Targeting inhibition of microtubule affinity regulating kinase 4 by Harmaline: Strategy to combat Alzheimer's disease.

Microtubule-affinity regulating kinase 4 (MARK4) is linked with the development of cancer, diabetes and neurodegenerative diseases. Due to its direct role in the hyperphosphorylation of tau protein, MARK4 is considered as an attractive target to fight Alzheimer's disease (AD) and neuroinflammation. In the present study, we have selected Harmaline (HAR), an alkaloid of Paganum harmala, to investigate its MARK4 inhibitory potential and its binding mechanism. Molecular docking and fluorescence binding studies were carried out to estimate the binding affinity of the HAR with the MARK4. We observed an excellent binding affinity of HAR to the MARK4 (K = 107 M-1), further complemented by isothermal titration calorimetric measurements. In addition, HAR significantly inhibits the kinase activity of MARK4 (IC50 value of 4.46 µM). Structural investigations suggested that HAR binds to the active site pocket and forms several non-covalent interactions with biologically important residues of MARK4. All-atom molecular dynamics simulation studies further advocated that the MARK4-HAR complex is stabilized throughout the trajectory of 200 ns and causes a little conformational change. All these findings suggest that HAR is a potential MARK4 inhibitor that can be implicated in managing MARK4-associated diseases, including AD.

Harmaline toxicity on dorsal striatal neurons and its role in tremor.

Harmaline is one of the ß-carboline derivative compounds that is widely distributed in the food chain and human tissues. Harmine, a dehydrogenated form of harmaline, appeared to have a higher concentration in the brain, and appeared to be elevated in essential tremor (ET) and Parkinson's disease. Exogenous harmaline exposure in high concentration has myriad consequences, including inducing tremor, and causing neurodegeneration of Purkinje cells in the cerebellum. Harmaline-induced tremor is an established animal model for human ET, but its underlying mechanism is still controversial. One hypothesis posits that the inferior olive-cerebellum pathway is involved, and CaV3.1 T-type Ca2+ channel is a critical target of action. However, accumulating evidence indicates that tremor can be generated without disturbing T-type channels. This implies that additional neural circuits or molecular targets are involved. Using in vitro slice Ca2+-imaging and patch clamping, we demonstrated that harmaline reduced intracellular Ca2+ and suppressed depolarization-induced spiking activity of medium spiny striatal neurons (MSN), and this effect of harmaline can be partially attenuated by sulpiride (5 µM). In addition, the frequencies of spontaneous excitatory post-synaptic currents (sEPSCs) on MSNs were also significantly attenuated. Furthermore, the induced tremor in C57BL/6 J mice by harmaline injections (i.p. 12.5-18 mg/kg) was also shown to be attenuated by sulpiride (20 mg/kg). This series of experiments suggests that the dorsal striatum is a site of harmaline toxic action and might contribute to tremor generation. The findings also provide evidence that D2 signaling might be a part of the mechanism underlying essential tremor.

Harmine is a potent antimalarial targeting Hsp90 and synergizes with chloroquine and artemisinin.

Previous studies have shown an antimalarial effect of total alkaloids extracted from leaves of Guiera senegalensis from Mali in West Africa. We independently observed that the beta-carboline alkaloid harmine obtained from a natural product library screen inhibited Plasmodium falciparum heat shock protein 90 (PfHsp90) ATP-binding domain. In this study, we confirmed harmine-PfHsp90-specific affinity using surface plasmon resonance analysis (dissociation constant [K(d)] of 40 µM). In contrast, the related compound harmalol bound human Hsp90 (HsHsp90) (K(d) of 224 µM) more tightly than PfHsp90 (K(d) of 7,010 µM). Site-directed mutagenesis revealed that Arg98 in PfHsp90 is essential for harmine selectivity. In keeping with our model indicating that Hsp90 inhibition affords synergistic combinations with existing antimalarials, we demonstrated that harmine potentiates the effect of chloroquine and artemisinin in vitro and in the Plasmodium berghei mouse model. These findings have implications for the development of novel therapeutic combinations that are synergistic with existing antimalarials.

Heme peroxidases are responsible for the dehydrogenation and oxidation metabolism of harmaline into harmine.

Harmaline and harmine are ß-carboline alkaloids with effective pharmacological effects. Harmaline can be transformed into harmine after oral administration. However, enzymes involved in the metabolic pathway remain unclear. In this study, harmaline was incubated with rat liver microsomes (RLM), rat brain microsomes (RBM), blood, plasma, broken blood cells, and heme peroxidases including horseradish peroxidase (HRP), lactoperoxidase (LPO), and myeloperoxidase (MPO). The production of harmine was determined by a validated UPLC-ESI-MS/MS method. Results showed that heme peroxidases catalyzed the oxidative dehydrogenation of harmaline. All the reactions were in accordance with the Hill equation. The reaction was inhibited by ascorbic acid and excess H2O2. The transformation of harmaline to harmine was confirmed after incubation with blood, plasma, and broken blood cells, rather than RLM and RBM. Harmaline was incubated with blood, plasma, and broken cells liquid for 3 h, and the formation of harmine became stable. Results indicated an integrated metabolic pathway of harmaline, which will lay foundation for the oxidation reaction of dihydro-ß-carboline. Moreover, the metabolic stability of harmaline in blood should not be ignored when the pharmacokinetics study of harmaline is carried out.

Ultrastructural and hormonal changes related to harmaline-induced treatment in Arabidopsis thaliana (L.) Heynh. root meristem.

Harmaline is an indole alkaloid with demonstrated phytotoxicity and recognized pharmacological applications. However, no information is available concerning its mode of action on plant metabolism. Therefore, the present work evaluated bioherbicide mode of action of harmaline on plant metabolism of Arabidopsis thaliana (L.) Heynh. Harmaline induced a strong inhibitory activity on root growth of treated seedlings, reaching IC50 and IC80 values of 14 and 29 µM, respectively. Treated roots were shorter and thicker than control and were characterized by a shorter root meristem size and an increase of root hairs production. Harmaline induced ultrastructural changes such as increment of cell wall thickness, higher density and condensation of mitochondria and vacuolization, appearance of cell wall deposits, increment of Golgi secretory activity and higher percentage of aberrant nuclei. The ethylene inhibitor AgNO3 reversed high root hair appearance and increment of root thickness, and pTCSn::GFP transgenic line showed fluorescence cytokinin signal in stele zone after harmaline treatment that was absent in control, whereas the auxin signal in the transgenic line DR5 was significantly reduced by the treatment. All these results suggest that the mode of action of harmaline could be involving auxin, ethylene and cytokinin synergic/antagonistic action.

In-silico Prediction of the Beta-carboline Alkaloids Harmine and Harmaline as Potent Drug Candidates for the Treatment of Parkinson's disease.

BACKGROUND: Parkinson's disease (PD) is a progressive neurodegenerative disease manifested by core symptoms of loss of motor control and postural instability. Loss of dopaminergic neurons is the cause of PD, thus enhancing dopamine level by pharmacological treatment is one of the key treatment strategies for PD. However, the limitations of current treatment strategies open the possibility of novel drug candidates for the treatment of PD. OBJECTIVE: To investigate the anti-PD potential of Harmine and Harmaline. We aim to evaluate the therapeutic potential of Harmine and Harmaline by in-silico approaches; molecular docking, pharmacokinetic and Prediction of Activity Spectra for Substances (PASS) analysis were used for evaluating the therapeutic potential of Harmine and Harmaline and standard drug levodopa (L-DOPA). METHODS: Auto dock vina was used for molecular docking of all three compounds against D2- and D3- dopamine receptors. The pharmacokinetics (PKs) and toxicity profile were predicted by pkCSM, and the pharmacological activity was predicted by PASS analysis. RESULTS: Molecular docking showed a higher binding affinity of Harmine and Harmaline as compared to L-DOPA, and these results were supported by in-silico pharmacokinetic and toxicity profiling. Moreover, PASS analysis showed anti-PD activity of Harmine and Harmaline. CONCLUSION: Harmine and Harmaline exhibit higher binding affinity towards D2- and D3- dopamine receptors compared to L-DOPA, and PKs and toxicity profile support their potential as drug candidates for PD therapy.

Anxiolytic and antidepressant effects of ACPA and harmaline co-treatment.

Depression and anxiety disorders are among the most common illnesses and a close relationship between them has been found. Because the psychotropic effects and abuse liability of cannabis prevent its therapeutic application in depression and anxiety states, we decided to investigate the effects of the combination of ineffective doses of cannabinoid CB1 receptor agonist arachidonylcyclopropylamide (ACPA) and ß-carbolines on anxiety- and depression-related behaviors in male NMRI mice. Anxiety- and depression-related behaviors were assesses using elevated plus maze (EPM) and forced swim test (FST), respectively. Intraperitoneal administration of ACPA (1 mg/kg) decreased the percentage of time spent in the open-arms (%OAT) and the number of entries to the open-arms (OAE) in the EPM, indicating an anxiogenic-like effect. ACPA also decreased immobility time in the FST compared to the control group, suggesting an antidepressant-like effect. ß-carbolines including harmane (5 and 10 mg/kg), norharmane (5 mg/kg) and harmaline (2.5 and 5 mg/kg) produced an anxiogenic-like response, while the highest dose of harmane or harmaline and the middle dose of norharmane induced an antidepressant-like behavior. Furthermore, co-administration of a subthreshold dose of ACPA (0.5 mg/kg) and harmaline (1.25 mg/kg), but not harmane or norharmane (both at the dose of 2.5 mg/kg), caused anxiolytic- and antidepressant-like behaviors and decreased locomotor activity. Our findings suggest a therapeutic potential for combined ineffective doses of ACPA and harmaline on anxiety- and depression-related processes.

Interspecies metabolic diversity of harmaline and harmine in in vitro 11 mammalian liver microsomes.

The ß-carboline alkaloids harmaline and harmine are widely present in hallucinogenic plants with great potential for treating depression, Parkinson's disease, and Alzheimer's disease. The present study was to elucidate metabolic difference of harmaline and harmine in 11 mammalian liver microsomes in order to quantitate species-specific metabolic profiles. Using the probe substrate reaction, the enzymatic activities for 8 CYP450 isozymes of 11 liver microsomes were characterized. Combining ultra performance liquid chromatography combined with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UPLC-ESI-Q/TOF-MS) and ultra performance liquid chromatography combined with electrospray ionization quadrupole tandem mass spectrometry (UPLC-ESI-MS/MS) methods, 18 metabolites for harmaline and 11 for harmine were identified. The metabolism patterns differences of them presented discrepancy in the quality and quantity of metabolites. It was notable that O-sulfate conjugation was detected in all species except sheep. The intrinsic clearance CLint, LM values for the metabolites harmine and harmol in rabbits (37.5 and 42.4 µL/min/mg) were higher than those in other animals, while dogs (16.2 and 16.7 µL/min/mg) and humans (16.0 and 16.3 µL/min/mg) exhibited similar in vitro metabolic clearance. These observations suggested that harmaline and harmine were rapidly metabolized in liver microsomes of rat, mouse, and rabbit; moderately metabolized in human and dog; while weakly metabolized in sheep. Comprehensive analysis of the metabolism indicated that dogs and humans showed considerable similarity in the elimination of parent drugs, metabolic profiles, and catalytic processes. To summarize, these findings illustrated that in vitro studies of harmaline and harmine metabolic profiles in different species are helpful for the proper selection and interpretation of animal models for pharmacological and toxicological evaluation, and will ultimately provide useful guidance for the development of ß-carboline alkaloids. Copyright © 2016 John Wiley & Sons Ltd.

Heme peroxidases are responsible for the dehydrogenation and oxidation metabolism of harmaline into harmine / 中国天然药物

Harmaline and harmine are β-carboline alkaloids with effective pharmacological effects. Harmaline can be transformed into harmine after oral administration. However, enzymes involved in the metabolic pathway remain unclear. In this study, harmaline was incubated with rat liver microsomes (RLM), rat brain microsomes (RBM), blood, plasma, broken blood cells, and heme peroxidases including horseradish peroxidase (HRP), lactoperoxidase (LPO), and myeloperoxidase (MPO). The production of harmine was determined by a validated UPLC-ESI-MS/MS method. Results showed that heme peroxidases catalyzed the oxidative dehydrogenation of harmaline. All the reactions were in accordance with the Hill equation. The reaction was inhibited by ascorbic acid and excess H2O2. The transformation of harmaline to harmine was confirmed after incubation with blood, plasma, and broken blood cells, rather than RLM and RBM. Harmaline was incubated with blood, plasma, and broken cells liquid for 3 h, and the formation of harmine became stable. Results indicated an integrated metabolic pathway of harmaline, which will lay foundation for the oxidation reaction of dihydro-β-carboline. Moreover, the metabolic stability of harmaline in blood should not be ignored when the pharmacokinetics study of harmaline is carried out.

Microwave-assisted extraction of three bioactive alkaloids from Peganum harmala L. and their acaricidal activity against Psoroptes cuniculi in vitro.

ETHNOPHARMACOLOGICAL RELEVANCE: Peganum harmala L. is a perennial herbaceous, glabrous plant that grows in semi-arid conditions, steppe areas and sandy soils. It is used to treat fever, diarrhoea, subcutaneous tumours, arthralgia, rheumatism, cough, amnesia and parasitic diseases in folk medicines. In this paper, we aimed to develop a simpler and faster method for the extraction of three alkaloids from Peganum harmala L. than other conventional methods by optimizing the parameters of a microwave-assisted extraction (MAE) method, and to investigate the acaricidal activities of three compounds against Psoroptes cuniculi. MATERIALS AND METHODS: After optimizing the operating parameters with the single factor experiment and a Box-Behnken design combined with a response-surface methodology, a MAE method was developed for extracting the alkaloids from the seeds, and a high-performance liquid chromatography was used to quantify these compounds. An in vitro experiments were used to study the acaricidal activities. RESULTS: The optimal conditions of MAE method were as follows: liquid-to-solid ratio 31.3:1mL/g, ethanol concentration 75.5%, extraction time 10.1min, temperature 80.7°C, and microwave power 600W. Compared to the heat reflux extraction (HRE, 60min) and the ultrasonic-assisted extraction (UAE, 30min) methods, MAE method require the shortest time (10min) and obtain the highest yield of three compounds (61.9mg/g). Meanwhile, the LT50 values for the vasicine (1.25 and 2.5mg/mL), harmaline (1.25 and 2.5mg/mL), harmine (1.25 and 2.5mg/mL) and MAE extract (100mg/mL) against Psoroptes cuniculi were 12.188h, 9.791h, 11.994h, 10.095h, 11.293h, 9.273h and 17.322h, respectively. CONCLUSIONS: The MAE method developed exhibited the highest extraction yield within the shortest time and thus could be used to extract the active compounds from Peganum harmala L. on an industrial basis. As the active compounds of Peganum harmala L., vasicine, harmalin and harmine presented the marked acaricidal activities against Psoroptes cuniculi, and could be widely applied for the treatments of acariasis in animals.

DNA binding and apoptotic induction ability of harmalol in HepG2: Biophysical and biochemical approaches.

Harmalol administration caused remarkable reduction in proliferation of HepG2 cells with GI50 of 14.2 µM, without showing much cytotoxicity in embryonic liver cell line, WRL-68. Data from circular dichroism (CD) and differential scanning calorimetric (DSC) analysis of harmalol-CT DNA complex shows conformational changes with prominent CD perturbation and stabilization of CT DNA by 8 °C. Binding constant and stoichiometry was calculated using the above biophysical techniques. The Scatchard plot constructed from CD data showed cooperative binding, from which the cooperative binding affinity (K'ω) of 4.65 ± 0.7 × 10(5) M(-1), and n value of 4.16 were deduced. The binding parameter obtained from DSC melting data was in good agreement with the above CD data. Furthermore, dose dependent apoptotic induction ability of harmalol was studied in HepG2 cells using different biochemical assays. Generation of ROS, DNA damage, changes in cellular external and ultramorphology, alteration of membrane, formation of comet tail, decreased mitochondrial membrane potential and a significant increase in Sub Go/G1 population made the cancer cell, HepG2, prone to apoptosis. Up regulation of p53 and caspase 3 further indicated the apoptotic role of harmalol.

Studies of the interaction of the fluorophores harmine and harmaline with calf thymus DNA.

The binding to calf thymus DNA of the hallucinogen harmine and one of its analogues harmaline was studied by absorption spectrophotometry and fluorescence quenching analysis. Viscosity measurements were also carried out. For both molecules, quenched and unquenched sites on DNA are present. For each type of binding site, the value of the product of the number of sites times the association constant was determined. Harmine is more strongly bound than harmaline. Viscosity measurements indicate intercalation in the case of harmine only.

Harmaline distribution in single muscle fibres and the inhibition of sodium efflux.

Harmaline, a known inhibitor of the (Na+ + K+)-ATPase in cell membranes, inhibited 50% of the 22Na efflux from barnacle muscle fibres at an extracellular concentration of 2.4 mM. Injected harmaline inhibited 50% of the efflux at an estimated intracellular concentration of about 8 mM . kg-1, assuming complete equilibration with no binding. Total fibre harmaline was measured in separate fibres by ultraviolet spectrophotometry. Fibres in 3 mM harmaline saline accumulated harmaline with a half-time of 17 min and a final total fibre concentration of 6-12 mM . kg-1. In harmaline-free saline this accumulated harmaline was lost exponentially with a half-time of 35 min; injected harmaline was lost exponentially from fibres with a half-time of 50 min. It is proposed that harmaline crosses the fibre membrane as the uncharged base and that its apparent accumulation against a concentration gradient is mainly due to intracellular binding with an additional contribution from a transmembrane ph gradient. It is concluded that, in fibres exposed to harmaline saline, the intracellular concentration can reach a sufficiently high value, as judged from the results of the injection experiments, to inhibit Na+ efflux at an interior-facing site on the fibre membrane. In contrast, harmaline appears to inhibit the Na+-dependent uptake of L-glutamate at an extracellular site.

Various alkaloid profiles in decoctions of Banisteriopsis caapi.

Twenty nine decoctions of Banisteriopsis caapi from four different sources and one specimen of B. caapi paste were analyzed for N,N-dimethyltryptamine (DMT), tetrahydroharmine (THH), harmaline and harmine. Other plants were also used in the preparation of these products, typically Psychotria viridis, which provides DMT. There were considerable variations in alkaloid profiles, both within and between sample sources. DMT was not detected in all samples. Additional THH may be formed from both harmine and harmaline during the preparation of these products. The alkaloid composition of one decoction sample did not change significantly after standing at room temperature for 80 days, but the initial acidic pH was neutralized by natural fermentation after 50 days.

Beta-carboline and quinoline alkaloids in root cultures and intact plants of Peganum harmala.

Alkaloid profiles of root and shoot cultures, seedlings and mature plants were analysed by capillary GLC and GLC-MS. beta-Carboline alkaloids, such as harmine, harmaline dominate in normal and root cultures transformed by Agrobacterium rhizogenes, as well as in roots and fruits of the plant. In shoots, flowers and shoot cultures quinoline alkaloids such as peganine, deoxypeganine, vasicinone and deoxyvasicinone widely replace the beta-carboline alkaloids. In root cultures, the formation of beta-carboline alkaloids can be induced by methyljasmonate and several other elicitors indicating that these alkaloids are part of the reactive chemical defence system of Peganum harmala.

Effects of harmine on transmembrane potentials of guinea-pig atrial muscle.

1 The effects of harmine 8.3 X 10(-5) M on membrane potentials of guinea-pig atrial muscle were analyzed and compared with those of harmaline. Transmembrane potentials of contractile fibres were measured during exposure to the drug at 30 degrees C. 2 In preparations superfused with 5.4 mmol K+-Tyrode, harmine produced a progressive reduction in the amplitude of the action potential (AP), in the absence of any change in resting potential (RP) and a prolongation of the duration of the action potential (APD). 3 Harmaline produced an enhancement of AP; RP was not affected and APD was prolonged. 4 The amplitude of slow responses elicited by noradrenaline in 16.2 mmol K+-Tyrode was enhanced by harmine. 5 It is proposed that dehydrogenation of harmaline to harmine reverses the initial stimulatory action of harmaline on AP because the depressant action on the fast component of the upstroke prevails over the stimulatory effect on the slow component.

Characterization and quantitative autoradiography of [3H]tryptamine binding sites in rat brain.

[3H]Tryptamine binds with high affinity (Kd = 9.1 nM, Bmax = 54 fmol/mg wet wt.) to tissue sections of rat brain. The binding occurs rapidly and is reversible. Low concentrations of the beta-carbolines harmaline (IC50 = 25 nM) and tetrahydronorharman (tetrahydro-beta-carboline, IC50 = 50 nM) inhibit [3H]tryptamine binding. Serotonin (5-HT, IC50 = 2600 nM) as well as the 5-HT receptor antagonists methysergide and metergoline displace [3H]tryptamine at much higher concentrations from brain slices. The distribution of [3H]tryptamine binding sites in sections of rat brain has been analyzed by quantitative autoradiography. The highest density of binding sites is found in the nucleus (n.) interpeduncularis, a slightly lower one in the locus coeruleus. Moderately labelled are the n. accumbens septi, n. septi lateralis, n. medialis habenulae, n. tractus olfactorii lateralis, the central region of the amygdala, n. caudatus/putamen, n. reuniens and the hippocampal formation. A low density of binding sites is detected in the cerebral cortex and the subiculum. Even less binding sites are found in the n. dorsalis raphe and the substantia nigra. The pattern of distribution of [3H]tryptamine binding sites differs from that of [3H]5-HT (5-HT1), [3H]ketanserin (5-HT2) as well as [3H]imipramine binding sites. These data suggest unique tryptamine binding sites.

Harmaline competitively inhibits [3H]MK-801 binding to the NMDA receptor in rabbit brain.

Harmaline, a beta-carboline derivative, is known to produce tremor through a direct activation of cells in the inferior olive. However, the receptor(s) through which harmaline acts remains unknown. It was recently reported that the tremorogenic actions of harmaline could be blocked by the noncompetitive NMDA channel blocker, MK-801. This study examined whether the blockade of harmaline's action, in the rabbit, by MK-801 was due to a pharmacological antagonism at the MK-801 binding site. This was accomplished by measurement of [3H]MK-801 binding in membrane fractions derived from tissue containing the inferior olivary nucleus and from cerebral cortex. Harmaline completely displaced saturable [3H]MK-801 binding in both the inferior olive and cortex with apparent IC50 values of 60 and 170 microM, respectively. These IC50 values are consistent with the high doses of harmaline required to produce tremor, e.g., 10-30 mg/kg. Non-linear curve fitting analysis of [3H]MK-801 saturation experiments indicated that [3H]MK-801 bound to a single site and that harmaline's displacement of [3H]MK-801 binding to the NMDA receptor was competitive as indicated by a shift in Kd but not in Bmax. In addition, a Schild plot gave a slope that was not significantly different from 1 indicating that harmaline was producing a displacement of [3H]MK-801 from its binding site within the NMDA cation channel and not through an action at the glutamate or other allosteric sites on the NMDA receptor. These findings provide in vitro evidence that the competitive blockade of harmaline-induced tremor by MK-801 occurs within the calcium channel coupled to the NMDA receptor. Our hypothesis is that harmaline produces tremor by acting as an inverse agonist at the MK-801 binding site and thus opening the cation channel.

Biotransformation of 1-methyl-1,2,3,4-tetrahydro-beta-carboline-1-carboxylic acid to harmalan, tetrahydroharman and harman in rats.

1-Methyl-1,2,3,4-tetrahydro-beta-carboline-1-carboxylic acid (1-carboxytetrahydroharman, 1-CTHH) has been detected in the brain of rats following intracerebroventricular injection of tryptamine and pyruvic acid. We now report the metabolism of this compound. Following intraperitoneal injection of 1-CTHH into rats, harmalan was found to be the major metabolite besides tetrahydroharman (THH) and harman. A high concentration of THH was measured in the lung while most of harman was found in the urine. Harmalan and THH could be detected in the brain in low concentrations. The products were separated following extraction from tissues by high-performance liquid chromatography (HPLC) on a reversed phase C18-DB column. The identity of the metabolites was confirmed by mass spectrometry (MS) analysis. The results demonstrate the role of 1-CTHH as a precursor of the biologically active compounds harmalan, THH and harman.

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.