Whereas small-fibre sensory neuropathies might ultimately lead to cell death and loss of sensation, they first progress through a phase, which might last for years, characterized by the presence of
analgesia-resistant neuropathic
dysesthesias and
pain. Much previous research has addressed these two phases as separate phenomena mediated by presumably discrete biochemical mechanisms. We hypothesized that activity in signalling pathways that ultimately lead to apoptosis plays a critical role in the generation of
neuropathic pain, before death of sensory neurons becomes apparent. We have tested the hypothesis that activator and
effector caspases, defining components of programmed cell death (apoptosis) signalling pathways, also contribute to
pain-related behaviour in animals with small-fibre
peripheral neuropathies and that the
death receptor ligand, tumour
necrosis factor-alpha, and its downstream second messenger,
ceramide, also produce
pain-related behaviour via this mechanism. In two models of painful
peripheral neuropathy, HIV/
AIDS therapy (induced by the
nucleoside reverse transcriptase inhibitor,
dideoxycytidine), and
cancer chemotherapy (induced by
vincristine)
peripheral neuropathy, and for
pain-related behaviour induced by tumour
necrosis factor-alpha and its second messenger,
ceramide, inhibition of both activator (1, 2, 8 and 9) and effector (3)
caspases attenuates
neuropathic pain-related behaviour, although has no effect in
streptozotocin-
diabetic neuropathy and control rats. We conclude that during a latent phase, before apoptotic cell death is manifest, the
caspase signalling pathway can contribute to
pain in small-fibre
peripheral neuropathies, and that inflammatory/immune mediators also activate these pathways. This suggests that these pathways are potential targets for novel pharmacological agents for the treatment of inflammatory as well as
neuropathic pain.