Homeostasis, thymic hormones and aging

The thymic-pituitary axis constitutes a bidirectional circuit where the asc ending feedback loop is effected by thymic factors of epithelial origin. Th e aim of the present article is, first, to introduce the idea of an immune- neuroendocrine homeostatic network in higher animals. Next, the relevance o f the thymus in th is network a nd the possible role of this gland in the n euroendocrine imbalances associated with aging are discussed. A number of s tudies are next reviewed which show that the endocrine thymus produces seve ral bioactive molecules, generally called thymic hormones, which in additio n to possessing immunoregulatory properties are also active on nervous and endocrine circuits. In particular, the reported activities of thymosin frac tion five, thymosin alpha 1 and thymosin beta 4 on beta-endorphin, adrenoco rticotropic hormone, glucocorticoids, luteinizing hormone-releasing hormone and luteinizing hormone secretion in different animal and cell models are reviewed. The known hypophysiotropic actions of other thymic hormones like thymulin, homeostatic thymus hormone and thymus factor are also summarized, and the impact of aging on pituitary responsiveness to thymic hormones is discussed. As a conclusion, it is proposed that in addition to its central role in the regulation of the immune function, the thymus gland may extend its influence to nonimmunologic components of the body, including the neuro endocrine system. The early onset of thymus involution might, therefore, ac t as a triggering event which would initiate the gradual decline in homeost atic potential that characterizes the aging process.