Fenótipos relacionados à dor em peixe-zebra: uma caracterização neurocomportamental utilizando o modelo de dor visceral induzida pelo ácido acético

Abstract

The zebrafish (Danio rerio) is a small freshwater teleost belonging to the Cyprinidae family, which has been studied in different scientific areas. This species has evolutionarily conserved genes and a relatively complex behavioral repertoire, which can be modulated by several pharmacological drugs, such as algogens and analgesic drugs. Although recent studies have shown that zebrafish is an emerging model organism for assessing nociception, specific phenotypes that may indicate local pain are still poorly understood. This thesis aimed to characterize specific behavioral phenotypes in the presence of algogens to validate a model of visceral pain induced by intraperitoneal administration of acetic acid. In a first study, we demonstrated that intraperitoneal injection of acetic acid (2.5 and 5.0%) induced a response similar to the abdominal constriction in rodents, which was assessed by measuring the abdominal curvature index. All doses tested (0.5–5.0%) reduced the distance traveled and vertical activity in the novel tank test. The duration of freezing increased after 5.0% acetic acid, while fish injected with 1.0, 2.5 and 5.0% spent more time in the top area of the tank. Both morphine (an opioid analgesic) and diclofenac (a nonsteroidal anti-inflammatory drug, NSAID) prevented acetic acid-induced changes (5.0%) in the body curvature index, while naloxone antagonized the analgesic effects of morphine. Although morphine attenuates pain-like responses in zebrafish, there are no data showing whether the opioid receptor antagonism prolongs pain duration in the absence of an exogenous opioid. In a second report, we investigated whether a common opioid antagonist, naloxone, affects the constriction-like response. Animals were injected intraperitoneally with acetic acid (5.0%), naloxone (1.25 mg / kg; 2.5 mg / kg; 5.0 mg / kg) or acetic acid with naloxone to investigate changes in the body curvature for 1 h. As expected, acetic acid elicited pain responses in zebrafish for 30 min, while no effect was observed after PBS injection. Although naloxone alone does not change the frequency and duration of this behavior, it dose-dependently prolongs the acetic acid-induced abdominal curvature response, suggesting that endogenous opioids may have a key role in recovering this specific phenotype in the acute visceral pain model. Finally, in the third study, we performed a systematic review on the importance of zebrafish models for pain-related studies. Albeit the anatomical differences between teleost fishes and mammals, orthologs of genes involved in nociception (e.g., opioid receptors, transient potential receptors, acid-sensitive ion channels, and cannabinoid receptors) show a high degree of genetic similarity. These data support the involvement of an evolutionarily conserved cellular machinery for nociceptive responses in zebrafish. Overall, the results obtained here a support a new strategy to assess pain-related behavioral parameters in zebrafish with high predictive, face, and construct validity.