In this experiment, acetic acid was injected into lips of trout. Cortisol levels significantly rose in the fish. Cortisol results in the release of endorphins and endogenous opioids that can act as painkillers within the central nervous system.
Unusually, there was no increased activity when alarm pheromone was then introduced into tank of the fish.
Afterwards, the fish were returned to a tank:
"Our findings provide new evidence that fish are considerably affected by pain ... and that the perception is not just a simple nociceptive¹ reflex."
"These findings may reflect an ability to prioritize motivational drivers in fish, and as such provide evidence for central processing of pain rather than merely showing a nociceptive reflex."
¹ A nociceptor is a sensory receptor that reacts to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process is called nociception.