Cytokine modulation of pituitary hormone secretion.

A rapidly expanding body of evidence indicates that cytokines do indeed regulate pituitary hormone secretion. Recent studies with cytokines in vivo and in vitro support the idea that cytokines are the principal mediators of the neuroendocrine responses previously observed in infectious and inflammatory states. The dominant route of this modulation appears to be via the brain and hypothalamus, although a role for direct effects on the pituitary has not been excluded. These effects may be mediated by circulating cytokines, endogenously produced cytokines, or both. A number of receptor systems and second messengers may be involved, and a role for arachidonate metabolite pathways appears particularly likely. A final question: Of what use to the organism is the ability of immune activation to control pituitary hormone secretion? For some pituitary secretions there is a reasonable basis for speculation. Glucocorticoids serve to limit the severity of immune responses and recent studies argue that defects in this pathway permit the expression of autoimmune disease. Inhibition of thyroid function may limit the catabolic side effects of infectious illness. Stimulation of growth hormone could have the same effect, and growth hormone and prolactin may serve to enhance some immune responses.