Intrathecal clonidine suppresses phosphorylation of the N-methyl-D-aspartate receptor NR1 subunit in spinal dorsal horn neurons of rats with neuropathic pain.
BACKGROUND
Intrathecal (IT) administration of the alpha-2 adrenoceptor agonist, clonidine, produces significant analgesic effects. Although several mechanisms underlying clonidine-induced analgesia have been proposed, the possible interaction with N-methyl-D-aspartate (NMDA) receptors as a major antinociceptive mechanism has not been addressed. We designed the present study to determine whether clonidine or other analgesics can affect spinal NMDA receptor activation in rats with chronic constriction injury (CCI)-induced neuropathy.


METHODS
Rats underwent unilateral CCI, and received IT clonidine (1, 5, 20 microg/rat), [D-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO, mu opioid receptor agonist, 1 microg/rat), gabapentin (anticonvulsant, 100 microg/rat) or vehicle 2 wks later. After drug injection, we measured the pain response to thermal or mechanical stimuli and used immunohistochemistry to evaluate spinal cord phosphorylated NMDA-receptor subunit 1 (pNR1) expression.


RESULTS
Two weeks after CCI surgery, rats displayed significant mechanical allodynia and thermal hyperalgesia, and the spinal cord dorsal horn showed a significant increase in the number of pNR1 immunoreactive neurons. IT injection of clonidine (20 microg/rat), DAMGO and gabapentin potently reduced mechanical allodynia and thermal hyperalgesia. Importantly, IT clonidine, but not IT DAMGO or gabapentin, dose-dependently reduced CCI-induced pNR1 expression in all lamina of the spinal cord dorsal horn by 30 min after injection. In addition, IT injection of the alpha-2 adrenoceptor antagonist, idazoxan (40 microg/rat) 10 min before clonidine injection completely reversed clonidine's antihyperalgesic and antiallodynic effects, as well as clonidine's suppressive effect on CCI-induced NR1 phosphorylation in the spinal cord dorsal horn.


CONCLUSIONS
Our data indicate that IT clonidine's antihyperalgesic/antiallodynic effect on neuropathic pain is associated with a significant reduction in spinal NMDA receptor phosphorylation and suggests a potentially novel mechanism of clonidine's action.
Roh
Dae-Hyun
DH
Department of Veterinary Physiology, College of Veterinary Medicine and BK21 Program for Veterinary Science, Seoul National University, Seoul, South Korea.
Kim
Hyun-Woo
HW
Yoon
Seo-Yeon
SY
Seo
Hyoung-Sig
HS
Kwon
Young-Bae
YB
Han
Ho-Jae
HJ
Beitz
Alvin J
AJ
Lee
Jang-Hern
JH
D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage* / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000588:Amines / Q000494:pharmacology; D000700:Analgesics / Q000008:administration & dosage* / Q000494:pharmacology; D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology; D003000:Clonidine / Q000008:administration & dosage* / Q000494:pharmacology; D003509:Cyclohexanecarboxylic Acids / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D000077206:Gabapentin; D019329:Idazoxan / Q000494:pharmacology; D009437:Neuralgia / Q000378:metabolism*; D010766:Phosphorylation; D020671:Posterior Horn Cells / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D017450:Receptors, Opioid, mu / Q000037:antagonists & inhibitors; D013116:Spinal Cord / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D000588:Amines; D000700:Analgesics; D000927:Anticonvulsants; D003509:Cyclohexanecarboxylic Acids; C409466:NR1 NMDA receptor; D016194:Receptors, N-Methyl-D-Aspartate; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D005680:gamma-Aminobutyric Acid; D000077206:Gabapentin; D003000:Clonidine; D019329:Idazoxan