ABSTRACT
I. INTRODUCTION The neuroendocrine and immune systems are linked through a regulatory loop that permits a bi-directional communication between them [1,2]. These interactions are mediated by hormones produced by neuroendocrine organs (hypothalamus, pituitary gland) acting on immune cells or cytokines produced by the hematopoietic tissues that exert regulatory influences on neuroendocrine structures. The accumulated evidence suggests that these interactions are central to the maintenance of organism homeostasis during stress, inflammation, or infection [3-5]. For example, interleukin-1 (IL-1), derived from immune cells, acts on the hypothalamus and pituitary gland to stimulate secretion of adrenocorticotropin (ACTH) [6,7]. On the other hand, hormones secreted by the pituitary gland such as growth hormone (GH) and prolactin (PRL) modulate immune cell responses via specific hormone receptors present on lymphocytes [8-10] (Fig. 1). In fact, recent evidence suggests that several neuroendocrine hormones and their receptors are expressed by immune cells and that cytokines and their receptors are produced by neuroendocrine and certain peripheral organs. Thus, secretion of these substances provides a milieu that facilitates local (intracrine/autocrine/paracrine) interactions [8]. These hormones or cytokines stimulate specific receptors and signaling intermediates to regulate immune or neuroendocrine function. The focus of this chapter is with the various aspects of signaling that are common among the neuroendocrine and immune systems. We will address the issue of hormones and receptors common between the systems, effects of hormones on immune cells, effects of cytokines on neuroendocrine tissues (primarily the hypothalamus and pituitary gland),
Figure 1 Bidirectional communication between the neuroendocrine system (hypothalamus, pituitary) and the immune system. Hormones released by the hypothalamus stimulate or inhibit pituitary hormone release. The hypothalamic or pituitary hormones affect the functioning of the immune system through specific receptors. In addition to cytokines, immune cells also release neurohormones, which affect the functioning of immune system. See text for additional details.
