Chronic inflammatory pain upregulates expression of P2Y2 receptor in small-diameter sensory neurons

Roles of ionotropic purinergic (P2X) receptors in chronic pain have been intensively investigated. However, the contribution of metabotropic purinergic (P2Y) receptors to pathological pain is controversial. In the present study, using single cell RT-PCR (reverse transcription-polymerase chain reaction) and single cell nested-PCR techniques, we examined the expression of P2X(2), P2X(3), P2Y(1) and P2Y(2) mRNA transcripts in retrogradely labeled cutaneous sensory neurons from mouse lumber dorsal root ganglia (DRGs) following peripheral inflammation. The percentage of cutaneous sensory neurons expressing P2Y(2) mRNA transcripts increased after complete Freund's adjuvant (CFA) treatment. Particularly, the P2Y(2) mRNA transcripts were more frequently detected in small-diameter cutaneous neurons from CFA-treated mice than those from control mice. Coexpression of P2Y(2) and P2X (P2X(2) or P2X(3)) mRNAs was more frequently observed in cutaneous sensory neurons from CFA-treated mice relative to controls. Pain behavioral tests showed that the blockade of P2Y receptors by suramin attenuated mechanical allodynia evoked either by CFA or uridine triphosphate (UTP), an endogenous P2Y(2) and P2Y(4) agonist. These results suggest that chronic inflammatory pain enhances expression of P2Y(2) receptor in peripheral sensory neurons that innervate the injured tissue and the activation of P2Y receptors contributes to mechanical allodynia following inflammation.