Influence of Age on the Control of Thyrotropin Secretion by Thyrotropin-Releasing Hormone in the Male Rat

Abstract

Alterations with age in the control of thyrotropin (TSH) secretion by thyrotropin-releasing hormone (TRH) were evaluated at the hypothalamic and pituitary levels in young (3–5 months) and old (22–24 months) male rats. In the hypothalamus, TRH was quantified in the median eminence and in the mediobasal hypothalamus; in the adenohypophysis the membrane receptors for TRH were evaluated as well as the accumulation of TRH in the gland. As for TSH, its concentration was determined in the anterior pituitary gland and in plasma. In the hypothalamus, the concentration of TRH did not differ between young and old rats in the whole mediobasal hypothalamus, but it was significantly less in the old rats at the level of the median eminence (29.9 ± 2.8 vs. 52.2 ± 4.3 ng/mg protein). In the adenohypophysis, the density of receptors for TRH was greater in the old than in the young rats (23.2 ± 3.2 vs. 13.7 ± 1.1 fmol MeTRH/mg gland)-with no change in the affinity constant - and the amount of TRH detected was larger (10.8 ± 1.8 vs. 2.8 ± 0.6 pg/mg gland), illustrative of an age-related increase in TRH accumulation in the pituitary gland. The latter results are contrasting with the findings of unchanged pituitary and plasma concentrations of TSH as well as unmodified TSH response to TRH in old rats. The present data concerning TRH and the analogy with previous observations regarding dopamine in old rats are indicative of reduced neuronal activities with age at the hypothalamic level associated with impairments in the processing of the hypothalamic hormones at the pituitary level.

Age-dependent different action of curcumin in thyroid of rat.

“…FT3 and FT4 serum levels in old rats significantly fall down in comparison with young animals. These results are in line with the findings of Donda et al [15], Cizza et al [24] and Moreira et al [25] who demonstrated a decrease in FT3 and FT4 serum levels in 18-month-old rats. Those authors confirmed a gradual decrease in total T3 and T4 levels with age in rats.…”

“…On the basis of literature data it is known that the level of the thyroid hormone falls down, while TSH content is not changed in old rats [15,16]. The biosynthetic activity of the thyroid gland was examined by measuring free thyroxine and triiodothyronine levels, as well as the colloid volume, the percentage of thyroid follicles and the height of secretory epithelium.…”

Aging and Subclinical Thyroid Dysfunction

“…The effect of aging on pituitary deiodinases type 1 and type 2 (D1, D2) is still awaiting further confirmatory studies. Donda and Lemarchand-Beraud found an increase of D1 and deiodinase type 2 (D2) activities in the pituitary of old male rats, while we found both pituitary deiodinase activities to be decreased in the old female rat [36]. No further information seems to be available, although suggestions of a "partial central hypothyroidism" and less efficient response of the hypothalamic-pituitary axis to lower circulating thyroid hormones are found often enough [32].…”

“…Hypothalamic TRH content is reduced in aged rats [32,35] and thyrotroph response to TRH is mostly reported as decreased, both in rats [29,32,36] and [37,38] in humans. Non-stimulated TSH concentration has also been reported as relatively diminished by aging in a large population of older persons without hyperthyroidism, and in aging patients with resistance to thyroid hormone (and their non-affected relatives) [27].…”

“…"Normal" circulating levels of TSH are frequently seen in aged rats, in spite of their low serum thyroid hormone levels [30,31,36]. This may be attributed to the secretion of a TSH with increased sialylation and diminished biological activity [39] as reported in some types of central hypothyroidism [40] and/or to a diminished response of the thyroid to TSH (less TSH receptors or defective transduction of its signal).…”