Reduction in activity by noxious chemical stimulation is ameliorated by immersion in analgesic drugs in zebrafish.
There have been suggestions that analgesics be used by fish researchers. But in the absence of dose-response data for morphine, this suggestion seems imprudent. The purpose of the present study was to develop a dose-response relationship in fish using six doses of morphine. The response (movement of the fins or tail) to a noxious stimulus (electrical shock to the face region) was monitored before and after a dose of morphine intraperitoneally (i.p.). The i.p. dose of morphine ED(50) in rainbow trout was 6.7 ± 0.8 mg/kg (n = 12 at each dose). The plasma morphine concentration EC(50) was 4.1 ± 1.5 mg/L. In a second experiment, rainbow trout tested with equal amounts of morphine and naloxone (30 mg/kg) showed that the antinociceptive effect of morphine was blocked by naloxone. It has been suggested that stress-induced analgesia has been a confounding factor in some fish studies. However, plasma cortisol levels in our study indicated that stress was not a confounding factor in the present experiments. The ED(50) for morphine in fish was higher than that reported for humans or other mammals. Our observation showing a dose-response relation for morphine using a noxious stimulus supports arguments for its effectiveness as an antinociceptive drug in fish.