Monoamine Oxidase Inhibition by Plant-Derived β-Carbolines; Implications for the Psychopharmacology of Tobacco and Ayahuasca

The monoamine oxidases (MAOs) are flavin-containing amine oxidoreductases responsible for metabolism of many biogenic amine molecules in the brain and peripheral tissues. Whereas serotonin is the preferred substrate of MAO-A, phenylethylamine is metabolized by MAO-B, and dopamine and tyramine are nearly ambivalent with respect to the two isozymes. β-Carboline alkaloids such as harmine, harman(e), and norharman(e) are MAO inhibitors present in many plant materials, including foodstuffs, medicinal plants, and intoxicants, notably in tobacco (Nicotiana spp.) and in Banisteriopsis caapi, a vine used in the Amazonian ayahuasca brew. The β-carbolines present in B. caapi may have effects on neurogenesis and intrinsic antidepressant properties, in addition to potentiating the bioavailability of the hallucinogen N,N-dimethyltryptamine (DMT), which is often present in admixture plants of ayahuasca such as Psychotria viridis. Tobacco also contains physiologically relevant concentrations of β-carbolines, which potentially contribute to its psychopharmacology. However, in both cases, the threshold of MAO inhibition sufficient to interact with biogenic amine neurotransmission remains to be established. An important class of antidepressant medications provoke a complete and irreversible inhibition of MAO-A/B, and such complete inhibition is almost unattainable with reversible and competitive inhibitors such as β-carbolines. However, the preclinical and clinical observations with synthetic MAO inhibitors present a background for obtaining a better understanding of the polypharmacologies of tobacco and ayahuasca. Furthermore, MAO inhibitors of diverse structures are present in a wide variety of medicinal plants, but their pharmacological relevance in many instances remains to be established. Keywords: Banisteriopsis caapi; Nicotiana; ayahuasca; dimethyltryptamine (DMT); harmine; monoamine oxidase (MAO); tobacco; β-carbolines

Monoamine Oxidase Inhibition by Plant-Derived β-Carbolines; Implications for the Psychopharmacology of Tobacco and Ayahuasca.

The monoamine oxidases (MAOs) are flavin-containing amine oxidoreductases responsible for metabolism of many biogenic amine molecules in the brain and peripheral tissues. Whereas serotonin is the preferred substrate of MAO-A, phenylethylamine is metabolized by MAO-B, and dopamine and tyramine are nearly ambivalent with respect to the two isozymes. β-Carboline alkaloids such as harmine, harman(e), and norharman(e) are MAO inhibitors present in many plant materials, including foodstuffs, medicinal plants, and intoxicants, notably in tobacco (Nicotiana spp.) and in Banisteriopsis caapi, a vine used in the Amazonian ayahuasca brew. The β-carbolines present in B. caapi may have effects on neurogenesis and intrinsic antidepressant properties, in addition to potentiating the bioavailability of the hallucinogen N,N-dimethyltryptamine (DMT), which is often present in admixture plants of ayahuasca such as Psychotria viridis. Tobacco also contains physiologically relevant concentrations of β-carbolines, which potentially contribute to its psychopharmacology. However, in both cases, the threshold of MAO inhibition sufficient to interact with biogenic amine neurotransmission remains to be established. An important class of antidepressant medications provoke a complete and irreversible inhibition of MAO-A/B, and such complete inhibition is almost unattainable with reversible and competitive inhibitors such as β-carbolines. However, the preclinical and clinical observations with synthetic MAO inhibitors present a background for obtaining a better understanding of the polypharmacologies of tobacco and ayahuasca. Furthermore, MAO inhibitors of diverse structures are present in a wide variety of medicinal plants, but their pharmacological relevance in many instances remains to be established

Dopamine release, diffusion and uptake : A computational model for synaptic and volume transmission

Computational modeling of dopamine transmission is challenged by complex underlying mechanisms. Here we present a new computational model that (I) simultaneously regards release, diffusion and uptake of dopamine, (II) considers multiple terminal release events and (III) comprises both synaptic and volume transmission by incorporating the geometry of the synaptic cleft. We were able to validate our model in that it simulates concentration values comparable to physiological values observed in empirical studies. Further, although synaptic dopamine diffuses into extra-synaptic space, our model reflects a very localized signal occurring on the synaptic level, i.e. synaptic dopamine release is negligibly recognized by neighboring synapses. Moreover, increasing evidence suggests that cognitive performance can be predicted by signal variability of neuroimaging data (e.g. BOLD). Signal variability in target areas of dopaminergic neurons (striatum, cortex) may arise from dopamine concentration variability. On that account we compared spatio-temporal variability in a simulation mimicking normal dopamine transmission in striatum to scenarios of enhanced dopamine release and dopamine uptake inhibition. We found different variability characteristics between the three settings, which may in part account for differences in empirical observations. From a clinical perspective, differences in striatal dopaminergic signaling contribute to differential learning and reward processing, with relevant implications for addictive- and compulsive-like behavior. Specifically, dopaminergic tone is assumed to impact on phasic dopamine and hence on the integration of reward-related signals. However, in humans DA tone is classically assessed using PET, which is an indirect measure of endogenous DA availability and suffers from temporal and spatial resolution issues. We discuss how this can lead to discrepancies with observations from other methods such as microdialysis and show how computational modeling can help to refine our understanding of DA transmission. Author summary The dopaminergic system of the brain is very complex and affects various cognitive domains like memory, learning and motor control. Alterations have been observed e.g. in Parkinson's or Huntington's Disease, ADHD, addiction and compulsive disorders, such as pathological gambling and also in obesity. We present a new computational model that allows to simulate the process of dopamine transmission from dopaminergic neurons originated in source brain regions like the VTA to target areas such as the striatum on a synaptic and on a larger, volume-spanning level. The model can further be used for simulations of dopamine related diseases or pharmacological interventions. In general, computational modeling helps to extend our understanding, gained from empirical research, to situations were in vivo measurements are not feasible.Peer reviewe

Neural Pathways and Receptor Mechanisms Mediating Stimulation-Evoked Striatal Dopamine Release: Relevance to Deep Brain Stimulation as a Treatment for Parkinsons Disease

Dopaminergic neurons of the nigrostriatal dopaminergic system, projecting from the substantia nigra compacta (SNc) to the striatum, serve a critical role in mediating voluntary motor control.Parkinson’s disease is a neurological disorder characterized by progressive degeneration of these dopamine neurons, which leads to dopaminergic deficiencies in the striatum.Reduced striatal dopamine transmission is thought to increase inhibitory basal ganglia output to the thalamus and subsequently reduce excitation of cortical motor areas, resulting in impaired motor functioning.Despite unclear mechanisms, deep brain stimulation (DBS) is an established neurosurgical approach for effectively treating the parkinsonian motor symptoms.Currently the subthalamic nucleus (STN) is the most commonly targeted site in these procedures, while the pedunculopontine tegmental nucleus (PPT) is emerging as a therapeutically beneficial target when stimulated alone or in combination with the STN.Thus, the connectivity between these nuclei and the nigrostriatal dopamine system is the focus of the present paper, with the overarching hypothesis being that the therapeutic benefits of STN/PPT DBS are mediated, at least in part, by activation of surviving nigrostriatal neurons, resulting in striatal dopamine release.The present study investigated several neural pathways and receptor mechanisms involved in mediating STN or PPT stimulation-evoked striatal dopamine release using in vivo fixed potential amperometry with carbon-fiber recording microelectrodes in the striatum of urethane-anesthetized mice.Overall, results indicate that STN stimulation evokes striatal dopamine release directly via excitatory glutamatergic inputs to SNc dopamine cells as well as indirectly by activating excitatory glutamatergic and cholinergic STN-PPT-SNc pathways, while PPT stimulation evokes striatal dopamine release directly by activating glutamatergic and cholinergic pathways to SNc dopamine cells as well as indirectly via activation of glutamatergic and cholinergic PPT-STN-SNc projections.Understanding the influence of the STN and PPT on SNc dopamine cell activity and output of the basal ganglia-thalamocortical motor circuit may lead to novel pharmaceutical therapies as well as a better understanding of the underlying mechanisms of clinical DBS, which could then improve the therapeutic efficacy of treatments for Parkinson’s disease

NICOTINIC RECEPTOR MODULATION OF DOPAMINE TRANSPORTERS

The current project examined the ability of nicotine to modulate dopamine transporter (DAT) function. Initial experiments determined the dose-response for nicotine to modulate dopamine (DA) clearance in rat striatum and medial prefrontal cortex (MPFC) using in vivo voltammetry and determined if this effect was mediated by nicotinic receptors (nAChRs). In both striatum and MPFC, nicotine increased DA clearance in a mecamylamine-sensitive manner, indicating nAChR-mediation. The effect of acute nornicotine on DAT function was also determined. In contrast to nicotine, nornicotine in a dose-related manner decreased striatal DA clearance in a mecamylamine-sensitive manner, indicating nAChR mediation. To determine if tolerance developed to the nicotine effect nicotine, separate groups of rats were injected once daily for 5 days with nicotine or saline. DA clearance in striatum and MPFC was determined 24 hrs after the last injection. Nicotine increased DA clearance only 10-15% in the group repeatedly administered nicotine, demonstrating that tolerance developed. To determine if nicotine altered striatal DAT efficiency, following nicotine injection, DAT density and maximal velocity of [3H]DA uptake was determined using [3H]GBR12935 binding and saturation analysis of [3H]DA uptake in rat striatum, respectively. Nicotine did not alter the Bmax or Kd of maximal binding of [3H]GBR12935 binding. However, an increase in Vmax was observed at 10 and 40 min following nicotine injection, suggesting that nicotine increases DAT efficiency. To determine if systemic nicotine enhanced DAT function via an action at nAChRs on striatal DA terminals, [3H]DA uptake was determined in striatum in vitro in the absence or presence of nicotine in the buffer. Nicotine did not alter the Vmax for [3H]DA uptake in vitro, suggesting that the nicotine-induced increase in DAT function observed in vivo is mediated by nAChRs on DA cell bodies or another site which indirectly alters DAT function. To determine if the increase in DAT efficiency was due to increased surface expression of striatal DAT, biotinylation and Western blot analyses were performed. Nicotine did not alter striatal DAT, suggesting that the nicotine-induced increase in DA clearance in vivo and DAT efficiency in vitro is not the result of increased trafficking of this protein to the cell surface

How Addictive Drugs Disrupt Presynaptic Dopamine Neurotransmission

The fundamental principle that unites addictive drugs appears to be that each enhances synaptic dopamine by means that dissociate it from normal behavioral control, so that they act to reinforce their own acquisition. This occurs via the modulation of synaptic mechanisms that can be involved in learning, including enhanced excitation or disinhibition of dopamine neuron activity, blockade of dopamine reuptake, and altering the state of the presynaptic terminal to enhance evoked over basal transmission. Amphetamines offer an exception to such modulation in that they combine multiple effects to produce nonexocytic stimulation-independent release of neurotransmitter via reverse transport independent from normal presynaptic function. Questions about the molecular actions of addictive drugs, prominently including the actions of alcohol and solvents, remain unresolved, but their ability to co-opt normal presynaptic functions helps to explain why treatment for addiction has been challenging

Pharmacological Evaluation of Choline on α4β2 Neuronal Nicotinic Acetylcholine Receptors

Choline is the precursor and metabolite of acetylcholine (ACh), the endogenous neurotransmitter responsible for activation of neuronal nicotinic acetylcholine receptors (nAChRs). Choline is known to activate some nAChRs, which suggests that it may play a role in neurotransmission beyond being a precursor/metabolite. Using electrophysiological techniques, this study investigates effects of choline on currently known α4β2 nAChRs stochiometries ― (α4)3(β2)2 and (α4)2(β2)3. The results suggests that choline activates (α4)3(β2)2 nAChRs with EC50 of 0.4mM and a maximal efficacy of 4%. In contrast, it did not produce any response on (α4)2(β2)3. When co-applied with ACh we revealed that low concentrations of choline potentiated currents induced by 1 µM and 10 µM ACh, but inhibited by choline in concentrations of 1mM or higher. Choline was also tested on mutant receptors engineered to only have α4α4 or α4β2-like binding sites in (4)3(23M)2 or (43M)3(2)2 respectively. On these mutant constructs, choline activated the (4)3(23M)2 receptors with EC50 of 0.3mM and a maximum response of 4%. In contrast, choline had no effect on (43M)3(2)2 nAChRs. Overall our results show that at least one α4α4 binding site is required to notice choline activity on α4β2 nAChRs. Choline also modulates ACh-mediated activation of nAChRs