ABSTRACT
I. INTRODUCTION The endocrine and immune systems are closely linked via an elaborate communication system constituted by an array of cytokines and neuropeptides which interact to modulate the integrated response of an organism to infection [1,2]. This bi-directional network between the two systems allows hormones and neuropeptides to affect immune function and, in turn, cytokines to induce neuroendocrine changes [3,4]. This tight communication is made possible, because both systems ‘‘speak’’ a common language by sharing a common set of ligands and receptors of classical hormones, neuropeptides, and immunoregulatory mediators [5]. Indeed, in addition to possessing numerous hormonal receptors classically associated with endocrine tissues, the cells of the immune system produce numerous hormones and neuropeptides [6-10]. Thus, in addition to the wellknown immunosuppressive effects of adrenal glucocorticoids, many neuropeptides and hormones are involved in the modulations of the immune processes. For example, hypothalamic hormones such as corticotropin-releasing-hormone (CRH) and somatostatin as well as pituitary hormones such as growth hormone (GH) and prolactin (PRL) have been shown to modulate the immune system function. CRH, the major stressintegrating peptide, does act centrally as an immunosuppressant agents [11-13]. This central immunosuppressive effect is possibly linked to an effect of CRH on the central sympathetic nervous system, but is independent of circulating glucocorticoids since it is still active in adrenalectomized animals. CRH has also been shown to be produced peripherally at inflammatory sites by cells of the immune system. However, in contrast
to its immunosuppressive effect at central level, CRH produced peripherally has proinflammatory effects. Interestingly, by blocking CRH action at inflammatory sites, administration of neutralizing antibodies against CRH or of Antalarmin, a CRH antagonist, diminishes or prevents the inflammatory process [11]. Somatostatin, another hypothalamic hormone, has also been found to be produced in the periphery, at inflammatory sites, by cells of the immune system [10]. Somatostatin exerts anti-inflammatory actions and may probably participate in the anti-inflammatory action of glucocorticoids [14]. The two pituitary hormones, GH and PRL, are also produced by cells of the immune system and have been shown to be potent immunoenhancers [15-19]. Thus, hormones produced by the immune system do locally modulate numerous immune functions and participate in the regulation of inflammation. Being produced in relatively small amounts, these hormones produce essentially autocrine and paracrine actions; exceptionally, they can also function in an endocrine fashion by acting on other cells at a distance, causing some clinical endocrine syndromes. One case of Cushing’s syndrome due to an ectopic secretion of ACTH by a granulamatous mass has been reported [20] as well as one case of acromegaly due to a non-Hodgkin’s lymphoma producing GH [21].
