Purinergic receptor P2Y1 regulates polymodal C-fiber thermal thresholds and sensory neuron phenotypic switching during peripheral inflammation.

We have recently found that, following complete Freund's adjuvant (CFA)-induced inflammation, cutaneous polymodal nociceptors (CPM) lacking the transient receptor potential vanilloid 1 (TRPV1) are sensitized to heat stimuli. In order to determine possible mechanisms playing a role in this change, we examined gene expression in the L2/L3 sensory ganglia following CFA injection into the hairy hind paw skin and found that G-protein-coupled purinoreceptor P2Y1 expression was increased. This receptor is of particular interest, as most CPMs innervating mouse hairy skin bind isolectin B4, which co-localizes with P2Y1. Additionally, our recent findings have shown that cutaneous CPMs in P2Y1-/- mice displayed significantly reduced thermal sensitivity. Together, these findings suggested a possible role for P2Y1 in inflammation-induced heat sensitization in these fibers. To test this hypothesis, we utilized our in vivo small interfering RNA technique to knock down the inflammation-induced increase in P2Y1 expression and then examined the functional effects using ex vivo recording. We found that the normal reduction of heat thresholds in CPM fibers induced by CFA was completely blocked by inhibition of P2Y1. Surprisingly, inhibition of P2Y1 during inflammation also significantly increased the number of CPM neurons expressing TRPV1 without a change in the total number of TRPV1-positive cells in the L2 and L3 dorsal root ganglia. These results show that the inflammation-induced enhanced expression of P2Y1 is required for normal heat sensitization of cutaneous CPM fibers. They also suggest that P2Y1 plays a role in the maintenance of phenotype in cutaneous afferent fibers containing TRPV1.