Background: A previous study from our laboratories showed that a significant reduction in spinal N-methyl-D-aspartate (NMDA) receptor NR1 subunit phosphorylation (pNR1) is associated with the antiallodynic effect produced by intrathecal (IT) injection of the alpha-2 adrenoceptor agonist, clonidine, in neuropathic rats. In this study, we determined whether the spontaneous pain and increased pNR1 expression induced by NMDA injection are reduced by IT injection of either clonidine or the mu-opioid receptor agonist, [D-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO).
Methods: We examined the effect of clonidine (20 microg/rat) or DAMGO (1 microg/rat) injection on IT NMDA-induced spontaneous nociceptive behavior and pNR1 expression in the spinal dorsal horn. We also determined whether the effect of clonidine is mediated by alpha-2A or alpha-2C adrenoceptors. Finally, rat spinal cords were immunohistochemically processed for double staining of pNR1 and alpha-2A or alpha-2C adrenoceptors or mu-opioid receptors.
Results: The NMDA-induced increase in both pNR1 expression and nociceptive behavior was significantly reduced by IT clonidine but not DAMGO. This analgesic effect of clonidine was blocked by administration of either an alpha-2A (BRL44408, 30 microg/rat) or an alpha-2C (JP-1302, 50 microg/rat) adrenoceptor antagonist. In addition, immunocytochemistry revealed that spinal pNR1 immunoreactive cells co-contain alpha-2A and alpha-2C adrenoceptors.
Conclusions: These results demonstrate that the IT NMDA-induced increase in pNR1 expression and nociceptive behavior is significantly reduced by activation of alpha-2 adrenoceptors, but not mu-opioid receptors, in the spinal cord dorsal horn. Furthermore, these findings suggest that the modulation of spinal NR1 phosphorylation is linked to the effect of IT clonidine on postsynaptic neuronal activity.