Morphine, beta-endorphin and [D-Ala2] Met-enkephalin inhibit oxytocin release by acetylcholine and suckling

Area of Science:

• Neuroendocrinology
• Lactation Physiology
• Opioid Peptide Research

Background:

• Morphine is known to inhibit suckling-induced oxytocin (OT) release in lactating mammals.
• Beta-endorphin and enkephalins share functional similarities with morphine, suggesting they might also influence OT release.

Purpose of the Study:

• To investigate the effects of beta-endorphin and enkephalins on oxytocin release in lactating rats.
• To determine if these opioid peptides modulate OT release in response to both pharmacological and physiological stimuli.

Main Methods:

• Experiments were conducted on anesthetized lactating rats.
• Oxytocin release was stimulated via intraventricular injection of acetylcholine (ACh) or through the natural stimulus of suckling.
• Milk-ejection responses were used to quantify OT release, comparing them to known intravenous (IV) OT doses.

Main Results:

• Both beta-endorphin and a specific enkephalin analog, [D-Ala2]Met-enkephalin, significantly inhibited ACh-induced OT release.
• These opioid peptides also suppressed OT release stimulated by suckling.
• Naloxone, an opioid antagonist, effectively reversed the inhibitory effects of beta-endorphin and [D-Ala2]Met-enkephalin on OT release.

Conclusions:

• Beta-endorphin and enkephalins play a role in regulating oxytocin release during lactation.
• Opioid peptide-mediated inhibition of oxytocin release can be counteracted by naloxone, indicating involvement of opioid receptors.
• These findings shed light on the complex neuroendocrine control of lactation and milk ejection.