1. INTRODUCTION:

Plants are a rich source of bioactive molecules; partly as a result of their immobile nature. A driving force for evolution of higher photosynthetic organisms is the need to respond promptly and effectively to a variety of stimuli, both biotic and abiotic. It is estimated that more than two thirds of the world’s population relies on plant derived drugs. Some 7000 medicinal compounds used in the western pharmacopeia are derived from plants. In the USA, approximately 25% of all prescription drugs used contain one or more bioactive compounds derived from vascular plants. The largest fraction of global biodiversity is located in the neotropics with the atlantic forest and Amazon being a rich untapped reservoir of plants that may lead to new drug discovery⁽¹⁾.

The atlantic forest is considered one of the key global hotspots of biodiversity, hosting thousands of endemic species⁽²⁾. Unexplored biodiversity and high levels of endemism combined with ethnopharmacological knowledge from traditional communities⁽³⁾ lead to high potential for drug discovery in this region. Some plants from the Neotropics are already booming in the international scenario. Bioactive molecules are often isolated from Rubiaceae species. In South America, three genera are of special interest due to the bioactivities of their phytochemicals along with their importance to local human populations: Uncaria, Cinchona, and Psychotria. The numerous bioactivities of alkaloids from species in these genera include emetic, cytotoxic, analgesic, anxiolytic, antidepressant, psychotropic, antipsychotic, antipyretic, anti-inflammatory, antioxidant, antimutagenic, immunomodulatory, vasorelaxant, antiviral, antimicrobial, and antiprotozoal. Indole alkaloids are widely represented in these plant groups. Detailed studies in planta and in vitro on the dynamics of biosynthesis, accumulation, and distribution of some of these metabolites in Psychotria have revealed complex regulatory controls, encompassing responses to both developmental and environmental signals.

1.1 Psychotria Genus:

The genus Psychotria is one of the largest genera of flowering plants and the largest within Rubiaceae, with estimated 1000 to 1650 species distributed worldwide. Psychotria species often accumulate indole alkaloids. This genus is taxonomically complex due to a lack of conspicuous morphological differentiating feature. A number of Psychotria species yielded bioactive extracts. Some examples include antibiotic activity in extracts from P. microlabastra and P. capensis (Africa), antiviral activity in P. serpens (China) and antiviral/antifungal and anti-inflammatory activities found in P. hawaiiensis and P. insularum (Central America), Antioxidant activity found in P. Nilgiriensis used in the teartment of Rheumatism. Active molecules produced by Psychotria species include naphtoquinones, peptides, benzoquinones, pigments and alkaloids. Perhaps the best known compound isolated from Psychotria species is the alkaloid emetine. Emetine is an isoquinoline alkaloid extracted from P. ipecacuanha (ipecac) bark, a plant used by traditional communities as stimulant and “antidote to opium” and in the treatment of intoxication due to its emetic effect. Synthetic analogs of emetine, which have less adverse effects, are currently used in the treatment of amoebiasis. Emetine is cytotoxic, inhibiting protein synthesis, and may have applications in drug-induced apoptosis. Another well-known Psychotria species is the one used in the preparation of the hallucinogenic drink “ayahuasca” P. viridis. The decoction is prepared using the plant in combination with the vine Banisteriopsis caapi. (Spruce ex Griseb) (Malpighiaceae). Psychotria viridis and B. caapi are rich sources of the proto-alkaloid Dimethyltryptamine (DMT) and the carboline harmine, respectively. Both substances are psychoactive, and the two have a strong synergism when administered together, possibly due to inhibitory effects of harmine on monoamine oxidase, a DMT detoxifying enzyme.

2. BIOACTIVE ALKALOIDS FROM SOUTH AMERICAN PSYCHOTRIA:

2.1 Psychotria ipecacuanha (Brot.) Stokes:

Emetine and Cephaeline: P. ipecacuanha, native to South and Central America, is a well-known species used as emetic, expectorant and treatment of amebic dysentery⁽⁴⁾. Traditional communities use this plant for intoxication treatment due to its emetic effect, as well as stimulant and “antidote to opium”⁽⁵⁾. This plant contains ipecac alkaloids, comprising the monoterpene isoquinoline alkaloids emetine, cephaeline, and psychotrine being the main components responsible for emetic properties. In addition, emetine is also used for amebic dysentery treatment, displaying cytotoxic activity, inhibiting protein synthesis, and with potential applications in drug-induced apoptosis. Currently, synthetic analogues of emetine with less adverse effects are used in the treatment of amoebiasis⁽⁶⁾. These ipecac alkaloids are present in all parts of the plant but in a greater quantity in the roots. Initial ipecac alkaloid biosynthesis is analogous to other MIA, with secologanin and dopamine, instead of tryptamine, condensation. After some methylations through O-methyl transferases, cephaeline is generated; and with addition of another methyl group, emetine is formed.

2.2 Psychotria viridis (Ruiz and Pav.):

Dimethyltryptamine P. viridis grows naturally in Amazonian tropical forests in Central and South America, and the leaves are used as one of the main components in the preparation of the hallucinogenic drink “ayahuasca”. This plant is a rich source of the psychedelic indole-alkaloid DMT⁽⁷⁾ in the mixture that also contains b-carboline alkaloids provided by Banisteriopsis caapi (Malpighiaceae), mainly harmine, harmaline, and tetrahydroharmine, that besides psychoactive properties, act as reversible MAOIs. MAO acts as a detoxifying enzymes⁽⁸⁾.P. carthagenensis is regarded as a substitute for P. viridis, although no evidence of alkaloids in plants or activity of key enzymes of secondary metabolism in cell cultures were detected in P. carthagenensis from Southern Brazil.

2.3 Psychotria colorata (Willd. ex Roem. and Schult.) Mull. Arg.:

Hodgkinsine, (+)-Chimonanthine, meso-Chimonanthine, and Psychotridine. An ethnobotanical survey identified several species used as painkillers among traditional rural communities from Brazilian Amazon, and different parts of P. colorata were reported for therapeutical uses among the Amazonian traditional rural communities, being flowers used for earache treatment and roots/fruits for abdominal pain⁽⁹⁾. Similar qualitative composition is present in flowers and leaves of P. colorata, being the highest content of alkaloids found in flowers followed by leaves and lower concentrations in roots. Preliminary tests with P. colorata extract from flowers indicated alkaloids as the major responsible for its analgesic effect, probably with opioid-like activity. Hodgkinsine, a pyrrolidine indole alkaloid, was identified as the major component among several pyrrolidine indole and quinoline alkaloids present in the flowers of this species. Hodgkinsine was first isolated from Hodgkinsonia frutescens (Rubiaceae), and it is a potent analgesic. Hodgkinsine is also present in P. muscosa and in the Indonesian P. malayana (also as the major alkaloid)⁽¹⁰⁾. The pyrrolidine indole alkaloids (+)-chimonanthine, meso-chimonanthine, and psychotridine also showed analgesic activity⁽¹¹⁾, but the P. colorata quinoline alkaloids did not show positive results for nociceptive activity.

2.4 Psychotria umbellata Vell.: Psychollatine:

P. umbellata; has a dose-dependent opioid-like analgesic activity. Psychollatine is a monoterpene indole alkaloid, the major alkaloid from P. umbellate⁽¹²⁾. The alkaloid accumulation seems to be regulated by developmental stage with the highest levels in inflorescences and the lowest in seeds, followed by fruit⁽¹³⁾ Psychollantine showed anxiolytic activity and antidepressant effect. Other properties of psychollatine include antimutagenic properties in Saccharomyces cerevisiae and antioxidant activity. Due to its properties, psychollatine has high pharmacological potential. Post-harvest stability experiments showed high stability of the molecule in temperatures below 65°C, high lighting an extra useful feature in future alkaloid applications.

2.5 Psychotria brachyceras Mull. Arg.: Brachycerine

P. brachyceras contains Brachycerine is a MIA, present only in shoots. Antioxidant activity against most physiologically relevant ROS in plants is a feature of this alkaloid^(14-16). Unlike Nicotiana sylvestris, in which nicotine is whole-plant-induced, brachycerine is accumulated only in damaged sites. In vitro assays showed that brachycerine has antimutagenic activity in S. cerevisiae. The other interesting results were nonspecific analgesic activity of P. brachyceras leaf ethanolic extract.

2.6 Psychotria myriantha Mull. Arg.: Strictosidinic Acid:

P. myriantha is common in Southern Brazilian forests and produces the MIA strictosidinic acid, being the major alkaloid present in P. myriantha leaves. Oral administration of strictosidinic acid in mice has analgesic and antipyretic effects, and it is also an efficient inhibitor of PMN chemotaxis in vivo, having anti-inflammatory properties⁽¹⁷⁾. Opioid-like analgesic activity was observed after intraperitoneal injection of this alkaloid in hot-plate and tail-flick models. Opioid-like denomination suggests morphine-similar effect, being blocked with naloxone-like antagonists.

2.7 Psychotria leiocarpa (Cham. and Schltdl.):

P. leiocarpa is native to Argentina, Paraguay, and Brazil. The MIA GPV is the major alkaloid present in the ethanolic extract of this plant, and it is detected only in shoots being the highest levels in leaves, reaching up to 2.5% DW. Nonspecific analgesic activity from foliar crude ethanolic extract was reported in tail-flick test⁽¹⁸⁾. An uncommon configuration for MIAs, with a glucose residue attached to the indole ring nitrogen, was first described in GPV.

2.8 Psychotria suterella Mull. Arg.: Lyaloside, Naucletine, and Strictosamide:

P. suterella is found in the coast of tropical forests in Southern Brazil. Lyaloside, naucletine, and strictosamide are MIAs present in P. suterella leaves, being absent in roots. In the tail-flick assay, no analgesic effects of the alkaloid extract obtained from leaves (100 or 300mg kg^-1), or isolated lyaloside (10 or 30mg kg^-1) were observed; in addition, the higher doses of extract and lyaloside lead to convulsions followed by death⁽¹⁹⁾.

2.9 Psychotria nuda (Cham. and Schltdl.) Wawra: Strictosamide:

P. nuda is common in tropical forests from Southern Brazil. The MIA strictosamide is the major alkaloid from P. nuda⁽²⁰⁾, being previously isolated from other plants. It is considered an important biosynthetic precursor of the antitumor alkaloid camptothecin, from Camptotheca acuminata. From South American Psychotria species, strictosamide was isolated from P. leiocarpa, P. suterella, P. bahiensis, and P. myriantha^(21-24). The alkaloid-rich extract obtained from P. nuda, as well as isolated strictosamide (major alkaloid from P. nuda), shows in vitro antichemotactic activity⁽¹⁷⁾. PMNs are recognized as the first cells to arrive at the sites of inflammation, these results being related to anti-inflammatory effects.

2.10 Psychotria nilgiriensis Deb and Gang:

Psychotria nilgiriensis is large shrub or small tree 4m in height and grows in high elevation evergreen forests between 2000 and 2300m. Its tender is consumed along with honey for its action against rheumatism. Acetone extract of P. nilgiriensis fruit and root contains notable chemical compounds that are responsible for its antioxidant activity. Acetone extract of fruit has significant antimicrobial activity against tested pathogens⁽²⁵⁾.

3. REGULATION OF PSYCHOTRIA ALKALOID BIOSYNTHESIS:

Alkaloid biosynthesis can be a very complex and highly regulated biological process. It may encompass various cell types⁽²⁶⁾, intensify in certain developmental stages, and be influenced by circumstantial factors, like nutritional status or stress^(27-29). Some alkaloid biosynthetic pathways, due to their importance, are meticulously studied, as is the case with Catharanthus roseus, Nicotiana tabacum, Papaver somniferum, and many others. The main goal of alkaloid (and other phytochemicals) research is to establish the genes and enzymatic steps needed for building a valuable compound, as well as what influences its production rate. This information could be used, for example, to manage phytochemical exploration of intact plants or to engineer controlled bioprocesses for their synthesis (e.g., bioreactors)⁽³⁰⁾. Despite progresses in alkaloid research, some of the most demanded alkaloids are still produced in large scale by land-cultivating alkaloid-containing plants and purifying the compounds, in a very expensive and laborious process. There are many examples of strong inhibition or total lack of biosynthesis of alkaloids and other phytochemicals in such conditions, including for Psychotria species⁽³⁰⁾ reinforcing the evidence for tight regulation of alkaloid biosynthesis and requirement of specialized tissues for its completion. This underlines the need to investigate distribution of alkaloids among tissues of adult plants as well as to determine the influence of various treatments on alkaloid production. Seedlings of in vitro cultured P. leiocarpa were exposed to different sucrose concentrations in the medium along with presence or absence of white light. GPV production, restricted to shoots, was strongly induced by light and inhibited by sucrose supply, indicating that photoautotrophic metabolism plays a major role in alkaloid accumulation. GPV also seems to be developmentally regulated since its content in older (150 days) seedlings is significantly higher than in younger ones (100 days).

The alkaloid is most concentrated in leaves and reproductive structures. Bioactive Alkaloids from South American Psychotria and Related Rubiaceae. Brachycerine accumulation in P. brachyceras also is restricted to shoots. It is most concentrated in inflorescences (0.3% over dry weight) but is also present in fully expanded leaves (0.18%), stems (0.18%), younger leaves (0.12%), and fruit pulp (0.04%). Leaf brachycerine concentration in adult field-grown plants varies seasonally, with lower concentrations in summer months. In vitro germinated seeds yield seedlings with alkaloid accumulation as observed in leaves of adult plants. Brachycerine accumulation is highly responsive to environmental signals. Mechanic damage and also jasmonic acid, a hormone related to signaling of herbivory, induce brachycerine production in cuttings. The same happens with UV radiation but with higher fold induction. Brachycerine-I, a strong antioxidant, and it’s up regulation in stress situations suggests a protective role for it in planta. The alkaloid is strongly accumulated in inflorescences and fruit. An in vitro somatic embryogenesis protocol was established for P. umbellata. It employs a hormonal balance and physical variable manipulation, notably light, that is capable of driving direct embryo development from somatic cells of rhizogenic calli, resulting, after further growth and acclimation to in vitro conditions, in fully differentiated plants. P. umbellata plants established in in vitro cultures had psychollatine concentrations similar to field-grown shrubs⁽³⁰⁾.

4. CONCLUSION:

The simple expansion of chemical investigations to include related species from a different biome chemotaxonomocal survey of Psychotria species of southern brazil involving plants that even lack popular names, revealed several other bioactive alkaloids. Further research on the biological activity of these new compounds, elucidation of their biosynthetic origins and regulation of production, along with studies on plant propagation, may convert relatively unknown species in viable sources of new drugs. In addition different strategies of accumulation are found in these plants and alkaloids, ranging from highly constitutive to strongly inducible upon environmental stimuli. Therefore it is likely that distinct transcription factors orchestrate the expression of genes encoding enzymes leading to the production of these related alkaloids. Clearly, in spite of significant progress, large efforts are still required to fully understand Psychotria alkaloid production and its multitude of applications.

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Received on 15.12.2019 Modified on 30.01.2020

Research J. Pharm. and Tech. 2020; 13(11):5484-5488.

DOI: 10.5958/0974-360X.2020.00957.9