Histometric investigation of the activity of the pituitary-interrenal axis in juvenile coho salmon, Oncorhynchus kisutch walbaum

The activity of the pituitary-interrenal axis in juvenile coho salmon (Oncorhynchus kisutch) and the relationship of this activity to the numbers of circulating leucocytes has been studied, using histological and histometric techniques. To this end, the effects of injections of mammalian ACTH, of Cortisol acetate and of dexamethasone on the interrenal tissue, on the epsilon cells in the pituitary gland, and on the differential leucocyte counts were investigated. In addition, the activity of the pituitary-interrenal axis and the related changes in leucocyte counts were determined throughout their year of stream residence. Further, the response of this axis, along with corresponding hematological changes to environmental alterations in the laboratory, were studied. Injections of ACTH resulted in a dosage-related stimulation of the interrenal tissue of coho salmon fry. On the other hand, injections of Cortisol acetate and of dexamethasone produced a marked atrophy of the interrenal tissue. All dosages of either ACTH or Cortisol acetate decreased the mean nuclear diameters of epsilon cells, and resulted in their degranulation. In addition, a decrease in numbers of circulating small lymphocytes and thrombocytes resulted from administration of all dosages of ACTH, Cortisol acetate or dexamethasone. It is proposed that the interrenal tissue of coho fry is capable of marked variations in activity, that this tissue is under pituitary control, and that a negative-feedback mechanism operates between the interrenal and the pituitary gland. Additionally, changes in pituitary-adrenocortical activity are reflected in characteristic alterations in numbers of certain circulating leucocytes. The pituitary-interrenal axis of juvenile coho salmon in their natural habitat is inactive, from the time of emergence in spring, and through summer and early fall, compared with the winter and spring samples of yearling coho. In addition, numbers of circulating small lymphocytes were decreased in the winter and spring samples of yearling coho compared with summer and autumn samples. It is suggested that the increased activity of the pituitary-interrenal axis along with changes in leucocyte counts observed in the winter sample of juvenile coho salmon are related to cold-temperature acclimation. Furthermore, it is proposed that the increased pituitary-adrenocortical activity noted in the latest of the three spring samples of yearling coho salmon is associated with the transformation from parr to smolt. The interrenal tissue of juvenile coho salmon maintained in continuous darkness, or exposed to a continuously flashing light for varying numbers of days, was generally more active than that of corresponding control fish subjected to a twelve hour photoperiod. Additionally, small-lymphocyte and thrombocyte counts for darkness-maintained and flash-exposed fish were lower than values for corresponding control samples. On the other hand, no consistent differences in activity of the interrenal tissue were found when fish maintained in continuous light for varying numbers of days and corresponding control fish were compared. It was observed that the pituitary-interrenal axis of juvenile coho salmon was initially stimulated following transfer of the fish from holding tanks to an altered environment. In addition, the activity of the interrenal tissue was increased by exposure of these fish to cold water temperatures; this increased interrenal activity was accompanied by a lymphopenia. It is suggested that the pituitary-interrenal axis of juvenile coho salmon is involved in cold-temperature acclimation. It is concluded that the pituitary-interrenal axis of juvenile coho salmon undergoes marked fluctuations in activity as a result of environmental alterations within the laboratory. Furthermore, an increase in pituitary-adrenocortical activity during acclimatization is characteristically reflected in a decrease in number of.circulating small lymphocytes.Science, Faculty ofZoology, Department ofGraduat