The measurement of melanin-concentrating hormone in trout blood

Two methods are described for measuring the titres of melanin-concentrating hormone (MCH) in trout plasma. One involves the extraction of MCH from 1-ml plasma onto C18 Sep Pak cartridges, after which the eluted peptide is measured by conventional radioimmunoassay. In the alternative method, antibodies are bound onto immunobeads which are added to 0.5 ml plasma. After incubation for 24 hr, the beads are washed to remove the plasma and are incubated with 125I-labelled MCH; the following day, the labelled beads are separated by centrifugation, washed, and counted. The relative advantages of each method is discussed. Using these two methods, it is shown that the plasma concentration of the hormone is significantly higher in fish from white tanks (>50 pmol/litre) than in fish from black tanks (∼ 10 pmol/litre) or those kept in the dark (∼ 5 pmol/litre). The plasma concentration of MCH changes rapidly when trout are moved from one coloured background to another, indicating its involvement in physiological colour change.