Age-related decrease of somatostatin receptor number in the normal human thymus

The thymus exhibits a pattern of aging oriented toward a physiological invo lution. The structural changes start with a steady decrease of thymocytes, whereas no major variations occur in the number of thymic epithelial cells (TEC). The data concerning the role of hormones and neuropeptides in thymic involution are equivocal. We recently demonstrated the presence of somatos tatin (SS) and three different SS receptor (SSR) subtypes in the human thym us. TEC selectively expressed SSR subtype 1 (sst(1)) and sst(2A). In the pr esent study we investigated whether SSR number is age related in the thymus . Binding of the sst(2)-preferring ligand I-125-Tyr(3)-octreotide was evalu ated in a large series of normal human thymuses of different age by SSR aut oradiography and ligand binding on tissue homogenates. The score at autorad iography and the number of SSR at membrane homogenate binding (B-max) were inversely correlated with the thymus age (r =-0.84, P < 0.001; r =-0.82, P < 0.001, respectively). The autoradiographic score was positively correlate d with the B-max values (r = 0.74, P < 0.001). Because the TEC number in th e age range considered remains unchanged, the decrease of octreotide bindin g sites might be due to a reduction of sst(2A) receptor number on TEC. The age-related expression of a receptor involved mainly in controlling secreti ve processes is in line with the evidence that the major changes occurring in TEC with aging are related to their capabilities in producing thymic hor mones. In conclusion, SS and SSR might play a role in the involution of the human thymus. These findings underline the links between the neuroendocrin e and immune systems and support the concept that neuropeptides participate in development of cellular immunity in humans.