Chronic pain has long been attributed to an alteration in the anatomy and physiology of neuronal pathways.
Specifically, the subpopulation of primary afferents responsible for pain transmission (nociceptors) which express binding sites for the plant lectin Bandeiraea simplicifolia I isolectin B4 (IB4) has been shown to play a direct role in the development of neuropathic pain. Nerve injury results in a decrease in IB4 binding in both the dorsal root ganglion and the superficial dorsal horn of the spinal cord suggesting either death of the IB4 binding nociceptors, a downregulation of their expression of the IB4 binding site or perhaps structural alteration in the IB4 binding site itself.
Recent anatomical studies in our laboratory have provided evidence that IB4 binding nociceptors do actually survive partial nerve injury (although they do show signs of damage) but have a reduced ability to bind IB4, thereby explaining the decrease in IB4 binding previously reported. It is possible that these surviving, yet injured, primary sensory neurones contribute to the hyper excitation believed to underlie neuropathic pain sensory symptoms.
Previous studies in fields such as cancer research have used the technique of targeted cell death to selectively kill particular cell types in order to understand their contribution to disease states. This technique involves linking a cell toxin with a vehicle molecule which selectively binds to unique receptors present only in the membrane of a particular type of cell. Once the linked toxin-vehicle conjugate binds to the appropriate receptor in the membrane of the targeted cell type, the cell takes both the vehicle and the toxin inside and the toxin causes the targeted cell to die, thereby causing targeted cell death.
Targeted cell death offers a novel way to study the contribution of a particular cell type to a disease process by selectively removing its contribution to the disease development. In some cases targeted cell death may even hold the key to disease relief, as separate reports of targeted nociceptor death and targeted microglial suppression have shown symptom reduction after nervous system damage.
Saporin is a toxin that causes cell deterioration and death by interfering with protein synthesis. It is unable to enter cells without a vehicle, but conjugating it to an appropriate vehicle can make it an excellent candidate for targeted cell death. Thus a conjugate of saporin and IB4 would conceptually act to selectively ablate the nociceptors that express the IB4 binding site. Studies have shown that a single injection of IB4-SAP into the sciatic nerve results in a decrease in baseline pain sensitivity as well as a delay in the onset of neuropathic pain symptoms.
However, whilst it has been shown that the nociceptors ablated by IB4-SAP do not recover, it has been reported that even in the face of IB4 binding nociceptor absence, neuropathic pain symptoms do eventually appear up to 4 weeks post- nerve injury. These findings give leverage to the current belief that the traditional idea of neuropathic pain being a mere result of altered neuronal processing is too simplistic and also explains why pain therapies targeting neurons alone have been less than adequate to date. It is now widely accepted that, along with neurons, glial cells, including microglia and astrocytes, play a significant role in the induction and maintenance of chronic pain symptoms.
Microglial infiltration specifically is reported in the superficial dorsal horn of the spinal cord after a variety of nerve injury methods. Moreover, just like alterations in neuronal processing, alterations in microglial infiltration has been associated with changes in neuropathic pain symptoms suggesting chronic pain conditions may rely on a neuron-glial link for development and maintenance.
In support of this theory we have very recently demonstrated that targeted ablation of the subpopulation of IB4 binding primary sensory neurons prior to nerve injury reduces the microglial infiltration in the superficial dorsal horn of the spinal cord that results after injury. These results not only support the concept that IB4 binding nociceptors are key players in neuropathic pain development but also provides evidence for the concept that a nociceptor-glial link is pivotal to the perpetuation of neuropathic pain resulting from nerve injury.
The aim of current research in this laboratory is to further elucidate this neuropathic pain perpetuating neuron-glial link by examining peripheral and central markers of glial activation as well as glial infiltration in both the presence and absence of IB4 binding nociceptors in various models of neuropathic pain using neural tracing and neural ablation strategies fused with lectin- and immune-histochemical identification techniques.