Both enkephalins and dynorphin relaxed rabbit arteries by blocking nerve-driven contraction, with maximum effects varying between blood vessel types.
Pre-junctional inhibitionOpioids block nerve signals at the blood vessel wall to reduce contraction
What the researchers found
In rabbit ear and saphenous (leg) arteries, both delta-selective opioid agonists (leu-enkephalin and met-enkephalin) and kappa-selective agonists (dynorphin-(1-13) and ethylketocyclazocine) inhibited nerve-stimulation-evoked contractions.
Delta agonists showed a vessel-specific difference: their maximum inhibitory effect was significantly less in the saphenous artery than in the ear artery. Kappa agonists worked similarly in both vessels.
Yohimbine (an alpha-2 adrenergic blocker) did not enhance opioid inhibitory effects at any stimulation frequency in either vessel. This disproved the hypothesis that opioid receptors work by boosting alpha-2 receptor inhibition of noradrenaline release.
Why it matters
This study showed opioid peptides directly relax blood vessels by blocking nerve-driven contractions. The vessel-specific differences for delta agonists suggest different parts of the circulatory system respond differently to the same opioid signals. This matters for understanding why opioids cause blood pressure changes.
How the study worked
In vitro ring segments of rabbit ear and saphenous arteries were transmurally stimulated electrically. Opioid peptides were applied and contractile responses measured. Two stimulation patterns: short (8 Hz, 1 second) and long (2 Hz, 25 seconds). Yohimbine tested alpha-2 receptor interaction.
What this study cannot tell us
In vitro study with isolated rabbit blood vessels. Results may not apply to intact animals or humans. Only two vessel types were tested. The physiological concentrations of endogenous opioid peptides at blood vessel nerve terminals are unknown.
How to read the evidence
Preliminary in-vitro study comparing two vessel types with receptor-selective agonists.
When this study was published
Published in 1988 — established opioid receptor function in blood vessel innervation.
The bigger picture
Opioid receptors on blood vessel nerves represent a local regulatory mechanism. This could explain regional blood flow changes during pain and contribute to blood pressure effects of opioid drugs.
Questions still open
- Do vascular opioid receptors contribute to opioid-induced hypotension?
- Can peripheral opioid receptors be targeted for blood flow disorders?
Common questions
What does pre-junctional mean?
Why do different vessels respond differently?
Read the original research
Comparison of the pre-junctional effect of opioids in two blood vessels of the rabbit.
European journal of pharmacology, 158(1-2), 21-8
Citation
Gan, E A; Duckles, S P. (1988). Comparison of the pre-junctional effect of opioids in two blood vessels of the rabbit.. European journal of pharmacology, 158(1-2), 21-8.