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Terpenes and Essential Oils: Health Risks and Benefits

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A special issue of International Journal of Molecular Sciences (ISSN 1422-0067). This special issue belongs to the section "Bioactives and Nutraceuticals".

Deadline for manuscript submissions: closed (31 August 2020) | Viewed by 70415

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Special Issue Editor

Prof. Lenka Skalova

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Guest Editor

Charles University, Prague Praha, Czech Republic

Special Issue Information

Dear Colleagues,

Public interest in natural therapies has increased significantly over past decades. Numerous herbs and essential oils which are widely used in folk medicine contain terpenes as their major components. These compounds bear diverse linear and cyclic carbon skeletons and have attracted the increasing interest of scientists in multiple disciplines. Terpenes have also been reported to possess a broad spectrum of biological activities, including antibacterial, anticancer, insecticidal, and anti-inflammatory effects. Some of them have been registered as new drugs and are used in pharmacotherapy, e.g., artemisinin in malaria treatment. However, terpenes can also exert serious toxicities, and many cases of herb-induced liver injuries are reported every year. Well-known examples of dangerous terpenes are hepatotoxic pulegone, neurotoxic α-thujone, and nephrotoxic cantharidin.

This Special Issue aims to provide a forum for the dissemination of the latest information on the in vitro and in vivo testing of the biological activities of terpenes as well as essential oils, in view of possible pharmacological and/or toxicological applications. Researchers are cordially invited to submit work relevant to these topics for the Special Issue of International Journal of Molecular Sciences.

Prof. Lenka Skalova
Guest Editor

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Keywords

monoterpenes
sesquiterpenes
diterpenes
terpenes lactones
essential oils
anticancer
anti-inflammatory
antioxidant
antibacterial
anthelmintic activity
toxicity
pro-oxidative effect
pro-inflammatory effect
drug-interactions

Published Papers (13 papers)

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Research

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16 pages, 2425 KiB

Open AccessArticle

Carvone Enantiomers Differentially Modulate IgE-Mediated Airway Inflammation in Mice

by Jaime Ribeiro-Filho, Juliana da Silva Brandi, Hermann Ferreira Costa, Karina Carla de Paula Medeiros, Jacqueline Alves Leite, Damião Pergentino de Sousa and Márcia Regina Piuvezam

Int. J. Mol. Sci. 2020, 21(23), 9209; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21239209 - 03 Dec 2020

Cited by 9 | Viewed by 2143

Abstract

Carvone is a monoterpene found in nature in the form of enantiomers (S- and R-). While previous research has demonstrated the anti-inflammatory and anti-allergic effects of carvone, the influence of carvone enantiomeric composition on its anti-allergic activity remains to be investigated. This study aimed to evaluate the anti-allergic activity of carvone enantiomers in a murine model of airway allergic inflammation induced by sensitization and challenge with ovalbumin (OVA). The oral treatment with R-carvone or S-carvone 1 h before each challenge inhibited the number of leukocytes and eosinophils in the bronchoalveolar lavage (BAL). R-carvone inhibited leukocyte infiltration and mucus production in the lung, which was correlated with decreased production of OVA-specific IgE in the serum and increased concentrations of IL-10 in the BAL. On the other hand, the administration of S-carvone had little inhibitory effect on inflammatory infiltration and mucus production in the lung, which might be associated with increased production of IFN-γ in the BAL. When administered 1 h before each sensitization, both enantiomers inhibited eosinophil recruitment to the BAL but failed in decreasing the titers of IgE in the serum of allergic mice. Our data indicate that carvone enantiomers differentially modulated IgE-mediated airway inflammation in mice. In conclusion, unlike S-carvone, R-carvone has the potential to be used in anti-allergic drug development. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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17 pages, 3365 KiB

Open AccessArticle

TMPE Derived from Saffron Natural Monoterpene as Cytotoxic and Multidrug Resistance Reversing Agent in Colon Cancer Cells

by Kamila Środa-Pomianek, Anna Palko-Łabuz, Andrzej Poła, Mirosława Ferens-Sieczkowska, Olga Wesołowska and Agata Kozioł

Int. J. Mol. Sci. 2020, 21(20), 7529; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21207529 - 13 Oct 2020

Cited by 4 | Viewed by 2180

Abstract

Terpenes constitute one of the largest groups of natural products. They exhibit a wide range of biological activities including antioxidant, anticancer, and drug resistance modulating properties. Saffron extract and its terpene constituents have been demonstrated to be cytotoxic against various types of cancer cells, including breast, liver, lung, pancreatic, and colorectal cancer. In the present work, we have studied anticancer properties of TMPE, a newly synthesized monoterpene derivative of β-cyclocitral—the main volatile produced by the stigmas of unripe crocuses. TMPE presented selective cytotoxic activity to doxorubicin-resistant colon cancer cells and was identified to be an effective MDR modulator in doxorubicin-resistant cancer cells. Synergy between this derivative and doxorubicin was observed. Most probably, TMPE inhibited transport activity of ABCB1 protein without affecting its expression level. Analysis of TMPE physicochemical parameters suggested it was not likely to be transported by ABCB1. Molecular modeling showed TMPE being more reactive molecule than the parental compound—β-cyclocitral. Analysis of electrostatic potential maps of both compounds prompted us to hypothesize that reduced reactivity as well as susceptibility to electrophilic attack were related to the lower general toxicity of β-cyclocitral. All of the above pointed to TMPE as an interesting candidate molecule for MDR reversal in cancer cells. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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15 pages, 2007 KiB

Open AccessArticle

Antibacterial Activity of Selected Essential Oil Compounds Alone and in Combination with β-Lactam Antibiotics Against MRSA Strains

by Paweł Kwiatkowski, Łukasz Łopusiewicz, Agata Pruss, Mateusz Kostek, Monika Sienkiewicz, Radosław Bonikowski, Iwona Wojciechowska-Koszko and Barbara Dołęgowska

Int. J. Mol. Sci. 2020, 21(19), 7106; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21197106 - 26 Sep 2020

Cited by 18 | Viewed by 3383

Abstract

This study aimed to determine the effect of selected essential oil compounds (EOCs) on the antibacterial activity of β-lactam antibiotics (βLAs) against methicillin-resistant Staphylococcus aureus (MRSA) strains. The following parameters were studied: antibiotic susceptibility testing, detection of mecA gene and evaluation of genotypic relativity of isolates using molecular techniques, analysis of chemical composition applying Fourier-transform infrared (FTIR) spectroscopy, and determination of antibacterial activity of EOCs alone and in combination with βLAs against MRSA strains using microdilution and checkerboard methods. It was found that all isolates expressed MRSA and resistance phenotypes for macrolides, lincosamides, and streptogramins B. All isolates harbored the mecA gene and belonged to three distinct genotypes. Eight of the 10 EOCs showed efficient antimicrobial activity against the MRSA reference strain. The analysis of interaction between EOCs and βLAs against the MRSA reference strain revealed a synergistic and additive effect of the following combinations: methicillin (Met)-linalyl acetate (LinAc), penicillin G (Pen)-1,8-cineole (Cin), and Pen-LinAc. Analysis of EOC-βLA interactions showed a synergistic and additive effect in the following combinations: Met-LinAc (against low- and high-level βLAs resistance strains), Pen-Cin, and Pen-LinAc (against low-level βLAs resistance strains). It was also confirmed that changes in phosphodiester, -OH, -CH₂ and -CH₃ groups may change the interactions with βLAs. Moreover, the presence of two CH₃O- moieties in the Met molecule could also play a key role in the synergistic and additive mechanism of LinAc action with Met against MRSA strains. Direct therapy using a Met-LinAc combination may become an alternative treatment method for staphylococcal infections caused by MRSA. However, this unconventional therapy must be preceded by numerous cytotoxicity tests. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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19 pages, 4047 KiB

Open AccessArticle

Innate Immune Response against Staphylococcus aureus Preincubated with Subinhibitory Concentration of trans-Anethole

by Paweł Kwiatkowski, Bartosz Wojciuk, Iwona Wojciechowska-Koszko, Łukasz Łopusiewicz, Bartłomiej Grygorcewicz, Agata Pruss, Monika Sienkiewicz, Karol Fijałkowski, Edward Kowalczyk and Barbara Dołęgowska

Int. J. Mol. Sci. 2020, 21(11), 4178; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21114178 - 11 Jun 2020

Cited by 16 | Viewed by 4468

Abstract

The study aimed to analyze morphological and functional changes of Staphylococcus aureus cells due to trans-anethole (a terpenoid and the major constituent of fennel, anise, or star anise essential oils) exposition, and their consequences for human neutrophils phagocytic activity as well as IL-8 production (recognized as the major chemoattractant). The investigation included the evaluation of changes occurring in S. aureus cultures, i.e., staphyloxanthin production, antioxidant activities, cell size distribution, and cells composition as a result of incubation with trans-anethole. It was found that the presence of trans-anethole in the culture medium reduced the level of staphyloxanthin production, as well as decreased antioxidant activities. Furthermore, trans-anethole-treated cells were characterized by larger size and a tendency to diffuse in comparison to the non-treated cells. Several cell components, such as phospholipids and peptidoglycan, were found remarkably elevated in the cultures treated with trans-anethole. As a result of the aforementioned cellular changes, the bacteria were phagocytized by neutrophils more efficiently (ingestion and parameters associated with killing activity were at a higher level as compared to the control system). Additionally, IL-8 production was at a higher level for trans-anethole modified bacteria. Our results suggest that trans-anethole represents a promising measure in combating severe staphylococcal infections, which has an important translational potential for clinical applications. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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14 pages, 7309 KiB

Open AccessArticle

Preliminary Assessment of the Mucosal Toxicity of Tea Tree (Melaleuca alternifolia) and Rosemary (Rosmarinus officinalis) Essential Oils on Novel Porcine Uterus Models

by Martina Bertocchi, Antonella Rigillo, Alberto Elmi, Domenico Ventrella, Camilla Aniballi, Diana G. Scorpio, Maurizio Scozzoli, Giuliano Bettini, Monica Forni and Maria Laura Bacci

Int. J. Mol. Sci. 2020, 21(9), 3350; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21093350 - 09 May 2020

Cited by 7 | Viewed by 2921

Abstract

Antimicrobial resistance, an ever-growing global crisis, is strongly linked to the swine production industry. In previous studies, Melaleuca alternifolia and Rosmarinus officinalis essential oils have been evaluated for toxicity on porcine spermatozoa and for antimicrobial capabilities in artificial insemination doses, with the future perspective of their use as antibiotic alternatives. The aim of the present research was to develop and validate in vitro and ex vivo models of porcine uterine mucosa for the evaluation of mucosal toxicity of essential oils. The in vitro model assessed the toxicity of a wider range of concentrations of both essential oils (from 0.2 to 500 mg/mL) on sections of uterine tissue, while the ex vivo model was achieved by filling the uterine horns. The damage induced by the oils was assessed by Evans Blue (EB) permeability assay and histologically. The expression of ZO-1, a protein involved in the composition of tight junctions, was assessed through immunohistochemical and immunofluorescence analysis. The results showed that low concentrations (0.2–0.4 mg/mL) of both essential oils, already identified as non-spermicidal but still antimicrobial, did not alter the structure and permeability of the swine uterine mucosa. Overall, these findings strengthen the hypothesis of a safe use of essential oils in inseminating doses of boar to replace antibiotics. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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8 pages, 237 KiB

Open AccessArticle

Vepris macrophylla (Baker) I. Verd Essential Oil: An Antifungal Agent against Phytopathogenic Fungi

by Laura Giamperi, Anahi Elena Ada Bucchini, Donata Ricci, Bruno Tirillini, Marcello Nicoletti, Rianasoambolanoro Rakotosaona and Filippo Maggi

Int. J. Mol. Sci. 2020, 21(8), 2776; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21082776 - 17 Apr 2020

Cited by 7 | Viewed by 1997

Abstract

Rutaceae are widely used in ethnomedicine to treat infectious diseases in humans and plants. In this study, the antifungal activity of the Vepris macrophylla leaf essential oil (VEO) and its main components, citral and citronellol, was evaluated against six phytopathogenic fungi. In addition, the possible action of VEO on the synthesis of mycotoxins was evaluated as well. To determine the antifungal activity of VEO we used the agar dilution method and VEO showed inhibitory activity against all the tested fungi. In particular, VEO resulted to be fungicidal against Phytophthora cryptogea and Fusarium avenaceum. For all other fungi VEO exhibited fungistatic activity and the weakest effect was observed on Alternaria solani. Citral was very effective against P. cryptogea, F. avenaceum, F. poae and F. graminearum. On the other hand, citronellol showed good activity towards P. cryptogea and F. avenaceum and weaker activity towards F. poae and F. graminearum. It can be concluded that VEO can be considered a promising antifungal agent, especially against P. cryptogea and F. avenaceum, suggesting a possible use in the formulation of new selective and natural fungicides. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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14 pages, 2894 KiB

Open AccessArticle

Role of 5-HT1A Receptor in the Anxiolytic-Relaxant Effects of Bergamot Essential Oil in Rodent

by Laura Rombolà, Damiana Scuteri, Chizuko Watanabe, Shinobu Sakurada, Kengo Hamamura, Tsukasa Sakurada, Paolo Tonin, Maria Tiziana Corasaniti, Giacinto Bagetta and Luigi Antonio Morrone

Int. J. Mol. Sci. 2020, 21(7), 2597; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21072597 - 09 Apr 2020

Cited by 33 | Viewed by 4367

Abstract

The essential oil obtained by the fresh fruit of Citrus bergamia Risso et Poiteau is used worldwide in aromatherapy to reduce pain, facilitate sleep induction, and/or minimize the effects of stress-induced anxiety. Preclinical pharmacological data demonstrate that bergamot essential oil (BEO) modulates specific neurotransmissions and shows an anxiolytic-relaxant effect not superimposable to that of the benzodiazepine diazepam, suggesting that neurotransmissions, other than GABAergic, could be involved. Several studies on essential oils indicate a role for serotonergic (5-HT) neurotransmission in anxiety. Interestingly, among serotonergic receptors, the 5-HT1A subtype seems to play a key role in the control of anxiety. Here, we report that modulation of the 5-HT1A receptor by selective agonist ((±)8-OH-DPAT) or antagonist (WAY-100635) may influence some of the anxiolytic-relaxant effects of BEO in Open Field and Elevated Plus Maze tests. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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13 pages, 2940 KiB

Open AccessArticle

Interaction of the Coffee Diterpenes Cafestol and 16-O-Methyl-Cafestol Palmitates with Serum Albumins

by Federico Berti, Luciano Navarini, Elena Guercia, Ana Oreški, Alessandra Gasparini, Jeremy Scoltock and Cristina Forzato

Int. J. Mol. Sci. 2020, 21(5), 1823; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21051823 - 06 Mar 2020

Cited by 5 | Viewed by 2876

Abstract

The main coffee diterpenes cafestol, kahweol, and 16-O-methylcafestol, present in the bean lipid fraction, are mostly esterified with fatty acids. They are believed to induce dyslipidaemia and hypercholesterolemia when taken with certain types of coffee brews. The study of their binding to serum albumins could help explain their interactions with biologically active xenobiotics. We investigated the interactions occurring between cafestol and 16-O-methylcafestol palmitates with Bovine Serum Albumin (BSA), Human Serum Albumin (HSA), and Fatty Free Human Serum Albumin (ffHSA) by means of circular dichroism and fluorimetry. Circular Dichroism (CD) revealed a slight change (up to 3%) in the secondary structure of fatty-free human albumin in the presence of the diterpene esters, suggesting that the aliphatic chain of the palmitate partly occupies one of the fatty acid sites of the protein. A warfarin displacement experiment was performed to identify the binding site, which is probably close but not coincident with Sudlow site I, as the affinity for warfarin is enhanced. Fluorescence quenching titrations revealed a complex behaviour, with Stern–Volmer constants in the order of 10³–10⁴ Lmol⁻¹. A model of the HSA-warfarin-cafestol palmitate complex was obtained by docking, and the most favourable solution was found with the terpene palmitate chain inside the FA4 fatty acid site and the cafestol moiety fronting warfarin at the interface with site I. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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13 pages, 2327 KiB

Open AccessArticle

Development of pH-Responsive Biopolymeric Nanocapsule for Antibacterial Essential Oils

by Sylvie Skalickova, Tereza Aulichova, Eva Venusova, Jiri Skladanka and Pavel Horky

Int. J. Mol. Sci. 2020, 21(5), 1799; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21051799 - 05 Mar 2020

Cited by 11 | Viewed by 3626

Abstract

It is generally believed that antibacterial essential oils have the potential to become one of the alternatives in preventing diarrheal diseases of monogastric animals. The disadvantage is their low efficiency per oral due to easy degradation during digestion in the stomach. This study compares the efficacy of chitosan, alginate-chitosan, guar gum-chitosan, xanthan gum-chitosan and pectin-chitosan nanocapsules to the synthesis of pH-responsive biopolymeric nanocapsule for Thymus vulgaris, Rosmarinus officinalis and Syzygium aromaticum essential oils. Using spectrophotometric approach and gas chromatography, release kinetics were determined in pH 3, 5.6 and 7.4. The growth rates of S. aureus and E. coli, as well as minimal inhibition concentration of essential oils were studied. The average encapsulation efficiency was 60%, and the loading efficiency was 70%. The size of the nanocapsules ranged from 100 nm to 500 nm. Results showed that chitosan-guar gum and chitosan-pectin nanocapsules released 30% of essential oils (EOs) at pH 3 and 80% at pH 7.4 during 3 h. Similar release kinetics were confirmed for thymol, eugenol and α-pinene. Minimal inhibition concentrations of Thymus vulgaris and Syzygium aromaticum essential oils ranged from 0.025 to 0.5%. Findings of this study suggest that the suitable pH-responsive nanocapsule for release, low toxicity and antibacterial activity is based on chitosan-guar gum structure. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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15 pages, 2247 KiB

Open AccessArticle

Sesquiterpenes Are Agonists of the Pregnane X Receptor but Do Not Induce the Expression of Phase I Drug-Metabolizing Enzymes in the Human Liver

by Michaela Šadibolová, Tomáš Zárybnický, Tomáš Smutný, Petr Pávek, Zdeněk Šubrt, Petra Matoušková, Lenka Skálová and Iva Boušová

Int. J. Mol. Sci. 2019, 20(18), 4562; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms20184562 - 14 Sep 2019

Cited by 2 | Viewed by 3572

Abstract

Sesquiterpenes, the main components of plant essential oils, are bioactive compounds with numerous health-beneficial activities. Sesquiterpenes can interact with concomitantly administered drugs due to the modulation of drug-metabolizing enzymes (DMEs). The aim of this study was to evaluate the modulatory effects of six sesquiterpenes (farnesol, cis-nerolidol, trans-nerolidol, α-humulene, β-caryophyllene, and caryophyllene oxide) on the expression of four phase I DMEs (cytochrome P450 3A4 and 2C, carbonyl reductase 1, and aldo-keto reductase 1C) at both the mRNA and protein levels. For this purpose, human precision-cut liver slices (PCLS) prepared from 10 patients and transfected HepG2 cells were used. Western blotting, quantitative real-time PCR and reporter gene assays were employed in the analyses. In the reporter gene assays, all sesquiterpenes significantly induced cytochrome P450 3A4 expression via pregnane X receptor interaction. However in PCLS, their effects on the expression of all the tested DMEs at the mRNA and protein levels were mild or none. High inter-individual variabilities in the basal levels as well as in modulatory efficacy of the tested sesquiterpenes were observed, indicating a high probability of marked differences in the effects of these compounds among the general population. Nevertheless, it seems unlikely that the studied sesquiterpenes would remarkably influence the bioavailability and efficacy of concomitantly administered drugs. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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Review

Jump to: Research

38 pages, 9333 KiB

Open AccessReview

Monoterpenes and Their Derivatives—Recent Development in Biological and Medical Applications

by Mariola Zielińska-Błajet and Joanna Feder-Kubis

Int. J. Mol. Sci. 2020, 21(19), 7078; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21197078 - 25 Sep 2020

Cited by 144 | Viewed by 10950

Abstract

Monoterpenes, comprising hydrocarbons, are the largest class of plant secondary metabolites and are commonly found in essential oils. Monoterpenes and their derivatives are key ingredients in the design and production of new biologically active compounds. This review focuses on selected aliphatic, monocyclic, and bicyclic monoterpenes like geraniol, thymol, myrtenal, pinene, camphor, borneol, and their modified structures. The compounds in question play a pivotal role in biological and medical applications. The review also discusses anti-inflammatory, antimicrobial, anticonvulsant, analgesic, antiviral, anticancer, antituberculosis, and antioxidant biological activities exhibited by monoterpenes and their derivatives. Particular attention is paid to the link between biological activity and the effect of structural modification of monoterpenes and monoterpenoids, as well as the introduction of various functionalized moieties into the molecules in question. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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32 pages, 1758 KiB

Open AccessReview

Therapeutic Potential of Volatile Terpenes and Terpenoids from Forests for Inflammatory Diseases

by Taejoon Kim, Bokyeong Song, Kyoung Sang Cho and Im-Soon Lee

Int. J. Mol. Sci. 2020, 21(6), 2187; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21062187 - 22 Mar 2020

Cited by 123 | Viewed by 12178

Abstract

Forest trees are a major source of biogenic volatile organic compounds (BVOCs). Terpenes and terpenoids are known as the main BVOCs of forest aerosols. These compounds have been shown to display a broad range of biological activities in various human disease models, thus implying that forest aerosols containing these compounds may be related to beneficial effects of forest bathing. In this review, we surveyed studies analyzing BVOCs and selected the most abundant 23 terpenes and terpenoids emitted in forested areas of the Northern Hemisphere, which were reported to display anti-inflammatory activities. We categorized anti-inflammatory processes related to the functions of these compounds into six groups and summarized their molecular mechanisms of action. Finally, among the major 23 compounds, we examined the therapeutic potentials of 12 compounds known to be effective against respiratory inflammation, atopic dermatitis, arthritis, and neuroinflammation among various inflammatory diseases. In conclusion, the updated studies support the beneficial effects of forest aerosols and propose their potential use as chemopreventive and therapeutic agents for treating various inflammatory diseases. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

36 pages, 1688 KiB

Open AccessReview

The Effects of Essential Oils and Terpenes in Relation to Their Routes of Intake and Application

by Sachiko Koyama and Thomas Heinbockel

Int. J. Mol. Sci. 2020, 21(5), 1558; https://0-doi-org.brum.beds.ac.uk/10.3390/ijms21051558 - 25 Feb 2020

Cited by 55 | Viewed by 14085

Abstract

Essential oils have been used in multiple ways, i.e., inhaling, topically applying on the skin, and drinking. Thus, there are three major routes of intake or application involved: the olfactory system, the skin, and the gastro-intestinal system. Understanding these routes is important for clarifying the mechanisms of action of essential oils. Here we summarize the three systems involved, and the effects of essential oils and their constituents at the cellular and systems level. Many factors affect the rate of uptake of each chemical constituent included in essential oils. It is important to determine how much of each constituent is included in an essential oil and to use single chemical compounds to precisely test their effects. Studies have shown synergistic influences of the constituents, which affect the mechanisms of action of the essential oil constituents. For the skin and digestive system, the chemical components of essential oils can directly activate gamma aminobutyric acid (GABA) receptors and transient receptor potential channels (TRP) channels, whereas in the olfactory system, chemical components activate olfactory receptors. Here, GABA receptors and TRP channels could play a role, mostly when the signals are transferred to the olfactory bulb and the brain. Full article

(This article belongs to the Special Issue Terpenes and Essential Oils: Health Risks and Benefits)

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