Source Paper
Chronic Morphine Induces Downregulation of Spinal Glutamate Transporters: Implications in Morphine Tolerance and Abnormal Pain Sensitivity
Jianren Mao, Backil Sung, Ru-Rong Ji, Grewo Lim
Journal of Neuroscience • 2002
NMDA Receptor Antagonism Study
Objective: Assessment of whether noncompetitive NMDA receptor antagonist MK-801 blocks PDC-potentiated morphine tolerance and thermal hyperalgesia
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Materials6
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Protocol Steps
Chronic morphine administration
Administer morphine chronically to rats through either intrathecal boluses or continuous infusion to induce dose-dependent downregulation of glutamate transporters
Note: Administration route can be either intrathecal boluses or continuous infusion
View evidence from paper
“Chronic morphine administered through either intrathecal boluses or continuous infusion induced a dose-dependent downregulation of GTs”
Assess glutamate transporter downregulation
Measure downregulation of glutamate transporters (EAAC1 and GLAST) in the rat's superficial spinal cord dorsal horn following morphine administration
Note: Specific transporters measured are EAAC1 and GLAST in the superficial spinal cord dorsal horn
View evidence from paper
“Chronic morphine administered through either intrathecal boluses or continuous infusion induced a dose-dependent downregulation of GTs (EAAC1 and GLAST) in the rat's superficial spinal cord dorsal horn”
Test naloxone blockade of GT changes
Administer naloxone to determine if morphine-induced glutamate transporter changes are mediated through opioid receptors
Note: Naloxone should block glutamate transporter changes if they are opioid receptor-mediated
View evidence from paper
“This GT downregulation was mediated through opioid receptors because naloxone blocked such GT changes”
Assess hyperalgesic response to exogenous glutamate
Measure hyperalgesic response to exogenous glutamate in morphine-treated rats with reduced spinal glutamate transporters, including magnitude and time course
Note: Response should show increased magnitude and prolonged time course in morphine-treated rats
View evidence from paper
“the hyperalgesic response to exogenous glutamate was enhanced, including an increased magnitude and a prolonged time course, in morphine-treated rats with reduced spinal GTs”
Establish temporal correlation
Establish temporal correlation between spinal glutamate transporter downregulation and development of morphine tolerance and thermal hyperalgesia
Note: Temporal relationship should be documented between GT downregulation and behavioral outcomes
View evidence from paper
“the downregulation of spinal GTs exhibited a temporal correlation with the development of morphine tolerance and thermal hyperalgesia”
Test PDC potentiation effects
Administer PDC (glutamate transporter inhibitor) to potentiate the development of morphine tolerance and thermal hyperalgesia
Note: PDC should enhance both morphine tolerance and thermal hyperalgesia development
View evidence from paper
“the GT inhibitor l-trans-pyrrolidine-2-4-dicarboxylate (PDC) potentiated, whereas the positive GT regulator riluzole reduced, the development of both morphine tolerance and thermal hyperalgesia”
Test riluzole reduction effects
Administer riluzole (positive glutamate transporter regulator) to reduce the development of morphine tolerance and thermal hyperalgesia
Note: Riluzole should reduce both morphine tolerance and thermal hyperalgesia development
View evidence from paper
“the GT inhibitor l-trans-pyrrolidine-2-4-dicarboxylate (PDC) potentiated, whereas the positive GT regulator riluzole reduced, the development of both morphine tolerance and thermal hyperalgesia”
Test MK-801 blockade of PDC effects
Administer MK-801 (noncompetitive NMDA receptor antagonist) to determine if it blocks the PDC-potentiated morphine tolerance and thermal hyperalgesia
Note: MK-801 should block both morphine tolerance and thermal hyperalgesia that are potentiated by PDC, indicating NMDA receptor involvement
View evidence from paper
“the noncompetitive NMDAR antagonist MK-801 blocked both morphine tolerance and thermal hyperalgesia that were potentiated by PDC”