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

Abstract

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.

Figures

Figure 1. Time course of glycaemia in non-diabetic (Non-D), non-supplemented STZ-diabetic (Non-suppl. STZ-D) and MgSO4-supplemented diabetic (MgSO4-suppl. STZ-D) rats

Values are expressed as means ±s.e.m. in g l−1 (n = 10 for each group). Statistical analysis was performed by one-way ANOVA followed by a Tukey–Kramer test. ***P < 0.001 vs. non-D rats. #P < 0.05 vs. Non-suppl. STZ-D rats.

Figure 2. Time course of body weight in non-diabetic (Non-D), non-supplemented STZ-diabetic (Non-suppl. STZ-D) and MgSO4-supplemented diabetic (MgSO4-suppl. STZ-D) rats

Values are expressed as means ±s.e.m. in g (n = 10 for each group). Statistical analysis was performed by one-way ANOVA followed by a Tukey–Kramer test. **P < 0.01, ***P < 0.001 vs. non-D rats; #P < 0.05 vs. Non-suppl. STZ-D rats.

Figure 3. Insulin plasma concentrations in non-diabetic (Non-D), non-supplemented STZ-diabetic (Non-suppl. STZ-D) and MgSO4-supplemented STZ-diabetic (MgSO4-suppl. STZ-D) rats at week 4 of the experiment

Values are expressed as means ±s.e.m. in μg l−1 (n = 6–8/group). Statistical analysis was performed by one-way ANOVA followed by a Tukey–Kramer test. *P < 0.05 vs. Non-D rats.

Figure 4. Time course of mechanical sensitivity measured by paw pressure-induced vocalization threshold (VT) variations in non-diabetic (Non-D), non-supplemented STZ-diabetic (Non-suppl. STZ-D) and MgSO4-supplemented (MgSO4-suppl. STZ-D) rats

Results are expressed as means ±s.e.m. in g (n = 10 for each group). Statistical analysis was performed by one-way ANOVA followed by a Tukey–Kramer test. **P < 0.01, ***P < 0.001 vs. Non-D rats; ###P < 0.001 vs. Non-suppl. STZ-D rats.

Figure 5. Expression of phosphorylated NR1 (pNR1) and total NR1 in spinal cord dorsal horn extracts from non-diabetic (Non-D), non-supplemented STZ-diabetic (Non-suppl. STZ-D) and MgSO4-supplemented (MgSO4-suppl. STZ-D) rats at week 4 of the experiment

A, representative Western blots showing pNR1 (upper panel) or total NR1 (lower panel) expression; B and C, percentage changes of pNR1 and NR1 levels. Data are means ±s.e.m. expressed as a percentage of values measured in Non-D rats (% of controls), for 3 Non-D rats, 3 MgSO4-suppl. STZ-D rats and 3 Non-suppl. STZ-D rats. Statistical analysis was performed by one-way ANOVA followed by a Tukey–Kramer test. ***P < 0.001 vs. N rats; ##P < 0.01 vs. Non-suppl. STZ-D rats.