Enkephalins Found in Ancient Fish Brains — But Not Dynorphin, Suggesting Different Evolutionary Origins | Research

The takeaway

Authentic met-enkephalin and leu-enkephalin were detected in holostean fish and lungfish brains, but dynorphin-related peptides were not — suggesting the enkephalin system is evolutionarily more ancient.

Enkephalins present, dynorphins absent
Two ancient fish lineages had confirmed enkephalins but no detectable dynorphin-related peptides

What the researchers found

Authentic met-enkephalin and leu-enkephalin were detected in holostean fish and lungfish brains. Dynorphin-related peptides were not detected in either species.

Why it matters

These ancient fish sit at key points in vertebrate evolution. Finding enkephalins but not dynorphins suggests the enkephalin system is more evolutionarily ancient.

How the study worked

Brain extracts were fractionated by Sephadex G-50 chromatography and analyzed by reverse-phase HPLC with radioimmunoassay for multiple opioid peptides.

What this study cannot tell us

Negative results for dynorphin could reflect low abundance rather than true absence. Detection methods from 1991 were less sensitive than modern techniques. Only two species studied.

How to read the evidence

Preliminary comparative study limited by 1991-era detection sensitivity. Negative results should be interpreted cautiously.

When this study was published

Published in 1991. Opioid peptide evolution has been further mapped with genomic approaches.

The bigger picture

Tracing opioid peptide evolution helps us understand which aspects of the opioid system are ancient and conserved vs. recently evolved. The enkephalin system appears to be a more fundamental vertebrate feature.

Questions still open

At what point in vertebrate evolution did the dynorphin system appear?
Are the undetected dynorphins truly absent or just below detection limits?

Common questions

Why study opioid peptides in fish?

Fish evolved hundreds of millions of years before mammals. Finding which opioid peptides they have tells us which parts of the opioid system are ancient and which evolved later.

Could the dynorphins just be at undetectable levels?

Possibly. The detection methods of 1991 were less sensitive than modern techniques. However, the clear presence of enkephalins and absence of dynorphins across two species suggests a real difference in their evolutionary appearance.

Read the original research

The phylogeny of Met-enkephalin and Leu-enkephalin: studies on the holostean fish Lepisosteus platyrhincus and the Australian lungfish, Neoceratodus forsteri.

General and comparative endocrinology, 84(2), 228-36

View on PubMed ↗

Citation

McDonald, L K; Joss, J M; Dores, R M. (1991). The phylogeny of Met-enkephalin and Leu-enkephalin: studies on the holostean fish Lepisosteus platyrhincus and the Australian lungfish, Neoceratodus forsteri.. General and comparative endocrinology, 84(2), 228-36.