Central Ghrelin Affects Pituitary-Thyroid Axis: Histomorphological and Hormonal Study in Rats

Abstract

Body weight depends on the balance between energy intake and consumption. An interaction between ghrelin and thyroid function has been reported only in pathophysiological states. We examined whether intracerebroventricular (ICV) administration of ghrelin affects the structure and function of the pituitary-thyroid axis in young adult male rats. Ghrelin (0.3 nmol/5 microl PBS) or an equal volume of PBS were injected every 24 h into the lateral cerebral ventricle for 5 days. Two hours after the last treatment the animals were killed, their pituitaries and thyroids excised and prepared for further histological, immunohistochemical and morphometric investigation. Serum TSH levels were measured by RIA, while the total T(4) and T(3) levels were examined by ECLIA. Ghrelin treatment increased pituitary weight (p < 0.05) when compared to the controls, with no effect on the thyroid weight. Smaller, degranulated TSH-immunopositive cells were noticed within the pituitaries of ghrelin-treated animals; their cellular and nuclear volume as well as the relative volume density of thyrotrophs decreased (p < 0.05) in comparison to the control values. The level of serum TSH was reduced (p < 0.05). In the thyroid parenchyma of ghrelin-treated rats, an increased number of hypofunctioning follicles was noticed, characterized by flattened, weakly Tg-immunoreactive epithelium and colloid distension. The relative volume densities of the follicles and colloid increased (p < 0.05), while the thyroid index of activation rate and the serum level of total T(4) decreased (p < 0.05). In conclusion, centrally applied ghrelin modulated the immunohistomorphometric features of pituitary TSH cells and decreased the level of serum TSH, consequently changing thyroid morphology and function, by reducing the T(4) hormone level in the serum.