There is still the need for efficacious therapies for pain. In the search for new therapeutic options, plants are a major source of novel biomolecules. Monoterpenes constitute 90% of essential oils, and there is a growing interest in understanding the mechanisms underlying their pharmacological activity. This systematic review reports what is so far known about the analgesic activity of monoterpenes and also provides an overview of their mechanisms of action. The search terms analgesia, anti-inflammatory, anaesthetic and antioxidant were used to retrieve English language articles in SCOPUS, PUBMED and EMBASE published between 1990 and 2012. Forty-five papers were found concerning the potential analgesic activity of 27 monoterpenes. The data reviewed here suggest these compounds are possible candidates for the treatment of painful conditions.

We describe the antinociceptive and anti-inflammatory properties of citronellol (CT) in rodents. CT, a monoterpene alcohol, is a naturally occurring monoterpene compound prevalent in essential oils of various aromatic plant species, such as Cymbopogon citratus. In mice, when evaluated against acetic-acid-induced abdominal writhing, CT (25, 50 and 100 mg/kg, i.p.) reduced (P < 0.001) the amount of writhing compared to the control group. In the formalin test, CT also significantly inhibited both the early (neurogenic pain) and the late (inflammatory pain) phases of formalin-induced licking (P < 0.001). When assessed in a thermal model of pain, CT (100 mg/kg, i.p.) caused a significant increase (P < 0.05) in the latency response on the hot-plate test. Such results were unlikely to be caused by motor abnormality. The anti-inflammatory activity of CT was investigated through carrageenan-induced pleurisy in mice. Pretreatment with CT was able to inhibit both neutrophil infiltration and the increase in TNF-α level in the exudates from carrageenan-induced pleurisy. In in vitro experiments, CT (1 and 100 μg/ml) also decreased nitric oxide production by LPS-stimulated macrophage. Together, these results indicate that CT is effective as an analgesic compound in various pain models, with its action probably mediated by the inhibition of peripheral mediators as well as central inhibitory mechanisms that could be related to its strong antioxidant effect observed in vitro.

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The use of CDs can improve the action of anti-inflammatory compounds and it can also be a way to reduce the side effects, therapeutic doses and toxicity.

The anti-infl ammatory and redox protective effects of the citronellal (CT) were evaluated using in vivo and in vitro tests. Intraperitoneal (i.p.) administration of CT (50, 100, and 200 mg/kg) inhibited (p < 0.05) the carrageenan-induced leukocyte migration to the peritoneal cavity. Additionally, the carrageenan-and arachidonic acid-induced rat hind paw edema was signifi cantly inhibited (p < 0.05) by i.p. administration of 100 and 200 mg/kg of the compound. When the redox activity was evaluated, CT (200 mg/kg) signifi cantly reduced hepatic lipoperoxidation (p < 0.001), as well as oxidation of plasmatic (p < 0.05) and hepatic (p < 0.01) proteins. The results of the present study support the hypothesis that CT possesses anti-infl ammatory and redox protective activities. It is suggested that its effects are associated with the inhibition of the enzymes in the arachidonic acid pathway, which prevent cell migration by inhibiting leukotriene production, edema formation and the increase of reactive oxygen species in tissues. Therefore, CT is of potential benefi t to manage infl ammatory disorders and correlated damages caused by oxidant agents.