Intraplantar injection of bergamot essential oil induces peripheral antinociception mediated by opioid mechanism

Abstract

This study investigated the effect of bergamot essential oil (BEO) containing linalool and linalyl acetate as major volatile components in the capsaicin test. The intraplantar injection of capsaicin (1.6 μg) produced a short-lived licking/biting response toward the injected paw. The nociceptive behavioral response evoked by capsaicin was inhibited dose-dependently by intraplantar injection of BEO. Both linalool and linalyl acetate, injected into the hindpaw, showed a significant reduction of nociceptive response, which was much more potent than BEO. Intraperitoneal (i.p.) and intraplantar pretreatment with naloxone hydrochloride, an opioid receptor antagonist, significantly reversed BEO- and linalool-induced antinociception. Pretreatment with naloxone methiodide, a peripherally acting μ-opioid receptor preferring antagonist, resulted in a significant antagonizing effect on antinociception induced by BEO and linalool. Antinociception induced by i.p. or intrathecal morphine was enhanced by the combined injection of BEO or linalool. The enhanced effect of combination of BEO or linalool with morphine was antagonized by pretreatment with naloxone hydrochloride. Our results provide evidence for the involvement of peripheral opioids, in the antinociception induced by BEO and linalool. Combined administration of BEO or linalool acting at the peripheral site, and morphine may be a promising approach in the treatment of clinical pain.

Introduction

The essential oil of bergamot (BEO; Citrus bergamia, Risso) is one of the most common essential oil and most familiar to the general public. BEO is obtained by cold pressing of the epicarp and in part of the mesocarp of the fresh fruit of bergamot. BEO consists of a volatile fraction (93–96%) and a non volatile fraction (4–7%); the former fraction contains monoterpene and sesquiterpene hydrocarbons and oxygenated derivatives such as linalool and linalyl acetate, and the latter fraction contains waxes, polymethoxylated flavones, coumarins and psoralens such as bergamottin and bergapten (Mondello et al., 1993, Dugo et al., 2000). Recently, BEO has been shown to reduce neuronal damage caused by excitotoxic stimuli (Corasaniti et al., 2007). (−) Linalool is the natural occurring enantiomer of the monoterpene compound found in the essential oil extracted from aromatic plants such as sauge, lavender, rose wood, thyme and bergamot. As previously reported (Peana et al., 2002, Peana et al., 2003, Peana et al., 2004), (−) linalool administration produced anti-inflammatory and antinociceptive activities in several behavioral assays. However, the antinociceptive efficacy of intraplantar BEO and linalool on capsaicin-induced nociceptive response is unknown.

The capsaicin (8-methly-N-vanillyl-6-noneamide) test is widely used as a model of pain in mice (Sakurada et al., 1992), rats (Pelissier et al., 2002) and humans (Hughes et al., 2002). We previously reported that subcutaneous (s.c.) injection of capsaicin into the hindpaw produced a short-lasting paw-licking/biting response, which was dose-dependently inhibited by intrathecally (i.t.) administered morphine (Sakurada et al., 1994). Activation of primary afferent nociceptors by capsaicin causes the release of nociceptive transmitters, substance P and glutamate from the dorsal spinal cord in vivo and in vitro (Gamse et al., 1979, Ueda et al., 1993, Sorkin and McAdoo, 1993). In addition, it has been shown that capsaicin excites the C-fiber population of nociceptive afferents through transient receptor potential vanilloid type-1 (TRPV-1) receptors located in C-fiber type nociceptors (Di Marzo et al., 2002, Szallasi et al., 2007).

In this study, the effects of BEO on capsaicin-evoked acute pain were investigated in comparison with linalool. In addition, here intraplantar injection BEO and linalool were tested for (1) production of antinociception in a capsaicin pain model, (2) assessment of the involvement of peripheral opioid system, and (3) modulation of the antinociceptive effect of morphine.