Use of a shuttle box model to assess the behavioral and analgesic effects of opioid injections in goldfish (Carassius auratus).

Objective: To evaluate if opioid-induced behavioral effects, such as sedation, can be detected using a shuttle box experimental apparatus and whether thermal preference following noxious stimulation using mustard oil is reversed by morphine administration in fish.
Methods: 5 goldfish (Carassius auratus) underwent 2 randomized blinded experimental trials, with a crossover study design. First, opioid effects were tested in a shuttle box without painful stimulus. Fish were injected 5 days apart with butorphanol at 0.4 or 10 mg/kg, morphine at 5 or 10 mg/kg, or saline IM. After 30 minutes, each fish was placed in a shuttle box for 2 hours with a temperature gradient of 26 to 28 °C. Temperature preference, time spent immobile, and swimming velocity were assessed. The second trial consisted of cutaneous noxious stimulation using mustard oil immersion for 5 minutes followed by an assessment of thermal preference for 4 minutes in the shuttle box after either morphine at 10 mg/kg or saline IM injections. Linear mixed models were used to compare treatment groups.
Results: Before noxious stimulation, a low dose of morphine caused sedation compared with control group and high-dose morphine and butorphanol treatments. Immersion in mustard oil caused fish to spend more time in the cold area in the control group. Morphine administration reversed this pattern.
Conclusions: The sedative and analgesic effects of opioids were detected through this model.
Clinical Relevance: The shuttle box model could be used to assess the analgesic effects of other opioids in goldfish while reducing biases associated with the sedative and stimulatory effects of these drugs.