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

Magnesium gjør at nervesystemet blir mindre sensitivt i studie på rotter med nevropati. Dosen er beregnet til å være ca 147 mg pr dag (24t), som er veldig mye relativt til kroppsvekten på en mus på 10-20g. Om vi regner det om til menneskevekt blir det megadoser.

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-dependant 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.

Diabetes is also the most common pathological state in which secondary magnesium (Mg) deficiency occurs. Indeed, Mg deficiency has been described in 25–30% and 13.5–47.7% of type 1 and type 2 diabetic patients, respectively (Garland, 1992Tossielo, 1996Corsonelloet al. 2000Engelen et al. 2000Rodriguez-Moran & Guerrero-Romero, 2003Pham et al.2007) and its incidence is correlated to diabetes complications (De Leeuw, 2001). Mg is an ATPase allosteric effector involved in inositol transport (Grafton et al. 1992) and the impaired Na+/K+-ATPase activity in peripheral nerves of diabetic animals (Garland, 1992) plays a role in the pathophysiology of diabetic neuropathy (Li et al. 2005).

In the central nervous system, Mg has voltage-dependent blocking properties that play an important role in pain processing at the N-methyl-d-aspartate (NMDA) receptor channel complex (Mayer et al. 1984Xiao & Bennett, 1994Begon et al. 2000). In vitro, this blockade operates at extracellular Mg concentrations of less than 1 mm (Mayer et al. 1984), i.e. within the ranges found in human and animal cerebrospinal fluid and plasma (Morris, 1992). The excess release of glutamate from central nociceptor terminals due to nerve damage releases Mg blockade and activates NMDA receptors known to trigger painful sensations (hyperalgesia, allodynia) and alter the sensitivity of postsynaptic cells, resulting in central sensitization (Bennett, 2000). This central sensitization involving the NMDA receptor can be induced in rats in vivo by Mg depletion (Alloui et al. 2003). Several studies suggest that phosphorylation of the NMDA receptor NR1 subunit is correlated to the presence of signs of neuropathy and to persistent pain following nerve injury (Gao et al.2005Ultenius et al. 2006Gao et al. 2007Roh et al. 2008).

One week after STZ or distilled water injection, the animals were assigned to the following three experimental groups:

  • MgSO4-supplemented STZ-D group: STZ-D rats receiving MgSO4 (296 mg l−1 of Mg) in drinking water for 3 weeks,
  • Non-supplemented STZ-D group: STZ-D rats given tap water,
  • Control non-diabetic group: rats given tap water.

Water intake was 10-fold and sixfold higher in non-supplemented and MgSO4-supplemented STZ-D rats, respectively, compared with non-diabetic rats. Water intake was significantly lower in MgSO4-supplemented STZ-D rats than non-supplemented STZ-D rats (Table 1). Consequently, urine excretion was 24-fold higher in non-supplemented STZ-D rats than non-diabetic rats. The MgSO4-supplemented STZ-D rats also developed polyuria corresponding to a 15-fold increase in urine excretion compared with non-diabetic rats, but which was nevertheless lower than the increase in non-supplemented STZ-D rats (Table 1). Polyuria in STZ-D rats was significantly correlated to water intake (P < 0.001).

Parameter Non-diabetic Non-suppl. STZ-D MgSO4-suppl. STZ-D
Water intake (ml (24 h)−1) 35.22 ± 2.36 376.6 ± 32.87*** 214.4 ± 30.87***,###
Urine excretion (ml (24 h)−1) 12.45 ± 1.51 300.1 ± 24.16*** 184.4 ± 25.23***,##
Food intake (g (24 h)−1) 30.7 ± 1.83 54.66 ± 3.67** 42.06 ± 6.21
Total Mg intake (mg (24 h)−1) 61.40 ± 3.66 109.32 ± 7.34*** 147.58 ± 1.58***,###

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

Parameter Non-diabetic Non-suppl. STZ-D MgSO4-suppl. STZ-D
Tactile hypersensitivity
Week 2 0/10 3/10 0/10#
Week 4 0/10 6/10* 0/10#
Thermal hypersensitivity
Week 2 0/10 6/10* 0/10#
Week 4 0/10 6/10* 0/10#

This study clearly showed that Mg supplementation prevents tactile and thermal allodynia and attenuates and delays mechanical hyperalgesia in STZ-D rats. This effect was mediated, at least in part, by the prevention of NMDA receptor NR1 subunit phosphorylation in STZ-D rats. However, the study also showed that Mg supplementation did not improve most of the biological and clinical signs of diabetes despite restoration of normal insulin secretion.

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