**2. Essential oils**

The active molecules, involved in plant defence mechanisms, are derived from secondary metabolism. Besides, they are not directly involved in plant growth but have evolved to provide natural protection against attacks by microbes or insects [11]. Above and beyond, some of such secondary metabolites are concentrated in the oil sacs, which are essential oil-secreting pockets [12]. In consequence, exploring essential oils for molecules with biological activity seems to be an interesting avenue.

#### **2.1 Definition**

The eighth edition of the French Pharmacopoeia defines EOs as 'products of generally fairly complex composition containing volatile products contained in plants and more or less modified during preparation'. There are various processes for extracting these volatile principles. At the time, EOs were also referred to as 'essences' or 'volatile oils'. Since the ninth edition (1972), the Pharmacopoeia now only uses the term 'essential oil' [13–15]. Additionally, essential oils have shown to be complex natural mixtures of volatile secondary metabolites, isolated from plants by hydrodistillation or mechanical expression [16]; they are obtained from leaves, seeds, buds, flowers, twigs, herbs, bark, wood, roots or fruit, but alike from the gums that run off the trunks of trees. Above and beyond, hydrodistillation is still the most widely used method of producing essential oils, in particular for commercial purposes [17]. In addition, secondary metabolites are extracted from plants by steam distillation. The volume of essential oil recovered depends on the distillation yield, which varies for the same plant depending on the season [18]. Likewise, essential oils can be obtained by cold expression, as in the case of citrus fruits. New techniques have been developed to increase production yields, in respect such as extraction using liquid carbon dioxide at low temperature and high pressure [19] or extraction assisted by ultrasound or microwaves [20].

*Methods for Evaluating the* In Vivo *Analgesic and Anti-Inflammatory Activity of Essential Oils DOI: http://dx.doi.org/10.5772/intechopen.113832*

#### **2.2 Chemical composition of essential oils**

Contrary to what might be suggested by its name, a pure and natural essential oil does not contain any fats. It is made up of molecules with a carbon skeleton. As well, essential oils represent complex mixtures that may contain more than 300 different compounds [21–24]. In virtue of which, such compounds are volatile molecules, the vast majority of which belong to the terpene family. Only the most volatile terpenes, that is. those whose molecular weight is not too high, are used [23]. Terpene compounds are products of secondary metabolism. There are mainly monoterpenes (C10) and sesquiterpenes (C15). Several thousand compounds have been described and have shown to be classified according to their number of rings (acyclic, mono- and bicyclic compounds) together with the nature of the functions they carry (alcohols, aldehydes, ketones, ether-oxides) [25].

## **2.3 Physico-chemical properties of essential oils**

In general, essential Oils are colourless or pale yellow in their liquid state at ordinary temperatures. However, all EOs are volatile, fragrant and flammable; their specific gravity is usually less than one [26]. Only three officinal EOs have a specific gravity greater than that of water: cinnamon, clove and sassafras. They are sparingly soluble in water, soluble in alcohols and in most organic solvents. They are alterable and are very sensitive to oxidation [27].

Essential oils share organoleptic properties (characteristics of a substance that can be perceived by the sense organs: flavour, odour, appearance and consistency of the object) such as being liquid at room temperature, volatile and water vapour permeable [28].

Modern scientific work has made it possible to better understand the essences and to precisely define their different constituents and their physico-chemical characteristics, revealing the principle of their long-known therapeutic action. The physiological role of essential oils in the herb kingdom remains unknown. However, the molecular diversity of the metabolites they contain endows them with a wide range of biological roles. Additionally, several essential oils, such as cinnamon, cayenne pepper, bay leaf and oregano oils, have antioxidant properties [29, 30].

An anti-inflammatory effect has been described for essential oils of citrus cultivars. The results suggest that *C. japonica* and *C. maxima* are promising candidates for relieving inflammatory diseases. These research findings provide the scientific basis for using essential oils from citrus cultivars to reduce inflammatory symptoms [31]. Above and beyond, the antifungal activities of numerous essential oils, including thyme, citronella, cinnamon and tea tree oils [17], have been described. The efficacy of oils extracted from yarrow, *Achillea fragrantissima* [32] and *A. milefolium* [33] against the pathogenic yeast *Candida albicans*, has also been demonstrated.

Certain essential oils have anti-tumour activity and are used in the preventive treatment of certain types of cancer. The essential oil, isolated from the seeds of *Nigella sativa* L., demonstrates cytotoxic activity *in vitro* against various tumour cell lines. *In vivo*, it limits the proliferation of liver metastases and delays the death of mice that have developed the p815 tumour [34].

#### **2.4 Analgesic and anti-inflammatory activity of essential oils**

A number of pain-relieving essential oils can help us to overcome certain ailments, in respect such as dental pain, headaches or chronic inflammation. The essential oils

presented below can effectively be used to fight pain. Most of them have analgesic, analgesic and anaesthetic mechanisms of action. Lemon eucalyptus essential oil acts as an anti-inflammatory thanks to its citronellal content of over 65%, which helps to relieve pain. Clove essential oil is one of the ultimate pain-relieving essential oils. Besides, its eugenol content of over 80% gives it formidable analgesic properties. It is recommended for relieving dental pain, and peppermint essential oil has multiple properties that provide fast and effective pain relief. Firstly, it acts as a local analgesic not only thanks to its menthol content but also as a mild anaesthetic [35].

The bioactive constituents of essential oil extracted from many medicinal plants were known to provide protection against prolonged inflammation and improve the health of mortals. In this regard, the anti-inflammatory properties of these plants are of immense importance to the drug discovery process [36, 37]. Specifically, lavender essential oil has been shown to inhibit inflammation by inhibiting the nascence of TNF (tumour necrosis factor) and NF-kB (nuclear factor kappa–light chain enhancer of activated B cells) in the murine brain and human umbilical vein endothelial cells. [38]. Linalool and cinnamaldehyde present in native cinnamon leaf essential oil were found to be anti-inflammatory against endotoxin introduced into mice [39].
