The involvement of RVM ionotropic glutamate receptors in descending modulation of persistent inflammatory hyperalgesia

Abstract

Descending pathways from the rostral ventromedial medulla (RVM) modulate spinal nociceptive transmission. Descending pain modulation exhibits dynamic functional changes in response to persistent pain. Glutamate receptors in the RVM are involved in descending pain modulation. However, their roles in descending modulation of persistent inflammatory hyperalgesia are unclear. Behavioral studies showed that intra-RVM microinjection of NMDA produced descending modulation that depended upon the post-inflammatory time period. At 3 hr post-inflammation, NMDA induced facilitation at a lower dose (10 pmol) and an inhibition at a higher dose (1000 pmol). At 24 hr post-inflammation, NMDA (0.1--1000 pmol) produced a dose-dependent inhibition. AMPA (0.1--100 pmol) produced only dose-dependent inhibition at both 3 hr and 24 hr. Importantly, there is a leftward shift in dose-response curves of NMDA- and AMPA-produced modulation at 24 hr post-inflammation, compared to that at 3 hr, suggesting a time-dependent increase in glutamatergic neurotransmission in the RVM after inflammation.;We further identified the molecular mechanisms underlying the enhanced AMPA receptor function. RT-PCR analysis indicated an upregulation of GluR1-flip (5 hr--24 hr), GluR2-flip (24 hr) and GluR2-flop (24 hr) mRNAs in the RVM after inflammation. Western blots demonstrated that GluR1 protein levels were upregulated at 24 hr-3 days post-inflammation. GluR2 protein levels remained unchanged after inflammation. Immunohistochemistry demonstrated an increase in GluR1-like immunoreactivity localized to the RVM at 24 hr post-inflammation. Phosphorylation of GluR1 Ser831 plays an important role in regulation of AMPA receptor function. Western blots indicated a rapid and prolonged increase in phospho-Ser831 GluR1 levels in the RVM after inflammation. The onset of upregulation of GluR1 phosphorylation was blocked by pre-emptive local anesthesia of the injured area and attenuated by pretreatment of NMDA receptor antagonists, PKC and CaMKII inhibitors in the RVM. These findings suggest that AMPA receptors in the RVM undergo selective transcriptional, translational and post-translational modulations following inflammation. These findings support the notion that there is a time-dependent dynamic change in descending modulation after inflammation. This study demonstrates that ionotropic glutamate receptors in the RVM are involved in the descending modulation of inflammatory hyperalgesia and play a role in plasticity in descending pain modulation after persistent pain.