A novel fluorinated tryptamine with highly potent serotonin 5-HT1A receptor agonist properties.

Synthesis and biological evaluation of a novel fluorinated tryptamine analogue are described. This new compound 1-(4-fluoro-5-methoxyindol-3-yl)pyrrolidine (2) was found to be a potent serotonin 5-HT1A agonist.

Effect of ring fluorination on the pharmacology of hallucinogenic tryptamines.

A series of fluorinated analogues of the hallucinogenic tryptamines N,N-diethyltryptamine (DET), 4-hydroxy-N,N-dimethyltryptamine (4-OH-DMT, psilocin), and 5-methoxy-DMT was synthesized to investigate possible explanations for the inactivity of 6-fluoro-DET as a hallucinogen and to determine the effects of fluorination on the molecular recognition and activation of these compounds at serotonin receptor subtypes. The target compounds were evaluated using in vivo behavioral assays for hallucinogen-like and 5-HT(1A) agonist activity and in vitro radioligand competition assays for their affinity at 5-HT(2A), 5-HT(2C), and 5-HT(1A) receptor sites. Functional activity at the 5-HT(2A) receptor was determined for all compounds. In addition, for some compounds functional activity was determined at the 5-HT(1A) receptor. Hallucinogen-like activity, evaluated in the two-lever drug discrimination paradigm using LSD-trained rats, was attenuated or abolished for all of the fluorinated analogues. One of the tryptamines, 4-fluoro-5-methoxy-DMT (6), displayed high 5-HT(1A) agonist activity, with potency greater than that of the 5-HT(1A) agonist 8-OH-DPAT. The ED(50) of 6 in the two-lever drug discrimination paradigm using rats trained to discriminate the 5-HT(1A) agonist LY293284 was 0.17 micromol/kg, and the K(i) at [(3)H]8-OH-DPAT-labeled 5-HT(1A) receptors was 0.23 nM. The results indicate that fluorination of hallucinogenic tryptamines generally has little effect on 5-HT(2A/2C) receptor affinity or intrinsic activity. Affinity at the 5-HT(1A) receptor was reduced, however, in all but one example, and all of the compounds tested were full agonists but with reduced functional potency at this serotonin receptor subtype. The one notable exception was 4-fluoro-5-methoxy-DMT (6), which had markedly enhanced 5-HT(1A) receptor affinity and functional potency. Although it is generally considered that hallucinogenic activity results from 5-HT(2A) receptor activation, the present results suggest a possible role for involvement of the 5-HT(1A) receptor with tryptamines.

Further studies on oxygenated tryptamines with LSD-like activity incorporating a chiral pyrrolidine moiety into the side chain.

The enantiomers of 3-(N-methylpyrrolidin-2-ylmethyl)-5-methoxyindole, 1, and 3-(N-methylpyrrolidin-2-ylmethyl)-4-hydoxyindole, 3, were prepared using an asymmetric synthesis that employed (+)- or (-)-proline. A new approach was developed that had certain advantages over the synthesis originally reported for the isomers of 1. (+/-)-3-(N-Methylpyrrolidin-3-yl)-4-hydroxyindole, 5, was also prepared as a rigid analogue of psilocin and compared with its 5-methoxy counterpart 4. Radioligand competition assays were used to assess the affinity of compounds for the 5-HT(2A) receptor labeled with the agonist ligand [(125)I]DOI and the antagonist ligand [(3)H]MDL100907. Two-lever drug discrimination assays in rats trained to discriminate either LSD or DOI from saline were employed to assess the hallucinogen-like behavioral properties of these rigid tryptamine analogues. The receptor binding assay results clearly demonstrated a stereochemical preference for the R enantiomers that did not discriminate the position of the oxygen function. The receptor is 10-20-fold more selective for the R isomers. The affinities of the R enantiomers were virtually identical for both 1 and 3 at the agonist-labeled receptor, while racemic 4 and 5 had about one-tenth the affinity. The drug discrimination data in both LSD- and DOI-trained rats paralleled the binding data using [(125)I]DOI displacement. Both (R)-1 and (R)-3 are about equipotent, comparable to DOI in activity but about 10-fold less potent than LSD. Compound 4 produced only partial substitution, even at a dose nearly 5-fold higher than for (R)-1. Based on conformational energies, it seems doubtful that these compounds bind to the 5-HT(2A) receptor in an ergoline-like conformation. The results also suggest that both 1 and 3 would possess LSD-like psychopharmacology in humans.

Thieno[3,2-b]- and thieno[2,3-b]pyrrole bioisosteric analogues of the hallucinogen and serotonin agonist N,N-dimethyltryptamine.

The synthesis and biological activity of 6-[2-(N, N-dimethylamino)ethyl]-4H-thieno[3,2-b]pyrrole (3a) and 4-[2-(N, N-dimethylamino)ethyl]-6H-thieno[2,3-b]pyrrole (3b), thienopyrroles as potential bioisosteres of N,N-dimethyltryptamine (1a), are reported. Hallucinogen-like activity was evaluated in the two-lever drug discrimination paradigm using LSD- and DOI-trained rats. Neither 3a nor 3b substituted for LSD or DOI up to doses of 50 micromol/kg. By comparison, 1a fully substituted in LSD-trained rats. However, 3a and 3b fully substituted for the 5-HT1A agonist LY293284 ((-)-(4R)-6-acetyl-4-(di-n-propylamino)-1,3,4, 5-tetrahydrobenz[c,d]indole). Both 3a and 3b induced a brief "serotonin syndrome" and salivation, an indication of 5-HT1A receptor activation. At the cloned human 5-HT2A receptor 3b had about twice the affinity of 3a. At the cloned human 5-HT2B and 5-HT2C receptors, however, 3a had about twice the affinity of 3b. Therefore, thiophene lacks equivalence as a replacement for the phenyl ring in the indole nucleus of tryptamines that bind to 5-HT2 receptor subtypes and possess LSD-like behavioral effects. Whereas both of the thienopyrroles had lower affinity than the corresponding 1a at 5-HT2 receptors, 3a and 3b had significantly greater affinity than 1a at the 5-HT1A receptor. Thus, thienopyrrole does appear to serve as a potent bioisostere for the indole nucleus in compounds that bind to the serotonin 5-HT1A receptor. These differences in biological activity suggest that serotonin receptor isoforms are very sensitive to subtle changes in the electronic character of the aromatic systems of indole compounds.

Drug discrimination studies of the interoceptive cues produced by selective serotonin uptake inhibitors and selective serotonin releasing agents.

MMAI (5-methoxy-6-methyl-2-aminoindan) is a nonneurotoxic, highly selective neuronal serotonin (5-HT) releasing agent. MMAI and other 5-HT releasing agents produce a robust discriminative cue in drug discrimination (DD) studies. The selective serotonin reuptake inhibitors (SSRIs) sertraline and citalopram may also serve as discriminative stimuli, but acquisition of their discrimination required almost twice as much time as for MMAI. In vitro, 5-HT release by MMAI can be blocked by selective SSRIs. However, in the present DD studies, pretreatment with fluoxetine, sertraline, or citalopram 60 min before the training drugs MMAI or (+)-MBDB produced only partial inhibition of the discriminative cue. In substitution tests, sertraline and citalopram partially mimicked the training drugs, whereas only 40% substitution occurred with fluoxetine in MMAI or (+)-MBDB trained rats. In generalization tests, the tricyclic antidepressants imipramine and clomipramine partly substituted for the sertraline, citalopram, and MMAI stimuli. The increase in extracellular 5-HT produced by SSRIs leads to a subtle or feeble drug cue that is apparently difficult for an animal to recognize. This observation contrasts with the 5-HT releasing agents, which clearly produce robust cues that are easily recognized by the animals. However, mechanism(s) responsible for the discriminative stimulus effects of SSRIs and 5-HT releasing agents seem to be similar, at least in part.

Substituted naphthofurans as hallucinogenic phenethylamine-ergoline hybrid molecules with unexpected muscarinic antagonist activity.

A series of substituted racemic naphthofurans were synthesized as "hybrid" molecules of the two major prototypical hallucinogenic drug classes, the phenethylamines and the tryptamines/ergolines. Although it was hypothesized that these new agents might possess high affinity for the serotonin 5-HT2A/2C receptor subtypes, unexpected affinity for muscarinic receptors was observed. The compounds initially synthesized for this study were (+/-)-anti- and syn-4-amino-6-methoxy-2a,3,4,5-tetrahydro-2H-naphtho[1,8-bc]furan (4a,b), respectively, and their 8-bromo derivatives 4c,d, respectively. The brominated primary amines 4c,d were assayed initially for activity in the two-lever drug discrimination (DD) paradigm in rats trained to discriminate saline from LSD tartrate (0. 08 mg/kg). Also, 4c,d were evaluated for their ability to compete against agonist and antagonist radioligands at cloned human 5-HT2A, 5-HT2B, and 5-HT2C receptors. After the syn diastereomers were found to have the highest activity in these preliminary assays, the N-alkylated analogues syn-N,N-dimethyl-4-amino-6-methoxy-2a,3,4, 5-tetrahydro-2H-naphtho[1,8-bc]furan (4e) and syn-N, N-dipropyl-4-amino-6-methoxy-2a,3,4,5-tetrahydro-2H-naphtho[1, 8-bc]furan (4f) were prepared and assayed for their affinities at [3H]ketanserin-labeled 5-HT2A and [3H]-8-OH-DPAT-labeled 5-HT1A sites. All of the molecules tested had relatively low affinity for serotonin receptors, yet a preliminary screen indicated that compound 4d had affinity for muscarinic receptors. Thus, 4b,d,e were evaluated for their affinity at muscarinic M1-M5 receptors and also assessed for their functional characteristics at the M1 and M2 isoforms. Compound 4d had affinities of 12-33 nM at all of the muscarinic sites, with 4b,e having much lower affinity. All three compounds fully antagonized the effects of carbachol at the M1 receptor, while only 4d completely antagonized carbachol at the M2 receptor. The fact that the naphthofurans lack LSD-like activity suggests that they do not bind to the serotonin receptor in a way such that the tricyclic naphthofuran nucleus is bioisosteric with, and directly superimposable upon, the A, B, and C rings of LSD. This also implies, therefore, that the hallucinogenic phenethylamines cannot be directly superimposed on LSD in a common binding orientation for these two chemical classes, contrary to previous hypotheses.

Synthesis and pharmacological evaluation of ring-methylated derivatives of 3,4-(methylenedioxy)amphetamine (MDA).

The three isomeric ring-methylated derivatives of the well-known hallucinogen and entactogen MDA (1a) were synthesized and evaluated for pharmacological activity as monoamine-releasing agents and as serotonin agonists. The 2-methyl derivative 2a and the 5-methyl derivative 2b were found to be more potent and more selective than the parent compound in inhibiting [3H]-serotonin accumulation in rat brain synaptosomal preparations. Their activity in vivo was confirmed in rats trained to discriminate serotonin-releasing agents and hallucinogens from saline. The results indicate that compounds 2a,b are among the most potent 5-HT-releasing compounds known and show promise as lead compounds in the search for antidepressant drugs that release serotonin rather than inhibit its uptake.

Indan analogs of fenfluramine and norfenfluramine have reduced neurotoxic potential.

N-Ethyl-5-trifluoromethyl-2-aminoindan (ETAI) and 5-trifluoromethyl-2-aminoindan (TAI) were synthesized to examine the effects of side-chain cyclization on the pharmacology of the anorectic drugs fenfluramine (FEN) and norfenfluramine (norFEN), respectively. ETAI and TAI inhibited synaptosomal accumulation of 5-HT but were less effective at inhibiting catecholamine uptake than FEN or norFEN, respectively. In vivo, ETAI and TAI were less neurotoxic than FEN or norFEN; decreases in the number of [3H]paroxetine-labeled 5-HT uptake sites were 50% less than the decreases produced by FEN or norFEN. Rats treated with ETAI. TAI, FEN, and norFEN lost 10-15% of their pretreatment body weight over a 4-day period, while saline-treated control animals gained 8%. In two-lever drug discrimination (DD) assays in rats, TAI fully substituted for the 5-HT releaser/uptake inhibitor, (+)-MBDB [(+)-N-methyl-1-(1,3-benzodioxol-5-yl)-2-aminobutane]. ETAI produced only partial substitution in this test. Neither TAI nor ETAI mimicked (+)-amphetamine in the DD assay. These studies demonstrate that incorporation of the side-chain of phenylisopropylamines into the five-membered ring of a 2-aminoindan changes both the molecular pharmacology and the neurotoxic profile of FEN and norFEN, but does not diminish the drugs' ability to reduce body weight.

Dihydrobenzofuran analogues of hallucinogens. 4. Mescaline derivatives.

Dihydrobenzofuran and tetrahydrobenzodifuran functionalities were employed as conformationally restricted bioisosteres of the aromatic methoxy groups in the prototypical hallucinogen, mescaline (1). Thus, 4-(2-aminoethyl)-6,7-dimethoxy-2,3-dihydrobenzofuran hydrochloride (8) and 1-(8-methoxy-2,3,5,6-tetrahydrobenzo[1,2-b:5,4-b']difuran-4-yl)-2- aminoethane hydrochloride (9) were prepared and evaluated along with 1 for activity in the two-lever drug discrimination (DD) paradigm in rats trained to discriminate saline from LSD tartrate (0.08 mg/kg). Also, 1, 8, and 9 were assayed for their ability to displace [3H]ketanserin from rat cortical homogenate 5-HT2A receptors and [3H]8-OH-DPAT from rat hippocampal homogenate 5-HT1A receptors. In addition, these compounds were evaluated for their ability to compete for agonist and antagonist binding to cells expressing cloned human 5-HT2A, 5-HT2B, and 5-HT2C receptors. Finally, agonist efficacy was assessed by measurement of phosphoinositide hydrolysis in NIH 3T3 cells expressing the rat 5-HT2A or 5-HT2C receptors. Although 1 fully substituted for LSD in the DD assays (ED50 = 33.5 mumol/kg), neither 8 nor 9 substituted for LSD, with just 50% of the rats administered 8 selecting the drug lever, and only 29% of the rats administered 9 selecting the drug lever. All of the test compounds had micromolar affinity for the 5-HT1A and 5-HT2A receptors in rat brain homogenate. Curiously, the rank order of affinities of the compounds at 5-HT2A sites was opposite their order of potency in the behavioral assay. An evaluation for ability to stimulate phosphoinositide turnover as a measure of functional efficacy revealed that all the compounds were of approximately equal efficacy to serotonin in 5-HT2C receptors. At 5-HT2A receptors, however, 8 and 9 were significantly less efficacious, eliciting only 61 and 45%, respectively, of the maximal response. These results are consistent with the proposed mechanism of action for phenethylamine hallucinogens, that such compounds must be full agonists at the 5-HT2A receptor subtype. In contrast to the 2,5-dimethoxy-substituted phenethylamines, where rigidification of the methoxy groups had no deleterious effect on activity, the loss of activity in the 3,4,5-trioxygenated mescaline analogues may suggest that the 3 and 5 methoxy groups must remain conformationally mobile to enable receptor activation.

5-HT2A/2C receptor agonists potentiate the discriminative cue of (+)-amphetamine in the rat.

The possible effect of 5-HT2A/2C receptor agonists on an amphetamine-induced behavioral response was examined using the two-lever drug discrimination paradigm. The experiments were designed to investigate an interaction of the hallucinogenic 5-HT2A/2C agonists lysergic acid diethylamide (LSD) and 2,5-dimethoxy-4-iodoamphetamine (DOI), with the discriminative stimulus elicited by a relatively low dose of (+)-amphetamine (1.35 micromol/kg, 0.25 mg/kg, which produced approximately 50% selection of the drug lever). DOI and LSD did not produce amphetamine-like responding at any dose tested or time of administration. However, LSD alone was able to induce a drug-appropriate response in two of nine amphetamine-trained rats. Simultaneous administration of DOI or LSD with amphetamine was not significantly different from the response produced by amphetamine alone. Pre-administration of DOI (3 hr) or of LSD (2 hr) before amphetamine, however, evoked significant enhancement of the amphetamine cue. The results suggest that the enhanced behavioral response to amphetamine may be due either to an increased sensitivity of dopaminergic neurons in the mesolimbic area, or to an enhanced release of dopamine by amphetamine.

The Effect of Selective Serotonin Releasing Agents in the Chronic Mild Stress Model of Depression in Rats.

Chronic exposure to mild unpredictable stress has been found to depress the consumption of, and preference for, highly palatable sucrose solution in rats. Stress-induced behavioral deficits may be maintained for a long time, however chronic administration of clinically effective antidepressants can restore normal behavior. This is the first report showing that Sprague-Dawley rats can be used in this model. A preference deficit in this strain of rats took at least 7 weeks to develop; about twice the time required when hooded Lister or Wistar rats are used in this model. Water consumption was not effected by chronic exposure to the mild stress regime and/or by chronic administration of the selective serotonin (5-HT) releasing agent MMAI (5-methoxy-6-methyl-2-aminoindan). The stress-induced deficit in sucrose intake was completely reversed by chronic treatment with MMAI (5 mg/kg, 2 x day) over 3 weeks in the two-bottle tests. In single-bottle tests, chronic treatment with the selective 5-HT releasers, MMAI (5 mg/kg, 2 x day) or MTA (p-methylthioamphetamine; 5 mg/kg, 2 x day), reversed the deficit in rewarded behavior (anhedonia) measured as a decrease in the consumption of 1% sucrose solution in the chronic mild stress model of depression in rats. With the experimental procedure employed, and at a dose of 10 mg/kg/day of 5-HT releasers, the magnitude and onset of this effect were greater than observed following similar administration of the selective 5-HT reuptake inhibitor (SSRI) sertraline (10 mg/kg/day), used as a standard anti-depressant drug.

Dihydrobenzofuran analogues of hallucinogens. 3. Models of 4-substituted (2,5-dimethoxyphenyl)alkylamine derivatives with rigidified methoxy groups.

Tetrahydrobenzodifuran functionalities were employed as conformationally restricted bioisosteres of the aromatic methoxy groups in prototypical hallucinogenic phenylalkylamines 1 and 2. Thus, a series of 8-substituted 1-(2,3,6,7-tetrahydrobenzo[1,2-b:4,5-b']difuran-4-yl)-2-aminoal kanes (7a-e) were prepared and evaluated for activity in the two-lever drug discrimination paradigm in rats trained to discriminate saline from LSD tartrate (0.08 mg/kg) and for the ability to displace [3H]ketanserin from rat cortical homogenate 5-HT2A receptors and [3H]-8-OH-DPAT from rat hippocampal homogenate 5-HT1A receptors. In addition, 1-(8-bromo-2,3,6,7-tetrahydrobenzo[1,2-b:4,5-b']difuran-4-yl)-2-am inopropane (7b), which was found to be extremely potent in the rat in vivo assays, was evaluated for its ability to compete with [125I]DOI and [3H]ketanserin binding to cells expressing cloned human 5-HT2A, 5-HT2B, and 5-HT2C receptors. All of the dihydrofuranyl compounds having a hydrophobic substituent para to the alkylamine side chain had activities in both the in vitro and in vivo assays that equaled or surpassed the activity of the analogous conformationally flexible parent compounds. For example, 7b substituted for LSD in the drug discrimination assay with an ED50 of 61 nmol/kg and had Kj values in the nanomolar to subnanomolar range for the displacement of radioligand from rat and human 5-HT2 receptors, making it one of the most potent hallucinogen-like phenylalkylamine derivatives reported to date. The results suggest that the dihydrofuran rings in these new analogues effectively model the active binding conformations of the methoxy groups of the parent compounds 1 and 2. In addition, the results provide information about the topography and relative orientation of residues involved in agonist binding in the serotonin 5-HT2 receptors.

Reinforcing effects of certain serotonin-releasing amphetamine derivatives.

The present study was designed to characterize further the rewarding and aversive properties of 3,4-methylenedioxymethamphetamine (MDMA), the alpha-ethyl homologue of MDMA (MBDB), fenfluramine, and the selective serotonin releasing agent 5-methoxy-6-methyl-2-aminoindan (MMAI) using the conditioned place preference paradigm (CPP). Extracellular dopamine (DA) and its metabolite DOPAC were also measured in the nucleus accumbens after systemic drug administration, using in vivo microdialysis in freely moving rats. MDMA produced a positive dose-dependent effect in the CPP test, which was maximal at doses of 5 and 10 mg/kg. MBDB also induced a positive CPP, with a maximum effect at 10 mg/kg. The conditioning effect of MBDB was more than 2.5-fold weaker compared with MDMA. Fenfluramine evoked place aversion at doses of 4, 6, and 10 mg/kg. This effect of fenfluramine was independent of dose. MMAI at doses of 1.25, 2.5, and 5 mg/kg produced no significant effect on place conditioning. At doses of 10 and 20 mg/kg, MMAI produced an effect similar to fenfluramine: Place aversion was independent of dose. In the microdialysis experiments, MDMA significantly elevated extracellular DA and induced a decrease of DOPAC in the nucleus accumbens. Thus, activation of dopaminergic systems may be responsible for the rewarding properties of MDMA-like drugs. In contrast to the effects seen with MDMA, no difference in extracellular DA or DOPAC was seen after injection of MBDB, fenfluramine, or MMAI, even though MBDB weakly induced a place preference. The mechanism responsible for the development of place aversion by fenfluramine or MMAI is unknown at this time and requires further study.

Stereoselective pharmacological effects of lysergic acid amides possessing chirality in the amide substituent.

Studies of the affinities for serotonin 5-HT2A and 5-HT1A receptor subtypes of lysergic acid amides prepared from chiral 2-aminoalkanes showed a stereoselective preference at both receptor types for the amides with alkyl groups containing the R configuration. The 5-HT2A receptor was less tolerant of long alkyl groups than was the 5-HT1A subtype. In vivo assays in rats trained to discriminate LSD from saline also showed that amides with alkyl groups having the R configuration were most potent.

Psychostimulant-like effects of p-fluoroamphetamine in the rat.

The present study was undertaken to compare the pharmacological properties of p-fluoroamphetamine with those of amphetamine and of other halogenated amphetamines, using several in vivo and in vitro tests. These included substitution testing in (+)-amphetamine (1 mg/kg, 5.4 mu mol/kg, i.p.)-, (+)-N-methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine [(+)-MBDB] (1.75 mg/kg, 7.8 mu mol/kg, i.p.)-, and 5-methoxy-6-methyl-2-aminoindan (MMAI) (1.71 mg/kg, 8 mu mol/kg, i.p.)-trained rats, [3H]5-HT and [3H]dopamine uptake inhibition in whole brain synaptosomes, and changes in striatal extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) as measured by in vivo microdialysis in freely moving rats. In drug discrimination substitution tests, p-fluoroamphetamine fully mimicked (+)-amphetamine (ED50 0.43 mg/kg, 2.11 mu mol/kg), whereas 'no substitution' was observed in rats trained to discriminate the serotonin (5-hydroxytryptamine, 5-HT)-releasing agents (+)-MBDB or MMAI from saline. p-Chloroamphetamine did not substitute for amphetamine but fully substituted for the (+)-MBDB and MMAI cues (ED50 0.17 mg/kg, 0.82 mu mol/kg, and 0.14 mg/kg, 0.69 mu mol/kg, respectively). p-Fluoroamphetamine, in comparison with p-chloroamphetamine and p-iodoamphetamine, showed much stronger inhibition of [3H]dopamine than [3H]5-HT uptake into rat brain synaptosomes but was less selective than amphetamine. p-Fluoroamphetamine (7.0 mg/kg, i.p.), 1 h after administration, strongly elevated (849% of baseline) extracellular dopamine in rat striatum measured using in vivo microdialysis. Amphetamine (2 mg/kg, i.p.) increased extracellular dopamine in rat striatum with a maximum at the same time as did p-fluoroamphetamine, but the latter gave a smaller increase. The data presented suggest that p-fluoroamphetamine resembles amphetamie more than it does the 5-HT-releasing type amphetamines.

Complex stimulus properties of LSD: a drug discrimination study with alpha 2-adrenoceptor agonists and antagonists.

The influence of several alpha 2-adrenergic agents on the discriminative stimulus (DS) properties of lysergic acid diethylamide (LSD) was studied in rats trained to discriminate 0.08 mg/kg (186 nmol/kg) of LSD from saline in a two-lever operant paradigm. Only yohimbine fully mimicked LSD with an ED50 of 2.05 mg/kg (5.24 mumol/kg). Yohimbine's 5-HT1A agonist properties may be responsible for this substitution. Other alpha 2-adrenoceptor antagonists, idazoxan with an agonist/antagonist profile at 5-HT1A receptors and RS 26026-197, a highly selective alpha 2-adrenoceptor antagonist, failed to produce substitution. Clonidine, an alpha 2-adrenoceptor agonist, did not substitute for LSD but the response rate was dose-dependently reduced. None of the alpha 2-adrenergic agents used for pretreatment before LSD inhibited the response to the LSD training dose. Coadministration of clonidine with LSD produced a leftward shift of the dose-response relationship of LSD without a significant change in the slope of the dose-response line. Simultaneous administration of alpha 2-adrenergic agents with LSD shifted the dose-response curve to the left only when the adrenergic agent also possessed at least moderate affinity for the 5-HT1A receptor. In addition, radioligand competition experiments were performed that showed LSD to have relatively high affinity (Ki = 37 nM) for [3H]clonidine-labeled sites in rat cortex with lower affinity for [3H]yohimbine labeled sites. While previous studies have suggested that the nature of the LSD cue may be essentially expressed by 5-HT2 receptor activation, the present data show that this cue can be modulated by effects of LSD at 5-HT1A and at other monoamine neurotransmitter receptors.

Stereoselective LSD-like activity in a series of d-lysergic acid amides of (R)- and (S)-2-aminoalkanes.

The 3-pentyl-, (R)- and (S)-2-pentyl-, 2-hexyl-, and 2-heptylamides of d-lysergic acid were synthesized and evaluated in biochemical and behavioral assays for LSD-like activity. In radioligand competition studies, the (R)-lysergamides were consistently more potent than the (S)-amides in displacing [3H]ketanserin from 5-HT2A receptors in rat cortical homogenate and in displacing [3H]-8-OH-DPAT ([3H]-8-hydroxy-2-(di-n- propylamino)tetralin) from rat hippocampal 5-HT1A receptors. As the amide alkyl was lengthened from pentyl to heptyl, the affinity of the (R)-isomers for 5-HT2A sites decreased, while affinity for 5-HT1A sites was maximal for the (R)-2-hexyllysergamide. In rats trained to discriminate 0.08 mg/kg LSD tartrate from saline, a similar stereoselective effect was noted in which the (R)-alkylamides were more potent than the (S)-isomers in producing the LSD-like discriminative stimulus effect. However, as the amide alkyl substituent was increased in length, LSD-like activity decreased, with only partial substitution for training drug being observed for the (R)-hexylamide. The (R)- and (S)-pentyllysergamides were also assayed for their ability to activate intracellular phosphoinositide hydrolysis. Consistent with the binding and behavioral studies, these assays showed that both isomers are potent agonists at the 5-HT2A receptor, but that the (R)-pentyllysergamide is approximately 20 times more active than the (S)-pentyllysergamide in stimulating phosphoinositide turnover.

1-(2,5-Dimethoxy-4-(trifluoromethyl)phenyl)-2-aminopropane: a potent serotonin 5-HT2A/2C agonist.

A method was found to synthesize 1-(2,5-dimethoxy-4-(trifluoromethyl) phenyl)-2-aminopropane, 5, and its des-alpha-methyl congener 2-(2,5-dimethoxy-4-(trifluoromethyl)phenyl)aminoethane, 6, the trifluoromethyl analogs of substituted hallucinogenic phenethylamine derivatives such as 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (3, DOI) that are potent 5-HT2A/2C agonists. In our hands, 5 and 6 have proven to have affinity for [3H]ketanserin or [125I]-3-labeled 5-HT2A/2C sites in rat cortex comparable to or higher than the analogous bromo or iodo analogs. Similarly, 5 and 6 had potency comparable to or slightly greater than that of their bromo or iodo congeners in the two-lever drug discrimination assay in rats trained to discriminate saline from LSD tartrate. The agonist properties of 5 and 6 were evaluated by measuring the accumulation of [3H]inositol monophosphate in cultured cells selectively expressing either 5-HT2A or 5-HT2C receptors. In comparison to serotonin (5-HT), compounds 3 (DOI), 5, and 6 were equally efficacious and full agonists at the 5-HT2C receptor. Similarly, 3 and 5 produced equivalent responses at the 5-HT2A receptor as compared to 5-HT. In contrast, 6, the alpha-desmethyl analog of 5, was only half as potent at stimulating inositol monophosphate accumulation at the 5-HT2A receptor. In conclusion, the title compound 5 and its alpha-desmethyl congener 6 appear to be the most potent of the so-called hallucinogenic amphetamine 5-HT agonists reported to date. Further, the reduced efficacy of 6 at the 5-HT2A receptor may offer at least a partial explanation for the observed higher in vivo potencies of alpha-methyl-substituted compounds in this series.

Behavioral effects of the highly selective serotonin releasing agent 5-methoxy-6-methyl-2-aminoindan.

The behavioral effects of 5-methoxy-6-methyl-2-aminoindan (MMAI) were examined using the drug discrimination procedure and direct observation for classification of the characteristic syndrome induced by MMAI. The stimulus effects of MMAI were studied in 5 different groups of rats trained to discriminate MMAI (1.71 mg/kg; 8 microM/kg), MDMA (3,4-methylenedioxymethamphetamine; 1.75 mg/kg; 7.6 microM/kg), (+)-MBDB ((+)-N-methylamino-(1,3-benzodioxo-5-yl)-2-butanamine; 1.75 mg/kg; 7.18 microM/kg), (+)-amphetamine (1 mg/kg; 5.4 microM/kg), or LSD ((+)-lysergic acid diethylamide tartrate; 0.08 mg/kg; 186 nM/kg) from saline. In substitution tests in rats trained to discriminate MMAI from saline, all the compounds which fully mimicked MMAI were serotonin (5-hydroxytryptamine, 5-HT) releasing agents. This substitution is symmetrical for MDMA and (+)-MBDB. Nevertheless, the dose-response curve of MMAI is parallel to those of (+)-fenfluramine (m-trifluoromethyl-N-ethylamphetamine) and p-chloroamphetamine. The results also show that MMAI lacks amphetamine-like and LSD-like discriminative stimulus effects, suggesting that MMAI is neither a psychostimulant nor a hallucinogen. Tests of the discriminability of MMAI after 5-HT depletion with the selective serotonin synthesis inhibitor, p-chlorophenylalanine (2 x 200 mg/kg i.p., pretreatment 72 h before test), showed only saline appropriate responding. Prolonged block (ca. 1 week) of the MMAI cue by p-chlorophenylalanine further supports the conclusion that endogenous 5-HT is essential for MMAI discrimination. Fluoxetine (10 mg/kg) or paroxetine (2.5 mg/kg), both selective 5-HT uptake inhibitors, reduced the discriminability of MMAI to 40% and 50%, respectively. None of the antagonists (ketanserin, methiothepin, pindolol, yohimbine, haloperidol) used in antagonism tests inhibited the stimulus properties of MMAI. These results and data from radioligand binding studies support the conclusion that direct activation or inhibition of known neurotransmitter receptors did not play a significant role in the discriminative cue of MMAI. The administration of 5, 10, or 20 mg/kg of MMAI to rats induced a behavioral syndrome consisting of hypolocomotion with accompanying catalepsy-like posture, turning, Straub tail, flat body posture, and suppressed sleeping time. In general, this is qualitatively similar to what is seen after administration of 5-HT precursors or 5-HT receptor agonists. In conclusion, the data from the drug discrimination study and the behavioral syndrome induced by MMAI suggest that MMAI is a potential selective releaser of serotonin.