Abstract
Proteinase-activated receptors (PARs) are a family of G-protein-coupled receptors that are activated by endogenous serine proteinases that cleave the N-terminal domain of the receptor unmasking a “tethered ligand” sequence. Trypsin and other agonists at PAR2 act on peripheral nerves to augment the transfer of nociceptive information. We tested whether PAR2 agonists also exert a spinal pronociceptive effect by i.t. administering the selective ligand, Ser-Leu-Ile-Gly-Arg-Leu-NH2 (SLI-GRL). This produced thermal and mechanical hyperalgesia in rats and mice and augmented mechanical and thermal hyperalgesia seen in the formalin inflammatory pain test. Effects of SLIGRL were abrogated in PAR2-deficient mice and were not seen with the inactive control peptide, Leu-Arg-Gly-Ile-Leu-Ser-NH2. Surprisingly, electrophysiological studies, using whole-cell recording from rat substantia gelatinosa neurons, failed to demonstrate an increase in excitatory transmission or neuronal excitability following treatment with SLIGRL or trypsin. In fact, the actions of trypsin were consistent with a decrease in dorsal horn excitability. SLIGRL and trypsin did, however, depolarize and increase the excitability of large, medium and small primary afferent, dorsal root ganglion neurons. The effects were associated with an increase in conductance at hyperpolarized potentials and a decrease in conductance at depolarized potentials. PAR2-like immunoreactivity was found in DRG but not in spinal dorsal horn. These results suggest that activation of DRG neuron cell bodies may account for the pronociceptive actions of i.t. applied PAR2 agonists. They also imply that pathophysiological release of PAR2-activating proteases in the vicinity of DRG neurons may produce profound effects on nociceptive processing in vivo.
Footnotes
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This work was supported by the Canadian Institutes of Health Research.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.107.129171.
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ABBREVIATIONS: PAR, proteinase-activated receptor; SLIGRL, PAR2 ligand (SLIGRL-NH2; Ser-Leu-Ile-Gly-Arg-Leu-NH2); SP, substance P; LRGILS, reverse sequence (control) PAR2 ligand (LRGILS-NH2; Leu-Arg-Gly-Ile-Leu-Ser-NH2); DRG, dorsal root ganglion; DMEM, Dulbecco's modified Eagle's medium; AP, action potential; EPSC, excitatory postsynaptic current; sEPSC, spontaneous EPSC; PBS, phosphate-buffered saline; TRPV, transient receptor potential vanilloid; I-V, current-voltage.
- Received July 24, 2007.
- Accepted October 3, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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