Diabetes Mellitus (DM) is a chronic systemic disease affecting about 5-6% of the U.S. population between the ages of 25-64. Complications of DM are numerous and affect almost every organ of the body, resulting in diverse pathological conditions including abnormal response to painful stimuli. The role of endogenous opioid peptide, metenkephalin (ME), in the mediation of the analgesic response in various experimental animal models has been documented. Studies from our laboratory have shown that streptozotocin (STZ)-induced diabetes in male rats resulted in a gradual increase in the analgesic threshold, reached a maximum at week 6-8, and remained elevated for up to 14 weeks of diabetes. This analgesic response was associated with an increase in the concentration of free ME in certain brain regions and the spinal cord. Chronic insulin replacement therapy initiated after the development of hypalgesia normalized not only glucose levels, body weight and the analgesic response in diabetic rats, but also the endogenous ME levels in the brain regions studied. The object of the present proposal is to test the hypothesis that the hypalgesia and the elevated levels of ME observed in diabetic rats are resulted from enhanced synthesis of ME coupled with a decrease in its turnover rate in various brain regions, spinal cord and adrenal medulla after the induction of diabetes. In the present investigation, we will determine; 1) the time-course of the changes in ME level by measuring PreproME mRNA, Cryptic and free, ME and its metabolite (Tyr-Gly-Gly); 2) at what time point the initiation of insulin therapy after the induction of diabetes will normalize the changes in ME activity; and 3) whether changes in ME activity results in up or down regulation of the delta-opioid receptors in the same brain regions and tissues. In addition, diabetics, diabetics + insulin and control animals will be tested for the changes in their analgesic threshold every two weeks to correlate this response with changes in the ME levels. The results of these studies should provide more focused information of the effect of diabetes on ME levels, as well as the analgesic response mediated by this opioid peptide in this systemic disease.
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