Published in:
01-09-2009 | Review Article
Is thermal nociception only sensed by the capsaicin receptor, TRPV1?
Author:
Akio Hiura
Published in:
Anatomical Science International
|
Issue 3/2009
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Abstract
Mammalian heat pain perception is well documented as a molecular event in the primary afferent neurons expressing TRPV1. Six types of thermo-TRPs were found, i.e., TRPV1–4, TRPM8 and TRPA1. The former TRPV1, 2 and TRPV3, 4 are sensitive to noxious heat and warmth, and the latter two are sensitive to cool or cold, respectively. We attempted to provide a hypothesis to explain the paradox in which TRPV1 knockout mice and capsaicin-pretreated mice with severe loss of small dorsal root ganglion (DRG) neurons behave normally to noxious heat. From the general view that TRPV1 is preferentially expressed in C-fibers responding to a moderate thermal threshold (>43°C) and TRPV2 in Aδ-fibers to high threshold temperatures (>52°C), the above phenomenon is perplexing. Woodbury et al. (J Neurosci 24:6410–6415,
2004) offered two pain transduction mechanisms, one being TRPV1/2-independent and the other TRPV1-dependent. The former detects noxious heat under normal conditions without the presence of TRPV1 or TRPV2, and the latter requires TRPV1 under pathophysiological conditions. Unidentified isolectin B4 (IB4)-positive but TRPV1-negative small neurons with a higher noxious heat threshold are feasible, because a spliced isoform of TRPV1 responsive to noxious heat (47°C) but not responsive to either proton or capsaicin is present in human and rat sensory neurons. Thus, the IB4-positive but TRPV1-negative small sensory neurons must have a crucial role in the noxious heat response.