Cytokines in Neuropathic Pain and Associated Depression

Neuropathic pain occurs as a result of lesion or disease affecting the somatosensory nervous system and is present in a diverse set of peripheral and central pathologies such as nerve trauma, diabetic neuropathy, post-herpetic neuralgia, chemotherapy-induced peripheral neuropathy, spinal cord injury and multiple sclerosis. Debilitating symptoms including allodynia, hyperalgesia and spontaneous pain have a substantial negative impact on patients' quality of life. The currently available therapeutic treatments are generally ineffective and characterised by poor response rates. Accumulating evidence suggests that neuroinflammation and cytokine signalling play a critical role in neuropathic pain. Numerous experimental studies have demonstrated that certain pro-inflammatory cytokines are elevated in neuropathic pain conditions, and administration of these cytokines can elicit pain hypersensitivity in the absence of injury or disease. This phenomenon is also apparent in the 'sickness response', which encompasses a broad inflammatory response to disease and injury and involves a series of physiological and behavioural changes including pain hypersensitivity. Interestingly, the 'sickness response' is also similar in nature to some of the defining characteristics of the depressed state of affective disorder. In this review, we explore links that may relate the co-existence of depression in neuropathic pain patients with the activity of cytokines and discuss the role of several key pro-inflammatory and anti-inflammatory cytokines in neuropathic pain.