The transient receptor potential (TRP) gene superfamily consists of a large set of tetrameric channels permeable to monovalent and/or divalent cations. In mammals, several of the 28 TRP channel family members are expressed in subpopulations of peripheral sensory neurons and are involved in the transduction of thermal, mechanical and chemical stimuli, with documented roles in normal and pathological nociception
However, both the cellular physiology of TRP canonical (TRPC) channels in sensory neurons and their exact role in nociception have yet to be clarified. The TRPC subtypes are non-selective calcium-permeable cationic channels ubiquitously expressed in most tissues. They integrate several types of intracellular stimuli, including PLC and PKC activity, DAG levels, intracellular calcium levels and PIP2 levels into changes in membrane potential and calcium entry
. Of particular interest is the role of TRPC3 in somatosensory pathways because of recent evidence pointing to an almost exclusive expression of this channel in IB4+ nociceptors, a large population of C-fiber nonpeptidergic sensory neurons known to contribute to inflammatory pain
One of the hallmarks of inflammatory pain is the sensitization of peripheral nociceptors innervating the inflamed area. During the inflammation process, induced either by chemical agents, injury or infection, a stimulus-response shift is observed in nociceptors, resulting in an enhanced response to noxious stimuli (hyperalgesia) or in a nocifensive response to normally innocuous stimuli (allodynia)
. The many chemicals released at the site of inflammation form a “soup” that triggers oedema, itchiness, redness, and sensitization
. This “inflammatory soup” consists of a diverse set of signaling molecules which are capable of inducing hyperexcitability by activating their cognate membrane receptors at the surface of nociceptors
. Many of these receptors include well characterized Gq-protein coupled receptors (GqPCR), such as the P2Y2 purinoceptor, bradykinin B2 and protease-activated PAR2 receptor
[6, 7]. The coupling of these GqPCRs to the phospholipase C (PLC) pathway produces two bioactive compounds upon stimulation, diacylglycerol (DAG) and inositol trisphosphate (IP3), both of which act to increase intracellular calcium levels through two distinct mechanisms: one is referred to as store-operated calcium entry (SOCE) and involves the activation of calcium channels by IP3-induced depletion of endoplasmic reticulum calcium stores. The other, receptor-operated calcium entry (ROCE), involves the activation of calcium-permeable channels directly by DAG
[8–11]. Hence, the production of these two secondary messengers via PLC activation results in both intracellular calcium levels increase and enhanced entry of extracellular calcium across the plasma membrane following depletion of calcium stores. Crucially, one common resulting action of these two pathways is the downstream activation of calcium-dependent protein kinase C (PKC) isoforms mediating the phosphorylation and sensitization of voltage- and ligand-gated channels involved in acute and chronic inflammatory responses
The signaling pathways of known pro-inflammatory receptors in DRG neurons such as P2Y2 and PAR2 are not completely characterized, but an increase in intracellular calcium levels is critical for the activation of their downstream effectors
[8, 10, 14]. It has recently been shown that an abnormal persistent increase in intracellular calcium levels mediates the transition from acute to chronic pain in inflammatory pancreatitis
[15, 16]. Thus the regulation of intracellular calcium levels, primarily through SOCE, could be a key mechanism in preventing sensitization. While most TRPC channel subtypes play a role in ROCE, some members of the TRPC family have been implicated in SOCE signaling as well
[17, 18]. Moreover, members of the TRPC3/6/7 channel subfamily have been shown to be functionally coupled to the PLC pathway in various mammalian cell lines
[19–22]. In addition, TRPC channels have been linked to the activation of pro-inflammatory bradykinin B2 receptors in non-neural cells
[23, 24], suggesting that this class of calcium-permeable cation channels might be involved in both SOCE and ROCE signaling mechanisms downstream of the PLC pathway in neurons as well.
The present report focuses on the ill-defined role of TRPC channels in mammalian sensory pathways. Our data show that TRPC3, highly expressed in adult rodent DRG, is coupled to several classes of pro-inflammatory metabotropic receptors and plays a significant role in calcium homeostasis and sensitization in primary nociceptors, through its involvement in both SOCE and ROCE.