Met-enkephalin and leu-enkephalin were found in distinct neuron populations in dogfish shark brain, but dynorphin-related peptides were not detected.
Separate neuron populationsMet-enkephalin and leu-enkephalin were in distinct neurons in shark brain — different functions even in ancient species
What the researchers found
Both met-enkephalin and leu-enkephalin are present in separate neuron populations in the dogfish shark brain, but dynorphin-related peptides were not detected.
Why it matters
Finding enkephalins in such an ancient species confirms these opioid peptides play fundamental roles in brain function that have been preserved for hundreds of millions of years of evolution.
How the study worked
Immunohistochemical staining of dogfish shark (Scyliorhinus canicula) brain sections using antibodies against multiple opioid peptides to map their distribution.
What this study cannot tell us
Single species study using immunohistochemistry, which can have cross-reactivity issues. Absence of staining does not definitively prove peptide absence.
How to read the evidence
Preliminary — single species immunohistochemistry study. Negative results for dynorphin could reflect detection limitations.
When this study was published
Published in 1994 (32 years ago). Comparative opioid studies have since been expanded with genomic approaches.
The bigger picture
Sharks represent one of the most ancient vertebrate lineages. Finding enkephalins but not dynorphin helps date when different opioid systems evolved and confirms enkephalins as the oldest opioid peptide family.
Questions still open
- Do sharks lack the prodynorphin gene entirely, or is expression below detection limits?
- What functions do enkephalins serve in shark neurobiology?
Common questions
Why study opioids in sharks?
Why are met-enkephalin and leu-enkephalin in separate neurons?
Read the original research
Immunocytochemical evidence for the presence of Met-enkephalin and Leu-enkephalin in distinct neurons in the brain of the elasmobranch fish Scyliorhinus canicula.
The Journal of comparative neurology, 347(4), 585-97
Citation
Vallarino, M; Bucharles, C; Facchinetti, F; Vaudry, H. (1994). Immunocytochemical evidence for the presence of Met-enkephalin and Leu-enkephalin in distinct neurons in the brain of the elasmobranch fish Scyliorhinus canicula.. The Journal of comparative neurology, 347(4), 585-97.