Interleukin-1alpha regulates substance P expression and release in adult sensory neurons.

Nerve injury frequently results in development of chronic, dysesthetic pain and allodynia (painful sensation in response to benign stimulation). Following nerve injury, spinal cord glia become activated and secrete a number of inflammatory cytokines, including interleukin-1 (IL-1), which exists as two genetically distinct proteins, IL-1alpha and IL-1beta. To investigate whether neuropeptide expression could be altered by exposure to these cytokines, dorsal root neurons from mature rats were grown in culture and substance P (SP) expression was analyzed. IL-1alpha and IL-1beta both increased neuronal content of SP. Interestingly, IL-1alpha was significantly more efficient than IL-1beta in inducing SP expression. Cultured neurons exposed to either cytokine secreted substantially more SP with capsaicin stimulation than did control cultures, supporting a physiologic role for these inflammatory cytokines after nerve injury. However, when IL-1beta was added in combination with IL-1alpha to cultured neurons, the amount of SP expressed was significantly lower than that induced by IL-1alpha alone. Evidence is presented that both cytokines alter SP expression via the IL-1 receptor, and that the signaling pathway involves nerve growth factor (NGF) expression and transcription. In summary, IL-1alpha was significantly more efficient than IL-1beta at up-regulating SP expression than IL-1beta. Taken together, these observations suggest an important role for IL-1alpha in the events following nerve injury.