Thyroid status affects the regulation of prolactin mRNA accumulation by tri-iodothyronine and thyrotrophin-releasing hormone in cultured rat anterior pituitary cells

The effects of tri-iodothyronine (T3) and TRH on prolactin mRNA accumulation in monolayer pituitary cell cultures prepared from both euthyroid and hypothyroid rats were investigated. Basal prolactin mRNA concentrations and prolactin release into culture medium were increased in hypothyroid cultures, the increase being related to the duration of hypothyroidism in vivo. The inhibitory effects of T3 seen in euthyroid cells were preserved in cells derived from hypothyroid animals, and the degree of inhibition was greater in cells from the most severely hypothyroid rats. However, the stimulation of prolactin synthesis and secretion induced by TRH in euthyroid cultures was not found in the hypothyroid cells. Hypothalamic and anterior pituitary TRH content were measured in similarly hypothyroid and euthyroid rats. A large hypothalamic pool of TRH was found, which was unchanged in hypothyroidism, whereas anterior pituitary TRH content was increased in the hypothyroid rats. The consequent down-regulation of anterior pituitary TRH receptors may explain the poor response of prolactin to TRH seen in vitro.