Intrathecal magnesium sulfate attenuates algogenic behavior and spinal amino acids release after kainic acid receptor activation in rats

Magnesium attenuates chronic hypersensitivity and spinal cord NMDA receptor phosphorylation in a rat model of diabetic neuropathic pain

Neuropathic pain is a common diabetic complication affecting 8-16% of diabetic patients. It is characterized by aberrant symptoms of spontaneous and stimulus-evoked pain including hyperalgesia and allodynia. Magnesium (Mg) deficiency has been proposed as a factor in the pathogenesis of diabetes-related complications, including neuropathy. In the central nervous system, Mg is also a voltage-dependent blocker of the N-methyl-d-aspartate receptor channels involved in abnormal processing of sensory information. We hypothesized that Mg deficiency might contribute to the development of neuropathic pain and the worsening of clinical and biological signs of diabetes and consequently, that Mg administration could prevent or improve its complications. We examined the effects of oral Mg supplementation (296 mg l(-1) in drinking water for 3 weeks) on the development of neuropathic pain and on biological and clinical parameters of diabetes in streptozocin (STZ)-induced diabetic rats. STZ administration induced typical symptoms of type 1 diabetes. The diabetic rats also displayed mechanical hypersensitivity and tactile and thermal allodynia. The level of phosphorylated NMDA receptor NR1 subunit (pNR1) was higher in the spinal dorsal horn of diabetic hyperalgesic/allodynic rats. Magnesium supplementation failed to reduce hyperglycaemia, polyphagia and hypermagnesiuria, or to restore intracellular Mg levels and body growth, but increased insulinaemia and reduced polydipsia. Moreover, it abolished thermal and tactile allodynia, delayed the development of mechanical hypersensitivity, and prevented the increase in spinal cord dorsal horn pNR1. Thus, neuropathic pain symptoms can be attenuated by targeting the Mg-mediated blockade of NMDA receptors, offering new therapeutic opportunities for the management of chronic neuropathic pain.

Modulation of P2X3 receptors by Mg2+ on rat DRG neurons in culture

On nociceptive neurons the commonest response to ATP is a rapidly desensitizing current mediated by P2X(3) receptors and believed to be involved in certain forms of pain. P2X(3) receptor recovery from desensitization is a slow process. We studied whether Mg(2+) might modulate such ATP-evoked currents on rat cultured DRG neurons, and thus account for its analgesic action in vivo. Transient increases in extracellular Mg(2+) strongly and reversibly depressed ATP currents which had not recovered from desensitization. Ca(2+)-free solution had the same action as Mg(2+). High Mg(2+) or Ca(2+)-free modulation depended on exposure length to modified divalent cation solutions, whereas it was independent from membrane potential or intracellular Ca(2+) buffering. Paired-pulse protocols showed that high Mg(2+) or Ca(2+)-free medium delayed ATP receptor recovery from desensitization, while leaving desensitization onset apparently unchanged. Tests with various concentrations of Ca(2+) and Mg(2+) showed that the depressant action by Mg(2+) was primarily due to functional antagonism of a facilitatory effect of Ca(2+) on ATP receptor function. The present results suggest that, on sensory neurons, P2X(3) receptors could be inhibited by high Mg(2+) or lack of Ca(2+), representing a negative feedback process to limit ATP-mediated nociception.

Changes in the levels of nitric oxide synthase and protein kinase C gamma following kainic acid receptor activation in the rat spinal cord

In this study, we evaluated the levels of nitric oxide synthase, both neuronal and induced (nNOS and iNOS, respectively), cyclooxygenase-1 and 2 (COX-1 and COX-2) and protein kinase C gamma (PKCgamma) and correlated these with algogenic behavior following spinal kainic acid (KA) receptor activation in rats. Thirty adult male Sprague-Dawley rats were randomly assigned into six groups (n=5). Groups A, B, and C received 0.5 g kainic acid intrathecally and were analyzed at 3, 6, 24 h after injection, respectively. Groups D, E, and F received saline and were analyzed at 3, 6, 24 h after injection, respectively. We observed for behavioral changes in the rats following intrathecal KA injection and analyzed the protein levels of NOS, COX and PKCgamma by Western blotting techniques. Importantly, we clarified the potential roles of PKCgamma in the regulation of nNOS and COX-2 following intrathecal injection with KA in the rat spinal cord. COX-2 protein was detected but not significantly changed in the lumbosacral spinal cord at 3, 6, and 24 h following intrathecal KA injection (P>0.05). In contrast, nNOS protein was detected at higher levels in comparison with normal spinal cord at 6 and 24 h after intrathecal administration of KA (P<0.05). PKCgamma also increased significantly at 3, 6, and 24 h after intrathecal KA injection when compared with the baseline level (P<0.05). On the other hand, COX-1 and iNOS were not detected in either normal or KA treated spinal cords. These results provide strong in vivo evidence to support the idea that nNOS but not COX-2, plays an important role in spinal KA receptor activation. Furthermore, up-regulation of PKCgamma is involved in KA induced algogenic behavior in rats.