Opioid mydriasis: cross-tolerance between morphine and enkephalins

Area of Science:

• Pharmacology
• Neuroscience
• Opiate Receptor Research

Background:

• Morphine and enkephalins are endogenous and exogenous opioids that interact with opiate receptors.
• Mydriasis (pupil dilation) is a known physiological effect of certain opioid agonists.
• Cross-tolerance between different opioids suggests interaction with common receptor pathways.

Purpose of the Study:

• To investigate the occurrence of cross-tolerance between morphine and enkephalins (met-enkephalin and D-Ala-D-Leu-enkephalin) in the induction of mydriasis in mice.
• To explore the potential involvement of multiple opiate receptor systems in the mydriatic effects of these compounds.

Main Methods:

• Establishing morphine tolerance in mice.
• Administering met-enkephalin and D-Ala-D-Leu-enkephalin (DADL) intracerebroventricularly (ICV) to both tolerant and non-tolerant mice.
• Quantifying and comparing the mydriatic responses (pupil dilation) in both groups.

Main Results:

• Morphine-tolerant mice exhibited a significantly reduced mydriatic response to ICV administration of met-enkephalin and DADL.
• Maximal mydriasis in tolerant animals was approximately 30% for met-enkephalin and 50% for DADL compared to non-tolerant controls.
• These findings provide early evidence of cross-tolerance between morphine and enkephalins in vivo.

Conclusions:

• The study demonstrates cross-tolerance between morphine and enkephalins in mice, specifically concerning mydriasis.
• The results support the hypothesis that multiple opiate receptor systems may mediate the pupillary effects of morphine and enkephalins.
• This research contributes to understanding the complex interactions within the endogenous opioid system and its modulation by exogenous opioids.