Stress-Induced Inhibition of the Hypothalamic-Pituitary-Thyroid Axis Is Attenuated in the Aged Fischer 344/N Male Rat

Abstract

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 and thyroid-stimulating hormones, which are influenced by stress, the plasma levels of these two hormones were also determined during immobilization (Immo). To accomplish this, young (3-month-old) and old (23-month-old) male 344/N Fischer rats were immobilized for 2 h; blood was collected by cannulation from the tail artery at different intervals during Immo (0, 5, 30, 60, and 120 min), and brains were removed at the end of Immo. The basal plasma levels of thyroid-stimulating hormone were similar in both groups, but were significantly and progressively inhibited by Immo in young, but not in old rats. The baseline plasma levels of total triiodothyronine were slightly lower in old than in young rats; Immo caused a significant decrease of total triiodothyronine levels only in the young animals. The baseline plasma levels of free triiodothyronine were similar and were not altered by Immo in either age group. The paraventricular nucleus thyrotropin-releasing hormone mRNA levels were lower in old than in young rats under basal conditions; under stress they were significantly inhibited in young, but remained unchanged in old rats. The basal thyroid-stimulating hormone beta mRNA levels in the anterior pituitary were significantly lower in old than in young rats, but were not affected by Immo in either age group. The plasma prolactin levels were similar at baseline and were significantly increased by Immo in both age groups, but significantly more in old than in young rats. The plasma growth hormone levels were also similar at baseline; they were significantly decreased by Immo to a similar extent in both age groups. In summary, these data indicate that the stress-induced decrease in plasma thyroid-stimulating hormone is in part mediated at the level of the hypothalamic thyrotropin-releasing hormone neuron and that this phenomenon is attenuated in the aged rat.

Age and Gender Influence Basal and Stress-Modulated Hypothalamic-Pituitary-Thyroidal Function in Fischer 344/N Rats

“…This differs somewhat from our previous report, in which there were nonsignificant increases in pituitary TSH (3 mRNA in old rats after IMMO (control: 196 ± 39 dpm/mg; IMMO: 276 ± 77 dpm/mg [6]). The regulation of TSH (3 subunit is a complex phenomenon involving both stimulatory and inhibitory elements.…”

“…As compared with male rats, females exhibit no difference in hypothalamic content of thyrotropin-releasing hormone (TRH) [1], increased or decreased pituitary thyroid-stimulating hormone (TSH) content [2,3], unchanged [1] or decreased [2] plasma observation that hypothalamic TRH mRNA expression is decreased concomitantly during immobilization stress in the F344 male rat suggests that this inhibitory effect is in part centrally mediated [6].…”

“…In male rats, these peripheral changes have been associated with decreased gene expression of hypo thalamic TRH and pituitary TSFIp, and with diminished circulating levels of TSH, indicative of central hypothy roidism [9]. Immobilization-mediated inhibition of the HPT axis is markedly attenuated in old versus young F344/N male rats [6]. Whether stress inhibits the activity of the HPT axis in females similarly to males, and wheth er the stress-induced inhibition of this axis is attenuated at the older age in females as in males remains unknown.…”

Effects of Chronic Social Stress on Neuroendocrine Responsiveness to Challenge with Ethanol, Dexamethasone and Corticotropin-Releasing Hormone

“…While adrenal steroids (cortisol in humans and corticosterone (CORT) in rodents) are important mediators of the stress response, current interest has focused on other potentially important stress-related hormones. For example, the levels of anterior pituitary hormone prolactin (PRL) in blood was increased by both physical and psychological stressors of humans [3,4,5] and of experimental animals [6,7,8]. …”

The Effects of Acute and Chronic Immobilization Stress and Aloe vera Extract on Serum Levels of TSH, T3 and T4 in Male Rats

“…However, there are other studies that show thyroid hormone levels decrease during chronic immobilization stress (7,30). Other studies also argue that chronic immobilization stress decreases plasma levels of TSH and hypothalamic TRH mRNA levels in young rats (32,33). In justifying the lack of effect of chronic immobilization stress on levels of hormones T3, T4 and TSH, adaptation of rats with this stress (13) may fail in changing the activity of thyroid gland during the implementation of this type of stress.…”