Interactions between the neuroendocrine and immune systems: common hormones and receptors.

The studies reviewed here support a molecular basis for bidirectional communication between the immune and neuroendocrine systems. The main findings can be summarized as follows: First, cells of the immune system can synthesize biologically active neuroendocrine peptide hormones. Second, immune cells also possess receptors for many of these peptides. Third, these same neuroendocrine hormones can influence immune function; and fourth, lymphokines can influence neuroendocrine tissues. Although recent studies have begun to unravel the biochemistry of bidirectional communication between the immune and neuroendocrine systems, there are still missing parts in this puzzle. Among the important questions that must be resolved are the identification of factors that trigger the synthesis of neuroendocrine hormones by immune cells. Are these events operating similar to or in balance with pituitary cells? Drugs that interfere with either pathway may be useful. Second, it will be of value to understand the factors controlling neuroendocrine hormone receptor expression on immune cells. A better understanding of the spectrum of positive and negative regulatory events for both systems may determine the ultimate behavior of immune and neuroendocrine cells. In addition, since leukocytes can produce hormones and also have receptors for the same hormones (e.g., ACTH and GH), it is possible that these immunocytes may also influence their own function in an autocrine-like fashion. We have postulated that the immune system can serve as a sensory organ for external stimuli that cannot be detected by the nervous system (Blalock 1984). Thus, the immune system recognizes stimuli such as bacteria, viruses or tumors, whereas the nervous system detects classical sensory stimuli. The contribution of extrapituitary sites of hormone production and function may provide new clues to define psychological and/or pathological states in the pathophysiology of infectious diseases and tumors.