Abstract
The amphibian Bv8 and the mammalian prokineticin 1 (PROK1) and 2 (PROK2) are new chemokine-like protein ligands acting on two G protein-coupled receptors, prokineticin receptor 1 (PKR1) and 2 (PKR2), participating to the mediation of diverse physiological and pathological processes. Prokineticins (PKs), specifically activating the prokineticin receptors (PKRs) located in several areas of the central and peripheral nervous system associated with pain, play a fundamental role in nociception. In this paper, to improve the understanding of the prokineticin system in the neurobiology of pain, we investigated the role of PKR2 in pain perception using pkr2 gene-deficient mice. We observed that, compared to wildtype, pkr2-null mice were more resistant to nociceptive sensitization to temperatures ranging from 46 to 48 °C, to capsaicin and to protons, highlighting a positive interaction between PKR2 and the non-selective cation channels TRPV1. Moreover, PKR2 knock-out mice showed reduced nociceptive response to cold temperature (4 °C) and to mustard oil-induced inflammatory hyperalgesia, suggesting a functional interaction between PKR2 and transient receptor potential ankyrin 1 ion (TRPA1) channels. This notion was supported by experiments in dorsal root ganglia (DRG) cultures from pkr1 and–pkr2-null mice, demonstrating that the percentage of Bv8-responsive DRG neurons which were also responsive to mustard oil was much higher in PKR1−/− than in PKR2−/− mice. Taken together, these findings suggest a functional interaction between PKR2 and TRP channels in the development of hyperalgesia. Drugs able to directly or indirectly block these targets and/or their interactions may represent potential analgesics.
Highlights
Prokineticins (PKs) are secreted regulatory proteins of 80–90 amino acids that display structural and functional characteristics similar to those of chemokines
Contains the PCR amplification products of the mutant gene; lanes wild type (WT) contain the PCR amplification products obtained with primers for wild-type prokineticin receptor 2 (PKR2) gene; lanes NEO contain the PCR amplification products obtained with primers for the mutant gene; lanes WT + NEO indicate multiplex PCR for both the WT and the mutant gene; lane M contains DNA molecular weight markers. (B) Immunofluorescence staining showing the presence of PKR2 in dorsal root ganglia (DRG) neurons from WT and PKR1À/À mice
The results showed that the development of CFA-induced thermal hyperalgesia was impeded by the disruption of pkr1 as well as pkr2 genes (Fig. 5B; the same effect in pkr2-null mice (Time): F(5,89) = 47.79, p < 0.0001; Group: F(2,89) = 91.85, p < 0.0001; Interaction: F(10,89) = 28.54, p < 0.0001; post-hoc p < 0.001 PKR1À/À vs WT and PKR2À/À vs WT), while only the pkr2 disruption blocked the development of CFA-induced tactile allodynia (Fig. 5C; Time: F(8,134) = 150.22, p < 0.0001; Group: F(2,134) = 107.23, p < 0.0001; Interaction: F(16,134) = 21.56, p < 0.0001; post-hoc p < 0.001 PKR2À/À vs WT)
Summary
Prokineticins (PKs) are secreted regulatory proteins of 80–90 amino acids that display structural and functional characteristics similar to those of chemokines. Studies from pkr1-null mutant mice explored the role of PKR1 receptors in nociceptive signalling and in nociceptor sensitization, demonstrating that, when compared with WT littermates, the lack of pkr gene was able to significantly impair nociception following exposure to noxious heat, mechanical stimuli, capsaicin, and protons (Negri et al, 2006). In the same work it was demonstrated that interaction between PKR1 and TRPV1, both co-expressed in a significant proportion of small-diameter neurons in dorsal root ganglia (DRG), represents a new example of GPCR and TRPV1 interaction mediating hyperalgesia and allodynia (Tominaga and Tominaga, 2005) Following both acute and chronic inflammatory stimuli, pkr1-null mice exhibited reduced development of hyperalgesia, with a decrease in PROK2 upregulation due to the inflammatory process, as previously demonstrated (Giannini et al, 2009), confirming the involvement of the PKR1 receptor in initiating and maintaining inflammatory pain. In view of the role of the prokineticin system in pain and hyperalgesia, the present study aims to further explore the role of PKR2 receptors in pain perception following noxious chemical, thermal and inflammatory stimulation using pkr2-deficient mice
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