Peripheral Mechanism Underlying The Contribution Of VR1 Activation To The Transduction Of Noxious Information

A number of studies have shown that capsaicin produces a membrane response by opening a specific non-selective cation channels, named VR1, in the small dorsal root ganglion (DRG) neurons. VR1 selectively expressed on small- to medium-diameter DRG neurons (nociceptors) and it was recently considered as a molecular integrating multiple pain-producing stimuli. By means of the whole-cell patch-clamp recording technique, immunohistochemistry, pharmacological method and behavior tests, the present study was designed to determine the contribution of VR1 activation to the transduction of noxious stimuli, especially of noxious heat after inflammation, exploring the molecular mechanism underlying the activation and sensitization of nociceptors. The main results are as follows:Capsaicin-evoked current and voltage responses, and the effect of drug with analgesic action on such membrane responses were investigated in isolated dorsal root ganglion (DRG) neurons with small diameters. The present study showed that capsaicin induced inward current and depolarization of membrane potential with firing of action potentials which was blocked totally by competitive antagonist of VR1, capsazepine. In addition, neomycin, an analgesic, dose-dependently inhibited capsaicin-evoked current and voltage responses in small DRG neurons by inhibiting activation of VR1 directly or1. indirectly. All these data indicated that in small DRG neurons, VR1-involved peripheral mechanism plays an important role in activation of nociceptor and peripheral analgesia by neomycin. 2. The behavioral study indicated that: (1) intraplantar injection of the competitive antagonist of VR1, capsazepine, had no effect on the baseline response to noxious heat stimuli in the normal rat. (2) capsazepine treatment not only blocked capsaicin-induced thermal hyperalgesia, but also attenuated carrageenan-induced thermal hyperalgesia. At higher dose, it also prevented the development of carrageenan-induced hyperalgesia. (3) after systemic treatment with RTX to destroy VR1-expressed small- and medium- diameter DRG neurons selectively, carrageenan failed to produce thermal hyperalgesia. All these results suggested that VR1 is critical for the expression of inflammation-induced heat hyperalgesia.3. The present work showed for the first time that PKC activation facilitated capsaicin-evoked responses in rat DRG neurons. We presume that the PKC-induced phosphorylation of VR1 in nociceptors is likely to be involved in the fast regulation of capsaicin-evoked membrane responses. Hence, PKC is not only involved in the development of the central sensitization, but also plays an important role in the sensitization of primary nociceptive neurons. PKC-induced phosphorylation and subsequent sensitization of VR1 may be one of the mechanisms underlying the contribution of VR1 in the expression of inflammation-associated hyperalgesia.Double labeling study exhibited that the proteins encoded by VR1 and p-PKC( (Ser-729) were both expressed in nociceptors with co-localization to a great extent. Our quantitative analysis showed for the first time that p-PKC( (Ser-729) expression in DRG neurons was significantly up regulated after both carrageenan- and CFA-induced acute and chronic inflammation, while VR1 expression had no significant change. Hence, we presumed that inflammation-4. induced up regulation of p-PKC( expression may contribute to the development of inflammatory thermal hyperalgesia by phosphorylating VR1 directly or indirectly. However, the plastic changes in VR1 expression is not involved in the critical role of VR1 in the expression of inflammation-induced thermal hyperalgesia.