOI (4-iodo-2,5-dimethoxyphenylisopropylamine), and the indolealkylamines, such as DMT (dimethyltryptamine) and 5-OMe-DMT (5-methoxy-dimethyltryptamine), strongly correlate with their in vitro 5-HT2 receptor binding affinities in rat cortical homogenates. In order to ascertain if this correlation applies to human 5-HT2 receptors as well, we examined the affinities of 13 psychoactive compounds at 3H-ketanserin-labelled 5-HT2 receptors in human cortical samples. Both radioligand binding and autoradiographical procedures were used. As in rat brain, d-LSD was the most potent displacer of 3H-ketanserin specific binding with a Ki of 0.9 nM. The phenylisopropylamine DOI also displayed high affinity (Ki of 6 nM). Stereospecific interactions were found with DOB; (-) DOB had a Ki of 17 nM while (+) DOB had a Ki of 55 nM. The behaviorally active compound DOM (4-methyl-2,5-phenylisopropylamine) had an affinity of 162 nM while its behaviorally less active congener iso-DOM had an affinity of 6299 nM. The indolealkylamines 5-OMe-DMT and DMT competed with moderate affinities (207 and 462 nM, respectively). In general, Hill coefficients were significantly less than unity which is consistent with an agonist interaction with 5-HT2 receptors. MDMA, a substituted amphetamine analog was inactive with a Ki of greater than 10 microM. A strong correlation was found for the hallucinogen affinities and human hallucinogenic potencies (r = 0.97). Also, human and rat brain 5-HT2 receptor affinities were strongly correlated (r = 0.99). These results strongly support the hypothesis that the hallucinogenic effects of these drugs in humans are mediated in whole or in part via 5-HT2 receptors. Furthermore, these studies imply that treatment with 5-HT2 receptor antagonists may be effective in reversing the hallucinogenic effects caused by the ingestion of LSD and LSD-like drugs.

2-(Alkylamino)tetralin derivatives: interaction with 5-HT1A serotonin binding sites.

8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) is a selective 5-HT1A serotonin agonist. Derivatives of 8-OH-DPAT with amine substituents larger or more bulky than n-propyl appear to be inactive in a presynaptic biochemical assay measuring agonist-induced feedback inhibition of 5-HT synthesis but have never been examined in brain binding assays. A series of N-phenylalkyl derivatives of 8-methoxy-2-aminotetralin was evaluated at [3H]-8-OH-DPAT-labeled 5-HT1A sites in rat brain hippocampal membranes. All of the phenylalkyl derivatives displayed significant affinity for these sites and, of the agents examined, the 3-phenylpropyl 8-hydroxy analogue appears to be optimal and had an affinity (Ki = 1.9 nM) comparable to that of 8-OH-DPAT (Ki = 1.2 nM). In addition, the presence of an oxygen-containing substituent at the 8-position of the tetralin ring is not necessary for good affinity, and secondary amines and tertiary amines displayed equal affinity at central 5-HT1A binding sites. 5-HT1A sites are found both pre- and postsynaptically; thus, differences observed in the biochemical assay as compared to the results of the present binding study could be due to different structural requirements of these two receptors. This seems unlikely, however, because there was little difference in the affinities of several selected analogues for striatal versus hippocampal binding sites. Because we have now demonstrated that amine substituents larger than propyl, and an unsubstituted 8-position, are well tolerated by central 5-HT1A sites, future studies aimed at the development of new serotonergic tetralin analogues need not be limited to N-propyl or 8-hydroxy derivatives of 2-aminotetralin.

Arylpiperazine derivatives as high-affinity 5-HT1A serotonin ligands.

Although simple arylpiperazines are commonly considered to be moderately selective for 5-HT1B serotonin binding sites, N4-substitution of such compounds can enhance their affinity for 5-HT1A sites and/or decrease their affinity for 5-HT1B sites. A small series of 4-substituted 1-arylpiperazines was prepared in an attempt to develop agents with high affinity for 5-HT1A sites. Derivatives where the aryl portion is phenyl, 2-methoxyphenyl, or 1-naphthyl, and the 4-substituent is either a phthalimido or benzamido group at a distance of four methylene units away from the piperazine 4-position, display high affinity for these sites. One of these compounds, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine (18), possesses a higher affinity than 5-HT and represents the highest affinity (Ki = 0.6 nM) agent yet reported for 5-HT1A sites.

A preliminary investigation of the psychoactive agent 4-bromo-2,5-dimethoxyphenethylamine: a potential drug of abuse.

4-Bromo-2,5-dimethoxyphenethylamine (alpha-desMe DOB) is a psychoactive agent that may possess significant abuse potential. Because of its structural similarity to the established hallucinogen 1-(4-bromo-2,5-dimethoxyphenyl)-2-aminopropane (DOB), and because almost no pharmacological data are available on this agent, we undertook this preliminary investigation. alpha-DesMe DOB (Ki = 1 nM), like DOB itself (Ki = 0.79 nM), displays a high affinity for [3H]DOB-labeled central 5-HT2 serotonin receptors. However, unlike DOB, the alpha-desmethyl derivative also binds with significant affinity to 5-HT1A, 5-HT1B, and 5-HT1C serotonin receptors and, as such, is less selective than DOB. In drug discrimination studies using rats trained to discriminate either DOM (i.e., the 4-methyl analog of DOB) or R(-)DOB from saline, stimulus generalization occurred in both groups of animals. However, stimulus generalization was associated with extensive disruption of behavior, alpha-DesMe DOB may produce stimulus effects similar, but not identical, to those of DOM and R(-)DOB; in addition, this agent may be capable of producing other, as yet undefined, central effects at comparable doses. These other effects may be reflective of the lack of selectivity of alpha-desMe DOB for 5-HT2 serotonin receptors. Because other hallucinogenic agents display high affinity for 5-HT2 serotonin receptors and result in stimulus generalization in DOM- and/or DOB-trained animals, it is tentatively concluded that alpha-desMe DOB is a psychoactive agent with at least some hallucinogenic or DOB-like properties.

Correlated asymmetries in striatal D1 and D2 binding: relationship to apomorphine-induced rotation.

Long-Evans derived rats were tested for nocturnal, amphetamine-induced and apomorphine-induced rotation (circling behavior); the rats' left and right striata were subsequently dissected and D1 and D2 receptor densities (Bmax) were assayed in the same striatal homogenates using [3H]SCH-23390 and [3H]N-methylspiperone, respectively. D1 and D2 Bmax values were correlated (r = 0.68). Moreover, left-right asymmetries in D1 and D2 Bmax values were more highly correlated (r = 0.84). Although asymmetries in D1 and D2 binding were not by themselves related to rotational behavior, an asymmetry in the ratio or balance of D1 and D2 binding was associated with the direction of apomorphine-induced rotation: the D1/D2 ratio of Bmax values was significantly higher in the striatum ipsilateral to the preferred direction of apomorphine-induced rotation. These results suggest that normal variations in numbers of D1 and D2 receptors are determined by a common mechanism, that D1 and D2 receptors are functionally coupled, and that, with respect to activation of striatal receptors, D1 is inhibitory and D2 is excitatory. The effects of apomorphine, a mixed D1 and D2 agonist, appear to reflect the balance between D1 and D2 receptors.

N,N-di-n-propylserotonin: binding at serotonin binding sites and a comparison with 8-hydroxy-2-(di-n-propylamino)tetralin.

8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) is a serotonergic agonist with high affinity and selectivity for a particular population of central serotonin (5-HT) binding sites (i.e., 5-HT1A sites). Because the selectivity of 8-OH-DPAT may be due to the terminal amine substituents, the di-n-propyl analogue of 5-HT (i.e., 4) and of 5-methoxytryptamine (i.e., 5) were prepared and compared with 8-OH-DPAT with respect to their binding profile. Unlike 8-OH-DPAT, neither compound 4 nor 5 displays selectivity for 5-HT1A vs 5-HT2 sites. Consistent with these results, stimulus generalization occurs with 5 both in rats trained to discriminate 8-OH-DPAT from saline and in rats trained to discriminate the 5-HT2 agonist DOM from saline. The results of this study suggest that it is not the N,N-dipropyl groups that account for selectivity, but, rather, it is some feature associated with the pyrrole portion of the indolylalkanamines that is important.

Indolealkylamine analogs share 5-HT2 binding characteristics with phenylalkylamine hallucinogens.

Twenty-one indolealkylamines, some of which are known to be psychoactive in man, were examined for their binding interactions with rat brain cortical 5-HT2 receptors labeled with the antagonist radioligand [3H]ketanserin in order to develop structure-activity relationships for binding at these sites. Features investigated included aromatic, alpha-methyl and terminal amine substituents. 4-Methoxy and 5-methoxy substitution impart a higher affinity than 6- or 7-methoxy substitution; a 7-hydroxyl group essentially abolishes affinity whereas a 7-methyl or 7-bromo group enhances affinity. alpha-Methylation has little effect on affinity and, in the one case examined, the S(+) isomer of alpha-methyltryptamine was essentially equipotent with its racemate and twice as potent as its R(-) enantiomer. Terminal amine methylation results in a small but progressive decrease in affinity in the order: primary amine greater than dimethylamine greater than diethylamine. Similarities were noted between these structural requirements for binding and those of the phenalkylamines. Selected compounds (5-methoxytryptamine, N,N-dimethyltryptamine, 5-methoxy-N,N-diethyltryptamine and 5-methoxy-N,N-dimethyltryptamine) were further examined by two-site analysis of displacement studies for [3H]ketanserin specific binding. Hill coefficients were significantly less than unity and computer-assisted analysis indicated that a two-site model better fit the data than a one-site model. In displacement studies using the putative agonist radioligand [3H]DOB to label 5-HT2 receptors affinities were 10-100-fold higher than those using [3H]ketanserin. These results are also consistent with earlier findings using psychoactive phenalkylamines in competition studies for radiolabelled 5-HT2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)