Having previously debunked the claim that fish don’t feel pain, the University of Liverpool’s Dr Lynne Sneddon has become a leading figure in the movement to reduce, replace and refine the use of animals in scientific research (University of Liverpool, 2017).

Uncomfortable with the increasing use of adult fish in pain research, she and her colleague Dr Javier Lopez-Luna at the University of Liverpool’s Institute of Integrative Biology have led a new study to investigate a potential alternative.

The researchers noted “Previous studies have identified multiple subtypes of pain receptors in zebrafish…even as early as a few days post-fertilization.” They wanted to see if they could they replace the adult fish that are used in research with larvae that are only a few days old. This relied on them being able to prove that the larvae respond to pain and any discomfort could be relieved.

The study, published in the Journal of Experimental Biology, involved exposing five day post-fertilisation zebrafish embryos to dilute concentrations of acetic acid and citric acid, both of which are known to irritate adult fish, and tracking the larvae’s activity with software produced by researchers from the University of Liverpool’s Department of Electrical Engineering and Electronics.

Analysing the minute fish’s motion, they noticed that the larvae became less active in the two most dilute concentrations of acetic acid (0.01 and 0.1%). However, the most concentrated acetic acid (0.25%) and all three concentrations of citric acid (0.1, 1 and 5%) stimulated the fish to swim harder and farther, possibly in a bid to escape the uncomfortable sensation. But when the team administered pain relief to the disturbed fish larvae, in the form of aspirin, morphine and lidocaine, their discomfort appeared to be relieved and their behaviour returned to normal.

Now, having confirmed that larval fish are capable of experiencing pain and benefit from pain relief, the researchers recommend that larval zebrafish can be used as a model for the study of pain and pain receptors, sparing many of the adult fish that are currently used in toxicity tests.