Spinal Serotonin and 5HT6 Receptor Levels during Development of Neuropathy and Influence of Blockade of these Receptors on Thermal Hyperalgesia in Diabetic Mice

Abstract

Little is known about the role of 5-HT 6 receptors in the pathophysiology of neuropathic pain. The aim of this study is firstly, to investigate the influence of spinal and systemic 5-HT 6 receptors on thermal hyperalgesia, one of the most significant symptoms of neuropathy occurring in diabetes; and secondly to determine spinal lumbar serotonin and 5-HT 6 receptor levels during development of diabetic neuropathy in mice. Diabetes was produced in Balb/c mice with a single injection of streptozocin (150 mg/kg, i.p.). Using the hot plate test, the 5-HT 6 antagonist SB-258585 was given systemically (3, 10, 30 mg/kg) and intrathecally (0.01, 0.1, 1 nmol/mouse) to determine its effect on thermal hyperalgesia. Furthermore, on days 7 and 15 of diabetes, development of thermal hyperalgesia was evaluated in relation to changes in spinal serotonin and 5-HT 6 receptor levels by using LC/MS/MS and Western blot analyses, respectively. Two-way analysis of variance and unpaired t-tests were used to evaluate data from hot-plate tests and 5-HT levels/ 5-HT 6 receptor expression, respectively. Thermal hyperalgesia was observed in neuropathic mice, starting from day 5 after streptozocin administration. On day 15, systemic, but not intrathecal, SB-258585 attenuated thermal hyperalgesia in neuropathic mice. Spinal serotonin levels did not change during development of hyperalgesia after induction of diabetes, whereas spinal 5-HT 6 receptor levels were significantly reduced on days 7 and 15. Our findings show that systemic, but not spinal, blockade of 5-HT 6 receptors may exert antihyperalgesic effects in neuropathic mice and suggest that systemic 5-HT 6 receptors contribute to the pathophysiology of diabetic neuropathy.