The inhibition of prolactin secretion due to intrahypothalamic pituitary grafts is reversed by estradiol benzoate but not by progesterone

Anterior pituitary (AP) tissue grafted into the hypothalamus inhibited the luteotrophic response to mating and prevented pseudopregnancy (PSP) and pregnancy. All normal rats given 10 micrograms estradiol benzoate (EB) on estrus became PSP (15 days) while the same treatment caused 10-day PSPs in 20/21 grafted rats. Doses of 30 micrograms EB or 10 micrograms EB plus reserpine (1 mg/kg) resulted in 15-day PSP in grafted rats. By contrast progesterone (P; 10 mg on estrus) did not prolong cycles in rats with hypothalamic grafts though it did in 50% of normals. Earlier studies showed that PSP or pregnancy was restored in the grafted rats by blockade of dopamine (DA) secretion. The results above show that EB was similarly effective in restoring PSP while P was not, suggesting that EB both raised prolactin and lowered DA while P was unable to lower DA in rats with AP grafts in the hypothalamus.