Using the sciatic nerve CCI model in TRPV1−/− mice we uncover a substantial role of TRPV1 in neuropathic mechanical pain. We observed ∼41% reversal of CCI-induced mechanical allodynia by spinal application of the TRPV1 antagonist BCTC in RTX-treated mice (Figure 4D and 4E) revealing that endogenous activation of spinal TRPV1, possibly by GPCRs (Kim et al., 2009) or arachidonic acid (AA) metabolites (Gibson et al., 2008) such as 12-hydroperoxyeicosatetraenoic acid (12-HPETE)
(Figure S5) contributes to the maintenance of PD-0332991 mw chronic mechanical allodynia after neuropathic nerve injury. Our observations also correlate with findings showing that TRPV1 antagonists with greater CNS penetration are more potent for reducing mechanical allodynia (Cui et al., 2006 and Patapoutian et al., 2009). Finally, we have shown that by targeting spinally mediated chronic pain we can avoid the side effects of peripheral TRPV1 blockade on temperature homeostasis (Steiner et al., 2007). Our results help to clarify prior controversy surrounding the role of TRPV1 by explaining how it is that TRPV1 antagonists can reduce neuropathic mechanical pain (Cui et al.,
2006 and Patapoutian et al., 2009) even though TRPV1-expressing primary sensory neurons do not convey physiological mechanical pain (Cavanaugh et al., 2009). By using RTX to ablate TRPV1-expressing primary afferents, we were able to functionally isolate the contribution of postsynaptic TRPV1; however, further study into spinal TRPV1-mediated plasticity may require conditional TRPV1 knockout in DRG neurons. A recent study using a Roxadustat cost TRPV1
reporter mouse showed that there are very few cells in the CNS that express TRPV1 (Cavanaugh et al., 2011); our results using both immuno EM and electrophysiology show that a subpopulation of interneurons in the SG are among these. The TRPV1-mediated currents in these SG neurons were small (∼17 pA PDK4 on average), corresponding to activation of only a few dozen TRPV1 channels. Nevertheless, we find that this sparse expression of postsynaptic TRPV1 channels in a key population of neurons has major functional consequences, playing a critical role in mediating mechanical allodynia. Together with TRPV1-mediated synaptic plasticity recently demonstrated in hippocampus (Gibson et al., 2008), dentate gyrus (Chávez et al., 2010), and nucleus accumbens (Grueter et al., 2010), this work provides further evidence for the functional significance and physiological implications of TRPV1 in the CNS. In particular, our results show that TRPV1 expression in a key population of spinal cord neurons underlies a critical role as modulator of pain transmission in spinal circuits distinct from its well-known role as a molecular transducer of pain in primary sensory neurons. Detailed protocols are listed in Supplemental Experimental Procedures.