Arendt-Nielsen L., Yarnitsky D. (2009). Experimental and clinical applications of quantitative sensory testing applied to skin, muscles and viscera. J. Pain 10 Genome-wide association analysis have revealed that TRPM8 is associated with susceptibility to migraine without aura ( Freilinger et al., 2012), and reduced TRPM8 expression (rs10166942 carriers) helps reduce migraine risk in humans ( Gavva et al., 2019), which may provide the rationale for the topical menthol as an alternative treatment option for migraine patients. A randomized, triple-blind, placebo-controlled, crossed-over study demonstrated the efficacy, safety, and relative tolerability of cutaneous application of 10% menthol solution in the treatment of migraine without aura ( Borhani Haghighi et al., 2010). An open-label pilot study showed that 52% of subjects experienced at least one statistically significant improvement in migraine severity 2 h after using topical menthol ( St Cyr et al., 2015). Due to the limitations of these studies, larger placebo-controlled clinical trials are needed. Freilinger T., Anttila V., de Vries B., Malik R., Kallela M., Terwindt G. M., et al. (2012). Genome-wide association analysis identifies susceptibility loci for migraine without aura. Nat. Genet. 44 Colvin L. A., Bull F., Hales T. G. (2019). Perioperative opioid analgesia-when is enough too much? A review of opioid-induced tolerance and hyperalgesia. Lancet 393 Andersson D. A., Nash M., Bevan S. (2007). Modulation of the cold-activated channel TRPM8 by lysophospholipids and polyunsaturated fatty acids. J. Neurosci. 27

As effectual as the TRPM8’s anti-inflammatory effect may seem, however, there are still some doubts about it at present. In an in vitro study, human bronchial epithelial cells’ activation of TRPM8 promotes the expression of IL-6, IL-8, and TNF-α ( 82). TRPM8 mRNA and protein expression in bronchial tissues were upregulated by cigarette smoke extract. Based on the upregulation, cigarette smoke extract synergistically amplifies inflammatory factor release ( 82). Furthermore, many researchers hold a critical opinion on TRPM8’s anti-inflammatory effect, indicating that the relationship between TRPM8 and inflammation remains to be further discussed ( 70). Seemingly, not only does TRPM8 have an anti-inflammatory effect, but it also plays a role in pro-inflammatory properties under some circumstances. Other channels of the TRP subfamily related to menthol Within the context of chronic inflammation, there has been an interest in the potential therapeutic properties of menthol to alleviate inflammation and antioxidant damage. A study by Shahid et al. ( 62) explored the role of volatile organic compounds within urine samples taken from patients with or without interstitial cystitis. The condition of interstitial cystitis is characterized by an inflammatory response affecting the bladder, leading to pronounced urinary symptoms, which are often chronic in nature ( 63). Examination of volatile organic compounds and comparison between those with or without interstitial cystitis identified that levels of menthol differed between the groups, with lower levels in individuals with the condition ( 62). This may be interpreted as lower levels of menthol correlating with higher levels of inflammatory reactions in the urinary tract. However, the establishment of causation is challenging due to the design of the study, and therefore, menthol responses may have been influenced by inflammation of other aspects of the biochemical response to cystitis. These findings are intriguing, as they suggest that menthol may be a potential anti-inflammatory agent in the urinary tract. It has been previously noted in pathological specimens of bladders with inflammatory conditions that levels of TRPM8 expression are elevated in such situations compared to healthy bladders ( 64). Therefore, the increase in these receptors may play a role in sensitization and symptoms, while potentially offering an opportunity for targeting with menthol. Hence, the data suggest that menthol exposure in the urinary tract may be associated with anti-inflammatory effects, potentially mediated by TRPM8. Chemotherapy-induced peripheral neuropathy is a severe and painful adverse reaction of cancer treatment in patients. CIPN that occurs during chemotherapy, sometimes requiring dose reduction or cessation, impacting on survival ( Colvin, 2019). Menthol activated TRPM8 channels are a promising therapeutic target in CIPN ( Colvin, 2019). In Fallon et al. (2015), in a proof-of-concept study, determined that 82% of evaluable patients had an improvement in their total pain scores after 4−6 weeks of treatment with topical 1% menthol cream, and 50% had a clinically relevant reduction in pain scores of at least 30%. Cortellini et al. (2017) reported the remarkable treatment with menthol cream of a male patient with a history of metastatic colon cancer and previous chemotherapies who had neuropathy that impaired his quality of life and limited further chemotherapy. Another clinical study ( {"type":"clinical-trial","attrs":{"text":"NCT01855607","term_id":"NCT01855607"}}NCT01855607, as shown in Table 2) assessed whether 6 weeks of treatment with topical menthol twice daily would reduce CIPN in patients have completed adjuvant or neo-adjuvant Taxane based breast cancer therapy or Oxaliplatin based colon cancer chemotherapy. Recently, a phase II study ( {"type":"clinical-trial","attrs":{"text":"NCT04276727","term_id":"NCT04276727"}}NCT04276727, as shown in Table 2) used a special brain scan called functional magnetic resonance imaging (fMRI) to help determine whether topical menthol therapy has potential for CIPN patients. In a word, CIPN patients may benefit from the use of menthol, either during the treatment of patients complain of subjective improvement, lead to a better quality of life, and can be implemented without interruption of chemotherapy and effective chemotherapy dose delivery, which in turn lead to longer survival, is likely to be an effective potential palliative treatment option.

Effect of TRP channels on the anti-inflammatory effect of menthol

Engelhard D., Hofer P., Annaheim S. (2019). Evaluation of the effect of cooling strategies on recovery after surgical intervention. BMJ Open Sport Exerc. Med. 5:e000527. 10.1136/bmjsem-2019-000527

Osteoarthritis (OA) is the main cause of disability in the elderly ( Glyn-Jones et al., 2015). Topp et al. compare the ability to complete functional tasks and knee pain while completing functional tasks among 20 patients with knee OA after topical application of either 3.5% menthol gel or an inert placebo gel, but no differences were found in functional tasks or pain ( Topp et al., 2013). {"type":"clinical-trial","attrs":{"text":"NCT03888807","term_id":"NCT03888807"}}NCT03888807 attempted to determine the effects of Biofreeze vs. placebo on walking gait characteristics and walking pain in patients with bilateral knee OA but announced early termination due to difficulty in enrolling subjects. In addition, {"type":"clinical-trial","attrs":{"text":"NCT03888807","term_id":"NCT03888807"}}NCT03888807, {"type":"clinical-trial","attrs":{"text":"NCT04351594","term_id":"NCT04351594"}}NCT04351594, and {"type":"clinical-trial","attrs":{"text":"NCT01565070","term_id":"NCT01565070"}}NCT01565070 have been evaluating the effect of Biofreeze on knee OA, but no results have been obtained. Fernandes E. S., Fernandes M. A., Keeble J. E. (2012). The functions of TRPA1 and TRPV1: Moving away from sensory nerves. Br. J. Pharmacol. 166 Menthol has demonstrated anti-inflammatory effects within the gastric mucosa in an experimental model of ethanol-induced gastric ulcers ( 37). Ethanol-induced gastric ulcers are characterized by marked inflammatory responses following the generation of reactive oxygen species and a vigorous immune reaction ( 48). The study found that menthol administration led to an increase in anti-inflammatory interleukin (IL)-10 expression, while there was a reduction in pro-inflammatory markers, including IL-6 and tumor necrosis factor-α (TNF-α). The changes in these inflammatory markers were all statistically significant compared with baseline levels (P<0.001). This study investigated the antioxidant, anti-apoptotic, and anti-inflammatory effects of menthol, providing the potential to differentiate these effects within a model whereby all three factors contribute to ulceration and associated symptoms ( 37). Importantly, menthol administration led to a reduction in markers of antioxidant activity and a reduction in the neutrophilic immune response in the gastric mucosa, which collectively suggests a gastroprotective effect. Hans M., Wilhelm M., Swandulla D. (2012). Menthol suppresses nicotinic acetylcholine receptor functioning in sensory neurons via allosteric modulation. Chem. Senses 37

Colburn R. W., Lubin M. L., Stone D. J., Jr., Wang Y., Lawrence D., D’Andrea M. R., et al. (2007). Attenuated cold sensitivity in TRPM8 null mice. Neuron 54 Harrison J. L., Davis K. D. (1999). Cold-evoked pain varies with skin type and cooling rate: A psychophysical study in humans. Pain 83 Caspani O., Zurborg S., Labuz D., Heppenstall P. A. (2009). The contribution of TRPM8 and TRPA1 channels to cold allodynia and neuropathic pain. PLoS One 4:e7383. 10.1371/journal.pone.0007383 Wasner et al., 2004, 2008; Namer et al., 2005, 2008; Seifert and Maihöfner, 2007; Altis et al., 2009; Flühr et al., 2009; Binder et al., 2011; Mahn et al., 2014

Fallon M. T., Storey D. J., Krishan A., Weir C. J., Mitchell R., Fleetwood-Walker S. M., et al. (2015). Cancer treatment-related neuropathic pain: Proof of concept study with menthol–a TRPM8 agonist. Support. Care Cancer 23 Basso L., Aboushousha R., Fan C. Y., Iftinca M., Melo H., Flynn R., et al. (2019). TRPV1 promotes opioid analgesia during inflammation. Sci. Signal. 12:eaav0711. 10.1126/scisignal.aav0711Sundar et al. ( 69) and Kaur et al. ( 70) have noted that tobacco flavoring can influence the inflammatory response of cigarette smoke inhalation in the lungs. A variety of flavors added to tobacco have been associated with decreased viability of cells, decreased cell numbers in cultures, and increased levels of inflammation after exposure compared with unflavored tobacco, including when flavored with menthol ( 69). It is proposed that menthol acts on the TRPA1 receptor to activate an inflammatory response in lung parenchyma based on cell experiments and animal models of cigarette smoke inhalation ( 30, 71). Markers of inflammation elevated with menthol flavored smoke inhalation included cyclo-oxygenase-2 (COX-2) and prostaglandin levels, which are recognized as drivers of an acute local inflammatory reaction in various tissue types ( 30). Hawthorn M., Ferrante J., Luchowski E., Rutledge A., Wei X. Y., Triggle D. J. (1988). The actions of peppermint oil and menthol on calcium channel dependent processes in intestinal, neuronal and cardiac preparations. Aliment. Pharmacol. Ther. 2 Diver M. M., Cheng Y., Julius D. (2019). Structural insights into TRPM8 inhibition and desensitization. Science 365 Studies showed that TRPM8 might affect in inflammation process by mediating the release of inflammatory cytokines ( 47, 81). NF-κB mediated TRPM8’s TNF-α inhibition ( 81). The study found that TRPM8 expression enhanced TRPM8-NF-κB interaction, decreasing NF-κB nuclear localization. On this basis, the suppression inhibited TNF-α gene transcription. TRPM8 agonists, such as menthol and icilin, are found to decrease the release of cytokines IL-1β, IL-6, and TNF-α from dendritic cells ( 47). The application of menthol to cutaneous tissue and submucosal tissue is common in clinical practice for the alleviation of pain and to promote a cooling effect ( 54). However, there is increasing interest in the potential for menthol to exert anti-inflammatory effects when applied to the skin and associated tissues, including inflammation associated with pathological conditions and processes. Andersen et al. ( 39) reported that the application of a 40% menthol-based cream to an area of irritation and inflammation on the forearm leads to a reduction in pain, neurogenic inflammation, and hyperalgesia (high responsiveness to sensations). This study utilized topical 10% trans-cinnamaldehyde application to elicit the cutaneous symptoms, with the treatment of menthol simultaneously noted to reduce pain intensity (P<0.01), neurogenic inflammation (P<0.01), and hyperalgesia (P<0.05) compared with symptoms when menthol was not applied.

Taken together, studies exploring the protective effects of menthol against inflammatory damage, or association with protection against chronic inflammation, are indicative of potential therapeutic applications of the compound. However, it is vital to note the limitations of the studies completed to date. One main limitation is that most of the studies are either in vitro or based on animal models of human disease, which may limit our direct application to human pathology. While menthol has been extensively used therapeutically in humans, demonstration of a clear anti-inflammatory effect should be sought and evaluated using biochemical markers or inflammation in a pathological context. Furthermore, while the evidence for the anti-inflammatory effects of menthol is compelling, additional insights may be needed to clarify the mechanisms of action and the clinical relevance of any anti-inflammatory effects. This is especially noteworthy considering the evidence for the pro-inflammatory impact of inhaled menthol when combined with cigarette smoking ( 42). Inflammatory cascades are complex and closely related to immunological function and antioxidant activity in multiple tissues; further delineation of these mechanisms and pathways is needed to truly appreciate the potential for menthol, which appears to be a molecule with multiple cellular targets, as a therapeutic anti-inflammatory compound. Author contributions Some perspectives propose that menthol may exert its anti-inflammatory effect by interacting with the dense network of nerves that are embedded in the dermal-epidermal junction of the skin. In connection with the nerve and inflammation impact, nerve endings detect cutaneous stimulation, trigger inflammatory responses, produce vasoactive neuropeptides, and transmit pain signals ( 59). After an injury, neuropeptides and noradrenaline released from nerve terminals in the skin activate receptors on the surface of keratinocytes, resulting in the production of inflammatory cytokines and nerve growth factor, as well as the degranulation of mast cells. Superimposed upon that, mast cell degranulation produces several inflammatory mediators, including tryptase, proteases, histamine, cytokines, and eicosanoids, all of which are theoretically capable of producing inflammation and sensitization ( 60). Hilfiger L., Triaux Z., Marcic C., Héberlé E., Emhemmed F., Darbon P., et al. (2021). Anti-hyperalgesic properties of menthol and pulegone. Front. Pharmacol. 12:753873. 10.3389/fphar.2021.753873

Conflict of interest

Chen Y., Geis C., Sommer C. (2008). Activation of TRPV1 contributes to morphine tolerance: Involvement of the mitogen-activated protein kinase signaling pathway. J. Neurosci. 28 Cold hypersensitivity alleviated, no change in receptive field size was observed or in heat, dynamic brush, or electrically evoked responses