Novel agonistic action of mustard oil on recombinant and endogenous porcine transient receptor potential V1 (pTRPV1) channels

Abstract

Neurogenic components play a crucial role in inflammation and nociception. Mustard oil (MO) is a pungent plant extract from mustard seed, horseradish and wasabi, the main constituent of which is allylisothiocyanate. We have characterized the action of MO on transient receptor potential V1 (TRPV1), a key receptor of signal transduction pathways in the nociceptive system, using fura-2-based [Ca²⁺]_i imaging and the patch-clamp technique in a heterologous expression system and sensory neurons. In human embryonic kidney (HEK) 293 cells expressing porcine TRPV1 (pTRPV1), MO evoked increases of [Ca²⁺]_i in a concentration-dependent manner. A high concentration of MO elicited irreversible cell swelling. Capsazepine, ruthenium red and iodoresiniferatoxin dose-dependently suppressed the MO-induced [Ca²⁺]_i increase. MO elicited outward rectified currents in pTRPV1-expressing HEK 293 cells with a reversal potential similar to that of capsaicin. [Ca²⁺]_i responses to MO were completely abolished by the removal of external Ca²⁺. MO simultaneously elicited an inward current and increase of [Ca²⁺]_i in the same cells, indicating that MO promoted Ca²⁺ influx through TRPV1 channels. In cultured porcine dorsal root ganglion (DRG) neurons, MO elicited a [Ca²⁺]_i increase and inward current. Among DRG neurons responding to MO, 85% were also sensitive to capsaicin. The present data indicate that MO is a novel agonist of TRPV1 channels, and suggest that the action of MO in vivo may be partly mediated via TRPV1. These results provide an insight into the TRPV1-mediated effects of MO on inflammation and hyperalgesia.

Introduction

A neurogenic component has been reported to play a pivotal role in the pathological mechanisms of numerous inflammatory diseases [1], [2] and nociception [3]. The vanilloid receptor 1 (VR1), which is now referred to as transient receptor potential V1 (TRPV1), is a non-selective cation channel found in sensory neurons and functions as a molecular integrator of pain perception. TRPV1 is activated by protons, heat and capsaicin, which is the pungent agent in chilli peppers which produces itching, picking and burning sensations [4], [5], [6]. Because mice genetically lacking the TRPV1 receptor exhibit impaired nociception [7], [8], TRPV1 is considered to be a key component of signal transduction pathways in the nociceptive system.

Many naturally occurring pungent compounds have been reported to be activators of TRPV1 [9]. These include the vanilloid compounds such as the cactus extract resiniferatoxin, the ginger extracts zingerone and gingerol, and the nutmeg and clove oil constituent eugenol [10]. More recently, some endogenous activators of TRPV1, such as endovanilloids, have been identified [11]. Both agonist and antagonist strategies are thought to have merit for the treatment of a whole range of conditions ranging from inflammatory and neuropathic pain, to bladder dysfunction and irritable bowel syndrome [12].

Mustard oil (MO) is one of the pungent components of mustard seed, horseradish and wasabi [13] and its major active constituent is allylisothiocyanate [14]. Topical application of MO to the skin activates underlying sensory nerve endings, resulting in the production of burning pain, inflammation and hypersensitivity to thermal and mechanical stimuli [15], [16], [17]. MO causes a neurogenic inflammation, which is not observed in a denervated component of the rat hind paw skin [18]. However, the cellular targets and mechanisms of action of MO are still not fully elucidated. A recent report indicates that ankylin transmembrane protein 1 (ANKTM1) or TRPA1 [19], a member of the TRP channel family implicated in the detection of noxious cold, is activated by MO [13]. Similar to capsaicin, instillation of MO into the colon induces visceral pain behavior, hyperalgesia and plasma extravasation [20], [21], [22]. It is still a matter of debate whether this occurs through a direct action on sensory neurons or through a specific membrane receptor [9]. Since TRPVI-deficient mice still retain sensitivity to MO [23], capsaicin and MO have been suggested to excite nociceptors through distinct molecular mechanisms. On the other hand, it is suggested that MO and capsaicin act through convergent cellular signaling pathways, because inflammatory responses produced by these irritants show partial cross-desensitization [24]. In fact, in a mouse model of visceral pain induced by colonic instillation of MO, similar activation of spinal extracellular-regulated kinases (ERK1/2) in response to capsaicin is observed [25]. Using this model, a TRPV1 antagonist causes significant inhibition of MO-induced nociception [26]. Therefore, it is of interest to examine whether MO stimulates TRPV1 channels directly or indirectly.

Recently, we cloned a porcine orthologue of TRPV1 (pTRPV1) and analyzed its functional properties using a heterologous expression system [27]. We showed that pTRPV1 was a non-selective cation channel sensitive to a number of vanilloid agonists, including capsaicin and endovanilloids, heat and protons. From pharmacological characteristics, it has been shown that there were remarkable species differences; e.g., PPAHV, an agonist for TRPV1, can stimulate rodent [28], [29], [30], [31] and porcine TRPV1 [27] but not human [29] and guinea pig orthologues [32]. Capsazepine, a TRPV1 antagonist, does not antagonize proton- and heat-induced responses in rodent TRPV1 [29], [30] but does antagonize in human [29], guinea pig [32] and porcine TRPV1 [27]. It is apparent that chicken TRPV1 is not sensitive to capsaicin [33] and rabbit TRPV1 is less sensitive to capsaicin [34]. Although it has been reported that rat TRPV1-transfected cells are insensitive to MO [13], [35], due to the presence of significant evidence for species differences in the molecular pharmacology, it is worth investigating whether MO stimulates other TRPV1 orthologues.

In the present study, to elucidate the molecular mechanisms underlying the action of MO on nociceptors, we investigated whether MO activates TRPV1 in a heterologous expression system and sensory neurons, using pTRPV1-expressing human embryonic kidney (HEK) 293 cells and porcine dorsal root ganglion (DRG) neurons, respectively. To examine TRPV1 activity, we used fura-2-based [Ca²⁺]_i imaging and whole-cell patch-clamp techniques.