Abstract

Introduction

Pain and analgesia have been documented in fishes. Reviewed research illustrates the similarities of fish nociception to that of mammals. A three-phase study evaluating novel methods for assessing analgesia in rainbow trout (Oncorhynchus mykiss) and determining pharmacokinetics of two common analgesic drugs is discussed.

Nociception in Fish

Fishes possess mammalian-like anatomic, biochemical, and functional components for the transmission, mediation, and central processing/modulation of painful stimuli.² Teleost fishes possess peptides, similar to corticotropin-releasing-factor in mammals, that regulate the hypothalamic-pituitary-adrenal axis, activate opioid receptors, and are key integrators of the neuroendocrine and immune systems.^21,22 Proteins analagous to mammalian GABA-benzodiazepine receptors have been documented.¹⁴ Substance P-like immunoreactivity, likely contributing to nociception, is present and distributed similarly in fishes as in mammals.^3,31 Additionally, many studies in fishes have confirmed the presence of µ, δ, and κ opioid receptors, and endogenous opioid activity that modulates response to stressors.^6,24,26,30 Social stressors in teleosts produce a marked opioid-mediated immunosuppression, lessened by administration of opioid antagonists.^13,24

Prostaglandin-mediated inflammatory mechanisms in fishes also appear to be equivalent to those in mammals. Teleosts possess cyclooxygenase (COX) enzymes identical to those of mammals,^16,18,27 which are inhibited by COX-2-inhibiting non-steroidal anti-inflammatory drugs (NSAIDs). Prostaglandins exert a protective effect on the gastric mucosa and are decreased by COX-2 inhibitors in fishes.¹²

Goldfish show an escape response to electric shock, which is lessened by the administration of morphine and increased by naloxone.^11,17 Cod and steelhead react to electric stimulation, but were found to react less after intranasal administration of opioids and NSAIDs.⁵

Nociception or Pain?

Although fishes have been shown to avoid known sources of pain, suggesting a “pain memory” and thus a coordinated response,^4,23 there is disagreement over whether central processing or “awareness” of pain occurs, which is necessary for the experience of fear or suffering^7,25. This debate centers on the presumed lack of a “consciousness” in fishes, due to an absence of areas in the cerebral cortex responsible for these functions in humans.²⁹ In contrast, several recent studies in the Netherlands have addressed the welfare aspects of fish slaughter methods.^20,28 The authors believe that, regardless of the presence of fish consciousness and despite this semantic controversy, humane medical practice dictates the incorporation of appropriate analgesics into medical treatments and clinical procedures for fishes. Despite documentation of analgesia produced by chemical agents^5,11 there is a paucity of information about effects, pharmacokinetics, and dosages for commonly available analgesic agents.

New Methods to Evaluate Analgesia in Fish

Most studies assessing pain and analgesia in fishes have relied on behavioral avoidance responses to noxious stimuli, primarily electric shock.^5,11,17 In other species, the ability of an analgesic drug to reduce the minimum alveolar/anesthetic concentration (MAC) of an inhalant anesthetic agent has been used as an objective measurement of the drug’s analgesic properties.^8,9,19 To the authors’ knowledge, no study has attempted a similar study design using aquatic anesthetic agents with non-air breathing animals.

We have completed the first phase of a study, which determined a “minimum gill concentration” (MGC) of MS 222 in rainbow trout (Oncorhynchus mykiss) to determine the effect of varying dosages of butorphanol and ketoprofen on this MGC. During this MGC study, fish were placed in one or more of five tanks filled with MS 222 in varying concentrations. Once immobilization was achieved, a 22-gauge hypodermic needle was inserted into the caudal peduncle near the caudal fin as a noxious stimulus. If there was an escape response associated with the application of the stimulus, the fish was transferred to an increased concentration of MS 222 and the process repeated. This titration process continued until the concentration of MS 222 required to prevent a response to the stimulus was determined (MGC). One half of the fish were then given butorphanol, and the remainder given ketoprofen, both by IM injection in the caudal epaxial musculature.

Measurable relevant plasma biochemical changes in response to pain or other stressors in teleosts include an increase in cortisol, glucose, and lactate.^1,10,15 Sodium, potassium, and chloride concentrations have been variously reported to increase or decrease with stress.^10,15 The degree of changes in these parameters in response to stress is cumulative and depends upon the severity and duration of the stressors.¹ The second phase of our study will evaluate whether antinociception in rainbow trout is reflected by reduced alterations in these parameters. If so, fish that receive an effective analgesic before a noxious stimulus should exhibit significant differences in these biochemical parameters compared with a control group, implying a reduction in stress. Analgesic medication being the sole variable, this reduction in stress would be defined as analgesia.

Effective Analgesic Dosages in Fish

Since opioid receptors and mediators and prostaglandin-mediated inflammatory processes have been found to be similar in fishes and mammals, we expect that butorphanol and ketoprofen will have analgesic effects in fish. No pharmacokinetic studies for either drug have been reported in non-mammalian species. The third phase of our study will determine the pharmacokinetics of both butorphanol and ketoprofen in rainbow trout.

Conclusions

Despite a common assumption that the capacity to perceive pain is related to an animal’s position in the phylogenetic hierarchy, research has shown that pain perception in fishes and other non-mammalian vertebrates is likely to be analagous to that of mammals. It is imperative that we develop appropriate protocols for the inclusion of analgesics in piscine treatments.

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