Dual effects of Rho-kinase inhibitors on a rat model of inflammatory pain

Rho GTPases in the Physiology and Pathophysiology of Peripheral Sensory Neurons

Numerous experimental studies demonstrate that the Ras homolog family of guanosine triphosphate hydrolases (Rho GTPases) Ras homolog family member A (RhoA), Ras-related C3 botulinum toxin substrate 1 (Rac1) and cell division cycle 42 (Cdc42) are important regulators in somatosensory neurons, where they elicit changes in the cellular cytoskeleton and are involved in diverse biological processes during development, differentiation, survival and regeneration. This review summarizes the status of research regarding the expression and the role of the Rho GTPases in peripheral sensory neurons and how these small proteins are involved in development and outgrowth of sensory neurons, as well as in neuronal regeneration after injury, inflammation and pain perception. In sensory neurons, Rho GTPases are activated by various extracellular signals through membrane receptors and elicit their action through a wide range of downstream effectors, such as Rho-associated protein kinase (ROCK), phosphoinositide 3-kinase (PI3K) or mixed-lineage kinase (MLK). While RhoA is implicated in the assembly of stress fibres and focal adhesions and inhibits neuronal outgrowth through growth cone collapse, Rac1 and Cdc42 promote neuronal development, differentiation and neuroregeneration. The functions of Rho GTPases are critically important in the peripheral somatosensory system; however, their signalling interconnections and partially antagonistic actions are not yet fully understood.

Intra-Articular Blockade of P2X7 Receptor Reduces the Articular Hyperalgesia and Inflammation in the Knee Joint Synovitis Especially in Female Rats

Synovitis is a key factor in joint disease pathophysiology, which affects a greater proportion of women than men. P2X7 receptor activation contributes to arthritis, but whether it plays a role in articular inflammatory pain in a sex-dependent manner is unknown. We investigated whether the P2X7 receptor blockade in the knee joint of male and female rats reduces the articular hyperalgesia and inflammation induced by a carrageenan knee joint synovitis model. Articular hyperalgesia was quantified using the rat knee joint incapacitation test and the knee joint inflammation, characterized by the concentration of cytokines tumor necrosis factor-α, interleukin-1β, interleukin-6, and cytokine-induced neutrophil chemoattractant-1, and by neutrophil migration, was quantified using enzyme-linked immunosorbent assay and by myeloperoxidase enzyme activity measurement, respectively. P2X7 receptor blockade by the articular coadministration of selective P2X7 receptor antagonist A740003 with carrageenan significantly reduced articular hyperalgesia, pro-inflammatory cytokine concentrations, and myeloperoxidase activity induced by carrageenan injection into the knee joint of male and estrus female rats. However, a lower dose of P2X7 receptor antagonist was sufficient to significantly induce the antihyperalgesic and anti-inflammatory effects in estrus female but not in male rats. These results suggest that P2X7 receptor activation by endogenous adenosine 5'-triphosphate is essential to articular hyperalgesia and inflammation development in the knee joint of male and female rats. However, female rats are more responsive than male rats to the antihyperalgesic and anti-inflammatory effects induced by P2X7 receptor blockade.

PERSPECTIVE

P2X7 receptors could be promising therapeutic targets in the treatment of knee joint disease symptoms, especially in women, who are more affected than men by these conditions.

Tissue-selective inflammation in the oral cavity of the rat

In the current study, carrageenan (CG; 100-1000 μg/site) was injected intraorally in the cheeks of Holtzman or Wistar rats to evaluate the consequences of administration of a non-immunogenic stimulus in the orofacial region. Subsequent inflammation was measured as oedema (increased thickness of the cheek wall using digital calipers), relative to the other cheek injected with saline. Oedema formation and tissue collection for histopathological studies were assessed at 0.5, 1, 2, 3, 4, 6, 24, 48, 72, 96, 120 and 144 h after injection. In parallel, other groups of rats were injected with CG in the hind paw, to provide a reference response. The inhibitor of prostaglandin biosynthesis, indomethacin, and antagonists of histamine, serotonin and NK1 receptors were injected s.c., 0.5 h before CG. CG induced a dose-related oedema more rapidly from 0 to 2 h which lasted for at least 72 h, showing a biphasic profile (peak at 2 and 24 h), compared with the monophasic oedema induced in rat paws (maximal duration of 24 h). Histopathological analysis of the CG-injected cheek revealed oedema formation with little leukocyte recruitment at 1-3 h, mast cell degranulation at 6 h, and a mixed polymorphonuclear and mononuclear cell infiltrate by 24 h. Histamine and serotonin antagonists and indomethacin, but not the NK1 antagonist, decreased cheek oedema in the first 4 h following carrageenan. Taken together, our data indicated important differences in the pattern of inflammation between the oral cavity and the paw which will determine the therapeutic approach to the treatment of inflammatory conditions in the oral cavity.

Rho/ROCK acts downstream of lysophosphatidic acid receptor 1 in modulating P2X3 receptor-mediated bone cancer pain in rats

Background

Lysophosphatidic acid receptor 1 and Rho/ROCK signaling is implicated in bone cancer pain development. However, it remains unknown whether the two signaling pathways function together in P2X₃ receptor-mediated bone cancer pain.

Results

In this study, using a rat model of bone cancer, we examined the expression of P2X₃ and lysophosphatidic acid receptor 1 in rat dorsal root ganglion neurons and further dissected whether lysophosphatidic acid receptor 1 and Rho/ROCK-mediated pathways interacted in modulating rat pain behavior. Bone cancer was established by inoculating Walker 256 cells into the left tibia of female Wistar rats. We observed a gradual and yet significant decline in mean paw withdrawal threshold in rats with bone cancer, but not in control rats. Our immunohistochemical staining revealed that the number of P2X₃- and lysophosphatidic acid receptor 1-positive dorsal root ganglion neurons was significantly greater in rats with bone cancer than control rats. Lysophosphatidic acid receptor 1 blockade with VPC32183 significantly attenuated decline in mean paw withdrawal threshold. Flinching behavior test further showed that lysophosphatidic acid receptor 1 inhibition with VPC32183 transiently but significantly attenuated α,β-meATP-induced increase in paw lift time per minute. Rho inhibition by intrathecal BoTXC3 caused a rapid reversal in decline in mean paw withdrawal threshold of rats with bone cancer. Flinching behavior test showed that BoTXC3 transiently and significantly attenuated α,β-meATP-induced increase in paw lift time per minute. Similar findings were observed with ROCK inhibition by intrathecal Y27632. Furthermore, VPC32183 and BoTXC3 effectively aborted the appearance of lysophosphatidic acid-induced calcium influx peak.

Conclusions

Lysophosphatidic acid and its receptor LPAR₁, acting through the Rho-ROCK pathway, regulate P2X₃ receptor in the development of both mechanical and spontaneous pain in bone cancer.