Anti-Hyperalgesic Properties of Menthol and Pulegone

Abstract

<b>Context:</b> Menthol, the main monoterpene found in <i>Mentha piperita</i> L. (<i>M. piperita</i>) is known to modulate nociceptive threshold and is present in different curative preparations that reduce sensory hypersensitivities in pain conditions. While for pulegone, a menthol-like monoterpene, only a limited number of studies focus on its putative analgesic effects, pulegone is the most abundant monoterpene present in <i>Calamintha nepeta</i> (L.) Savi (<i>C. nepeta</i>), a plant of the Lamiaceae family used in traditional medicine to alleviate rheumatic disorders, which counts amongst chronic inflammatory diseases. <b>Objectives:</b> Here, we analyzed the monoterpenes composition of <i>C. nepeta</i> and <i>M. piperita</i>. We then compared the putative anti-hyperalgesic effects of the main monoterpenes found, menthol and pulegone, in acute inflammatory pain conditions. <b>Methods:</b> <i>C. nepeta</i> and <i>M. piperita</i> extracts were obtained through pressurized liquid extraction and analyzed by gas chromatography-mass spectrometry. The <i>in vitro</i> anti-inflammatory activity of menthol or pulegone was evaluated by measuring the secretion of the tumour necrosis factor alpha (TNF α) from LPS-stimulated THP-1 cells. The <i>in vivo</i> anti-hyperalgesic effects of menthol and pulegone were tested on a rat inflammatory pain model. <b>Results:</b> Pulegone and menthol are the most abundant monoterpene found in <i>C. nepeta</i> (49.41%) and <i>M. piperita</i> (42.85%) extracts, respectively. <i>In vitro</i>, both pulegone and menthol act as strong anti-inflammatory molecules, with EC50 values of 1.2 ± 0.2 and 1.5 ± 0.1 mM, respectively, and exert cytotoxicity with EC50 values of 6.6 ± 0.3 and 3.5 ± 0.2 mM, respectively. <i>In vivo</i>, 100 mg/kg pulegone exerts a transient anti-hyperalgesic effect on both mechanical (pulegone: 274.25 ± 68.89 g, <i>n</i> = 8; vehicle: 160.88 ± 35.17 g, <i>n</i> = 8, <i>p</i> < 0.0001), thermal heat (pulegone: 4.09 ± 0.62 s, <i>n</i> = 8; vehicle: 2.25 ± 0.34 s, <i>n</i> = 8, <i>p</i> < 0.0001), and cold (pulegone: 2.25 ± 1.28 score, <i>n</i> = 8; vehicle: 4.75 ± 1.04 score, <i>n</i> = 8, <i>p</i> = 0.0003). In a similar way, 100 mg/kg menthol exerts a transient anti-hyperalgesic effect on both mechanical (mechanical: menthol: 281.63 ± 45.52 g, <i>n</i> = 8; vehicle: 166.25 ± 35.4 g, <i>n</i> = 8, <i>p</i> < 0.0001) and thermal heat (menthol: 3.65 ± 0.88 s, <i>n</i> = 8; vehicle: 2.19 ± 0.26 s, <i>n</i> = 8, <0.0001). <b>Conclusion:</b> Here, we show that both pulegone and menthol are anti-inflammatory and anti-hyperalgesic monoterpenes. These results might open the path towards new compound mixes to alleviate the pain sensation.