Group I metabotropic glutamate receptors in spinal cord injury: roles in neuroprotection and the development of chronic central pain.

Spinal cord injury (SCI) initiates a cascade of biochemical events that leads to an increase in extracellular excitatory amino acid (EAA) concentrations, which results in glutamate receptor-mediated excitotoxic events. An important division of these glutamate receptors is the metabotropic glutamate receptor (mGluR) class, which is divided into three groups. Of these three groups, group I (mGluR1 and mGluR5) activation can initiate a number of intracellular pathways that lead to increased extracellular EAA concentrations. To evaluate subtypes of group I mGluRs in SCI, we administered AIDA (group I antagonist), LY 367385 (mGluR1 specific antagonist), or MPEP (mGluR5 specific antagonist) by interspinal injection to adult male Sprague-Dawley rats (175-200 g) immediately following injury at T10 with an NYU impactor (12.5-mm drop, 10-g rod, 2 mm in diameter). AIDA- and LY 367385-treated subjects had improved locomotor scores and demonstrated an attenuation in the development of mechanical allodynia as measured by von Frey stimulation of the forelimbs; however, LY 367385 potentiated the development of thermal hyperalgesia. MPEP had no effect on locomotor recovery or mechanical allodynia, but attenuated the development of thermal hyperalgesia. AIDA and LY 367385 treatment resulted in a significant increase in tissue sparing compared to the vehicle-treated group at 4 weeks following SCI. These results suggest that mGluRs play an important role in EAA toxicity and have different acute pathophysiological roles following spinal cord injury.