PAR2 activation in the dura causes acute behavioral responses and priming to glyceryl trinitrate in a mouse migraine model.

BACKGROUND: Migraine is a severely debilitating disorder that affects millions of people worldwide. Studies have indicated that activation of protease-activated receptor-2 (PAR2) in the dura mater causes headache responses in preclinical models. It is also well known that vasodilators such as nitric oxide (NO) donors can trigger migraine attacks in migraine patients but not controls. In the current study we examined whether activation of PAR2 in the dura causes priming to the NO donor glyceryl trinitrate (GTN). METHODS: A preclinical behavioral model of migraine was used where stimuli (PAR2 agonists: 2at-LIGRL-NH2 (2AT) or neutrophil elastase (NE); and IL-6) were applied to the mouse dura through an injection made at the intersection of the lamdoidal and sagittal sutures on the skull. Following dural injection, periorbital von Frey thresholds and facial grimace responses were measured until their return to baseline. GTN was then given by intraperitoneal injection and periorbital hypersensitivity and facial grimace responses observed until they returned to baseline. RESULTS: We found that application of the selective PAR2 agonist 2at-LIGRL-NH2 (2AT) onto the dura causes headache-related behavioral responses in WT but not PAR2-/- mice with no differences between sexes. Additionally, dural PAR2 activation with 2AT caused priming to GTN (1 mg/kg) at 14 days after primary dural stimulation. PAR2-/- mice showed no priming to GTN. We also tested behavioral responses to the endogenous protease neutrophil elastase, which can cleave and activate PAR2. Dural neutrophil elastase caused both acute responses and priming to GTN in WT but not PAR2-/- mice. Finally, we show that dural IL-6 causes acute responses and priming to GTN that is identical in WT and PAR2-/- mice, indicating that IL-6 does not act through PAR2 in this model. CONCLUSIONS: These results indicate that PAR2 activation in the meninges can cause acute headache behavioral responses and priming to an NO donor, and support further exploration of PAR2 as a novel therapeutic target for migraine.

A Role for Protease Activated Receptor Type 3 (PAR3) in Nociception Demonstrated Through Development of a Novel Peptide Agonist.

The protease activated receptor (PAR) family is a group of G-protein coupled receptors (GPCRs) activated by proteolytic cleavage of the extracellular domain. PARs are expressed in a variety of cell types with crucial roles in homeostasis, immune responses, inflammation, and pain. PAR3 is the least researched of the four PARs, with little known about its expression and function. We sought to better understand its potential function in the peripheral sensory nervous system. Mouse single-cell RNA sequencing data demonstrates that PAR3 is widely expressed in dorsal root ganglion (DRG) neurons. Co-expression of PAR3 mRNA with other PARs was identified in various DRG neuron subpopulations, consistent with its proposed role as a coreceptor of other PARs. We developed a lipid tethered PAR3 agonist, C660, that selectively activates PAR3 by eliciting a Ca2+ response in DRG and trigeminal neurons. In vivo, C660 induces mechanical hypersensitivity and facial grimacing in WT but not PAR3-/- mice. We characterized other nociceptive phenotypes in PAR3-/- mice and found a loss of hyperalgesic priming in response to IL-6, carrageenan, and a PAR2 agonist, suggesting that PAR3 contributes to long-lasting nociceptor plasticity in some contexts. To examine the potential role of PAR3 in regulating the activity of other PARs in sensory neurons, we administered PAR1, PAR2, and PAR4 agonists and assessed mechanical and affective pain behaviors in WT and PAR3-/- mice. We observed that the nociceptive effects of PAR1 agonists were potentiated in the absence of PAR3. Our findings suggest a complex role of PAR3 in the physiology and plasticity of nociceptors. PERSPECTIVE: We evaluated the role of PAR3, a G-protein coupled receptor, in nociception by developing a selective peptide agonist. Our findings suggest that PAR3 contributes to nociception in various contexts and plays a role in modulating the activity of other PARs.

The cellular basis of protease-activated receptor 2-evoked mechanical and affective pain.

Protease-activated receptor 2 (PAR2) has long been implicated in inflammatory and visceral pain, but the cellular basis of PAR2-evoked pain has not been delineated. Although PAR2-evoked pain has been attributed to sensory neuron expression, RNA-sequencing experiments show ambiguous F2rl1 mRNA detection. Moreover, many pharmacological tools for PAR2 are nonspecific, acting also on the Mas-related GPCR family (Mrg) that are highly enriched in sensory neurons. We sought to clarify the cellular basis of PAR2-evoked pain. We developed a PAR2-conditional knockout mouse and specifically deleted PAR2 in all sensory neurons using the PirtCre mouse line. Our behavioral findings show that PAR2 agonist-evoked mechanical hyperalgesia and facial grimacing, but not thermal hyperalgesia, are dependent on PAR2 expression in sensory neurons that project to the hind paw in male and female mice. F2rl1 mRNA is expressed in a discrete population (~4%) of mostly small-diameter sensory neurons that coexpress the Nppb and IL31ra genes. This cell population has been implicated in itch, but our work shows that PAR2 activation in these cells causes clear pain-related behaviors from the skin. Our findings show that a discrete population of DRG sensory neurons mediate PAR2-evoked pain.

Protease activated receptor 2 (PAR2) activation causes migraine-like pain behaviors in mice.

BACKGROUND: Pain is the most debilitating symptom of migraine. The cause of migraine pain likely requires activation of meningeal nociceptors. Mast cell degranulation, with subsequent meningeal nociceptor activation, has been implicated in migraine pathophysiology. Degranulating mast cells release serine proteases that can cleave and activate protease activated receptors. The purpose of these studies was to investigate whether protease activated receptor 2 is a potential generator of nociceptive input from the meninges by using selective pharmacological agents and knockout mice. METHODS: Ratiometric Ca++ imaging was performed on primary trigeminal and dural cell cultures after application of 2at-LIGRL-NH2, a specific protease activated receptor 2 agonist. Cutaneous hypersensitivity and facial grimace was measured in wild-type and protease activated receptor 2-/- mice after dural application of 2at-LIGRL-NH2 or compound 48-80, a mast cell degranulator. Behavioral experiments were also conducted in mice after dural application of 2at-LIGRL-NH2 (2AT) in the presence of either C391, a selective protease activated receptor 2 antagonist, or sumatriptan. RESULTS: 2at-LIGRL-NH2 evoked Ca2+ signaling in mouse trigeminal neurons, dural fibroblasts and in meningeal afferents. Dural application of 2at-LIGRL-NH2 or 48-80 caused dose-dependent grimace behavior and mechanical allodynia that were attenuated by either local or systemic application of C391 as well as in protease activated receptor 2-/- mice. Nociceptive behavior after dural injection of 2at-LIGRL-NH2 was also attenuated by sumatriptan. CONCLUSIONS: Functional protease activated receptor 2 receptors are expressed on both dural afferents and fibroblasts and activation of dural protease activated receptor 2 produces migraine-like behavioral responses. Protease activated receptor 2 may link resident immune cells to meningeal nociceptor activation, driving migraine-like pain and implicating protease activated receptor 2 as a therapeutic target for migraine in humans.

Protease-activated receptor 2 activation is sufficient to induce the transition to a chronic pain state.

Protease-activated receptor type 2 (PAR2) is known to play an important role in inflammatory, visceral, and cancer-evoked pain based on studies using PAR2 knockout (PAR2(-/-)) mice. We have tested the hypothesis that specific activation of PAR2 is sufficient to induce a chronic pain state through extracellular signal-regulated kinase (ERK) signaling to protein synthesis machinery. We have further tested whether the maintenance of this chronic pain state involves a brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (trkB)/atypical protein kinase C (aPKC) signaling axis. We observed that intraplantar injection of the novel highly specific PAR2 agonist, 2-aminothiazol-4-yl-LIGRL-NH2 (2-at), evokes a long-lasting acute mechanical hypersensitivity (median effective dose ∼12 pmoles), facial grimacing, and causes robust hyperalgesic priming as revealed by a subsequent mechanical hypersensitivity and facial grimacing to prostaglandin E2 (PGE2) injection. The promechanical hypersensitivity effect of 2-at is completely absent in PAR2(-/-) mice as is hyperalgesic priming. Intraplantar injection of the upstream ERK inhibitor, U0126, and the eukaryotic initiation factor (eIF) 4F complex inhibitor, 4EGI-1, prevented the development of acute mechanical hypersensitivity and hyperalgesic priming after 2-at injection. Systemic injection of the trkB antagonist ANA-12 similarly inhibited PAR2-mediated mechanical hypersensitivity, grimacing, and hyperalgesic priming. Inhibition of aPKC (intrathecal delivery of ZIP) or trkB (systemic administration of ANA-12) after the resolution of 2-at-induced mechanical hypersensitivity reversed the maintenance of hyperalgesic priming. Hence, PAR2 activation is sufficient to induce neuronal plasticity leading to a chronic pain state, the maintenance of which is dependent on a BDNF/trkB/aPKC signaling axis.

Reactive oxygen species and lipid peroxidation inhibitors reduce mechanical sensitivity in a chronic neuropathic pain model of spinal cord injury in rats.

Chronic neuropathic pain is a common consequence of spinal cord injury (SCI), develops over time and negatively impacts quality of life, often leading to substance abuse and suicide. Recent evidence has demonstrated that reactive oxygen species (ROS) play a role in contributing to neuropathic pain in SCI animal models. This investigation examines four compounds that reduce ROS and the downstream lipid peroxidation products, apocynin, 4-oxo-tempo, U-83836E, and tirilazad, and tests if these compounds can reduce nocioceptive behaviors in chronic SCI animals. Apocynin and 4-oxo-tempo significantly reduced abnormal mechanical hypersensitivity measured in forelimbs and hindlimbs in a model of chronic SCI-induced neuropathic pain. Thus, compounds that inhibit ROS or lipid peroxidation products can be used to ameliorate chronic neuropathic pain. We propose that the application of compounds that inhibit reactive oxygen species (ROS) and related downstream molecules will also reduce the behavioral measures of chronic neuropathic pain. Injury or trauma to nervous tissue leads to increased concentrations of ROS in the surviving tissue. Further damage from ROS molecules to dorsal lamina neurons leads to membrane excitability, the physiological correlate of chronic pain. Chronic pain is difficult to treat with current analgesics and this research will provide a novel therapy for this disease.