Treatment of Pain in Fish

This chapter provides an overview of our current understanding of clinical analgesic use in fish. Recently, the efficacy and pharmacokinetics of several analgesic drugs for use in fish have been investigated, and the most important data indicates that μ-opioid agonist drugs (e.g, morphine) are consistently effective as analgesics across fish species. In addition, bath application of some analgesic drugs may be useful, which affords multiple methods for delivering analgesics to fish. Although few published studies of non-steroidal anti-inflammatory drugs administered to fish show promise, we have much to learn about the analgesic efficacy of most drugs in this class.

Sex identification of the ornamental amazon fish Astronotus ocellatus by videoceloscopy and gonadal biopsy

This study was conducted to evaluate and validate the efficacy and safety of videoceloscopy and gonadal biopsy as sexing methods for the A. ocellatus. A total of 31 adult individuals were used. Florfenicol (50 mg/kg) and morphine (5 mg/kg) were administered intramuscularly during the pre-surgical period. Animals were maintained in a supine position preceding a ventral midline incision and endoscope optics were then utilized for gonad visualization and sex identification. A gonadal fragment was collected using laparoscopic forceps and conditioned in 10 % formalin. To suture the cavity, polyamide yarn was used in a simple and continuous pattern. At 15 days subsequent to surgery, healing was evaluated, and the stitches were removed. Videoceloscopy accuracy and gonadal biopsy effectiveness were 97 % and 83 %, respectively. Total time devoted in the videoceloscopy, gonadal biopsy and surgery was longer for animals identified as males compared to females The survival rate was 100 %. There were differences regarding food consumption at 24 and 36 h post-surgery when compared to control specimens (pre-surgical) Regarding position in the water column, differences were observed at 24 and 72 h after surgery when compared individually to the control specimens. There were differences for interaction behavior at 24, 36 and 60 h, and regarding search for hiding places at 12 and 24 h after surgery in relation to the control specimens. The applied videoceloscopy and gonadal biopsy surgical techniques are, therefore, effective and safe for A. ocellatus sexing procedures.

Zebrafish early life stages as alternative model to study 'designer drugs': Concordance with mammals in response to opioids

The number of new psychoactive substances (NPS) on the illicit drug market increases fast, posing a need to urgently understand their toxicity and behavioural effects. However, with currently available rodent models, NPS assessment is limited to a few substances per year. Therefore, zebrafish (Danio rerio) embryos and larvae have been suggested as an alternative model that would require less time and resources to perform an initial assessment and could help to prioritize substances for subsequent evaluation in rodents. To validate this model, more information on the concordance of zebrafish larvae and mammalian responses to specific classes of NPS is needed. Here, we studied toxicity and behavioural effects of opioids in zebrafish early life stages. Synthetic opioids are a class of NPS that are often used in pain medication but also frequently abused, having caused multiple intoxications and fatalities recently. Our data shows that fentanyl derivatives were the most toxic among the tested opioids, with toxicity in the zebrafish embryo toxicity test decreasing in the following order: butyrfentanyl>3-methylfentanyl>fentanyl>tramadol> O-desmethyltramadol>morphine. Similar to rodents, tramadol as well as fentanyl and its derivatives led to hypoactive behaviour in zebrafish larvae, with 3-methylfentanyl being the most potent. Physico-chemical properties-based predictions of chemicals' uptake into zebrafish embryos and larvae correlated well with the effects observed. Further, the biotransformation pattern of butyrfentanyl in zebrafish larvae was reminiscent of that in humans. Comparison of toxicity and behavioural responses to opioids in zebrafish and rodents supports zebrafish as a suitable alternative model for rapidly testing synthetic opioids.

Mapping of Morphine-Induced OPRM1 Gene Expression Pattern in the Adult Zebrafish Brain

Morphine is a potent analgesic opiate commonly used in treating pain, and it is also a substance of abuse and highly addictive. Hence, it is vital to discover the action sites of morphine in the brain to increase its efficacy of treatment. In the present study, we aimed at identifying comprehensive neuroanatomical locations that are sensitive to morphine in the adult zebrafish (Danio rerio). We performed in situ hybridization to localize the mu opioid receptor (oprm1) gene and to map the morphine sensitive brain areas using neuronal PAS domain-containing protein 4a (npas4a), an early gene marker. Real-time PCR was used to detect changes in mRNA levels of oprm1 and npas4a in control and acute morphine treated fish (2 mg/L; 20 min). Intense positive oprm1 signals were seen in the telencephalon, preoptic area, habenula, hypothalamic area and periventricular gray zone of the optic tectum. Acute morphine exposure significantly increased oprm1 and npas4a mRNA levels in the medial zone of dorsal telencephalon (Dm), ventral region of the ventral telencephalon (Vv), preoptic area, and in the hypothalamus but a decrease in oprm1 and npas4a signals in the dorsal habenula. This study provides a detailed map of oprm1 localization in the brain, which includes previously unreported oprm1 in the habenula of teleost. Presence of oprm1 in multiple brain sites implies multiple action targets of morphine and potential brain functions which could include reward, cognitive and negative emotions.

Evolution of nociception and pain: evidence from fish models

In order to survive, animals must avoid injury and be able to detect potentially damaging stimuli via nociceptive mechanisms. If the injury is accompanied by a negative affective component, future behaviour should be altered and one can conclude the animal experienced the discomfort associated with pain. Fishes are the most successful vertebrate group when considering the number of species that have filled a variety of aquatic niches. The empirical evidence for nociception in fishes from the underlying molecular biology, neurobiology and anatomy of nociceptors through to whole animal behavioural responses is reviewed to demonstrate the evolutionary conservation of nociception and pain from invertebrates to vertebrates. Studies in fish have shown that the biology of the nociceptive system is strikingly similar to that found in mammals. Further, potentially painful events result in behavioural and physiological changes such as reduced activity, guarding behaviour, suspension of normal behaviour, increased ventilation rate and abnormal behaviours which are all prevented by the use of pain-relieving drugs. Fish also perform competing tasks less well when treated with a putative painful stimulus. Therefore, there is ample evidence to demonstrate that it is highly likely that fish experience pain and that pain-related behavioural changes are conserved across vertebrates. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Automated monitoring of behaviour in zebrafish after invasive procedures

Fish are used in a variety of experimental contexts often in high numbers. To maintain their welfare and ensure valid results during invasive procedures it is vital that we can detect subtle changes in behaviour that may allow us to intervene to provide pain-relief. Therefore, an automated method, the Fish Behaviour Index (FBI), was devised and used for testing the impact of laboratory procedures and efficacy of analgesic drugs in the model species, the zebrafish. Cameras with tracking software were used to visually track and quantify female zebrafish behaviour in real time after a number of laboratory procedures including fin clipping, PIT tagging, and nociceptor excitation via injection of acetic acid subcutaneously. The FBI was derived from activity and distance swum measured before and after these procedures compared with control and sham groups. Further, the efficacy of a range of drugs with analgesic properties to identify efficacy of these agents was explored. Lidocaine (5 mg/L), flunixin (8 mg/L) and morphine (48 mg/L) prevented the associated reduction in activity and distance swum after fin clipping. From an ethical perspective, the FBI represents a significant refinement in the use of zebrafish and could be adopted across a wide range of biological disciplines.

Ethical considerations in fish research

Fishes are used in a wide range of scientific studies, from conservation research with potential benefits to the species used to biomedical research with potential human benefits. Fish research can take place in both laboratories and field environments and methods used represent a continuum from non-invasive observations, handling, through to experimental manipulation. While some countries have legislation or guidance regarding the use of fish in research, many do not and there exists a diversity of scientific opinions on the sentience of fish and how we determine welfare. Nevertheless, there is a growing pressure on the scientific community to take more responsibility for the animals they work with through maximising the benefits of their research to humans or animals while minimising welfare or survival costs to their study animals. In this review, we focus primarily on the refinement of common methods used in fish research based on emerging knowledge with the aim of improving the welfare of fish used in scientific studies. We consider the use of anaesthetics and analgesics and how we mark individuals for identification purposes. We highlight the main ethical concerns facing researchers in both laboratory and field environments and identify areas that need urgent future research. We hope that this review will help inform those who wish to refine their ethical practices and stimulate thought among fish researchers for further avenues of refinement. Improved ethics and welfare of fishes will inevitably lead to increased scientific rigour and is in the best interests of both fishes and scientists.

Anaesthetics and analgesics used in adult fish for research: A review

The number of fish used in research has increased in the last decades. Anaesthesia is required when fish must be held immobile and it is crucial to promote fish welfare, because these vertebrates can show signs of stress and/or pain during handling, transport, tagging, sampling and invasive procedures. The use of an inadequate anaesthetic protocol can compromise not only the welfare of the fish, but also the reliability of the research results. Thus, the development of suitable anaesthetic regimes for each fish species is important. This article reviews the main anaesthetic and analgesic agents used in adult fish in a research setting.

Lack of postexposure analgesic efficacy of low concentrations of eugenol in zebrafish

OBJECTIVE

To test the postexposure analgesic efficacy of low doses of eugenol in zebrafish.

STUDY DESIGN

Prospective experimental study.

ANIMALS

A total of 76 large adult zebrafish (Danio rerio).

METHODS

Fish swimming behavior (median velocity, freeze time, high-speed swimming and distance moved in the vertical direction) was recorded in a 1.6 L video arena before and after exposure to eugenol (0, 1, 2, 5, 10 and 20 mg L^-1). In a second experiment, fish were anesthetized with 2-phenoxy-ethanol and treated with an injection of 5% acetic acid (noxious stimulus), and then exposed to 0, 1, 2 and 5 mg L^-1 eugenol. The fish swimming behavior was also recorded.

RESULTS

The higher doses (10 and 20 mg L^-1) reduced the median velocity, high-speed swimming and distance moved in the vertical direction, and increased the freeze time. Zebrafish behavior was not altered by eugenol (1, 2 and 5 mg L^-1) after noxious stimulation.

CONCLUSIONS AND CLINICAL RELEVANCE

The change in the behavior of zebrafish associated with a noxious stimulus can be monitored and is a good model for studying analgesia in fish. Eugenol (10 and 20 mg L^-1) induced zebrafish sedation. The response after a noxious stimulus was not affected by postexposure to lower doses, and thus we cannot recommend its use as an analgesic.

Evidence-Based Advances in Aquatic Animal Medicine

Fish and aquatic invertebrates deserve evidence-based medicine. Pharmacologic information is available; most pharmacokinetic studies are derived from the aquaculture industry and extrapolated to ornamental fish. Conversely, advanced diagnostics and information regarding diseases affecting only ornamental fish and invertebrates require more peer-reviewed experimental studies; the examples of carp edema virus, sea star wasting disease, seahorse nutrition, and gas bubble disease of fish under human care are discussed. Antinociception is also a controversial topic of growing interest in aquatic animal medicine. This article summarizes information regarding new topics of interest in companion fish and invertebrates and highlights some future avenues for research.

Reduction in activity by noxious chemical stimulation is ameliorated by immersion in analgesic drugs in zebrafish

Research has recently demonstrated that larval zebrafish show similar molecular responses to nociception to those of adults. Our study explored whether unprotected larval zebrafish exhibited altered behaviour after exposure to noxious chemicals and screened a range of analgesic drugs to determine their efficacy to reduce these responses. This approach aimed to validate larval zebrafish as a reliable replacement for adults as well as providing a high-throughput means of analysing behavioural responses. Zebrafish at 5 days post-fertilization were exposed to known noxious stimuli: acetic acid (0.01%, 0.1% and 0.25%) and citric acid (0.1%, 1% and 5%). The behavioural response of each was recorded and analysed using novel tracking software that measures time spent active in 25 larvae at one time. Subsequently, the efficacy of aspirin, lidocaine, morphine and flunixin as analgesics after exposure to 0.1% acetic acid was tested. Larvae exposed to 0.1% and 0.25% acetic acid spent less time active, whereas those exposed to 0.01% acetic acid and 0.1–5% citric acid showed an increase in swimming activity. Administration of 2.5 mg l−1 aspirin, 5 mg l−1 lidocaine and 48 mg l−1 morphine prevented the behavioural changes induced by acetic acid. These results suggest that larvae respond to a noxious challenge in a similar way to adult zebrafish and other vertebrates and that the effect of nociception on activity can be ameliorated by using analgesics. Therefore, adopting larval zebrafish could represent a direct replacement of a protected adult fish with a non-protected form in pain- and nociception-related research.

Comparative analgesic efficacy of morphine sulfate and butorphanol tartrate in koi (Cyprinus carpio) undergoing unilateral gonadectomy

OBJECTIVE

To identify pain-related behaviors and assess the effects of butorphanol tartrate and morphine sulfate in koi (Cyprinus carpio) undergoing unilateral gonadectomy. Design-Prospective study.

ANIMALS

90 adult male and female koi.

PROCEDURES

Each fish received saline (0.9% NaCl) solution (which is physiologically compatible with fish) IM, butorphanol (10 mg/kg [4.5 mg/lb], IM), or morphine (5 mg/kg [2.3 mg/lb], IM) as an injection only (6 fish/treatment); an injection with anesthesia and surgery (12 fish/treatment); or an injection with anesthesia but without surgery (12 fish/treatment). Physiologic and behavioral data were recorded 12 hours before and at intervals after treatment.

RESULTS

Compared with baseline values, the saline solution-surgery group had significantly decreased respiratory rates (at 12 to 24 hours), food consumption assessed as a percentage of floating pellets consumed (at 0 to 36 hours), and activity score (at 0 to 48 hours). Respiratory rate decreased in all butorphanol-treated fish; significant decreases were detected at fewer time points following morphine administration. In the butorphanol-surgery group, the value for food consumption initially decreased but returned to baseline values within 3 hours after treatment; food consumption did not change in the morphine-surgery group. Surgery resulted in decreased activity, regardless of treatment, with the most pronounced effect in the saline solution-surgery group. Changes in location in water column, interactive behavior, and hiding behavior were not significantly different among groups. Butorphanol and morphine administration was associated with temporary buoyancy problems and temporary bouts of excessive activity, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Butorphanol and morphine appeared to have an analgesic effect in koi, but morphine administration caused fewer deleterious adverse effects. Food consumption appeared to be a reliable indicator of pain in koi.

Pain in aquatic animals

Recent developments in the study of pain in animals have demonstrated the potential for pain perception in a variety of wholly aquatic species such as molluscs, crustaceans and fish. This allows us to gain insight into how the ecological pressures and differential life history of living in a watery medium can yield novel data that inform the comparative physiology and evolution of pain. Nociception is the simple detection of potentially painful stimuli usually accompanied by a reflex withdrawal response, and nociceptors have been found in aquatic invertebrates such as the sea slug Aplysia. It would seem adaptive to have a warning system that allows animals to avoid life-threatening injury, yet debate does still continue over the capacity for non-mammalian species to experience the discomfort or suffering that is a key component of pain rather than a nociceptive reflex. Contemporary studies over the last 10 years have demonstrated that bony fish possess nociceptors that are similar to those in mammals; that they demonstrate pain-related changes in physiology and behaviour that are reduced by painkillers; that they exhibit higher brain activity when painfully stimulated; and that pain is more important than showing fear or anti-predator behaviour in bony fish. The neurophysiological basis of nociception or pain in fish is demonstrably similar to that in mammals. Pain perception in invertebrates is more controversial as they lack the vertebrate brain, yet recent research evidence confirms that there are behavioural changes in response to potentially painful events. This review will assess the field of pain perception in aquatic species, focusing on fish and selected invertebrate groups to interpret how research findings can inform our understanding of the physiology and evolution of pain. Further, if we accept these animals may be capable of experiencing the negative experience of pain, then the wider implications of human use of these animals should be considered.

Physiological and behavioural responses to noxious stimuli in the Atlantic cod (Gadus morhua)

In the present study, our aim was to compare physiological and behavioural responses to different noxious stimuli to those of a standardized innocuous stimulus, to possibly identify aversive responses indicative of injury detection in a commercially important marine teleost fish, the Atlantic cod. Individual fish were administered with a noxious stimulus to the lip under short-term general anaesthesia (MS-222). The noxious treatments included injection of 0.1% or 2% acetic acid, 0.005% or 0.1% capsaicin, or piercing the lip with a commercial fishing hook. Counts of opercular beat rate (OBR) at 10, 30, 60, 90 and 120 min and observations of behaviour at 30 and 90 min post-treatment were compared with pre-treatment values and with control fish injected with physiological saline, an innocuous stimulus. Circulatory levels of physiological stress indicators were determined in all fish at 120 minutes post-treatment. All treatments evoked temporarily increased OBR that returned to pre-treatment levels at 60 minutes (saline, 0.005% capsaicin, hook), 90 minutes (0.1% acetic acid, 0.1% capsaicin), or 120 minutes (2% acetic acid), but with no significant differences from the control group at any time point. Fish treated with 0.1% and 2% acetic acid and 0.1% capsaicin displayed increased hovering close to the bottom of the aquaria and fish given 2% acetic acid and 0.1% capsaicin also displayed a reduced use of shelter. The only effect seen in hooked fish was brief episodes of lateral head shaking which were not seen pre-treatment or in the other groups, possibly reflecting a resiliency to tissue damage in the mouth area related to the tough nature of the Atlantic cod diet. There were no differences between groups in circulatory stress indicators two hours after treatment. This study provides novel data on behavioural indicators that could be used to assess potentially aversive events in Atlantic cod.

Behavioural effects of the commonly used fish anaesthetic tricaine methanesulfonate (MS-222) on zebrafish (Danio rerio) and its relevance for the acetic acid pain test

The pros and cons of using anaesthesia when handling fish in connection with experiments are debated. A widely adopted practice is to wait thirty minutes after anaesthesia before behavioural observations are initiated, but information about immediate effects of a treatment is then lost. This is pertinent for responses to acute stressors, such as acid injection in the acetic acid pain test. However, omission of anaesthetics in order to obtain data on immediate responses will compromise the welfare of fish and contribute to experimental noise due to stress. We therefore tested the effect of tricaine methanesulfonate on the behaviour of zebrafish. We predicted that tricaine (MS 222) would decrease swimming velocity and that the control fish would show an increased level of anxiety- and stress-related behaviours compared to the tricaine group. Following acclimatization to the test tank, baseline behaviour was recorded before immersion in either tricaine (168 mg l⁻¹, treatment group, N = 8) or tank water (control group, N = 7). Latencies to lose equilibrium and to lose response to touch were registered. The fish was then returned to the test tank, and the latency to regain equilibrium was registered in anaesthetized fish. When equilibrium was regained, and at five, thirty and sixty minutes after the fish had been returned to the test tank, behaviour was recorded. The tricaine fish showed the following responses (mean ± sd): latency to lose equilibrium 22.6 s±3.9; latency to lose response to touch 101.9 s±26.8; latency to regain equilibrium 92.0 s±54.4. Contrary to our predictions, neither treatment caused a change in any of the behaviours registered. This indicates that tricaine has no effect on several commonly used behavioural parameters, and that it may be unnecessary to postpone behavioural observations to 30 min after anaesthesia.

A Novel Behavioral Fish Model of Nociception for Testing Analgesics

Pain is a major symptom in many medical conditions, and often interferes significantly with a person's quality of life. Although a priority topic in medical research for many years, there are still few analgesic drugs approved for clinical use. One reason is the lack of appropriate animal models that faithfully represent relevant hallmarks associated with human pain. Here we propose zebrafish (Danio rerio) as a novel short-term behavioral model of nociception, and analyse its sensitivity and robustness. Firstly, we injected two different doses of acetic acid as the noxious stimulus. We studied individual locomotor responses of fish to a threshold level of nociception using two recording systems: a video tracking system and an electric biosensor (the MOBS system). We showed that an injection dose of 10% acetic acid resulted in a change in behavior that could be used to study nociception. Secondly, we validated our behavioral model by investigating the effect of the analgesic morphine. In time-course studies, first we looked at the dose-response relationship of morphine and then tested whether the effect of morphine could be modulated by naloxone, an opioid antagonist. Our results suggest that a change in behavioral responses of zebrafish to acetic acid is a reasonable model to test analgesics. The response scales with stimulus intensity, is attenuated by morphine, and the analgesic effect of morphine is blocked with naloxone. The change in behavior of zebrafish associated with the noxious stimulus can be monitored with an electric biosensor that measures changes in water impedance.