Research has found that opioids, such as morphine, have been effective in reducing pain from heat in rodents, cats, and poultry. Morphine is a strong pain-killer in people and other mammals, so much so, that it is used as a standard test to compare other opioids against.
In this experiment, heat was applied to the skin of goldfish. The temperature was raised in small increments until the fish responded as if to swim away. When the goldfish received an injection of morphine, they were able to withstand a significantly higher level of heat.
"In rodents, cats and poultry, tests of heat nociception¹ have been used successfully to assess the efficacy of different analgesics, including opioids (Hughes, 1990;Dixon et al., 2002; Robertson et al., 2003; Morgan et al., 2006; Taylor et al., 2007)."
"Morphine has strong analgesic properties in mammals and is the standard against which other opioid analgesics are assessed (Rang et al., 1999)."
"A belt made of chamois leather and vet-wrap with a small patch of gauze held the heater and sensor in contact with the fish skin."
"For all tests, the temperature was increased at a mean rate of 0.9 8C s-¹"
"An escape response was defined as a response that would have propelled the fish away from the source of stimulation had its movement not been limited."
"Test showed that the thermal threshold 30 min after injection was significantly higher when the fish received morphine than when they received saline injection"
"Thus, the findings supported the current hypothesis, and the earlier observations by Bardach (1956), that goldfish, like birds, reptiles and mammals, perceive heat as noxious, and that the avoidance responses observed were caused by activation of heat sensitive nociceptors, and not by thermal receptors."
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.