Role of thymic peptides as transmitters between the neuroendocrine and immune systems

Thymic peptides, a heterogenous family of polypeptidic hormones synthesized within the thymus, not only exert important regulatory effects within both the immune and neuroendocrine systems but are also themselves subject to c ontrol by hormones derived from the hypothalamic-pituitary-adrenal axis (HP A) and other endocrine glands. Regarding thymic hormonal function, thymulin production is up-regulated by several hormones, including prolactin, growt h hormone and thyroid hormones. Other aspects of the physiology of thymic e pithelial cells can also be modulated by hormones and neuropeptides, partic ularly cytokeratin expression, cell growth and production of extracellular matrix proteins, thus characterizing the pleiotrophic action of these molec ules on the thymic epithelium. Conversely, thymic-derived peptides also reg ulate hormone release from the HPA axis and may act directly on target endo crine glands of this axis, modulating gonadal tissues. In addition, it has recently been shown that thymulin can modulate some peripheral nervous sens ory functions, including those related to sensitivity to pain. According to the dose given, thymulin induces or reduces hyperalgesia related to both m echanical and thermal nociceptors and thus represents an important interfac e between the immune, endocrine and nervous systems.