Critical role of the rostral ventromedial medulla in early spinal events leading to chronic constriction injury neuropathy in rats.
Neuropathic pain is a major clinical problem, and several animal models have been developed to investigate its mechanisms and its treatment. In this report, the role of the rostral ventromedial medulla (RVM) in the early events of the chronic constriction injury (CCI) model was investigated in behavioral and electrophysiological experiments. Placing the 4 CCI ligatures around the sciatic nerve induced large discharges and residual ongoing activity in spinal nociceptive neurons. Two weeks after CCI ligation, the rats showed behavioral hyperalgesia and allodynia as well as increased ongoing activity and responsiveness of spinal nociceptive neurons to innocuous and noxious stimuli. Blockade of excitatory synapses in the RVM by a kynurenate microinjection (2 nmol in 0.5 muL) 5 minutes before placement of the sciatic ligatures had no immediate effect on spinal neuronal activity but largely prevented the activation of spinal neurons. In kynurenate microinjected rats, behavioral hyperalgesia and allodynia developed slowly and incompletely, which corresponded with an incompletely developed hyperexcitability of spinal neurons. To the best of our knowledge, these results show for the first time that the initial response to nerve damage requires facilitation from the RVM.


CONCLUSIONS
The present and previous findings indicate that descending facilitation from brainstem nuclei critically contributes to the spinal hyperexcitability that underlies neuropathic pain. The present results indicate that this contribution begins at the very moment the nerve is damaged and should be prevented and treated accordingly.
Sanoja
Raul
R
Instituto Venezolano de Investigaciones Cientificas, Caracas, Venezuela.
Vanegas
Horacio
H
Tortorici
Victor
V
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D003250:Constriction; D004195:Disease Models, Animal; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D007736:Kynurenic Acid / Q000494:pharmacology; D007796:Laminectomy; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects / Q000503:physiopathology*; D009474:Neurons / Q000502:physiology*; D010147:Pain Measurement; D017288:Pain Threshold / Q000187:drug effects / Q000502:physiology*; D010812:Physical Stimulation / Q000379:methods; D051381:Rats; D017207:Rats, Sprague-Dawley; D011930:Reaction Time / Q000502:physiology; D020426:Sciatic Neuropathy / Q000188:drug therapy / Q000209:etiology / Q000473:pathology*; D013116:Spinal Cord / Q000473:pathology*; D013997:Time Factors
D018691:Excitatory Amino Acid Antagonists; D007736:Kynurenic Acid