The heat and capsaicin receptor, TRPV1, is required for the detection of painful heat by primary afferent pain fibers (nociceptors), but the extent to which functional TRPV1 channels are expressed in the central nervous system (CNS) is debated. As previous evidence is based largely on indirect physiological responses to capsaicin, here we genetically modified the TrpV1 locus to reveal, with excellent sensitivity and specificity, the distribution of TRPV1 in all neuronal and non-neuronal tissues. In contrast to reports of widespread and robust expression in the CNS, we find that neuronal TRPV1 is largely restricted to nociceptors in primary sensory ganglia, with minimal expression in a few discrete brain regions, most notably in a contiguous band of cells within and adjacent to the caudal hypothalamus. We confirm hypothalamic expression in the mouse using several complementary approaches, including in situ hybridization, calcium imaging, and electrophysiological recordings. Additional in situ hybridization experiments in rat, monkey and human brain demonstrate that the restricted expression of TRPV1 in the CNS is conserved across species. Outside of the CNS, we find TRPV1 expression in a subset of arteriolar smooth muscle cells within thermoregulatory tissues. Here, capsaicin increases calcium uptake and induces vasoconstriction, an effect that likely counteracts the vasodilation produced by activation of neuronal TRPV1.

Alteration in the intracellular signal transduction pathway in primary afferent neurons may contribute to pain hypersensitivity. We demonstrated that very rapid phosphorylation of extracellular signal-regulated protein kinases (pERK) occurred in DRG neurons that were taking part in the transmission of various noxious signals. The electrical stimulation of Adelta fibers induced pERK primarily in neurons with myelinated fibers. c-Fiber activation by capsaicin injection induced pERK in small neurons with unmyelinated fibers containing vanilloid receptor-1 (VR-1), suggesting that pERK labeling in DRG neurons is modality specific. Electrical stimulation at the c-fiber level with different intensities and frequencies revealed that phosphorylation of ERK is dependent on the frequency. We examined the pERK in the DRG after application of natural noxious stimuli and found a stimulus intensity-dependent increase in labeled cell size and in the number of activated neurons in the c- and Adelta-fiber population. Immunohistochemical double labeling with phosphorylated ERK/VR-1 and pharmacological study demonstrated that noxious heat stimulation induced pERK in primary afferents in a VR-1-dependent manner. Capsaicin injection into the skin also increased pERK labeling significantly in peripheral fibers and terminals in the skin, which was prevented by a mitogen-activated protein kinase/ERK kinase inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis(2-aminopheylthio)butadiene (U0126). Behavioral experiments showed that U0126 dose-dependently attenuated thermal hyperalgesia after capsaicin injection and suggested that the activation of ERK pathways in primary afferent neurons is involved in the sensitization of primary afferent neurons. Thus, pERK in primary afferents by noxious stimulation in vivo showed distinct characteristics of expression and may be correlated with the functional activity of primary afferent neurons.

Proteinase-activated receptor (PAR) 2 is expressed on a subset of primary afferent neurons and involved in inflammatory nociception. Transient receptor potential vanilloid subfamily 1 (TRPV1) is a sensory neuron-specific cation channel that responds to capsaicin, protons, or heat stimulus. Here, we show that TRPV1 is coexpressed with PAR2 but not with PAR1 or PAR3, and that TRPV1 can functionally interact with PAR2. In human embryonic kidney 293 cells expressing TRPV1 and PAR2, PAR2 agonists increased capsaicinor proton-evoked TRPV1 currents through a PKC-dependent pathway. After application of PAR2 agonists, temperature threshold for TRPV1 activation was reduced from 42°C to well below the body temperature. PAR2-mediated Fos expression in spinal cord was decreased in TRPV1-deficient mice. The functional interaction was also observed in mouse DRG neurons and proved at a behavioral level. These represent a novel mechanism through which trypsin or tryptase released in response to tissue inflammation might trigger the sensation of pain by PAR2 activation.

Microglia in the spinal cord may play an important role in the development and maintenance of neuropathic pain. A metabotropic ATP receptor, P2Y 12 , has been shown to be expressed in spinal microglia constitutively and be involved in chemotaxis. Activation of p38 mitogen-activated protein kinase (MAPK) occurs in spinal microglia after nerve injury and may be related to the production of cytokines and other mediators, resulting in neuropathic pain. However, it remains unknown whether any type of P2Y receptor in microglia is involved in the activation of p38 MAPK and the pain behaviors after nerve injury.Using the partial sciatic nerve ligation (PSNL) model in the rat, we found that P2Y 12 mRNA and protein increased in the spinal cord and peaked at 3 d after PSNL. Double labeling studies revealed that cells expressing increased P2Y 12 mRNA and protein after nerve injury were exclusively microglia. Both pharmacological blockades by intrathecal administration of P2Y 12 antagonist and antisense knockdown of P2Y 12 expression suppressed the development of pain behaviors and the phosphorylation of p38 MAPK in spinal microglia after PSNL. The intrathecal infusion of the P2Y 12 agonist 2-(methythio) adenosine 5Ј-diphosphate trisodium salt into naive rats mimicked the nerve injury-induced activation of p38 in microglia and elevated pain behaviors.These data suggest a new mechanism of neuropathic pain, in which the increased P2Y 12 works as a gateway of the following events in microglia after nerve injury. Activation of this receptor by released ATP or the hydrolyzed products activate p38 MAPK pathway and may play a crucial role in the generation of neuropathic pain.

Proinflammatory agents trypsin and mast cell tryptase cleave and activate PAR2, which is expressed on sensory nerves to cause neurogenic inflammation. Transient receptor potential A1 (TRPA1) is an excitatory ion channel on primary sensory nerves of pain pathway. Here, we show that a functional interaction of PAR2 and TRPA1 in dorsal root ganglion (DRG) neurons could contribute to the sensation of inflammatory pain. Frequent colocalization of TRPA1 with PAR2 was found in rat DRG neurons. PAR2 activation increased the TRPA1 currents evoked by its agonists in HEK293 cells transfected with TRPA1, as well as DRG neurons. Application of phospholipase C (PLC) inhibitors or phosphatidylinositol-4,5-bisphosphate (PIP 2 ) suppressed this potentiation. Decrease of plasma membrane PIP 2 levels through antibody sequestration or PLC-mediated hydrolysis mimicked the potentiating effects of PAR2 activation at the cellular level. Thus, the increased TRPA1 sensitivity may have been due to activation of PLC, which releases the inhibition of TRPA1 from plasma membrane PIP 2 . These results identify for the first time to our knowledge a sensitization mechanism of TRPA1 and a novel mechanism through which trypsin or tryptase released in response to tissue inflammation might trigger the sensation of pain by TRPA1 activation.