Neuroendocrine-immune interactions: immunoregulatory signals mediated by neurohumoral agents

While we generally think of the brain and nervous system as central to most basic life processes, the concept of immune regulation or modulation is a relatively new idea. The novelty of such an association may be rooted in classic concepts of neurologic function describing direct innervation of controlled tissues and stimulation across synapses. The involvement of the neuroendocrine system in the precise control of metabolic and a variety of cellular functions should preface its involvement in defense against and/or surveillance for aberrant cell replication. Moreover, a principal characteristic of any control mechanism is feedback from the affected system (be it an organ or single cell). In this framework, it is not unreasonable to expect bidirectional interactions between the nervous and immune systems. Direct innervation of lymphoid tissues was described a number of years ago. More recently, immunoregulatory function has been demonstrated in vitro with a variety of neuroendocrine molecules such as the biogenic amines, SP, CGRP, SOM, vasopressin, ACTH, the endorphins, enkephalins, neurotensin, NGF and VIP. Now it has been shown that many of these same or similar neuroregulatory molecules are produced by cells of the immune system. The possibility that neurotransmitters or peptides, or both, may play a role in vivo in the maintenance of immunocompetence is supported by the finding that specific receptors for the neurohumoral modulators are present on the surface of immunocompetent cells. Current hypotheses speculate that feedback control mechanisms are manifested through the production of lymphokines, PGs and leukotrienes. Though it has not been possible to clearly demonstrate the reciprocal interaction between the neuroendocrine and immune systems in vivo, the evidence to date points to its inevitability.