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Immunohistochemical study of opioid receptors after chronic morphine treatment in rats
1Department of Pharmacology, National Defense Medical Center, Taipei, Taiwan, ROC.
Abstract:
Previously, we have used the biochemical receptor binding method to investigate whether down-regulation of the opioid receptor is a mechanism for morphine tolerance, and we were led to a negative conclusion. In the current study, we further used immunohistochemistry to reinvestigate this issue. Male Sprague-Dawley rats (250-300 g) were chronically treated with morphine s.c. for 2, 4 or 6 days, using an escalating dosage paradigm (5-45 mg), which resulted in a 1.8 to 4.0-fold increase in AD50. Rat brains were removed, frozen, coronally sectioned (14 microm) and processed for mu- or delta-opioid receptor immunohistochemistry using the Avidin-Biotin Complex (ABC) method. No significant decrease in mu-opioid receptor (MOR) immunodensity was found in most of the brain regions, which were enriched with MOR after chronic treatment with morphine except for the anteroventral thalamic nucleus in the ventrolateral part (AVVL). No significant change in delta-opioid receptor (DOR) immunodensity after chronic treatment with morphine was found either. Therefore, our conclusion is that down regulation of opioid receptors may not be an important mechanism for morphine tolerance.
Insights
Down-regulation of opioid receptors is not a key factor in developing morphine tolerance. This study found no significant decrease in mu-opioid receptor or delta-opioid receptor levels after chronic morphine treatment.
Area of Science:
- Neuroscience
- Pharmacology
Background:
- Morphine tolerance is a complex phenomenon.
- Previous studies suggested opioid receptor down-regulation as a potential mechanism.
Purpose of the Study:
- To investigate the role of opioid receptor down-regulation in morphine tolerance using immunohistochemistry.
- To re-evaluate previous findings on receptor binding methods.
Main Methods:
- Chronic morphine administration to Sprague-Dawley rats over 2, 4, or 6 days with escalating doses.
- Immunohistochemistry (Avidin-Biotin Complex method) to assess mu-opioid receptor (MOR) and delta-opioid receptor (DOR) immunodensity in rat brains.
- Quantification of receptor levels in various brain regions.
Main Results:
- Chronic morphine treatment led to a 1.8 to 4.0-fold increase in AD50, indicating tolerance.
- No significant decrease in MOR immunodensity was observed in most brain regions.
- No significant changes in DOR immunodensity were detected after chronic morphine treatment.
Conclusions:
- Opioid receptor down-regulation is unlikely to be a major mechanism contributing to morphine tolerance.
- Immunohistochemistry provides further evidence against receptor down-regulation as a primary cause of tolerance.