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Updated: Aug 19, 2026
Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities
Published on: July 29, 2014
HS014, a selective melanocortin-4 (MC4) receptor antagonist, modulates the behavioral effects of morphine in mice
N Eser Ercil1, Ruggero Galici, Robert A Kesterson
1Department of Molecular Physiology and Biophysics, Vanderbilt University, 702 Light Hall, Nashville, TN 37232, USA.
Rationale:
Melanocortin and opioid systems regulate feeding as well as other behaviors; however, the relationship between the two systems is not yet defined. Since agonist-induced stimulation of melanocortin receptors blocks the behavioral effects of mu opioid receptor agonists, and melanocortin-4 (MC4) receptors and mu opioid receptors share a similar anatomical distribution in the central nervous system, MC4 receptor blockade may increase opioid responsiveness.
Objectives:
The goal of this study was to test the hypothesis that blockade of MC4 receptors increases the behavioral effects of morphine.
Methods:
The effects of HS014 (0.0032, 0.032, and 1 nmol, i.c.v.), a selective MC4 antagonist, on morphine-induced (3.2, 10, and 32 mg/kg, i.p.) locomotor activity (measured in the open field for 15 min) and antinociception (measured in the hot plate at 55 degrees C) were assessed in C57Bl/6 mice. In addition, the effects of morphine were evaluated in A(y) mice, a genetic model for MC4 receptor blockade.
Results:
The dose-effect curve of morphine for locomotor activity was shifted downwards in C57Bl/6 mice pretreated with HS014 and in A(y) mice. The dose-effect curve of morphine for antinociception was shifted two- and threefold to the left in C57Bl/6 mice pretreated with HS014 and in A(y) mice, respectively.
Conclusions:
These results indicate that blockade of MC4 receptors increases the antinociceptive effects of morphine without changing the potency of morphine for locomotor activity, suggesting that MC4 receptor antagonists may be candidate drugs that can be clinically used for the treatment of pain.
Insights
Blocking melanocortin-4 (MC4) receptors enhances morphine's pain-relieving effects without affecting locomotor activity. This suggests MC4 receptor antagonists could be useful for pain management.
Area of Science:
- Neuroscience
- Pharmacology
Background:
- Melanocortin and opioid systems interact to regulate behavior, but their precise relationship is unclear.
- Melanocortin-4 (MC4) receptors and mu opioid receptors share central nervous system distribution, suggesting potential interactions.
- Previous studies indicate MC4 receptor stimulation can inhibit opioid effects, implying blockade might enhance them.
Purpose of the Study:
- To investigate if blocking MC4 receptors potentiates the behavioral effects of morphine.
- To test the hypothesis that MC4 receptor antagonism increases opioid responsiveness.
Main Methods:
- Assessed morphine's effects on locomotor activity and antinociception in C57Bl/6 mice pretreated with a selective MC4 antagonist (HS014).
- Evaluated morphine's effects in A(y) mice, a genetic model with inherent MC4 receptor blockade.
- Locomotor activity was measured in an open field, and antinociception was assessed using the hot plate test.
Main Results:
- Morphine's dose-effect curve for locomotor activity was shifted downward in mice with blocked MC4 receptors.
- Morphine's antinociceptive potency increased two- to threefold when MC4 receptors were blocked.
- These effects were observed both with pharmacological MC4 blockade (HS014) and genetic MC4 blockade (A(y) mice).
Conclusions:
- MC4 receptor blockade significantly enhances the pain-relieving (antinociceptive) effects of morphine.
- Blocking MC4 receptors does not alter morphine's impact on locomotor activity.
- MC4 receptor antagonists show potential as adjuncts for clinical pain management.
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