Modulation of sensory function by high glucose-induced oxidative stress: Role of TRPV1 receptors

Abstract

Diabetes can have a devastating effect on sensory nerves often leading to clinical symptoms of pain abnormalities termed diabetic sensory neuropathy (DSN). Converging evidence demonstrate that oxidative stress plays a pivotal role in DSN as a result of elevated reactive oxygen species (ROS) induced by hyperglycemia and lead to sensory neuron injury. However, the cellular and molecular mechanisms initiating neuronal dysfunction in DSN are still poorly understood. The present thesis investigated the effects of high glucose-induced oxidative stress on sensory function mediated by the activation of purinergic (P2X) and vanilloid (TRPV1) receptors expressed in nociceptive neurons from neonatal mouse dorsal root ganglion (DRG). Primary cultures of lumbar DRG neurons were prepared to characterize sensory function during control (5 mM glucose) and high glucose (25 mM glucose) conditions using electrophysiological, live imaging and biochemical techniques. Interesting, while characterizing TRPV1 channels following repetitive applications of capsaicin, I noticed two distinct responses in TRPV1 activity: a ‘monophasic’ response, commonly observed as a run-down of peak currents; and a ‘biphasic’ response characterized as a gradual potentiation followed by a run-down of peak currents. The latter response is a novel behaviour that had not been characterized under control condition, and this prompted further investigations. I revealed that the gradual potentiation was independent of calcium entry, and was mediated by PKC. Under the high glucose condition, the two responses remained unaffected; however, I observed an enhancement of TRPV1-mediated currents, while P2X-mediated currents seemed to be unaffected. Furthermore, I demonstrated that the enhancement of current can be prevented with the use of antioxidants. This suggests that the enhancement of TRPV1 activity by high glucose- induced oxidative stress may play an essential role in peripheral sensitization in diabetes.