Comparison of the spinal neuropathic pain induced by intraspinal injection of N-methyl-d-aspartate and quisquate in rats

Abstract

Objective: Excitatory amino acids play important roles in the development of secondary pathology following spinal cord injury (SCI). This study was designed to evaluate morphological changes in the dorsal horn of the spinal cord and assess profiles of pain behaviors following intraspinal injection of N-methyl-D-aspartate (NMDA) or quisqualate (QUIS) in rats.

Methods: Forty male Sprague-Dawley rats were randomized into three groups : a sham, and two experimental groups receiving injections of 125 mM NMDA or QUIS into their spinal dorsal horn. Following injection, hypersensitivity to cold and mechanical stimuli, and excessive grooming behaviors were assessed serially for four weeks. At the end of survival periods, morphological changes in the spinal cord were evaluated.

Results: Cold allodynia was developed in both the NMDA and QUIS groups, which was significantly higher in the QUIS group than in the NMDA group. The mechanical threshold for the ipsilateral hind paw in both QUIS and NMDA groups was significantly lower than that in the control group. The number of groomers was significantly higher in the NMDA group than in the QUIS group. The size of the neck region of the spinal dorsal horn, but not the superficial layer, was significantly smaller in the NMDA and QUIS groups than in the control group.

Conclusion: Intraspinal injection of NMDA or QUIS can be used as an excitotoxic model of SCI for further research on spinal neuropathic pain.

Keywords: NMDA; Neuropathic pain; Quisqualate; Rat; Spinal cord injury.

Fig. 1

The effects of intraspinal NMDA or QUIS injections or sham surgery on the responses to cold (A and B) and mechanical (C and D) stimuli delivered to the hind paws. Baseline responses were measured 3 days prior to receiving intraspinal injections/sham surgery. Cold and mechanical allodynia developed after intraspinal injection of NMDA or QUIS. Positive responses were defined as sustained signs of cold allodynia and/or measured withdrawal threshold of less than 5 g (mechanical allodynia). Data are expressed as the number of animals or mean±SEM (^*p<0.05 compared with the control group; ^†p<0.05 compared with the NMDA group). NMDA : N-methyl D-aspartate, QUIS : quisqualate, SEM : standard errors of the mean.

Fig. 2

Excessive grooming behaviors developed in animals that received intraspinal NMDA or QUIS injections. The number of groomers is significantly higher in the NMDA group than in the QUIS group (A), but there is no significant difference in the severity of grooming between the two groups (B). Data in A are expressed as number of animals; in B, boxes show interquartile ranges and bars denote 10th and 90th percentiles (^*p<0.05 compared with the control group; ^†p<0.05 compared with the NMDA group). NMDA : N-methyl D-aspartate.

Fig. 3

Representative cresyl violet-stained spinal cord sections of sham-operated control rats (A), NMDA-injected rats (B), and QUIS-injected rats (C). Dilation of the central canal, ipsilateral neuronal loss of lamina III and V, and intraspinal cavities are seen in B and C. Scale bar in A (inset) equals 500 µm. NMDA : N-methyl D-aspartate, QUIS : quisqualate.

Fig. 4

The estimated size of the dorsal horn of the spinal gray matter is shown. The results are based on histological reconstructions from spinal cords of seven animals per group. Inset : schematic representation of the spinal cord showing the superficial (lamina I and II) and neck (lamina III and V) regions. Neuronal loss is observed in the neck region of the spinal dorsal horn following intraspinal injection of NMDA or QUIS. Values are mean±SEM (^*p<0.05 compared with the control group). NMDA : N-methyl D-aspartate, QUIS : quisqualate, SEM : standard errors of the mean.