Increased running response to morphine in morphine-pretreated mice.

Area of Science:

• Neuroscience
• Pharmacology
• Behavioral Science

Background:

• Opioid drugs like morphine are widely used for pain relief.
• Understanding the long-term effects of opioid administration, such as sensitization, is crucial for clinical practice.
• Previous research has explored opioid-induced behavioral changes, but the mechanisms of sensitization require further investigation.

Purpose of the Study:

• To investigate the development and characteristics of morphine-induced behavioral sensitization in B6AF1/J mice.
• To explore the role of opioid receptors and other drugs in modulating this sensitization.
• To determine if morphine-induced sensitization is associated with analgesic tolerance.

Main Methods:

• B6AF1/J mice and their parental strains (C57Bl/6J and A/J) were administered morphine sulfate or other drugs at various doses and frequencies.
• Locomotor activity (running response) was measured following drug administration.
• The tail-flick assay was used to assess analgesic tolerance.
• The effects of opioid antagonists and other psychostimulants were examined.

Main Results:

• Repeated morphine administration (twice daily for 5 days or weekly for 2-3 weeks) increased the running response in B6AF1/J mice up to threefold.
• This sensitization was dose-dependent, lasted up to a month after cessation of treatment, and was not observed in young mice (<15 days old).
• Morphine sensitization was blocked by naloxone but not by dextrorphan, and it did not lead to analgesic tolerance. Pretreatment with morphine also enhanced responses to amphetamine and cocaine.

Conclusions:

• Repeated morphine administration induces significant and long-lasting behavioral sensitization in mice, mediated by opioid receptors.
• This sensitization is distinct from analgesic tolerance and can cross-sensitize with other psychostimulants.
• The findings highlight the complex neuroadaptive changes associated with chronic opioid exposure.