Opioid Peptides Raised Levels of a Heart Hormone That Controls Blood Pressure

What the researchers found

Morphine injected into the brain (intracerebroventricular) was 10 times more potent than intravenous morphine at increasing plasma ANP (atrial natriuretic peptide), a hormone released by the heart that lowers blood pressure and promotes sodium excretion.

Leu-enkephalin decreased plasma ANP concentrations, the opposite effect of morphine.

Dynorphin and beta-endorphin (given either into the brain or into the vein) did not change ANP levels at all.

All four opioids increased plasma norepinephrine and epinephrine (stress hormones), but morphine caused increases 10 to 50 times greater than the others.

Ganglionic blockade (cutting the nerve connection from the brain to the heart) significantly reduced morphine's ability to increase ANP, confirming the effect requires an intact autonomic nervous system.

Why it matters

This showed that different opioids have very different effects on cardiovascular hormones. Morphine increases ANP (which lowers blood pressure), while enkephalin decreases it. This may partly explain why morphine lowers blood pressure and has specific cardiovascular effects distinct from other opioids.

How the study worked

Awake, freely moving Sprague-Dawley rats received intracerebroventricular or intravenous injections of morphine, leu-enkephalin, dynorphin, or beta-endorphin. Plasma ANP and catecholamines (norepinephrine, epinephrine) were measured by radioimmunoassay before and after injection. Ganglionic blockade with chlorisondamine tested the role of the autonomic nervous system.

What this study cannot tell us

Tested in rats, not people. Brain injection bypasses normal routes. Only acute single doses were tested. The ganglionic blockade experiment is crude and affects many systems beyond the opioid-ANP pathway.

How to read the evidence

Preliminary animal study in freely moving rats — physiologically relevant but not confirmed in humans.

When this study was published

Published in 1988 — early evidence for opioid-natriuretic peptide crosstalk.

The bigger picture

The connection between brain opioid receptors and heart ANP release adds another dimension to understanding blood pressure regulation and may explain some cardiovascular effects of opioid drugs.

Questions still open

• Do chronic opioid users have altered ANP levels?
• Could this pathway be used therapeutically for heart failure?

Common questions

Read the original research

Effects of morphine and opioid peptides on plasma levels of atrial natriuretic peptide.

Life sciences, 43(10), 851-8

Citation

Crum, R L; Brown, M R. (1988). Effects of morphine and opioid peptides on plasma levels of atrial natriuretic peptide.. Life sciences, 43(10), 851-8.