Prolactin (PRL) is a major anterior pituitary (AP) hormone that is necessary for mammary gland development, lactation, and female fertility. The synthesis and release of PRL from specialized pituitary cells (lactotrophs) is tightly regulated and primarily under tonic inhibition by dopamine (DA), produced by the tuberoinfundibular dopaminergic neurons (TIDA) of the hypothalamus. In turn, PRL regulates its own release by acting on the TIDA neurons in a short feedback loop to increase inhibition by DA and maintain homeostasis. Superimposed on this negative regulation are the stimulatory effects of estrogen. Estrogen (E2) stimulates PRL synthesis, secretion, and lactotroph proliferation by direct action at the level of the AP. E2 also stimulates PRL indirectly at the level of the hypothalamus by inhibiting TIDA activity. Finally, E2 can also modify lactotroph response to DA and other PRL-regulatory factors. The normal growth and function of lactotrophs depends on an interactive balance between DA and E2. However, the relative contribution of E2 regulation directly on the pituitary lactotrophs versus hypothalamic DA, particularly during various physiological states in the female, is not well defined and current animal models are unable to distinguish this. Thus, there is a need for a new model with which to study the regulatory role of E2 action at the level of the pituitary versus the hypothalamus.
Through transgenic Cre/loxP technology and homologous recombination, we have generated a selective knockout mouse model in which the primary mediator of E2 action in the lactotroph, estrogen receptor alpha (ERα), is deleted only in pituitary lactotrophs, while conserving E2 action at the TIDA neurons. Herein is the generation of this animal model and its characterization. Results show the successful generation of mice with the ERα knockout targeted to lactotrophs but not in other ERα expressing tissues of these animals.
Estrogen responsiveness of the selective knockout mouse was evaluated. In vivo studies, utilizing a model of spontaneous prolactinoma formation, showed no significant differences in AP growth between mice with the selective deletion of ERα and those without. DA levels in the AP were low in both groups, suggesting that loss of dopaminergic inhibition contributes more to lactotroph proliferation than does a direct action of E2 however further investigation is required to support this conclusion. In vitro studies demonstrated that lactotrophs with ERα deletion did not respond to E2 or E2 antagonist with regard to cell content of PRL, highlighting the direct action of E2 on the lactotroph in modulating PRL synthesis.
Our mouse model, with selective loss of ERα in pituitary lactotrophs, and E2 action at the level of the hypothalamus maintained, will provide an effective tool in elucidating the primary effects of estrogen, as well as other factors, on PRL regulation.