Involvement of the Proinflammatory Cytokines Tumor Necrosis Factor-a, IL-1b, and IL-6 But Not IL-8 in the Development of Heat Hyperalgesia: Effects on Heat-Evoked Calcitonin Gene-Related Peptide Release from Rat Skin

Proinflammatory cytokines contribute to the development of inflammatory and neuropathic pain and hyperalgesia in many in vivo models. The rat skin model was used to investigate the effects of proinflammatory cytokines on the basal and heatevoked release of calcitonin gene-related peptide from nociceptors in 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-1b, tumor necrosis factor (TNF)-a, 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-1b of 2.7 ng/ml and for TNF-a of 3.1 ng/ml. The maximum IL-1b effect reached almost 600% of the heat-evoked release, and the maximum TNF-a effect induced a rise in CGRP release of 350%. In contrast to IL-1b and TNF-a, 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-1b 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 vitro and 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.