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Assessment of Morphine-induced Hyperalgesia and Analgesic Tolerance in Mice Using Thermal and Mechanical Nociceptive Modalities
Published on: July 29, 2014
Microglial involvement in inflammatory pain hypersensitivity in adult rats after adolescent morphine exposure
Fatemeh Jalali1, Kawsar Alami1, Shiva Hashemizadeh2
1Department of Physiology, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, , Iran.
Background And Aim:
Adolescent morphine exposure (AME) is increasingly recognized as a risk factor for long-term alterations in pain processing, predisposing individuals to persistent pain in adulthood. Microglia are key regulators of neuroimmune signaling and central sensitization, yet their contribution to inflammatory pain hypersensitivity following adolescent opioid exposure remains unclear. This study examined whether inhibiting microglial activation during adolescence prevents the development of inflammatory pain hypersensitivity induced by chronic morphine exposure.
Methods:
Male Wistar rats received escalating doses of morphine (2.5-25 mg/kg, s.c., twice daily) during adolescence (postnatal days [PND] 28-37), with or without the microglial inhibitor minocycline. After a one-month drug-free washout period, inflammatory pain sensitivity was assessed in adulthood using the formalin test. Nociceptive scores, paw licking, and flinching behaviors were quantified across test phases. Multivariate behavioral patterns were analyzed using principal component analysis (PCA). Lumbar spinal cord expression of inflammatory and neurotrophic genes was measured by RT-qPCR two hours after formalin injection.
Results:
Adolescent morphine exposure produced persistent inflammatory pain hypersensitivity in adulthood, characterized by enhanced nociceptive scores, licking, and flinching behaviors, particularly during the late inflammatory phase of the formalin test. Cotreatment with minocycline during adolescence significantly attenuated morphine-induced hyperalgesia and partially normalized nociceptive behavioral profiles in PCA. At the molecular level, adolescent morphine exposure paradoxically reduced spinal Tlr4, Il-12, and Bdnf expression in adulthood despite heightened pain behaviors, and minocycline did not prevent morphine effects.
Conclusions:
These findings demonstrate that microglial activity during adolescence plays a potential role in the long-term enhancement of inflammatory pain sensitivity after morphine exposure, yet the behavioral hypersensitivity occurs independently of classical inflammatory transcriptional upregulation in the spinal cord. Microglia remain a potential therapeutic target, but the relevant functional state is not captured by the classical markers assessed.
Insights
Adolescent morphine exposure causes long-term pain hypersensitivity, but inhibiting microglia during adolescence did not prevent this effect. Microglial activation may contribute to persistent pain, though not through classical spinal inflammatory markers.
Area of Science:
- Neuroscience
- Pain Research
- Neuroimmunology
Background:
- Adolescent morphine exposure (AME) is linked to adult pain processing alterations.
- Microglia regulate neuroimmune signaling and central sensitization.
- The role of microglia in AME-induced inflammatory pain hypersensitivity is unclear.
Purpose of the Study:
- To investigate if inhibiting microglial activation during adolescence prevents long-term inflammatory pain hypersensitivity induced by chronic morphine exposure.
- To examine the contribution of adolescent microglial activity to persistent pain.
Main Methods:
- Male Wistar rats received escalating morphine doses during adolescence, with or without minocycline.
- Inflammatory pain sensitivity was assessed in adulthood using the formalin test.
- Spinal gene expression (inflammatory and neurotrophic) was analyzed post-formalin injection.
Main Results:
- AME induced persistent inflammatory pain hypersensitivity in adulthood.
- Minocycline co-treatment during adolescence partially attenuated morphine-induced hyperalgesia.
- AME paradoxically reduced spinal Tlr4, Il-12, and Bdnf expression in adulthood, effects not prevented by minocycline.
Conclusions:
- Adolescent microglial activity may contribute to long-term inflammatory pain sensitivity after morphine exposure.
- Behavioral hypersensitivity appears independent of classical spinal inflammatory transcriptional changes.
- Microglia are a potential therapeutic target, but their relevant functional state requires further investigation beyond classical markers.
