Abstract

Teleost fish demonstrate the neurophysiological capacity to experience pain and analgesia. Current methods of assessing pain and analgesia in fish suffer from methodological constraints. A common, objective model to assess the effects of analgesic drugs is determination of the minimum anesthetic concentration (MAC), defined as the minimum concentration of anesthetic at which 50 percent of a population fails to respond to a noxious stimulus. To lay groundwork for the use of a MAC depression model in teleost fish, we investigated the feasibility and utility of a MAC determination model in goldfish (Carassius auratus).

A population of 44 sarasa comet goldfish was tested (average weight 30 grams). Fish were individually immersed in MS222 for 4 minutes and subsequently evaluated for response to a supramaximal stimulus (25-gauge needle insertion into epaxial muscles). MS222 concentration for each fish was determined according to an up-down method of population sampling, starting at 160 parts per million (ppm) and proceeding in increments of 10 ppm based on a positive or negative response of the previous fish. Initial MAC (MAC_i) was determined 6 times in 1 week. As part of a separate ongoing project, fish were then anesthetized with MS222 for 8 minutes weekly for 7 weeks. MAC was also determined following saline injection (MAC_sal) at week 3. At week 8, final MAC (MAC_f) was re-determined 6 times within 1 week.

MAC_f (mean 182 ppm, SD 7.5 ppm) was significantly different from MAC_i (mean 162 ppm, SD 7.5 ppm) (p = 0.018). MAC_sal (170 ppm) was not significantly different from MAC_i (p > 0.05) but was significantly different from MAC_f (p<0.001). We conclude that MS222 allowed consistent and repeatable MAC determination in the short term, but MAC increased over time and/or repeated exposure. Repeated exposure to MS222 may increase anesthetic needs in fish, and may also limit the usefulness of a MAC depression model for studying analgesic effect of drugs in teleost fish.