Agents that act by different mechanisms modulate the activity of protein kinase CβII isozyme in the rat spinal cord during peripheral inflammation

Prooxidant-induced c-Src/nuclear factor kappa B-coupled signalling in sensory ganglia mediates cutaneous hyperalgesia

BACKGROUND

Persistent pain resulting from peripheral injury/inflammation is associated with altered sensitivity to cutaneous stimuli, which can manifest as hyperalgesia. The role of oxidant stress in the development, progression and maintenance of hyperalgesia is still not understood. Furthermore, there appears to be a relationship between c-Src kinase in the pain pathway and oxidative stress.

METHODS

We have used a novel prooxidant inflammatory pain model that involves potassium peroxychromate (PPC), a unique prooxidant that produces the same reactants as activated phagocytes. This model was used to investigate the role of oxidant-activated c-Src in mediating hyperalgesia. We compared the effects of PP2 (a Src family kinase inhibitor) and c-Src siRNA on behavioural hyperalgesia with sodium stibogluconate (SSG) (a non-receptor tyrosine phosphatase inhibitor) and AG 1478 (a receptor tyrosine kinase inhibitor).

RESULTS

PP2 and c-Src siRNA attenuated PPC-induced thermal hyperalgesia, while SSG enhanced it. AG 1478 had no effect. PP2 decreased the levels of IL-1β, c-Src/inhibitory kappa B kinase complex formed and prostaglandin E2 produced in the dorsal root ganglia (DRG) ipsilateral to the inflamed paw, while SSG increased the levels of these parameters. c-Src siRNA decreased Src expression and activity in the DRG ipsilateral to the inflamed paw.

CONCLUSIONS

These results confirm that prooxidant-activated c-Src plays a role in initiating and maintaining hyperalgesia by regulating a stimulus-response coupling between the inflamed tissue and the DRG in the pain pathway. Our data also suggest that oxidant-induced dysregulation of c-Src/nuclear factor kappa B coupling may contribute to our understanding of the transition from acute to chronic dysfunctional pain state seen in many human diseases.

Anti-inflammatory and analgesic effects displayed by peptides derived from PKI55 protein, an endogenous protein kinase C inhibitor

We recently characterized the PKI55 protein as an endogenous protein kinase C (PKC) inhibitor and investigated, in vitro, the potential anti-inflammatory actions of its N-terminal peptides 1-16 (peptide 5), 1-8 (peptide 8) and 1-5 (peptide 9). We showed their ability to inhibit chemotaxis in human polymorphonuclear leukocytes activated by the N-formyl tripeptide for-Met-Leu-Phe-OMe. In this work, we evaluated the anti-inflammatory and the analgesic effects of the selected peptides by in vivo experiments carried out in the mouse. The peptides 5, 8 and 9 (0.1 and 10 nmol i.c.v.) were effective in both the parameters chosen to test the anti-inflammatory activity, i.e., the xylene-induced ear edema and the acetic acid-induced infiltration of neutrophils in the peritoneal cavity. In addition, they displayed analgesic effect, evaluated by the acetic acid-induced writhing test. All the peptides' effects were shared by the reference compounds, dexamethasone and indomethacin (10 mg kg(-1) i.p.), but not by the 9-scramble peptide (10 nmol i.c.v.). The peptide 9, which represents the shortest active sequence of the PKI55 protein, was tested in the ear edema model even following intraperitoneal (i.p.) administration and proved to be effective in the range doses 3-30 mg kg(-1). Moreover, an increase in plasma corticosterone levels was detected in mice treated with the peptide 9, but not with the 9-scramble peptide (both at 10 nmol i.c.v.). The anti-inflammatory and analgesic effects of the PKI55-derived synthetic peptides, possibly related both to PKC inhibition and hypothalamic-pituitary-adrenal axis activation, deserve further investigation in view of potential therapeutic exploitation.

Proinflammatory cytokines mediating burn-injury pain

Thermal burns induce pain at the site of injury, mechanical hyperalgesia, associated with a complex time-dependent inflammatory response. To determine the contribution of inflammatory mediators to burn injury-induced mechanical hyperalgesia, we measured dynamic changes in the levels of three potent hyperalgesic cytokines, interleukin IL-1 beta, IL-6, and tumor necrosis factor-alpha (TNFalpha), in skin of the rat, following a partial-thickness burn injury. Only IL-6 demonstrated a sustained increase ipsilateral but not contralateral to the burn, correlating with the prolonged ipsilateral mechanical hyperalgesia. Spinal intrathecal injection of oligodeoxynucleotides antisense for gp130, a receptor subunit shared by members of the IL-6 family of cytokines, attenuated both burn- and intradermal IL-6-induced hyperalgesia, as did intradermal injection of anti-IL-6 function blocking antibodies. These studies suggest that IL-6 is an important mediator of burn-injury pain.

PKC activation sets an upper limit to the functional plasticity of GABAergic transmission induced by endogenous neurosteroids

The activity of GABAergic inhibitory interneurones located in lamina II of the spinal cord is of fundamental importance for the processing of peripheral nociceptive messages. We have recently shown that 3alpha-hydroxy ring A-reduced pregnane neurosteroids [3alpha5alpha-neurosteroids (3alpha5alphaNS)], potent allosteric modulators of GABA(A) receptors (GABA(A)Rs), are synthesized in the spinal cord and limit thermal hyperalgesia during inflammatory pain. Because changes in the expression of calcium-dependent protein kinases [protein kinase C (PKC)] are observed during pathological pain in the spinal cord, we examined the possible interactions between PKC and 3alpha5alphaNS at synaptic GABA(A)Rs. Using patch-clamp recordings of lamina II interneurones in the spinal cord of 15-20-day-old rats, we showed that synaptic inhibition mediated by GABA(A)Rs and its modulation by 3alpha5alphaNS in lamina II of the spinal cord largely depend on activation of PKC. Our experimental results suggested that activation of PKC locks synaptic GABA(A)Rs in a functional state precluding further positive allosteric modulation by endogenous and exogenous 3alpha5alphaNS. This effect was fully prevented by coadministration of chelerythrin, an inhibitor of PKC. Furthermore, application of chelerythrin alone rendered synaptic GABA(A)Rs hypersensitive to endogenously produced or exogenously applied 3alpha5alphaNS. These findings confirmed that there was a significant production of endogenous 3alpha5alphaNS in lamina II of the spinal cord but also indicated that PKC-dependent phosphorylation processes were tonically activated to control GABA(A)R-mediated inhibition under resting conditions. We therefore can conclude that PKC activation sets an upper limit to the functional plasticity of GABAergic transmission induced by endogenous neurosteroids.