PLASMA SOMATOLACTIN CONCENTRATIONS IN SALMONID FISH ARE ELEVATED BY STRESS

The preliminary finding that plasma levels of somatolactin (SL) were m arkedly elevated following stress caused by confinement in chinook sal mon (Oncorhynchus tshawytscha) prompted a more detailed study of SL dy namics during stress. SL levels have been determined in the plasma of rainbow trout (Oncorhynchus mykiss) during exposure to acute (0-30 min ) and short (0-24 h) periods of stress resulting from handling and con finement. The results show that SL levels increase rapidly within minu tes following the onset of stress, reach a peak between 1 and 2 h, dec line over the next 3 h, and then show an additional increase again by 24 h. During acute stress caused by confinement, the increase in plasm a SL levels occurred within 2 min, thus showing a more rapid response than cortisol. This suggests that the response is mediated directly by the hypothalamus and is not a result of a feedback mechanism. The res ults also demonstrate that SL secretion in response to stress is at le ast partially under genetic control. In the short-term stress experime nt, progeny of fish selected as high responders or low responders to s tress, based on the magnitude of the plasma cortisol levels induced by stress, were used, and these fish showed similarly accentuated or att enuated release of SL following stress. These results clearly demonstr ate that nonspecific environmental stress causes rapid activation of S L-secreting cells in the pars intermedia, suggesting that this hormone has an important role in the adaptive response of fish to stress.