Received for publication July 25, 2007.
Revised October 2, 2007.
Accepted for publication October 3, 2007.

Intrathecal Administration of Proteinase-Activated Receptor-2 Agonists Produces Hyperalgesia by Exciting the Cell Bodies of Primary Sensory Neurons

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 Proteinase Activated Receptor-2 (PAR₂) act on peripheral nerves to augment the transfer of nociceptive information. We tested whether PAR₂ agonists also exert a spinal pro-nociceptive effect by intrathecally administering the selective ligand, SLIGRL-NH₂ (SLIGRL). 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, LRGILS-NH₂ (LRGILS). 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 pro-nociceptive actions of intrathecally-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.

Key words: Dorsal Horn, Dorsal Root Ganglion, PAR2, Primary Afferent, Substantia Gelatinosa, Trypsin