Changes in circulating levels of neuroendocrine and thymic hormones during aging in rats: A correlation study

It is well established that during early life the thymus gland and the neuroendocrine system influence each other's maturation. Furthermore, there is a growing body of evidence indicating that the immune and neuroendocrine systems also function as a bidirectional network during adult life. In order to assess possible changes in the thymicneuroendocrine network during aging, we undertook to measure and correlate the circulating levels of several neuroendocrine and thymic hormones in young (3 month) and old (26 month) male Sprague-Dawley rats. Sequential plasma samples were obtained from chronically cannulated, nonstressed animals every 30 min for 5 h. Two days later rats were killed between 11:30 a.m. and 1:30 p.m. and trunk serum was obtained. All hormones were measured by radioimmunoassay. Growth hormone (GH), prolactin (PRL), thyrotropin (TSH) and corticosterone were measured in plasma, whereas thyroxine (T4), triiodothyronine (T3), thymosin α1 (Tα1) and thymosin β4 (Tβ4) were determined in trunk serum. The circulating levels of T3, PRL, corticosterone and Tβ4 did not show significant differences between young and old rats, whereas GH, T4, Tα1, and thymus weight showed a significant age-related reduction. The anterior pituitary (AP) weight and plasma TSH were significantly higher in old than in young rats. Three pairs of parameters showed highly significant levels of linear correlation: AP weight vs. Tα1; thymus weight vs. T4 and Tα1 vs. T4 (p < 0.01, p < 0.001 and p < 0.001, respectively). Our results suggest that thymus involution during aging has a significant impact on serum Tα1 levels, but not on Tβ4, and that some age changes in thyroid and thymic function are closely correlated in the rat.