Aromatherapy and Neurotransmission: Behavioral and Neurochemical Studies

Aromatherapy is the use of concentrated essential oils extracted from herbs, flowers,and other plant parts to treat various diseases (Segen, 1998). This complementarytherapy is becoming increasingly popular worldwide, and a number of essential oilsare currently in use in the management of chronic pain, depression, anxiety, somecognitive disorders, insomnia, and stress-related disorders (see Perry and Perry,2006; Setzer, 2009; Umezu, 2012). However, although essential oils are currently inuse to treat various mental disorders, their central nervous system (CNS)–acting effectshave not been clarified, and the pharmacology of the essential oils and their singlechemical constituents remains largely undiscovered. Among the properties of purefragrance compounds and essential oils, the emotional and behavioral modulations,although there are many anecdotal or empirically speculated efficacious effects, are often difficult to examine and demonstrate in scientifically controlled conditions. Individual constituents reach the blood, cross the blood–brain barrier, and enter the CNS following inhalation, dermal application (Brooker et al., 1997), intraperitoneal or subcutaneous injection, and oral administration (Orafidiya et al., 2004). The inhalation of essential oils can induce effects through the CNS (e.g., lung absorption and bloodstream transport) or stimulation of the olfactory system and secondary activation of brain regions. For example, it is well known that olfaction influences the behavior of mammals, and the olfactory system has a direct or indirect connection to a variety of CNS components, including the hypothalamus, hippocampus, and limbic system (Broughan, 2002; Edge, 2003; Weier and Beal, 2004). Consequently, the inhalation of fragrance produces a variety of physiological effects, including brain (Lorig, 2000; see Kiecolt-Glaser et al., 2008), endocrine (Komori et al., 1995a; see Kiecolt-Glaser et al., 2008), and immune (Haze et al., 2002; see Kiecolt-Glaser et al., 2008) functions. Thus, it can be hypothesized that inhalation of an essential oil can be used as a therapeutic intervention for correcting a reversible malfunction of the body by direct activation of a targeted brain pathway (see Zhao et al., 2005). In consideration of the diffusion of aromatherapy, in the last decades many studies on essential oils and related compounds have been conducted, and some studies have suggested that essential oils affect the modulation of the central neurotransmitter system. This review discusses the most current knowledge on the neurochemical effects of the most popular essential oils in experimental studies.