Immune-neuroendocrine interactions during aging: age-dependent thyrotropin-inhibiting activity of thymosin peptides

Thymosin fraction 5 (TF-5), a partially purified thymic preparation, has been previously shown to have luteinizing hormone-releasing hormone (LH-RH)-releasing activity in perfused rat hypothalamus as well as in vivo stimulatory effect on the pituitary-adrenal axis in prepubertal monkeys. We report here the effect of TF-5 on the TSH-thyroid axis in young (3 months) and old (25 months) Sprague-Dawley male rats. Conscious free-moving animals carrying an indwelling atrial cannula received a single dose of 5 mg/kg body wt. of either bovine serum albumin (BSA) or TF-5 via the cannula. In the young rats, TF-5 induced a marked reduction of plasma thyrotropin (TSH) which was significantly greater than the normal circadian decline observed in the BSA-treated controls. The old males displayed high basal levels of TSH which showed no circadian rhythmicity, and did not respond to TF-5. Thyroxine (T4), triiodothyronine (T3), corticosterone, and prolactin levels were not affected by TF-5 at the dose levels tested. The old rats had significantly lower basal levels of T4, but not T3, than their young counterparts. The synthetic peptides thymosin alpha-1 and serum thymic factor, which are components of TF-5, had no effect on the above hormones when injected in doses up to 5 micrograms/kg body wt. Acute thymectomy in 3-month-old males induced a significant increase in basal levels of TSH without affecting plasma T4 or T3. These results suggest that the thymus has an inhibitory action on TSH in the rat, which is not mediated by the thyroid gland.(ABSTRACT TRUNCATED AT 250 WORDS)