Proinflammatory cytokines contribute to the development of inflammatory and neuropathic pain and hyperalgesia in manyin vivo models. The rat skin model was used to investigate the effects of proinflammatory cytokines on the basal and heat-evoked release of calcitonin gene-related peptide from nociceptorsin vitro. In contrast to the excitatory effects of cytokines observed in vivo, none of the cytokines tested evoked any calcitonin gene-related peptide (CGRP) release at normal skin temperature of 32°C. However, the cytokines IL-1β, tumor necrosis factor (TNF)-α, and IL-6 but not IL-8 induced a pronounced and transient sensitization of the heat-evoked CGRP release from nociceptors in vitro. This heat sensitization was dose dependent, with EC₅₀ for IL-1β of 2.7 ng/ml and for TNF-α of 3.1 ng/ml. The maximum IL-1β effect reached almost 600% of the heat-evoked release, and the maximum TNF-α effect induced a rise in CGRP release of 350%. In contrast to IL-1β and TNF-α, IL-6 did not induce heat sensitization when applied alone but was only effective in the presence of soluble IL-6 receptor. This suggests a constitutive expression of signaling receptors for TNF and IL-1β and the signal transduction molecule gp130 but not IL-6 receptor or IL-8 receptor. Furthermore, the acute cytokine signaling observed in the present study was independent of transcriptional pathways because sensitization occurred on short latency in vitroand under conditions that excluded chemotactic accumulation of immune cells from blood vessels. Our results demonstrate that interleukins may play an important role in the initiation of heat hyperalgesia in inflammation and neuropathy.

The pleiotropic cytokine interleukin-6 (IL-6) is emerging as a molecule with both beneficial and destructive potentials. It can exert opposing actions triggering either neuron survival after injury or causing neurodegeneration and cell death in neurodegenerative or neuropathic disorders. Importantly, neurons respond differently to IL-6 and this critically depends on their environment and whether they are located in the peripheral or the central nervous system. In addition to its hub regulator role in inflammation, IL-6 is recently emerging as an important regulator of neuron function in health and disease, offering exciting possibilities for more mechanistic insight into the pathogenesis of mental, neurodegenerative and pain disorders and for developing novel therapies for diseases with neuroimmune and neurogenic pathogenic components.

Interleukin 1β (IL‐1β) is a proinflammatory cytokine that maintains thermal hyperalgesia and facilitates the release of calcitonin gene‐related peptide from rat cutaneous nociceptors in vivo and in vitro. Brief applications of IL‐1β to nociceptive neurons yielded a potentiation of heat‐activated inward currents (I _heat ) and a shift of activation threshold toward lower temperature without altering intracellular calcium levels. The IL‐1β‐induced heat sensitization was not dependent on G‐protein‐coupled receptors but was mediated by activation of protein kinases. The nonspecific protein kinase inhibitor staurosporine, the specific protein kinase C inhibitor bisindolylmaleimide BIM ₁ , and the protein tyrosine kinase inhibitor genistein reduced the sensitizing effect of IL‐1β whereas negative controls were ineffective. RT‐PCR and in situ hybridization revealed IL‐1RI but not RII expression in neurons rather than surrounding satellite cells in rat dorsal root ganglia. IL‐1β acts on sensory neurons to increase their susceptibility for noxious heat via an IL‐1RI/PTK/ PKC‐dependent mechanism.—Obreja, O., Rathee, P. K., Lips, K. S., Distler, C., Kress, M. IL‐1J potentiates heat‐activated currents in rat sensory neurons: involvement of IL‐1RI, tyrosine kinase, and protein kinase C. FASEB J . 16, 1497–1503 (2002)

Inflammatory mediators not only activate “pain-”sensing neurons, the nociceptors, to trigger acute pain sensations, more important, they increase nociceptor responsiveness to produce inflammatory hyperalgesia. For example, prostaglandins activate G_s-protein-coupled receptors and initiate cAMP- and protein kinase A (PKA)-mediated processes. We demonstrate for the first time at the cellular level that heat-activated ionic currents were potentiated after exposure to the cAMP activator forskolin in rat nociceptive neurons. The potentiation was prevented in the presence of the selective PKA inhibitor PKI_14–22, suggesting PKA-mediated phosphorylation of the heat transducer protein. PKA regulatory subunits were found in close vicinity to the plasma membrane in these neurons, and PKA catalytic subunits only translocated to the cell periphery when activated. The translocation and the current potentiation were abolished in the presence of an A-kinase anchoring protein (AKAP) inhibitor. Similar current changes after PKA activation were obtained from human embryonic kidney 293t cells transfected with the wild-type heat transducer protein vanilloid receptor 1 (VR-1). The forskolin-induced current potentiation was greatly reduced in cells transfected with VR-1 mutants carrying point mutations at the predicted PKA phosphorylation sites. The heat transducer VR-1 is therefore suggested as the molecular target of PKA phosphorylation, and potentiation of current responses to heat depends on phosphorylation at predicted PKA consensus sites. Thus, the PKA/AKAP/VR-1 module presents as the molecular correlate of G_s-mediated inflammatory hyperalgesia.