Hypophysectomy depresses immune functions in rainbow trout

As the immune system is known to be influenced by the endocrine system, the effects of hypophysectomy on immune functions were examined in the rainbow trout ( Oncorhynchus mykiss ). Superoxide anion (O 2 − ) production, accompanied by phagocytosis, was significantly decreased in leucocytes isolated from the head kidney 7 days after hypophysectomy. Significant reduction was also observed in plasma immunoglobulin (Ig) M levels, whereas no change was observed in plasma lysozyme activity. The number of Ig-secreting leucocytes in peripheral blood had decreased after hypophysectomy, although total leucocyte number was not affected. The percentage of Ig-producing leucocytes as assessed by flow cytometry using a monoclonal antibody to trout IgM showed significant reduction in the head kidney. However, hypophysectomy did not affect the number of Ig-producing leucocytes in spleen, thymus or peripheral blood. By RT-PCR, expression of two growth hormones (GH I and II) and prolactin (PRL) mRNA was detected in lymphoid tissues, such as head kidney, spleen, thymus and intestine, as well as in leucocytes from blood and head kidney, indicating the local production of these hormones. These results indicate important roles of hypophyseal hormones produced not only in the pituitary, but also in the lymphoid tissues, in the maintenance of the immune functions in trout.