Morphine Tolerance and Thermal Hyperalgesia Assessment
behavioralratNot specified
Objective: Examine the role of spinal glutamate transporters in the development of morphine tolerance and associated thermal hyperalgesia through chronic morphine administration
Materials & Equipment Checklist
7 items1 from ConductScience
Gather these items before starting the experiment. Check off items as you prepare.
Equipment1
Not specified • Not specified • Not specified • Not mentioned
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Protocol Steps
View Abstract
Tolerance to the analgesic effects of an opioid occurs after its chronic administration, a pharmacological phenomenon that has been associated with the development of abnormal pain sensitivity such as hyperalgesia. In the present study, we examined the role of spinal glutamate transporters (GTs) in the development of both morphine tolerance and associated thermal hyperalgesia. 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. This GT downregulation was mediated through opioid receptors because naloxone blocked such GT changes. Morphine-induced GT downregulation reduced the ability to maintain in vivo glutamate homeostasis at the spinal level, because 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. Moreover, the downregulation of spinal GTs exhibited a temporal correlation with the development of morphine tolerance and thermal hyperalgesia. Consistently, 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. The effects from regulating spinal GT activity by PDC were at least in part mediated through activation of the NMDA receptor (NMDAR), because the noncompetitive NMDAR antagonist MK-801 blocked both morphine tolerance and thermal hyperalgesia that were potentiated by PDC. These results indicate that spinal GTs may contribute to the neural mechanisms of morphine tolerance and associated abnormal pain sensitivity by means of regulating regional glutamate homeostasis.
1
Chronic morphine administration via intrathecal boluses
Administer morphine through intrathecal boluses in a dose-dependent manner
Not specifiedNot specified
Note: This is one of two administration routes tested
View evidence from paper
“Chronic morphine administered through either intrathecal boluses or continuous infusion induced a dose-dependent downregulation”
2
Chronic morphine administration via continuous infusion
Administer morphine through continuous infusion in a dose-dependent manner
Not specifiedNot specified
Note: This is the alternative administration route tested
View evidence from paper
“Chronic morphine administered through either intrathecal boluses or continuous infusion induced a dose-dependent downregulation”
3
Assess glutamate transporter downregulation
Examine downregulation of glutamate transporters (EAAC1 and GLAST) in the rat's superficial spinal cord dorsal horn
Not specifiedNot specified
Note: Measure in response to morphine treatment
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”
4
Test naloxone blockade of glutamate transporter changes
Administer naloxone to determine if opioid receptors mediate glutamate transporter downregulation
Not specifiedNot specified
Note: Confirms opioid receptor involvement
View evidence from paper
“This GT downregulation was mediated through opioid receptors because naloxone blocked such GT changes”
5
Measure hyperalgesic response to exogenous glutamate
Assess hyperalgesic response to exogenous glutamate in morphine-treated rats with reduced spinal glutamate transporters
Not specifiedNot specified
Note: Measure both magnitude and time course of 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”
6
Assess temporal correlation with morphine tolerance and thermal hyperalgesia
Evaluate the temporal relationship between glutamate transporter downregulation and development of morphine tolerance and thermal hyperalgesia
Not specifiedNot specified
Note: Establish correlation between molecular and behavioral changes
View evidence from paper
“the downregulation of spinal GTs exhibited a temporal correlation with the development of morphine tolerance and thermal hyperalgesia”
7
Test glutamate transporter inhibitor (PDC) effects
Administer PDC to potentiate the development of morphine tolerance and thermal hyperalgesia
Not specifiedNot specified
Note: PDC is a glutamate transporter inhibitor
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”
8
Test positive glutamate transporter regulator (riluzole) effects
Administer riluzole to reduce the development of morphine tolerance and thermal hyperalgesia
Not specifiedNot specified
Note: Riluzole is a positive glutamate transporter regulator
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”
9
Test NMDA receptor antagonist (MK-801) blockade
Administer MK-801 to determine if NMDA receptor activation mediates PDC-potentiated morphine tolerance and thermal hyperalgesia
Not specifiedNot specified
Note: MK-801 is a noncompetitive NMDA receptor antagonist
View evidence from paper
“the noncompetitive NMDAR antagonist MK-801 blocked both morphine tolerance and thermal hyperalgesia that were potentiated by PDC”