STRESS-INDUCED INHIBITION OF THE HYPOTHALAMIC-PITUITARY-THYROID AXIS IS ATTENUATED IN THE AGED FISCHER 344 N MALE-RAT/

Aging is associated with a progressive decrement in the basal activity of the hypothalamic-pituitary-thyroid axis in male Fischer 344/N rats . The aim of this study was to determine whether stress influences the activity of this axis in young and old rats. As prolactin and growth hormone share some regulatory mechanisms with thyrotropin-releasing an d thyroid-stimulating hormones, which are influenced by stress, the pl asma levels of these two hormones were also determined during immobili zation (Immo). To accomplish this, young (3-month-old) and old (23-mon th-old) male 344/N Fischer rats were immobilized for 2 h; blood was co llected by cannulation from the tail artery at different intervals dur ing Immo (0, 5, 30, 60, and 120 min), and brains were removed at the e nd of Immo. The basal plasma levels of thyroid-stimulating hormone wer e similar in both groups, but were significantly and progressively inh ibited by Immo in young, but not in old rats. The baseline plasma leve ls of total triiodothyronine were slightly lower in old than in young rats; Immo caused a significant decrease of total triiodothyronine lev els only in the young animals. The baseline plasma levels of free trii odothyronine were similar and were not altered by Immo in either age g roup. The paraventricular nucleus thyrotropin-releasing hormone mRNA l evels were lower in old than in young rats under basal conditions; und er stress they were significantly inhibited in young, but remained unc hanged in old rats, The basal thyroid-stimulating hormone beta mRNA le vels in the anterior pituitary were significantly lower in old than in young rats, but were not affected by Immo in either age group. The pl asma prolactin levels were similar at baseline and were significantly increased by Immo in both age groups, but significantly more in old th an in young rats. The plasma growth hormone levels were also similar a t baseline; they were significantly decreased by Immo to a similar ext ent in both age groups. In summary, these data indicate that the stres s-induced decrease in plasma thyroid-stimulating hormone is in part me diated at the level of the hypothalamic thyrotropin-releasing hormone neuron and that this phenomenon is attenuated in the aged rat.