Terpenes are a diverse class of oils and organic compounds that are produced in many plants as secondary metabolites mainly for defence purposes, but also for favouring pollination.
In the Cannabis plant, a large number of terpenes (120) have been identified; they attribute the aroma of each cannabis strain, and it has been hypothesised that terpene ratio inherently change the properties of the cannabis plant. (1)
Recent data suggest that some of the terpenes found within the cannabis plant possess anti-inflammatory and analgesic activity which is additional to the effects of classical phytocannabinoids and may generate synergistic interactions. (2)
In this short review we will examine the latest scientific evidences supporting the use of myrcene, a common cannabis terpene, for pain and inflammatory conditions.
What is β-myrcene
β-myrcene is an acyclic monoterpene commonly found in nature (along with other terpenes) in the essential oil of lemongrass (Cymbopogon citratus), hop (Humulus lupus), verbena (Verbena Officinalis), mango (Mangifera Indica), thyme (Thymus Vulgaris), bay (Laurus Nobilis) and Cannabis Sativa.
Myrcene's volatile nature makes it somehow difficult to utilise on its own, and it is commonly used as an intermediate by the fragrance industry for the production of derivative terpenes. (3)
It has been widely tested for safety, investigating genotoxicity of the monoterpene and screened by salmonella/microsome assay, demonstrating that it is not a mutagenic compound. (4)
Myrcene and analgesia
β-myrcene has been extensively used throughout history by folk medicine as a pain-killer.
Most commonly, traditional medicine have been using essential oils which contain high percentages of myrcene along with an array of other terpenes.
Myrcene's anti-nociceptive effects as a single molecule have been demonstrated by Rao and collegues already in 1990; Their study showed that intraperitoneal (ip) injections of 10 & 20 mg/Kg-1 and subcutaneus (sc) injections of 20 & 40 mg/Kg-1 of myrcene were sufficient to significantly inhibit pain perception in mice both in the acetic acid writhing test (peripheral analgesia), as well as in the hot plate test (51.5 +- 0.5 Co), typically used to investigate Central Nervous System (CNS) modulation.
They reported that 1 mg/Kg-1 of naloxone (opioid receptors inverse agonist, used for reversing opiate overdose) and 2 mg/Kg-1 of yohimbine (adrenergic receptor antagonist, used for sedation) antagonised the beneficial effects of myrcene. The group henceforth suggested that the analgesic effects of myrcene are "probably mediated by α2-adrenoceptor stimulated release of endogenous opioids". (5)
However, this final conclusion has been subject to controversy, with other studies reporting different results.
Another group tested the neurobehavioural effects of 1g/kg-1 of β-myrcene on rats and mice, drawing the conclusion that benzodiazepine-like effects have to be excluded, and that a direct activity at Central Nervous System is unlikely.
Evaluation of exploratory, emotional and anxiolytic activity in rodents (with tests such as elevated plus maze, conditioned avoidance, open space etc) showed that β-myrcene does not provoke any mind-altering behaviour. (6)
Thus, it is improbable the hypothesis of Rao et al. of myrcene's activation of endogenous opioids via α2-adrenoceptor (which would hence modify locomotor activity and anxiety states), (7) but one may speculate that myrcene's analgesic actions are mediated via release of opioid peptides, endocannabinoids and anti-inflammatory cytokines acting upon opioid receptors present on primary afferent neurons to block pain transmission.
Such local opioid-mediated effect is limited to the periphery and thus does not have the adverse systemic effects of centrally mediated opioid and benzodiazepines analgesia. (8)
Moreover, as we will discuss next, β-myrcene shows significant anti-inflammatory effects via inhibition of cyclooxygenase (COX) pathways, which is observed by the diminished pain over time.
This hypothesis seem to find correspondence with the literature on this compound.
In 2008, the essential oil obtained by the leaves of Brazilian Eremanthus erythropappus has been tested for its antinociceptive effects.
The oil is composed by a large number of terpenes, including an high percentage of monoterpenes (39.12%) with anti-inflammatory and antinociceptive activity, amongst which, β-myrcene, (10.03%) was prevalent. The investigators demonstrated that analgesia acted both on first phase pain (acute, CNS-mediated) as well as on second phase (inflammatory). (9)
Pain reactions in mice were induced via chemical stimulation with formalin injection in the paw, after which the group tested different doses of essential oil, showing that 400 mg/Kg-1 inhibited licking at the paw, which is a marker for decreased pain perception.
In order to further investigate the nature of these results, the group of Sousa and colleagues tested the effects of E. erythropappus oil on the hot-plate (55 ± 1°C). Doses of 200 mg/Kg-1 and 400 mg/Kg-1 of the essential oil induced significant antinociceptive effects only when the drug was administered 30, 60 and 90 minutes before hot-plate test, with maximum effects reached with pre-treatment 60 minutes before the hot-plate challenge.
The hot-plate latency demonstrated centrally-mediated effects. (10) However, when a pretreatment of 1 mg/Kg-1 of Naloxone (subcutaneous) was administered, it resulted in inhibition of morphine (5 mg Kg-1)- induced analgesia, but it failed to prevent the antinociceptive effects induced by the essential oil. (9)
These results, in apparent discordance with the above-mentioned study of Rao et al., may find an explanation with the difference between myrcene as a single molecule and myrcene within a phytocomplex, which includes several other terpenes (such as in the E. erythropappus oil, which also express b-pinene, b-caryophyllene and 30 more compounds).
It is probable that β-myrcene analgesic effects act both centrally and peripherally, but that synergistic interactions with other terpenes may prevent opioid-mediated activity or boost endocannabinoid-derived central actions.
Furthermore, myrcene analgesic and anti-inflammatory actions can be attributed to reduction of peripheral nociception by inhibition of prostaglandin release.
E. erythropappus oil was also shown to inhibit abdominal writhing induced by acetic acid in mice. This mechanism is controlled by COX pathway and prostaglandin synthesis, which were both inhibited by the oil. (9)
This speculation fits accordingly with another study, which examines the anti-inflammatory effect of oral administration of lemongrass tea, as well as of essential oil from lemongrass leaves on rats treated with subplantar injections of carrageneean (an irritant) and Prostglandin E2 (PGE2).
Myrcene was the main analgesic compound identified, and its peripheral effects were indeed capable of inhibiting hyperalgesia induced by PGE2 in the rat paw test, as well as contortions provoked by iloprost in mice. However, repeated administration of the essential oil failed to cause tolerance, which is a key feature of the central analgesic effects of morphine.
The study concluded that lemongrass used as a pain killer by folk medicine in Brazil acted via peripheral analgesia, although some sedative effects of β-myrcene have been recognised both by a study on sleep aid preparations from hop, as well as by myrcene-enhanced barbituric effects. (11,12,13)
Additionally to its unique analgesic property, myrcene contained within the cannabis plant may potentiate the innate anti-nociceptive properties of cannabinoids. It appears that the monoterpene lowers resistance across the blood brain barrier, improving permeability, so that β-myrcene itself and many other chemicals (including the analgesic THC) may cross the barrier more effectively.
Β-myrcene has been in fact used succesfully as a permeation agent in a transdermal patch, which delivers cannabinoids into the bloodstream. (14)
Moreover, terpenes were suggested to modulate the affinity of THC for the CB1 receptor, which contributes to the improved analgesic effects of whole plant cannabis preparations over single-molecules. (2,15)
Cannabinoid receptors CB1 are also widely expressed in the periaqueductal grey matter (PAG) and in the substantia gelatinosa of the spinal cord, both key sites for modulating nociceptive input from primary afferent neurons. (16,17)
Myrcene and Inflammation
The anti-inflammatory effects of myrcene have been evaluated in a number of studies.
Β-myrcene is the main monoterpene of the essential oil from a Mexican plant of the Asteracee specie: Porophyllum Ruderale. This essential oil was administered orally to mice suffering from pleurisy, (inflammation of the lungs lining, pleura), induced by injection of zymosan and lipopolysaccharide (LPS).
The investigators reported that myrcene could inhibit the LPS-induced inflammation, including cell migration, which is a key feature of pleurisy and generally of inflammatory response. Moreover, β-myrcene was found to have immunoregulatory activity, inhibiting the production of nitrogen oxide (NO), as well as cytokines Interferon gamma (IFNγ) and Interleukin-4 (IL-4), which is normally overproduced during lungs inflammation (18,19).
These results were matched accordingly by another group which examined the effects of myrcene on acute inflammation in mice, utilising 200 and 400 mg/Kg-1 of essential oil E. erythopappus on a carrageenan-induced pleurisy. They verified that indeed the monoterpene inhibited mobilisation of cells, particularly of leukocytes, as well as significantly decreasing exudate volume. (9)
Further evidences of the anti-inflammatory actions of myrcene were reported by the same group, which administered 200 and 400 mg/Kg-1 of essential oil E. erythopappus in mice previously injected with carrageen (an irritant) subplantarly. Paw oedema is typically induced by an overproduction of histamine, serotonin, bradikinin and a number of prostaglandins; By inhibiting prostaglandins, β-myrcene was capable of reducing the oedema by 15.18%. (9)
A Korean essential oil from the flowers of Magnolia sieboldii, provided additional evidence of the anti-inflammatory effects of the hydrocarbon myrcene.
This oil contains 12.72% myrcene, along with other 60 terpenes, and was capable of totally inhibiting NO and PGE2 production induced by LPS in immune cells (macrophages). (20)
Matching these results was the investigation carried out on the essential oil from Distichoselinum tenuifolium, which is commonly used in Spain for treating skin infections and dermatitis. B-myrcene is the main compound present in the oil (varying plant to plant from 47.7% to 84.6%). This oil was shown to inhibit significantly NO production in macrophages without affecting cell viability, that is, at non-cytotoxic doses. (21)
B-myrcene is a natural compound with a stellar safety profile which could significantly improve immune functions, as well as decrease pain sensation in patients suffering from inflammatory as well as chronic pain.
Given the known anti-inflammatory and antinociceptive effects of cannabis, it is important to discern the qualities that can provide the correct terpene synergy to manage different disorders such as ostheoarthritis, neuropathic pain or dermatitis. (2,21,22)
Some cannabis strain more than others contain high levels of myrcene, usually those with more "sedative" effects, which, by the recreational world, have been called "indica plants".
Although the mechanism of function of myrcene is not fully understood yet, it is evident its promising role in the treatment of inflammation and pain.
Better understanding of myrcene interactions within the cannabis plant and its clinical effect in humans may contribute to the developing of specific cannabis genetics to target at best inflammatory disorders.
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