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
Exogenous Glutamate Hyperalgesic Response Testing
Objective: Assessment of enhanced hyperalgesic response to exogenous glutamate in morphine-treated rats with reduced spinal glutamate transporters
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Materials6
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Protocol Steps
Chronic morphine administration
Administer morphine chronically to rats through intrathecal boluses or continuous infusion to induce dose-dependent downregulation of glutamate transporters
Note: Two administration routes tested: 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 treatment
Note: Two specific transporter types measured: EAAC1 and GLAST in 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 transporter changes
Administer naloxone to determine if opioid receptors mediate the morphine-induced glutamate transporter downregulation
Note: Naloxone should block glutamate transporter changes if mediated through opioid receptors
View evidence from paper
“This GT downregulation was mediated through opioid receptors because naloxone blocked such GT changes”
Assess hyperalgesic response to exogenous glutamate
Administer exogenous glutamate to morphine-treated rats with reduced spinal glutamate transporters and measure hyperalgesic response including magnitude and time course
Note: Measure both increased magnitude and prolonged time course of hyperalgesic response
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”
Evaluate temporal correlation with morphine tolerance and thermal hyperalgesia
Monitor the temporal relationship between glutamate transporter downregulation and the development of morphine tolerance and thermal hyperalgesia
Note: Establish temporal correlation between transporter 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 effects on morphine tolerance and thermal hyperalgesia
Administer PDC (glutamate transporter inhibitor) to determine its effects on potentiating morphine tolerance and thermal hyperalgesia development
Note: PDC should potentiate both morphine tolerance and thermal hyperalgesia
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 effects on morphine tolerance and thermal hyperalgesia
Administer riluzole (positive glutamate transporter regulator) to determine its effects on reducing morphine tolerance and thermal hyperalgesia development
Note: Riluzole should reduce both morphine tolerance and thermal hyperalgesia
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 NMDA receptor activation mediates PDC-potentiated morphine tolerance and thermal hyperalgesia
Note: MK-801 should block both morphine tolerance and thermal hyperalgesia potentiated by PDC if NMDA receptor mediated
View evidence from paper
“the noncompetitive NMDAR antagonist MK-801 blocked both morphine tolerance and thermal hyperalgesia that were potentiated by PDC”