Morphine acts directly on the central nervous system to relieve pain.
Naloxone is able to block the activity of morphine in humans.
This experiment shows that naloxone also blocks morphine in fish. Additionaly, it shows that Prolyl-leucyl-glycinamide (MIF-1) has a similar effect.
"Prolyl-leucyl-glycinamide (MIF-1), the C-terminal tripeptide of oxytocin, and naloxone were administered intracranially to goldfish."
"Threshold levels of pain were determined by the voltage necessary to produce an agitated swimming response (ASR). Both MIF-1 and naloxone were found to significantly reduce the analgesic effects of morphine."