EGCG Prevents the Loss of Pontine Noradrenergic Neurons Induced by Diabetes: A Role in Diabetic Neuropathic Pain

Abstract

Diabetes is a major health problem with an alarming increasing prevalence, and is the most frequent cause of neuropathy worldwide. Neuropathy affects 50–60% of diabetic patients, being a major life-quality impairment for a quarter of these patients. Diabetic neuropathic pain (DNP) is characterized by spontaneous pain, mechanical hyperalgesia and tactile allodynia and is accompanied by functional and neurochemical changes at the peripheral nerves, spinal cord and supraspinal pain control areas. Regarding the effects of diabetic neuropathy in the central somatossensory system, it was shown that streptozotocin (STZ)-diabetic rats present spontaneous hyperactivity and hyperexcitability of spinal nociceptive neurons, which may be subserving the exacerbated pain responses. The spinal functional changes and pain may be due to increased peripheral input(2), changes in spinal nociceptive modulatory mechanisms and altered supraspinal descending pain modulation. Noradrenergic descending pain modulation seems to be impaired since STZ-diabetic rats present decreased numbers of noradrenergic neurons at the A5 and A7 pontine cell groups, along with lower levels of noradrenaline at the spinal cord and higher behavioral responses to pain. This is consistent with the strong noradrenergic projection from A5 and A7 neurons to the spinal dorsal horn and the modulation of nociceptive transmission by local release of noradrenaline. The mechanisms underlying the decrease in noradrenergic neurons in the brainstem during diabetes remain unclear. Our recent findings that diabetes induces oxidative stress damage in neurons from those areas, lead us to hypothesize that it may contribute to their loss. Thereafter, with the present study we aimed to evaluate the effects of Epigallocathechin Gallate (EGCG), a potent antioxidant present in green tea, on spinal noradrenaline levels, on A5 and A7 noradrenergic neurons and on behavioral pain responses of STZ-diabetic rats.