Morphine uptake, disposition, and analgesic efficacy in the common goldfish (Carassius auratus)

The purposes of the present study were to examine the rate of morphine uptake in goldfish (Carassius auratus (L., 1758)) when administered via the ater, to calculate the pharmacokinetics of morphine when administered intraperitoneally, and to determine whether morphine could act as an analgesic. When administered via the water, morphine uptake was very slow, and the concentration accumulated in the plasma was <1% of that in water after 2 h. Furthermore, changing water pH or hardness caused small changes in morphine uptake from the water, but plasma levels remained <1% of water concentrations after 2 h exposure. The pharmacokinetics of morphine administered intraperitoneally (40 mg/kg) revealed a half-time for elimination of 37 h and a mean residence time of 56 h. Finally, morphine acted as an analgesic when administered via the water as demonstrated by significantly decreased rubbing behaviour in response to the presence of a noxious stimulus (subcutaneous injection of 0.7% acetic acid). Although morphine appeared to have analgesic properties in goldfish, morphine administered via ambient water is not recommended because of its slow rate of uptake.