Citrus Essential Oils in Aromatherapy: Therapeutic Effects and Mechanisms

Abstract

Citrus is one of the main fruit crops cultivated in tropical and subtropical regions worldwide. Approximately half (40-47%) of the fruit mass is inedible and discarded as waste after processing, which causes pollution to the environment. Essential oils (EOs) are aromatic compounds found in significant quantities in oil sacs or oil glands present in the leaves, flowers, and fruit peels (mainly the flavedo part). Citrus EO is a complex mixture of ~400 compounds and has been found to be useful in aromatic infusions for personal health care, perfumes, pharmaceuticals, color enhancers in foods and beverages, and aromatherapy. The citrus EOs possess a pleasant scent, and impart relaxing, calming, mood-uplifting, and cheer-enhancing effects. In aromatherapy, it is applied either in message oils or in diffusion sprays for homes and vehicle sittings. The diffusion creates a fresh feeling and enhances relaxation from stress and anxiety and helps uplifting mood and boosting emotional and physical energy. This review presents a comprehensive outlook on the composition, properties, characterization, and mechanism of action of the citrus EOs in various health-related issues, with a focus on its antioxidant properties.

Keywords: aromatherapy; characterization of citrus EOs; citrus essential oils; natural aromatic compounds; therapeutic effects of citrus EOs.

Figure 1

Market of essential oils obtained from citrus fruits, (2020) (Note: Bar graph was created for major countries involved in import and export; the map was created using ArcGIS 10.8.1 with UTM projection taking WGS84 datum) [24].

Figure 2

Pathways followed by citrus EOs for aromatherapy.

Figure 3

Inhalation of the citrus EOs and response delivery to the central nervous system through the olfactory lobe and respiratory and circulatory system. (a) Activation of nasal olfactory chemoreceptors (b) Direct absorption of the EO active molecules into the neuronal pathway (c) Absorption of EO active molecules in the alveolar blood circulation.

Figure 4

Metabolism of d-limonene and α-pinene in the liver, absorption of citrus EO components into the circulatory system through the intestinal villi.

Figure 5

Mechanisms of gastroprotection, anti-cancer, anti-tumor, anti-inflammation, anti-microbial, and lipolytic actions of citrus EO components. Caspase (key apoptosis-inducing protein) [85,86,87,88,89,90,91,92]. Abbreviations; PPAR-α (Peroxisome proliferator-activated receptor alpha), bcl 2 (B-cell lymphoma protein 2), Bax (bcl 2-associated X), NF-ΚB (Nuclear factor-κB), LXR-β (Liver X receptor beta), TG8 LDL (Triglycerides 8 Low-density lipoprotein), FBG (fasting blood glucose), ROS (Reactive Oxygen Species), TNF-α (tumor necrosis factor alpha), ILs (Interleukins), ATP (Adenosine triphosphate).

Figure 6

Absorption of citrus EOs to the deeper layers of the skin, molecular structures of the skin penetration enhancers (frequently employed in topical lotions/ointments for facilitating transdermal drug delivery), and molecules participating in anti-inflammatory, anti-microbial, and anti-carcinoma activities.

Figure 7

Oxidative stress in the cell: Events and consequences-I: Somatic cell.

Figure 8

Oxidative stress in the cell: Events and consequences-II: Nerve cell.

Figure 9

Therapeutic effect of citrus EOs aromatherapy.

Figure 10

The pathophysiological mechanism of migraine induced by 5-HT. (1) Platelet aggregation trigger release of 5-HT and ADP in blood plasma. (2) High level of plasma 5-HT causes reversible vasoconstriction followed by conversion of 5-HT to its metabolite 5-HIAA. The latter is excreted in urine. (3) Low level of plasma 5-HT stimulates perivascular neurons to release neuropeptides (NO, PG, SP, NKA, CGRP) causing vasodilation of cerebral veins. The latter leads to migraine symptoms.

Figure 11

Neuroprotective mechanisms of α-pinene in migraine [168]. The α-pinene can reduce LPS-induced inflammation in macrophages. α-pinene can block phosphorylation of MAPKs (ERK/JNK) in macrophages and reduce the level of active (soluble IKK). This can prevent degradation of the NF-ĸB/IĸB complex. Also, α-pinene can hinder NF-ĸB phosphorylation and formation of the P65/p50/NF-ĸB complex that leads to its nuclear translocation and induction of inflammatory genes to generate cytokines. Abbreviations; TNF-α (tumor necrosis factor alpha), IL-1β (Interleukin-1β), IL-6 (Interleukin), Cox-2 (Cyclooxygenase-2), Inos (Inducible nitric oxide synthase).

Figure 12

Pathological targets in diseased conditions of dementia, Alzheimer’s, and Parkinson’s.

Figure 13

Mechanism of action of citrus EOs to inhibit acetylcholinesterase (AChE), thereby increasing levels and duration of acetylcholine in the brain and assisting with cognition (learning and memory retention). Abbreviation; ACh—acetylcholine; AChE—acetylcholinesterase; nAChr—nicotinic acetylcholine receptors; EOs—Citrus essential oil components.

Figure 14

Syntheses of neurotransmitter molecules, viz., GABA (γ- Aminobutyric acid), dopamine, and serotonin (also called as 5-HT) and the mechanism of neurotransmission. AADC also known as DDC. Abbreviations; GAD (glutamate decarboxylase), TH (Tyrosine hydroxylase), AADC (aromatic amino acid decarboxylase), DDC (DOPA decarboxylase), TPH2 (s tryptophan hydroxylase 2).

Figure 15

Neurotransmission pathways in GABAergic, DAergic, and 5-HTergic neurons and Citrus EO components that activate neurotransmission and exhibit anti-proliferative effects on human neuroblastoma cell growth.

Figure 16

Therapeutic effects of majorly occurring component in citrus essential oil [202,208,209,210,211].