Slugs showed naloxone-reversible stress-induced analgesia mediated by endorphin and enkephalin pathways — proving opioid pain control evolved over 500 million years ago.
500+ million years conservedOpioid pain control system functional in invertebrate slugs
What the researchers found
Stress-induced analgesia in slugs appears mediated by endogenous opioid peptides, particularly enkephalins and beta-endorphin. The opioid pain system is evolutionarily ancient.
Why it matters
Finding opioid-based pain relief in slugs shows this system existed hundreds of millions of years before mammals. It is one of the most ancient and conserved peptide systems in biology.
How the study worked
Slugs were stressed by tail pinch and tested on a hot plate. Various opioid peptides were injected into the foot. Opioid antagonists and enzyme inhibitors were used to confirm the mechanism.
What this study cannot tell us
Slugs have a very simple nervous system compared to mammals. The term 'analgesia' is used loosely since slugs may not experience pain the way mammals do.
How to read the evidence
Preliminary animal study in an invertebrate model — demonstrates evolutionary conservation.
When this study was published
Published in 1989 — established opioid analgesia in an invertebrate species.
The bigger picture
The opioid pain system is not just a mammalian feature — it exists in invertebrates. This extreme evolutionary conservation underscores its fundamental biological importance.
Questions still open
- At what point in evolution did the kappa (dynorphin) system emerge?
- Can simple invertebrate models accelerate opioid pain research?
Common questions
Do invertebrates feel pain?
Why is evolutionary conservation important?
Read the original research
The involvement of opioid peptides in stress-induced analgesia in the slug Arion ater.
Peptides, 10(1), 9-13
Citation
Dalton, L M; Widdowson, P S. (1989). The involvement of opioid peptides in stress-induced analgesia in the slug Arion ater.. Peptides, 10(1), 9-13.