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Pain. 2014 Dec;155(12):2662-72. doi: 10.1016/j.pain.2014.09.033. Epub 2014 Oct 2.

TRPV4 is necessary for trigeminal irritant pain and functions as a cellular formalin receptor.

Author information

1
Department of Neurology, Duke University, Durham, NC 27710, USA.
2
Pain Center and Department of Anesthesiology, Washington University, St Louis, MO 63110, USA.
3
Department of Neurobiology, Duke University, Durham, NC 27710, USA.
4
Department of Neurology, Duke University, Durham, NC 27710, USA; Department of Neurobiology, Duke University, Durham, NC 27710, USA; Department of Anesthesiology, Duke University, Durham, NC 27710, USA; Clinics for Pain and Palliative Care, Duke University, Durham, NC 27710, USA. Electronic address: wolfgang@neuro.duke.edu.

Abstract

Detection of external irritants by head nociceptor neurons has deep evolutionary roots. Irritant-induced aversive behavior is a popular pain model in laboratory animals. It is used widely in the formalin model, where formaldehyde is injected into the rodent paw, eliciting quantifiable nocifensive behavior that has a direct, tissue-injury-evoked phase, and a subsequent tonic phase caused by neural maladaptation. The formalin model has elucidated many antipain compounds and pain-modulating signaling pathways. We have adopted this model to trigeminally innervated territories in mice. In addition, we examined the involvement of TRPV4 channels in formalin-evoked trigeminal pain behavior because TRPV4 is abundantly expressed in trigeminal ganglion (TG) sensory neurons, and because we have recently defined TRPV4's role in response to airborne irritants and in a model for temporomandibular joint pain. We found TRPV4 to be important for trigeminal nocifensive behavior evoked by formalin whisker pad injections. This conclusion is supported by studies with Trpv4(-/-) mice and TRPV4-specific antagonists. Our results imply TRPV4 in MEK-ERK activation in TG sensory neurons. Furthermore, cellular studies in primary TG neurons and in heterologous TRPV4-expressing cells suggest that TRPV4 can be activated directly by formalin to gate Ca(2+). Using TRPA1-blocker and Trpa1(-/-) mice, we found that both TRP channels co-contribute to the formalin trigeminal pain response. These results imply TRPV4 as an important signaling molecule in irritation-evoked trigeminal pain. TRPV4-antagonistic therapies can therefore be envisioned as novel analgesics, possibly for specific targeting of trigeminal pain disorders, such as migraine, headaches, temporomandibular joint, facial, and dental pain, and irritation of trigeminally innervated surface epithelia.

KEYWORDS:

ERK; Formalin model; Formalin receptor; Irritant; MEK; TRPV4; Trigeminal ganglion; Trigeminal pain; Whisker pad

PMID:
25281928
PMCID:
PMC4295825
DOI:
10.1016/j.pain.2014.09.033
[Indexed for MEDLINE]
Free PMC Article

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