One of the most significant health problems in our country is the inadequate treatment of pain, especially the chronic abnormal pains often associated with nerve injury (neuropathic pain). Injuries to nerves in experimental models of neuropathic pain elicit peripheral, spinal and supraspinal neural plasticity characterized in part by increased expression of spinal dynorphin. Progress made in our previous funding period suggests that enhanced expression of dynorphin resulting from experimental nerve injury acts in a non-opioid fashion to promote pain. Some of the relevant observations which support a pronociceptive role of dynorphin in the post-nerve injury state include: (a) spatial and temporal correlation of increased spinal dynorphin expression across multiple, anatomically relevant spinal segments following nerve injury, (b) inhibition of nerve-injury induced pain by antiserum to dynorphin, but not control serum, and (c) demonstration of sustained nerve-injury induced pain in wild-type (WT), but not in prodynorphin """"""""knock-out"""""""" (KO) mice. Our studies show that dynorphin (2-17) (which does not bind to opioid receptors) promotes calcium accumulation in DRG cells in culture and activates protein kinase C (PKC) in spinal cord. Moreover, des-Tyr fragments of dynorphin enhance capsaicin-stimulated CGRP release in DRG cells and in spinal cord tissue preparations. The overall hypothesis of our work is that spinal dynorphin is pronociceptive and it a critical mediator which serves to maintain pathological post-nerve injury pain. Specifically we propose a mechanism by which dynorphin maintains post-nerve injury pain by increasing spinal excitability, in part, by enhancing the release of excitatory transmitters.
Our aims are designed to systematically test this hypothesized mechanism.
Aim 1 will determine whether nerve-injury induced changes in dynorphin expression are the result of modulatory influences from supraspinal sites.
Aim 2 will focus on measurement of nerve-injury induced release of dynorphin at the spinal level.
Aim 3 will use cultured DRG cells to test if dynorphin enhances evoked CGRP release through activation of PKC.
Aim 4 will use spinal cord tissue preparations taken from sham- or nerve-injured rats, as well as from sham- or nerve-injured WT and prodynorphin KO mice to determine the role of endogenous, pathological levels of dynorphin on basal and evoked CGRP release.
Aim 5 will focus on the role of dynorphin in the release of excitatory amino acids in sham-operated and nerve-injured rats. The experiments in Aim 5 will be carried out in the laboratory of Dr. Tony Yaksh through a Consortium Arrangement. All of these studies focus on the central hypothesis of pronociceptive actions of dynorphin in the post nerve-injury state and test a proposed mechanism which addresses how dynorphin may act under conditions of nerve-injury to maintain pain. As most clinical conditions of neuropathic pain are treated in the post-injury (i.e., maintenance) state, such information may allow approaches to limit or reverse the pathological actions of dynorphin in maintaining neuropathic pain.
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