Russian scientists held fish in a water chamber. They applied painful short bursts of electricity to the tail fin. The tail jerked in response.
Pain-killers were given to the fish. This then allowed higher voltages to be applied before the fish responded.
However, if a chemical known to block pain-killers was then given, the fish again jerked their tails at the lower voltages.
After the experiment, different species took varying times to recover. The sturgeon swam slowly and preferred to lie at the bottom of the tank - this lasted for 5 days.
The scientists concluded that fish feel pain, which affects their physiology and behaviour.
"Optico-mechanical system was used to record the response to painful electrical stimulation before and after administration of analgetic agents."
"The fish was semirigidly fixed in a flow chamber (in the region of the mouth and pectoral fins). The gills were continuously moistened with water. The stimulating electrodes were inserted into the caudal fin blade in order to exclude the direct stimulation of muscle fibers."
"The recording apparatus was a movable wire "fork" embracing the caudal peduncle in the posterior third of the body. In response to painful stimulation (bursts of short pulses 0.5 ms of current 0.5-2.0 mA, with frequency 300/s), the fish moved its caudal peduncle and deviated "the fork" from the zero point."
"Drugs were administrated by different ways - peritoneally, subcutaneously, intranasally."
ANALGESICS REDUCED PAIN IN TROUT, COD AND CARP
"In rainbow trout, intranasal administration of dermorphine 0.20-0.75 mg/kg caused a concentration-dependent decrease in the pain sensitivity by 12-55%. The analgesic effect was usually observed within 10 min after administration and it lasted for at least 1 h (up to 2-3 h in some fish)."
"In cod, intranasal administration of beta-casomorphine 2.5-12.5 mg/kg and peritoneal one 10-30 mg/kg decreased the pain sensitivity by 15-37% and 14-35%, respectively."
STURGEON TOOK 5 DAYS TO RECOVER AFTER PAIN EXPERIMENT
"Preliminary experiments have shown that sturgeon possess of nociception¹ as well as bony fishes. Its reacted to painful electrical stimuli with the same behavior – a jerk of the tail. Their nociceptive thresholds were comparable to that of carp. The pattern of response was the same after administration of the 100 nmol/g tramadol solution to carp and sturgeon."
"However, the period of recovery in sturgeon lasted more than in carp, for at least 3-5 days – fish demonstrated slowly swimming and prefered to lie at the bottom."
KNOWN ANALGESTIC BLOCKERS REDUCED INCREASED PAIN LEVELS
"In carp, nociceptive thresholds significantly increased following the intramuscular injection of agonists mu, delta, and kappa opioid receptors, tramadol 10-100 nmol/g, DADLE 10-50 nmol/g, and U-50488 30-80 nmol/g, respectively."
"Antinociceptive effects of opioid agents were blocked or significantly reduced by pretreatment with naloxone."
"Local subcutaneous injections of 2% solution of novocainum blocked the nociceptive reactions."
"Our results indicate that, like higher vertebrates, fish also develop a prolonged analgesia in response to agonists of the opioid mu receptors. Hence, fish have an antinociceptive system consisting of the opioid receptors similar to those in terrestrial vertebrates. The opioid receptors were first found in mammals and shown to mediate the effects of morphine and its derivates (analgesia, addiction, etc.). They are also targets for endogenous opioid peptides: enkephalins and endorphins."
FISH HAVE RECEPTORS FOR THEIR OWN NATURAL ANALGESICS
"In mammals, the analgesic effects are primarily mediated through mu opioid receptors. Our results indicate that in fish, the same receptors are responsible for increasing the pain threshold. The decrease in pain sensitivity under the action of nonopioid preparations analginum and sidnophenum as well as analgesy caused by stress, illustrates the presence in fishes of other endogenous analgesic systems in addition to the opioid system."
PAIN CONTROL SIMILAR TO HIGHER VERTEBRATES
"It is reasonably safe to suggest that fish similar to higher vertebrates have nociceptive and both opioid and nonopioid antinociceptive systems that take part in the control of fish behavior and physiological status on the level of central mechanisms."
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.