An essential oil is defined as the extract obtained by chemical processes or by a suitable mechanical process without heating the plant. They are volatile, colourless, aromatic oily liquids. They are present in very small amount but are widely used in food, cosmetic and pharmaceutical industries due to their aromatic characteristics. Essential oils have a complex composition, containing several components. The main components identified in essential oils include terpenes, monoterpenes, sesquiterpenes, propenylphenols.
Inflammation is a normal protective response instigated by tissue injury or infection and functions to pathogens in the body and to evacuvate dead or damaged tissues. There is much evidence that some essential oils possess anti-inflammatory activity. For instance, chamomile essential oil has been utilised for centuries as an anti-inflammatory and also for alleviating the symptoms associated with eczema, dermatitis and other pronounced irritation. However, there are other examples of essential oils (eucalyptus, rosemary, lavender, millefolia) along with other plants (pine, clove and myrrh) that have been used as blended formulations as anti-inflammatory agents. The anti-inflammatory activity of essential oils may be attributed not only to their antioxidant activities but also to their interactions with signalling cascades involving cytokines and regulatory transcription factors, and on the expression of pro-inflammatory genes.
A polyunsaturated fatty acid known as Arachidonic acid is released from cell membranes by phospholipase A2 under the stimulus of several factors associated with inflammation. It is metabolized by lipoxygenase (LOX) and cyclooxygenase (COX) pathways in different eicosanoids such as leukotrienes (LTs) and prostaglandins (PGs) in diverse cells and thromboxane A2 in platlets. The prostaglandins amplify the pain mechanism and enhance vascular permeability whereas the leukotrienes contract the smooth muscles of blood vessels, enhance vascular permeability and mediate proinflammatory and allergic responses. The Torreya nucifera oil, principally constituted by α- pinene, limonene, and δ-3-carene, was a COX-2 selective inhibitor having critical inhibitory impacts on prostaglandin generation. 1,8-Cineole, a terpene oxide, present in many essential oils display in numerous fundamental oils is appeared to hinder leukotrienes and prostaglandins, both pathways of arachidonic acid metabolism. Aloe vera, cinnamon leaf, juniperus berry, lavender, thyme, anise star and eucalyptus, essential oils, in which 1,8-cineole, p-cymene, thymol, limonene, and eugenol predominated, demonstrated strong lipoxygenase inhibitory effects.
Interleukin-1β (IL-1β) and tumour necrosis factor-α (TNF-α) are two important pro-inflammatory cytokines with a relevant role in the inflammatory disorders. The major source of TNF-α is the cells of monocyte/macrophage lineage although T lymphocytes, neutrophils and mast cells also produce this pro-inflammatory cytokine. The gram positive bacteria cell membrane contains Lipoteichoic acid (LTA) along with peptidoglycan (PG) which contributes to the stimulation in the production of different cytokines. The capacity to repress IL-1β and IL-6 production however not TNF-α is credited to 1,8-cineole, santoline, spathulenol and caryophyllene oxide, major constituents of the essential oil of Cinnamomum osmophloeum leaves. Cinnamaldehyde from Cinnamomum osmophloeum also suppressed the production of these cytokines from LPS stimulated human blood monocytes derived primary macrophages.
The anti-inflammatory activities of the essential oils are very much recorded; nevertheless their utilisation can be hampered because of the chemical variability of the oils. A few elements including climatic conditions, type of extraction used, vegetative development of the plant, the plant part used, and harvesting time of the aromatic plant can be considered as responsible for fluctuations in their chemical compositions.