Tohko Iida

Sensitization of TRPV1 by EP1 and IP reveals peripheral nociceptive mechanism of prostaglandins.

Prostaglandin E2 (PGE2) and prostaglandin I2 (PGI2) are major inflammatory mediators that play important roles in pain sensation and hyperalgesia. The role of their receptors (EP and IP, respectively) in inflammation has been well documented, although the EP receptor subtypes involved in this process and the underlying cellular mechanisms remain to be elucidated. The capsaicin receptor TRPV1 is a nonselective cation channel expressed in sensory neurons and activated by various noxious stimuli. TRPV1 has been reported to be critical for inflammatory pain mediated through PKA- and PKC-dependent pathways. PGE2 or PGI2increased or sensitized TRPV1 responses through EP1 or IP receptors, respectively predominantly in a PKC-dependent manner in both HEK293 cells expressing TRPV1 and mouse DRG neurons. In the presence of PGE2 or PGI2, the temperature threshold for TRPV1 activation was reduced below 35°C, so that temperatures near body temperature are sufficient to activate TRPV1. A PKA-dependent pathway was also involved in the potentiation of TRPV1 through EP4 and IP receptors upon exposure to PGE2 and PGI2, respectively. Both PGE2-induced thermal hyperalgesia and inflammatory nociceptive responses were diminished in TRPV1-deficient mice and EP1-deficient mice. IP receptor involvement was also demonstrated using TRPV1-deficient mice and IP-deficient mice. Thus, the potentiation or sensitization of TRPV1 activity through EP1 or IP activation might be one important mechanism underlying the peripheral nociceptive actions of PGE2 or PGI2.

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

Transient receptor potential vanilloid 4 (TRPV4), a cation channel responsive to hypotonicity, can also be activated by warm temperatures. Moreover, TRPV4-/- mice reportedly exhibit deficits in inflammation-induced thermal hyperalgesia. However, it is unknown whether TRPV4 or related transient receptor potential channels account for warmth perception under injury-free conditions. We therefore investigated the contribution of TRPV4 to thermosensation and thermoregulation in vivo. On a thermal gradient, TRPV4-/- mice selected warmer floor temperatures than wild-type littermates. In addition, whereas wild-type mice failed to discriminate between floor temperatures of 30 and 34°C, TRPV4-/- mice exhibited a strong preference for 34°C. TRPV4-/- mice also exhibited prolonged withdrawal latencies during acute tail heating. TRPV4-/- and wild-type mice exhibited similar changes in behavior on a thermal gradient after paw inflammation. Circadian body temperature fluctuations and thermoregulation in a warm environment were also indistinguishable between genotypes. These results demonstrate that TRPV4 is required for normal thermal responsiveness in vivo.

TRPV1 activation and induction of nociceptive response by a non-pungent capsaicin-like compound, capsiate.

Capsiate is a capsaicin-like ingredient of a non-pungent cultivar of red pepper, CH-19 sweet. To elucidate the mechanisms underlying the non-pungency of capsiate, we investigated whether capsiate activates the cloned capsaicin receptor, TRPV1 (VR1). In patch-clamp experiments, capsiate was found to activate TRPV1 expressed transiently in HEK293 cells with a similar potency as capsaicin. Capsiate induced nociceptive responses in mice when injected subcutaneously into their hindpaws with a similar dose dependency as capsaicin. These data indicate that the non-pungent capsiate is an agonist for TRPV1 and could excite peripheral nociceptors. In contrast to this, capsiate did not induce any significant responses when applied to the skin surface, eye or oral cavity of mice, suggesting that capsiate requires direct access to nerve endings to exhibit its effects. Capsiate was proved to have high lipophilicity and to be easily broken down in normal aqueous conditions, leading to less accessibility to nociceptors. Another highly lipophilic capsaicin analogue, olvanil, was similar to capsiate in that it did not produce irritant responses when applied to the skin surface, although it could activate TRPV1. Taken together, high lipophilicity and instability might be critical determinants for pungency and so help in understanding the effects of capsaicin-related compounds.

Attenuated fever response in mice lacking TRPV1

TRPV1, the capsaicin receptor, is expressed not only in nociceptive neurons, but also in other locations, including the hypothalamus. Studies involving systemic or intrahypothalamic capsaicin administration have suggested a role for TRPV1 in body temperature control. To explore this possibility, we examined thermoregulatory responses in TRPV1-/- mice. These mutant animals exhibited no obvious changes in circadian body temperature fluctuation, tolerance to increased (35 degrees C) or decreased (4 degrees C) ambient temperature or ethanol-induced hypothermia. In contrast, fever production in response to the bacterial pyrogen, lipopolysaccharide (LPS) was significantly attenuated in TRPV1-/- mice. Despite this finding, we detected no significant differences between TRPV1-/- and control mice in the extent of LPS-induced c-Fos expression in numerous fever-related brain subregions. These results suggest that TRPV1 participates in the generation of polyphasic fever, perhaps at sites outside the brain.

Enhanced thermal avoidance in mice lacking the ATP receptor P2X3

&NA; P2X3 is an ATP‐gated cation channel subtype expressed by a subpopulation of primary sensory neurons. In vivo spinal cord recordings in mice lacking P2X3 (Symbol) have suggested that this protein may be important for the coding of peripheral warm stimuli. To explore this possibility more thoroughly, we examined behavioral and electrophysiological responses to thermal stimuli in Symbol mice. As previously reported, recording from the spinal cord dorsal horn of anesthetized Symbol mice revealed a blunted response of wide dynamic range neurons to hind paw heating. When placed in a thermal gradient, however, Symbol mice exhibited an unexpectedly enhanced avoidance of both hot and cold temperatures, relative to controls. In the tail immersion test, mutant mice exhibited shorter withdrawal latencies at temperatures above and below thermoneutrality. Consistent with these changes, Symbol mice exhibited enhanced induction of spinal cord c‐FOS following hind paw heating to 45 °C. Thus, gain‐ and loss‐of‐function thermosensory phenotypes coexist in Symbol mice. No changes in thermal preference were observed in wild‐type mice injected subcutaneously with the P2X3 antagonist, A317491 or intrathecally with the P2X3 and P2X1 antagonist TNP‐ATP. The reason for this apparent discrepancy is unclear, but we cannot exclude the possibility that compensatory events contribute, at least in part, to the Symbol phenotype. Regardless, this study illustrates the utility of thermal preference assays as part of a comprehensive approach to the analysis of mouse thermosensation. Symbol. No caption available Symbol. No caption available Symbol. No caption available Symbol. No caption available Symbol. No caption available Symbol. No caption available Symbol. No caption available