Involvement of increased excitatory amino acids and intracellular Ca2+ concentration in the spinal dorsal horn in an animal model of neuropathic pain

Neuropathic pain following nerve injury is believed to involve excitatory amino acids (EAAs) and Ca2+-mediated neuronal plastic changes in the central nervous system (CNS). This study was designed to investigate the changes in glutamate and aspartate contents in the dorsal half of the spinal cord following chronic constrictive injury (CCI) of the rat common sciatic nerve. We also examined the changes in intracellular calcium ion concentration (Ca2+i) of the spinal dorsal horn in transverse spinal slices in the same animal model. Thermal and mechanical hyperalgesia were observed on day 2 and thereafter following CCI (P < 0.0001). In the CCI rats to which 0.5 mg/kg of i.p. MK-801 was given 30 min prior to CCI and subsequently three daily treatments with 0.5 mg/kg of i.p. MK-801, the development of thermal and mechanical hyperalgesia was suppressed for a period of up to 7 days; however, hyperalgesia appeared on day 10 and day 14 (P < 0.001). In CCI rats, significant increases were observed in glutamate and aspartate contents on the ipsilateral side of the dorsal horn to nerve ligation on days 4, 7 and 14 (P < 0.001). Moreover, significant increases in Ca2+i in the spinal dorsal horn were also observed in the superficial (lamina I-II) and deep layers (lamina V-VI) on the ipsilateral side to nerve ligation on days 4, 7 and 14 after nerve ligation in the spinal slices (P < 0.0001). The treatment with i.p. MK-801 suppressed the increases in the contents of glutamate and aspartate and in Ca2+i on days 4 and 7. However, the ipsilateral contents of glutamate and aspartate significantly increased on day 14 (P < 0.001 and 0.003, respectively); the increased Ca2+i was also observed on day 14 (P < 0.001), and the spatial pattern of the increased regions was similar to untreated CCI rats. We interpret these results to indicate that neuropathic hyperalgesia induced by CCI in the rat is associated with an increase in glutamate and aspartate contents and the subsequent activation of NMDA receptors, followed by an increase in Ca2+i within dorsal horn of the spinal cord.