Stress-induced elevation of plasma α-MSH and endorphin in brown trout, Salmo trutta L.

Handling and confinement caused a pronounced elevation in the plasma cortisol levels of brown trout. This response was more rapid, and the elevation greater, at 13.4 than at 5° although basal cortisol levels were also higher at the warmer water temperature. The large increase in plasma cortisol caused by handling and confinement was not accompanied by any changes in the plasma levels of either α-MSH or endorphin. However, when handling and confinement was combined with a thermal shock, not only was there a rapid and pronounced elevation in plasma cortisol, but there were also concomitant and sustained rises in the plasma levels of both α-MSH and endorphin. The levels of α-MSH and endorphin induced by the thermal shock were considerably higher than those recorded in long-term, black-adapted brown trout, the only other circumstance in fish known to cause an elevation of the plasma levels of these two peptides. These results indicate that handling and confinement only activated the corticotrophs of the pars distalis, not the melanotrophs of the neurointermediate lobe, whereas when combined with a thermal shock, both cell types were activated.