Transient receptor potential ankyrin 1 (TRPA1), responding to noxious cold and pungent compounds, is implicated in the mediation of nociception, but little is known about the processing of the TRPA1-mediated nociceptive information within the trigeminal sensory nuclei (TSN) and the spinal dorsal horn (DH). To address this issue, we characterized the TRPA1-positive (+) neurons in the trigeminal ganglion (TG) and investigated the distribution of TRPA1(+) afferent fibers and their synaptic connectivity within the rat TSN and DH by using light and electron microscopic immunohistochemistry. In the TG, TRPA1 was expressed in unmyelinated and small myelinated axons and also occasionally in large myelinated axons. Many TRPA1(+) neurons costained for the marker for peptidergic neurons substance P (26.8%) or the marker for nonpeptidergic neurons IB4 (44.5%). In the CNS, small numbers of axons and terminals were immunopositive for TRPA1 throughout the rostral TSN, in contrast to the dense network of positive fibers and terminals in the superficial laminae of the trigeminal caudal nucleus (Vc) and DH. The TRPA1(+) terminals contained clear round vesicles, were presynaptic to one or two dendrites, and rarely participated in axoaxonic contacts, suggesting involvement in relatively simple synaptic circuitry with a small degree of synaptic divergence and little presynaptic modulation. Immunoreactivity for TRPA1 was also occasionally observed in postsynaptic dendrites. These results suggest that TRPA1-dependent orofacial and spinal nociceptive input is processed mainly in the superficial laminae of the Vc and DH in a specific manner and may be processed differently between the rostral TSN and Vc.

No abstract

Eugenol is widely used in dentistry as a local analgesic agent, because of its ability to allay tooth pain. Interestingly, eugenol shares several pharmacological actions with local anesthetics which include inhibition of voltage-gated sodium channel (VGSC) and activation of transient receptor potential vanilloid subtype 1 (TRPV1). In the present study, we investigated the effects of eugenol on pain behaviors in orofacial area, and as an attempt to elucidate its mechanism we characterized inhibitory effects of eugenol on VGSCs in trigeminal ganglion (TG) neurons. TG neurons were classified into four types on the basis of their neurochemical and electrophysiological properties such as cell size, shapes of action potential (AP), isolectin-B(4) (IB(4)) binding, and were analyzed for the association of their distinctive electrophysiological properties and mRNA expression of Na(v)1.8 and TRPV1 by using single-cell RT-PCR following whole-cell recordings. Subcutaneous injection of eugenol reduced the thermal nociception and capsaicin-induced thermal hyperalgesia in a dose-dependent manner. Eugenol also diminished digastric electromyogram evoked by noxious electrical stimulation to anterior tooth pulp, which was attributable to the blockade of AP conduction on inferior alveolar nerve. At cellular level, eugenol reversibly inhibited APs and VGSCs in IB(4)+/TRPV1+/Na(v)1.8+ nociceptive TG neurons (Type I-Type III) and IB(4)-/TRPV1-/Na(v)1.8- nociceptive TG neurons (Type IV). Both TTX-resistant I(Na) in Type I-Type III neurons and TTX-sensitive I(Na) in Type IV neurons were sensitive to eugenol. Taken together, these results suggest that eugenol may serve as local anesthetics for other pathological pain conditions in addition to its wide use in dental clinic.

No abstract

We have previously reported a novel method for producing chronic nociceptive behavior in rats following compression of the trigeminal ganglion. In the present study, we have further studied the role of demyelination in the development of prolonged nociceptive behavior in the trigeminal territory. For this purpose, lysophosphatidic acid (LPA) was injected into the trigeminal ganglia of male Sprague-Dawley rats weighing between 250 and 260 g. Under pentobarbital sodium anesthesia, the rats were mounted onto a stereotaxic frame and 3 microL of LPA (1 nmol) solution was injected into the trigeminal ganglion to produce demyelination. This treatment decreased the air-puff thresholds both ipsilateral and contralateral to the injection site, which persisted until postoperative day 100 and returned to the preoperative levels 130 days after the LPA injection. The LPA injection also produced a significant ipsilateral hyper-responsiveness to pin-prick stimulation. The effects of DGPP, an LPA1/3 receptor antagonist, and Y-27632, a Rho kinase inhibitor, upon LPA-induced mechanical allodynia and hyperalgesia were also investigated. Pretreatment with DGPP blocked both mechanical allodynia and ipsilateral hyperalgesia. However, pretreatment with Y-27632 blocked only ipsilateral and contralateral mechanical allodynia. These results thus indicate that a targeted blockade of LPA receptor and Rho kinase pathways are potentially important new treatments for demyelination-induced trigeminal neuralgia-like nociception.