Somatostatin receptors and their ligands in the human immune system

Maintenance of homeostasis is essential for survival of the mammalian organism. For a long time it was believed that the different systems in the human body act independently from each other to achieve this goal. However, during the last decades it has become more evident that the different systems in the human body integrate and regulate different functions in close interaction. Numerous studies have provided evidence that the immune, endocrine and neural systems interact to maintain homeostasis. The first suggestion that neuropeptides and neurohormones might play regulatory roles in the immune system came with the detection of corticotropin (ACTH), which is a peptide hormone, in the immune system. Since then, expression of various neuroendocrine hormones, neuropeptides and their receptors has been described in cells of the immune system (for a review see. In addition, lymphoid organs are extensively innervated and neuropeptides can reach and might act on lymphocytes and macrophages via this route. On the other hand, expression of cytokines and its receptors, which were thought to be restricted to the immune system, has been described in the neuroendocrine system (3). These findings point to the existence of bi-directional regulatory pathways between the immune and neuroendocrine systems and have led to an increasing number of studies, evaluating neuroendocrine-immune interactions. These interactions are very complex. Many reciprocal interactions are still poorly understood and many mechanisms are still difficult to interpret. However, it has become more evident that the neuroendocrineimmune interactions are very important in maintaining homeostasis and that disturbances in these links may be involved in pathological conditions. For instance, infections are regarded by the neuroendocrine system as stressors and the neuroendocrine system has to maintain homeostasis of the body. Activation of the immune system by an agent not only evokes potential dangers to the agent, but potentially also to the integrity of the host, because overly vigorous responses may kill the host in the process of controlling an infection. Therefore, the neuroendocrine system has to constantly monitor and, if necessary, regulate the immune cell functions. On the other hand, the immune system needs the neuroendocrine system to help determining how to respond in case of an invading agent. Any disturbances in the cross-talks between both systems could be very important in the development or maintenance of disease and a clearer understanding of these interactions may have important therapeutical applications.

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