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.

Thermal nociception and TRPV1 function are attenuated in mice lacking the nucleotide receptor P2Y2.

Recent studies indicate that ATP and UTP act at G protein-coupled (P2Y) nucleotide receptors to excite nociceptive sensory neurons; nucleotides also potentiate signaling through the pro-nociceptive capsaicin receptor, TRPV1. We demonstrate here that P2Y(2) is the principal UTP receptor in somatosensory neurons: P2Y(2) is highly expressed in dorsal root ganglia and P2Y(2)-/- mice showed profound deficits in UTP-evoked calcium transients and potentiation of capsaicin responses. P2Y(2)-/- mice were also deficient in the detection of painful heat: baseline thermal response latencies were increased and mutant mice failed to develop thermal hypersensitivity in response to inflammatory injury (injection of complete Freund's adjuvant into the hindpaw). P2Y(2) was the only Gq-coupled P2Y receptor examined that showed an increase in DRG mRNA levels in response to inflammation. Surprisingly, TRPV1 function was also attenuated in P2Y(2)-/- mice, as measured by the frequency and magnitude of capsaicin responses in vitro and behavioral responses to capsaicin administration in vivo. However, TRPV1 mRNA levels and immunoreactivity were not reduced, and behavioral sensitivity to capsaicin could be largely restored in P2Y(2)-/- mice by pretreatment with bradykinin, suggesting that normal function of TRPV1 requires ongoing modulation by G protein-coupled receptors. These results indicate that nucleotide signaling through P2Y(2) plays a key role in thermal nociception.