Warm Temperature-sensitive Transient Receptor Potential Vanilloid 4 (TRPV4) Plays an Essential Role in Thermal Hyperalgesia

Abstract

Animals sense various ranges of temperatures by cutaneous thermal stimuli. Transient receptor potential vanilloid 4 (TRPV4) is a cation channel activated at a warm temperature (over 30°C) in exogenously expressed cells. We found in the present study that TRPV4 is essential in thermal hyperalgesia at a warm temperature in vivo. TRPV4؊/؊ and TRPV4؉/؉ mice exhibited the same latency of escape from 35-50°C hotplates. Neuronal activity in the femoral nerve, however, revealed that the number and activity level of neurons decreased in response to a warm temperature in TRPV4؊/؊ mice. TRPV4؊/؊ mice displayed a significantly longer latency to escape from the plates at 35-45°C when hyperalgesia was induced by carrageenan without changes in foot volumes. TRPV4 therefore determines the sensitivity rather than the threshold of painful heat detection and plays an essential role in thermal hyperalgesia.Sensation of warm to hot temperatures is an essential afferent neural activity in animals. An insight into the molecular basis of this sensation has been obtained by discovery of a capsaicin receptor, transient receptor potential vanilloid 1 (TRPV1) 1 (1). TRPV1 encodes a nonselective cation channel protein originally identified as the receptor for capsaicin, the principal pungent constituent in hot peppers. In addition to capsaicin and related vanilloid compounds, this channel is activated by acidic pH or by painfully hot temperatures. Responses to noxious heat are diminished in mice lacking TRPV1 (2). Detectable responses to hot temperatures (42-52°C) are absent in sensory neurons cultured from these animals. Therefore, TRPV1 plays an essential role in the detection of hot temperature. However, a significant component of heat responsiveness remains in TRPV1-knock-out mice and in skin-nerve preparations explanted from them (2, 3). These findings indicate the existence of TRPV1-independent mechanisms of heat detection.The mechanisms by which mammals detect innocuous warm temperatures are even less well understood than the mechanism underlying noxious thermosensation. A subset of sensory nerve fibers responsive to heat ranging from 30 to 42°C has been identified in vivo in a number of mammalian species (4, 5), although the heat transduction mechanisms accounting for such responsiveness have not been clarified. Recently, it has been reported that two TRPV1-related ion channel proteins, TRPV3 and TRPV4, can be activated by mild temperature elevations exceeding ϳ30 -35°C (6, 7). TRPV3 and TRPV4 are located in cultured keratinocytes and play an essential role in the detection of warm temperatures and hypoosmolarity in vitro (8).We previously found that TRPV4 might be a mediator of sensory neuron responsiveness to hypoosmolarity as well as a contributor to mechanical nociception (9 -11) in vivo by using mice lacking TRPV4. However, the role of TRPV4 in the detection of warm temperatures in vivo remains obscure.The mice lacking TRPV1 showed impairment in detection of chemically induced inflammatory pain when tested on hotplates over 50°...

Altered Thermal Selection Behavior in Mice Lacking Transient Receptor Potential Vanilloid 4

“…Peripheral nerve recordings have suggested that there might be a decrease in warmth-evoked electrical activity in TRPV4 Ϫ/Ϫ mice (Todaka et al, 2004). Moreover, we demonstrated that keratinocytes cultured from these animals lack the TRPV4-like warmth-activated currents observed in wildtype keratinocytes (Chung et al, 2004).…”

“…Ϫ/Ϫ mice failed to exhibit the same reduction in hot-plate escape latency as wild-type mice (Todaka et al, 2004). The same study suggested that there was a reduction in the activity of warmth-sensitive peripheral nerve fibers in these mice, even in the uninjured state.…”

“…In contrast, no changes in escape latency from heat stimuli were observed in either the hotplate (Suzuki et al, 2003;Todaka et al, 2004) or radiant pawheating (Liedtke and Friedman, 2003) assays. After subcutaneous injection of capsaicin or carrageenan, however, TRPV4 Ϫ/Ϫ mice showed longer escape latencies from a hot surface, relative to wild-type controls (Todaka et al, 2004). Thus, the precise contributions of TRPV4 to thermosensation and thermoregulation in vivo remain unclear.…”

“…Moreover, we demonstrated that keratinocytes cultured from these animals lack the TRPV4-like warmth-activated currents observed in wildtype keratinocytes (Chung et al, 2004). In contrast, no changes in escape latency from heat stimuli were observed in either the hotplate (Suzuki et al, 2003;Todaka et al, 2004) or radiant pawheating (Liedtke and Friedman, 2003) assays. After subcutaneous injection of capsaicin or carrageenan, however, TRPV4 Ϫ/Ϫ mice showed longer escape latencies from a hot surface, relative to wild-type controls (Todaka et al, 2004).…”

TRP Channels and Pruritus

“…>30 °C for TRPV4) [273][274][275][276][282][283][284] and their deletion results in altered sensation of thermal stimuli [285][286][287] -although recent results suggest that their contribution to heat sensation is not essential in mice [288]. Of further importance, the thermo-sensory functions of TRPV3 and TRPV4 are apparently linked to keratinocytes; namely, the moderate warm temperature elevation, that increases the activity of TRPV3 and/or TRPV4 expressed in keratinocytes, resulted in the release of ATP release which, in turn, can transmit the thermal information toward sensory neurons [289][290][291][292].…”