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

Hiroshi Todaka, Junichi Taniguchi, Jun-ichi Satoh, Atsuko Mizuno, Makoto Suzuki
2004 Journal of Biological Chemistry  
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
more » ... 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°C (3). Carrageenan is a widely used reagent known for the ability to induce an acute inflammation. Carrageenan is a sulfated polysaccharide extracted mainly from an alga. When injected into the hind paw of a rat, foot volume had increased at 3 h after injection, the gain of which was used as an indicator of the inflammation or as an assay to test antiinflammatory drugs (12). In the first 1.5 h of the early phase of inflammation, thermal hyperalgesia is observed without remarkable edema. Thus use of such inflammatory agents may amplify the role of TRPV4 in thermal sensation in vivo. We thus examined the contribution of TRPV4 to the detection of warm temperatures and to chemically induced hyperalgesia using TRPV4-knock-out mice. EXPERIMENTAL PROCEDURES Animals-Mice were maintained on ad libitum diet chow and tap water for drinking. TRPV4-knock-out mice were backcrossed into a C57BL/6J inbred strain (Clea Japan Inc., Tokyo) to 4 -5 generations. The male TRPV4 null (TRPV4Ϫ/Ϫ) mice and the wild type (TRPV4ϩ/ϩ) mice were used at 8 -12 weeks of age for hotplate tests and at 10 -24 weeks of age for recordings of neuronal discharges. Foot volumes before and after injection of capsaicin, carrageenan, or a vehicle were measured by immersion of the foot in water. The Animal Experiment Committee at the Jichi Medical School approved all of the experimental procedures used in this study. Hotplate Test-The hotplate test was performed by a conventional method. A mouse placed on a thermal plate jumped over a fence (13 cm in height) to escape from the temperature. The duration (s) until escape was measured. The test was performed in triplicate for each mouse. Acute Inflammatory Responses-Injections of chemical reagents were performed under brief anesthesia with ether inhalation. For carrageenan hypersensitivity, carrageenan (20 mg/ml, Sigma) was suspended in an isotonic saline solution and injected into the plantar surfaces of both hind paws in a volume of 20 l by using a 26-gauge needle under brief anesthesia with ether. After 20 min, mice were tested for thermal sensitivity by the hotplate test. For capsaicin hypersensitivity, capsaicin (20 mg/ml, Sigma) in an isotonic saline solution containing 10% ethanol and 0.5% Tween 20 was injected into both hind paws in a volume of 20 l. After 20 min, thermal sensitivity was tested as described above. Recording and Analysis-Recordings were performed under anesthesia with urethane (1 g/kg, intraperitoneal). To apply thermal stress immediately, an aluminum pipe perfused with temperature-controlled
doi:10.1074/jbc.m406260200 pmid:15187078 fatcat:pvtlodygbvc6xfj66a5b6x5avi