Neuropathic pain is one of the most common and troublesome complications afflicting diabetic patients. Since diabetic neuropathic pain often is not adequately relieved by classical analgesics, it represents an important unmet medical need. The etiology of painful diabetic neuropathy remains poorly understood and likely involves both peripheral and central mechanisms. Generation of ectopic discharges from C-fiber afferent nerves and sensitization of spinal dorsal horn neurons may play a critical role in the development of neuropathic pain in diabetes. This exploratory (R21) proposal is designed as a first step to investigate mechanisms of neuropathic pain in a rodent model of diabetes. The major objective of this proposal is to study the mechanisms of increased spinal excitatory tone relevant to neuropathic pain in diabetes. Increased spinal glutamate synthesis and decreased inhibition of spinal glutamate release by the descending noradrenergic system may represent a functional basis for the development of diabetic neuropathic pain and diminished analgesic effect of opioids and alpha2 adrenergic receptor agonists. Two hypotheses will be tested in this proposal: 1) Depletion of capsaicin-sensitive afferent nerves eliminates afferent ectopic discharges and attenuates increased spinal glutaminase activity, hyperexcitability of spinal dorsal horn neurons, and neuropathic pain in diabetes; and 2) The inhibitory action of morphine or alpha2 adrenergic receptor agonists on spinal glutamate release is diminished in diabetic neuropathy. Electrophysiological records of single-unit activity of afferents and spinal neurons, quantitative measurements of neurotransmitters and proteins, and behavioral assessment of pain will be used to test these novel hypotheses. The proposed studies will improve our understanding of the mechanisms of neuroplasticity associated with diabetic neuropathic pain. This new information also could provide a rationale for development of new therapies for patients with diabetic neuropathic pain.
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