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James Parkinson first described a disease characterized by tremor in 1817. Today, Parkinson’s disease is the second most common neurodegenerative disease, after Alzheimer’s. Parkinson’s disease is characterized by neuronal death in the substantia nigra, which is Latin for ‘black substance,’ as it appears darker than neighboring brain regions due to the high levels of neuromelanin in its dopaminergic neurons[1].
Dopaminergic neurons are neurons that carry receptors for the neurotransmitter dopamine, which is involved in motor coordination, reward, and habituation. Dopamine is also the neurotransmitter whose action most characterizes drug addiction – hence the term ‘dope.’ In drug addiction, enhanced dopamine transmission causes excitotoxicity of dopaminergic neurons. Failing to downregulate their dopamine receptors, neurons die of over-stimulation[2]. In some ways, dopamine can be thought of an endogenous neurotoxin[3].
The search for drugs to prevent or repair damage to dopaminergic neurons is important to sufferers of the debilitating disease Parkinson’s, to those who have damaged their brains with recreational drug use, and to the emerging community interested in cognitive enhancement. This article briefly reviews a promising, little-known molecule who’s exceptional regenerative ability on dopaminergic neurons was discovered in recent years.
CARBOLINES IN NATURE
Carbolines are a heterogenous group of naturally occurring pyridoindole compounds. Classified according to their skeleton as alpha, beta, gamma, or delta-carbolines, depending on the location of the nitrogen in their pyridine ring, the beta-carbolines (BCs) are best studied because of the rarity of alpha, gamma, and delta-carbolines in nature. BCs, which can be synthesized from tryptamine and tryptamine-like compounds, are characterized as either tetrahydro-BCs or aromatic BCs[4].
BCs have been found in fruits[5], grilled bacon, fish[6], and sausages[7]. In meats, the formation of BCs is accelerated during charcoal-grilling, where aromatic BCs are found in high quantities[8]. BCs are also found in coffee, tobacco smoke, and alcohol[9], as well as endogenously in the human body, where they have been detected in milk, urine[10], blood[11], and cerebrospinal fluid[12].
THE TOXICITY OF SOME BETA-CARBOLINES
Many derivatives of BCs resemble 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the neurotoxin commonly used in academic studies to produce a Parkinsonian-like phenotype in rodent brains. Several BCs, particularly methylated forms such as 2,9-dimethyl-beta-carbolinium ion (2,9-dime-BC+), have proven to be neurotoxic to dopaminergic neurons[13]. Nonmethylated BCs can also be metabolized into neurotoxic forms by the enzymes 2N-methyltransferase and 9N-methyltransferase[14]. Thankfully, the subject of this article, 9-methyl-beta-carboline (9-me-BC), has not been observed to be acted on by 2N-methyltransferase.
THE USES OF BETA-CARBOLINES
BCs of the harman and norharman varieties exhibit heterogenous actions on the nervous system. Some act on the inhibitory GABA receptors, producing antidepressant[15], anxiolytic[16][17], and anti-epileptic effects[18]. Others act on glycine[19], serotonin[20], acetylcholine[21], and dopamine[22] neurotransmitter systems.
They have particularly remarkable potential as a substrate for the dopamine transporter (DAT)[23][24] and as inhibitors of the monoamine oxidase enzymes (A and B)[25][26] that degrade monoamine neurotransmitters, such as dopamine, norepinephrine, and serotonin. To prevent the degradation of the hallucinogen N,N-Dimethyltryptamine (DMT) by monoamine oxidase upon its oral consumption, BCs from the harmala plants are included in all preparations of the traditional ayahuasca beverage[27]. In tobacco smoke, harman and norharman BCs exert potent dopaminergic effects[28].
9-ME-BC’S BIOLOGICAL FUNCTION ON DOPAMINERGIC NEURONS
At high doses (0.25 millimole/kg), 9-methyl-beta-carboline (9-me-BC, the subject of this article), had previously been shown to be neurotoxic in mouse models[29]. However, in 2007 a German team of MDs showed that lower doses of 9-me-BC (0.48 micromole/kg) were neuroprotective in rats in vitro[30]. 9-me-BC is transported into dopaminergic neurons by the dopamine transporter (DAT), after which tyrosine hydroxylase (TH) neurons increase (by 30% at 90 micromole) and protein expression of TH increased by 75%[31]. TH is the rate-limiting enzyme crucial to the synthesis of the neurotransmitter dopamine, and early loss of TH activity is characteristic of the development of Parkinson’s disease[32].
9-me-BC has been shown to upregulate a variety of neurotrophic and morphogenic factors, including dopaminergic transcription factors engrailed in homeobox 1 (En1)[33], the sonic hedgehog (Shh)[34], nuclear receptor related 1 (Nurr1)[35], wingless-type mouse mammary tumor virus (MMTV) integration site family, member 1 (Wnt1)[36] and member 5a (Wnt5a)[37], and paired-like homeodomain transcription factor 3 (Pitx3)[38]. Nurr1 activity is particularly attractive, as Nurr1 is required for the development and maintenance of dopaminergic neurons[39], and the Nurr1 gene is under-expressed in Parkinson’s disease patients[40].
9-me-BC increased gene expression of the neurotrophic factors brain derived neurotrophic factor (BDNF), conserved dopamine neurotrophic factor (CDNF), cerebellin 1 (CBLN1), ciliary neurotrophic factor (CNTF), neurotrophin-3, and nerve growth factor (NGF) in astrocyte cell cultures[41][42]. These factors modulate the health of dopaminergic neurons - CDNF has been shown to regenerate dopaminergic neurons[43], while CBLN improves the synaptic plasticity and integrity of neurons[44].
9-me-BC also reduced gene expression of the pro-apoptotic (i.e. enhancing cell death) members of the p53 pathway, death-domain-associated protein Fas, the growth arrest and DNA-damage-inducing protein 45a (GADD45A), and caspase 3[45]. It also increases ATP content of dopaminergic cultures by upregulating nicotinamide adenine dinucleotide dehydrogenase (NADH) activity of the monomeric complex I of mitochondria[46].
Note that inhibition of the dopamine transporter (DAT) or PKA/C blocks the stimulatory effect of 9-me-BC on dopaminergic neurons[47], indicating their integral role in its effect.
9-ME-BC AND MONOAMINE OXIDASE
Monoamine oxidase is an enzyme that degrades dopamine in the brain. Monoamine oxidase’s (MAO) two isoforms both deaminate dopamine, while monoamine oxidase A preferentially deaminates norepinephrine and serotonin[48]. Elevated MAO-B[49] has been shown to degrade dopaminergic neurons while the inhibition of MAO-A[50] and MAO-B[51] has been shown to protect neurons from apoptosis. Strikingly, 9-me-BC inhibits both isoforms of MAO[52].
9-ME-BC AND INFLAMMATION
Inflammatory processes are integral to the degradation of dopaminergic neurons and the use of anti-inflammatory drugs have been associated with reduced incidence of Parkinson’s disease[53] (and Alzheimer’s disease[54]). In addition to its neurotrophic and neuroprotective effects on dopaminergic neurons, 9-me-BC is also anti-inflammatory.
Lipopolysaccharides (LPS), the notorious endotoxins produced by gastrointestinal bacteria, were studied in vitro with dopaminergic neurons to determine whether 9-me-BC could reduce their pro-inflammatory effect. LPS increased gene expression of the inflammatory cytokines and receptors CCL-1, CCL-2, CCL-4, CCL-5, complement factor 3 (C3), CVCL1, , beta 2 microglobulin (b2m),chemokine ligand 10 (CVCL-10), nitric oxide synthase 2 (NOS2), heat shock protein 90-kDa alpha (HSP90AB1), tumor necrosis factor (TNF), Fc receptor of IgE (FCER1G), interleukin 1-beta (IL-1B) and interleukin-18 (IL-18), all of which 9-me-BC sharply inhibited. It is hypothesized that 9-me-BC’s anti-inflammatory properties are mediated by the anti-inflammatory interleukin-10[55].
CONCLUSION
9-me-BC’s neuroprotective, neurotrophic, and anti-inflammatory qualities are profound, highly desirable, and rarely found in a single molecule.
Despite its attractiveness to researchers and cognitive enhancement aficionados, research on the molecule appears to have ceased for nearly a decade. In a future episode of The Live Long Podcast, I hope to interview one of the researchers who produced the admirable work on the molecule to learn more about it and determine why research on it has ceased. Nonetheless, the molecule is used in cognitive enhancement communities. If you do use it, be sure to avoid sunlight[56].
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